Broiler Bacterial Enteritis: Diagnosis and Management

At a Glance

Bacterial enteritis in broiler flocks presents as a syndrome of diarrhoea, poor feed conversion, reduced weight gain, and increased mortality. The primary bacterial agents involved are avian pathogenic Escherichia coli (APEC) causing colibacillosis and Salmonella spp. causing salmonellosis. Diagnosis requires systematic post-mortem examination, bacteriological culture, and antimicrobial sensitivity testing. Management depends on accurate pathogen identification, targeted antimicrobial therapy guided by sensitivity results, and rigorous biosecurity measures to prevent recurrence. The table below summarises the key features distinguishing these two common causes of bacterial enteritis in broilers.

Clinical Presentation and Differential Diagnosis

Bacterial enteritis in broilers presents with non-specific signs that overlap with other enteric diseases. Flock health managers must recognise the syndrome and initiate a systematic diagnostic approach. The most common bacterial causes are colibacillosis caused by avian pathogenic Escherichia coli and salmonellosis caused by Salmonella enterica serovars. Other potential causes include clostridial enteritis, necrotic enteritis, coccidiosis, viral enteritis, and nutritional imbalances.

Clinical signs of bacterial enteritis include watery or mucoid diarrhoea, pasted vents, huddling, ruffled feathers, depression, reduced feed intake, and uneven growth. Mortality may increase gradually or suddenly depending on the pathogen and flock immunity. Affected birds often show poor feed conversion and increased culling rates. Respiratory signs such as sneezing, coughing, and dyspnoea may accompany colibacillosis because APEC frequently causes respiratory tract infection as a primary or secondary pathogen.

The World Organisation for Animal Health (WOAH) provides international standards for the diagnosis and reporting of avian diseases, including salmonellosis and colibacillosis. Flock health managers should consult the WOAH Terrestrial Manual for detailed diagnostic procedures and notification requirements for notifiable Salmonella serovars.

Differential diagnosis requires careful observation of clinical signs, post-mortem lesions, and laboratory results. Coccidiosis produces bloody or mucoid diarrhoea with intestinal thickening and petechiae. Necrotic enteritis causes sudden mortality, dark diarrhoea, and a characteristic foul odour with intestinal mucosal necrosis. Viral enteritis from rotavirus, reovirus, or astrovirus produces watery diarrhoea without the fibrinous lesions typical of colibacillosis. Nutritional enteritis from poor-quality feed or mycotoxins causes diarrhoea without infectious lesions.

Diagnostic Sampling and Laboratory Confirmation

Accurate diagnosis of bacterial enteritis requires systematic sampling and laboratory submission. The flock health manager must collect appropriate samples from affected birds, transport them correctly, and request the right tests. Delayed or improper sampling can lead to false-negative results and inappropriate treatment decisions.

Post-Mortem Examination

Perform post-mortem examination on freshly dead or euthanised birds showing typical clinical signs. Examine at least five birds per flock, selecting birds that have been dead less than four hours or are moribund. Record all gross lesions systematically. For colibacillosis, look for fibrinous pericarditis, perihepatitis, airsacculitis, yolk sac infection in young chicks, and omphalitis. For salmonellosis, examine the liver for necrotic foci, the spleen for enlargement, and the caeca for typhlitis or caecal cores.

Sample Collection and Transport

Collect samples aseptically using sterile instruments and containers. For bacteriological culture, submit liver, spleen, bone marrow, and pericardial swabs for colibacillosis investigation. For salmonellosis, submit caecal tonsils, liver, spleen, and cloacal swabs. Litter samples and boot swabs can detect Salmonella carriage in the flock environment.

Place samples in sterile containers with transport medium if delivery to the laboratory will exceed two hours. Refrigerate samples at 4°C during transport. Do not freeze samples because freezing kills bacteria and reduces culture success. Submit samples to an accredited veterinary diagnostic laboratory within 24 hours of collection.

Laboratory Tests

Request aerobic bacterial culture and identification from the submitted tissues. The laboratory should perform antimicrobial sensitivity testing on all significant isolates using disk diffusion or minimum inhibitory concentration methods. For Salmonella, request serotyping to identify the specific serovar, which is important for epidemiological tracking and biosecurity decisions.

Polymerase chain reaction (PCR) testing can detect bacterial DNA directly from tissues or faeces, providing rapid results within 24 hours. PCR is particularly useful for detecting Salmonella in litter or environmental samples. However, PCR does not provide antimicrobial sensitivity data, so culture remains essential for treatment guidance.

Interpretation of Results

Interpret laboratory results in the context of clinical signs and post-mortem lesions. Isolation of E. coli from pericardium, liver, or bone marrow confirms colibacillosis when accompanied by typical lesions. Isolation of Salmonella from any tissue confirms salmonellosis. However, isolation of E. coli from the intestine alone may represent normal flora and does not confirm colibacillosis.

A systematic review and meta-analysis on the efficacy of vaccination against colibacillosis in broiler production (PloS one, 2024) provides evidence that vaccination can reduce the incidence of colibacillosis. Flock health managers should consider vaccination history when interpreting diagnostic results.

Antimicrobial Therapy

Antimicrobial therapy for bacterial enteritis must be based on culture and sensitivity results. Empirical treatment without sensitivity testing risks treatment failure and promotes antimicrobial resistance. The flock health manager must work with a veterinarian to select the appropriate antimicrobial, dose, route, and duration.

Principles of Antimicrobial Selection

Select an antimicrobial based on the sensitivity pattern of the isolated pathogen. Use the narrowest spectrum antimicrobial that is effective against the specific isolate. Avoid using critically important antimicrobials for human medicine as first-line treatments. The World Organisation for Animal Health provides guidance on the prudent use of antimicrobials in animal agriculture.

Consider the pharmacokinetics of the antimicrobial. Some antimicrobials achieve high concentrations in the intestine and are suitable for enteric infections. Others are absorbed systemically and are better for septicaemic infections. The route of administration matters: water medication is practical for large flocks but may result in variable intake, while injectable antimicrobials ensure accurate dosing but are labour-intensive.

Antimicrobial Resistance

Antimicrobial resistance is a growing concern in broiler production. A study on the current status of β-lactam antibiotic use and characterization of β-lactam-resistant Escherichia coli from commercial farms by integrated broiler chicken operations in Korea (Poultry science, 2023) documented high levels of β-lactam resistance among E. coli isolates from broiler farms. This finding underscores the importance of sensitivity testing before treatment.

A systematic review and meta-analysis on the efficacy of antibiotic treatment in controlling colibacillosis in broiler production (PloS one, 2025) provides evidence that antimicrobial treatment can reduce mortality and clinical signs when the pathogen is sensitive to the chosen drug. However, the review also highlights the variability in treatment outcomes depending on resistance patterns.

Treatment Protocols

Treatment protocols must be prescribed by a veterinarian and must comply with local regulations regarding antimicrobial use, withdrawal periods, and record-keeping. The flock health manager should maintain accurate records of all antimicrobial treatments, including the drug name, dose, route, duration, and withdrawal period.

For colibacillosis, common antimicrobial classes include penicillins, tetracyclines, fluoroquinolones, and sulphonamides. The choice depends on sensitivity results. For salmonellosis, fluoroquinolones and third-generation cephalosporins are often effective, but resistance is increasing. Treatment duration is typically 3-5 days, but severe cases may require longer therapy.

Treatment Failure

Treatment failure occurs when clinical signs do not improve within 48-72 hours of starting therapy. Common causes include antimicrobial resistance, incorrect diagnosis, concurrent viral or parasitic infection, poor drug administration, inadequate dose or duration, and re-infection from the environment.

When treatment fails, re-culture affected birds and repeat sensitivity testing. Review the biosecurity programme to identify sources of re-infection. Consider the possibility of mixed infections involving multiple pathogens. Escalate to a veterinary specialist if the flock does not respond to two consecutive treatment courses.

Biosecurity Measures

Biosecurity is the foundation of bacterial enteritis prevention and control. A systematic review on the role of biosecurity to prevent or control colibacillosis in broiler production (Poultry science, 2024) provides evidence that comprehensive biosecurity programmes reduce the incidence and severity of colibacillosis outbreaks. The same principles apply to salmonellosis control.

Farm-Level Biosecurity

Implement all-in/all-out production to break the cycle of infection between flocks. Clean and disinfect houses thoroughly between flocks, including feed and water systems. Use a disinfectant that is effective against E. coli and Salmonella. Allow adequate downtime between flocks, typically 14-21 days depending on the production system.

Control rodents, wild birds, and insects that can carry and transmit Salmonella and E. coli. Rodent control programmes should include bait stations, exclusion measures, and regular monitoring. Wild bird exclusion requires netting of vents and openings. Insect control focuses on darkling beetles and flies that can carry bacteria between houses.

Litter Management

Litter management directly affects bacterial enteritis risk. Wet litter promotes bacterial growth and increases the risk of enteric disease. Maintain litter moisture below 30% through proper ventilation, drinker management, and litter amendments. Remove wet litter promptly and replace with dry material.

Litter acidification can reduce bacterial populations. A study on the effect of on-farm litter acidification treatments on Campylobacter and Salmonella populations in commercial broiler houses in northeast Georgia (Poultry Science, 2006) demonstrated that acidification treatments can reduce pathogen levels in litter. However, the effectiveness depends on the product used, application rate, and litter condition.

Water Sanitation

Water is a common vehicle for bacterial transmission in broiler houses. Test water quality regularly for bacterial contamination, particularly E. coli and coliforms. Maintain water sanitation through chlorination, acidification, or other approved treatments. Clean drinkers daily to prevent biofilm formation that can harbour bacteria.

Feed Hygiene

Feed can introduce Salmonella into broiler flocks. Source feed from mills that implement Salmonella control programmes, including heat treatment and testing. Store feed in clean, dry conditions to prevent contamination. Use feed additives such as organic acids or probiotics that may reduce Salmonella colonisation.

A study on the cereal type in feed influences Salmonella Enteritidis colonization in broilers (Poultry Science, 2009) found that feed composition can affect Salmonella colonisation. Flock health managers should consider feed formulation as part of an integrated Salmonella control programme.

Hatchery Biosecurity

Hatchery contamination can introduce bacterial enteritis pathogens to day-old chicks. Ensure that hatcheries implement strict biosecurity protocols, including egg sanitation, fumigation, and environmental monitoring. Vaccination of parent flocks against colibacillosis can reduce the transmission of APEC to progeny.

A retrospective observational study from 2016 to 2019 in Finland on vaccinating parent flocks against colibacillosis reduces broiler mortality (Preventive veterinary medicine, 2024) provides evidence that parent flock vaccination can reduce broiler mortality. This approach should be considered as part of a comprehensive colibacillosis control programme.

Vaccination Strategies

Vaccination is an important tool for controlling bacterial enteritis in broiler flocks. Vaccines are available for both colibacillosis and salmonellosis, but their use and effectiveness vary depending on the production system and pathogen serovars present.

Colibacillosis Vaccines

Colibacillosis vaccines are available for parent flocks and, in some regions, for broilers. Autogenous vaccines prepared from farm-specific APEC isolates can provide targeted protection. Commercial vaccines containing multiple APEC serogroups are also available.

A systematic review and meta-analysis on the efficacy of vaccination against colibacillosis in broiler production (PloS one, 2024) provides evidence that vaccination can reduce the incidence of colibacillosis. However, the effectiveness depends on the match between vaccine serogroups and field isolates. Flock health managers should work with a veterinarian to select the appropriate vaccine based on local epidemiology.

Salmonella Vaccines

Salmonella vaccines are used primarily in parent and layer flocks to reduce egg and environmental contamination. Live attenuated vaccines and killed vaccines are available for several Salmonella serovars, including Salmonella Enteritidis and Salmonella Typhimurium.

A study on the effect of oral administration of a homologous hilA mutant strain on the long-term colonization and transmission of Salmonella Enteritidis in broiler chickens (Vaccine, 2008) demonstrated that vaccination can reduce colonisation and transmission. However, vaccination alone is not sufficient to eliminate Salmonella from a flock and must be combined with biosecurity measures.

Vaccine Limitations

Vaccines have limitations that flock health managers must understand. Vaccination does not provide 100% protection and does not eliminate the need for biosecurity. Vaccine efficacy can be reduced by immunosuppression, poor nutrition, stress, and concurrent disease. Vaccines may not protect against all serovars or serogroups present on the farm.

Records and Measurements

Accurate records are essential for diagnosing bacterial enteritis, monitoring treatment response, and evaluating biosecurity effectiveness. The flock health manager should maintain systematic records for each flock.

Clinical Records

Record daily mortality, culling, and clinical signs. Note the onset, duration, and severity of diarrhoea, respiratory signs, and depression. Record feed and water intake daily. A sudden drop in feed intake often precedes clinical signs of enteritis by 24-48 hours.

Post-Mortem Records

Record post-mortem findings for each bird examined. Use a standardised form that includes bird age, weight, and all organ systems. Photograph significant lesions for reference and training. Record the number of birds examined and the percentage showing each lesion type.

Laboratory Records

Maintain a database of all laboratory results, including culture, sensitivity, and PCR results. Track antimicrobial resistance patterns over time to identify emerging resistance trends. Record serovar information for Salmonella isolates to monitor the epidemiology on the farm.

Treatment Records

Record all antimicrobial treatments, including the drug name, dose, route, duration, withdrawal period, and batch number. Record the number of birds treated, the total amount of drug used, and the treatment outcome. This information is essential for regulatory compliance and for evaluating treatment effectiveness.

Biosecurity Records

Record all biosecurity activities, including cleaning and disinfection dates, disinfectant used, downtime between flocks, rodent control measures, and visitor logs. Monitor and record litter moisture, ammonia levels, and ventilation rates. These records help identify biosecurity gaps that may contribute to disease outbreaks.

Common Failure Patterns

Understanding common failure patterns in bacterial enteritis management helps flock health managers avoid mistakes and improve outcomes.

Delayed Diagnosis

Delayed diagnosis is a common failure pattern. Flock health managers may attribute early signs of enteritis to coccidiosis or nutritional problems and delay laboratory investigation. By the time bacterial enteritis is confirmed, mortality may be high and treatment less effective. Submit samples for laboratory testing as soon as enteric disease is suspected.

Inappropriate Sampling

Inappropriate sampling leads to false-negative results. Submitting only faecal samples may miss systemic infections. Submitting dead birds that have been dead for many hours may yield overgrowth of post-mortem invaders. Submit freshly dead or euthanised birds and collect tissues aseptically.

Empirical Antimicrobial Use

Empirical antimicrobial use without sensitivity testing is a major cause of treatment failure and resistance development. Flock health managers may use the same antimicrobial repeatedly, selecting for resistant strains. Always perform sensitivity testing before starting antimicrobial therapy.

Incomplete Treatment Course

Incomplete treatment courses occur when clinical signs improve but the flock health manager stops treatment early. This practice selects for resistant bacteria and increases the risk of relapse. Complete the full treatment course as prescribed by the veterinarian.

Biosecurity Gaps

Biosecurity gaps allow re-infection after treatment. Common gaps include inadequate cleaning and disinfection, insufficient downtime, rodent infestation, and contaminated feed or water. Conduct a biosecurity audit after each disease outbreak to identify and correct gaps.

Ignoring Concurrent Disease

Concurrent disease, such as coccidiosis, infectious bursal disease, or mycoplasmosis, can predispose birds to bacterial enteritis. Treating only the bacterial component without addressing predisposing factors leads to recurrent disease. Investigate and manage all concurrent health problems.

Welfare and Safety Context

Bacterial enteritis has significant welfare implications for broiler flocks. Affected birds experience pain, discomfort, and distress from diarrhoea, dehydration, and systemic infection. Mortality can be high in untreated flocks. Prompt diagnosis and treatment are essential for welfare.

Welfare Indicators

Monitor welfare indicators during bacterial enteritis outbreaks. These include mortality rate, culling rate, body condition score, gait score, and litter quality. Birds with severe diarrhoea may develop pasted vents, which can lead to cloacal obstruction and death. Provide appropriate litter management and ventilation to maintain dry conditions.

Euthanasia Criteria

Establish clear euthanasia criteria for birds that are unlikely to recover. Birds that are unable to stand, have severe dehydration, or show signs of septicaemia should be euthanised promptly using approved methods. The World Organisation for Animal Health provides standards for humane euthanasia of poultry.

Food Safety

Bacterial enteritis pathogens, particularly Salmonella, pose food safety risks to consumers. Flock health managers must implement control measures to reduce Salmonella contamination of broiler meat. This includes biosecurity, vaccination, feed hygiene, and monitoring programmes.

The World Organisation for Animal Health provides international standards for the control of Salmonella in poultry production. Flock health managers should be aware of their regulatory obligations regarding Salmonella monitoring and reporting.

Occupational Safety

Flock health managers and farm workers are at risk of exposure to zoonotic pathogens, including Salmonella and E. coli. Use personal protective equipment when handling sick birds, collecting samples, and cleaning houses. Practice good hand hygiene and avoid eating or drinking in poultry houses.

Professional Escalation Criteria

Flock health managers should escalate cases to a veterinary specialist or diagnostic laboratory under specific circumstances. The following criteria indicate the need for professional escalation.

Urgent Escalation

Escalate immediately if any of the following occur:

• Mortality exceeds 1% per day for two consecutive days
• Clinical signs affect more than 10% of the flock
• Birds show neurological signs such as torticollis, ataxia, or paralysis
• There is suspicion of a notifiable disease, such as highly pathogenic avian influenza or Newcastle disease
• Antimicrobial treatment fails to produce improvement within 48 hours

Routine Escalation

Escalate for routine veterinary consultation in the following situations:

• First occurrence of bacterial enteritis on the farm
• Recurrent bacterial enteritis in consecutive flocks
• Isolation of Salmonella serovars that are notifiable or of public health significance
• Antimicrobial sensitivity results showing multidrug resistance
• Need for autogenous vaccine development

Laboratory Escalation

Escalate to a diagnostic laboratory for the following:

• Confirmation of bacterial enteritis diagnosis
• Serotyping of Salmonella isolates
• Antimicrobial sensitivity testing
• Molecular typing for epidemiological investigation
• Investigation of unusual or severe disease presentations

Practical Decision Framework for Antimicrobial Stewardship in Broiler Bacterial Enteritis

Antimicrobial stewardship in broiler production requires a structured decision framework that balances animal welfare, treatment efficacy, and resistance prevention. Flock health managers face pressure to treat sick flocks quickly, but hasty antimicrobial use without diagnostic confirmation leads to treatment failure and resistance development. A systematic decision framework helps managers make evidence-based choices at each stage of a bacterial enteritis outbreak.

The Five-Step Antimicrobial Decision Framework

This framework guides the flock health manager from initial suspicion of bacterial enteritis through treatment completion and outcome evaluation. Each step includes specific actions, records, and escalation criteria.

Step 1: Clinical Assessment and Outbreak Classification

When a flock shows signs of enteritis, the manager must classify the outbreak severity within the first 12 hours of observation. Use the following classification system based on daily mortality rate, percentage of birds affected, and clinical sign duration.

Mild outbreak: Mortality below 0.3% per day, less than 5% of birds showing clinical signs, diarrhoea present for less than 24 hours. Birds remain active and continue eating and drinking.

Moderate outbreak: Mortality between 0.3% and 0.7% per day, 5% to 10% of birds affected, diarrhoea present for 24 to 48 hours. Some birds show depression and reduced feed intake.

Severe outbreak: Mortality above 0.7% per day, more than 10% of birds affected, diarrhoea present for more than 48 hours. Many birds show depression, huddling, and marked reduction in feed and water intake.

Record the outbreak classification in the flock health record. This classification determines the urgency of diagnostic sampling and the need for immediate veterinary consultation. For severe outbreaks, contact a veterinarian within 4 hours of classification. For mild outbreaks, proceed with diagnostic sampling within 24 hours.

Step 2: Diagnostic Sampling and Laboratory Submission

Submit samples for bacteriological culture and sensitivity testing before any antimicrobial treatment. The only exception is a severe outbreak where mortality exceeds 1% per day, in which case the veterinarian may authorise immediate treatment while samples are collected simultaneously.

Sampling protocol for each outbreak classification:

For mild outbreaks, collect samples from five affected birds. Euthanise three birds showing clinical signs and collect two freshly dead birds that have been dead less than 4 hours. Submit liver, spleen, and intestinal contents for culture. Request aerobic bacterial culture with identification and sensitivity testing.

For moderate outbreaks, collect samples from eight affected birds. Euthanise five birds and collect three freshly dead birds. Submit liver, spleen, bone marrow, and pericardial swabs. Request culture, sensitivity testing, and Salmonella serotyping if Salmonella is isolated.

For severe outbreaks, collect samples from ten affected birds. Euthanise seven birds and collect three freshly dead birds. Submit the same tissues as for moderate outbreaks plus caecal tonsils. Request culture, sensitivity testing, Salmonella serotyping, and PCR for Salmonella detection if rapid results are needed.

Sample transport and documentation: Place each tissue sample in a separate sterile container. Label each container with flock identification, bird number, tissue type, and collection date. Complete a laboratory submission form that includes flock history, clinical signs, post-mortem findings, and any recent antimicrobial use. Transport samples to the laboratory within 24 hours at 4 degrees Celsius. If delivery will exceed 24 hours, contact the laboratory for guidance on alternative transport methods.

Step 3: Antimicrobial Selection and Treatment Initiation

Antimicrobial selection must be based on sensitivity results whenever possible. The framework provides three treatment pathways depending on the availability of sensitivity data.

Pathway A: Sensitivity-guided treatment (preferred)

When sensitivity results are available within 48 to 72 hours of sample submission, select the antimicrobial with the narrowest spectrum that shows sensitivity against the isolated pathogen. Use the following criteria for antimicrobial selection:

Select a first-line antimicrobial that is not classified as critically important for human medicine by the World Organisation for Animal Health. Examples include amoxicillin, tetracycline, or sulphonamide-trimethoprim combinations if sensitivity is confirmed.

Select a second-line antimicrobial only when the pathogen shows resistance to all first-line options. Second-line options include fluoroquinolones or third-generation cephalosporins. Use these only under veterinary prescription and with strict adherence to withdrawal periods.

Select a third-line antimicrobial only when the pathogen shows resistance to first-line and second-line options. Third-line options include colistin or aminoglycosides. Use these only under specialist veterinary guidance and with documented justification.

Pathway B: Empirical treatment with concurrent sensitivity testing

When clinical signs are moderate to severe and the manager cannot wait 48 to 72 hours for sensitivity results, the veterinarian may authorise empirical treatment while samples are being processed. The framework requires the following conditions for empirical treatment:

The outbreak classification must be moderate or severe. Samples must have been collected and submitted before treatment starts. The veterinarian must select an antimicrobial based on the farm's historical sensitivity patterns from the past 12 months. The manager must record the reason for empirical treatment in the flock health record.

If sensitivity results become available during the treatment course and show resistance to the empirical antimicrobial, the manager must contact the veterinarian immediately to discuss changing the treatment. Do not continue an antimicrobial that the sensitivity results show as resistant.

Pathway C: No antimicrobial treatment

For mild outbreaks where birds are eating and drinking normally, the manager may choose not to treat with antimicrobials. This decision requires the following conditions:

The outbreak classification must be mild. Samples must be collected and submitted for culture and sensitivity testing. The manager must implement enhanced biosecurity measures, including increased litter management, water sanitation, and ventilation. The manager must monitor the flock daily and reclassify the outbreak if clinical signs worsen.

If the outbreak progresses to moderate or severe within 48 hours, the manager must contact a veterinarian and initiate treatment following Pathway A or B.

Step 4: Treatment Administration and Monitoring

Once treatment begins, the manager must monitor the flock daily and record specific parameters to evaluate treatment response.

Daily monitoring parameters:

Record mortality count and calculate daily mortality percentage. Record the number of birds showing clinical signs, including diarrhoea, depression, and huddling. Estimate feed intake by weighing feed remaining in feeders or measuring feed disappearance. Estimate water intake by measuring water meter readings or checking drinker line levels. Assess litter moisture by hand squeeze test or moisture meter.

Treatment response evaluation at 48 hours:

At 48 hours after treatment initiation, evaluate the flock using the following criteria:

Good response: Mortality has decreased by at least 50% from the pre-treatment level. Clinical signs have improved in at least 75% of affected birds. Feed and water intake have returned to normal or are increasing.

Partial response: Mortality has decreased by 25% to 50%. Clinical signs have improved in 50% to 75% of affected birds. Feed and water intake are improving but not yet normal.

Poor response: Mortality has not decreased or has increased. Clinical signs have not improved or have worsened. Feed and water intake remain low or have decreased further.

Decision at 48 hours:

For good response, continue the treatment course as prescribed. Complete the full duration, typically 3 to 5 days. Do not stop treatment early even if clinical signs have resolved.

For partial response, contact the veterinarian. Discuss extending the treatment duration, increasing the dose if within the label range, or adding supportive therapy such as electrolytes or probiotics.

For poor response, contact the veterinarian immediately. Re-culture affected birds and repeat sensitivity testing. Review the biosecurity programme for sources of re-infection. Consider the possibility of concurrent disease such as coccidiosis or viral infection.

Step 5: Treatment Completion and Outcome Evaluation

After completing the full treatment course, evaluate the overall outcome and record the results for future reference.

Outcome classification:

Successful outcome: Mortality returned to baseline within 7 days of treatment start. Clinical signs resolved completely. Feed conversion ratio for the flock remained within target range. No recurrence of clinical signs within 14 days of treatment completion.

Partially successful outcome: Mortality decreased but did not return to baseline. Some birds continued to show mild clinical signs. Feed conversion ratio was above target but improved from pre-treatment levels. No recurrence of severe clinical signs.

Unsuccessful outcome: Mortality did not decrease or increased. Clinical signs persisted or worsened. Feed conversion ratio remained poor. Recurrence of clinical signs occurred within 14 days of treatment completion.

Record keeping for each treatment event:

Record the following information in the flock health record: outbreak classification at start, date and time of sample collection, laboratory name and accession number, pathogen isolated and sensitivity pattern, antimicrobial used including drug name, dose, route, and duration, batch number of the antimicrobial, number of birds treated, total amount of antimicrobial used, withdrawal period and date when withdrawal period ends, treatment response at 48 hours, outcome classification, and any complications or concurrent diseases identified.

Records and Measurements for Antimicrobial Stewardship

The flock health manager must maintain a dedicated antimicrobial stewardship record for each farm. This record tracks antimicrobial use over time and helps identify emerging resistance patterns.

Antimicrobial Use Register

Create a register that lists every antimicrobial treatment event for the past 24 months. Include the following columns: flock identification, date treatment started, date treatment ended, antimicrobial name, dose in milligrams per kilogram body weight, route of administration, duration in days, total amount used in grams or kilograms, number of birds treated, pathogen isolated, sensitivity pattern, and outcome classification.

Review this register quarterly with the farm veterinarian. Identify trends in antimicrobial use, such as increasing use of second-line or third-line antimicrobials. Identify recurring pathogens and their resistance patterns. Use this information to adjust the farm's biosecurity programme and vaccination strategy.

Farm-Specific Antibiogram

Develop a farm-specific antibiogram that summarises the sensitivity patterns of bacterial isolates from the farm over the past 12 months. The antibiogram should list each pathogen and the percentage of isolates sensitive to each antimicrobial tested. Update the antibiogram every 6 months or after every 20 isolates, whichever comes first.

Use the antibiogram to guide empirical treatment decisions when sensitivity results are not yet available. If the antibiogram shows that more than 80% of E. coli isolates from the farm are sensitive to amoxicillin, then amoxicillin is a reasonable empirical choice. If sensitivity is below 50%, choose a different first-line antimicrobial.

Treatment Outcome Database

Maintain a database of treatment outcomes for each antimicrobial used on the farm. Record the pathogen, sensitivity pattern, antimicrobial used, and outcome classification for each treatment event. Analyse this database annually to identify which antimicrobials are most effective for specific pathogens on the farm.

Common Failure Patterns in Antimicrobial Stewardship

Understanding common failure patterns helps managers avoid mistakes that lead to treatment failure and resistance development.

Delayed Sample Collection

Managers often delay sample collection while waiting to see if clinical signs improve on their own. This delay allows the pathogen to multiply and spread within the flock. By the time samples are collected, the outbreak may be severe and treatment less effective. Collect samples within 12 hours of first observing clinical signs.

Incomplete Laboratory Submission

Managers sometimes submit only faecal samples or only dead birds without collecting tissues from euthanised birds. Faecal samples may contain normal flora that overgrows the pathogen. Dead birds that have been dead for many hours may contain post-mortem invaders that confuse culture results. Submit tissues from freshly euthanised birds and include multiple tissue types.

Ignoring Farm-Specific Resistance Patterns

Managers may choose an antimicrobial based on general recommendations without considering the farm's own resistance patterns. A systematic review and meta-analysis on the efficacy of antibiotic treatment in controlling colibacillosis in broiler production (PloS one, 2025) provides evidence that treatment outcomes vary depending on resistance patterns. Use the farm-specific antibiogram to guide empirical choices.

Stopping Treatment Early

Managers often stop antimicrobial treatment when clinical signs improve, typically after 2 to 3 days. This practice selects for bacteria that survived the initial treatment and increases the risk of relapse with resistant organisms. Complete the full treatment course as prescribed, even if birds appear healthy.

Reusing the Same Antimicrobial

Managers may use the same antimicrobial for every outbreak because it worked in the past. This practice selects for resistance over time. A study on the current status of beta-lactam antibiotic use and characterization of beta-lactam-resistant Escherichia coli from commercial farms by integrated broiler chicken operations in Korea (Poultry science, 2023) documented high levels of resistance associated with repeated use of the same antimicrobial class. Rotate antimicrobial classes based on sensitivity results.

Neglecting Concurrent Disease

Managers may focus only on the bacterial component of enteritis without addressing predisposing factors. Coccidiosis, infectious bursal disease, mycoplasmosis, and poor environmental conditions all predispose birds to bacterial enteritis. Treating the bacterial infection without managing these factors leads to recurrent disease. Investigate and manage all concurrent health problems.

Welfare and Safety Context for Antimicrobial Stewardship

Antimicrobial stewardship directly affects bird welfare and food safety. Poor stewardship leads to treatment failure, prolonged suffering, and increased mortality. It also contributes to antimicrobial resistance, which threatens the effectiveness of treatments for both animals and humans.

Welfare Implications of Treatment Delay

Delayed treatment prolongs the period during which birds experience pain, discomfort, and distress from diarrhoea, dehydration, and systemic infection. Birds with bacterial enteritis show reduced activity, increased huddling, and decreased feed intake. These signs indicate poor welfare. The World Organisation for Animal Health provides standards for animal welfare in poultry production systems. Prompt diagnosis and treatment are essential for maintaining welfare.

Welfare Implications of Treatment Failure

Treatment failure results in continued or worsening clinical signs. Birds may develop severe dehydration, septicaemia, and death. Managers must establish clear euthanasia criteria for birds that are unlikely to recover. Birds that are unable to stand, have severe dehydration, or show signs of septicaemia should be euthanised promptly using approved methods.

Food Safety Implications

Antimicrobial use in broilers can lead to residues in meat if withdrawal periods are not observed. Managers must record the withdrawal period for each antimicrobial used and ensure that birds are not sent to slaughter before the withdrawal period has expired. The World Organisation for Animal Health provides guidance on the prudent use of antimicrobials to minimise residues and resistance.

Salmonella contamination of broiler meat is a significant food safety concern. Antimicrobial treatment of Salmonella-infected flocks may reduce clinical signs but does not eliminate Salmonella carriage. Treated birds may still shed Salmonella and contaminate the environment and carcasses. Managers must implement comprehensive Salmonella control programmes that include biosecurity, vaccination, and monitoring, as recommended by the World Organisation for Animal Health.

Occupational Safety Implications

Farm workers handling sick birds and collecting samples are at risk of exposure to zoonotic pathogens, including Salmonella and E. coli. Use personal protective equipment, including gloves, boots, and coveralls, when handling birds or samples. Wash hands thoroughly after handling birds or entering poultry houses. Do not eat, drink, or smoke in poultry houses.

Professional Escalation Criteria for Antimicrobial Stewardship

The flock health manager should escalate antimicrobial stewardship issues to a veterinarian or specialist under specific circumstances.

Urgent Escalation

Escalate immediately if any of the following occur: mortality exceeds 1% per day for two consecutive days despite treatment, clinical signs worsen after 48 hours of treatment, sensitivity results show resistance to all first-line and second-line antimicrobials, the same pathogen shows resistance to three or more antimicrobial classes, or the flock requires a third-line antimicrobial.

Routine Escalation

Escalate for routine veterinary consultation in the following situations: first use of a second-line or third-line antimicrobial on the farm, need to update the farm-specific antibiogram, recurrent bacterial enteritis in consecutive flocks despite treatment, isolation of Salmonella serovars that are notifiable or of public health significance, or need for autogenous vaccine development.

Laboratory Escalation

Escalate to a diagnostic laboratory for the following: confirmation of bacterial enteritis diagnosis when initial culture is negative, serotyping of Salmonella isolates, molecular typing for epidemiological investigation when outbreaks recur, investigation of unusual resistance patterns, or confirmation of treatment failure when clinical signs persist despite sensitivity-guided treatment.

Frequently Asked Questions

What are the first signs of bacterial enteritis in broilers?

The first signs include watery or mucoid diarrhoea, pasted vents, huddling, ruffled feathers, depression, and reduced feed intake. Flock health managers may notice a sudden drop in feed consumption 24-48 hours before clinical signs become apparent. Mortality may increase gradually or suddenly depending on the pathogen and flock immunity.

How do I distinguish colibacillosis from salmonellosis in broilers?

Distinguishing these conditions requires post-mortem examination and laboratory testing. Colibacillosis typically produces fibrinous pericarditis, perihepatitis, and airsacculitis. Salmonellosis produces enlarged liver and spleen with necrotic foci, typhlitis, and caecal cores. Bacteriological culture and serotyping provide definitive diagnosis.

What samples should I collect for bacterial enteritis diagnosis?

Collect liver, spleen, bone marrow, and pericardial swabs for colibacillosis investigation. For salmonellosis, collect caecal tonsils, liver, spleen, and cloacal swabs. Litter samples and boot swabs can detect Salmonella carriage in the flock environment. Submit samples in sterile containers with transport medium if delivery exceeds two hours.

How long should I treat bacterial enteritis with antimicrobials?

Treatment duration depends on the antimicrobial used, the severity of disease, and the response to therapy. Typical treatment courses last 3-5 days. Complete the full course as prescribed by the veterinarian, even if clinical signs improve. Stopping treatment early selects for resistant bacteria and increases the risk of relapse.

Why did my antimicrobial treatment fail?

Treatment failure can result from antimicrobial resistance, incorrect diagnosis, concurrent viral or parasitic infection, poor drug administration, inadequate dose or duration, and re-infection from the environment. Re-culture affected birds and repeat sensitivity testing when treatment fails. Review the biosecurity programme to identify sources of re-infection.

Can I prevent bacterial enteritis through vaccination?

Vaccination can reduce the incidence and severity of colibacillosis and salmonellosis but does not provide complete protection. Vaccination must be combined with biosecurity measures, good management practices, and monitoring programmes. Work with a veterinarian to select the appropriate vaccine based on local epidemiology.

How do I control Salmonella in my broiler flock?

Salmonella control requires a comprehensive programme including biosecurity, feed hygiene, water sanitation, litter management, rodent and wild bird control, vaccination, and monitoring. Implement all-in/all-out production with thorough cleaning and disinfection between flocks. Test feed and water regularly for Salmonella contamination.

When should I call a veterinarian for bacterial enteritis?

Call a veterinarian immediately if mortality exceeds 1% per day for two consecutive days, clinical signs affect more than 10% of the flock, birds show neurological signs, or antimicrobial treatment fails within 48 hours. Call for routine consultation on first occurrence, recurrent disease, or isolation of notifiable Salmonella serovars.

Related Veterinary Guides

• Broiler Litter Management
• Broiler Chicken Farming Flock Management From Placement To Processing
• Poultry Mortality Investigation And Flock Records
• Swarm Prevention And Management
• Broiler Breeder Flock Management And Fertility Records

References and Further Reading

• World Organisation for Animal Health
• Merck Veterinary Manual. Merck Veterinary Manual.
• Animal Health and Welfare. World Organisation for Animal Health.
• A systematic review and meta-analysis on the efficacy of vaccination against colibacillosis in broiler production.. PloS one, 2024.
• A systematic review on the role of biosecurity to prevent or control colibacillosis in broiler production.. Poultry science, 2024.
• A systematic review and meta-analysis on the efficacy of antibiotic treatment in controlling colibacillosis in broiler production.. PloS one, 2025.
• Current status of β-lactam antibiotic use and characterization of β-lactam-resistant Escherichia coli from commercial farms by integrated broiler chicken operations in Korea.. Poultry science, 2023.
• Vaccinating parent flocks against colibacillosis reduces broiler mortality - A retrospective observational study from 2016 to 2019 in Finland.. Preventive veterinary medicine, 2024.
• High sequence similarity between avian pathogenic E. coli isolates from individual birds and within broiler chicken flocks during colibacillosis outbreaks.. Veterinary microbiology, 2022.
• Effect of on-farm litter acidification treatments on Campylobacter and Salmonella populations in commercial broiler houses in northeast Georgia. Poultry Science, 2006.
• The cereal type in feed influences Salmonella Enteritidis colonization in broilers. Poultry Science, 2009.
• Reduction of Salmonella Typhimurium in experimentally challenged broilers by nitrate adaptation and chlorate supplementation in drinking water. Journal of Food Protection, 2003.
• Isolation and Characterization of Potential Salmonella Phages Targeting Multidrug-Resistant and Major Serovars of Salmonella Derived From Broiler Production Chain in Thailand. Frontiers in Microbiology, 2021.
• The effect of oral administration of a homologous hilA mutant strain on the long-term colonization and transmission of Salmonella Enteritidis in broiler chickens. Vaccine, 2008.

This article is educational and is not a substitute for veterinary diagnosis or treatment. Contact a veterinarian for advice about an individual animal.