Equine Strangles: Outbreak Investigation, Isolation, and Return to Herd

At a Glance

Strangles is a highly contagious bacterial infection of horses caused by Streptococcus equi subspecies equi. The disease spreads through direct contact, contaminated equipment, and environmental contamination. Effective outbreak management requires rapid diagnostic confirmation, strict isolation, environmental decontamination, and systematic screening before horses return to the general population. The table below summarizes key management phases and actions.

Understanding Strangles and Its Impact

Strangles is one of the most frequently diagnosed infectious diseases in horses worldwide. The causative bacterium Streptococcus equi subspecies equi colonizes the upper respiratory tract and associated lymph nodes. Clinical signs include fever, nasal discharge, and abscess formation in submandibular and retropharyngeal lymph nodes. The disease can cause significant morbidity and occasional mortality, particularly when abscesses rupture internally or when horses develop complications such as purpura hemorrhagica or metastatic abscess formation. The Merck Veterinary Manual provides an overview of strangles and other equine respiratory diseases for horse owners.

The World Organisation for Animal Health (WOAH) recognizes strangles as a significant equine disease requiring reporting and control measures in many jurisdictions. The disease has substantial economic consequences for horse operations due to treatment costs, lost training time, and prolonged movement restrictions. A 2023 overview in Microbial pathogenesis describes strangles as a disease with global distribution and significant welfare implications.

Clinical Recognition and Initial Response

Early Clinical Signs

The incubation period for strangles ranges from 3 to 14 days after exposure. The first clinical sign is typically a fever of 39.5 to 41 degrees Celsius. Within 24 to 48 hours of fever onset, affected horses develop serous nasal discharge that becomes mucopurulent. Lymph node enlargement follows, most commonly in the submandibular and retropharyngeal regions. Abscesses may form and rupture externally within 7 to 14 days of initial fever.

Veterinarians and farm managers should monitor all horses on the premises for fever when strangles is suspected. Daily temperature recording of all horses in contact with suspect cases is essential. Any horse with a temperature above 38.5 degrees Celsius should be considered potentially infected and isolated pending diagnostic testing.

Differential Diagnoses

Several other respiratory conditions can mimic strangles. Equine influenza, equine herpesvirus, and other bacterial infections of the upper respiratory tract may present with fever and nasal discharge. Lymph node enlargement is more specific to strangles but can occur with other conditions. Diagnostic testing is necessary to confirm the diagnosis before implementing outbreak control measures.

Immediate Isolation Protocol

When strangles is suspected, the first action is to isolate the affected horse from all other horses. The isolation area should be physically separated from the main horse population by at least 10 meters if possible. Dedicated equipment including buckets, feed tubs, halters, and grooming tools must be assigned to the isolation area and not shared with other horses.

Personnel attending to isolated horses should wear disposable gloves and boots that remain in the isolation area. Hand washing stations with soap and water or alcohol-based hand sanitizer should be available at the isolation area entrance. Movement of personnel from isolation areas to other horse areas should follow a strict order of contact, with isolation horses attended last.

Diagnostic Confirmation

Sample Collection

Polymerase chain reaction (PCR) testing is the primary diagnostic method for strangles. Samples can be collected from nasal swabs, guttural pouch lavage, or abscess contents. Nasal swabs are easiest to collect but may have lower sensitivity in horses without active nasal shedding. Guttural pouch lavage samples have higher sensitivity for detecting carrier horses.

For horses with active clinical signs, a deep nasal swab from the nasopharynx is appropriate. The swab should be inserted into the ventral nasal meatus to the level of the medial canthus of the eye. For guttural pouch sampling, a flexible catheter is passed through the nasal passage into the guttural pouch opening, and sterile saline is infused and aspirated.

Abscess contents can be collected by needle aspiration of intact abscesses or by swabbing the draining tract of ruptured abscesses. Samples should be placed in appropriate transport media and shipped to a diagnostic laboratory with refrigeration.

Interpreting PCR Results

A positive PCR result confirms the presence of Streptococcus equi DNA. However, PCR cannot distinguish between live bacteria and dead bacterial DNA. Horses that have recently recovered from strangles may test positive for several weeks after clinical resolution due to residual DNA in the respiratory tract.

A negative PCR result from a horse with clinical signs does not completely rule out strangles. False negatives can occur if the sample is collected too early in the disease course, if the swab does not contact infected tissue, or if the horse is in the early stages of abscess formation without active shedding.

Culture and Sensitivity Testing

Bacterial culture of Streptococcus equi is less sensitive than PCR but provides additional information. Culture can confirm the presence of live bacteria and allows for antimicrobial susceptibility testing if needed. Culture is particularly useful for testing guttural pouch samples from horses being screened for carrier status before returning to the herd.

Biosecurity Measures During an Outbreak

Zoning and Movement Control

Establishing clear zones on the property is critical for outbreak control. The isolation zone contains all confirmed and suspect cases. A buffer zone surrounds the isolation area and includes pathways and equipment used by personnel attending to isolated horses. The clean zone contains all horses that have had no known exposure to infected animals.

Movement between zones must be controlled. Personnel should not move from the isolation zone to the clean zone without changing clothing and footwear and washing hands. Equipment should not be moved from the isolation zone to other areas without thorough cleaning and disinfection.

Personnel Hygiene

All personnel entering the isolation zone should wear dedicated clothing and footwear that remains in that zone. Disposable coveralls and boots are practical options. Hand hygiene with soap and water or alcohol-based hand sanitizer is required before and after contact with each horse.

A 2022 study in Tierarztliche Praxis. Ausgabe G, Grosstiere/Nutztiere on management and hygiene measures during outbreaks of strangles and other infectious diseases emphasizes the importance of strict hygiene protocols for controlling spread. Personnel should be trained on these protocols and compliance should be monitored.

Equipment and Feed Management

Dedicated equipment for the isolation zone includes buckets, feed tubs, halters, lead ropes, grooming tools, and medical supplies. These items should not be removed from the isolation zone until the outbreak is declared over and all items have been disinfected.

Feed and water should be provided in individual containers that are cleaned and disinfected daily. Hay and grain should be stored in the isolation zone to avoid cross-contamination from feed storage areas.

Environmental Disinfection

Cleaning Before Disinfection

Organic material such as manure, bedding, and feed residues must be removed before disinfection. Streptococcus equi can survive in organic material for extended periods. All surfaces in the isolation area including walls, floors, and stall partitions should be cleaned with detergent and water before applying disinfectant.

Disinfectant Selection

Streptococcus equi is susceptible to many common disinfectants when used according to manufacturer instructions. Phenolic compounds, quaternary ammonium compounds, and bleach solutions are effective. The specific disinfectant chosen should be appropriate for the surface being treated and should be used at the correct concentration and contact time.

A 1995 review in Revue scientifique et technique (International Office of Epizooties) on disinfecting equine facilities provides guidance on appropriate disinfectants for different surfaces and situations. Disinfectants should be applied after thorough cleaning and allowed to remain in contact with surfaces for the recommended time.

Pasture and Outdoor Area Management

Streptococcus equi can survive in the environment for several weeks under favorable conditions. Pastures and outdoor areas where infected horses have been should be rested for at least 4 weeks before reintroducing clean horses. Sunlight and drying help reduce bacterial survival in outdoor environments.

Manure and bedding from isolation areas should be composted or disposed of in a manner that prevents access by other horses. Composting at temperatures above 55 degrees Celsius for several weeks can kill Streptococcus equi.

Isolation Duration and Monitoring

Minimum Isolation Period

The minimum isolation period for horses with strangles is 4 weeks after the last clinical sign resolves. Clinical signs include fever, nasal discharge, and lymph node abscesses. The isolation period may need to be extended if horses continue to shed bacteria after clinical recovery.

A 2015 study in Australian veterinary journal on practical management of strangles outbreaks recommends that isolation continue until guttural pouch sampling confirms the horse is no longer shedding Streptococcus equi. This approach reduces the risk of introducing carrier horses back into the general population.

Daily Monitoring During Isolation

Horses in isolation should be examined daily for clinical signs. Temperature should be recorded at the same time each day. Nasal discharge, lymph node swelling, and appetite should be assessed. Any horse that develops new clinical signs should have its isolation period restarted from the date of the new sign.

Criteria for Ending Isolation

Isolation can be considered for ending when the following criteria are met:

• No clinical signs for at least 4 weeks
• Two negative PCR tests from guttural pouch samples collected at least 7 days apart
• No new cases in the isolation group for at least 4 weeks

If guttural pouch sampling is not possible, a longer isolation period of 6 to 8 weeks after clinical recovery may be considered, but this approach carries higher risk of reintroducing infection.

Guttural Pouch Sampling for Carrier Detection

Importance of Carrier Detection

Carrier horses are those that harbor Streptococcus equi in their guttural pouches without showing clinical signs. These horses can shed bacteria intermittently and serve as a source of infection for other horses. Detection of carriers is essential for preventing reintroduction of strangles after an outbreak.

A 2023 study in Mathematical biosciences and engineering : MBE on dynamics analysis of strangles with asymptomatic infected horses and long-term subclinical carriers highlights the role of carriers in maintaining infection within populations. Carrier detection through guttural pouch sampling is the most reliable method for identifying these horses.

Sampling Technique

Guttural pouch sampling requires sedation and restraint. A flexible catheter is passed through the nasal passage into the guttural pouch opening. Sterile saline is infused and aspirated. The aspirated fluid is submitted for PCR testing and bacterial culture.

The procedure should be performed by a veterinarian experienced in guttural pouch sampling. Complications are rare but can include epistaxis or trauma to the nasal passages.

Interpreting Carrier Screening Results

A positive PCR result from guttural pouch fluid indicates the presence of Streptococcus equi DNA. Bacterial culture can confirm whether live bacteria are present. Horses with positive culture results are considered active carriers and should remain isolated.

Horses with positive PCR but negative culture may have residual bacterial DNA from a recent infection. These horses should be retested in 2 to 4 weeks. If the PCR remains positive, further investigation is warranted.

Return to Herd Protocol

Stepwise Reintroduction

Horses that have completed isolation and have negative guttural pouch screening can be considered for return to the herd. Reintroduction should be gradual to allow monitoring for any delayed signs of infection.

The first step is to move the horse to a transition area where it has contact with a small number of sentinel horses. These sentinel horses should be monitored for clinical signs for 2 weeks. If no signs develop, the horse can be moved to the general population.

Vaccination Considerations

Vaccination against strangles is available in some regions. The vaccine can reduce the severity of clinical signs but does not prevent infection or shedding. Vaccination should be discussed with a veterinarian based on the specific circumstances of the farm and the horses involved.

Vaccination is not a substitute for biosecurity measures and should be used as part of a comprehensive strangles control program. Horses that have recovered from strangles may have immunity for several months to years, but reinfection can occur.

Record Keeping

Accurate records of all horses involved in the outbreak should be maintained. Records should include:

• Identification of each horse
• Dates of clinical signs
• Diagnostic test results
• Isolation start and end dates
• Guttural pouch sampling results
• Vaccination history

These records are important for managing the current outbreak and for planning future prevention strategies.

Common Failure Patterns in Outbreak Management

Incomplete Isolation

The most common failure in outbreak management is incomplete isolation. This can occur when horses are moved between groups before the isolation period is complete, when equipment is shared between isolation and clean areas, or when personnel move between areas without proper hygiene.

Premature Return to Herd

Returning horses to the general population before confirming they are no longer shedding Streptococcus equi is a frequent cause of recurrent outbreaks. The minimum 4 week isolation period after clinical recovery is a guideline, not a guarantee. Guttural pouch sampling provides more reliable evidence that a horse is safe to return.

Environmental Contamination

Streptococcus equi can survive in the environment for weeks. Inadequate cleaning and disinfection of stalls, equipment, and common areas can lead to reinfection of horses after they return. Thorough cleaning and disinfection of all contaminated areas is essential.

Carrier Horses

Undetected carrier horses can reintroduce strangles to a herd weeks or months after an outbreak appears to be resolved. Screening all horses that were involved in the outbreak, including those that did not show clinical signs, is important for identifying carriers.

Welfare and Safety Considerations

Pain Management

Horses with strangles experience pain from lymph node abscesses and pharyngitis. Abscesses should be allowed to mature and rupture naturally when possible. Warm compresses can help promote abscess maturation and provide comfort.

Nonsteroidal anti-inflammatory drugs may be used to reduce fever and pain, but should be used under veterinary guidance. The use of these medications should be recorded, and withdrawal periods should be observed if the horse is intended for food production.

Complications Requiring Veterinary Attention

Several complications of strangles require immediate veterinary attention:

• Abscesses that do not rupture externally and may rupture internally
• Difficulty breathing due to enlarged retropharyngeal lymph nodes
• Purpura hemorrhagica, characterized by swelling and bruising
• Metastatic abscess formation in other body sites
• Colic or other signs of systemic illness

Veterinarians should be contacted immediately if any of these complications are suspected.

Euthanasia Considerations

In severe cases with complications such as internal abscess rupture or purpura hemorrhagica, euthanasia may be the most humane option. This decision should be made in consultation with a veterinarian based on the horse's quality of life and prognosis.

Professional Escalation Criteria

When to Contact a Veterinarian

A veterinarian should be contacted in the following situations:

• First suspicion of strangles in a horse
• Any horse with fever and lymph node swelling
• Horses with difficulty breathing or swallowing
• Horses that do not improve within 7 days of supportive care
• Any horse with signs of complications such as purpura hemorrhagica

When to Contact Regulatory Authorities

In some jurisdictions, strangles is a reportable disease. Farm managers should check with their local veterinary authority about reporting requirements. Even if not required by law, reporting outbreaks to local equine veterinary networks can help prevent spread to other farms.

When to Seek Specialist Consultation

Complex cases or outbreaks that are not controlled with standard measures may benefit from consultation with a veterinary internal medicine specialist. The American College of Veterinary Internal Medicine (ACVIM) provides resources for finding specialists in equine internal medicine.

Practical Decision Framework for Strangles Outbreak Triage and Resource Allocation

Managing a strangles outbreak requires rapid, consistent decisions about resource allocation, isolation priorities, and testing schedules. Without a structured framework, farm managers may delay critical actions or apply resources inefficiently. This section provides a practical decision framework based on outbreak severity classification, a record system for tracking individual horse status, and troubleshooting methods for common management failures. The framework is designed for use by farm managers and veterinarians working together during an outbreak.

Outbreak Severity Classification System

Classifying the severity of a strangles outbreak at the time of recognition helps determine the intensity of response measures and the allocation of personnel, equipment, and testing resources. The classification system below uses observable criteria that can be assessed within the first 24 to 48 hours of outbreak recognition.

Low Severity Outbreak

A low severity outbreak is characterized by a single confirmed case with no evidence of spread to other horses. The affected horse shows mild clinical signs including fever below 40 degrees Celsius, minimal nasal discharge, and lymph node enlargement without respiratory distress. No other horses on the premises have developed fever or clinical signs within 14 days of exposure to the index case.

Management response for low severity outbreaks includes isolation of the affected horse, daily temperature monitoring of all contact horses, and diagnostic testing of the index case. Environmental disinfection is limited to the stall and equipment used by the affected horse. Movement restrictions apply only to the affected horse and its direct contacts. The Merck Veterinary Manual provides guidance on basic isolation protocols for individual cases.

Moderate Severity Outbreak

A moderate severity outbreak involves two to five confirmed or suspect cases within a single barn or paddock group. Affected horses show moderate clinical signs including fever between 40 and 41 degrees Celsius, mucopurulent nasal discharge, and lymph node abscesses that may require drainage. Some contact horses may develop fever without other clinical signs.

Management response for moderate severity outbreaks requires establishment of a dedicated isolation zone, assignment of dedicated personnel, and daily temperature monitoring of all horses on the premises. Diagnostic testing should include PCR testing of all horses with fever or clinical signs. Environmental disinfection extends to all areas accessed by affected horses including aisles, common areas, and equipment. Movement restrictions apply to the entire barn or paddock group.

High Severity Outbreak

A high severity outbreak involves more than five confirmed cases, spread across multiple barns or paddock groups, or the presence of severe clinical signs including respiratory distress, internal abscess formation, or complications such as purpura hemorrhagica. The outbreak may involve multiple age groups or immunocompromised horses.

Management response for high severity outbreaks requires immediate consultation with a veterinarian and potentially a veterinary internal medicine specialist. The American College of Veterinary Internal Medicine (ACVIM) provides resources for finding equine internal medicine specialists. A comprehensive biosecurity plan must be implemented across the entire premises. All horses should be tested to determine infection status. Environmental disinfection covers all facilities. Movement restrictions apply to the entire premises, and notification of neighboring horse operations is recommended.

Resource Allocation Decision Matrix

The following matrix guides resource allocation decisions based on outbreak severity and available resources. Farm managers should assess their situation against the matrix at the start of an outbreak and reassess weekly.

Escalation and De-escalation Criteria

Outbreak severity classification is not static. Farm managers should reassess severity daily based on new cases, clinical progression, and testing results.

Escalation criteria that warrant moving to a higher severity classification include:

• Development of new cases in a previously unaffected group
• Worsening clinical signs in any affected horse
• Detection of complications such as respiratory distress or purpura hemorrhagica
• Failure of isolation measures to prevent spread within 7 days
• Positive PCR results in horses previously considered unexposed

De-escalation criteria that allow moving to a lower severity classification include:

• No new cases for 14 days
• All affected horses showing clinical improvement
• Negative PCR results from all horses in the affected group
• Completion of environmental disinfection with negative environmental samples if collected

A 2015 study in Australian veterinary journal on practical management of strangles outbreaks emphasizes that de-escalation should be gradual and based on objective criteria instead of time alone.

Record System for Individual Horse Tracking

Accurate record keeping is essential for managing strangles outbreaks. The following record system provides a structured approach to tracking individual horse status throughout the outbreak. Records should be maintained in a logbook or digital spreadsheet accessible to all personnel involved in outbreak management.

Horse Identification and Baseline Information

Each horse on the premises should have a record containing:

• Horse name, age, breed, and identification method (microchip, brand, or markings)
• Location on premises (barn, stall number, paddock)
• Vaccination history for strangles if applicable
• Previous strangles infection history
• Baseline temperature range (recorded when healthy)

Daily Monitoring Record

For each horse, a daily record should include:

• Date and time of observation
• Rectal temperature
• Presence or absence of nasal discharge (serous, mucopurulent, or none)
• Lymph node assessment (normal, enlarged, abscessed, draining)
• Appetite assessment (normal, reduced, absent)
• Attitude assessment (normal, depressed, severely depressed)
• Any other clinical signs observed
• Name of person completing the observation

Diagnostic Testing Record

For each horse that undergoes testing, the record should include:

• Date of sample collection
• Type of sample (nasal swab, guttural pouch lavage, abscess contents)
• Laboratory name and accession number
• Test type (PCR, culture, or both)
• Test result and date received
• Interpretation of result
• Recommended follow-up testing date

Isolation Status Record

For each horse in isolation, the record should include:

• Date isolation started
• Reason for isolation (clinical signs, exposure, positive test)
• Location of isolation area
• Isolation end date
• Criteria met for ending isolation
• Date of return to general population

Treatment Record

For horses receiving treatment, the record should include:

• Date treatment started
• Medication name, dose, route, and frequency
• Duration of treatment
• Any adverse reactions observed
• Withdrawal period if applicable

Common Failure Patterns and Troubleshooting Methods

Even with careful planning, strangles outbreak management can fail. The following troubleshooting methods address common failure patterns observed in practice.

Failure Pattern 1: Spread to Previously Unaffected Groups

When strangles spreads to a group of horses that was previously considered unexposed, the most likely causes are:

• Incomplete isolation of affected horses
• Shared equipment or personnel between groups
• Environmental contamination from runoff or shared water sources
• Undetected carrier horses in the unaffected group

Troubleshooting steps:

1. Review movement records for all personnel and horses for the preceding 14 days
2. Identify any shared equipment, water sources, or feed storage areas
3. Test all horses in the newly affected group with PCR from nasal swabs
4. Implement stricter zoning with physical barriers between groups
5. Assign completely separate personnel to each zone
6. Disinfect all shared areas and equipment

A 2022 study in Tierarztliche Praxis. Ausgabe G, Grosstiere/Nutztiere on management and hygiene measures during outbreaks of strangles and other infectious diseases emphasizes that personnel movement is the most common route of between-group transmission.

Failure Pattern 2: Recurrent Cases After Apparent Resolution

When new cases appear after the outbreak appeared to be resolved, the most likely causes are:

• Premature ending of isolation for recovered horses
• Undetected carrier horses in the population
• Environmental contamination that was not adequately disinfected
• Introduction of a new infected horse from outside the premises

Troubleshooting steps:

1. Review isolation end dates and criteria for all recovered horses
2. Test all horses that were involved in the outbreak with guttural pouch PCR and culture
3. Re-inspect all facilities for areas that may not have been adequately cleaned and disinfected
4. Review biosecurity protocols for incoming horses
5. Extend the outbreak monitoring period by an additional 4 weeks

A 2023 study in Mathematical biosciences and engineering : MBE on dynamics analysis of strangles with asymptomatic infected horses and long-term subclinical carriers highlights that carrier horses can shed bacteria intermittently, making them difficult to detect without repeated testing.

Failure Pattern 3: Persistent Positive PCR Results in Recovered Horses

When recovered horses continue to test positive on PCR from guttural pouch samples, the most likely causes are:

• Residual bacterial DNA from the recent infection
• Active carrier state with live bacteria in the guttural pouch
• Contamination of the sample during collection
• Laboratory error

Troubleshooting steps:

1. Repeat guttural pouch sampling with strict aseptic technique
2. Submit samples for both PCR and bacterial culture
3. If PCR positive and culture negative, retest in 2 to 4 weeks
4. If PCR positive and culture positive, the horse is an active carrier and should remain isolated
5. Consider guttural pouch lavage with appropriate solutions under veterinary guidance to clear bacterial material

Failure Pattern 4: Environmental Recontamination

When environmental samples or clinical cases suggest recontamination of cleaned areas, the most likely causes are:

• Incomplete removal of organic material before disinfection
• Use of ineffective disinfectant or incorrect concentration
• Insufficient contact time for disinfectant
• Recontamination from infected horses or personnel

Troubleshooting steps:

1. Review cleaning protocols to ensure organic material is completely removed before disinfection
2. Verify that disinfectant is being used at the correct concentration and contact time
3. Test disinfectant solution for potency if using diluted stock solutions
4. Implement a two-step cleaning and disinfection protocol with a detergent wash followed by disinfectant
5. Restrict access to cleaned areas until they are completely dry

A 1995 review in Revue scientifique et technique (International Office of Epizootics) on disinfecting equine facilities emphasizes that organic material neutralizes many disinfectants and must be removed before disinfection is effective.

Practical Implementation Steps for the Decision Framework

Implementing the decision framework requires preparation before an outbreak occurs. The following steps should be completed during routine farm management.

Step 1: Develop Premises Maps and Zone Plans

Create a map of the premises showing all horse housing areas, feed storage, water sources, and personnel pathways. Identify potential isolation areas that are physically separated from the main horse population. Mark locations for hand washing stations, boot baths, and equipment storage for each zone.

Step 2: Establish Communication Protocols

Designate a single person as the outbreak coordinator. This person is responsible for communicating with the veterinarian, laboratory, and regulatory authorities if required. Establish a communication tree for notifying all personnel about outbreak status changes.

Step 3: Stock Outbreak Supplies

Maintain a supply of outbreak management materials including:

• Disposable gloves, coveralls, and boots
• Disinfectant appropriate for Streptococcus equi
• Digital thermometers for each zone
• Sample collection supplies (swabs, transport media, coolers)
• Signage for isolation areas
• Logbooks or digital record templates

Step 4: Train Personnel

Train all personnel on the outbreak management protocols before an outbreak occurs. Training should include:

• Recognition of early clinical signs of strangles
• Proper hand hygiene and personal protective equipment use
• Temperature recording procedures
• Sample collection procedures if personnel will be involved
• Record keeping requirements

Step 5: Conduct Regular Drills

Conduct annual drills to practice outbreak response. Drills should include:

• Simulated recognition of a suspect case
• Implementation of isolation protocols
• Communication with the veterinarian
• Record keeping exercises

Limitations of the Decision Framework

The decision framework described in this section has several limitations that farm managers should recognize.

First, the framework is based on published evidence and expert opinion but has not been validated in controlled studies. Individual farm circumstances may require adjustments to the recommended protocols.

Second, the framework assumes access to diagnostic testing and veterinary consultation. Farms in remote areas or with limited resources may need to adapt the protocols based on available services.

Third, the framework does not address specific management considerations for different types of horse operations such as breeding farms, training facilities, or boarding stables. Each operation type has unique risk factors and management constraints that should be considered.

Fourth, the framework provides general guidance but cannot replace professional veterinary judgment. Veterinarians should be involved in all major decisions about outbreak management, including severity classification, testing protocols, and return to herd decisions.

The World Organisation for Animal Health (WOAH) provides additional guidance on disease reporting and control measures that may apply in specific jurisdictions. Farm managers should check with their local veterinary authority about reporting requirements and available resources.

Records and Measurements for Framework Evaluation

To evaluate the effectiveness of the decision framework, farm managers should maintain the following records and measurements.

Outcome Measures

• Total number of horses affected during the outbreak
• Duration of the outbreak from first case to last case resolution
• Number of horses that required veterinary treatment for complications
• Mortality rate if any horses died or were euthanized
• Total cost of outbreak management including testing, treatment, and lost productivity

Process Measures

• Time from first clinical sign to isolation of suspect case
• Time from sample collection to receipt of test results
• Compliance with daily monitoring protocols
• Number of biosecurity breaches observed
• Number of horses that required extended isolation due to persistent positive tests

Follow-up Measures

• Number of new cases in the 6 months following outbreak resolution
• Results of follow-up testing in recovered horses at 3 and 6 months
• Changes to farm management practices implemented after the outbreak

These records provide objective data for evaluating the effectiveness of outbreak management and identifying areas for improvement in future outbreaks. A 2023 overview in Microbial pathogenesis on strangles in equines emphasizes that systematic data collection during outbreaks contributes to improved understanding of disease dynamics and control measures.

Frequently Asked Questions

How long does strangles survive in the environment?

Streptococcus equi can survive in the environment for several weeks under favorable conditions. The bacteria survive longer in moist, cool environments and are killed by drying and sunlight. Thorough cleaning and disinfection of contaminated areas is essential for preventing environmental transmission.

Can horses get strangles more than once?

Yes, horses can get strangles more than once. Immunity after natural infection is variable and may last from several months to several years. Reinfection can occur, particularly if the horse is exposed to a different strain of Streptococcus equi or if immunity has waned.

What is the difference between strangles and bastard strangles?

Strangles refers to the typical form of the disease with abscess formation in the lymph nodes of the head and neck. Bastard strangles is a term used when abscesses form in other parts of the body, such as the lungs, liver, or abdominal lymph nodes. Bastard strangles is more difficult to treat and has a poorer prognosis.

How is strangles diagnosed in horses?

Strangles is diagnosed based on clinical signs and confirmed by laboratory testing. PCR testing of nasal swabs, guttural pouch lavage, or abscess contents is the most common diagnostic method. Bacterial culture can also be used and provides information about antimicrobial susceptibility.

What disinfectants kill Streptococcus equi?

Many common disinfectants are effective against Streptococcus equi when used correctly. Phenolic compounds, quaternary ammonium compounds, and bleach solutions are effective. Disinfectants must be applied to clean surfaces and allowed to remain in contact for the recommended time.

How long should a horse be isolated after strangles?

The minimum isolation period is 4 weeks after the last clinical sign resolves. However, isolation should continue until guttural pouch sampling confirms the horse is no longer shedding Streptococcus equi. Two negative PCR tests from guttural pouch samples collected at least 7 days apart are recommended before ending isolation.

Can strangles be treated with antibiotics?

Antibiotic treatment for strangles is controversial. Antibiotics may be used in the early stages of infection before abscess formation, but they can also interfere with abscess maturation and immune response. Antibiotic use should be guided by a veterinarian based on the individual case.

What is a strangles carrier horse?

A carrier horse is one that harbors Streptococcus equi in its guttural pouches without showing clinical signs. Carrier horses can shed bacteria intermittently and infect other horses. Detection of carriers requires guttural pouch sampling and PCR testing or bacterial culture.

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References and Further Reading

• aaep.org
• www.merckvetmanual.com
• www.acvim.org
• Merck Veterinary Manual. Merck Veterinary Manual.
• Animal Health and Welfare. World Organisation for Animal Health.
• [Management and hygiene measures during an outbreak of herpes, influenza, strangles or infections with multidrug resistant bacteria].. Tierarztliche Praxis. Ausgabe G, Grosstiere/Nutztiere, 2022.
• Strangles in equines: An overview.. Microbial pathogenesis, 2023.
• Disinfecting equine facilities.. Revue scientifique et technique (International Office of Epizootics), 1995.
• Strangles: taking steps towards eradication.. Veterinary microbiology, 2013.
• Dynamics analysis of strangles with asymptomatic infected horses and long-term subclinical carriers.. Mathematical biosciences and engineering : MBE, 2023.
• Practical management of strangles outbreaks in horses.. Australian veterinary journal, 2015.

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