# Small Ruminant Parasite Control: Diagnostic Strategies and Anthelmintic Stewardship


## Key Takeaways

- Anthelmintic resistance (AR) in small ruminant gastrointestinal nematodes is a global threat necessitating a shift from calendar-based treatments to diagnostic-driven parasite control, focusing on measured indicators of host impact and parasite transmission.
- Targeted Selective Treatment (TST) is a cornerstone of sustainable parasite control, leveraging the principle of refugia by treating only animals exhibiting clinical signs or production losses, thereby preserving susceptible parasite populations and slowing resistance selection.
- FAMACHA scoring is a validated tool for identifying anemic sheep and goats due to *Haemonchus contortus* infection, guiding selective treatment; however, its efficacy is limited in mixed infections where other nematode genera cause pathology without significant anemia.
- Quantitative fecal egg counts (FEC) using methods like McMaster are essential for flock-level risk assessment, evaluating treatment response, and informing treatment timing, with interpretation thresholds varying based on parasite species, season, and production system.
- The Fecal Egg Count Reduction Test (FECRT) is the standard field method for auditing anthelmintic efficacy and detecting AR, requiring careful adherence to protocols including adequate group size, pre-treatment egg count thresholds, and appropriate post-treatment sampling intervals.
- Accurate bodyweight estimation and species-specific dosing are critical to avoid underdosing, a primary driver of AR selection, with goats exhibiting faster drug clearance rates than sheep, necessitating adjusted dosing and FECRT interpretation.

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Gastrointestinal nematode (GIN) infection remains the most economically consequential parasitic disease of grazing sheep and goats worldwide. The widespread emergence of anthelmintic resistance (AR) has transformed parasite control from a routine husbandry task into a diagnostic challenge requiring species-specific reasoning and quantitative monitoring. This article provides the practicing veterinarian with a structured approach to diagnosing parasite burdens, selecting animals for treatment, and implementing stewardship frameworks that preserve remaining anthelmintic efficacy. It answers three clinical questions: which animals need treatment, how can treatment decisions be validated, and what monitoring parameters distinguish sustainable control from accelerating resistance selection.

The content assumes familiarity with ruminant medicine and focuses on diagnostic reasoning instead of drug protocols. Guidance is organized around the principle that parasite control decisions should be driven by measured indicators of host impact and parasite transmission, not by calendar-based assumptions. Regional differences in parasite species, climate, and production systems require adaptation of the frameworks presented here, and the evidence base for several diagnostic thresholds remains contested, as noted where relevant.

## At a Glance

| Parameter | Clinical Decision or Fact |
|---|---|
| FAMACHA score | Mucous membrane color scale used to identify anemic sheep and goats for targeted selective treatment of *Hemonchus contortus* |
| Fecal egg count (FEC) | Quantitative measure of nematode egg shedding, used for flock-level risk assessment and treatment response evaluation |
| Fecal egg count reduction test (FECRT) | Standardized field test comparing pre- and post-treatment FECs to estimate anthelmintic efficacy |
| Refugia | Parasite population unexposed to anthelmintic, maintenance of refugia slows resistance selection |
| Targeted selective treatment (TST) | Treating only animals meeting defined clinical or production-based criteria instead of whole-flock treatment |
| Larval culture | Speciation of nematode eggs via third-stage larvae, distinguishes *Hemonchus* from other genera |
| Anthelmintic class history | Prior drug exposure on the farm determines which class to test first and informs resistance risk assessment |

## The Resistance Imperative

Anthelmintic resistance in small ruminant nematodes is no longer a regional concern but a global production threat. Reports of AR now span the European continent with high within-country prevalence in several nations, and multi-drug resistance in sheep and goats threatens the viability of small ruminant industries in some regions of the world. The problem extends across every anthelmintic class, and the rate of new drug development has not kept pace with resistance evolution. The genome of *Hemonchus contortus*, the most economically important small ruminant parasite, reveals extensive drug-metabolizing gene families that help explain its remarkable propensity to develop resistance.

The clinical consequence is straightforward: a veterinarian cannot assume that a drug works because it worked previously. Every treatment decision carries an implicit hypothesis about drug efficacy, and that hypothesis must be tested periodically with quantitative methods. This shifts the diagnostic focus from identifying infected animals to characterizing the resistance status of the parasite population on each farm.

## Refugia and the Logic of Selective Treatment

The concept of refugia underpins all modern approaches to sustainable parasite control. Refugia are the parasite stages not exposed to anthelmintic at the time of treatment, including larvae on pasture, inhibited larvae in the host, and worms in untreated animals. These unexposed parasites maintain genes for drug susceptibility within the population. When the entire flock is treated, all surviving worms are those carrying resistance alleles, and the next generation is drawn disproportionately from resistant parents.

Targeted selective treatment (TST) exploits this principle by treating only the proportion of animals that will most benefit, leaving the remainder as a source of susceptible parasites. The success of TST depends entirely on the accuracy of the indicators used to identify those animals. Research has evaluated anemia, milk production, and liveweight gain as practical markers, with results suggesting that these can reduce anthelmintic usage while maintaining animal performance. The diagnostic challenge is selecting the right indicator for the dominant parasite genus on each farm.

## FAMACHA Scoring for Hemonchus Control

The FAMACHA system addresses the specific pathology of *H. contortus*, a blood-feeding abomasal parasite that causes anemia before significant weight loss or diarrhea. The system uses a five-point scale of mucous membrane color, from red (score 1) to white (score 5), to identify animals requiring treatment. It is published and validated for use in sheep and goats in regions where *Hemonchus* is the primary pathogen, and it is most reliable when hemonchosis is the dominant parasitic condition.

Clinical application requires training and periodic refresher assessment. The veterinarian must confirm that anemia is attributable to hemonchosis instead of other causes such as liver fluke, nutritional deficiency, or chronic disease. FAMACHA scoring is performed at intervals determined by seasonal transmission risk, and animals scoring 3 or higher are candidates for treatment. The system is less useful in mixed infections where other genera contribute substantially to pathology, and it does not detect subclinical production losses from non-hematophagous parasites.

## Quantitative Fecal Diagnostics

Fecal egg counting provides the quantitative foundation for parasite control decisions. The McMaster technique remains the standard method, with modifications to improve sensitivity for low-shedding animals. Composite sampling from multiple animals reduces cost but obscures individual variation, making it suitable for flock-level risk assessment instead of treatment selection. Individual counts are required when the goal is identifying which animals to treat under a TST protocol.

Larval culture adds genus-level information that egg counts cannot provide. Differentiation of third-stage larvae distinguishes *Hemonchus*, *Teladorsagia*, *Trichostrongylus*, and *Cooperia*, each with different pathogenicity, seasonality, and resistance profiles. This information determines whether FAMACHA scoring, weight-based criteria, or other indicators are appropriate for the farm. The combination of quantitative egg counts with periodic larval culture provides the diagnostic picture needed to design a control program matched to the local parasite community.

## Diagnostic Sequence for Parasite-Burden Assessment

The clinical assessment of a small ruminant parasite problem proceeds from individual animal examination to group-level diagnostics. Begin with signalment, body condition scoring, and a targeted physical examination. Mucous membrane color, submandibular or ventral edema, and the presence of diarrhea or fecal soiling direct the initial differential list. Anemia with bottle jaw in a grazing animal points strongly to hemonchosis, whereas poor condition with diarrhea in a lamb suggests trichostrongylosis or Teladorsagia infection. The FAMACHA system provides a standardized method for scoring anemia by comparing conjunctival color against a five-point card, with scores of 3, 4, and 5 indicating progressively severe anemia that warrants treatment in Hemonchus-endemic settings [Kenyan and colleagues describe the role of targeted selective treatments in refugia-based control](https://pubmed.ncbi.nlm.nih.gov/19450930/).

The diagnostic sequence must account for the parasite species present on the property. FAMACHA scoring is valid only where Hemonchus contortus is the dominant pathogen, because other common nematodes do not produce the same pattern of blood loss. In flocks where mixed infections predominate, FAMACHA identifies the animals most affected by hemonchosis but misses burdens of Trichostrongylus, Teladorsagia, and Nematodirus. The [MSD Veterinary Manual](https://www.msdvetmanual.com/) provides species-specific guidance on the clinical presentation and diagnostic approach for each of these parasites.

## Selecting the Diagnostic Tool

The choice between FAMACHA, fecal egg count (FEC), and other markers depends on the question being asked. FAMACHA answers the question of which individual animals need treatment for hemonchosis today. FEC answers the question of how heavily a group is contaminated and whether treatment has been effective. Body condition score and dag score answer the question of cumulative production loss. Each tool detects a different failure mode, and no single test covers all of them.

| Diagnostic Tool | What It Detects | Best Used For | Limitations |
|---|---|---|---|
| FAMACHA score | Anemia from Hemonchus blood loss | Selecting individual animals for treatment in Hemonchus-endemic flocks | Not useful for other nematode species, requires training and regular card replacement |
| Fecal egg count | Nematode egg output, group contamination level | Monitoring seasonal transmission, timing treatments, evaluating control success | Does not correlate perfectly with worm burden, requires laboratory equipment and time |
| Fecal egg count reduction test | Anthelmintic efficacy against the resident worm population | Confirming resistance, auditing treatment protocols | Requires two samples per group, unreliable if egg counts are low |
| Body condition score | Cumulative nutritional loss from parasitism and other causes | Identifying chronically affected animals, monitoring flock-level trends | Non-specific, many conditions cause poor condition |
| Dag score | Fecal soiling, often from Trichostrongylus or Nematodirus | Identifying animals needing crutching or treatment in scouring flocks | Subjective, affected by pasture type and fleece characteriztics |

The [increasing importance of anthelmintic resistance in European livestock](https://pubmed.ncbi.nlm.nih.gov/33277891/) documented through a meta-analysis of published and unpublished studies shows that resistance is present throughout Europe with high within-country prevalence in some regions. This finding changes the diagnostic approach: routine monitoring of anthelmintic efficacy is no longer optional but a required component of parasite control on every property.

## FEC Technique and Interpretation

Quantitative fecal egg counts require a McMaster or modified McMaster technique with a known sensitivity. Standard McMaster counting chambers with a 1:15 dilution and two-chamber examination provide a sensitivity of approximately 50 eggs per gram (epg). Where lower detection limits are needed, such as in the fecal egg count reduction test (FECRT), flotation methods with higher sensitivity, including the Wisconsin technique or a modified McMaster with increased sample volume, are preferred. The [MSD Veterinary Manual](https://www.msdvetmanual.com/) describes the standardized techniques and their performance characteriztics.

Sample handling determines result validity. Collect fresh fecal samples directly from the rectum or from the ground immediately after defecation. Refrigerate samples if processing is delayed beyond a few hours. Do not freeze samples, as freezing destroys eggs and produces false low counts. Composite samples from 8 to 10 animals per management group provide a practical group-level estimate, but individual samples are required for FECRT and for identifying high-shedding animals.

Interpretation thresholds vary with parasite species, season, and production system. A count of 500 epg in a lactating ewe in spring may warrant treatment in a Hemonchus-endemic region, whereas the same count in a dry ewe on clean pasture in winter may be inconsequential. The [FAO animal production and health guidance](https://www.fao.org/animal-production/en/) emphasizes that local epidemiological knowledge must inform threshold selection, as transmission patterns differ markedly between temperate and tropical systems.

## The FECRT as a Resistance Audit

The fecal egg count reduction test is the field-standard method for detecting anthelmintic resistance. The protocol requires a minimum of 10 to 15 animals per treatment group with pre-treatment egg counts above a threshold, typically 150 to 200 epg. Animals are randomised to treatment or untreated control groups, and post-treatment samples are collected 10 to 14 days after treatment for most anthelmintics. The percentage reduction is calculated from the arithmetic mean egg counts of the treated and control groups.

A reduction below 95% for macrocyclic lactones or below 98% for benzimidazoles indicates resistance, although these thresholds vary by region and by the specific resistance mechanism involved. The [review of drug resistance in nematodes of veterinary importance](https://pubmed.ncbi.nlm.nih.gov/15363441/) reported that multi-drug resistance in sheep and goats threatens the viability of small ruminant industries in some regions, making routine FECRT auditing essential.

The FECRT has important limitations. It cannot detect resistance when pre-treatment egg counts are low, and it is unreliable in goats because of their different drug pharmacokinetics. For goats, the same protocol is used but the interpretation thresholds are adjusted, and the [MSD Veterinary Manual](https://www.msdvetmanual.com/) should be consulted for species-specific guidance. Where resistance to multiple classes is suspected, molecular assays may supplement the FECRT, but these are not yet widely available in practice.

## Documentation and Decision Recording

Every diagnostic finding should be recorded in a format that supports longitudinal comparison. Record the date, the management group, the diagnostic tool used, the raw result, and the treatment decision that followed. For FECRT, record the anthelmintic class, the product used, the route of administration, and the weight-based dose calculation. This documentation allows the practitioner to detect declining efficacy over time before clinical failure occurs.

The [USDA APHIS animal health information](https://www.aphis.usda.gov/livestock-poultry-disease) and the [WOAH terrestrial animal health standards](https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-code-online-access/) provide frameworks for disease surveillance and reporting that apply to notifiable conditions. Parasite monitoring records also support quality assurance programs and certification schemes that require documented anthelmintic use.

## Species and System Modifications

Goats differ from sheep in several ways that change diagnostic interpretation. Goats have higher drug clearance rates, so anthelmintic doses are often higher on a body weight basis. Goats also tend to browse instead of graze, which affects their exposure to infective larvae on pasture. The [FAO animal production and health guidance](https://www.fao.org/animal-production/en/) notes that production systems, from intensive zero-grazing to extensive rangeland, fundamentally alter the epidemiology of nematode infection and therefore the appropriate diagnostic strategy.

Dairy operations require attention to milk withdrawal periods when anthelmintics are administered, and the diagnostic plan must be coordinated with the milking schedule. Organic production systems may restrict the use of certain anthelmintic classes, and the practitioner must verify the applicable standards before recommending treatment. In all cases, the diagnostic strategy should be reviewed at least annually and adjusted when FECRT results, seasonal patterns, or management changes warrant it.

## Recognized Failure Modes in Parasite Diagnosis

Diagnostic strategies fail in predictable ways. The most consequential failure is the misinterpretation of a negative fecal egg count (FEC) as proof of absence of parasitic disease. In sheep and goats, a zero or near-zero FEC does not exclude significant larval challenge, particularly where pasture contamination is recent or where parasites are in the prepatent period. The same limitation applies to inhibited larvae that resume development later in the season. A negative FEC must be interpreted against the grazing history, the season, and the signalment of the animal, not in isolation.

A second failure mode is the over-reliance on FAMACHA scoring in flocks where Hemonchus contortus is not the dominant parasite. The FAMACHA system detects anemia, and anemia is a specific but not sensitive marker for trichostrongylid infection. Where Teladorsagia or Trichostrongylus predominate, weight loss and diarrhea occur without pallor, and FAMACHA scores remain normal while production losses accumulate. The method is valid only where hemonchosis is the primary threat, and it must be validated locally before it is used as the sole screening tool.

A third failure is the use of the fecal egg count reduction test (FECRT) under conditions that invalidate the result. The test requires adequate group sizes, a pre-treatment FEC above a minimum threshold, and a post-treatment sample collected at the correct interval for the drug class used. When these conditions are not met, the calculated reduction is unreliable and may falsely suggest efficacy. The [meta-analysis of European anthelmintic resistance data](https://pubmed.ncbi.nlm.nih.gov/33277891/) shows that resistance is widespread and that underpowered or poorly timed FECRTs have contributed to delayed recognition of resistance on individual farms.

| Observation | Likely cause | Discriminating check |
|---|---|---|
| Negative FEC with clinical signs | Prepatent infection, inhibited larvae, or non-nematode cause | Repeat FEC in 10 to 14 days, examine history of pasture exposure, consider other pathogens |
| FAMACHA scores normal but poor performance | Non-Hemonchus nematodes or non-parasitic disease | Quantitative FEC with larval culture, assess body condition and production records |
| FECRT shows apparent efficacy but resistance emerges later | Test underpowered or interval incorrect | Repeat with adequate group size and correct sampling interval, request species identification |
| High FEC in a treated group | Treatment failure, incorrect dose, or recent reinfection | Verify dose and administration route, check weight estimates, perform FECRT |

## Common Clinical Errors and Corrective Action

Less experienced clinicians frequently underestimate the importance of accurate bodyweight estimation. Underdosing is the single most common management error that selects for resistance, because subtherapeutic drug exposure removes susceptible worms while allowing resistant survivors to reproduce. The corrective action is to weigh representative animals, not to estimate by eye, and to dose to the heaviest animal in the group.

A second error is the routine treatment of all animals at the same time, which eliminates refugia and accelerates resistance selection. The [review of targeted selective treatments](https://pubmed.ncbi.nlm.nih.gov/19450930/) describes how treating only the animals that most need intervention preserves susceptible parasite populations and slows resistance development. The corrective action is to adopt a treatment decision framework based on FAMACHA, body condition, or production parameters, and to accept that some animals will carry low-level infections.

A third error is the failure to distinguish between sheep and goat dosing requirements. Goats metabolise many anthelmintics more rapidly than sheep, and label doses for sheep are frequently subtherapeutic in goats. Current formularies and label references must be consulted before prescribing for goats, and the clinician should confirm whether the product is authorised for use in goats in the relevant jurisdiction.

## Limitations of the Evidence and Areas of Expert Disagreement

The evidence base for small ruminant parasite control is strong on the epidemiology of resistance but weaker on the comparative performance of diagnostic markers in different production systems. The [international review of integrated nematode control](https://pubmed.ncbi.nlm.nih.gov/10048828/) noted that research capacity in applied veterinary parasitology has contracted, and much of the current guidance derives from a limited number of well-studied systems, predominantly in temperate regions with high H. contortus prevalence.

Expert opinion differs on the optimal threshold for treatment intervention. Some authorities advocate treatment when FEC exceeds a fixed cut-off, while others prefer production-based criteria such as weight gain or milk yield. The [review of refugia-based strategies](https://pubmed.ncbi.nlm.nih.gov/19450930/) acknowledges that the markers studied for targeted selective treatment, including anemia, milk production, and liveweight gain, have variable predictive value across systems. The clinician should select a decision framework that fits the local parasite spectrum and production goals, and should document the rationale for that choice.

## Escalation and Referral Criteria

Referral or specialist consultation is warranted when resistance is suspected but the practice lacks the laboratory capacity to confirm it, when clinical disease persists despite apparently adequate control, or when a production unit experiences unexplained losses that standard diagnostics do not explain. Diagnostic laboratories should be involved for larval culture and species identification, for molecular assays where available, and for confirmatory FECRTs performed to a protocol that meets published standards.

Regulatory reporting obligations vary by jurisdiction. Where anthelmintic resistance is a notifiable condition, or where drug residues are suspected, the clinician should consult the relevant national authority. The [WOAH terrestrial animal health standards](https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-code-online-access/) and [USDA APHIS animal health information](https://www.aphis.usda.gov/livestock-poultry-disease) provide starting points for determining local requirements, but the clinician must confirm the applicable rules in their own jurisdiction.

## Frequently Asked Questions

### How Should I Prioritize Diagnostic Investment When the Flock Budget Is Limited?

Start with FAMACHA scoring, which requires no equipment beyond the eye mucosa guide and provides immediate treatment decisions for hemonchosis. When funds permit a single laboratory test, request a composite fecal egg count (FEC) from 10 to 15 animals instead of individual counts. This gives herd-level burden data at reduced cost. Reserve individual FECs for animals that fail FAMACHA-based treatment criteria or for pre- and post-drench sampling in a fecal egg count reduction test (FECRT). The FECRT requires two sampling time points and represents the highest-value investment for resistance surveillance, since it directly measures product efficacy on your farm.

### What Is the Minimum Equipment Needed for Reliable In-House FEC Work?

A compound microscope with 10x and 40x objectives, McMaster slides or a modified McMaster counting chamber, a balance accurate to 1 g, sieves, and flotation solution with a specific gravity near 1.20. Saturated salt solution is acceptable for strongyle eggs but may distort or fail to float some species. If a balance is unavailable, use a standardized scoop validated against known fecal weights. Centrifugation improves egg recovery but is not mandatory for routine McMaster counts. The limiting factor is consistent technique, so run duplicate counts on the first several samples and compare results before trusting single readings. Reference laboratories offer quality assurance for in-house results.

### How Do Diagnostic Approaches Differ for Goats Compared With Sheep?

Goats metabolise many anthelmintics more rapidly than sheep, so treatment failure may reflect pharmacokinetics instead of true resistance. This complicates FECRT interpretation. For goats, the FECRT threshold for suspected resistance is often set lower, and the [European anthelmintic resistance database](https://pubmed.ncbi.nlm.nih.gov/33277891/) documents higher reported resistance prevalence in goats across multiple classes. FAMACHA scoring remains valid for Hemonchus in goats, but goats are more likely to carry mixed infections where anemia is not the dominant sign. Periparturient ewes and does also differ in FEC patterns, so sample timing must be adjusted accordingly. Always record species when submitting samples and when interpreting results.

### What Records Should I Maintain for Each Parasite Diagnostic Event?

Record the date, production group, number of animals sampled, sampling method, FEC technique, raw counts, and calculated eggs per gram. For FAMACHA, record the score distribution across the group. When performing a FECRT, document the product, dose rate, route, batch number, and the interval between treatment and post-treatment sampling. Retain this information in a format that allows year-on-year comparison. The [MSD Veterinary Manual](https://www.msdvetmanual.com/) recommends tracking treatment history alongside diagnostic results to identify emerging resistance patterns. These records also support veterinary quality assurance programs and provide evidence for herd health planning discussions with producers.

### How Do I Explain Resistance Testing to a Producer Who Wants Immediate Treatment?

Frame the FECRT as a management audit instead of a delay in care. Explain that treating a resistant worm population removes susceptible worms, leaving resistant worms to reproduce, a mechanism detailed in the [refugia-based control review](https://pubmed.ncbi.nlm.nih.gov/19450930/). The test requires withholding treatment from a small group for a defined period, which some producers find counterintuitive. Emphasize that the information gained prevents repeated use of ineffective products, which costs more in the long term through production losses and purchased anthelmintics. Offer to treat clinically affected animals immediately and use only the healthy cohort for testing. This preserves animal welfare while generating the data needed for sustainable control.

### When Should I Refer a Parasite Problem to a Specialist or Laboratory?

Refer when FECRT results indicate resistance to multiple anthelmintic classes, when clinical disease persists despite apparently adequate treatment, or when you suspect an unusual parasite species outside your diagnostic experience. Specialist laboratories can perform larval culture and speciation, which distinguishes Hemonchus from Trichostrongylus and Teladorsagia, and can offer molecular assays for resistance-associated alleles. The [FAO animal production guidance](https://www.fao.org/animal-production/en/) notes that regional differences in parasite ecology affect control recommendations, so a specialist familiar with your area is preferable. Refer also when you need to rule out non-parasitic causes of poor performance, such as trace element deficiency or chronic disease, that mimic parasitism.

## Related Clinical & Scientific Guides

* [Rumen Health Assessment in Dairy Cows: Clinical and Subclinical Indicators](/knowledge/veterinary-medicine/food-animal-medicine/rumen-health-assessment-dairy-cows-clinical-subclinical-indicators)
* [Mastitis Control Programs in Dairy Herds: Monitoring and Prevention](/knowledge/veterinary-medicine/food-animal-medicine/mastitis-control-programs-dairy-herds-monitoring-prevention)
* [Swine Nutrition and Health: Feed-Related Disease Diagnosis](/knowledge/veterinary-medicine/food-animal-medicine/swine-nutrition-health-feed-related-disease-diagnosis)


## References and Further Reading

- [Increasing importance of anthelmintic resistance in European livestock: creation and meta-analysis of an open database.](https://pubmed.ncbi.nlm.nih.gov/33277891/). 2020.
- [The role of targeted selective treatments in the development of refugia-based approaches to the control of gastrointestinal nematodes of small ruminants.](https://pubmed.ncbi.nlm.nih.gov/19450930/). 2009.
- [International approaches to the concept of integrated control of nematode parasites of livestock.](https://pubmed.ncbi.nlm.nih.gov/10048828/). 1999.
- [Drug resistance in nematodes of veterinary importance: a status report.](https://pubmed.ncbi.nlm.nih.gov/15363441/). 2004.
- [The genome and transcriptome of Hemonchus contortus, a key model parasite for drug and vaccine discovery.](https://pubmed.ncbi.nlm.nih.gov/23985316/). 2013.
- [Bovine cryptosporidiosis: impact, host-parasite interaction and control strategies.](https://pubmed.ncbi.nlm.nih.gov/28800747/). 2017.
- [USDA APHIS Animal Health Information](https://www.aphis.usda.gov/livestock-poultry-disease). USDA APHIS.
- [FAO Animal Production and Health](https://www.fao.org/animal-production/en/). FAO.
- [MSD Veterinary Manual, Professional Edition](https://www.msdvetmanual.com/). MSD Veterinary Manual.

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> This article is educational professional reference material for veterinary audiences. It is not a substitute for veterinary diagnosis, individual clinical judgment, current product labeling, or applicable regulatory requirements.