Fecal Egg Count: A Potent Genetic Marker for Parasitic Resistance in Small Ruminants
Abstract
For decades, the severity of parasitic infection in small ruminants has been measured using fecal egg count (FEC). Genetic resistance to parasitic infection varies among individual animals and flocks, making this trait economically important, particularly in small ruminants. Heritability estimates for FEC across different breeds, populations, and time periods range from low to moderate. This paper reviews studies reporting genetic parameter estimates for FEC in different populations. It also focuses on FEC as a genetic marker and a means for selecting small ruminants for parasitic resistance — a threshold trait for which the underlying variable is fecal egg count. Non-genetic factors affecting FEC are also discussed herein.
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Introduction
Parasitic infections in small ruminants, particularly those affecting the gastrointestinal system, are a significant cause of morbidity and mortality, leading to extensive economic losses. These losses are primarily due to reduced growth rates, lower reproductive performance, decreased milk yield, and increased veterinary costs associated with the treatment of infected animals (Gourley et al., 2016). Moreover, gastrointestinal parasites contribute to a reduced ability of livestock to convert feed efficiently into meat and milk, further impacting the profitability of farming operations.
The primary method of managing and impeding parasite infections is the use of anthelmintics. However, the costs of such therapy are high, and another disadvantage is that the application of these drugs may lead to drug-resistant populations of parasites (Charon et al., 2002). There has also been increasing concern regarding residual drugs in animal-derived food products (Gowane et al., 2019). Several studies indicate that the problem of parasitic infection occurs most frequently in small ruminants (Augad et al., 2024; Malathi et al., 2021; Suarez et al., 2021; Shashank et al., 2019).
Gastrointestinal nematodes (GINs) pose a major threat to the production performance of small ruminants. The best strategy to control GINs is to appraise or breed sheep that are tolerant to parasites (Miller et al., 1998; Kemper et al., 2009). Extensive information is available regarding variation in sheep and goat resistance to GINs. The term "resistance" includes passive and active immunity. Passive resistance includes physical or chemical barriers that inhibit parasite entry into the host body. Active resistance involves innate and/or adaptive immune responses generally produced in response to infection (Coustau et al., 2000). McClure (2000) described host resistance as the ability of an individual animal to eradicate a parasitic infection and to prevent reinfection by utilising innate (non-specific) and acquired (learned and parasite-specific) immunity.
This review aims to: (1) summarize published heritability estimates for FEC in small ruminants, (2) discuss non-genetic factors influencing FEC, and (3) evaluate the potential of FEC as a genetic marker for breeding parasite-resistant animals.
Conclusion
Many traits in farm animals are complex and threshold in nature, meaning they cannot be measured directly. Parasitic resistance is one such trait, and fecal egg count (FEC) is considered a reliable indicator of this trait in livestock. Most research on parasitic resistance has been conducted in sheep and goats, as these species suffer major economic losses due to nematodes. They are more susceptible to gastrointestinal parasites due to the grazing nature of their management systems.
FEC, along with molecular genetic markers, can be effectively used for the selection of disease-resistant animals, thereby benefiting livestock owners. Incorporating FEC into breeding programs offers a sustainable approach to parasite control that reduces reliance on anthelmintics, thereby mitigating the development of drug-resistant parasite populations. For smallholder farmers in resource-limited settings where diagnostic facilities may be unavailable, simplified FEC protocols and genetic selection indices that include FEC as a trait could be developed. Future research should focus on establishing breed-specific FEC reference values, validating genetic markers associated with low FEC, and developing decision-support tools for integrating FEC into routine flock management.
References
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