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Pathogens (Basel, Switzerland)2026; 15(2); 156; doi: 10.3390/pathogens15020156

Relationship Between Mean Faecal Gastrointestinal Nematode Egg Excretion in Horses and Its Variability: Implications for Control.

Abstract: Faecal egg counts (FECs) are used to assess the intensity of gastrointestinal nematode (GIN) infections in herbivores. FEC distribution is aggregated, meaning that approximately 20% of animals harbour 80% of infections. In times of escalating anthelmintic resistance, it may be necessary to restrict treatment to the animals with the heaviest infections. This strategy is called targeted selective treatment (TST) and is relevant to GIN, for example. The difficulty lies in identifying which animals to treat. One solution is to select potentially at-risk animals based on age (for example, treating the young) or to perform individual faecal egg counts (though this is costly). We propose a solution for determining the suitability of selective treatment based on the level of FEC (200 or 500 eggs per gram of faeces). First, we demonstrated that the mean FEC in a group is strictly related to its variance (Taylor's power law) using published data and our own unpublished data on horses from France, Poland, and Mexico. The study focused on small and large strongyles in horses. Taylor's power law states that sample variance (Var) and the population mean are related by a simple equation: Var = a Mean^b or log(Var) = log(a) + b log(Mean). The influence of factors such as age, status (mare, stallion, yearling, etc.), day-to-day variability, and previous anthelmintic treatments did not alter this relationship. To reduce the number of FECs, we estimated the mean FEC on a composite faecal sample. We then calculated the variability and therefore the number of horses with an FEC above the chosen acceptable level. When the mean is high, the number of horses to be treated is also high and TST is not beneficial. When the FEC is average, TST may be worthwhile, either based on the FEC of individual horses or on the horse class at risk. Based on the percentage of horses with an FEC above the acceptable level, farmers can decide whether to treat all animals or establish a TST protocol. Caution should be exercised when using TST in the presence of large strongyles.
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Publication Date: 2026-02-02 PubMed ID: 41754409PubMed Central: PMC12942807DOI: 10.3390/pathogens15020156Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Overview

  • This study investigates the relationship between the average faecal egg count (FEC) of gastrointestinal nematodes in horses and the variability of these counts, aiming to inform targeted treatment strategies to control parasite infections.
  • By understanding this relationship, the research provides guidance on when selective treatment of only high-infection horses is effective versus when treating all animals might be necessary.

Background

  • Gastrointestinal nematode (GIN) infections in herbivores, including horses, are commonly assessed via faecal egg counts (FECs), which measure the number of nematode eggs per gram of faeces.
  • FEC data tend to be aggregated, meaning a small proportion of animals typically carry the majority of the parasite burden (approximately 20% of horses carry 80% of infections).
  • Anthelmintic resistance (resistance to deworming drugs) is increasing, making it important to reduce unnecessary treatments and strategically target only the animals with heavy infections.
  • Targeted selective treatment (TST) is a strategy that treats only the horses with the highest FECs, potentially limiting drug use and slowing resistance development.

The Challenge

  • The primary difficulty in applying TST is identifying which horses need treatment without testing every individual—testing all horses is costly and labor-intensive.
  • Approaches like treating based on age or other risk factors are commonly used, but may not always perfectly identify the heaviest infections.
  • This research seeks a method to evaluate when selective treatment is suitable based on the mean FEC and its variability within a group.

Key Methods and Findings

  • Data were collected from both published sources and new, unpublished datasets involving horses from France, Poland, and Mexico, focusing on infections with small and large strongyles—common nematodes in horses.
  • The study examined the relationship between the mean FEC and the variance in FEC within groups of horses, using Taylor’s power law, which states the variance in counts relates to the mean by a power function: Var = a Mean^b.
  • In logarithmic terms: log(Var) = log(a) + b log(Mean), verifying this relationship supports predictability of variance based on mean values.
  • This relationship held true regardless of variables including age, sex, daily fluctuations in egg excretion, and prior anthelmintic treatments.
  • To reduce testing effort, the researchers estimated the mean FEC from composite faecal samples created by pooling individual samples, which still allowed them to estimate the variability and identify how many horses might exceed a particular FEC threshold.

Implications for Treatment Strategies

  • When the average FEC in a group is high, many horses have counts above the threshold, meaning that most animals would need treatment—making TST less beneficial compared to treating all horses.
  • When the average FEC is moderate or low, TST becomes more valuable, because it is possible to accurately identify and treat only the small subset of heavily infected horses.
  • Farmers can use the estimated percentage of horses exceeding the chosen FEC threshold (e.g., 200 or 500 eggs per gram) to decide whether it is cost-effective and epidemiologically sound to apply TST or to treat the entire group.
  • Particular caution is advised when large strongyles are present, as the risk profile and control strategies may differ, implying TST might not be appropriate.

Conclusions

  • The study provides a valuable practical tool by linking mean FEC levels to infection spread within groups, enabling better-informed decisions on parasite control strategies.
  • It highlights the potential to reduce unnecessary anthelmintic use by selectively treating animals in groups with moderate infection levels, thereby mitigating resistance risk and treatment costs.
  • The approach using composite samples for mean FEC estimation could lower diagnostic costs while still providing actionable data on population-level parasite burdens.
  • Overall, this research supports more sustainable parasite management practices in equine populations through informed application of targeted selective treatment based on scientifically grounded variability analysis.

Cite This Article

APA
Cabaret J, Guerrero Molina C, Martínez-Ortiz-de Montellano C, Alcala Canto Y. (2026). Relationship Between Mean Faecal Gastrointestinal Nematode Egg Excretion in Horses and Its Variability: Implications for Control. Pathogens, 15(2), 156. https://doi.org/10.3390/pathogens15020156

Publication

Pathogens (Basel, Switzerland)
ISSN: 2076-0817
NlmUniqueID: 101596317
Country: Switzerland
Language: English
Volume: 15
Issue: 2
PII: 156

Researcher Affiliations

Cabaret, Jacques
  • SantéSocioVéto, 8 Pl. Carré de Busserolle, 37100 Tours, France.
Guerrero Molina, Cristina
  • Departamento de Parasitologia, Facultad de Medicina Veterinaria y Zootecnia, Universidad Nacional Autonoma de Mexico (UNAM), Ciudad de Mexico 04510, Mexico.
Martínez-Ortiz-de Montellano, Cintli
  • Departamento de Parasitologia, Facultad de Medicina Veterinaria y Zootecnia, Universidad Nacional Autonoma de Mexico (UNAM), Ciudad de Mexico 04510, Mexico.
Alcala Canto, Yazmin
  • Departamento de Parasitologia, Facultad de Medicina Veterinaria y Zootecnia, Universidad Nacional Autonoma de Mexico (UNAM), Ciudad de Mexico 04510, Mexico.

MeSH Terms

  • Animals
  • Horses
  • Feces / parasitology
  • Parasite Egg Count
  • Nematode Infections / veterinary
  • Nematode Infections / parasitology
  • Nematode Infections / drug therapy
  • Horse Diseases / parasitology
  • Horse Diseases / drug therapy
  • Nematoda / isolation & purification
  • Anthelmintics / therapeutic use

Conflict of Interest Statement

The authors declare no conflict of interest.

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