Parasites can harm the reproductive efficiency of horses – review

"Understanding the cause of impaired reproductive performance is essential in terms of maintaining productivity and efficient stud breeding strategies."
“Understanding the cause of impaired reproductive performance is essential in terms of maintaining productivity and efficient stud breeding strategies.”

Parasite burdens have a range of possible direct and indirect effects on the reproductive efficiency of horses, according to the authors of a just-published review.

Panagiota Tyrnenopoulou and her fellow researchers noted that significant changes have occurred in the equine industry in the past 50 years.

The liberalisation of rules regarding horse breeding and the increased diversity of equestrian activities have resulted in a rise in the number of horse breeds and breed registries.

Stallion and mare reproductive efficiency levels are among the core components of the economy of the equine industry.

“Improving our understanding of the causes and consequences of impaired reproductive performance in horses is, therefore, of prime interest,” they said.

Parasitic infections are ubiquitous in horses and consequently represent a significant component in terms of implementing appropriate management practices in those animals, they said.

Resistance to deworming drugs, known as anthelmintics, has been reported in worms recovered from horses. Despite that, until recently, protocols for the use of deworming drugs have been based on their exclusive and regular use.

Research findings indicate that surveillance-based control programmes and regular faecal egg counts are necessary for determining when treatment is needed, thereby reducing the reliance on anthelmintic treatments.

In their paper, published in the journal Parasitologia, the review team looked at the interactions between parasitic infections and reproductive efficiency in horses.

They said the prevention and control of parasitic infections are particularly important in horses, as they can affect reproductive ability.

Early studies have highlighted nutrition as an important factor in the reproductive performance of various species, with a minimum level of body fat required to ensure normal reproductive activity in mammals.

Ovarian inactivity is a significant cause of infertility in breeding mares and it has been long associated with the low body condition of the animals, they noted.

A relationship between parasite load and body condition is not always easy to demonstrate and predict, they said, particularly due to the variation in energy acquisition by horses. However, ovarian inactivity has been recorded in mares debilitated from intense parasitism that had brought the animals to a poor body condition.

The authors traversed the various stages of the reproductive cycle and the potential for reproduction to be impaired by parasites. They discussed ovarian activity and relevant body condition, breeding management, pregnancy and neonatal period.

They also discussed control strategies in pregnant mares and foals.

“It can be assumed that horses grazing together are infected with the same parasitic population. Nevertheless, large differences between them have been identified in the number of parasitic eggs excreted in their faeces,” they said.

Protocols for the administration of deworming treatments can be individualised among adult animals if the number of eggs shed in faecal samples is determined, usually after performing faecal egg counts.

“In general, examination of faecal samples from at least six animals in the group should be carried out at the time of treatment and 14 days later,” they said.

They noted that young horses are susceptible to a wide range of parasites. “Selective, surveillance-based helminth control strategies based on the evaluation of each individual animal’s susceptibility to parasitism cannot be applied in young horses,” they said.

“Innate differences in the susceptibility of the hosts do not have adequate competency through the immature immune system of foals.

“In general, before the establishment of acquired immunity, effective larvicidal anthelmintic protocols are important to ensuring the effective control of all stages of ascarid infections in young foals.”

However, the resistance to antiparasitic compounds of helminths recovered from foals complicates the design and establishment of appropriate anthelmintic schemes for foals.

In any case, the first deworming treatment should not be performed later than the age of two months.

“Moreover, as increased doses of drugs have often been recommended for foals, it is noteworthy that administration of unlicenced doses of drugs to horses is the responsibility and concern of the prescribing veterinarian with regard to the safety of the drug and maintenance of increased withdrawal periods.”

Some drugs, for example, moxidectin, are not licenced for use in young foals, which sets further hindrances in formulating anthelmintic programmes, they said.

Strongyloides westeri and Parascaris equorum are the helminths most frequently infecting horses in those ages. S. westeri often causes an asymptomatic infection, but P. equorum can cause more serious problems. Pyrantel and macrocyclic lactones are effective against adult parasites, while fenbendazole has been reported to show activity against adults and larvae of some parasites.

“Challenges are also present in controlling the parasites,” they wrote. “For example, P. equorum is a dose-limiting parasite for many anthelmintics, as it has a lower threshold for the development of anthelmintic resistance.”

Tapeworm infection should be taken into consideration in older foals and at the end of the grazing season, they said. Foals should be treated against Anoplocephala perfoliata during the first autumn or winter of their life.

Gasterophilus species and Oxyuris equi infections are seasonal and sporadic, and usually affecting older foals.

Macrocyclic lactones are the only currently marketed class of equine anthelmintics with efficacy against O. equi, they said.

The authors noted there were many anecdotal reports of pinworms being resistant to macrocyclic lactones; however, there are only a few documented cases in the literature thus far.

“The larval stages of Gasterophilus are highly susceptible to macrocyclic lactones and will be eliminated during regular deworming with these drugs. As fly activity ceases with the first frosts, appropriate treatment in late autumn would remove all larvae present within horses.

“A well-established programme throughout the neighbouring equine facilities is also necessary for control of these parasites.”

The researchers said parasites remain a significant threat to the health and welfare of horses.

“Understanding the cause of impaired reproductive performance is essential in terms of maintaining productivity and efficient stud breeding strategies.

“Further collaboration of parasitologists, stud managers and veterinarians is necessary in order to implement novel control strategies with a greater emphasis on the emerging challenges.”

The review team comprised Tyrnenopoulou, Petros Boufis and George Fthenakis, all with the University of Thessaly in Greece; and Elias Papadopoulos, with the Aristotle University of Thessaloniki, also in Greece.

Tyrnenopoulou, P.; Boufis, P.T.; Fthenakis, G.C.; Papadopoulos, E. Interactions between Parasitic Infections and Reproductive Efficiency in Horses. Parasitologia 2021, 1, 148-157.

The review, published under a Creative Commons License, can be read here

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