Interactions between Parasitic Infections and Reproductive Efficiency in Horses

Parasites remain a significant threat to horses’ health and welfare. The present paper reviews and discusses interactions between parasitic infections and reproductive efficiency in horses. The manuscript describes the interactions in a structured way, presenting the infecting parasites and the respective effects in the reproductive activity of horses. The following stages of the reproductive cycle are covered: ovarian activity and relevant body condition, breeding management (including effects in mares and stallions), pregnancy and neonatal period. A separate section covers the topic of transmission of pathogens to horses through parasites. Finally, parasite control strategies in relation to reproductive activity (strategies for breeding mares and foals) are discussed. Understanding the causality 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.


Introduction
During the past 50 years, significant changes have occurred in the equine industry around the world [1]. 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 [2,3]. Stallion and mare reproductive efficiency levels are among the core components of the economy of the equine industry [4][5][6]. Improving our understanding of the causes and consequences of impaired reproductive performance in horses is, therefore, of prime interest.
Parasitic infections are ubiquitous in horses and consequently represent a significant component in terms of implementing appropriate management practices in those animals [7]. Anthelmintic resistance has been reported in worms recovered from horses [8,9]. Nevertheless, until recently, protocols for the administration of anthelmintics in horses have been based on the exclusive and regular use of anthelmintic agents [10]. Research data indicate that surveillance-based control programmes and regular faecal egg counts are necessary for determining when treatment is needed, hence reducing the reliance on anthelmintic treatments [11,12].
The present paper reviews and discusses interactions between parasitic infections and reproductive efficiency in horses.

Ovarian Activity and Relevant Body Condition
The prevention and control of parasitic infections are particularly important in horses, as they affect reproductive ability [13,14]. Early studies have highlighted nutrition as an important factor in the reproductive performance of various species; a minimum level of body fat is required to ensure normal reproductive activity in mammals [15]. 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 [4]. Although a relationship between parasite load and body condition is not always easy to demonstrate and predict [16], particularly due to the variation in energy acquisition by horses, ovarian inactivity with anoestrus has been recorded in mares debilitated from intense parasitism that had brought the animals to a poor body condition [17][18][19].
Twin pregnancy in horses is often associated with complications; hence, it is not welcome. Mares carrying twins frequently lose one or both foetuses at an early or later stage of gestation and are more likely to suffer dystocia [20].

Breeding Management
Diseases caused by pathogens potentially transmitted during breeding cause significant concern to equine veterinarians [21]. Whether fertilisation would be achieved through mating or artificial insemination [2], some of these pathogens are particularly infective and can be transmitted between animals during direct horse-to-horse contact, through the use of contaminated semen or by indirect venereal contact, by means of contaminated equipment for semen collection and processing (including artificial vaginas and breeding phantoms) or even personnel (contaminated hands and clothing) participating in the semen collection process.
Trypanosoma equiperdum is a flagellate parasite, the causal agent of dourine, and the only protozoan pathogen causing a venereal disease in horses [22,23], as well as the only trypanosome not transmitted by an invertebrate vector. The disease is considered to have been eradicated in some countries of the world; cases of the disease are still diagnosed and reported in various countries in Asia, Africa, Central and South America and Russia [24]. In some countries of Europe, cases of dourine have not been reported for a long time  [25]. Nevertheless, during the previous decade, cases of the diseases were diagnosed in some countries of Southern and Eastern Europe [26,27].
Transmission of the parasite follows its presence in the seminal fluid and the mucous exudates of the penis and its sheath. Stallions are usually affected during the mating of mares, in which the organism was present in the vaginal mucus [23,25]. Clinical manifestations of dourine associated with the reproductive system include gelatinous infiltration in the vulva, the vaginal mucosa and the uterus, often coupled with non-painful oedema of the mammary glands in mares. In stallions, clinical sign can vary from a slight thickening of the scrotum, sheath and testicular tunica to the development of tough mass(es) of sclerotic tissue potentially resulting in testicular malformation [28]. Foals can become infected through the oral, nasal or conjunctival mucosa at birth, when coming in contact with vaginal discharge. Thenceforth, these infected foals can spread the organism further at later stages of their life [29], thus contributing to the dissemination of the parasite. Transmission to foals by ingestion of colostrum or milk is rare [30,31].
Among the common nematode parasites of horses, Oxyuris equi is widely distributed, historically throughout juvenile and immature populations of animals [32]. O. equi infection is often considered to be non-pathogenic; however, the local irritant effect of the eggs of the helminth deposited in the perineal region can cause considerable distress to an affected horse [33]. The intense itching by the irritant effect of the parasites may cause restlessness and impaired feeding, potentially leading to the deterioration of body condition.
With regard to stallions, it is noteworthy that the larvae of Habronema spp. or Draschia megastoma present on mucocutaneous junctions of the prepuce and penis, can cause ulcerated granulomatous lesions in those sites [34] that may lead to penile prolapse and reproductive disorders in stallions [35,36]. Moreover, Setaria equina, a filarioid nematode living in the abdominal cavity of horses is not considered to be pathogenic; however, some reports have presented incorrect migration of the parasite to the testes of stallions, which concluded with long-standing orchitis [37,38].

Pregnancy
In horses, the causal agents of abortion and foetopathy include a wide range of mainly bacterial and viral pathogens [39][40][41], although parasitic agents are also implicated in their aetiology.
Neospora caninum and Neospora hughesi are intracellular protozoan parasites responsible for reproductive losses in horses. Cases of abortion caused by N. caninum may be misdiagnosed to be caused by Toxoplasma gondii, as also reported in other species [42,43]. Seroprevalence of the infection has been found to vary widely, from 0% to 85% [44][45][46][47]. In horses, the infection usually causes a subclinical disease; nevertheless, less often, it can result in abortion. Further, transplacental infection of a borne foetus can occur that may later lead to stillbirth [44,46,48,49], although a case of congenital neosporosis has been reported in a one-month-old foal [50].
Less frequently, cases of infection by microsporidia (Encephalitozoon spp.) have also been reported, associated with placentitis and subsequent abortion in horses [51]. Nevertheless, although these parasites have an abortifacient effect in mares [52,53], there is little research regarding their significance in horses [54]. A case of infection with Encephalitozoon cuniculi associated with necrotising placentitis and abortion at the final stage of gestation has been reported [52].
Acanthamoeba spp. are protozoan species present in the environment [55]. Infections with amoebae generally occur through inhalations of cysts or exposure of skin lesions to contaminated elements of the environment followed by a haematogenous spreading of the parasite to the rest of the animal. Placentitis associated with Acanthamoeba hatchetti infection has been reported [56]; potential transmission scenarios of this organism include mechanical transport by setae or localised vascular invasion in the uterus.
Babesia caballi and Theileria equi are protozoan parasites, primarily transmitted by ixodid ticks (Rhipicephalus spp., Amblyomma spp., Dermacentor spp.) and infecting the horses. Rarely, these organisms can cause transplacental infection, followed by abortion or, alternatively, the birth of infected foals, which may occasionally show signs of disease [57,58]. It is noteworthy that in areas where T. equi is endemic, this organism is considered to be the major cause of equine abortions [57].

Neonatal Period
Strongyloides westeri is the first nematode parasite to infect foals and achieve patency, events that occur from the 3rd to the 20th day of life [59]. Infection by S. westeri can be achieved by means of three potential routes. Lactogenic transmission is considered the most significant route of infection in foals; third-stage larvae, arrested within body tissues of the mare, are reactivated and migrate to the mammary glands and are transmitted to foals through the milk [60,61]. Alternatively, foals can also ingest infective free-living larvae or may experience percutaneous infection [62,63].
Young foals (<2 months) are exposed to infection by Parascaris spp., although primarily animals over the age of 2 months and until weaning may be infected with this helminth. This parasite is considered as the most important parasite in foals; adult P. equorum and their larval stages are pathogenic for foals [64]. The infection is associated with respiratory disease, abnormal growth, colic, intestinal impaction or perforation, peritonitis and subsequent death [65][66][67]. An incident of jejunal impaction in a foal, caused by this parasite after administration of pyrantel pamoate, has also been described by Schusser et al. [68].
Regarding other parasites, cyathostomin infection is associated with clinical disease (poor growth, weight loss and diarrhoea) in older foals [69]; acute colitis may result from mass eruption of larval stages, throughout the grazing season, due to pasture larval contamination [70]. Anoplocephala perfoliata is a common equine tapeworm in the terminal part of the ileum and caecum associated with an increased risk of colic in adult horses and in older foals at the end of the grazing season [71,72]. Cryptosporidiosis (Cryptosporidium spp.) has been associated with foal diarrhea in immunocompromised animals [73][74][75]. Oocysts of Eimeria leuckarti have also been identified in faecal samples from foals. E. leuckarti has been incriminated as the cause of an intermittent diarrhoea, but the significance of this parasite is not fully known [76]. Gasterophilus spp. and O. equi do not achieve patency in foals younger than 2 months, and thus clinical signs characteristic of these parasitoses would not normally be expected [60].
During the neonatal period, foal health is fragile and these animals are particularly susceptible to pathogens. Hence, even mild parasitic burdens or the presence of strains with increased pathogenicity can lead to suboptimal growth and delayed development of foals.

Transmission of Pathogens to Horses through Parasites
Many arthropods (flies, midges, ticks, etc.) can transmit various pathogens, which may cause various reproductive disorders in horses [77]. Such examples include Trypanosoma evansi, which is reported to be mechanically transmitted in several ways; via biting or sucking insects (Tabanus, Haematobia, Hippobosca and Stomoxys), leaf nosed bats, carnivores, as well as by fomites [78]. Transplacental transmission has also been documented, but the mechanism responsible for crossing the placental barrier needs to be elucidated in greater depth [79].
The trematode helminthes Acanthatrium sp. and Lecithodendrium sp. can serve as vectors for Neorickettsia risticii, the agent of Potomac horse fever [80]. The disease occurs infrequently and can be associated with spontaneous abortion, although no evidence of placentitis has been reported in such cases [81,82].
In recent years, the terms 'mare reproductive loss syndrome' [83] and 'equine amnionitis and foetal loss' [84] have been used to describe abortions occurring during the midto-late stage of gestation without specific clinical or laboratory findings, with a view to differentiate them from other common causes of equine pregnancy loss [85]. In such cases, pasture exposure to eastern tent caterpillars (Malacosoma americanum) and processionary caterpillars (Ochragaster lunifer) has been considered as an important risk factor for the above conditions; moreover, abortions have been experimentally reproduced [86], although the mechanism causing abortion is still unknown. Setae of the processionary caterpillar were documented to migrate from the gastrointestinal tracts of exposed mares into the uterus and foetal membranes, ultimately causing abortion [86].

Control Strategies for Breeding Mares
Anthelmintic resistance to equine parasites is still a substantial challenge for veterinarians active in the field [87,88]. Previous studies have reported resistance (confirmed or suspected) of small strongyles to benzimidazoles, pyrantel salts and macrocyclic lactones [89][90][91][92][93]. Cyathostomins have nowadays become a growing concern, as in the past their pathogenic potential was over-shadowed by Strongylus vulgaris; features of the infection are not specific and can vary from weight loss and diarrhoea to severe inflammatory enteropathy occasionally leading to death [94,95]. It is also noteworthy that anthelmintic resistance among large strongyle species, which may be a cause of high mortality rate, has been infrequently reported in recent years, although some recent evidence points out to a potential re-emergence of the problem, as the result of inappropriate parasite control strategies [7,96,97]. In this context, therefore, broodmares are the most important animals for considering when designing control programmes in stud farms.
Targeted anthelmintic treatments in broodmares are principally focused on cyathostomin control [98]. As mature horses, broodmares vary in their innate resistance to cyathostomin infection and their potential for strongyle egg shedding. Grazing horses can be infected by cyathostomins, which often may account for up to 95% to 100% of the total worm burden in an individual [7,99]. Other helminthes often recovered from adult horses include P. equorum, O. equi and the large strongyle species: S. vulgaris, S. edentatus, S. equinus and Triodontophorus spp. Moreover, A. perfoliata can also be recovered, from 20% to 80% of animals [72,100].
Control of parasitic infections can effectively be performed by administration of macrocyclic lactones during the last stage of pregnancy, immediately prior to foaling. The scheme aims to reduce the risk of transfer of S. westeri to newborn foals through milk [101]. S. westeri patency and foal heat diarrhoea can develop concurrently during the second week of the life of foals, although no causality has thus far been established [64]. Therefore, the practice of routine anthelmintic treatment, in order to prevent Strongyloides-associated diarrhoea, could no longer be sustained in properly managed stud farms. During targeted regimes, administration of anthelmintics should be performed after confirming evidence of the presence of helminth eggs in the faeces of animals.
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 amount of parasitic eggs excreted in their faeces [102]. This distribution of parasite egg shedding among hosts is similar for all parasitic species and is referred to as 'over-dispersion'. Consequently, protocols for the administration of anthelmintic treatments can be individualised among adult animals (i.e., >3 years of age), 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 [103].
As mentioned above, the selective treatment strategies for the parasites of interest can be incorporated into the annual life cycle of a horse, which runs according to the gestation of the animal. In this context, one-or two-yearly treatments suffice for the prevention of infection by large strongyles [104]. With regard to broodmares shedding a low number of eggs, treatment with ivermectin (concurrently with or without praziquantel) in the autumn and in the spring, immediately before foaling or immediately post-partum, is considered a safe, useful and sustainable method [104][105][106].

Control Strategies for Foals
As mentioned above, 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 [64]. Innate differences in the susceptibility of the hosts do not have adequate competency through the immature immune system of foals [60].
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 helminthes recovered from foals [107][108][109] complicates the design and establishment of appropriate anthelmintic schemes for foals. In any case, the first anthelmintic treatment should not be performed later than the age of two months [104]. 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. Finally, some drugs (e.g., moxidectin) are not licenced for use in young foals, which sets further hindrances in formulating anthelmintic programmes.
S. westeri and P. equorum are the helminthes 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 [110]. Challenges are also present in controlling the parasites; for example, P. equorum is a dose-limiting parasite for many anthelmintics, as it has a lower threshold for the development of anthelmintic resistance [60].
Tapeworm infection should be taken into consideration in older foals and at the end of the grazing season. Foals should be treated against A. perfoliata during the first autumn or winter of their life [111].
Gasterophilus spp. and O. equi infections are seasonal and sporadic and usually affect older foals [60]. Macrocyclic lactones are the only currently marketed class of equine anthelmintics with efficacy against O. equi. There are many anecdotal reports of pinworms being resistant to macrocyclic lactones; however, there are only a few documented cases in the literature thus far [95,112]. 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, an 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.

Concluding Remarks
Parasites in horses remain a significant threat to the health and welfare of these animals. The present review discussed the possible direct and indirect effects of parasitism on the reproductive efficiency of horses. Understanding the causality 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.
Author Contributions: Conceptualization, P.T. and E.P.; writing-original draft preparation, P.T. and P.T.B.; writing-review and editing, P.T., G.C.F. and E.P. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.