Internal parasitic affections of the horse are numerous and highly varied, heterogeneous in terms of the nature of the parasite species responsible, of very discreet or spectacular clinical expression, benign or extremely serious, sometimes contagious, dangerous to humans, amenable to standard and effective treatment or, on the contrary, incurable. Horses are therefore often victims of various worms and are therefore good customers for vermifuges of all kinds. It’s worth defining and classifying them, before taking a closer look at some of them and outlining a few illustrative rules.
The several types of worms that threaten horses
Internal parasitic affections are pathological (“abnormal”) conditions defined by two criteria: the pathogens that cause them and the parts of the body concerned.
Parasitic affections are caused by pathogens qualified as parasites, i.e., any living species, microscopic or visible to the naked eye, unicellular or not, living or multiplying at the expense of a host species, in this case the horse. These may be worms (helminths causing helminthosis), insects or mites (arthropods responsible for entomosis and acariosis respectively), protozoa (unicellular species with an animal character causing protozoosis) or fungi (responsible for mycosis), the species and corresponding diseases studied by parasitologists. Bacteria and viruses, species also capable of living at the expense of a host species, and the diseases they cause, are traditionally studied by microbiologists and infectiologists.
These diseases concern the viscera and deep tissues of the organism (as opposed to superficial skin and mucous membrane diseases), and include digestive, respiratory, blood, vascular and nervous disorders.
Treating your horse with deworming and other worm remedies
Parasitology is just one part of medicine, and is based on the same principles and methods of analysis, which are based on consulting the animal, i.e.:
- gathering information about the animal itself – origin, age, sex, breed, lifestyle, diet, exercise, vaccinations, deworming, previous pathological conditions, current treatments, etc.;
- collecting information specific to the condition – age of disorders, treatments undertaken and their results, other associated symptoms observed, changes in behavior and appetite;
- general clinical examination of the animal, apparatus by apparatus, according to recognized medical rules and procedures: inspection, palpation-pressure, auscultation. These hypotheses need to be confirmed (or invalidated) by further examinations (often specific to parasitology), enabling a diagnosis to be made with certainty, and specific treatment (“antiparasitic”) and prophylaxis to be envisaged.
Example of entomosis in horses: gastropilosis
Gastrophilosis is a digestive disease caused by the development and migration of Gasterophilus insect larvae into the oral cavity, stomach, and intestine. They belong to the myiasis group (diseases caused by dipteran insect larvae). These parasitosis are very common in our regions: around 2/3 of horses are (or have been) parasitized by these arthropods, whatever their age, sex, or breed. However, only grazing animals are likely to be infested (see parasite study). In addition, myiasis can lead to gastric emptying disorders and abdominal pain (colic). They illustrate the strong pre-valence of certain parasites, their possible clinical repercussions, and the need for knowledge of the morphology and biology of the parasite species involved, to establish effective therapy and prophylaxis.
Adult gastropods are flies, free in the external environment, without mouthparts (they don’t feed) and laying their eggs in summer on horsehair, in various parts of the body depending on the species.
Horses become infected by ingesting the larvae that hatch on the surface of the skin. They migrate into the oral cavity, pharynx, and esophagus, attaching themselves with their hooks to the gastric mucosa, inflicting ulcerative lesions, and then to the intestine. In the spring of the following year, stage 3 larvae are expelled into the dung, transforming into pupae from which adults emerge. The parasitic stages are therefore only larvae that can be observed in the animal during the off-season.
How can I tell if my horse is infected with a worm?
Parasitized horses show:
- whitish elements, 1 mm long, attached to the hair (these eggs are clearly visible in summer in dark-coated animals, on cannons for example).
- a wide variety of digestive disorders: mostly discreet (a few abdominal pains or colic in winter, due to failure to empty the stomach) and disappearing spontaneously in spring; sometimes more violent colic, due to gastric ulcers and their complications (hemorrhages, abscesses).
- stage 3 larvae (around 2 cm long) with several rows of spines, truncated at one end, expelled in dung in spring.
Diagnosis is based entirely on observation of the parasite. Treatment involves the use of insecticides or endectocides administered by mouth and prescribed by a veterinarian. Prophylaxis would involve keeping animals away from adult flies, which is not practically feasible.
Example of helminthiasis in horses: trichonemosis
Trichomoniasis is a helminthiasis caused by the pathogenic action of roundworm larvae called trichonemes or cyathostomes. This helminthiasis is very frequent and widespread in temperate zones, sometimes clinically severe and difficult to treat.
Trichonemes belong to a group of roundworms known as strangles. Because of their small size (around 1cm long), trichonemes are commonly referred to as “small strangles”, as opposed to other parasites of completely different morphology and biology called “large strangles” (genus Strongylus).
Trichonemosis is a very special helminthiasis for several reasons:
- practically all horses are infested as soon as they can go out to pasture and ingest infesting stage 3 larvae. This means that coproscopic examinations (i.e., laboratory techniques for observing parasitic elements, such as eggs, in dung) are always positive;
- on the other hand, some horses will show trichonemosis disease, i.e., clinical symptoms due to the pathogenic power of trichonemes: highly infested animals, young horses meeting the parasite;
- sick animals are parasitized by the pathogenic larval stages of the parasite present in the intestinal mucosa. Coproscopic examinations, which detect elements disseminated in the droppings, are therefore of no interest;
- the disease is the consequence of massive destruction of the intestinal mucosa, itself caused by the exit of intra-mucosal larvae into the intestinal lumen: “diarrheal debacle”, very liquid fecal tinged red (hemorrhagic appearance) due to the presence of these expelled hematophagous larvae, significant water loss, dehydration, oedema of the sloping regions (ars, limbs), prostration.
Trichonemosis illustrates an important biological phenomenon: hypobiosis, in which the larvae stop developing in an intramucosal position throughout the winter, i.e., for several months. The cycle resumes during the summer months, so that the disease typically manifests itself at the end of winter. It’s as if the parasite “spent the bad season” in the horse, and only resumed its evolutionary cycle when external conditions were favorable for larval metamorphosis (from L [ to L3) in the pasture.
Treatment involves the use of larvicidal anthelmintics (according to specific protocols) combined with vigorous symptomatic therapy. Protecting the horse is extremely difficult, as Trichonema larvae persist in very wet pastures of good nutritional quality.
Today’s difficulties lie in the dilemma faced by horse owners and veterinarians: whether to institute frequently repeated antiparasitic treatments to rid the animal of its parasites, at the risk of developing chemoresistance (not to mention the cost of such programs), or not to define effective prophylaxis, thereby exposing oneself to fatal clinical cases, or to dispose of unusable animals. It’s the medical treatment of observed cases and the development of an integrated, well-managed prophylaxis plan for a given number of animals that make it possible to manage internal equine parasitic affections at the lowest possible cost.
