Parasite Insight
Strongylida:
Small strongyles, also known as cyathostomins, are now recognised as the most prevalent and pathogenic parasites affecting horses worldwide, with over 50 species identified. These parasites are a major concern due to their ability to cause significant health problems in horses, including colic, weight loss, and other digestive issues.
The eggs of both large strongyles and small strongyles (cyathostomins) are indistinguishable from one another under typical faecal examination. All members of the order Strongylida, which includes both strongyles in horses and Trichostrongyles in ruminants, produce similar-looking eggs. As a result, differentiating between species based solely on egg morphology is difficult, if not impossible. In practice, these eggs are typically classified as Strongyle-type eggs, which refers to any egg from the Strongylidae family, but provides no information about the specific species, whether large or small strongyles.
Historically, anthelmintic treatment strategies were developed to control large strongyles like Strongylus vulgaris, S. endentatus, and S. equinus. These treatments were highly effective in reducing infections and minimising their prevalence in well-managed herds. However, the frequent and repeated use of anthelmintics over the course of a horse’s life has become less effective in recent years. The once-popular approach of rotational deworming has inadvertently led to the selection of drug-resistant parasites, particularly small strongyles.
As a result, most of the parasite eggs observed during faecal exams today belong to various cyathostomin species—small strongyles that have become increasingly resistant to commonly used dewormers. This highlights the growing challenge of managing these parasites effectively.
While it is possible to culture faecal samples to identify the larvae and differentiate between small and large strongyles, this method is not routinely used. Both types of strongyles pose a threat to horse health, and high egg counts typically indicate that treatment is needed, regardless of whether the parasites are small or large strongyles.
To effectively manage small strongyles and address the growing problem of anthelmintic resistance, it is essential to move away from indiscriminate deworming practices. Instead, targeted deworming strategies, based on faecal egg counts and diagnostic testing, should be adopted, along with improved pasture management and integrated parasite control approaches.
Ascarids:
Unlike strongyles, Ascarid eggs (particularly Parascaris Univalens, commonly known as the equine roundworm or ascarid) are thick-shelled and highly resistant to freezing and drying, enabling them to survive for extended periods in the environment. The outer shell of the egg is sticky, which aids in the widespread distribution of these eggs. As a result, Ascarid eggs can be found almost everywhere in a horse’s environment, including pastures, stables, and feeding areas. This environmental persistence significantly increases the risk of infection, as horses can ingest the eggs through contaminated food, water, or surfaces.
In addition to small strongyles, Ascarids are a major threat to equine health, especially in foals and younger horses. Adult horses typically have low numbers of Ascarids due to the development of immunity over time. However, it is important to note that anthelmintic resistance in equine ascarids has become a growing concern. The reduced efficacy of dewormers and the development of multiple drug-resistance to Parascaris Univalens have been reported in many countries, including South Africa, making the management of this parasite more challenging.
Tapeworm:
The three types of tapeworms that can infect horses are Anoplocephala perfoliata, Anoplocephala magna, and Paranoplocephala mamillana, with A. perfoliata being by far the most common and problematic.
Tapeworm eggs develop in the lower segments of the worm’s body, which eventually separate and pass out with the faecal material. However, this process is not continuous, making the detection of tapeworm eggs through faecal examination unreliable. Additionally, tapeworm eggs do not float well using traditional faecal flotation methods, further complicating egg detection.
Among the three types of equine tapeworms, Anoplocephala perfoliata is the most common and problematic. It tends to localise at the ileocecal junction—the area where the small intestine, cecum, and colon meet. In high numbers, A. perfoliata can cause obstruction of the ileocecal valve, leading to mild colic. However, it is difficult to gauge the exact number of tapeworms present, as only about 50% of infected horses will shed tapeworm eggs in their faeces at any given time.
Praziquantel is the most common drug in treating tapeworm and although considered safe, it should not be overused. Frequent use of the same dewormer puts pressure on the parasite population, encouraging drug resistance to develop. Resistance occurs when parasites adapt to survive the ongoing exposure to dewormers, which is why it is important to use strategic deworming practices to minimise the risk of resistance.
Strongyloides Westeri: A Unique Parasite of Foals
Important Note: This is not the same as a large or small strongyle!
Strongyloides westeri is a parasitic roundworm that primarily affects foals by residing in the small intestine. Unlike strongyles, adult horses rarely harbour active infections. However, mares often carry dormant larval stages of S. westeri in their tissues, which can become activated during the birthing process. Upon delivery, the active larvae migrate into the mammary tissue, where they are subsequently transmitted to foals through colostrum or milk. This route of transmission is one of the most common ways foals become infected with Strongyloides westeri.
Additionally, foals and adult horses can acquire the infection through skin penetration by infective larvae, which is another mode of transmission, though less common.
Oxyuris equi is a common tapeworm-like parasite found in the large intestine of horses, particularly affecting the rectum and perianal region. Female worms migrate toward the anus to lay their eggs, which are coated with a sticky, irritating substance. This substance causes significant discomfort and leads to excessive scratching as the horse rubs its hindquarters against objects like fence posts, stall walls, and feed bunks. This behaviour often results in broken tail hairs and bald patches around the tail and hindquarters.
While the damage to a horse’s tail can be distressing, Oxyuris equi generally causes minimal impact on overall health and can be easily treated with anthelmintics.
The most effective method of identifying Oxyuris equi is to use transparent adhesive tape to collect eggs from the perianal area for microscopic examination. Occasionally, eggs may also be passed in the faeces and detected using faecal flotation methods.
Flukes:
Gastrodiscus is a genus of trematodes (flukes) that can infect horses. These parasites primarily affect the gastrointestinal tract, particularly the large intestine and cecum, and can lead to digestive disturbances in infected horses.
Symptoms: The clinical signs of Gastrodiscus infection can vary depending on the intensity of the infection and the horse’s overall health, but they may include: Diarrhea, often with a mucous consistency. Weight loss and poor condition, due to damage to the intestinal lining. Colic or abdominal discomfort due to inflammation in the gut. Reduced appetite or difficulty grazing, as the parasite causes digestive upset. Fatigue and lethargy, particularly in cases of chronic infection. In severe cases, especially when there is a large worm burden, the horse may experience more pronounced gastrointestinal distress and poor overall health.
***
Fasciola hepatica is the most commonly recognised liver fluke that can infect mammals including ruminants, as well as on the rare occassion, horses. It is primarily a liver-dwelling parasite and can cause fascioliasis, a disease that can damage the liver, bile ducts, and other parts of the gastrointestinal system.
Symptoms: Liver damage, leading to poor nutrient absorption, weight loss, and lethargy. Bile duct inflammation and liver fibrosis in chronic infections. Anemia and jaundice (yellowing of the skin or eyes) in severe cases. Colic, due to the inflammation of the liver or bile ducts. Diarrhea and digestive issues
***
Paramphistomum, also known as rumen flukes, typically affect the rumen of ruminants, but on rare occasions, they can infect horses if they ingest contaminated water or plants harboring the metacercariae.
Symtoms: In horses, the infection is usually asymptomatic, but in severe cases, it can lead to digestive disturbances and weight loss. In ruminants, rumen flukes can cause significant damage to the rumen lining, but in horses, they are less likely to cause severe disease.
Life Cycle of Fluke Parasites:
Eggs are excreted in the faeces of an infected host (mammals).
The eggs hatch in water and release miracidia, which must find an intermediate host (typically a snail).
Inside the snail, the larvae go through several stages before exiting the snail as cercariae (larval flukes).
The cercariae then encyst on vegetation as metacercariae.
Mammals become infected by ingesting contaminated plants (grazing) or drinking contaminated water containing metacercariae.
Once ingested, the metacercariae penetrate the intestinal wall and travel to their preferred area (liver, intestine, cecum, etc.), where they mature into adult flukes. Adult Liver Fluke and Gastrodiscus can be quite large, where Rumen fluke is smaller and can survive in the mammal for several months to years, depending on the severity of the infection.
