Parasitic Diseases of the Foal and Appropriate Treatment

Small intestinal ascarid impaction

Verminous small intestinal impactions are most commonly described in foals around 5 months of age but have been reported in horses as old as 5 years. The condition can occur spontaneously but is most often triggered by an effective anthelmintic treatment. The dead or paralysed ascarid worms are then carried down the small intestine and eventually clog the ileum. Worms are easily visualised on transabdominal ultrasonography, which can help diagnose the condition. Foals typically present with a painful colic that usually requires hospitalisation and intensive care. Complications include intestinal rupture, volvulus, and intussusception. Surgery can be attempted, but the prognosis for full recovery is guarded. A review of published cases found that only 11 of 37 cases that underwent surgery survived beyond 12 months after the incident.

Anthelmintic drug classes associated with this condition generally have a paralytic mode of action, whereas the benzimidazole drug class was only used prior to a single published incident. Attempts to relieve an ascarid impaction medically can be made if pain manifestations are manageable and foals do not have gastric reflux. Medical procedures often include a single dose of a benzimidazole combined with mineral oil passed via nasogastric tube. Such foals should be closely monitored and evaluated for gastric reflux and pain manifestations for the next 24–48 hours.

Tapeworm-associated colic

Anoplocephala perfoliata has been statistically associated with ileal colic in several independent case–control studies, and cases have been encountered in short yearlings. These cases include ileal impactions and ileocaecal intussusceptions and they have been reported from a variety of different geographical locations.

Despite often-made claims, only one study has reported an association between A. perfoliata and spasmodic colic. This may be in part because spasmodic colic is not a tightly defined clinical condition, and that many conditions can lead to the type of clinical signs often regarded as spasmodic. Similarly, there is no evidence supporting the often-held notion that tapeworms are a cause of intermittent colic in horses.

It should be kept in mind that A. perfoliata is commonly occurring and colic is a rare incident, so it is unclear to what extent tapeworm infection acts as a risk factor for colic. The clinical presentation of tapeworm-associated colic varies, but ileal impactions may require surgical intervention, and intussusceptions definitely will. Coprological diagnosis of A. perfoliata might demonstrate presence of mature parasites but does not determine any causal relationship. Similarly, an antibody ELISA merely reflects exposure to the parasite and not necessarily actual infection. A massive tapeworm burden may be visualised on ultrasound examination, but the caecum often contains pockets of air which will disturb the procedure.

Larval cyathostominosis

This syndrome is perhaps the best described parasitic disease complex in the scientific literature over the past 35 years; however, it is generally not encountered in foals. Work done in our laboratory has suggested that encysted cyathostomin larvae do not undergo arrested development in foals, which means that an accumulation of these stages does not occur at this age.

The disease is characterised by an acute profuse watery diarrhoea resulting in pronounced dehydration and hypoproteinaemia. Thickened large intestinal walls can often be visualised with ultrasonography and bloodwork typically reveals pronounced neutrophilia, hypergammaglobulinaemia and hypoalbuminaemia. This disease is most often seen in horses aged 1 to 5 years and during winter or early spring. Anthelmintic treatment with an effective non-larvicidal anthelmintic has been identified as a significant risk factor for the disease.

Treatment of acute cases consists of fluid therapy and anti-inflammatory treatment. Moxidectin is considered the anthelmintic of choice for this condition because of the larvicidal efficacy and the very limited inflammatory reaction observed post treatment. Given the widespread occurrence of benzimidazole resistance in cyathostomin parasites worldwide, the 5-day double dose of fenbendazole is unlikely to be effective.

Veterinarians and horse-owners often express concern over treating with an effective larvicidal anthelmintic in cases where large encysted burdens are suspected. Over a series of studies, we have investigated systemic and local immunological and inflammatory reactions to ivermectin, moxidectin, and the 5-day fenbendazole regimen in horses harbouring large cyathostomin burdens. Interestingly, we consistently saw very subtle reactions to deworming, if any at all, and the highest proinflammatory response was in the untreated control groups. Thus, in otherwise healthy horses, the risk of adverse reaction associated with the die-off of large larval burdens following anthelmintic treatment is minimal.

Non-strangulating intestinal infarctions

Non-strangulating intestinal infarctions are significantly associated with Strongylus vulgaris infection and are relatively common in areas where the parasite is endemic, such as Scandinavia, but the condition is quite rare in the United Kingdom. Disease can occur at all ages, including foals.

The condition was previously referred to as thromboembolic colic, but this term is misleading as only a subset of affected horses present with colic signs. Horses typically present with signs of peritonitis. Heart rates are often in the normal range or only slightly elevated, borborygmi are typically decreased or absent, and temperature can be normal or elevated. It is very important to perform abdominocentesis as early as possible to identify the peritonitis, whereas parasite diagnostics such as faecal egg counts and coprocultures will not be useful.

Surgery can be attempted with resection of the infarcted area, and horses have successfully survived to discharge and returned to full athletic function. Attempts to treat the condition medically, on the other hand, are usually unsuccessful. The challenge is that the presence and extent of given infarction cannot be determined without performing an exploratory laparotomy. Since a quick intervention is required for the best possible prognosis, an exploratory laparotomy should be recommended in cases of peritonitis from areas known to be endemic for S. vulgaris.

[...]