Hepatic Dicrocoelium dendriticum infection in a miniature horse

Abstract

A miniature horse which died following humane seizure from an Ontario farm was emaciated with serous atrophy of fat. Autopsy revealed hepatic atrophy and moderate periportal fibrosis. Eggs and trematodes seen in the liver were identified as Dicrocoelium dendriticum. This appears to be the first reported case of infection of equids in North America with Dicrocoelium dendriticum.

Dicrocoelium dendriticum, also known as the small liver fluke or lancet liver fluke, is a small liver fluke which typically infects ruminants. The fluke is a parasite of the gall bladder and bile ducts and is able to reproduce by both cross insemination and hermaphroditism. It has a complex life cycle involving 2 intermediate hosts — terrestrial snails and ants. Infecting terrestrial snails first, the eggs hatch into sporocysts, develop, and then become cercariae. The cercariae are shed from the snail in mucoid “slime balls” which are ingested by ants. This first stage takes 3 to 4 mo. Over the next 1 to 2 mo in ants they develop into metacercariae and sometimes localize in the ant’s nervous system. Temperatures less than 15°C cause the ant to cramp, keeping them on the tips of grass and predisposed to being eaten by grazing animals. In ruminants, the metacercariae excyst in the small intestine, then migrate into the bile ducts and gall bladder. Eggs are released in feces about 2 mo after infection, with its full life cycle taking about 6 mo (1).

The known geographic distribution of D. dendriticum is primarily Europe and Asia, with sporadic occurrence in North America (2). It is not regarded as native to North America, but has an invasion history, first anecdotally noted in Eastern Canada in the 1930s, where it was likely introduced from Europe, explaining why it is still relatively rare in Canada (3). In Ontario, infections are occasionally diagnosed in pastured sheep and cattle in southern parts of the province, with liver lesions observed at slaughter (4).

Dicrocoelium dendriticum can occasionally infect other animals, including pigs, dogs, rabbits, horses, and humans (1). Scientific publications regarding infection in horses and donkeys are primarily limited to brief mention in textbooks and review articles. However, there are references to infection of equids in Denmark (5), Nigeria (6), Switzerland (7), and Turkey (8,9). The authors have been unable to find any case reports in equids in North America.

This report describes the occurrence of D. dendriticum in a young miniature horse.

Case description

In the winter of 2017, the Ontario Humane Society investigated a small farm in southern Ontario following reports of neglect of care of resident animals. The animals on the farm included a miniature horse, 17 chickens, 3 goats, and 6 rabbits. The horse and 1 of the goats were subsequently submitted to the Animal Health Laboratory (AHL) at the University of Guelph for necropsy examination to document the condition of the animals and cause of death. The submitted goat was found dead; the submitted horse died several hours after seizure. The horse had been on the farm for 48 h, but its origin before this could not be determined. The horse and goat were frozen at the time of submission.

Both animals were emaciated and had serous atrophy of fat. The horse was a neutered male with 4 fully erupted and worn upper incisors, and no eruption of the lateral 2. Based on this dentition the horse was estimated to be about 4 y old. It had a body weight of 36 kg. The liver appeared small and weighed 500 g (1.4% of body weight).

Using the Cornell-Wisconsin Centrifugal Flotation technique on feces from the horse, low numbers of strongyle and Oxyuris equi eggs were observed. Extrapolating from a study using bovine feces and Haemonchus contortus, this would translate to between 1 and 30 eggs per gram (epg) of feces for each. Analytic sensitivity was reported to be 1 egg per 5 g of feces (10). Sensitivity using strongyle and O. equi eggs, or for trematodes has not been determined.

Routine histology with preparation of hematoxylin and eosin (H&E)-stained slides was performed on major organs. Microscopically, tissues from the horse displayed marked autolysis and freeze artifact. There was skeletal muscle atrophy and mild interstitial mononuclear nephritis, the latter being regarded as incidental. In the initial liver section a cross section of a small trematode containing eggs was seen (Figure 1A). The liver was re-examined grossly, and additional sections were collected for histology from different parts of the liver. In 1 of the 10 histology sections, an additional single trematode containing eggs was seen (Figure 1B) as well as a bile duct containing trematode eggs. In all 11 liver sections there was moderate periportal fibrosis (Figure 1C). Additionally, careful gross microdissection down the bile ducts recovered a single small trematode that was identified based on its morphology and the egg characteristics as D. dendriticum (Figures 1D and 1E). The dissected trematode was autolyzed and ~5 mm in length [normal in sheep: 4.70 to 5.60 mm in length (2)]. Eggs (n = 10) measured ~35.6 μm (range: 30.2 to 43.6 μm) × 21.8 μm (range: 18.9 to 23.7 μm) with a brown shell and were operculate (Figure 1D), consistent with D. dendriticum (2).

Figure 1.

A — Dicrocoelium dendriticum cross section in remnants of a bile duct. Eggs can be seen in the uterus of the trematode (arrows). Hematoxylin and eosin (H&E) stain. B — more longitudinal orientation of a partial D. dendriticum in a bile duct. The oral sucker is visible (arrow). H&E stain. C — periportal fibrosis (arrows) including necrotic cellular debris in a larger duct (large arrow). H&E stain. D — eggs of D. dendriticum released from the dissected frozen, unfixed, liver tissue. Wet mount. E — autolyzed body of recovered intact D. dendriticum. Wet mount.

A computer search of Province of Ontario and University of Guelph animal pathology records from May 1988 to December of 2017 found no other cases of trematodes in horses.

Discussion

The most common liver fluke reported in horses globally is Fasciola hepatica. Although infection is considered rare in horses, it is somewhat more common in donkeys (11). Equine D. dendriticum infection is likely very rare in Ontario, this being the first case seen at the AHL. Even in endemic areas, because horses are not a primary host and are not usually pastured with ruminants, infection would be expected to be rare. The infection is occasionally mentioned in textbooks and review articles, in which a primary reference is not cited. Documented cases of equine infection seem confined to Europe, some areas of Asia, and Africa (5–9). Unlike F. hepatica, D. dendriticum has 2 intermediate hosts (terrestrial snails and ants) and does not require a wet environment for its life cycle (12).

In sheep, the prepatent period for D. dendriticum is 10 to 12 wk. However, in horses the prepatent period is unknown. Even so, because the horse had only been on the farm in question for 48 h, it is not possible that the horse had acquired the infection on the farm on which it was found; exposure must have occurred on a previous farm. It should also be noted that the infection burden was very small relative to cases described in sheep (3), and although there was significant periportal fibrosis which may have been a response to the parasite, some of the periportal fibrous tissue may have been due to condensation of stroma after severe parenchymal atrophy. The D. dendriticum infection in this horse may have been contributory but was perhaps incidental to the emaciation and death, which was assumed to be due to malnutrition, as no other contributing causes were found on autopsy examination.

This report represents the first time D. dendriticum infection has been documented in a horse in North America. CVJ