Common Gastrointestinal Parasites of Small Ruminants

Haemonchus contortus, abomasum, sheep

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Courtesy of Dr. Raffaele Roncalli.

Heavy Haemonchus infection, postpartum goat

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Courtesy of Dr. Grace VanHoy.

Differentiating Haemonchus contortus from other trichostrong...

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Courtesy of Dr. Antoinette Marsh.

Haemonchus is prevalent in tropical or subtropical regions and in regions with summer rainfall, and H contortus is the most common pathogenic GI parasite in small ruminants. For descriptions and life cycles, see Common Parasites of the Abomasum in Cattle.

Haemonchosis in sheep may be classified as hyperacute, acute, or chronic. In hyperacute disease, death may occur within 1 week of heavy infection without notable clinical signs. Acute disease is characterized by severe anemia accompanied by generalized edema. Anemia is also characteristic of chronic infection with low worm burdens, and is accompanied by progressive weight loss.

Diarrhea is not a characteristic clinical sign of pure Haemonchus infection; when diarrhea is present, a mixed infection with other worm genera should be suspected. The abomasum is edematous, and in the chronic phase, gastric pH increases, leading to abomasal dysfunction. Periparturient rise occurs commonly with Haemonchus infections, and mature sheep and goats may develop fatal infections in late pregnancy and early lactation.

FAMACHA scoring (see Monitoring) is an important tool in the diagnosis, monitoring, and control of haemonchosis in herds of goat and sheep.

The brown stomach wormsTeladorsagia (formerly Ostertagia) circumcincta and Ostertagia trifurcata are more common in cooler areas with wet winters and temperate zones than are Haemonchus species. The lesions, pathogenesis, and clinical signs of infection by these parasites are similar to those of ostertagiasis in cattle (see Common Parasites of the Abomasum in Cattle). Teladorsagia adults are 14 mm long and brown, and the female’s tail is annulated.

Even subclinical infection depresses appetite, impairs gastric digestion, and decreases nutrient utilization. Teladorsagia is also involved in the periparturient rise in fecal egg counts in sheep and goats, and heavy infections may lead to diarrhea and depress milk production in dams. The output of eggs is the main source of contamination in lambs and kids. The same type of inhibited larval development that occurs in cattle has been observed with both Ostertagia and Haemonchus in small ruminants.

Trichostrongylus axei is also more common in temperate zones than are Haemonchus species. T axei infections in small ruminants show the same lesions, pathogenesis, and clinical signs as infections in cattle (see Common Parasites of the Abomasum in Cattle).

Mecistocirrus digitatus is a hematophagous trichostrongyle with a pathology similar to that of Haemonchus; however, M digitatus adults are much larger, ~40 mm. This parasite is typically present only in tropical climates.

Trichostrongylus colubriformis, Trichostrongylus vitrinus, and Trichostrongylus rugatus are found in the small intestine of small ruminants. The life cycle and appearance of eggs on fecal flotation are typical of a trichostrongyle, and the prepatent period is 18–21 days.

Anorexia, persistent diarrhea, and weight loss are the main clinical signs of Trichostrongylus infection. Villous atrophy (or stunting of villi) impairs digestion and malabsorption; protein loss occurs across the damaged mucosa.

Strongyloides papillosus is not a trichostrongyle, and it has a complex life cycle (seeCommon Parasites of the Small Intestine in Cattle). Heavy infections by adult worms cause a disease resembling trichostrongylosis. The means of infection is usually skin penetration; however, S papillosus can also infect the animal via milk.

Damage to the skin between the claws, produced by skin-penetrating larvae, resembles the early stages of footrot and may aid penetration by the causative agents of foot rot. Most infections are transitory and inconsequential.

Bunostomum trigonocephalum (hookworm) adults are found in the jejunum. The life cycle and clinical findings are essentially the same as for the cattle hookworm, with as few as 100 worms causing clinical signs.

Gaigeria pachyscelis is found in Africa and Asia and resembles Bunostomum in size (2–3 cm) and form. Larvae of G pachyscelis infect the host only by skin penetration. It is a voracious bloodsucker and highly pathogenic.

The species of Nematodirus found in the small intestine of sheep and goats are similar in morphology and life cycle to the Nematodirus spp found in cattle (see Common Parasites of the Small Intestine in Cattle). Clinical infections of Nematodirus are of considerable importance in the UK, New Zealand, and Australia, where lamb mortality may reach 20% in affected flocks if animals are untreated. The parasites are also endemic in some parts of the US, where they occasionally cause disease in young animals. The life cycle can be broken if lambing or kidding takes place in different areas each year. Most clinical infections occur in animals 6–12 weeks old.

Nematodirus battus is an important parasite of lambs in the UK and other parts of Europe and North America. Eggs hatch after a period of chill and then a rise in ambient temperature to a day/night mean of 10°C (50°F), which in temperate areas occurs in late spring. These hatching requirements mean that there is generally one generation of N battus per year; in the UK, however, occasional outbreaks in the fall have been reported.

The parasite can be highly pathogenic because large numbers of larvae hatch over a short period at a time when young lambs are beginning to consume sizable quantities of grass. Disease may be associated with developing larval stages and may occur within 2 weeks of challenge (the prepatent period is 15 days). Other Nematodirus spp often are found in low-rainfall regions (eg, the Karoo in South Africa and inland Australia) where other parasites are rare.

Nematodirosis is characterized by sudden-onset unthriftiness, profuse diarrhea, and marked dehydration, with death as early as 2–3 days after an outbreak begins. Nematodirosis is commonly confined to lambs or weaner sheep, but in low-rainfall country where outbreaks are sporadic, older sheep may have heavy infections.

Nematodirosis lesions usually consist of dehydration and a mild catarrhal enteritis; however, acute inflammation of the entire small intestine may develop. Counts of ≥ 10,000 worms, together with characteristic clinical signs and history, are indicative of clinical infections. Affected lambs may pass large numbers of eggs, which can be identified easily. If the onset of disease precedes maturation of the female worms, eggs will not be identified on fecal examination.

Aonchotheca (formerly Capillaria) longipes is closely related to Trichuris, but its eggs are slightly smaller and lighter brown, and the bipolar plugs are less prominent. A longipes has been detected on routine fecal examination; however, no specific pathogenicity has been reported in small ruminants. For more information regarding egg identification, seeCommon Parasites of the Small Intestine in Cattle.

Moniezia tapeworm passed in goat feces

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Courtesy of Megan Lagatta, RVT.

Moniezia eggs

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Courtesy of Dr. Grace VanHoy and Dr. Andrew Muir.

Moniezia expansa, Moniezia benedeni, and Moniezia caprae can all be found in small ruminants; however, the pathogenicity of Moniezia has long been debated. Many earlier observations, which associated Moniezia infection with diarrhea, emaciation, and weight loss, did not accurately differentiate between tapeworm infections and infection with certain small nematodes (eg, Trichostrongylus colubriformis).

Tapeworms are relatively nonpathogenic, but heavy infections can result in mild unthriftiness and GI disturbances. Diagnosis may be made if individual segments (which are much wider than they are long) are found in the feces, if lengths of adult tapeworms are observed protruding from the anus, or if the characteristic square to rectangular eggs are evident on fecal examination.

The life cycle of Moniezia spp is indirect and involves an oribatid mite that lives on the pasture. The prepatent period is 6–7 weeks. Infections are seasonal, aligned with mite activity, and unusual in animals older than 5–6 months.

Thysanosoma actinioides, adult, sheep

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Courtesy of Dr. Raffaele Roncalli.

Thysanosoma actinioides inhabits the duodenum, the bile ducts, and the pancreatic ducts of small ruminants. It is commonly found in sheep from the southern and western parts of the US and South America. The adult tapeworm can be up to 50 cm long, the head (scolex) is 1.5 mm, and the segments (proglottids) are much wider than they are long and have fringed posterior borders.

Although it has not been associated with disease, T actinioides is of economic importance because livers are condemned when tapeworms are found in the bile duct.