Many species of nematodes and cestodes cause parasitic gastritis and enteritis in sheep and goats. The most important of these are Haemonchus contortus, Teladorsagia (Ostertagia) circumcincta, Trichostrongylus axei, intestinal species of Trichostrongylus, Nematodirus spp, Bunostomum trigonocephalum, and Oesophagostomum columbianum. Cooperia curticei, Strongyloides papillosus, Trichuris ovis, and Chabertia ovina also may be pathogenic in sheep; these and related species are discussed under GI parasites of cattle (see Gastrointestinal Parasites of Cattle).
The principal stomach worms of sheep and goats are Haemonchus contortus, Teladorsagia (Ostertagia) circumcincta, Ostertagia trifurcata, Trichostrongylus axei (see Gastrointestinal Parasites of Cattle), and in some tropical regions, Mecistocirrus digitatus. Cross-transmission of Haemonchus between sheep and cattle can occur but not as readily as transmission between homologous species. Sheep are more susceptible to the cattle species than cattle are to the sheep species. For descriptions and life cycles, see Haemonchus, Ostertagia, and Trichostrongylus spp.
Haemonchus is most common in tropical or subtropical areas or in those areas with summer rainfall, whereas Ostertagia and T axei are more common in cooler winter rainfall areas. The latter species predominate in temperate zones.
Haemonchosis in sheep may be classified as hyperacute, acute, or chronic. In the hyperacute disease, death may occur within 1 wk of heavy infection without significant signs. The acute disease is characterized by severe anemia accompanied by generalized edema; anemia is also characteristic of the chronic infection, often of low worm burdens, and is accompanied by progressive weight loss. Diarrhea is not a sign of pure Haemonchus infection; the lesions are those associated with anemia. In cases in which diarrhea is present, there may be mixed infection with other worm genera. The abomasum is edematous and, in the chronic phase, gastric pH increases, which causes abomasal dysfunction. Mature sheep may develop heavy, even fatal, infections, particularly during lactation.
The lesions, pathogenesis, and signs of Ostertagia and T axei infections are similar to those found in cattle. Even subclinical infection depresses appetite, impairs gastric digestion, and reduces use of metabolizable energy and protein. Ostertagia is the principal genus involved in the periparturient rise in fecal egg counts in sheep, and heavy infections may cause diarrhea and depress milk production in ewes. This output of eggs serves as the main source of contamination for the lambs. The same type of inhibited development seen in cattle has been seen with both Ostertagia and Haemonchus in sheep.
The life cycle of intestinal Trichostrongylus (T colubriformis, T vitrinus, T rugatus) is direct. The developing larvae burrow superficially in the crypts of the mucosa and develop to egg-laying adults in 18–21 days.
Anorexia, persistent diarrhea, and weight loss are the main signs. Villous atrophy (or stunting of villi) results in impaired digestion and malabsorption; protein loss occurs across the damaged mucosa. There are no diagnostic lesions; a total worm count should be done to evaluate the condition.
Adult Bunostomum trigonocephalum (hookworm) are found in the jejunum. The life cycle and clinical findings are essentially the same as for the cattle hookworm (see Bunostomum sp), with as few as 100 worms causing clinical signs. Gaigeria pachyscelis is found in Africa and Asia and resembles Bunostomum in size and form (2–3 cm). Larvae of G pachyscelis infect the host only by skin penetration. G pachyscelis is a voracious bloodsucker and probably the most pathogenic hookworm.
The species of Nematodirus found in the small intestine of sheep are similar in morphology and life cycle to N helvetianus (see Nematodirus spp). Clinical infections 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 Rocky Mountain states of the USA, where they occasionally cause clinical disease in lambs.
In areas where clinical infections are common, the disease has a characteristic seasonal pattern. Many of the eggs passed by affected lambs lie dormant through the remainder of the grazing season and the winter, with large numbers of larvae appearing during the early grazing period of the following year. Thus, the lambs of one season contaminate the pastures for the next season’s lambs; fortunately, the life cycle can be broken if the same area is not used for lambing each year. Most clinical infections are seen in lambs 6–12 wk old.
N battus is seen in the UK and other parts of Europe and also in 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). This occurs in late spring in temperate areas. The hatching requirements mean that there is generally one generation of N battus per year, although in the UK, occasional outbreaks in the autumn 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 take in significant quantities of grass. Disease may be associated with developing larval stages and may be seen within 2 wk of challenge, ie during the prepatent period (15 days). Other Nematodirus spp often are found in low-rainfall regions (eg, the Karroo in South Africa and inland Australia) where other parasites are rarely seen.
Nematodirosis is characterized by sudden onset, “loss of bloom,” 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. The lesions usually consist of dehydration and a mild catarrhal enteritis, but acute inflammation of the entire small intestine may develop. Counts of ≥10,000 worms, together with characteristic signs and history, are indicative of clinical infections. Affected lambs may pass large numbers of eggs, which can be identified easily; however, because the onset of disease may precede the maturation of the female worms, this is not a constant finding.
The nodular worm of sheep, Oesophagostomum columbianum, has a similar morphology and life cycle to those of the nodular worm of cattle (see Oesophagostomum sp).
Diarrhea usually develops during the second week of infection. The feces may contain excess mucus as well as streaks of blood. As the diarrhea progresses, sheep become emaciated and weak. These signs often subside near the end of the prepatent period, but the continuing presence of numerous adult worms may result in a chronic infection in which signs may not develop for several months. The sheep become weak, lose weight despite a good appetite, and show intermittent diarrhea and constipation.
As immunity develops, nodules form around the larvae; they may become caseated and calcified. Nodule formation usually is more pronounced in sheep than in cattle. Affected sheep walk with a stilted gait and often have a humped back. Stenosis and intussusception may develop in severe cases. Diagnosis is difficult during the prepatent period, at which time it must be based largely on clinical signs.
Adult worms cause severe damage to the mucosa of the colon, with resulting congestion, ulceration, and small hemorrhages. Infected sheep are unthrifty; the feces are soft, contain much mucus, and may be streaked with blood. Immunity develops quickly, and outbreaks are seen only under conditions of severe stress.
Heavy infections with adult worms cause a disease resembling trichostrongylosis. Infection is usually by skin penetration but can also occur via the milk. Damage to the skin between the claws, produced by skin-penetrating larvae, resembles the early stages of footrot and may aid penetration of the causal agents of footrot. Most infections are transitory and inconsequential.
Heavy infections with whipworms are not common but may be seen in very young lambs or during drought conditions when sheep are fed grain on the ground. The eggs are very resistant. Congestion and edema of the cecal mucosa, accompanied by diarrhea and unthriftiness, are seen.
The pathogenicity of Moniezia expansa in sheep has long been debated. Many earlier observations, which associated this 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 by finding individual segments (which are much wider than long) in the feces or lengths of adult tapeworm protruding from the anus or by demonstrating the characteristic eggs on fecal examination. The life cycle involves an oribatid mite that lives in the pasture mat. The prepatent period is 6–7 wk. Infections are seasonal, in accordance with mite activity, and unusual in animals older than ~4–5 mo of age.
Thysanosoma actinioides, the “fringed tapeworm,” inhabits the small intestine, the bile ducts, and the pancreatic ducts. It is commonly found in sheep from the southern and western parts of the USA and also South America. Although it has not been associated with clinical disease, it is of economic importance because livers are condemned when tapeworms are found in the bile duct.