Stomach Worms in Pigs
Five genera of nematodes are found within the stomach of pigs. The trichostrongylid Hyostrongylus is relatively common, whereas the other four spirurids (Ascarops, Gnathostoma, Physocephalus, Simondsia) are less common or geographically limited. Although some (eg, Ascarops strongylina, Physocephalus sexalatus) are more obvious grossly, only Hyostrongylus rubidus is considered to be pathologically significant. H rubidus (the red stomach worm) is ~6 mm long, quite slender, and has a direct life cycle, features similar to those of Ostertagia ostertagi of cattle. The prepatent period is ~3 wk unless larval inhibition occurs; this may be induced by seasonal changes or repeated infections, explaining why hypobiotic larvae are usually found in older animals. As with Haemonchus contortus of sheep, relaxation of immunity associated with parturition allows inhibited larvae to resume development, leading to a periparturient rise in fecal egg counts. A strongylina and P sexalatus, the thick stomach worms, are 10–20 mm long, are much stouter than H rubidus, and have coprophagous beetles as intermediate hosts. The prepatent periods for the spirurids are in the range of 4–6 wk. Because of the free-living larval requirements (H rubidus) or the need for an intermediate host (all others), infections are confined to animals with pasture access or those kept in straw yards.
The pathogenesis of hyostrongylosis is similar to that of ostertagiosis of cattle, including the replacement of parietal cells by rapidly dividing undifferentiated cells, giving rise to nodules on the mucosal surface. Gastric pH increases as does mucus production, resulting in a catarrhal gastritis. Occasionally, gastric ulcerations of the glandular stomach occur but whether this is a direct result of the nematode infection is unclear. Light infections are usually asymptomatic. However, when present in large numbers or when the host’s condition is reduced by poor nutrition or other factors, these worms may cause variable appetite, anemia, diarrhea, or weight loss, and may contribute to a thin sow syndrome. H rubidus characteristically is found under a heavy catarrhal or mucous exudate. Resumed development of inhibited larvae may cause severe gastritis and, in addition, contaminate the environment of the young pigs. Egg excretion per female Hyostrongylus worm is generally much lower than that of other nematode genera.
Clinical signs other than unthriftiness are not obvious. Fecal examinations may show the distinctive ova of Physocephalus and Ascarops—small (35–40 × 17–20 mm), thick-shelled eggs containing active larvae. Hyostrongylus ova resemble those of other strongyle worms (eg, Oesophagostomum), and fecal cultures are required to obtain infective larvae for differential diagnosis.
At necropsy, adult worms, especially Physocephalus and Ascarops, are readily seen. Mucosal scrapings for microscopic examination are essential for detection of immature Hyostrongylus.
The same principles used for control of parasitic gastroenteritis of ruminants apply to the control of hyostrongylosis and should not depend solely on anthelmintic use. As an example, in temperate climates, an annual rotation of pastures with other livestock or crops would reduce pasture contamination. Care must be taken with rotating livestock if the pigs also harbor ascarid infections. Integration of anthelmintics depends on their availability and the season, as well as on other farming activities. The newer benzimidazoles, probenzimidazoles, and ivermectin are highly effective against adult and immature stages (including hypobiotic larvae) of Hyostrongylus. Implementing measures to control the intermediate hosts of the spirurid nematodes is usually not required or unproductive. Treatment for most has not been reported, although ivermectin has activity against adult Ascarops.