Five genera of nematodes occur 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 pathologically important. Hyostrongylus rubidus (the red stomach worm) is approximately 6 mm long, red, quite slender, and has a direct life cycle, features similar to those of Ostertagia ostertagi of cattle. The prepatent period is approximately 3 weeks unless larval inhibition occurs. This may be induced by seasonal changes or repeated infections, which is 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.
Ascarops 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 weeks. 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.
Clinical Findings of Stomach Worms in Pigs
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; however, whether this is a direct result of the nematode infection is unclear.
Light infections are usually asymptomatic. However, when worms are present in large numbers or when the host’s condition is worsened by poor nutrition or other factors, these worms may cause inappetance, anemia, or weight loss, and may contribute to the "thin sow syndrome." Resumed development of inhibited larvae may cause severe gastritis and, in addition, contaminate the environment of young pigs. Egg excretion per female Hyostrongylus worm is generally much lower than that of other nematode genera. Clinical signs associated with heavy infections of A strongylina or P sexalatus are similar to those of H rubidus.
Diagnosis of Stomach Worms in Pigs
Fecal examination (patent infections)
Postmortem examination (prepatent infections)
Clinical signs of stomach worms in pigs, 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.
Careful examination of the stomach at postmortem examination is necessary to identify H rubidus; although the worms are distinctively red in color, they are quite small.
At postmortem examination, adult worms, especially Physocephalus and Ascarops, are readily seen. H rubidus, characteristically, is found under a heavy catarrhal or mucous exudate in the stomach; therefore, mucosal scrapings for microscopic examination are essential for detection of immature Hyostrongylus.
Treatment and Control of Stomach Worms in Pigs
Anthelmintic use, as part of a parasite control program
Effective anthelmintics are available, the choice of which depends on what other parasites are present as well as potential withdrawal times. The same principles used for control of parasitic gastroenteritis of ruminants apply to the control of hyostrongylosis of pigs and should not depend solely on anthelmintic use. As an example, in temperate climates, an annual rotation of pastures with other production animals or crops decreases pasture contamination. Care must be taken with rotating production animals if pigs harbor ascarid infections. Integration of anthelmintics depends on their availability and the season, as well as on other farming activities. The benzimidazoles, probenzimidazoles, doramectin, 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 productive. Treatment for most has not been reported, although ivermectin has activity against adult Ascarops.
Stomach worms tend to occur in pigs raised outdoors; Hyostrongylus rubidus is the most common and most pathogenic of the stomach worms of swine.
Inappetance and weight loss associated with stomach worms can be exacerbated by poor nutrition.
Eggs can be found on fecal flotation; however, larval cultures may be necessary to differentiate H rubidus from Oesophagostomum.
Stomach worms are visible grossly at postmortem examination, although H rubidus may be overlooked because of its small size.