Several species of nematodes occur in the stomach of pigs; all except Hyostrongylus rubidus have indirect life cycles and are usually clinically unimportant except in large numbers. Clinical signs are usually absent in light infections of H rubidus; however, heavy infections can lead to decreased appetite, weight loss, and anemia. Infections are primarily confined to animals reared outdoors.
Five genera of nematodes occur within the stomach of pigs. The trichostrongylid Hyostrongylusis relatively common, whereas four spirurids (Ascarops, Gnathostoma, Physocephalus, and Simondsia) are less common or geographically limited. Some (eg, Ascarops strongylina and Physocephalus sexalatus) are more obvious grossly, with distinct spiral cuticular ridges on the anterior end (see and images); however, only Hyostrongylus rubidus is considered pathologically important, with infections leading to anemia, inappetance, and the development of ulcers (1, 2).
Anterior end of Physocephalus sp. Note spiral cuticular ridges.
Courtesy of Dr. Ellis Greiner.
Anterior end of Ascarops sp. Note spiral cuticular ridges.
Courtesy of Dr. Ellis Greiner.
Hyostrongylus rubidus (the red stomach worm) is approximately 6 mm long, red, quite slender, and has a direct life cycle. Infective larvae invade the gastric glands during development. The prepatent period is approximately 3 weeks, unless larval inhibition induced by seasonal changes or repeated infections occurs, and it has been found that higher temperatures and moisture increase H rubidustransmission (3).
Ascarops strongylina, Simondsia, and P sexalatus, the thick stomach worms, are 15–20 mm (0.6–0.8 inches) long, are much stouter than H rubidus, and have coprophagous beetles as intermediate hosts. Adults are found along the stomach mucosa, and the eggs found in feces are larvated, with a smooth, thick shell (30–40 × 15–20 mcm) (4). Adults of Simondsia are found in the mucosal glands of the stomach. Gnathostoma adults are slightly larger, up to 40 mm (1.5 inches) long, with spinose eggs that have a single polar plug and a brown, rough shell (70 × 40 mcm) (4).
The prepatent periods for Ascarops, Simondsia, and Physocephalus are approximately 4–6 weeks. The prepatent period for Gnathostoma is approximately 3 months and requires a second vertebrate intermediate host (4). 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 involves development of the larvae in gastric glands, which gives rise to nodules on the mucosal surface. Nodules can enlarge, or coalesce, and are often found in the gastric fundus (5).
Gastric pH increases with stomach worm infection, 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 (4).
Light infections of stomach worms are usually subclinical. However, when worms are present in large numbers or when the host’s condition is worsened by poor nutrition or other factors, these worms can cause inappetance, anemia, or weight loss, and they can contribute to the "thin-sow syndrome" (6). Resumed development of inhibited larvae can cause severe gastritis and can 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 (6).
Diagnosis of Stomach Worms in Pigs
Fecal examination (patent infections)
Direct visualization of worms on postmortem examination (prepatent infections)
Fecal culture (species identification)
Clinical signs of stomach worms in pigs, other than unthriftiness, are not obvious. Fecal examinations can show the distinctive ova of Physocephalus, Simondsia, and Ascarops—small (35–40 × 15–20 mm), smooth, thick-shelled eggs containing live larvae. Hyostrongylus ova resemble those of other strongylid worms (eg, Oesophagostomum or Globocephalus) (see ). Therefore, fecal cultures are required to obtain infective larvae for differential diagnosis.
Strongyle-type eggs recovered by fecal sedimentation. Eggs of Hyostrongylus, Oesophagostomum, and Globocephalus are morphologically similar. 400X total magnification.
Courtesy of Dr. Trey Tomlinson.
Careful examination of the stomach at postmortem examination is necessary to identify H rubidus. The worms are distinctively red in color; however, they are quite small.
At postmortem examination, adult spirurids are readily evident. 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 to treat stomach worms, 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; however, control should not depend solely on anthelmintic use. As an example, in temperate climates, 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 other farming activities. The avermectins are highly effective against adult and immature stages (including hypobiotic larvae) of Hyostrongylus (4). Additional anthelmintics include benzimidazoles, levamisole, dichlorvos, and pyrantel (4). Implementing measures to control the intermediate hosts of the spirurid nematodes is usually not required or productive.
Key Points
Stomach worms tend to occur in pigs raised outdoors; Hyostrongylus rubidus is the most common and most pathogenic of the stomach worms of swine.
Inappetence and weight loss associated with stomach worms can be exacerbated by poor nutrition.
Eggs can be found on fecal flotation; however, larval cultures might be necessary to differentiate H rubidus from Oesophagostomum.
Stomach worms are visible grossly at postmortem examination; however, H rubidus can be overlooked because of its small size.
References
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Bowman DD. Helminths. In Bowman, DD. Georgis' Parasitology for Veterinarians. 11th ed. WB Saunders; 2021:135-260. doi:10.1016/b978-0-323-54396-5.00013-1
Roepstorff A, Murrell KD. Transmission dynamics of helminth parasites of pigs on continuous pasture: Oesophagostomum dentatum and Hyostrongylus rubidus. Int J Parasitol. 1997;27(5):553-562. doi:10.1016/s0020-7519(97)00023-4
Brewer MT, Martin K, Greve JH. Internal parasites: helminths. In: Zimmerman JJ, Burrough ER, Karriker LA, Schwartz KJ, Zhang J, eds. Diseases of Swine. 12th ed. John Wiley & Sons; 2025:1179-1192. doi:10.1002/9781394179466.ch61
Stockdale PHG. The pathogenesis of Hyostrongylus rubidus in growing pigs. Br Vet.J. 1974;130(4):366-373. doi:10.1016/S0007-1935(17)35840-2
Roepstorff A, Nansen P, Hansen JW. Epidemiology, diagnosis and control of helminth parasites of swine. FAO Animal Health Manual. 1998;3:161. https://api.semanticscholar.org/CorpusID:82823545
