Infection of swine with parasites of the genus Oesophagostomum.
Nodular worms are distributed worldwide and, at least historically, frequently caused disease in swine, sheep and cattle. The species that occur in sheep and cattle do not pose a threat to swine. Roundworms, whipworms, and nodular worms are the most common internal parasites found in swine. Nodular worm parasitism usually only occurs in pigs raised in open lots or pasture but can rarely occur in confinement-raised pigs, including those without exposure to soil. The infection occurs in all age groups but pigs over three months old may be more susceptible.
At one time, natural sausage casings were made from swine intestines from which the mucosa had been removed. Nodules from nodular worms made the preparation of the casings a difficult or impossible task. Natural sausage casings from swine are seldom used anymore.
There are 5 species of Oesophagostomum. Oesophagostomum dentatum andOesophagostomum quadrispinulatum are the two species that cause most disease in swine in the United States. Three other species are largely restricted to southeastern states.
Adult Oesophagostomum are 1-2 cm long, have slightly curved bodies and are white to gray. Their life cycle is direct although there are 2 non-parasitic stages that precede the parasitic stages. Ova are morulated and thin-shelled, about 70 by 40 um. Under favorable conditions, non-parasitic stage larvae can persist on pastures for up to a year. Ingested L3 larvae penetrate the mucosa of the cecum and colon, molt to L4 larvae and return to the intestinal lumen in 6-20 days. Patency is reached as early as three weeks after ingestion; large numbers of ova are then passed in the feces.
Survival of the parasite is facilitated by the ability of non-parasitic stages of larvae to survive on pastures and by the large number of ova laid. During the periparturient period, the number of ova passed in the feces increases substantially. This increase continues throughout the lactation period and almost ensures that some of the piglets will be infected.
Although ingestion of L3 larvae from contaminated pastures is the usual route of transmission between pigs; introduction of infected animals is the primary mode of between-herd transmission.
Ingested third stage larvae (L3) penetrate the mucosa of the cecum and colon and cause petechial hemorrhages and mucosal irritation. Most of the larvae penetrate to the base of the mucosa, encyst there and destroy part of the adjacent muscularis mucosae. The parasites then reside in both the mucosa and submucosa. Some larvae penetrate much more deeply, even to the serosa where adhesions may develop.
Larvae in the wall of the intestine stimulate a marked inflammatory reaction and an effort is made by the host to wall them off. Repeated invasions probably result in hypersensitivity that contributes to the reaction. Inflammatory edema with thickening of the mucosa and thrombosis of lymphatics often results. Although many of the parasites molt and return to the surface of the mucosa, some parasites are walled off and eventually destroyed. The fibrotic nodules formed around the parasites may persist for some time. Secondary infections of the nodules often occur.
Most infections are asymptomatic. Others are concealed by signs of other parasitisms, especially ascariasis. Heavily infected animals may have diarrhea, decreased weight gain and poor feed efficiency. Deaths from nodular worm infection seldom occur.
The intestinal wall of a heavily parasitized pig is thickened and edematous. In the cecum and colon, nodules up to 20 mm in diameter are scattered in the mucosa/submucosa and some may be apparent on the serosa. Nodules often contain necrotic debris. Mature parasites that successfully migrate back to the lumen are 1-2 cm long. The lesions are easily confused with dilated lymphoglandular complexes which are common in the colon of pigs with chronic bacterial enteritis.
Diagnosis can be made on postmortem examination. Nodules in and on the large intestine and the presence of adult parasites in the lumen can be easily recognized. Lesions must be differentiated from abscessed lymphoglandular complexes.
Fecal exams can also be used for diagnosis. The eggs must be differentiated from grain mite eggs and from eggs of other Strongyloididae. Differentiation from Hyostrongylus rubidus may require larval culture.
Nodular worm infection usually is controlled by the use of anthelmintics. Deworming sows prior to farrowing will reduce future exposure of the piglets. In confinement operations, frequent removal of feces and good sanitation also reduce exposure of dams and piglets. In outdoor operations, lots and pastures that are heavily contaminated should be avoided. Pasture rotation, with tillage of land when not pastured, is helpful. A large number of anthelmintics are available for control. See the table, Anthelmintics and Parasiticides.