Infestation of swine with whipworms, Trichuris suis.
Trichuris suis infections occur in domestic and feral swine; infection has also been reported in some primates, including man. Infection is relatively common in swine raised on pasture or in dirt lots. In the United States, whipworms were one of the three most prevalent internal parasites of swine, based on examination of several thousand fecal samples submitted to laboratories between 1988 and 1994.
All age groups are susceptible but most infestations occur in swine less than six months old. In adults, clinical signs can be induced when infection occurs in the face of severe stress. However, whipworms commonly infest adult pigs at a subclinical level and serve as a major source of infection for other swine. All major swine-producing countries have whipworm.
Whipworm infections were common historically when more swine were raised outdoors on pasture or in previously used lots. In situations where that method of production is still practiced, whipworm infection is still common. Rearing swine in a confinement setting, without access to contaminated soil or feces from previous groups, has led to elimination of whipworms as a significant problem for most producers. In outdoor operations, newer anthelmintics have allowed for improved control over this once-troublesome parasite.
Trichuris suis is the etiologic agent of whipworms in swine. Mature female whipworms are six to eight cm long, and males are about half that length. The life span of adults is four to five months. The anterior two-thirds of the parasite is quite thin and, when seen at necropsy, is buried in the mucosa of the intestine; the posterior one third is visibly thicker and usually seen protruding into the intestinal lumen. Worms are not visible grossly until at least three weeks post-infection.
Microscopically, ova are approximately 65 by 25 µm, yellow to brown in color, double operculated, and easily recognized. Ova passed in feces become infective in three to four weeks. The prepatent period is seven to eight weeks but clinical signs can be observed within one to two weeks of severe infestation. Ova production tends to be sporadic and there are times when there are few ova in the feces.
Although adult worms have a relatively short life span and lay ova only sporadically, ova remain infective in most environments for up to six years. This long period of survival tends to perpetuate the species, even if pasture rotation is practiced. Adult swine tend to harbor low levels of infection resulting in a continual source of ova contamination in the environment.
After ingestion of larvated eggs, L3 larvae hatch and enter the mucosa of the anterior small intestine. The larvae reside there temporarily, return to the intestinal lumen for a period of time, then deeply reinvade the mucosa or submucosa of the cecum and colon. This penetration results in diffuse mucofibrinous to mucohemorrhagic typhlocolitis. Focal ulcerations may be complicated by secondary invasion of microorganisms that include salmonellae and the protozoan, Balantidium coli. Secondary infection often is an important part of the disease process.
The inflammatory reaction around the parasites results in edematous thickening of the wall of the gut and the formation of inflammatory nodules around some parasites. In heavy infections, a fibrinous pseudomembrane may form on the mucosa. Regional lymph nodes may be swollen as a consequence of drainage of toxic products.
Clinical effects are directly related to severity of infestation and presence of concurrent diseases. Mild infestations are subclinical. Signs include anorexia, mucoid or mucohemorrhagic diarrhea, dehydration, and possibly death of severely affected animals. Signs are most apparent two to four weeks following exposure to contaminated facilities.
During early stages (one to three weeks post-infestation) whipworms are not visible as they migrate within the mucosa. There is a mucoid to mucohemorrhagic typhlocolitis with excessive mucus, fibrin and blood on the mucosa. Lesions vary in extent from focal to diffuse and in acute stages, often resemble those of swine dysentery. The affected gut wall is thickened and edematous. Histopathology is required for diagnosis and will reveal the migrating larvae. Later, maturing whipworms become visible with careful gross examination. Worms often protrude from inflammatory nodules in the gut wall and purulent exudate may be visible around them. Gentle traction on parasites will free them from the gut wall, revealing the thin anterior end buried in the mucosa. Dilated lymphoglandular complexes should not be mistaken as lesions of either whipworms or nodular worms. Inflammation of the cecal and colonic walls can be quite extensive and severe even when only few adult worms are observed.
Diagnosis is accomplished by necropsy of a typically affected pig. Mature parasites are easily found in the cecum or colon and can be identified by their size and whip-like form. Immature worms can cause severe lesions if numerous. They are much smaller than adults. For identification, they require careful observation, perhaps with the aid of flotation over a dark background or use of a magnifying lens. Extensive lesions of the immature worms in mucosal migrations closely resemble and must be differentiated from those of swine dysentery, salmonellosis and proliferative enteritis, any of which may be concurrently present.
The presence of ova can be determined by laboratory fecal examinations by seven weeks after infestation but clinical signs and lesions will often be quite severe before ova are present. The double operculated eggs are distinctive. However, ova production often is sporadic and there may be few or no ova in a single fecal sample. The number of ova found on fecal examinations may not be a reliable indicator of the number of worms present.
Control can depend on avoidance of exposure to contaminated pastures or lots. Tillage of pastures or lots not being used can reduce the number of eggs that survive. In outdoor operations, it is helpful to treat the pregnant sows with an appropriate anthelmintic a week or two prior to farrowing and/or before moving them to clean pasture. This reduces exposure of the piglets to eggs. Raising swine in confinement, without access to soil, usually results in good control.
Not all anthelmintics have good efficacy against T. suis. This necessitates a careful evaluation of a product’s spectrum of activity before incorporating it into a control program.
See the table, Anthelmintics and Parasiticides.