Strongyloidesransomi (Threadworm) in Pigs

Strongyloides ransomi (pig threadworm) is a nematode parasite of pigs. It is found worldwide; however, it is of most importance in tropical and subtropical climates. All age groups can be infected; however, clinical signs are usually confined to nursing piglets.

Threadworms are unique among helminths, having both parasitic generations (females in the small intestine) and free-living generations (males and females in the surrounding environment). Transmission occurs either by skin penetration with larval migration to the lungs, emphasizing the importance of good hygiene, or via infective larvae in the milk of lactating sows. Lactogenic transmission is highly efficient in infecting newborn piglets. Even without reinfection of the sow, dormant larvae in the udder can be transmitted to several consecutive litters of piglets (1).

Strongyloides ransomi eggs

Image

Eggs of Strongyloides ransomi recovered by fecal flotation, 50 × 28 mcm. Eggs are larvated in fresh feces and smaller than strongyle eggs. 400X total magnification.

Courtesy of Dr. Heather Walden.

The adult female worms burrow into the wall of the small intestine and are found in tunnels in the epithelium at the base of the villi. The prepatent period is 6–10 days (1, 2). In light and moderate infections, pigs usually show no clinical signs. In heavy infections, diarrhea, anemia, and emaciation can occur most often in young pigs. Death can also occur, usually in piglets < 2 weeks old. Infection induces strong immunity; hence, infections in younger pigs typically lead to worse disease (2).

Diagnosis of threadworm infection is by demonstration of characteristic small, thin-shelled, larvated eggs (20–35 × 40–55 mcm) (see Strongyloides ransomi eggs image) in the feces. Feces should be collected from the rectum, because fecal droppings often are contaminated by free-living nematodes, which can have eggs indistinguishable from other Strongyloides spp eggs. Feces must be cooled immediately to prevent hatching (1).

At postmortem examination, adult threadworms can be found in scrapings from the intestinal mucosa, and immature worms can be recovered from minced tissues using the Baermann technique. Infective third-stage larvae can also be collected from feces after fecal culture and Baermann, and they have a morphologically distinct 'forked' tail (3).

Macrocyclic lactones, levamisole, and benzimidazoles are effective against adult threadworms, and somatic larvae are susceptible to ivermectin and benzimidazoles (1, 2). Good hygiene decreases larval development, including keeping pens dry and clean to minimize transmission (because larvae are susceptible to desiccation) and minimize multiplication of free-living generations in the pens.