Strongyloides ransomi (pig threadworm) is a ubiquitous and common nematode parasite of pigs; it is particularly pathogenic in suckling pigs, especially in tropical and subtropical climates. It is of less importance in adults. 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, emphasizing the importance of good hygiene, or via infective larvae in the colostrum of lactating sows. Lactogenic transmission is highly efficient in infecting newborn piglets. Even without reinfection of the sow, dormant larvae in the udder may be transmitted to several consecutive litters of piglets. The adult female worms burrow into the wall of the small intestine, being found in tunnels in the epithelium at the base of the villi. The prepatent period is 4–9 days, depending on the mode of infection. In light and moderate infections, the pigs usually show no signs. In heavy infections, diarrhea, anemia, and emaciation may be seen, and death may result. Infection induces strong immunity; hence, older pigs are usually not clinically affected.
Diagnosis is determined by demonstration of the characteristic small, thin-shelled, embryonated eggs (20–35 × 40–55 μm) in the feces. It is important that feces are collected from the rectum, because fecal droppings often are contaminated by free-living nematodes, which may have eggs indistinguishable from the Strongyloides eggs. Furthermore, feces must be cooled immediately to prevent hatching. At necropsy, adults may be found in scrapings from the intestinal mucosa, and immature worms may be recovered from minced tissues using the Baermann techniques.
The benzimidazoles and levamisole are effective against intestinal infections. If administered in the feed for several days before and after parturition, they reduce lactogenic transmission to suckling piglets. Ivermectin is effective against adults and, if given to the sow 1–2 wk before farrowing, suppresses larval excretion in the milk. A high level of hygiene is necessary to diminish larval development as well as multiplication of free-living generations in the pens.