Strongyloides ransomi (pig threadworm) is a nematode parasite of pigs found worldwide. It is of importance in tropical and subtropical climates. It is uncommon in most areas of the continental US. Although all age groups may be infected, 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 may be transmitted to several consecutive litters of piglets.
The adult female worms burrow into the wall of the pig's small intestine, and are found in tunnels in the epithelium at the base of the villi. The prepatent period is 7–9 days, or 4–5 days after lactogenic transmission. In light and moderate infections, the pigs usually show no clinical signs. In heavy infections, diarrhea, anemia, and emaciation may occur in pigs up to 3 months old; death may occur, usually in piglets <2 weeks of age. Infection induces strong immunity; hence, older pigs usually do not have clinical signs.
Diagnosis is by demonstration of the characteristic small, thin-shelled, embryonated eggs (20–35 × 40–55 mcm) in the feces. Feces must be collected from the rectum because fecal droppings often are contaminated by free-living nematodes, which may have eggs indistinguishable from Strongyloides eggs. Furthermore, feces must be cooled immediately to prevent hatching. At postmortem examination, adults may be found in scrapings from the intestinal mucosa, and immature worms may be recovered from minced tissues using the Baermann technique.
The benzimidazoles are effective against intestinal infections and, when administered in the feed for several days before and after parturition, decrease lactogenic transmission to suckling piglets. Ivermectin and doramectin are effective against adults. Ivermectin administered to the sow 1–2 weeks before farrowing suppresses larval excretion in the milk. A high level of hygiene is necessary to decrease larval development as well as multiplication of free-living generations in the pens.