Strongyloides ransomi: Intestinal threadworm

General Description: Threadworms are hair-like and 4 to 5 mm long. The only parasitic adult form is a parthenogenic female. Under favourable conditions, free-living males and females reproduce sexually outside the host.

Life Cycle: The intestinal threadworm is capable of infection both by skin penetration and by ingestion of infective larvae. The larvae penetrate the lung air spaces from the blood, migrate up the trachea toward the mouth, and are swallowed. They mature to adulthood in the small intestine, where females may lay eggs that faeces and then hatch to yield first-stage larvae, which develop in the manner described as typical for roundworms, to become infective L3 larvae that are eaten by the host during grazing, or penetrate through the skin. The acquisition of infective larvae through the colostrum is a common way of infection of newborn piglets. The prepatent period is 6 to 9 days.

Location: Small intestine.

Geographical Distribution: Worldwide, particularly common in warm climates.

Significance: Moderate to high. The threadworm causes extensive economic loss due to mortality and morbidity where climate and management practices favour its development.

Effect on Host: Threadworm damage is most important in very young pigs. Most of the damage is caused by migrating larvae. Damage includes larvae-caused coughing, muscle soreness, abdominal pain, and occasional sudden deaths. The pigs move very little and have poor appetites. Within several days, as larval worms mature to adults in the intestine, infected piglets begin to vomit and have moderate-to-severe bloody diarrhoea. With continual exposure, these signs may be present simultaneously. Mortality may be as high as 50%. Adult carrier pigs usually show no signs.

Diagnostic Information: Characteristic embryonated eggs in fresh faeces or the adult female worms in mucosal scrapings of intestines at necropsy are diagnostic.

Control: Cleaning the pig's living quarters can reduce free-living larvae and male and female adults. Breeder stock may be infected by dormant larvae in their subcutaneous fat. In this state, the threadworms are not affected by currently available drugs. Pregnancy and farrowing appear to stimulate the re-emergence of these larvae, which may then infect newborns via the colostrum. In only one week after infection, piglets are also passing threadworm eggs. Because eggs in the faeces of the sow or the piglets can develop into infective larvae in 24 hours, there is quick rise in worm burden. Therefore, it is obvious that the sow should be treated with an effective anthelmintic before farrowing. If faecal examinations show that infection is present amount the young suckling pigs, appropriate treatment must begin immediately.

Strongyloides– eggs		Adult female Strongyloides
Consolidation caused by larval migration in lungs		Tissue section showing larvae under skin