The life cycle of linguatulids involves two hosts. Larval development occurs in an intermediate host, which ingests eggs contained in the sputum, feces, or body cavity of definitive hosts (Hobmaier and Hobmaier, 1940; Cheng, 1986; John and Petri, 2006).
Eggs (90-70 microns) contain fully formed larvae. Females produce many eggs—several million—which are passed from the nasal discharge of definitive hosts (e.g., carnivorous reptiles, birds, mammals) to water or vegetation. Intermediate hosts (e.g., fish, cattle, sheep, rabbits, rodents, ungulates) then ingest infected water or vegetation. Eggs hatch in the intestine of intermediate hosts releasing the larvae, which then burrow through the intestinal wall and lodge themselves in the liver, lungs and other viscera. Within the viscera, the larvae mature to a pupa-like stage, then to a nymph (infective larva) stage in which hooks, annular rings, and spines develop. After maturation to the nymphal stage, the infective larva migrates to the pleural cavity (Hobmaier and Hobmaier, 1940; Cheng, 1986; John and Petri, 2006).
If the intermediate host (e.g., a fish, cow, sheep, rabbit, rodent, ungulate) is then eaten by a natural definitive host (e.g., a carnivorous reptile, bird, mammal) the infective larva migrate to the nasal cavities where they mature into adults. If the intermediate host is not eaten by a natural definitive host, there is no observed migration to the nasal cavity. Rather, the infective larvae migrate to the intestines, penetrate, and exist within the body cavity (Hobmaier and Hobmaier, 1940; Cheng, 1986; John and Petri, 2006).
Although cases of linguatulosis in humans is rare, humans may serve as both intermediate and definitive hosts—upon ingesting infective eggs and infective nymphs, respectively—for linguatulids (Ma et al., 2002).
Transmission to humans occurs by ingesting vegetables or water containing linguatulid eggs, or infected snake meat, sheep liver, or visceral lymph nodes (Hobmaier and Hobmaier, 1940; Cheng, 1986; John and Petri, 2006).
Linguatulids reside primarily in the respiratory ducts of carnivorous reptiles, birds, and mammals (John and Petri, 2006).
Clinical symptoms have been reported as appearing as early as 7 days or as late as several months after infection (Mehlhorn, 2004). In a life cycle study conducted using multiple animal hosts, performed on Linguatula rhinaria—a facultative parasite in humans and cosmopolitan parasite in livestock—eggs were demonstrated in nasal secretions following maturity within final hosts after 6 months. Adults lived about 2 years (Hobmaier and Hobmaier, 1940).