Aelurostrongylus abstrusus and Troglostrongylus brevior (feline lungworms) are parasitic nematodes that affect the respiratory tract of cats worldwide. Cats are the definitive host meaning the adult parasites reproduce sexually only in cats. The adult nematodes parasitize the bronchioles and alveolar ducts in the case of A. abstrusus or bronchi and bronchioles in case of T. brevior where they also lay their eggs. The eggs hatch and first-stage larvae (L1s) emerge, are coughed up, swallowed, and passed in the cat’s feces. The parasite’s life cycle is complex involving infection of and development within an intermediate host (i.e., Helix aspersa snails). Within an infected snail, the larva matures to L2 larva, and eventually to an infective L3 larva. A cat can be infected by ingesting an infected snail or more commonly by ingesting a paratenic host (i.e., bird, rodent, frog, etc.), which in turn have been parasitized by an arrested L3 larva by ingesting an infected snail. The life cycle of the parasite is completed after an infective L3 larva penetrates the cat’s gut wall and migrates through the peritoneal and thoracic cavities to the lungs.
Cats with access to outdoors are clearly at increased risk for lungworm infection due to predation. Clinical signs are usually absent. However, in heavy infections, cats develop a chronic cough accompanied by progressive breathing difficulty, loss of appetite, and emaciation.
The nematode parasites’s ecological niche is shared with that of the gastropod species and includes the gastropod species itself that acts as an intermediate host (i.e., H. aspersa snails). It was previously shown that H. aspersa could excrete infective L3 larva in the environment via the mucus of following death of the snail. Nevertheless, very little else is known about the biology of these parasites in their snail intermediate hosts. Therefore, researchers from University of Bari, Bari, Italy assessed the potential of nematode transmission from infected to naïve susceptible snails and evaluated the survival time of A. abstrusus and T. brevior L3 larva in the environment.
For the first time, these researchers showed the novel transmission of A. abstrusus L3s from infected to naïve H. aspersa. More in general, this is the first report of an intermediate host being infected by an infective L3 larva as opposed to a L1 larva. The impact of this transmission route on the overall patterns of parasite epidemiology and ecology is unknown and warrants further study. The phenomenon may significantly increase the survival time of infective larvae in the environment. This newly discovered mode of transmission has been termed intermediesis. [GO]
DiCesare A, DiFrancesco G, et al. Retrospective study on the occurrence of the feline lungworms Aelurostrongylus abstrusus and Troglostrongylus spp. in endemic areas of Italy. Vet J. 2015 Feb; 203(2):233-238.