Aelurostrongylus abstrusus, the lungworm of the cat, has a world wide distribution and has been reported from countries as far apart as America, Great Britain and Palestine. It has a complex life cycle insofar as a molluscan intermediate host is essential and it is possible that auxiliary hosts also play an important part. In Britain, the incidence of active infestation of cats with the parasite has been recorded as 19·4% (Lewis, 1927) and 6·6% (Hamilton, 1966) but the latter author found that, generally, the clinical disease produced by the parasite was of a mild nature. It is known that the average patent period of the infestation in the cat is 8–13 weeks and it seems likely that, in that time, a considerable number of first stage larvae would be evacuated. Information on that point is not available and the object of the following experiment was to ascertain the number of larvae produced by cats during the course of a typical infestation.

Two experiments were undertaken to assess the role of the mouse in the life-cycle of Aelurostrongyhis abslrusus. In the first, 18 mice were fed first stage larvae and killed one month later. At post-mortem and histopathological examinations larvae were not demonstrated and attempts to extract larvae from a proportion of the mice were also negative. In the second experiment, 24 mice were fed heavy doses of first stage larvae. Subsequent examinations failed to reveal the presence of larvae and post-mortem and histopathological examinations performed on two cats, each of which had been fed with six of the mice, failed to reveal the presence of lungworms. It is concluded that mice do not act as intermediate host for Aelurostrongyhis abstrusus.

After five to six weeks, and for a varying period thereafter, cats infested with Aelurostrongylus abstrusus evacuate first stage larvae in their faeces. Hobmaier and Hobmaier (1935), Blaisdell (1952) and MacKerras (1957) collected larvae for eight weeks, two years and seven months, respectively, after patency. Cameron (1927) found that larvae, extracted from faeces, lived for 11 days but were killed by freezing. Since the fate of such larvae is of importance with regard to the incidence of lungworm disease in the cat and because of the divergence of opinion concerning the viability of first stage larvae, it was considered necessary to establish the life-span of such larvae under various controlled conditions.