In order to understand better the mechanisms involved in the diverse clinical patterns in Microsporum canis-infected cats, the histopathological features were compared in symptomatic and asymptomatic infected cats. Additionally, the IgG immune response to a crude exo-antigen and purified keratinase of M. canis was studied by ELISA in cats of various clinical and mycological status. Acute and subacute perifolliculitis and folliculitis occurred more frequently in symptomatic than asymptomatic cats. The latter usually displayed signs of chronic inflammation and a marked infiltration of superficial dermis by mast cells, which would suggest that these animals present similarities to chronically dermatophytic humans or animals. When using a crude M. canis antigen, all infected cats were shown to have significantly higher levels of specific IgG when compared to culture negative and mechanical carrier-cats. In these non-infected animals, specific IgG was more frequently detected in adults than in young animals. No difference in anti-crude antigen specific IgG was observed between symptomatic and asymptomatic infected cats, indicating that the presence of IgG is probably unrelated to the clinical status of cats. Anti-keratinase specific IgG was only detected in one of the infected cats.

In order to understand better the host-parasite relationship and to compare with previous observations in Microsporum canis naturally infected cats, the humoral and cellular immune responses to both a crude exo-antigen and a 31.5 kDa purified keratinase were evaluated in 12 M. canis experimentally infected guinea pigs. Humoral and cellular responses were assessed by ELISA from days 0 to 56 postinfection (PI) and by measurement of delayed-type hypersensitivity (DTH) responses on days 14 and 57 PI, respectively. Additionally, immunohistochemical staining was performed and demonstrated that the keratinase was produced in infected guinea pig skin, as previously reported in cats. Despite a marked interindividual variation, all the guinea pigs produced specific IgG to the crude exo-antigen from day 21 PI onwards, but no anti-keratinase IgG was detected. Strongly positive DTH responses to the exo-antigen were observed on both dates, whereas the keratinase elicited no and weak DTH on days 14 and 57 PI, respectively. These results are in agreement with those previously described for naturally infected cats, and indicate that the 31.5 kDa keratinase is not a major antigen in M. canis infection.

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