Detection of Feline coronaviruses (FCoV) in circulating monocytes by direct immunofluorescence

DETECTION OF FELINE CORONAVIRUSES (FCoV) IN CIRCULATING MONOCYTES BY DIRECT IMMUNOFLUORESCENCE Paltrinieri S.*, Cammarata Parodi M.**, Cammarata G.**, Masotti A.*** *Istituto di Patologia Generale Veterinaria - Milano; **Istituto di Anatomia Patologica Veterinaria e Patologia Aviare - Milano; *** Private DVM - Milano In the pathogenesis of feline infectious peritonitis (FIP) spread of the coronaviruses (FIPV) through the body is ensured by circulating monocytes. Feline enteric coronaviruses (FeCV) seem to be unable to pass through epithelial cells. If Feline Coronaviruses (FCoV) are found in circulating monocytes this would differentiate the FIPV from the other coronaviruses of the cats. Therefore, we examined 62 cats without FIP-related symptoms, most of which were positive for circulating anti-FCoV antibodies, measured by an indirect fluorescent antibody test (IFA), from 3 different catteries (10, 31 and 21 cats), in which their first cases of FIP were recently diagnosed following exposure to the infection by contact with infected cats. In a small sample of blood from each cat, after the haematological analysis, red blood cells were lysed hypotonically. On the blood leukocytes obtained by cytocentrifugation direct immunofluorescence anti-FCoV (DIF) was performed, and the results were analyzed statistically to calculate the prevalences of the positivity for the catteries and to exclude the influence of pathophysiological and methodological variables. No haematological abnormalities were observed in cats from any of the 3 catteries. The prevalence of the FCoV positivity in blood monocytes was 22%. The age and the sex of the cats, the charachteristics of the blood sample (amount of blood, total and differential number of leukocytes, number of erythrocytes) and of the isolated cell populations (purity and recovery), and the presence of aspecific symptoms (moderate rhinitis, enteritis) were unrelated to positivity, while a risk factor such as contact with other infected catteries was highly significant (P<0.01). All the DIF+ve cats were also IFA+ve for circulating anti-FCoV antibodies: anti-FCoV titers were found also in some of the DIF -ve cats maybe due to a cross-reactivity with FeCV. The detection of FCoV on blood monocytes should be considered an useful tool to complete the clinico-serological diagnosis of FIP. Our results also seem to confirm the possibility of using the DIF test to detect the spread of the FCoV into the monocytes.