The outcome of coronavirus infection in domestic cats depends on the strain and dose of virus, and the age, immune competence and immune status of the cat. In particular, the ability of the cat to mount a strong cell-mediated immune (CMI) response to the virus is believed to be very important in determining the clinical sequelae to infection. Thus infection can cause a wide spectrum of disease, or very often, no disease at all ( See Picture ).
- Main route of virus entry is oronasal, leading to virus replication in the oropharynx and in the enterocytes at the tips of villi. Experimentally, cats can also be infected via the respiratory tract with aerosols. There is also some evidence of transplacental transmission although this is not common in the field and appears to play little role in the overall epidemiology of feline coronavirus infection.
- If the cat mounts a strong immune response, particularly a strong cell-mediated immune response and a good local immune response in the gut, or the virus is of low pathogenicity, then virus replication will be limited to the gut and clinical signs will be absent or limited to mild diarrhoea.
- If the cell-mediated immune response is not so strong, then a more pathogenic virus may escape from the gut, giving rise to systemic infection. More pathogenic viruses are especially able to grow in macrophages, and it is in macrophages and monocytes that virus is disseminated.
- If a moderate cell-mediated immune response is encountered, then the development of lesions is slow, giving rise to chronic, dry feline infectious peritonitis.
- If, however, the cat produces a still weaker cell-mediated immune response, then acute, wet FIP rapidly develops.
This model for the pathogenesis of feline coronavirus infection explains why cats with wet FIP often have some lesions of dry FIP at necropsy, and also explains anecdotal accounts of cats recovering from wet FIP, but going on to develop dry FIP, or even (rarely) to recover.
It also explains why concurrent FeLV infection increases the prevalence of clinical FIP in a colony with FCoV infection, and suggests that treatments involving immunosuppressive agents, e.g. corticosteroids, although they might provoke a short-term general improvement in the cat's demeanour, would depress the CMI response and cause more severe disease in the long run.
The role of antibody
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