Page 901 - Veterinary Immunology, 10th Edition

VetBooks.ir Some Selected Virus Diseases

Viruses, if they are to survive, must evade the immune responses.
They use many different mechanisms including, but not limited to

severe immunosuppression. These may result in significant
immunopathology.

Feline Infectious Peritonitis

Feline infectious peritonitis (FIP) is a fatal granulomatous disease of

wild and domestic cats caused by feline enteric coronavirus (FECV).
There are two distinct genotypes of feline enteric coronavirus,
avirulent and virulent. The avirulent genotype prefers to replicate
within intestinal epithelial cells, whereas the virulent genotype

prefers to replicate within macrophages. Macrophages also spread
the virus throughout the body. FIP presents in two major forms: (1)
an effusive (“wet”) form with peritonitis or pleuritis characterized
by the presence of large amounts of proteinaceous fluid in the body

cavities and associated with a vasculitis, and (2) a noneffusive
(“dry”) form characterized by multiple small granulomas on the
surface of the major abdominal organs. Both forms of the disease
are uniformly lethal, with affected cats dying between 1 week and 6

months.
The pathogenesis of FIP differs between the two forms of the
disease. After invading a cat, the virus first replicates in intestinal
epithelial cells. The virus shed by epithelial cells is then spread by

monocytes and taken up by phagocytic cells in the target tissues.
These target tissues include the serosa of the peritoneum and the
pleura, as well as the meninges and the uveal tract. The course of
the infection then depends on the nature of the immune response to

the virus—a phenomenon also seen in several bacterial diseases
(Chapter 26). Immunity to FIP virus is entirely cell mediated, and a
Th1 response is protective. A cat that mounts a good Th1 response
will become immune, regardless of the amount of antibodies it

makes. Some cats, however, mount a Th2 response to the viral spike
proteins. In these animals, antibodies enhance virus uptake by
macrophages, in which they then replicate. Virus-laden

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