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		Visions on the Future of Veterinary Virology Feline Infectious Peritonitis

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		· Introduction · Virologist's perspective · FCoV carrier state · From the FCoV carrier state to FIP · Laboratory tests - are they useful? · References
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Laboratory tests - are they useful?
The diagnosis of FIP has been covered in many textbooks and articles; in our view, the algorithm developed in the Zurich laboratory is still the best clinical guideline [10]. Although it includes titres in the decision tree, these constitute only a minor factor. A negative serologic result would not invalidate the diagnosis based on clinical and blood chemistry data.

Fig. 10 Algorithm used to facilitate the diagnosis of FIP

In the absence of clinical signs, serology is of no use for the prognosis in individual cats. A statistical correlation indeed exists between antibody titres and post-mortem confirmation of FIP three months after testing. However, about 40% of the animals with titres of <300 do develop FIP, and of those with titres exceeding 1000 only about one half succumb; in other words: about half of the tested animals that remained healthy showed the same high titre values as the cats at risk. Tossing a coin would have given a similar result.

How does the other face of that coin look? About 12% of the cats with titres <100 still developed FIP in the observation period. Based on these data, one in eight owners would have been sent home with the erroneous information that nothing will happen to his cat. Serology does not distinguish between harmless and FIP-inducing FCoV mutants, it only shows past - and in many cases still ongoing - infection. Any seropositive cat may succumb to FIP, irrespective of the titre. We now can explain this statistical correlation between high titers and poor prognosis: expansion of the coronavirus quasispecies cloud obviously not only provides much genomic material with the increased probability for FIP-inducing mutants to occur, it also provides the large antigenic mass to induce high levels of antibody. However, FIP-inducing mutants can always occur, also at low replication levels (with low antibody production), though with a lesser likelihood.

On the other hand: an uninfected cat - which is not synonymous with a seronegative cat - will not develop into a case of FIP. This may sound as a truism, nevertheless some consideration is justified. In the Hanover cattery study, a few seronegative kittens shed FCoV in both faeces and plasma, some in faeces only and others in plasma only. In concreto: 86% positive animals were detected using PCR, while serology showed only 71% positives. However, not a single seropositive animal tested PCR-negative [10].