The clinical presentation of FIP is extremely variable and this is reflected in the marked variability in the distribution of the vasculitis and pyogranulomatous lesions.
FIP has previously been classified as occurring in effusive and non-effusive (wet and dry) forms. This has some value in recognizing clinical presentations of FIP and contributing to diagnosis but it is clear that there is considerable overlap between the two forms. In cases with predominantly non-effusive features, investigation of possible accumulation of sub-clinical, small amounts of effusion can be helpful to provide samples for diagnostic testing.
Fever refractory to antibiotics, lethargy, anorexia and weight loss are common non-specific signs but occasional cases remain bright and retain body condition.
Ascites is the most obvious clinical manifestation of the effusive form [Holzworth, 1963, Wolfe & Griesemer 1966]. Thoracic and pericardial effusion may occur in combination with abdominal effusion. In a smaller proportion of cases effusion is restricted to the thorax and those cats usually present with dyspnoea. Serositis can involve the tunica vaginalis of the testes leading to scrotal enlargement. Non-effusive (or dry) FIP frequently represents a major diagnostic challenge. Non-specific signs of pyrexia, anorexia and lethargy may be the only signs, particularly in the early stages of disease. More specific signs will depend on the organs or tissues involved in the vasculitis and pyogranulomatous lesions. Abdominal organs are a common site for lesions. Renal involvement may lead to renomegaly detectable on palpation. Mural intestinal lesions in the colon or ileocaecoecolic junction occasionally occur and may be associated with chronic diarrhoea and vomiting. There may also be palpable enlargement of the mesenteric lymph nodes and this may be misinterpreted as neoplasia [Kipar et al, 1999]. Ocular involvement is common, leading to a variety of changes, such as iris colour, dyscoria or anisocoria secondary to iritis, sudden loss of vision and hyphaema. Keratic precipitates can also be seen and may appear as “mutton fat” deposits on the ventral corneal endothelium [Davidson, 2006]. The iris may show swelling, a nodular surface, and aqueous flare may be detected. On ophthalmoscopic examination chorioretinitis, fluffy perivascular cuffing (representing retinal vasculitis), dull perivascular puffy areas (pyogranulomatous chorioretinitis), linear retinal detachment and fluid blistering under the retina may be seen. Neurological signs are reported in around 10% or more of cats with FIP [Rohrer et al, 1993]. They reflect focal, multifocal, or diffuse involvement of the brain, the spinal cord and meninges. The most commonly reported signs are ataxia, hyperaesthesia, nystagmus, seizures, behavioural changes and cranial nerve defects [Kline et al, 1994; Timman et al, 2008]. Cutaneous signs have recently been reported occurring as multiple nodular lesions caused by pyogranulomatous-necrotising dermal phlebitis [Cannon et al, 2005] and skin fragility [Trotman et al, 2007]. A diffuse pyogranulomatous pneumonia is seen in some cases leading to severe dyspnoea [Trulove et al, 1992].