Objectives of the Presentation

Feline infectious peritonitis (FIP) is a baffling and clinical important disease of cats caused by a virulent biotype of feline coronavirus. This clinical syndrome appears to have emerged in the 1950s. Approximately 1 out of every 200 new feline cases seen at veterinary medical teaching hospitals are cats diagnosed with FIP.(1) Despite the fact that the disease is common, veterinarians are often faced with a diagnostic dilemma because there are no individual tests that are reliable for the diagnosis of FIP and the clinical picture is highly variable. This presentation will focus on the various clinical manifestations known to be associated with FIP.

Overview of the Issue

Most cats affected with FIP are young, particularly between 3 months and 2 years of age. Other risk factors include(2,4):

 Source: multi-cat home, shelter, cattery

 Pedigreed breeds: Abyssinian, Bengal, Birman, Himalayan, Ragdoll, Rex; may vary by country

 Genetic susceptibility

 Concurrent diseases, especially feline leukemia virus (FeLV) infection

 Stressors: re-homing, recent elective surgery, inter-cat conflicts, etc.

FIP presents in two clinical forms: effusive (wet) and non-effusive (dry). However, the two forms are not mutually exclusive and may occur in the same patient, although usually as a transition from one form to the other. Some investigators therefore designate a third, "mixed" form. The effusive form is traditionally the most common seen, although the non-effusive form appears to be increasing in prevalence in recent decades. Clinical signs depend on the predominant form of the disease and the organ systems affected. Some clinical signs are common to both forms:

 Lethargy

 Anorexia (partial or total)

 Weight loss (often out of proportion to the decrease in appetite)

 Chronic fluctuating fever non-responsive to antibiotics

 Poor growth rate in kittens

Overt clinical signs may be apparent for a few days to a few months, but in retrospect may have been preceded by a long period of vague ill health and poor growth in kittens. Generally the effusive form progresses more rapidly than the non-effusive form.

Clinical signs associated with the effusive form are due to immune complex damage of small blood vessels (vasculitis) with the resulting leakage of serum protein and fluid into body cavities. The most common clinical feature is ascites. In fact, FIP is the leading cause of ascites in young cats, proving a more common cause than cardiac disease, neoplasia, and hepatic or renal disease. The enlarged abdomen can contain a surprising amount of fluid, and may be mistaken for pregnancy by owners of female cats. Typically, the abdominal distension is non-painful and a fluid wave can be palpated. The effusion found in FIP is a non-septic exudate with distinct characteristics.(5)

 Straw to golden yellow color, viscous, clear to slightly cloudy, frothy when shaken

 High specific gravity (1.017-1.047)

 High protein (typically >3.5 g/dl, often 5-12 g/dl)

 Albumin: globulin ratio <0.45

 Low to moderate cellularity (<5000 cells/μl)

If pleural effusion occurs, the primary clinical signs may include dyspnea, tachypnea, open-mouth breathing, and cyanotic mucous membranes. Heart sounds will be muffled on thoracic auscultation. Other less common clinical pictures associated with effusive FIP include ocular or central nervous system (CNS) involvement, pericardial effusion, and scrotal enlargement due to extension of the peritonitis to the tunics surrounding the testes. Generalized synovitis may cause clinical signs of fever and intermittent lameness.

Clinical signs associated with the non-effusive form depend on the body system affected and are due to localized perivascular infiltrates of inflammatory cells (pyogranulomas) in the parenchyma of organs. Thoracic or abdominal effusions are either absent or too scant to be appreciated clinically. The most common clinical presentations involve the eyes or CNS. Ocular involvement may manifest as anterior uveitis with hyphema, hypopyon, aqueous flare, miosis, and keratic precipitates. Other findings include an irregularly shaped pupil, a change in iris color, chorioretinitis, retinal hemorrhage, or retinal detachment(6). Ocular disease may be the sole manifestation of FIP in affected cats, or it may be combined with CNS or abdominal involvement.

CNS involvement is common and may manifest as seizures, ataxia, nystagmus, tremors, depression, behavior or personality changes, paralysis or paresis, circling, head tilt, peripheral neuropathies, hyperesthesia, or urinary incontinence.(7) Clinical signs reflect the area of CNS involvement and are often multifocal. Affected cats are typically young (under 2 years) and come from multi-cat environments.(8) FIP is the most common inflammatory disease of the CNS in cats(9) and is a leading cause of spinal disease.(10) In one study of 24 cats with FIP with neurologic involvement, 75% had hydrocephalus at necropsy.(7) Finding hydrocephalus with CNS imaging such as CT scan is therefore highly suggestive of FIP.

Abdominal involvement with FIP may include granulomas in mesenteric lymph nodes, kidneys, or liver, as well as adhesions throughout the omentum and mesentery that may be palpable as masses and visible with ultrasound. Lesions in the gastrointestinal tract may cause diarrhea or sometimes vomiting or obstipation. Focal granulomas may be found in the ileum, ileocecocolic junction, or colon. Involvement of the cecum and colon produces a distinct form of FIP with signs of colitis (soft stools containing blood and mucus).(11)

Uncommon manifestations of non-effusive FIP include chronic fibrinous and necrotizing orchitis with bilateral scrotal enlargement.(12,13) Skin lesions due to coronavirus-induced vasculitis have been reported in a cat with FIP and concurrently infected with feline immunodeficiency virus.(14) Cutaneous papular lesions were seen late in the course of disease in a young cat with clinical signs of both effusive and non-effusive FIP.(15)

Summary

FIP can present a diagnostic dilemma to the clinician due to the variety of presenting clinical signs and the lack of definitive diagnostic tests. The clinician must exercise considerable diagnostic and physical examination skills in order to reach a diagnosis, especially in cats with noneffusive FIP.

References

1.  Rohrbach, B.W., et al., Epidemiology of feline infectious peritonitis among cats examined at veterinary medical teaching hospitals. J Am Vet Med Assoc, 2001. 218(7): p. 1111-5.

2.  Foley, J.E., et al., Risk factors for feline infectious peritonitis among cats in multiple-cat environments with endemic feline enteric coronavirus. J Am Vet Med Assoc, 1997. 210(9): p. 1313-8.

3.  Pesteanu-Somogyi, L., C. Radzai, and B. Pressler, Prevalence of feline infectious peritonitis in specific cat breeds. J Feline Med Surg, 2006. 8(1): p. 1-5.

4.  Foley, J. and N. Pedersen, The inheritance of susceptibility to feline infectious peritonitis in purebred catteries. Fel Pract, 1996. 24(1): p. 14-22.

5.  Paltrinieri, S., M.C. Parodi, and G. Cammarata, In vivo diagnosis of feline infectious peritonitis by comparison of protein content, cytology, and direct immunofluorescence test on peritoneal and pleural effusions. J Vet Diagn Invest, 1999. 11(4): p. 358-61.

6.  Doherty, M.J., Ocular manifestations of feline infectious peritonitis. J Am Vet Med Assoc, 1971. 159(4): p. 417-24.

7.  Kline, K., R. Joseph, and D. Averill, Feline infectious peritonitis with neurological involvement: clinical and pathological findings in 24 cats. J Am Anim Hosp Assoc, 1994. 30(Mar/Apr): p. 111-118.

8.  Foley, J.E., et al., Diagnostic features of clinical neurologic feline infectious peritonitis. J Vet Intern Med, 1998. 12(6): p. 415-23.

9.  Bradshaw, J., G. Pearson, and T. Gruffydd-Jones, A retrospective study of 286 cases of neurological disorders of the cat. J Comp Pathol, 2004. 131(2-3): p. 112-120.

10. Marioni-Henry, K., et al., Prevalence of diseases of the spinal cord in cats. J Vet Intern Med, 2004. 18(6): p. 851-858.

11. Harvey, C.J., J.W. Lopez, and M.J. Hendrick, An uncommon intestinal manifestation of feline infectious peritonitis: 26 cases (1986-1993). J Am Vet Med Assoc, 1996. 209(6): p. 1117-20.

12. Foster, R., J. Caswell, and N. Rinkardt, Chronic fibrinous and necrotic orchitis in a cat. Can Vet J, 1996. 37: p. 681-682.

13. Sigurdardottir, O.G., O. Kolbjornsen, and H. Lutz, Orchitis in a cat associated with coronavirus infection. J Comp Pathol, 2001. 124(2-3): p. 219-22.

14. Cannon, M.J., M.A. Silkstone, and A.M. Kipar, Cutaneous lesions associated with coronavirus-induced vasculitis in a cat with feline infectious peritonitis and concurrent feline immunodeficiency virus infection. J Feline Med Surg, 2005. 7(4): p. 233-6.

15. Declercq, J., et al., Papular cutaneous lesions in a cat associated with feline infectious peritonitis. Vet Dermatol, 2008. 19(5): p. 255-8.