Marley is a grey 6-year-old spayed DSH who's been brought to you because she's had diarrhea for three days. The owner maintains that Marley, an indoor-outdoor cat, has not been as active as usual and does not have much of an appetite. She was losing weight for a while but has recently put weight back on and has gotten fat around the middle. You palpate her distended abdomen. You quickly realize that this is not fat, but ascites.
Marley's temperature is somewhat elevated (103 degrees); her mucus membranes are pale, her eyes are sunken slightly, and the skin of her scruff tents briefly when you test it. You auscult her heart and lungs and find that both sounds are somewhat muffled. Furthermore, the lungs are crepitant and wet and her respiration rate is elevated.
When you palpate her abdomen, she doesn't seem to have pain but she becomes rather excited and dyspneic. You tell the owner that Marley has fluid in her abdomen and possibly in her chest and that it is making it difficult for her to breathe. You need to give her fluids and take some samples but you are concerned that further manipulation could excite her too much. You ask permission to hospitalize Marley in an oxygen cage so you can do your work more safely. The owner agrees.
In the oxygen cage, you place an IV catheter and start fluid therapy for the dehydration. You also decide to take a blood sample and to tap some of the fluid from her abdomen. The abdominal fluid is a thick, icteric exudate with some apparent fibrin strands. You check Marley's chart and find that she is up to date on her vaccines (rabies, FVRC, FeLV). You decide to do an in-house ELISA for FeLV anyway. All the same, you know what's going on here and the news isn't good.
Marley has Feline Infectious Peritonitis (FIP). Diagnosis of FIP is based on history and clinical signs. There's an antibody titer test but it's actually sensitive to a related virus (feline enteric coronavirus) so it's often misinterpreted. Gross examination of abdominal fluid can be helpful: the presence of fibrin in conjunction with the clinical signs is highly indicative but definitive diagnosis can only be made upon exploratory surgery. Histology is also helpful, especially examination of the necrotic foci on the peritoneum and discovery of a lymphocytic exudate.
FIP is not curable. Treatment is strictly palliative and supportive: once FIP has progressed to the point where pleural and abdominal effusion are present, the prognosis is poor. Concurrent infection with feline leukemia virus is common and such cases carry a very poor prognosis. There are commercial FIP vaccines available but their efficacy is not well established. The owner, faced with these facts, asks you to euthanize her.
Chronic exudative inflammation is not a unique form of inflammation. The word "exudative" indicates that there's an ongoing process involving the movement of protein and inflammatory cells out of the vasculature and into the interstitial and anatomic spaces of the body. "Chronic, exudative" implies that the inflammation may be long-standing but that it's still active: it hasn't settled into a quiescent state consisting mostly of macrophages, fibroblasts, and remodeling.
This mesenteric lymph node above left is obviously active; more to the point, the CT in which it's located (above right) is diffusely infiltrated by an inflammatory exudate with multifocal-to-coalescing regions of intense cellular inflammation. Higher magnification of the inflammatory area shows a significant infiltration of lymphocytes and neutrophils; there is some minor hemorrhage present as a result of tissue breakdown.
Exudates are typically associated with acute inflammation. Usually the affected region stops exuding protein and cells once tissue demand has been met, and then the inflammation will resolve. If the inflammation moves into a chronic, long-term stage and cells and protein are still exuding into the site, that can indicate a serious problem involving chronic antigenic stimulation.
Feline infectious peritonitis is caused by infection with a coronavirus. Classic FIP is further divided into three categories: effusive, non-effusive, and mixed. The effusive form, the most recognizable, involves effusion of abdominal fluids and marked deposition of fibrin throughout the abdomen. In this case, the the disease is the classic effusive form. Affected cats show nonspecific signs such as intermittent fever, diarrhea, and progressive weight loss. As the disease progresses and hepatic function is deranged, icterus and ascites become evident: lethargy and anorexia develop, as does a pleural effusion that impedes respiration and muffles the heart and lung sounds. The fluid in both the abdomen and the pleura is lymphocytic, fibrinous, viscous and sticky and will rapidly clot if exposed to air. Grossly, the omentum becomes adhered to itself and to visceral organs and many foci of necrosis develop on the peritoneum.
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