12th Ljudevit Jurak International Symposium on
Comparative Pathology
June 1-2, 2001

M. Reinacher
Institute of Veterinary Pathology, University Giessen, Giessen, Germany
FeLV-associated enteritis (FAE) is found in about 5-10% of spontaneously and persistently FeLV-infected cats at postmortem examination. It was characterized originally by histopathological alterations (1) in animals negative for all other known infectious pathogens but FeLV. Typical findings are cell degeneration in the intestinal crypt epithelium similar to feline infectious panleukopenia, a disease induced in young cats by parvovirus infection (parvovirus enteritis, PVE). In contrast to feline infectious panleukopenia, however, bone marrow and lymphatic tissues are not depleted but normal or even hyperplastic in FAE. Cats which died of FAE are on the average clearly older than cats with panleukopenia. The clinical course of FAE is usually slow and often undulating while untreated feline infectious panleukopenia usually has a lethal outcome within days.
Morphologically similar alterations as in FAE are found sometimes also in FeLV-negative cats. This histologically and immunohistologically defined entity  enteritis of unknown etiology (EUE)  has no connection with FeLV infection as proven by the exclusion of latent FeLV infection by PCR. In EUE more macrophages are found among the cell infiltrate in the intestinal mucosa than in FAE.
A third entity of feline enteritis is Coronavirus enteritis. It is characterized by degeneration and virus multiplication in the epithelium at the tips of the intestinal villi leaving the crypts unaltered. Furthermore, it can be differentiated from FAE and PVE by immunohistological demonstration of the etiological agent.
A striking difference between FeLV-positive cats with and without FeLV-associated enteritis is the expression of some structural virus proteins in the intestinal epithelium. Animals with FAE exhibit high expression of the surface proteins gp70 [SU] and p15(E) [TM] in these cells whereas FeLV-positive cats without FAE have no or only very little gp70 and p15(E) in enterocytes but a lot of p27.
An experimental model system in FeLV infection was described by Hoover and Mullins (2) which shows very striking similarities to FAE. This system was established in order to investigate FeLV strains with high immunosuppressive activities and was called, therefore, FeLV-FAIDS. The relevant FeLV strains exhibit altered glycosilation of gp70 resulting in slower processing of the protein simultaneously with accumulation of unintegrated variant DNA in the cells. Since gp70 and p15(E) derive from a common precursor our immunohistological results can be explained by this phenomenon. Some of these FeLV strains display nearly exclusive enterotropism resulting in similar morphology as described for FAE.
From these facts we conclude that FAE is a specific entity of non-neoplastic FeLV-associated diseases which has a similar pathogenesis as described in the experimental system FeLV-FAIDS.

(1) M. Reinacher, Vet. Pathol. 1987; 24:1
(2) E. Hoover et al., In?: Willett and Jarrett (eds)?: Feline immunology and immunodeficiency, Oxford Univ. Press 1995; p. 318