Diagnosis of feline leukemia virus and feline immunodeficiency virus infections

Feline leukemia virus is an oncogenic retrovirus that can result in a wide variety of neoplastic and non-neoplastic diseases, including immunosuppression. Diagnosis of FeLV infection can be achieved by several methods, including virus isolation; IFA assay of a peripheral blood smear; and detection of a viral protein (called p27) by ELISA testing of whole blood, plasma, serum, saliva, or tears. Commercially available ELISA kits have revolutionized FeLV testing and have become very popular as "in-house" procedures. This article discusses the interpretation of ELISA results and compares them with IFA assay findings. Feline immunodeficiency virus is a lentivirus that causes immunosuppression, but not neoplasia, in cats. It originally was called feline T-lymphotropic lentivirus. Differentiating FIV infection from the immunosuppressive type of FeLV infection requires virus isolation or serology. The most rapid method for diagnosis of FIV infection is ELISA testing for antiviral antibody.     

Clinical aspects of feline immunodeficiency and feline leukemia virus infection

Feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) are retroviruses with a global impact on the health of domestic cats. The two viruses differ in their potential to cause disease. FIV can cause an acquired immunodeficiency syndrome that increases the risk of developing opportunistic infections, neurological diseases, and tumors. In most naturally infected cats, however, FIV itself does not cause severe clinical signs, and FIV-infected cats may live many years without any health problems. FeLV is more pathogenic, and was long considered to be responsible for more clinical syndromes than any other agent in cats. FeLV can cause tumors (mainly lymphoma), bone marrow suppression syndromes (mainly anemia) and lead to secondary infectious diseases caused by suppressive effects of the virus on bone marrow and the immune system. Today, FeLV is less important as a deadly infectious agent as in the last 20 years prevalence has been decreasing in most countries.     

Feline leukemia virus and feline immunodeficiency virus.

Ophthalmic manifestations of FeLV or FIV infection can occur in all ocular tissues and may be manifestations of direct viral effects or secondary to viral-related malignant transformation. Additionally, the manifestations of common feline ophthalmic pathogens may be more severe and poorly responsive to therapy because of the immunosuppressive effects of FeLV or FIV infection. Prompt diagnosis of underlying viral infection in cats with ophthalmic disease is paramount for accurate diagnosis and prognosis and is required for appropriate therapeutic decision making.     

Epizootiologic association between feline immunodeficiency virus infection and feline leukemia virus seropositivity

Five hundred twenty-one feline serum samples submitted to the Texas Veterinary Medical Diagnostic Laboratory between Nov 1, 1988, and Jan 31, 1989 were tested for antibody to feline immunodeficiency virus (FIV) by use of an ELISA. The prevalence of FIV infection in this population was 11.3% (95% confidence interval: 8.6 to 14.0%). Serologic test results for FeLV were available for 156 of the 521 cats. A significant (P = 0.008) association between FIV infection and FeLV seropositivity was observed; FeLV-positive cats were nearly 4 times more likely to be seropositive for FIV than were FeLV-negative cats. The association remained statistically significant (P = 0.021) after adjusting for age and gender, using multiple-logistic regression analysis.     

Prevalences of feline leukaemia virus and feline immunodeficiency virus infections in cats in Sydney

Objective To determine prevalences of feline leukaemia virus (FeLV) and feline immunodeficiency virus (FIV) infections in ‘healthy’ cats that, through acute misadventure or other circumstance, were presented to veterinary practitioners. Prevalences of FeLV and FIV in this population were compared to those in a population of predominantly sick cats. Design and procedures Serum specimens were obtained over a 2-year period from 200 cats oldeer than 1 year of age presented to veterinary clinics for routine procedures, including cat fight injuries or abscesses, vehicular trauma, neutering, dental scaling, vaccination, grooming or boarding. An additional 894 sera were obtained over approximately the same period from specimens submitted by veterinarians to a private clinical pathology laboratory, mainly from sick cats suspected of having immune dysfunction, but including some sera from healthy cats being screened prior to FeLV vaccination. FIV antibody and FeLV antigen were detected in samples using commercial enzyme immunoassays. Results Amongst 200 ‘healthy’ cats, the prevalence of FeLV infection was 0 to 2%, and the prevalence of FIV was 6.5 to 7.5%, depending on the stringency of the criteria used to define positivity. FIV infection was significantly more prevalent in cats which resided in an inner city environment (P = 0.013). Of the 894 serum specimens submitted to the laboratory by practitioners, 11/761 (1.4%) were FeLV positive, while 148/711 (20.8%) were FIV positive. The prevalence of FIV was significantly higher in these predominantly ‘sick’ cats than in cats seen for routine veterinary procedures (P < 0.00001), while there was no difference in the prevalence of FeLV (P = 0.75) Conclusions The prevalence of FeLV and FIV in healthy cats may have been substantially overestimated in some previous Australian surveys. FeLV infection would appear to be a rare cause of disease in Australian cats. The higher prevalence of FIV positivity in sick as opposed to healthy cats infers that FIV infection contributes to the development of disease.     

Quantification of viral ribonucleic acid in plasma of cats naturally infected with feline immunodeficiency virus

OBJECTIVE: To assess plasma viral RNA concentration in cats naturally infected with feline immunodeficiency virus (FIV). ANIMALS: 28 FIV-infected cats. PROCEDURE: Cats were categorized into 1 of the 3 following stages on the basis of clinical signs: asymptomatic (nonclinical) carrier (AC; n = 11), acquired immunodeficiency syndrome-related complex (ARC; 9), or acquired immunodeficiency syndrome (AIDS; 8). Concentration of viral RNA in plasma (copies per ml) was determined by use of a quantitative competitive polymerase chain reaction (QC-PCR) assay. Total lymphocyte count, CD4+ cell and CD8+ cell counts, and the CD4+ cell count-to-CD8+ cell count ratio were determined by use of flow cytometry. RESULTS: Plasma viral RNA concentration was significantly higher in cats in the AIDS stage, compared with cats in AC and ARC stages. Most (5/7) cats in the AIDS stage had low total lymphocyte, CD4+ cell, and CD8+ cell counts. CONCLUSIONS AND CLINICAL RELEVANCE: Concentration of plasma viral RNA is a good indicator of disease progression in FIV-infected cats, particularly as cats progress from the ARC to the AIDS stage. Determination of CD4+ and CD8+ cell counts can be used as supportive indicators of disease progression.     

A review of feline leukemia virus and feline immunodeficiency virus seroprevalence in cats in Canada

Feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) are common and important infectious diseases of cats in Canada. Prevalence data are necessary to define prophylactic, management, and therapeutic measures for stray, feral and owned cats. Recently, comprehensive data on the seroprevalence of retrovirus infections of cats in Canada have become available and are reviewed. Further investigation into geographic variations in retrovirus seroprevalence within Canada is warranted, and may provide information to improve recommendations for testing and prevention. As well, more information is needed on FIV subtypes in Canada to improve diagnostics and vaccines, as well as to provide information on disease outcomes.     

Quality of different in-clinic test systems for feline immunodeficiency virus and feline leukaemia virus infection

Many new diagnostic in-house tests for identification of feline immunodeficiency virus (FIV) and feline leukaemia virus (FeLV) infection have been licensed for use in veterinary practice, and the question of the relative merits of these kits has prompted comparative studies. This study was designed to define the strengths and weaknesses of seven FIV and eight FeLV tests that are commercially available. In this study, 536 serum samples from randomly selected cats were tested. Those samples reacting FIV-positive in at least one of the tests were confirmed by Western blot, and those reacting FeLV-positive were confirmed by virus isolation. In addition, a random selection of samples testing negative in all test systems was re-tested by Western blot (100 samples) and by virus isolation (81 samples). Specificity, sensitivity, positive and negative predictive values of each test and the quality of the results were compared.     

Enzyme-linked immunosorbent assay methods for detection of feline leukemia virus and feline immunodeficiency virus.

Enzyme-linked immunosorbent assays have been widely used for diagnosis of FeLV and feline immunodeficiency virus (FIV) infections. Various ELISA kits for FeLV are available from several manufacturers. Although these tests are configured in a variety of formats, they are all direct antigen-detection systems for the viral core protein p27. On the other hand, ELISA for FIV exposure detects specific feline antibody to FIV. Basic immunoassay principles and the application of ELISA technology used in FeLV and FIV ELISA kits are described.     

Enhancement after feline immunodeficiency virus vaccination.

Cats were vaccinated with one of the three preparations: purified feline immunodeficiency virus (FIV) incorporated into immune stimulating complexes (ISCOMs), recombinant FIV p24 ISCOMs, or a fixed, inactivated cell vaccine in quil A. Cats inoculated with the FIV ISCOMs or the recombinant p24 ISCOMs developed high titres of antibodies against the core protein p24 but had no detectable antibodies against the env protein gp120 or virus neutralising antibodies. In contrast, all of the cats inoculated with the fixed, inactivated cell vaccine developed anti-env antibodies and four of five had detectable levels of neutralising antibody. However, none of the vaccinated cats were protected from infection after intraperitoneal challenge with 20 infectious units of FIV. Indeed there appeared to be enhancement of infection after vaccination as the vaccinated cats become viraemic sooner than the unvaccinated controls, and 100% of the vaccinated cats became viraemic compared with 78% of the controls. The mechanism responsible for this enhancement remains unknown.     

Prevalence of feline leukemia virus and antibodies to feline immunodeficiency virus in cats in Norway.

Serum samples from 224 Norwegian cats were analyzed for the presence of feline leukemia virus (FeLV) p27 common core antigen, and for antibodies to feline immunodeficiency virus (FIV). Ninety specimens originated from the serum bank at the central referral clinic at the Norwegian College of Veterinary Medicine, which had been collected during the years 1983-1989; 67 sera were submitted from veterinarian practitioners; while 67 sera originated from cats presented for euthanasia. The cats were classified into one "healthy" and one "sick" group. Only 2.2% of sick cats and 1.2% of healthy cats showed FeLV antigenemia, a finding which is lower than which has been reported from many other countries. The prevalence of FIV antibodies was 10.1% in sick cats and 5.9% in healthy cats. Antibodies to FIV was most prevalent in male cats (14.7%) than in female cats (2.1%), and more prevalent among domestic cats (12.0%) compared to pedigree cats (2.4%). Antibodies to FIV in the cats demonstrated increasing prevalence with increasing age. It may be concluded that FeLV causes minor problems in Norwegian cats, while FIV is present in a similar prevalence to what is reported from other countries.     

Epidemiology of feline leukaemia and feline immunodeficiency virus infections in the Czech Republic

Commercial serological sets were used for the examination of 727 cats kept in larger towns of the Czech Republic. FeLV antigen and antibodies to FIV were demonstrated in 96 (13.2%) and 42 (5.8%) of the animals, respectively. Seven (0.96%) animals were positive for both FeLV and FIV. Most of the FeLV and/or FIV positive patients were intact rambling males aged 1-4 years. Chronic gastrointestinal and respiratory diseases were found in 54.2% and 43.8% of the FeLV-positive patients, respectively. Chronic urinary tract diseases and generalized lymphadenopathy were found in 47.6% and 45.2% of the FIV-positive patients, respectively. The results of this first survey in the Czech Republic have shown prevalence values and clinical patterns similar to those reported formerly from other European countries.     

Efficacy and safety of a feline immunodeficiency virus vaccine

Fel-O-Vax FIV is an inactivated virus vaccine designed as an aid in the prevention of infection of cats, 8 weeks or older, by feline immunodeficiency virus (FIV). It contains two genetically distinct FIV strains. The efficacy of this vaccine was demonstrated in a vaccination-challenge study designed to meet various regulatory requirements for registering the vaccine. Eight-week-old kittens were vaccinated with an immunogenicity vaccine which contained minimal release levels of FIV antigens formulated with a proprietary adjuvant system. Twelve months later, all vaccinates and controls were challenged with a heterologous FIV strain. Following the vigorous challenge exposure, cats were monitored for FIV viremia. It was found that 16% of the vaccinated cats developed viremia while 90% of the controls became persistently infected with FIV, which demonstrated that the vaccine was efficacious and the protective immunity lasted for at least 12 months. The safety of the vaccine was demonstrated by a field safety trial in which only 22 mild reactions of short duration were observed following administering 2051 doses of two pre-licensing serials of Fel-O-Vax FIV to cats of various breeds, ages and vaccination histories. Thus, Fel-O-Vax FIV is safe and efficacious for the prevention of FIV infection in cats.     

Husbandry practices for cats infected with feline leukemia virus or feline immunodeficiency virus.

Cats that are persistently infected with FeLV or feline immunodeficiency virus but are not manifesting clinical signs of disease are at risk for developing a wide variety of immunosuppressive, degenerative, or neoplastic diseases. Infected cats should be isolated to prevent transmission of virus to healthy cats, and to protect infected cats from exposure to pathogens that can cause life-threatening secondary infections. Iatrogenic transmission of virus from infected cats in isolation to healthy cats may be reduced by strict adherence to handling, sanitation, and disinfection procedures. Husbandry practices that may delay the complications of infection include regular vaccination, provision of high-quality diets, reduction of stress, control of endoparasites and ectoparasites, and early and aggressive treatment of clinical signs of disease.