Diseases associated with spontaneous feline leukemia virus (FeLV) infection in cats

More than 2000 cats sent for necropsy in order to provide a diagnosis were investigated immunohistologically using paraffin sections for the presence of a persistent infection with feline leukemia virus (FeLV). The spectrum of neoplastic and non-neoplastic diseases associated significantly with FeLV infection was determined statistically. Three-quarters of the cats with persistent FeLV infections died of non-neoplastic diseases and about 23% died of tumors, nearly exclusively those of the leukemia/lymphoma disease complex. A strong association with liver degeneration, icterus and a FeLV-associated enteritis was found in addition to the known association with non-neoplastic diseases and conditions such as anemia, bacterial secondary infections and respiratory tract inflammations due to the immunosuppressive effect of FeLV, hemorrhages and feline infectious peritonitis. Surprisingly, diseases and conditions like feline infectious panleukopenia, enteritis (of other types than FeLV-associated enteritis and feline infectious panleukopenia), glomerulonephritis, uremia and hemorrhagic cystitis were not associated with persistent FeLV infection. Another unexpected finding was that most pathogenic infectious agents demonstrated in the cats were not FeLV-associated either. Thus, immunosuppression due to FeLV infection seems to make the animals susceptible to certain pathogenic infectious agents, but not to the majority.     

Tumor necrosis factor alpha levels in cats experimentally infected with feline immunodeficiency virus: effects of immunization and feline leukemia virus infection.

Tumor necrosis factor alpha (TNF alpha) levels were determined by enzyme-linked immunosorbent assay (ELISA) and by cell culture bioassay in supernatants of lipopolysaccharide-stimulated feline monocyte cultures and in cat serum samples. There was a good correlation between the results obtained by the two methods. From the fact that TNF alpha was neutralized quantitatively by antibodies to human TNF alpha in feline monocyte supernatants and in feline sera, it was concluded that feline TNF alpha immunologically cross-reacts with human TNF alpha and that the human TNF alpha ELISA can be used to quantitate feline TNF alpha. During the first 6 months after experimental feline immunodeficiency virus (FIV) infection no differences in serum TNF alpha values were observed between infected and non-infected cats. TNF alpha levels increased significantly after primary vaccination with a feline leukemia virus (FeLV) vaccine in FIV infected cats over those in the non-infected controls. During secondary immune response TNF alpha levels rose transiently for a period of a few days in both the FIV positive and the FIV negative cats. After FeLV challenge, TNF alpha levels increased in all animals challenged with virulent FeLV for a period of 3 weeks. This period corresponded to the time necessary to develop persistent FeLV viremia in the control cats. It was concluded from these experiments that in the asymptomatic phase of FIV infection no increased levels of TNF alpha are present, similar to the situation in asymptomatic HIV infected humans. Activation of monocytes/macrophages in FIV infected cats by stimuli such as vaccination or FeLV challenge readily leads to increased levels of TNF alpha.     

Isolation of feline leukemia virus from clinical specimens.

Specimens obtained from feline leukemia virus (FeLV)-positive cats were examined for infectious FeLV. Feline leukemia virus was detected by a focus-forming assay and confirmed by florescent antibody. Techniques of sample processing were evaluated and adjusted for optimum detection of FeLV. Low levels of FeLV were detected in 2 of 10 oral samples; however, the majority of these samples (17 of 27 tested) produced cytopathic effects in tissue culture which prevented Fe LV detection. Three of 24 urine samples and 1 of 20 rectal specimens were positive for FeLV. One milk sample contained high levels of FeLV.     

Inhibition of feline infectious peritonitis virus replication by recombinant human leukocyte (alpha) interferon and feline fibroblastic (beta) interferon

Replication of feline infectious peritonitis virus (FIPV) in feline cell cultures was inhibited after incubation of cells with either human recombinant leukocyte (alpha) interferon (IFN) or feline fibroblastic (beta) IFN for 18 to 24 hours before viral challenge exposure. Compared with virus control cultures, FIPV yields were reduced by ranges of 0.1 to 2.7 log10 or 2 to 5.2 log10 TCID50 in cultures treated with human alpha- or feline beta-IFN, respectively; yield reductions were IFN dose dependent. Sensitivity to the antiviral activities of IFN varied with cell type; feline embryo cells had greater FIPV yield reductions than did similarly treated feline kidney or feline lung cells. Comparison of the virus growth curves in IFN-treated and virus control cultures indicated marked reduction in intracellular and extracellular FIPV in IFN-treated cultures. Compared with virus control cultures, intracellular and extracellular infectivity in IFN-treated cultures was delayed in onset by 12 and 30 hours, respectively, and FIPV titers subsequently were reduced by 3 to 3.5 and 5 log10 TCID50, respectively. Frequently, immunofluorescent and electron microscopy of IFN-treated cells or cell culture fluids did not reveal virus; however, even in cultures without viral cytopathic changes, small amounts of virus occasionally persisted in cells.     

Feline leukemia virus detection by immunohistochemistry and polymerase chain reaction in formalin-fixed, paraffin-embedded tumor tissue from cats with lymphosarcoma.

The prevalence of feline leukemia virus (FeLV) antigen and DNA was assessed in formalin-fixed, paraffin-embedded tumor tissues from 70 cats with lymphosarcoma (LSA). Tissue sections were tested for FeLV gp70 antigen using avidinbiotin complex (ABC) immunohistochemistry (IHC); DNA was extracted and purified from the same tissue blocks for polymerase chain reaction (PCR) amplification of a 166 base pair region of the FeLV long terminal repeat (LTR). Results were related to antemortem FeLV enzyme-linked immunosorbent assay (ELISA) for serum p27 antigen, anatomic site of LSA, and patient age. Viral DNA was detected by PCR in 80% of cases and viral antigen by IHC in 57% of cases. Seventeen cases were PCR-positive and IHC-negative; one case was PCR-negative and IHC-positive. Clinical records included FeLV ELISA results for 30 of 70 cats. All 19 ELISA-positive cats were positive by PCR and IHC; of the 11 ELISA-negative cats that were negative by IHC, seven were positive by PCR. When evaluated according to anatomic site, FeLV DNA and antigen were detected less frequently in intestinal LSAs than in multicentric and mediastinal tumors. Lymphosarcoma tissues from cats < 7 yr were several fold more likely to be positive for FeLV antigen by IHC than were tumors from cats > or = 7 yr. However, there was no significant difference in PCR detection of FeLV provirus between LSAs from cats < 7 yr and those > or = 7 yr.(ABSTRACT TRUNCATED AT 250 WORDS)     

Association of feline leukemia virus with lymphosarcoma and other disorders in the cat.

Two hundred fifty Boston cats with disorders such as lymphosarcoma, myeloproliferative disease, anemia, glomerulonephritis, pregnancy abnormalities, feline infectious peritonitis, toxoplasmosis, and various bacterial infections were examined for feline leukemia virus (FeLV) by immunofluorescence. Antibody titers against feline oncornavirus-associated cell membrane antigen (FOCMA) were tested in 133 of these cats. The tests for FeLV and FOCMA antibody were also conducted among healthy cats not known to have been exposed to FeLV, as well as among healthy cats from households where FeLV was known to be present. Most of the cats with lymphosarcoma and the other aforementioned disorders were infected with FeLV and low FOCMA antibody titers. Healthy cats known to have been exposed to FeLV were often viremic, but those that remained healthy were able to develop high FOCMA antibody titers. Healthy cats without known prior exposure to FeLV were unlikely to be viremic but often had detectable FOCMA antibody titers, indicating that some exposure occurs under natural conditions in the Boston area. The association of FeLV with infections other than lymphosarcoma was assumed to be caused by the immunosuppresive effect of FeLV, thus allowing development of disease.     

Frequency and significance of feline leukemia virus infection in necropsied cats

Feline leukemia virus (FeLV) infection was diagnosed immunohistologically on paraffin-embedded tissues obtained from 1,095 necropsied cats. Significant association of FeLV infection was demonstrated by chi 2 and Fisher's tests with various conditions and diseases (ie, anemia, tumors of the leukemia/lymphoma complex, feline infectious peritonitis, bacterial infections, emaciation, FeLV-associated enteritis, lymphatic hyperplasia, and hemorrhage). Unexpected findings associated with FeLV infection were icterus, several types of hepatitis, and liver degeneration. A negative association with FeLV infection was found for most parasitic and viral infections, including feline panleukopenia. Neither positive nor negative associations were established for FeLV infection and most forms of nephritis, including severe glomerulonephritis. Feline leukemia virus-infected cats were significantly (Kruskal-Wallis test) older than were FeLV-negative cats with the same nonneoplastic FeLV-associated diseases.     

Relevance of feline calicivirus, feline immunodeficiency virus, feline leukemia virus, feline herpesvirus and Bartonella henselae in cats with chronic gingivostomatitis

Despite its common occurrence, the aetiology of chronic gingivostomatitis in cats remains uncertain. Aetiology is likely multifactorial, and several infectious agents may be associated with chronic gingivostomatitis. The purpose of this study was to investigate the prevalence of feline calicivirus (FCV), feline immunodeficiency virus (FIV), feline leukemia virus (FeLV), feline herpesvirus (FHV), and Bartonella henselae (B. henselae) in cats with chronic gingivostomatitis and in an age-matched control group. In addition, other factors, e. g., environmental conditions were investigated. In 52 cats with chronic gingivostomatitis and 50 healthy age-matched control cats, the presence of FCV ribonucleic acid (RNA), and FHV deoxyribonucleic acid (DNA) (polymerase chain reaction [PCR] from oropharyngeal swabs), and B. henselae DNA (PCR from oropharyngeal swabs and blood), as well as FeLV antigen (serum), and antibodies against FCV, B. henselae, and FIV (serum) were examined. FCV RNA was significantly more common in cats with chronic gingivostomatitis (53.8%, p < 0.001) than in controls (14.0%); a significant difference was also found in the prevalence of antibodies to FCV between the cats with chronic gingivostomatitis (78.8%, p = 0.023) and controls (58.0%). Of the other infectious agents investigated, there was no significant difference in the prevalence between the cats with chronic gingivostomatitis and the controls. The results of this study allow the conclusion that FCV, but no other infectious agents, is commonly associated with chronic gingivostomatitis in cats.     

Long-term impact on a closed household of pet cats of natural infection with feline coronavirus, feline leukaemia virus and feline immunodeficiency virus

A closed household of 26 cats in which feline coronavirus (FCoV), feline leukaemia virus (FeLV) and feline immunodeficiency virus (FIV) were endemic was observed for 10 years. Each cat was seropositive for FCoV on at least one occasion and the infection was maintained by reinfection. After 10 years, three of six surviving cats were still seropositive. Only one cat, which was also infected with FIV, developed feline infectious peritonitis (FIP). Rising anti-FCoV antibody titres did not indicate that the cat would develop FIP. The FeLV infection was self-limiting because all seven of the initially viraemic cats died within five years and the remainder were immune. However, FeLV had the greatest impact on mortality. Nine cats were initially FIV-positive and six more cats became infected during the course of the study, without evidence of having been bitten. The FIV infection did not adversely affect the cats' life expectancy.     

Infections with feline leukaemia virus detected upon post-mortem examination

Cats submitted for post-mortem examination have been studied for persistent FeLV infection using immune histological methods. Persistent FeLV infection turned out to cause the most frequent lethal infectious disease of the cat. In the post-mortem material, 16 per cent of the cats were found positive whereas 3 per cent can be assumed for the normal feline population. FeLV-positive animals die from FeLV-associated non-tumourous conditions in 75 per cent of the cases rather than from leucosis. The most important non-tumourous conditions are anaemia, feline infectious peritonitis, inflammation of the respiratory tract and liver degeneration. Important differences have been found between the various forms of leucosis which can also be distinguished in the small animal practice concerning their association with FeLV infection. Variations between 20 and 90 per cent have been found.     

Antiretroviral efficacy of a 98% solution of glycerol or ethylene oxide for inactivation of feline leukemia virus in bone

OBJECTIVE: To determine whether infectious retrovirus was inactivated in bones from FeLV-infected cats after ethylene oxide (ETO) sterilization or preservation in a 98% solution of glycerol in an in vitro cell culture system. SAMPLE POPULATION: Metatarsal bones obtained from 5 FeLV-infected cats and cultured with feline fibroblast cells. PROCEDURE: Metatarsal bones were treated with 100% ETO, a 98% solution of glycerol, or left untreated. Twenty-five flasks of feline fibroblast cells were assigned to 5 groups: negative control, positive control, ETO-treated bone, glycerol-treated bone, and untreated bone with 5 replicates/group for 4 passages. Media and cell samples were harvested from every flask at each passage to measure FeLV p27 antigen and the number of copies of provirus per 100 ng of DNA, respectively. RESULTS: All negative control and ETO-treated group replicates were negative for FeLV p27 antigen and provirus throughout the study. All positive control group replicates were positive for FeLV p27 antigen and provirus at passages 1 to 4. Untreated bone group replicates were positive for FeLV p27 antigen at passages 3 and 4 and provirus beginning at passage 2. Glycerol-treated group replicates had delayed cell replication and were negative for FeLV p27 antigen and provirus at passages 1 to 4 and 2 to 4, respectively. CONCLUSIONS AND CLINICAL RELEVANCE: Ethylene oxide sterilization of bone from FeLV-infected cats appeared to abrogate transmission of infectious retrovirus and effectively sterilized bone allografts. Impact for Human Medicine-Additional studies to confirm effectiveness of ETO treatment of allograft tissues for prevention of pathogen transmission via transplantation are warranted.     

Clonality analysis of various hematopoietic disorders in cats naturally infected with feline leukemia virus

The clonality analysis of the bone marrow cells was carried out by detecting the integrated proviruses of feline leukemia virus (FeLV) to understand the pathogenesis of FeLV-associated hematopoietic disorders in cats. Bone marrow cells from 4 cases with acute myeloid leukemia (AML), 9 cases with myelodysplastic syndromes (MDS), 2 cases with pure red cell aplasia (PRCA) and 3 healthy carriers infected with FeLV were subjected to Southern blot analyses using an exogenous FeLV probe. Clonal hematopoiesis was found in all the cases with AML and in 6 of the 9 cases with MDS, but not in the cases with both PRCA and healthy carriers infected with FeLV. In the 2 cases with MDS, it was thought that the same clones of the hematopoietic cells might proliferate before and after the progression of the disease irrespective of the changes of the hematological diagnoses by cytological examination. This study indicates that MDS in cats is a disease manifestation as a result of clonal proliferation of hematopoietic cells and can be recognized as a pre-leukemic state of AML.     

In vitro selective suppression of feline myeloid colony formation is attributable to molecularly cloned strain of feline leukemia virus with unique long terminal repeat

Molecularly cloned feline leukemia virus (FeLV)-clone 33 (C-33), derived from a cat with acute myelocytic leukemia (AML), was examined to assess its relation to the pathogenesis of AML and myelodysplastic syndrome (MDS). To evaluate in vitro pathogenicity of FeLV C-33, bone marrow colony-forming assay was performed on marrow cells infected with FeLV C-33 or an FeLV subgroup A strain (61E, a molecularly cloned strain with minimal pathogenicity). The myeloid colony-forming activity of feline bone marrow mononuclear cells infected with FeLV C-33 was significantly lower than that of cells infected with 61E. This suggests that FeLV C-33 has myeloid lineage-specific pathogenicity for cats, and that FeLV C-33 infection is useful as an experimental model for investigating pathogenesis of MDS and AML.     

Therapeutic effects of recombinant feline interferon-omega on feline leukemia virus (FeLV)-infected and FeLV/feline immunodeficiency virus (FIV)-coinfected symptomatic cats

The clinical efficacy of a recombinant feline interferon, rFeIFN-omega, was evaluated for the treatment of cats presented with clinical signs associated with feline leukemia virus (FeLV) infection and FeLV/feline immunodeficiency virus (FIV) coinfection in the field. In this multicentric, double-blind, placebo-controlled trial, 81 cats meeting the inclusion criteria were randomly placed into 2 groups and treated subcutaneously with rFelFN-omega (1 million [M]U/kg per day) or placebo once daily for 5 consecutive days in 3 series (day 0, 14, 60). The cats were monitored for up to 1 year for clinical signs and mortality. During the initial 4-month period, interferon (IFN)-treated cats (n = 39) had significantly reduced clinical scores compared with placebo (n = 42), with all cats having received concomitant supportive therapies. Compared with the control, the IFN-treated group showed significantly lower rates of mortality: 39% versus 59% (1.7-fold higher risk of death for controls) at the 9-month time point and 47% versus 59% (1.4-fold higher risk of death for controls) at the 12-month time point. The IFN treatment was associated with minor but consistent improvement in abnormal hematologic parameters (red blood cell count, packed cell volume, and white blood cell count), apparently underlying the positive effects of IFN on clinical parameters. These data demonstrate that rFeIFN-omega initially has statistically significant therapeutic effects on clinical signs and later on survival of cats with clinical signs associated with FeLV infection and FeLV/FIV coinfection.     

Modifications of the immunofluorescence assay for feline leukemia virus group-specific antigens.

Observations and minor modifications are presented concerning the immunofluorescence assay for feline leukemia virus (FeLV) group-specific antigens (GSA) in blood cells of cats. Data are given regarding absorption of goat FeLV GSA antiserum in vivo in cats, absorption of the antiserum in vitro with feline blood cells, and the comparative efficacy of various chemical fixatives in preservation of FeLV GSA for immunofluorescent staining. The best results were obtained with in vitro absorption of antiserum and methanol fixation of FeLV GSA in blood smears.     

Plasma electrophoretogram in feline immunodeficiency virus (FIV) and/or feline leukaemia virus (FeLV) infections

The electrophoretogram of 89 cats, including those infected by feline immunodeficiency virus (FIV+), feline leukaemia virus (FeLV+) and non-infected, showed statistically significant differences in several of the fractions. FIV+ cats had very high protein values (mean, 8.10 g/dl), mostly because of hypergammaglobulinemia (mean, 2.81 g/dl) as compared with non-infected animals and FeLV+. In addition, in these FIV+ animals, the albumin/globulins ratio (A/G) was very low (mean, 0.72). Statistically significant differences in A/G and alpha2-globulin fraction were observed in FeLV+ group (A/G mean, 0.88 +/- 0.08; alpha2-globulin, mean, 0.84 +/- 0.07 g/dl) when compared with non-infected group (A/G mean, 1.06 +/- 0.08; alpha2-globulin mean, 0.68 +/- 0.04 g/dl). The alpha1-globulin fraction was higher in double infected animals (FIV and FeLV positive, F-F) (3.55 g/dl), than in FeLV+ or FIV+ cats (3.10 and 3.07 g/dl respectively), but no statistical conclusions may be drawn from this fact because of the low number of F-F animals. This technique may help to assess the initial clinical status of retrovirus-infected cats, and the clinical course of these chronic diseases, specifically during and after suitable therapy.     

Incidence of persistent viraemia and latent feline leukaemia virus infection in cats with lymphoma

In the past, feline leukaemia virus (FeLV) infection, and also latent FeLV infection, were commonly associated with lymphoma and leukaemia. In this study, the prevalence of FeLV provirus in tumour tissue and bone marrow in FeLV antigen-negative cats with these tumours was assessed. Seventy-seven diseased cats were surveyed (61 antigen-negative, 16 antigen-positive). Blood, bone marrow, and tumour samples were investigated by two polymerase chain reaction (PCR) assays detecting deoxyribonucleic acid (DNA) sequences of the long terminal repeats (LTR) and the envelope (env) region of the FeLV genome. Immunohistochemistry (IHC) was performed in bone marrow and tumour tissue. None of the antigen-negative cats with lymphoma was detectably infected with latent FeLV. The prevalence of FeLV viraemia in cats with lymphoma was 20.8%. This suggests that causes other than FeLV play a role in tumorigenesis, and that latent FeLV infection is unlikely to be responsible for most feline lymphomas and leukaemias.     

The use of immunohistochemistry and the polymerase chain reaction for detection of feline leukemia virus and feline sarcoma virus in six cases of feline ocular sarcoma

Ocular sarcoma was diagnosed by light microscopic examination in enucleated globes ( n  = 4), orbital tissue biopsy ( n  = 1) and ocular evisceration contents ( n  = 1) from six cats. To determine if feline leukemia virus (FeLV) or a replication-defective FeLV, feline sarcoma virus (FeSV), was present in these ocular sarcomas, immunohistochemistry (IHC) and polymerase chain reaction (PCR) for FeLV were utilized. Immunohistochemical staining for FeLV glycoprotein 70 (gp70) was performed on all six formalin-fixed, paraffin-embedded tumors using an avidin–biotin complex technique. DNA was extracted from each specimen and a 166 bp region of the FeLV long-terminal repeat (LTR) was amplified by PCR. All tumors were composed primarily of spindle cells; two neoplasms had PAS-positive basement membrane enveloping areas of spindle cells. All tumors involved the uvea and five of six tumors showed transcleral extension, one of which invaded the optic nerve. Immunohistochemical staining for FeLV gp 70 was negative. PCR to amplify a portion of the FeLV LTR was negative. Based on these findings of these limited number of cases, FeLV/FeSV may not play a role in the tumorigenesis of feline ocular sarcomas. However, additional tumors representing all morphological subtypes should be investigated for the presence of viral antigen and DNA. It is important to determine the etiology and pathogenesis of these malignant ocular sarcomas. If the cell of origin and pathogenesis involve ocular and lenticular injury, and FeLV/FeSV is not present, then the clinical management of cases of feline ocular trauma, uveitis and glaucoma may prevent the development of this tumor.