An update on FIV and FeLV test performance using a Bayesian statistical approach

BACKGROUND

Screening tests for feline retroviruses are thought to have high sensitivity and specificity, although previous studies that evaluated these tests have limitations. Novel statistical approaches have been developed that allow the estimation of sensitivity and specificity in situations where the true state of the disease in individual animals cannot be assured.

OBJECTIVE

The purpose of this study was to evaluate the sensitivity and specificity of a variety of retrovirus tests, including some screening tests, in a population of cats potentially infected with either feline leukemia virus (FeLV) and/or feline immunodeficiency virus (FIV) by using a Bayesian statistical approach.

METHODS

Four hundred and ninety blood samples from cats being evaluated for FIV infection were tested by 2 rapid immunomigration tests (Witness single [WS], Witness combi [WC]) and a plate-based ELISA (Petcheck) for FIV antibody, and by a newly designed real-time polymerase chain reaction (PCR) assay for FIV provirus. Four hundred and ninety-five blood samples from cats being evaluated for FeLV infection were tested by 2 rapid immunomigration tests (WS, WC) and a plate-based ELISA (Petcheck) for FeLV antigen, and by a FeLV virus isolation technique. Results were then analyzed by using a Bayesian statistical method.

RESULTS

For FIV tests, median sensitivity estimates were 0.98 for WS, 0.97 for WC, 0.98 for ELISA, and 0.92 for PCR. Median specificity estimates were 0.96 for WS, 0.96 for WC, 0.93 for ELISA, and 0.99 for PCR. For FeLV tests, median sensitivity estimates were 0.97 for WS, 0.97 for WC, 0.98 for ELISA, and 0.91 for virus isolation. Median specificity estimates were 0.96 for WS, 0.96 for WC, 0.98 for ELISA, and 0.99 for virus isolation.

CONCLUSIONS

The use of Bayesian statistical methods overcomes a variety of methodologic problems associated with diagnostic test evaluations, including the lack of a definitive reference test. The sensitivity and the specificity of all 6 evaluated screening tests was high: however, specificity estimates were slightly lower than those reported by most recent studies.

Detection of feline leukemia virus RNA in saliva from naturally infected cats and correlation of PCR results with those of current diagnostic methods

A novel diagnostic test for feline leukemia virus (FeLV) RNA in saliva from naturally infected cats is described in this study. We evaluated different diagnostic tests and compared them with the widely used enzyme-linked immunosorbent assay (ELISA) for the detection of p27 in the diagnosis of FeLV. Blood samples from 445 cats were tested for the presence of provirus by real-time PCR and plasma and saliva specimens from those cats were tested for the presence of viral RNA by real-time reverse transcription (RT)-PCR and for the presence of p27 by ELISA. In comparison to conventional ELISA, the diagnostic sensitivity and specificity of the detection of salivary FeLV RNA by real-time RT-PCR were found to be 98.1 and 99.2%, respectively. Detection of viral RNA in saliva had a positive predictive value of 94.6% and a negative predictive value of 99.7%. The kappa value was 0.96, demonstrating an almost perfect agreement between both tests. Furthermore, we confirmed previous results showing that a number of cats which tested negative for the presence of p27 in plasma were in fact positive for the presence of DNA provirus in blood specimens (5.4%). However, 96.4% of these latently infected cats did not shed viral RNA in saliva; therefore, we assume that these cats are of relatively low clinical importance at the time of testing. This study shows considerable diagnostic value of the detection of saliva FeLV RNA in naturally infected cats. This new diagnostic method has advantages over the conventional ELISA, such as less invasive sample collection and no requirement for trained personnel.

Two cases of FeLV-associated dermatoses

Two cases of feline leukaemia virus (FeLV)-associated dermatosis are described. The first cat was affected by an ulcerative dermatitis identified as a giant-cell dermatosis. The second case was a cutaneous lymphoma. In both cases, FeLV antigens and FeLV genome were demonstrated in the affected skin immunologically and with polymerase chain reaction, respectively. The first case suggests that, like other retroviruses, at least some strains of FeLV can induce syncytium formation. As FeLV antigens and genome were demonstrated in a serologically negative cat, the second case suggests that focal skin FeLV replication may occur. FeLV-associated dermatoses are rare skin conditions that may be under-diagnosed.

Shedding of feline leukemia virus RNA in saliva is a consistent feature in viremic cats

The purpose of this investigation was to characterize the shedding pattern of feline leukemia virus (FeLV) RNA in saliva, and to correlate it with the proviral load in whole blood, viral load in plasma, levels of p27 in saliva and plasma, the isolation of infectious FeLV from saliva, and the titers of FeLV-specific antibodies of the IgG and IgA isotypes. We evaluated 24 experimentally FeLV-infected cats for these parameters using real-time RT-PCR and PCR, cell culture assay and sandwich ELISA. We observed that shedding of viral RNA in saliva was a consistent feature in viremic cats. Latently FeLV-infected cats, displaying a very low proviral load, did not shed infectious virus in saliva, but occasionally shed viral RNA. Consequently, salivary shedding of FeLV RNA may not necessarily indicate a transmission potential for susceptible cats. This study also confirmed previous results from our laboratory, showing that a negative result for p27 in plasma, or for viral RNA in plasma or saliva does not exclude FeLV infection, considering that blood cells from those cats contained provirus. We also showed that FeLV RNA and DNA were stable for more than 64 days in saliva samples stored at room temperature. We conclude that the detection of FeLV RNA in saliva may be a useful indicator of viremia, and that the detection of salivary viral RNA by RT-PCR could become a reliable tool for the diagnosis of FeLV infection, which is facilitated by the low invasive method of collection of the samples.

Quantitation of feline leukaemia virus viral and proviral loads by TaqMan real-time polymerase chain reaction

Feline leukaemia virus (FeLV) infection in cats is not only of veterinary importance but also a well-acknowledged animal model for studying the pathogenesis of retroviral disease. After virus exposure, different courses and outcomes of FeLV infection may prevail; they have been associated with cellular and humoral immune responses and the FeLV proviral load in peripheral blood. We hypothesized that the plasma viral RNA load might be an additional relevant indicator for the infection outcome. To quantify these loads, a real-time reverse transcriptase (RT) polymerase chain reaction (PCR) assay was developed. The assay amplifies FeLV-A, -B, and -C as some naturally infected cats could not be identified with a FeLV-A-based assay previously. The assay was applied to determine plasma FeLV RNA loads in cats infected both naturally and experimentally with FeLV. In addition, an improved real-time PCR assay for quantitation of FeLV proviral loads is described. The assays developed were more sensitive than ELISA and virus isolation in the early phase of infection. In addition, PCR allows the identification of provirus carriers that have overcome antigenaemia. Thus, for most effective detection of FeLV exposure and characterization of the infection in a cat, PCR assays are recommended as diagnostic tools.

Insertional polymorphisms of endogenous feline leukemia viruses

The number, chromosomal distribution, and insertional polymorphisms of endogenous feline leukemia viruses (enFeLVs) were determined in four domestic cats (Burmese, Egyptian Mau, Persian, and nonbreed) using fluorescent in situ hybridization and radiation hybrid mapping. Twenty-nine distinct enFeLV loci were detected across 12 of the 18 autosomes. Each cat carried enFeLV at only 9 to 16 of the loci, and many loci were heterozygous for presence of the provirus. Thus, an average of 19 autosomal copies of enFeLV were present per cat diploid genome. Only five of the autosomal enFeLV sites were present in all four cats, and at only one autosomal locus, B4q15, was enFeLV present in both homologues of all four cats. A single enFeLV occurred in the X chromosome of the Burmese cat, while three to five enFeLV proviruses occurred in each Y chromosome. The X chromosome and nine autosomal enFeLV loci were telomeric, suggesting that ectopic recombination between nonhomologous subtelomeres may contribute to enFeLV distribution. Since endogenous FeLVs may affect the infectiousness or pathogenicity of exogenous FeLVs, genomic variation in enFeLVs represents a candidate for genetic influences on FeLV leukemogenesis in cats.

Prevalence of infectious diseases in feral cats in Northern Florida

Objectives of this study were to determine prevalence of infection in feral cats in Northern Florida with a select group of infectious organisms and to determine risk factors for infection. Blood samples or sera from 553 cats were tested with a panel of antibody, antigen or PCR assays. Male cats were at higher risk for FIV, Mycoplasma haemofelis, and M. haemominutum. Infection with either FeLV or FIV was associated with increased risk for coinfection with the other retrovirus, M. haemofelis, or M. haemominutum. Bartonella henselae had the highest prevalence and was the only organism that did not have any associated risk for coinfection with other organisms. Feral cats in this study had similar or lower prevalence rates of infections than those published for pet cats in the United States. Thus, feral cats assessed in this study appear to be of no greater risk to human beings or other cats than pet cats.

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.

Genomically intact endogenous feline leukemia viruses of recent origin

We isolated and sequenced two complete endogenous feline leukemia viruses (enFeLVs), designated enFeLV-AGTT and enFeLV-GGAG. In enFeLV-AGTT, the open reading frames are reminiscent of a functioning FeLV genome, and the 5' and 3' long terminal repeat sequences are identical. Neither endogenous provirus is genetically fixed in cats but polymorphic, with 8.9 and 15.2% prevalence for enFeLV-AGTT and enFeLV-GGAG, respectively, among a survey of domestic cats. Neither provirus was found in the genomes of related species of the Felis genus, previously shown to harbor enFeLVs. The absence of mutational divergence, polymorphic incidence in cats, and absence in related species suggest that these enFeLVs may have entered the germ line more recently than previously believed, perhaps coincident with domestication, and reopens the question of whether some enFeLVs might be replication competent.

Detection of feline coronavirus antibody, feline immunodeficiency virus antibody, and feline leukemia virus antigen in ascites from cats with effusive feline infectious peritonitis

To investigate the usefulness of ascites as a material for viral tests in cats with effusive feline infectious peritonitis (FIP), we attempted to detect anti-feline coronavirus antibody, anti-feline immunodeficiency virus antibody, and feline leukemia virus antigen in ascites from 88 cats clinically suspected with effusive FIP. In each of these three viral tests, all cats positive for serum antibody/antigen were also positive for ascitic antibody/antigen, while cats negative for serum antibody/antigen were also negative for ascitic antibody/antigen. This finding indicates that ascites is useful for these viral tests.

Multicentric T-cell lymphoma associated with feline leukemia virus infection in a captive namibian cheetah (Acinonyx jubatus)

This case report describes a multicentric lymphoma in a 4 yr old female wildborn captive cheetah (Acinonyx jubatus) in Namibia after being housed in an enclosure adjacent to a feline leukemia virus (FeLV) infected cheetah that had previously been in contact with domestic cats. The year prior to the onset of clinical signs, the wild-born cheetah was FeLV antigen negative. The cheetah subsequently developed lymphoma, was found to be infected with FeLV, and then rapidly deteriorated and died. At necropsy, the liver, spleen, lymph nodes, and multiple other organs were extensively infiltrated with neoplastic T-lymphocytes. Feline leukemia virus DNA was identified in neoplastic lymphocytes from multiple organs by polymerase chain reaction and Southern blot analysis. Although the outcome of infection in this cheetah resembles that of FeLV infections in domestic cats, the transmission across an enclosure fence was unusual and may indicate a heightened susceptibility to infection in cheetahs. Caution should be exercised in holding and translocating cheetahs where contact could be made with FeLV-infected domestic, feral, or wild felids.

Centrosome Amplification and Chromosomal Instability in Feline Lymphoma Cell Lines

To evaluate the presence of centrosome amplification and the resulting chromosomal instability in cat tumors, a newly established feline lymphoma cell line and four already established feline lymphoma cell lines were examined using immunohistochemical analysis of centrosomes. The number of chromosomes were subsequently counted by metaphase spread. Moreover, to explore whether mutational inactivation of the p53 gene or inactivation of the P53 protein caused by mdm2 gene overexpression, occurred in the feline lymphoma cell lines, mutational analysis of the feline p53 gene was carried out. The expression of feline mdm2 mRNA was evaluated by reverse transcriptase-polymerase chain reaction (RT-PCR). Centrosome amplification and chromosomal instability was observed in three out of the five feline lymphoma cell lines. Of these three feline lymphoma cell lines, one had aberrations in the P53 amino-acid sequence, whereas the others had none. There was no significant difference in the expression of mdm2 mRNA between peripheral blood mononuclear cells (PBMC) obtained from a normal cat and that of the five feline lymphoma cell lines. These findings indicate that centrosome amplification also occurs in cat tumors and is strongly correlated with chromosomal instability, suggesting that the immunostaining of centrosomes could be an alternative method for the examination of the chromosomal instability. Furthermore, this study suggests the presence of unknown mechanism that leads to the centrosome amplification in feline lymphomas.

T-cell lymphoma of the tympanic bulla in a feline leukemia virus-negative cat

This report constitutes the first description of a T-cell lymphoma of the tympanic bulla in a cat. This feline leukemia virus (FeLV)-negative cat originally presented with signs referable to middle ear disease; it deteriorated rapidly after definitive diagnosis. Lymphoma of the middle ear is extremely rare in all species.

Haematogenous spread of Sporothrix schenckii in cats with naturally acquired sporotrichosis

The recovery of Sporothrix schenckii from blood samples is rare, and the diagnosis of systemic sporotrichosis is usually made at necropsy. In this report, S schenckii was isolated from two or more internal organs of nine necropsied cats with naturally acquired sporotrichosis. Haematogenous spread was demonstrated in vivo by the isolation of S schenckii from the peripheral blood of 17 (n = 49, 34.4 per cent) cats. Feline leukaemia virus (FeLV) was not detected, and co-infection with feline immunodeficiency virus (FIV), observed in nine cases (n = 43, 20.9 per cent), apparently did not affect the isolation of S schenckii from peripheral blood or from the internal organs.

Evidence of horizontal transmission of feline leukemia virus by the cat flea ( Ctenocephalides felis)

The feline leukemia virus (FeLV) is a naturally occurring and widespread retrovirus among domestic cats. The virus is mainly transmitted horizontally through saliva, blood and other body fluids by close contact between cats. Vectors other than cats, e.g. blood-sucking parasites, have not been reported. This study tested the vector potential of the cat flea ( Ctenocephalides felis) for FeLV. In a first feeding, fleas were fed for 24 h with blood from a FeLV-infected cat with persistent viremia. FeLV could be detected in the fleas, as well as in their feces. Fleas were then divided in two populations and fed in a second feeding for 5 h or 24 h with non-infected non-viremic blood. FeLV was again detected in the fleas and their feces. In addition, the two resulting blood samples of the second feeding were subsequently tested for FeLV and both samples were positive for FeLV RNA. The cat flea transmitted the FeLV from one blood sample to another. In a third feeding, the same populations of fleas were fed again with non-infected blood for 5 h or 24 h. This time FeLV was not detected in the fleas, or in the feces or blood samples. Results show that cat fleas are potential vectors for FeLV RNA in vitro and probably also in vivo.

Detection of the integrated feline leukemia viruses in a cat lymphoid tumor cell line by fluorescence in situ hybridization

Feline leukemia virus (FeLV) is a type-C retrovirus associated with lymphoid and hematopoietic malignancies in cats. The FeLV-induced tumors are thought to be caused, at least in part, by somatically acquired insertional mutagenesis in which the integrated provirus may activate a proto-oncogene or disrupt a tumor suppressor gene. This study was undertaken to enumerate and map the acquired proviral insertions in the genome of a feline thymic lymphoma cell line (FT-1) infected with FeLV. Fluorescence in situ hybridization (FISH) combined with tyramide signal amplification was applied on the chromosome specimen of FT-1 cells and normal cat lymphocytes, with an entire FeLV-A genome used as a probe. Specific hybridization signals were detected from only the metaphases of the FT-1 cells, not from those of normal cat lymphocytes. Statistically based on the Poisson's distribution, at least six loci of chromosomal regions, A2p23-p22, B2p15-p14, B4p15-p14, D4q23-q24, E1p14-p13, and E2p13-p12, appeared to be positive for FeLV integration. Consistently, Southern blot hybridization analysis using an FeLV LTR-U3 probe specific for exogenous FeLV showed the integration of at least six FeLV proviral genomes in FT-1 cells. The cytogenetic technique employed here will provide valuable molecular tags to reveal unidentified tumor-associated genes in FeLV-associated tumor cells.

Serological survey of Toxoplasma gondii, feline immunodeficiency virus and feline leukaemia virus in urban stray cats in Belgium

Three hundred and forty-six serum samples taken between 1998 and 2000 from urban stray cats in the city of Ghent were tested for antibodies to Toxoplasma gondii and feline immunodeficiency virus (FIV), and antigens of feline leukemia virus (FeLV). Of these 346 samples, 243 (70.2 per cent) were seropositive for Tgondii. Thirty-nine cats (11.3 per cent) had antibodies against FIV and 13 (3.8 per cent) had circulating antigens of FeLV. Fewer of the female cats had FIV and heavier cats had a higher seroprevalence of FIV. Exact logistic regression showed that cats that were infected with FIV were more likely to be infected with T gondii (P = 0.04), and the cats with FIV had a higher titre of Tgondii antibodies than FIV-negative animals. However, FeLV was not associated with either T gondii or FIV.

A trap, neuter, and release program for feral cats on Prince Edward Island

A new program to address the feral cat population on Prince Edward Island was undertaken during the spring and summer of 2001. Feral cats from specific geographic areas were trapped, sedated, and tested for feline leukemia virus and feline immunodeficiency virus. Healthy cats were neutered, dewermed, vaccinated, tattooed, and released to their area of origin. A total of 185 cats and kittens were trapped and tested during a 14-week period; 158 cats and kittens as young as 6 weeks of age were neutered and released. Twenty-three adult cats were positive for feline leukemia virus, feline immunodeficiency virus, or both, and were euthanized.

Prevalence of feline leukaemia virus and antibodies to feline immunodeficiency virus and feline coronavirus in stray cats sent to an RSPCA hospital

A total of 517 stray cats at an RSPCA veterinary hospital were tested for feline leukaemia virus (FeLV), feline coronavirus (FCoV) and feline immunodeficiency virus (FIV). The prevalence of FeLV was 3.5 per cent in all the cats, 1.4 per cent in healthy cats and 6.9 per cent in sick cats. FeLV positivity was associated only with disease of non-traumatic origin. Antibodies to FCoV were present in 22.4 per cent of the cats, and their prevalence was significantly higher in cats over two years old and in feral/semiferal cats. The prevalence of antibodies to FIV was 10.4 per cent in all the cats, 4.9 per cent in healthy cats and 16.7 per cent in sick cats. The prevalence of FIV antibodies was significantly higher in entire males and neutered males than in females, in cats over two years old compared with younger cats, and in cats suffering disease of non-traumatic origin rather than in healthy cats or cats suffering only from trauma. Sex, age and health status were each independently highly associated with FIV antibodies.

Longitudinal analysis of feline leukemia virus-specific cytotoxic T lymphocytes: correlation with recovery from infection

Feline leukemia virus (FeLV) is a common naturally occurring gammaretrovirus of domestic cats that is associated with degenerative diseases of the hematopoietic system, immunodeficiency, and neoplasia. Although the majority of cats exposed to FeLV develop a transient infection and recover, a proportion of cats become persistently viremic and many subsequently develop fatal diseases. To define the dominant host immune effector mechanisms responsible for the outcome of infection, we studied the longitudinal changes in FeLV-specific cytotoxic T lymphocytes (CTLs) in a group of naïve cats following oronasal exposure to FeLV. Using (51)Cr release assays to measure ex vivo virus-specific cytotoxicity, the emerging virus-specific CTL response was correlated with modulations in viral burden as assessed by detection of infectious virus, FeLV p27 capsid antigen, and proviral DNA in the blood. High levels of circulating FeLV-specific effector CTLs appeared before virus neutralizing antibodies in cats that recovered from exposure to FeLV. In contrast, persistent viremia was associated with a silencing of virus-specific humoral and cell-mediated host immune effector mechanisms. A single transfer of between 2 x 10(7) and 1 x 10(8) autologous, antigen-activated lymphoblasts was associated with a downmodulation in viral burden in vivo. The results suggest an important role for FeLV-specific CTLs in retroviral immunity and demonstrate the potential to modulate disease outcome by the adoptive transfer of antigen-specific T cells in vivo.