The Diagnosis of Feline Leukaemia Virus (FeLV) Infection in Owned and Group-Housed Rescue Cats in Australia

Abstract

A field study was undertaken to (i) measure the prevalence of feline leukaemia virus (FeLV) exposure and FeLV infection in a cross-section of healthy Australian pet cats; and (ii) investigate the outcomes following natural FeLV exposure in two Australian rescue facilities. Group 1 (n = 440) consisted of healthy client-owned cats with outdoor access, predominantly from eastern Australia. Groups 2 (n = 38) and 3 (n = 51) consisted of a mixture of healthy and sick cats, group-housed in two separate rescue facilities in Sydney, Australia, tested following identification of index cases of FeLV infection in cats sourced from these facilities. Diagnostic testing for FeLV exposure/infection included p27 antigen testing using three different point-of-care FeLV kits and a laboratory-based ELISA, real-time polymerase chain reaction (qPCR) testing to detect FeLV proviral DNA in leukocytes, real-time reverse-transcription PCR (qRT-PCR) testing to detect FeLV RNA in plasma, and neutralising antibody (NAb) testing. Cats were classified as FeLV-uninfected (FeLV-unexposed and presumptively FeLV-abortive infections) or FeLV-infected (presumptively regressive and presumptively progressive infections). In Group 1, 370 FeLV-unexposed cats (370/440, 84%), 47 abortive infections (47/440, 11%), nine regressive infections (9/440, 2%), and two progressive infections (2/440, 0.5%) were identified, and 12 FeLV-uninfected cats (12/440, 3%) were unclassifiable as FeLV-unexposed or abortive infections due to insufficient samples available for NAb testing. In Groups 2 and 3, 31 FeLV-unexposed cats (31/89, 35%), eight abortive infections (8/89, 9%), 22 regressive infections (22/89; 25%), and 19 progressive infections (19/89; 21%) were discovered, and nine FeLV-uninfected cats (9/89; 10%) were unclassifiable due to insufficient samples available for NAb testing. One of the presumptively progressively-infected cats in Group 3 was likely a focal FeLV infection. Two other presumptively progressively-infected cats in Group 3 may have been classified as regressive infections with repeated testing, highlighting the difficulties associated with FeLV diagnosis when sampling cats at a single time point, even with results from a panel of FeLV tests. These results serve as a reminder to Australian veterinarians that the threat of FeLV to the general pet cat population remains high, thus vigilant FeLV testing, separate housing for FeLV-infected cats, and FeLV vaccination of at-risk cats is important, particularly in group-housed cats in shelters and rescue facilities, where outbreaks of FeLV infection can occur.

2020 AAFP Feline Retrovirus Testing and Management Guidelines

“…14 In a study of 440 owned pet cats in Australia, 11% were classified as having presumptively abortive infection, 2% as presumptively regressive and 0.5% as presumptively progressive. 15 This suggests that abortive infection may be the most common outcome following exposure under typical conditions. In contrast, in two populations of cats in Australia (one group of 38 cats and one group of 51 cats) in which FeLV-infected and uninfected cats were co-mingled without separating healthy from clinically ill cats, 9% were classified as having abortive infection, 25% as regressive and 21% as progressive, suggesting that resistance to infection may be compromised by intense infectious pressure, comorbidities and a stressful environment.…”

“…In contrast, in two populations of cats in Australia (one group of 38 cats and one group of 51 cats) in which FeLV-infected and uninfected cats were co-mingled without separating healthy from clinically ill cats, 9% were classified as having abortive infection, 25% as regressive and 21% as progressive, suggesting that resistance to infection may be compromised by intense infectious pressure, comorbidities and a stressful environment. 15 Viral RNA is usually detectable in plasma by real-time reverse-transcriptase polymerase chain reaction (qRT-PCR) testing within 1 week of FeLV exposure, followed by pro viral dNA detection by PCR within 2 weeks of exposure and finally by FeLV antigen detection, which usually occurs by 30 days but can be longer in some cats. 16 It is not only cats with progressive infection that undergo the early phases, but also some cats with regressive infection.…”

“…Although not common after experimental infection, abortive infection seems to be more common in the field, as cats with natural infections can show evidence of FeLV antibodies in the absence of detectable viral RNA, proviral dNA or antigen, and without having received FeLV vaccines. 7,14,15,32 Although guidelines for prevention of infection have been available for decades, there remains a need to improve adherence to testing and vaccination recommendations.…”

Feline leukaemia virus infection: A practical approach to diagnosis

“…20 Several studies have since confirmed the presence of regressively FeLV-infected cats (ie, provirus-positive, free p27 antigen-negative) in different cat populations worldwide. 16,38,62,75 Provirus PCR on blood produces positive test results sooner after FeLV exposure than p27 antigen detection ( Table 2). In experimental studies, cats were provirus-positive 1-2 weeks after FeLV exposure (vs 3-6 weeks for p27 antigen).…”

Outcome of cats referred to a specialized adoption program for feline leukemia virus-positive cats

“…Moreover, several studies have demonstrated that FeLV test results can revert from positive to negative status, potentially signaling a regressive or abortive infection. 1,[8][9][10] Even when using high-accuracy tests, the complex natural history of FeLV infection makes it difficult to secure a definitive diagnosis, which presents an added challenge to the identification and effective management of FeLV-infected shelter cats. 10,11 Regressive infections are associated with a robust immune response that may, at times, suppress viral loads below the level of detection following an initial period of transient antigenemia.…”