Feline leukemia virus lifetime study: whole blood samples increase detection of low positive cats with extended long-term survival

Abstract

Objectives

Thousands of cats in the USA are newly diagnosed with feline leukemia virus (FeLV) each year, and known FeLV-infected cats are increasingly adopted from shelters. This study investigated optimal sample types to identify FeLV-infected cats and predictors of long-term survival in a cohort of FeLV-positive cats followed for up to 8 years after diagnosis.

Methods

Previously, 127 FeLV p27 antigen-positive cats were enrolled in a prospective study. Whole blood, plasma and serum were collected at enrollment and monthly for 6 months. All sample types were tested on SNAP FIV/FeLV Combo Test (SNAP) monthly, and results from microtiter plate ELISA (PetChek) for p27 antigen and a quantitative RT-PCR (qPCR) for proviral DNA were used for confirmation and classification of infection status (high positive, low positive or cryptic/negative). After the 6-month testing phase, cats entered a lifetime survival monitoring phase. Owner-reported status in the current study extended previous survival results by 4 years.

Results

Testing anticoagulated whole blood on SNAP at enrollment identified five and nine more FeLV-infected low positive cats (n = 29) than plasma or serum, respectively. Although some low positive (n = 11) cats demonstrated variable test results on SNAP with plasma and serum, others (n = 17) were SNAP positive with all three sample types and classified as low positive based on PetChek and qPCR results. After an additional 4 years of monitoring, low positive cats had not reached a median survival, with 19/29 (66%) cats still alive compared with 2/90 (2.2%) high positive cats.

Conclusions and relevance

Anticoagulated whole blood on SNAP was a sensitive indicator of FeLV infection relative to plasma and serum and therefore should be the preferred diagnostic sample for FeLV antigen testing. Combining the results of whole blood antigen testing, PetChek and qPCR identified cats as high positive, low positive or cryptic/negative, with high positive cats having higher risk for early mortality. Use of these diagnostic tools facilitates the management of FeLV as a chronic condition.

Keywords

Feline leukemia virus PCR ELISA antigen provirus diagnostic test whole blood

Introduction

Shelter medicine programs have played a leading role in demonstrating the adoptability of cats with either feline immunodeficiency virus (FIV) or feline leukemia virus (FeLV).^1,2 Adoption of FIV-positive cats has been popular for many years, owing to their extended duration of asymptomatic infection and minimal risk of transmission to other cohabitating cats.³ More recently, adoption of FeLV-infected cats has increased, despite their higher risk of illness and shortened lifespan.² In addition to purposeful adoption of FeLV-positive cats, newly acquired pets or currently owned cats may be diagnosed with FeLV at routine health screening or testing due to illness or FeLV exposure. Cats with FeLV infections have been shown to survive longer if homed alone or with few other cats rather than in large multi-cat households or shelter sanctuaries.⁴ This is presumably due to reduced stress and/or exposure to co-pathogens, but the course of an FeLV infection is also an important factor.⁴ Cats with progressive infections have extensive viral replication and poor immune control of the virus. They are more likely to have shorter survival times and be at risk of FeLV-associated illnesses within a few years of acquiring the infection.^5,6 In contrast, cats with low to undetectable FeLV p27 antigen levels and/or low to undetectable proviral DNA loads in circulation typically represent regressive or focal clinical courses of FeLV infections.^5,6 In regressive and focal infections, the immune response controls but does not eliminate the virus because the retrovirus promotes the integration of viral genes into the genome of the cat’s infected white blood cells or bone marrow hematopoietic precursor cells.⁷ Based on experimental infection studies, cats with regressive and focal infections typically demonstrate longer survival than cats with progressive infections.^8,9 Therefore, knowing a cat’s retroviral status is an essential component of their medical history and is of growing importance because more cats with retroviral infections are being adopted instead of being euthanized.¹⁰

Our work began in 2016 through a collaborative study with the Austin Pets Alive! (APA!) animal shelter and their FeLV rehoming program.^2,11 We investigated which types of diagnostic tests were most useful for FeLV screening and which combination of tests could provide an indication of long-term survival. A total of 127 FeLV-positive cats, initially identified by screening using SNAP FIV/FeLV Combo Test (SNAP; IDEXX Laboratories) at the shelter, were enrolled and had four different FeLV diagnostic tests performed monthly over 6 months.¹¹ Using the results from a quantitative FeLV p27 microtiter plate ELISA (PetChek) and a quantitative FeLV PCR (qPCR) for proviral DNA loads at enrollment (month 0), FeLV-infected cats were classified as either ‘high positive’ or ‘low positive’. A significant difference in survival was evident between the high positive and low positive groups when the cats were monitored for 4 years.¹¹ In the present paper, we evaluated the topic of sample type used for initial FeLV screening by comparing whole blood, plasma and serum at the time of original diagnosis.^6,12,13 The purpose of the study was to determine optimal sample types for identifying FeLV-infected cats and predictors of long-term survival in the original cohort of 127 FeLV-positive cats, now tracked for up to 8 years since diagnosis.

Materials and methods

This study was conducted in collaboration with the APA! animal shelter (Austin, TX). The population, study design, testing methods and determination of assay cutoffs have been described previously.¹¹ This study was approved by the Institutional Animal Care and Use Committee of the University of Florida, Maddie’s Shelter Medicine Program (protocol 201909584, 10 July 2016).

Cats

Upon admission to the APA! shelter, each cat was screened for FeLV p27 antigen using EDTA-anticoagulated whole blood with SNAP (SNAP FIV/FeLV Combo Test; IDEXX Laboratories). Cats testing positive were considered eligible for enrollment if they were at least 8 weeks of age, weighed at least 2 lbs (0.9 kg), and were sufficiently socialized for safe and low-stress handling. Enrollment typically occurred within 2 weeks of the positive screening test result and was completed for all cats over the course of 1 year. Cats participating in the study were placed in foster homes and formally adopted after completing the initial 6-month testing phase of the study. Cats were removed from the study if their temperament did not permit regular low-stress examinations or blood collection.

Blood sample collection

At the time of study enrollment (month 0) and monthly for the following 6 months (months 1–6), 3 ml of EDTA-anticoagulated whole blood and 1 ml of whole blood for processing to serum were collected.

Test methods

SNAP EDTA-anticoagulated whole blood and serum samples were stored refrigerated at the shelter until shipped to IDEXX (IDEXX R&D). Within 7 days of collection (median 5), an aliquot of anticoagulated whole blood was spun to plasma, and the whole blood, plasma and serum were tested on SNAP for the presence of FeLV p27 antigen and antibodies to FIV according to the manufacturer’s instructions. Laboratory personnel visually interpreted test results without knowledge of individual patient information and prior test results.

qPCR An aliquot of anticoagulated whole blood was submitted to the reference laboratory for quantitative, FeLV real-time PCR (FeLV Quant RealPCR Test; IDEXX Laboratories) to detect FeLV proviral DNA.¹⁴ If the qPCR was positive and the number of copies of FeLV proviral DNA was 4 × 10⁵ copies/ml or below, the load was considered low; if it was above 4 × 10⁵ copies/ml, the load was considered high.¹¹

PetChek The remaining plasma recovered from the anticoagulated whole blood was stored frozen at −20°C and reserved for batch testing on a microtiter plate ELISA for the detection of FeLV p27 antigen (FeLV Antigen by ELISA; IDEXX Laboratories). This ELISA is first performed as a screening assay to identify positive samples. Once identified, positive samples are tested using a confirmatory protocol with antigen neutralization to ensure high specificity of the ELISA results. The concentration of FeLV p27 antigen was quantified using a research algorithm on the confirmatory assay results.^14,15 If concentrations of soluble p27 antigen were 37.5 ng/ml or below, samples were designated as low; concentrations above 37.5 ng/ml were designated as high.¹¹

Classification

The classification of a cat’s status was determined by the qPCR result for FeLV proviral DNA in combination with the qualitative PetChek FeLV p27 antigen results at the time of study enrollment (month 0) as previously reported (Table 1).¹¹ Our earlier study demonstrated a strong positive correlation between the quantitative results for FeLV p27 antigen concentration and proviral DNA loads, and in the few instances where the two results were discordant (one high level and one low level), the decision was made to use the commercially available qPCR result for classification. Therefore, the qualitative (positive or negative) PetChek results were sufficient for classification. The resulting classification scheme specified the following: (1) high positive: positive PetChek result and high levels of proviral DNA (high qPCR result); (2) low positive: low level of proviral DNA present (low qPCR result), or no proviral DNA detected (negative qPCR result) but PetChek positive; and (3) cryptic/negative: positive FeLV p27 antigen test at shelter admission screening but negative on the PetChek and qPCR tests at the time of enrollment (month 0). This allowed for the possibility of FeLV infection with p27 concentration and proviral load below the level of detection on the sample collection day vs lack of infection caused by a false-positive result on the screening test.

Table 1

Cats that were feline leukemia virus (FeLV) p27 antigen positive on their initial shelter admission screening test with SNAP were classified as high positive, low positive or cryptic/negative when enrolled in the study (month 0) using qualitative results from the PetChek FeLV p27 antigen ELISA and the FeLV quantitative PCR (qPCR) for proviral DNA loads

Classification	SNAP at screening*	PetChek at month 0^†	qPCR at month 0^‡
High positive	Positive	Positive	High level positive
Low positive	Positive	Positive	Low level positive
		Positive	Negative
		Negative	Low level positive
Cryptic/negative	Positive	Negative	Negative

SNAP with anticoagulated whole blood

†

PetChek qualitative results with plasma

‡

Low level positive ⩽4 × 10⁵ copies/ml; high level positive >4 ×10⁵ copies/ml

Survival

After the 6-month testing phase, cats entered a lifetime survival monitoring phase in which cat owners were contacted by study personnel, either by email, telephone, text or mail, every 3 months through September 2020 (3–4 years after enrollment), and then every 6 months through December 2024 (7–8 years after enrollment) and asked if they still had their cats. Owners who no longer had their cats were asked to provide a date of death or loss, specify whether the cat died or was euthanized and speculate on cause of death. Appreciation for ongoing participation and incentivization to update survival information were provided by US$25 gift cards and cat toys valued at less than US$10. Survival time was calculated as the difference between the date of death or last contact and date of enrollment in the study.

Data visualization and statistics

Data visualizations were created with Python 3.12.5 using the package Matplotlib v3.8.4 (https://zenodo.org/records/10916799). Wilson/Brown 95% confidence intervals (CIs) were determined for proportion of positive SNAP results by sample type for low positive cats. A Kaplan–Meier analysis with a log-rank test and a Kruskal–Wallis test with multiple comparisons were used to evaluate differences in survival times between high positive, low positive and cryptic/negative classified cats. Statistical analyses were performed using GraphPad Prism 10 for Mac OS version 10.4.2 (534, 29 March 2025). Statistical significance was assessed at P <0.05. Cats lost to follow-up were censored at the time of last contact with adopters.

Results

Enrollment

A total of 127 cats were originally identified as FeLV positive on the SNAP test when screened on admission to the shelter.¹¹ At the time of enrollment (month 0), 121/127 cats were determined to be FeLV p27 antigen positive on SNAP using EDTA-anticoagulated whole blood, 113/121 samples were positive with plasma and 109/121 were positive with serum (Figure 1). Six samples were only positive for FeLV on SNAP using whole blood with 3/6 whole blood-only positive samples confirmed by qPCR (n = 1) or PetChek (n = 2). In addition, one cat that tested positive for FeLV p27 antigen with anticoagulated whole blood on SNAP at shelter admission screening, but negative at enrollment (month 0), tested positive on the FeLV qPCR with a low proviral load at month 0 (cat APA016).

Figure 1

Disposition of 127 feline leukemia virus (FeLV)-positive cats by SNAP test results and sample types. The 127 FeLV-positive cats were initially identified based on screening with SNAP at the time of shelter admission using EDTA-anticoagulated whole blood. Within 2 weeks of screening, cats were enrolled in the study (month 0) and had SNAP FeLV testing repeated with anticoagulated whole blood, plasma and serum. qPCR = quantitative PCR

All 127 FeLV-positive cats that were enrolled in the study had their infection status classified according to their PetChek p27 antigen results and qPCR proviral DNA loads at month 0 (classification results, as previously reported, are shown in the supplementary material).¹¹ In total, 90 (71%) cats were determined to be high positive, 29 (23%) cats were determined to be low positive and eight (6%) cats were classified as FeLV status cryptic/negative.¹⁶ Testing anticoagulated whole blood on SNAP at month 0 identified five and nine more FeLV-infected low positive cats than did plasma or serum, respectively. Whole blood detected 28/29 (97%, 95% CI 83–99), plasma detected 23/29 (79%, 95% CI 62–90) and serum detected 19/29 (66%, 95% CI 47–80), suggesting that whole blood was significantly more sensitive than serum.

High positive cats

High positive cats demonstrated largely consistent test results across methods over the 6 months of the study, including consistently positive SNAP results using all three sample types (Figure 2). Only 7/90 and 1/90 high positive cats had one and two low proviral DNA load results on subsequent monthly testing, respectively, and none ever tested negative by qPCR.

Figure 2

Test results for SNAP, PetChek and quantitative PCR (qPCR) are depicted for two high positive cats. High positive cats are defined as being SNAP p27 antigen positive at the time of screening and qPCR positive with high proviral loads at the time of enrollment (month 0). High positive cats demonstrated consistently positive test results, including high levels of p27 antigen on PetChek and proviral DNA on qPCR at nearly all test points. No sample type differences on SNAP were observed for high positive cats. The results key is shown in the white segment with SNAP results for whole blood (WB), plasma and serum shown from left to right in the outer ring (light gray) for each month of testing proceeding clockwise. Results are shown as either positive (+) or negative (–). The middle ring of the diagram (medium gray) depicts the results for PetChek, which are shown as either high (H), low (L) or negative (–). The inner ring (dark gray) contains the qPCR results, which are shown as either high (H), low (L) or negative (−)

Low positive cats

Within the low positive group, 17/29 (59%) cats had concordant positive results on SNAP for anticoagulated whole blood, plasma and serum at enrollment (APA047 and APA127 in Figure 3), very similar to what was observed for high positive cats. Low positive cats with concordant sample type results on SNAP tested positive on PetChek (n = 7), FeLV qPCR (n = 1) or both (n = 9), and this additional testing was necessary to distinguish them from high positive cats. Unlike high positive cats, some of these low positive cats demonstrated sample type variability with their SNAP results during the following 6 months (APA127 in Figure 3 and supplementary material). Of the remaining 12/29 (41%) low positive cats, 11 had discordant results on SNAP by sample type at enrollment (month 0) (APA084 in Figure 3). Nine of these cats were confirmed as positive by PetChek, one by the FeLV qPCR and one using both methods. The final low positive cat only tested positive by SNAP at screening and was only qPCR positive at enrollment (APA016 in Figure 3).

Figure 3

Test results for SNAP, PetChek and quantitative PCR (qPCR) are depicted for four low positive cats. Low positive cats are defined as being SNAP p27 antigen positive at the time of screening and either qPCR positive with low proviral loads or qPCR negative and PetChek p27 antigen positive at the time of enrollment (month 0). Low positive cats demonstrated consistent or variable test results for PetChek and qPCR across time points. Low positive cats may have concordant positive PetChek and qPCR results or discordant results. They may have concordant positive results for whole blood (WB), plasma and serum on SNAP or demonstrate sample type differences on SNAP. The results key is shown in the white segment with SNAP results for WB, plasma and serum shown from left to right in the outer ring (light gray) for each month of testing proceeding clockwise. Results are shown as either positive (+) or negative (–). The middle ring of the diagram (medium gray) depicts the results for PetChek, which are shown as either high (H), low (L) or negative (–). The inner ring (dark gray) contains the qPCR results, which are shown as either high (H), low (L) or negative (–)

Cryptic/negative cats

Of the eight cats classified as cryptic/negative, three cats tested positive on SNAP with only anticoagulated whole blood at enrollment, and none of them were positive by PetChek, qPCR or FeLV immunofluorescent assay (IFA) at month 0 despite having initial FeLV-positive SNAP results at screening (Figure 4). Five of these cats had additional anticoagulated whole blood positive results on subsequent SNAP tests over the next 6 months of monitoring. All cats in the cryptic/negative group had at least one result on the PetChek research ELISA in which FeLV p27 antigen was detectable but below the limit of quantitation. These results were reported as negative based on the diagnostic criteria for the assay (data not shown).¹⁴ One cat had a low positive p27 antigen result on PetChek at month 1, and none were ever positive for proviral DNA by qPCR.

Figure 4

Test results for SNAP, PetChek and quantitative PCR (qPCR) are depicted for two cryptic/negative cats. Cryptic/negative cats are defined as being SNAP p27 antigen positive at the time of screening and negative for proviral DNA by qPCR and negative for p27 antigen by PetChek at the time of enrollment (month 0). Some cryptic/negative cats demonstrated variable SNAP test results for p27 antigen with whole blood (WB) during the 6-month testing phase. The results key is shown in the white segment with SNAP results for WB, plasma and serum shown from left to right in the outer ring (light gray) for each month of testing proceeding clockwise. Results are shown as either positive (+) or negative (–). The middle ring of the diagram (medium gray) depicts the results for PetChek, which are shown as either high (H), low (L) or negative (–). The inner ring (dark gray) contains the qPCR results, which are shown as either high (H), low (L) or negative (–)

Survival

Survival, as assessed by contacting the owners every 3–6 months, was determined for the 127 cats for up to 8 years after diagnosis (an additional 4 years from the previous report). In total, 93 (73%) cats were deceased and 28 (22%) were still alive as of December 2024. The high positive and low positive groups each had three cats that were lost to follow-up because they went missing (three cats) or loss of contact with the owner (three cats). Overall median survival for the entire FeLV-positive population was 2.8 years. A significant difference (P <0.0001) remained between the survival times of high positive, low positive and cryptic/negative cats (Figure 5). Cats in the high positive group had a median survival of 1.4 years, with two (2%) cats still alive at the time of this report. In contrast, the low positive and cryptic/negative cats had not yet reached a median survival, with 19/29 (66%) and 7/8 (88%) cats still alive from these two groups, respectively. Despite the shorter median survival time of high positive cats, 26/90 (29%) cats were still alive at 3 years and 13/90 (14%) survived 5 years or longer. The distribution of survival times by group more clearly illustrates the outliers in the respective groups (Figure 6). Significant differences were noted between the median survival of high positive cats and either low positive (P <0.0001) or cryptic/negative (P = 0.0002) cats. No significant difference was found between the median survival of low positive and cryptic/negative cats (P >0.9999).

Figure 5

Kaplan–Meier survival curves for the 127 feline leukemia virus-positive cats categorized as high positive (n = 90; red), low positive (n = 29; blue) and cryptic/negative (n = 8; green). Each cat’s status was determined based on the test results obtained at the time of enrollment (month 0). Survival probability was significantly lower for high positive cats than for low positive or cryptic/negative cats (P <0.0001)

Figure 6

Distribution of survival times by high positive, low positive and cryptic/negative groups for the 127 feline leukemia virus-positive cats. Horizontal lines depict the median for each group. Median survival was significantly lower for high positive cats compared with low positive or cryptic/negative cats

Discussion

This study demonstrated the utility and sensitivity of using anticoagulated whole blood as a sample type for screening on a point-of-care FeLV p27 antigen test, when compared with plasma or serum and performed at a single point in time. A positive result followed by a quantitative FeLV real-time PCR for proviral DNA was sufficient to classify the infection status of a cat as either high positive or low positive when the PCR result was positive, and when the PCR result was negative, the microtiter plate ELISA for FeLV p27 antigen detection helped to identify additional low positive cats.¹¹ Although high positive cats demonstrated consistent positive results between anticoagulated whole blood, plasma and serum, this alone was not sufficient to classify a cat’s FeLV infection status since 17/29 (59%) low positive cats also had this testing pattern at enrollment. In our study, only low positive and cryptic/negative cats demonstrated discordant results between sample types on SNAP during the 6-month testing phase.

Our findings from this prospective study of adopted FeLV-infected cats revealed an overall median survival of 2.8 years, with high positive cats having the shortest median survival of 1.4 years and only 2/90 cats still known to be alive at more than 7 years. The low positive cats had not yet reached a median survival, with 19/29 cats still alive. Because the cats were admitted to the shelter with previously acquired FeLV infections of unknown duration, the survival times represent intervals since study enrollment, not total survival times since initial infection. Reported median survival times for naturally acquired FeLV infections in a study of over 800 FeLV-positive cats and 7000 age- and sex-matched uninfected controls were 2.4 years and 6.3 years, respectively.¹⁷

The enrolled FeLV-positive cats in this study infrequently demonstrated a change between high positive and low positive infection status over the 6 months of testing. High positive cats consistently tested high while low positive cats either had consistently low results or sometimes had negative results, suggesting that proviral DNA or p27 antigen fell below the level of detection at some time points. The infrequent change between high positive and low positive may be a reason why it was possible to use a single point-in-time test panel in our study to predict survival. Previous reports from vaccine efficacy studies that used molecular techniques to determine a cat’s FeLV infection status found that approximately 8 weeks after a challenge infection, a cat’s infection status was established and maintained for the following 2.5–3 years.^18
–20 Lower quantities of viral RNA and proviral DNA loads at 8 weeks after the challenge were associated with longer survival.¹⁹ Because lower quantities of proviral DNA and p27 antigen may fall below the level of detection in these assays, low positive cats may be missed with a single testing event. Retesting a healthy cat that is new to a household could be considered at a future date,⁵ but our study did not identify a specific interval that is likely to detect fluctuations in low positive status.

Cats that tested positive with anticoagulated whole blood on the point-of-care FeLV p27 antigen test, but did not test positive by microtiter plate ELISA or PCR, were considered cryptic/negative because proving that a cat is free of this retrovirus infection once a positive screening result is obtained can be challenging, and false-positive results are possible.⁸ The cryptic designation was borrowed from the human immunodeficiency virus (HIV) literature as it refers to the need for ultrasensitive techniques to detect HIV p24 antigen or viral RNA in confirmed HIV-infected individuals.¹⁶ More extensive use of antiviral therapies in individuals with HIV has increased the recognition of p24 antigen positive/virus negative status when viral loads are reduced to undetectable levels.^21,22 Likewise, FeLV p27 antigen positive and viral RNA or proviral DNA negative results are well recognized and have been described in cats with latent/focal infections.^23
–27 Discordant results between FeLV p27 antigen and proviral DNA have been reported in cats with hematological disease (immune mediated hemolytic anemia, primary bone marrow disease or insult) and systemic inflammation.^28,29 In our study, discrepancies were most often observed in healthy, low positive cats, making cross-reactivity due to the destruction of red blood cells or disease drivers of endogenous FeLV gag expression unlikely causes of false-positive results, as previously suggested.²⁹ Some low positive cats were SNAP positive with all three sample types and consistently positive on PetChek, yet were initially negative by qPCR only to test positive for proviral DNA later, indicating a true exogenous FeLV infection. Perhaps this suggests that the qPCR was falsely negative at some testing events despite utilizing a broadly adopted assay.¹⁵ However, research on HIV diagnostics has demonstrated that p24 antigen can be detected on erythrocytes by ELISA even when plasma viral loads are undetectable, suggesting that antigen tests can produce true-positive results in the absence of molecular confirmation.²² Finally, although focal/atypical infections are described as rare, they represent up to 10% of natural FeLV infections.⁶ This would be consistent with the 29/127 (23%) cats in our study that were classified as low positive, which includes both regressive and focal/atypical clinical courses, as regressive infections also account for approximately 10% of natural infections.³⁰

The terminology describing the clinical course of an FeLV infection (eg, progressive, regressive, focal) remains important for understanding the clinical state and pathogenesis, while the classification of infection status (eg, low positive, high positive) facilitates the interpretation of the diagnostic results from the different tests. Identifying low positive cats, and even cryptic/negative cats, with their anticipated longer survival times and risk for comorbidities as they age, is essential for re-evaluating their low vs high positive status should they present with a future illness. If they have not transitioned to high positive, they are unlikely to have a progressive clinical course of FeLV infection. Not recognizing these low positive and cryptic/negative cats may be one reason why some cats with hematological disease test positive for FeLV p27 antigen, yet do not have evidence of FeLV proviral DNA in circulation or in bone marrow, and demonstrate a full recovery with appropriate therapy.^28,29,31 Reactivation of focal FeLV infections and the development of progressive disease does remain a concern, and this has been described in experimental FeLV infections, including those evaluating vaccine efficacy.^27,32 Therefore, re-evaluating low vs high positive infection status is important in clinically ill cats.

A limitation of this study was that sampling was restricted to the minimally invasive collection of peripheral blood for FeLV testing. More invasive sampling of tissues such as bone marrow or lymphoid tissue may have revealed virus harbored in focal infections or the source of antigen or provirus found in circulation. This study did not assess viral RNA in saliva as a measure of FeLV infection due to the lack of sensitivity in cats with regressive, and presumably focal, infections.^33,34 Another limitation was the inability to perform additional FeLV testing or necropsies at the time of death to determine viral load at the end of life or the cause of death. Therefore, we were unable to establish if any low positive cats that died had transitioned to a high positive status or if mortality of high positive cats was due to a progressive clinical course of the disease.

Conclusions

This study demonstrated the benefit of using anticoagulated whole blood on a point-of-care test to screen for evidence of FeLV infection compared with separated plasma or serum. Additional testing with a quantitative, FeLV real-time PCR for proviral DNA and a microtiter plate ELISA for FeLV p27 antigen informed the likelihood for extended long-term survival. Knowledge of current testing recommendations and expected outcomes plays a pivotal role in the care of these retrovirus-infected patients. Providing veterinarians with prognostic information helps to establish accurate expectations of longevity with existing owners and potential adopters of FeLV-infected cats. These diagnostic protocols facilitate successful adoptions, enable conversations on single vs multi-cat households, and reinforce the importance of regular veterinary care for managing FeLV as a chronic condition.

Supplemental Material

Figure

Disposition of 127 FeLV-positive cats by sequential test results.

Supplemental Material

Table

Demographic, FeLV classification, survival and test results by cat.

Footnotes

Acknowledgements

We would like to thank the adopters of the FeLV-positive cats in the study for providing regular updates for their cats. We would like to thank the members of the APA! shelter, Natasha Hamman, Monica Frenden and Ellen Jefferson; the R&D staff at IDEXX, Genevieve Clark, Jancy Hanscom and Phyllis Tyrrell; and the staff at the University of Florida, Sylvia Tucker and Christina Reinhardt for their contributions to this study. Finally, we would like to thank Jon Ayers, former CEO of IDEXX Laboratories, for his enduring support for elevating the wellbeing of cats and the success of this project. Jon’s work for cats continues as board chairman of Panthera, a global nonprofit dedicated to the conservation of wild cat species and their habitats.

Accepted: 19 August 2025

Author note

This paper was presented in part at the 2024 International Society for Companion Animal Disease Symposium held in Vancouver, Canada on 14–16 October 2024.

Supplementary material

The following files are available as supplementary material:

Figure: Disposition of 127 FeLV-positive cats by sequential test results.

Table: Demographic, FeLV classification, survival and test results by cat.

Conflict of interest

Melissa Beall and Sean Turner are employees of IDEXX Laboratories.

Funding

This study received support from Maddie’s Fund and the EveryCat Health Foundation. A subset of the diagnostic testing was supported by IDEXX Laboratories.

Ethical approval

The work described in this manuscript involved the use of non-experimental (owned or unowned) animals. Established internationally recognized high standards (‘best practice’) of veterinary clinical care for the individual patient were always followed and/or this work involved the use of cadavers. Ethical approval from a committee was therefore not specifically required for publication in JFMS. Although not required, where ethical approval was still obtained, it is stated in the manuscript.

Informed consent

Informed consent (verbal or written) was obtained from the owner or legal custodian of all animal(s) described in this work (experimental or non-experimental animals, including cadavers, tissues and samples) for all procedure(s) undertaken (prospective or retrospective studies). For any animals or people individually identifiable within this publication, informed consent (verbal or written) for their use in the publication was obtained from the people involved.

ORCID iD

Melissa J Beall

Dana Moore

Sean Turner

Julie K Levy

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