FeLV in four European countries: infection markers and classification in 934 cats

Viruses · 2023

In plain language

The study analysed samples from 934 cats in Italy, Portugal, Germany and France, collected on a single date for each cat. Cats classified as having progressive infection had more copies of viral DNA than those classified as having regressive infection, although the ranges overlapped. Classification as regressive infection meant that viral DNA was detected but p27 antigen, a viral protein, was not. This study cannot establish how infection developed over the long term or why the distribution of infection courses differed between the countries studied.

Scientific detail

This observational clinical study, published in Viruses in 2023 (doi: 10.3390/v15081718), included 934 cats: 269 from Italy, 240 from Portugal, 318 from Germany and 107 from France. Saliva swabs were available for 815/934 cats and owner questionnaires for 777/934. The study related blood FeLV proviral DNA burden to antigenemia, viral RNA detection and laboratory-defined infection categories. The empirical findings included in the knowledge object are classified as level C evidence.

Cats classified as progressively infected had significantly higher proviral copy numbers than cats classified as regressively infected, although the reported ranges overlapped. This association does not establish a threshold that perfectly discriminates between the two categories or demonstrate causality. Among cats classified as progressively infected, blood p27 antigen concentration was positively correlated with viral RNA load in saliva. This correlation does not itself measure infectious virus or transmission probability, nor does it establish a causal relationship between the markers.

Regressive infection was operationally defined as p27 antigen-negative but provirus-positive. This laboratory criterion does not demonstrate stable regression over time, complete replication arrest or a particular integration-site configuration. Integration sites, proviral sequence integrity and the replication competence of the detected proviral DNA were not reported.

The principal limitations identified by the authors were sampling on a single date for each cat and selection bias arising from the inclusion of cats undergoing blood collection for diagnostic reasons. Long-term infection courses could not be established, and persistent latency, later reactivation or transitions between progressive and regressive infection could not be directly demonstrated. Infection timing was usually unknown, and proviral loads overlapped between groups. France was represented by one clinic; vaccination status was unknown for many cats, and the authors question the specificity of the anti-p15E antibody positivity threshold. The supplied text also contains inconsistent prevalence figures and an incomplete passage in Results section 3.4.

The reasons for the different distributions of infection categories between the sampled countries remain unknown. The authors discuss viral subgroups, host properties and endogenous FeLV load as possible contributors, but the study does not establish those explanations.

What we know

In this observational sample of cats, classification as progressive infection was associated with significantly higher proviral copy numbers than classification as regressive infection, with overlapping ranges. Within the progressive group, blood p27 concentration was positively correlated with salivary viral RNA load. The regressive category was defined as p27-negative and provirus-positive. These findings and criteria, classified as level C evidence, do not demonstrate individual long-term infection courses.

What we don't know

Longitudinal persistence, later reactivation and transitions between categories were not established in the sampled cats. The correlation between p27 and salivary RNA does not determine the amount of infectious virus or transmission probability. Integration sites, proviral sequence integrity and the replication competence of the detected proviral DNA were not reported. The causes of the differences between the sampled countries remain unknown.

Evidence

Provenance

This project does not diagnose or prescribe. Always consult your veterinarian.