No abstract

Foot-and-mouth disease (FMD) recombinant non-capsideal viral antigens 3A, 3B, 2C, 3D and 3ABC were assessed individually in an indirect enzyme-linked immunosorbent assay (I-ELISA) for their ability to screen for persistent infection-specific antibodies in cattle, regardless of vaccination condition. Results of sequential serum samples from non-vaccinated animals with experimentally induced persistent infection, and their correlation with virus isolation, indicated that the polypeptides 3A, 3B and 3ABC showed the most adequate characteristics for further field studies. Reliable performance of the I-ELISA with the selected antigen 3ABC was indicated by the distinct patterns observed for the frequency distribution values of naive and true positive samples. For regularly vaccinated livestock, a clear negative profile was proved in samples from regions without recent history of FMD. In contrast, at 90 and 900 days post-outbreak, coexistence of a positive and a negative population was established. These findings indicated that, irrespective of vaccination, the test allowed a classification of the herd-disease status. A high degree of agreement was observed between I-ELISA-3ABC and EITB results for clearly reactive and non-reactive sera. For samples with reactivity values close to that of the cut-off, the EITB profiles upheld the definition of the infection condition. On this basis, screening by I-ELISA-3ABC, together with confirmation of suspect or positive samples by EITB is proposed as an adequate and accurate approach for large-scale epidemiological surveillance.

The foot-and-mouth disease virus (FMDV) “carrier” state was defined by van Bekkum in 1959. It was based on the recovery of infectious virus 28 days or more post infection and has been a useful construct for experimental studies. Using historic data from 1,107 cattle, collected as part of a population based study of endemic FMD in 2000, we developed a mixed effects logistic regression model to predict the probability of recovering viable FMDV by probang and culture, conditional on the animal’s age and time since last reported outbreak. We constructed a second set of models to predict the probability of an animal being probang positive given its antibody response in three common non-structural protein (NSP) ELISAs and its age. We argue that, in natural ecological settings, the current definition of a ”carrier” fails to capture the dynamics of either persistence of the virus (as measured by recovery using probangs) or the uncertainty in transmission from such animals that the term implies. In these respects it is not particularly useful. We therefore propose the first predictive statistical models for identifying persistently infected cattle in an endemic setting that captures some of the dynamics of the probability of persistence. Furthermore, we provide a set of predictive tools to use alongside NSP ELISAs to help target persistently infected cattle.