The field effectiveness of routine and emergency vaccination with an inactivated vaccine against foot and mouth disease

High potency, inactivated foot and mouth disease (FMD) vaccines may be used in non endemic countries for emergency vaccination during outbreaks in order to prevent virus spread. In endemic countries either standard or high potency vaccines are used for routine vaccination. Despite their wide use there is a shortage of data on the field effectiveness of inactivated FMD vaccines. Epidemics of FMD caused by viruses of serotype O occur frequently in Israel, where a high potency (≥6PD(50)) vaccine is used for both routine and emergency vaccination. We investigated an outbreak of FMD caused by a virus of serotype O, which took place during 2011 in a feedlot and an adjacent dairy herd. Post outbreak testing of antibodies against non-structural protein demonstrated that infection occurred in 96% of the calves that received two doses of vaccine at least three months prior to the outbreak and more than 50% showed clinical signs consistent with FMD. Replacement heifers that had been vaccinated 3-5 times with the last vaccination administered 7 months prior to the outbreak were all infected and 18% showed clinical signs. Testing of cattle sera of the same vaccination status as the affected cattle demonstrated low neutralizing antibody (NA) titers against the field virus strain and an r(1) value of 0.37 compared to the vaccine strain. In contrast, cattle vaccinated only once but up to two weeks before the outbreak, were almost all protected from clinical disease and to a lesser extent, protected from FMD virus infection, despite low NA titers. We conclude that emergency vaccination was highly effective due to a mechanism not associated with NA, whereas routine vaccination with the same vaccine formulation provided only limited protection due to poor longevity of the elicited immunity and low matching with the field strain (despite an r(1) higher than 0.3).

Foot-and-mouth disease virus epitope dominance in the antibody response of vaccinated animals

Five neutralizing antigenic sites have been identified on the surface of serotype O foot-and-mouth disease virus (FMDV). A set of mAb neutralization-escape mutant viruses was used for the first time to evaluate the relative use of known binding sites by polyclonal antibodies from three target species: cattle, sheep and pigs. Antibodies to all five neutralizing antigenic sites were detected in all three species, with most antibodies directed against antigenic site 2, followed by antigenic site 1. In 76 % of cattle, 65 % of sheep and 58 % of pigs, most antibodies were directed against site 2. Antibodies specific to antigenic sites 3, 4 and 5 were found to be minor constituents in the sera of each of the target species. This implies that antigenic site 2 is a dominant neutralization immunogenic site in serotype O FMDV and may therefore be a good candidate for designing novel vaccines.

Serological survey for foot-and-mouth disease virus in wildlife in eastern Africa and estimation of test parameters of a nonstructural protein enzyme-linked immunosorbent assay for buffalo

In this study we estimate the seroprevalence of foot-and-mouth disease virus (FMDV) in wildlife from eastern and central Africa. Sera were sourced from between 1994 and 2002 from a rinderpest surveillance program. Our study compared a nonstructural protein enzyme-linked immunosorbent assay (Cedi test) with a virus neutralization test. The study shows that there is only a low seroprevalence of FMDV in sampled nonbuffalo species. The seroprevalence in the Cape buffalo was high for SAT2, lower for SAT1, and lowest for SAT3. As the SAT2 serotype was most prevalent, the Cedi test largely reflected the occurrence of SAT2-positive animals. The results also suggest that SAT2 became dominant around 1998, with a large increase in seroprevalence. The sensitivity and specificity of the Cedi test were estimated by comparison to the combined virus neutralization test results from all three SAT tests. A Bayesian implementation of the Hui-Walter latent class model was used to estimate the test parameters. The model permits estimation in the absence of a gold standard test. The final model, using noninformative priors and assuming conditional independence of test performance, estimated Cedi test sensitivity at 87.7% and specificity at 87.3%. These estimates are similar to those for domestic bovines; they suggest that the Cedi test is a useful tool for screening buffalo for infection with the various serotypes of FMDV.

Reduction of foot-and-mouth disease (FMD) virus load in nasal excretions, saliva and exhaled air of vaccinated pigs following direct contact challenge

In future, a policy of "vaccinate-to-live" may be included in the repertoire of foot-and-mouth disease (FMD) control measures and in support of this approach, we have investigated the hypothesis that vaccine-induced reduction in virus replication and excretion from pigs can be correlated to the severity of clinical signs of FMD by measuring excretion of virus in natural secretions and aerosols. The other aims of this study were to verify the existence of sub-clinical infection in vaccinated pigs, to evaluate the correlation between this and seroconversion to foot-and-mouth disease virus (FMDV) non-structural protein antibodies and to re-examine the occurrence of FMDV persistence in the oro-pharynx of pigs. Therefore, pigs were vaccinated (O1 Manisa) and challenged (O1 UKG) in a manner calculated to produce a broad range of clinical outcomes and were monitored for a minimum of another 33 days post-challenge. Eighty-one percent of the early (10 days vaccinated) challenged pigs and 25% of the late (29 days vaccinated) challenged pigs were clinically infected and all other vaccinated pigs were sub-clinically infected. Although vaccination could not provide complete clinical or virological protection, it reduced the severity of the disease, virus excretion and production of non-structural FMDV antibodies in vaccinated and subsequently infected pigs. As hypothesised, vaccine-induced reduction of virus replication and excretion was found to be correlated to the severity of clinical disease. RNA copies, but no live virus was detected from the pharyngeal and soft palate tissues of a minority of vaccinated and infected pigs beyond the acute stage of the infection.

Geographical and age-stratified distributions of foot-and-mouth disease virus-seropositive and probang-positive cattle herds in the Adamawa province of Cameroon

Six of the seven known serotypes of foot-and-mouth disease (FMD) virus occur in Africa. This paper describes the results of a population-based cross-sectional study of the seroprevalence of FMD and the persistence of the virus in cattle herds and associated sheep flocks in the Adamawa province of Cameroon. Antibody titres measured by the virus neutralising test indicated that serotypes O, A and SAT2 viruses had been circulating in the province. The estimates of apparent seroprevalence in cattle herds, based on five juvenile animals (eight to 24 months old) per herd, were 74.8 per cent for serotype SAT2, 30.8 per cent for serotype A and 11.2 per cent for serotype O, indicating recent exposure; the estimates based on animals more than 24 months of age were 91.1 per cent for SAT2, 83.6 per cent for A and 34.2 per cent for serotype O. Epithelial and oropharyngeal samples were collected from cattle and small ruminants, cultured and typed by ELISA; serotypes A and SAT2 were isolated from both types of sample. The herd-level estimate of apparent prevalence of probang-positive herds was 19.5 per cent and the animal-level estimate of apparent prevalence was 3.4 per cent. The geographical distribution of the seropositive herds based on juveniles suggested that recent SAT2 exposure was widespread and particularly high in the more northern and western parts of the province, whereas recent exposure to serotype A was patchy and more concentrated in the south and east. This distribution corresponded very closely with the distribution of herds from which virus was recovered by probang, indicating recent exposure or infection. No serotype O viruses were recovered from cattle, and the distribution of seropositive herds suggested very localised recent exposure. The apparent prevalence of probang-positive animals declined with the age of the animal and the period since the last recorded outbreak in the herd.

The application of new techniques to the improved detection of persistently infected cattle after vaccination and contact exposure to foot-and-mouth disease

Detection of antibodies to the non-structural proteins (NSP) of foot-and-mouth disease virus (FMDV) was compared with conventional serological and virological methods and with RT-PCR for the identification of FMDV carrier animals obtained after experimental contact challenge of vaccinated cattle. Transmission from carriers to sentinels was also monitored. Twenty FMDV vaccinated and five unvaccinated cattle were challenged by direct contact with five donor cattle excreting FMDV and monitored until 28 days post challenge-exposure . Twelve vaccinated and three unvaccinated animals were retained up to 24 weeks post exposure to FMDV in order to monitor viral persistence, transmission and antibody responses. In nine vaccinated animals, infection persisted beyond 28 days post exposure, virus being detected more frequently and for longer in oesophagopharyngeal samples from these animals when examined by RT-PCR rather than by virus isolation. Although recovery of FMDV RNA became increasingly sporadic over time, the number of RNA copies detected in positive samples declined only slowly. Two naïve sentinel cattle housed with the persistently infected animals between 93 and 168 days after the latter had been challenge-exposed to FMDV did not become infected. There were differences in the ability of commercially available serological tests to detect antibodies to FMDV non-structural proteins (NSP) in vaccinated and subsequently challenged cattle. Although no single test could identify all of the vaccinated cattle that became persistently infected, the most poorly recognised animals were those with the least evidence of virus replication based on other tests. The potential of the detection of antibodies to the 2B NSP of FMDV for diagnosing persistent FMDV infection was demonstrated.

Validation of a foot-and-mouth disease antibody screening solid-phase competition ELISA (SPCE)

This paper describes the validation of a solid-phase competition enzyme-linked immunosorbent assay (SPCE) for the serological detection of antibody to serotype O foot-and-mouth disease (FMD) in sheep, cattle and pigs. The specificity of the SPCE was calculated from the results of testing known negative sera from sheep, cattle and pigs (n=3030, 1418 and 1495, respectively). The mean percentage inhibition (PI) for known negative sheep, cattle and pig sera were 19.3, 24.1 and 20.8%, respectively. The specificity of the SPCE at a cut-off point (COP) of 60 PI was 99.50% for sheep sera, 99.44% for cattle sera and 100% for pig sera. The analytical sensitivity of the SPCE was examined by testing sera from sheep, cattle and pigs. Based on the testing of serial bleeds from experimentally infected animals, seroconversion at the 60 PI COP occurred between 4 and 9 days post-infection or -exposure, similar to that observed using the virus neutralisation test (VNT) with a COP of 1/45. When applied to 267 sheep and 143 pig samples, that were obtained in Great Britain (GB) during the 2001 FMD UK outbreak, the SPCE identified more positive samples than did the VNT. Estimates of the accuracy, repeatability and reproducibility of the SPCE were verified during the large-scale serosurveillance necessitated by the 2001 outbreak. Results from field and experimental sera showed that when compared against the VNT, the sensitivity of the SPCE was less affected by the choice of virus strain used in the test. Using the O(1) UKG 2001 FMD virus in the VNT with samples representative of the uninfected GB sheep population, the test specificity was 100% at a COP of 1/45.