The 1975 Foot-and-Mouth Disease Epidemic in Malta. II: the Detection of Carriers and Inapparent Infection

Abstract

SUMMARY During the course of the 1975 epidemic of type O foot-and-mouth disease (FMD) in Malta and in two follow-up visits, cattle, sheep, goats and pigs on neighbouring farms were sampled to determine the extent of inapparent infection and the effect of vaccination on the development of carriers. Oesophageal-pharyngeal (O-P) fluid samples were tested for virus and serum samples for ‘virus-infection-associated’ (VIA) antibody. Sampling was confined, with one exception, to premises which were free of clinical disease. FMD virus was detected in only three of 278 O-P fluid samples from ruminants on 23 premises during the course of the outbreak. The positive samples came from a single herd in which lesions were detected at the time of sampling. No FMD virus was recovered from 305 ruminant O-P samples taken two months after the last recorded case on premises close to those infected at the time of the outbreak. Antibody to VIA antigen was detected in 11 of 196 serum samples taken on premises at high risk during the outbreak and in 12 of 379 samples from the second visit, whereas 97 sera from the third visit a year later were all negative. Despite large amounts of virus excreted by pigs, the spread of virus during the 1975 epidemic of FMD in Malta appears to have been confined mostly to farms with clinical disease, since there was no evidence of occult infection in the surviving livestock sampled in areas of high risk of exposure.