Influence of exposure intensity on the efficiency and speed of transmission of foot-and-mouth disease

Foot-and-mouth disease virus (FMDV) can be spread by direct animal-to-animal contact, indirect contact facilitated by contaminated materials or by airborne spread. The rate of spread and the incubation period, as well as the severity of disease, depends on many variables including the dose received, the route of introduction, the virus strain, the animal species and the conditions under which the animals are kept. Quantitative data related to these variables are needed if model predictions are to be used in practical disease control. This experimental study quantifies the risk of transmission of FMDV in pigs exposed by contact, sheep exposed by indirect contact with pigs and sheep exposed to airborne FMDV. Groups of pigs were inoculated with the FMDV O UKG 34/2001 strain and susceptible pigs were then exposed to the inoculated animals at different stages of the infection cycle. The mean incubation period in the susceptible pigs ranged from 1 to 10 days. The length of the incubation period, severity of clinical disease and efficiency of spread were related to dose (i.e. infectiousness of source and intensity of contact). Low intensity transmission increased the proportion of subclinical or abortive infections. Local conditions are important in the efficiency and speed of transmission of FMDV. The results of the experiments described above suggest that transmission is frequency dependent rather than density dependent. The sheep experiments provided further evidence that development of infection and clinical disease is dependent upon local conditions. Dose, infectiousness, intensity of contact and local factors are thus important determinants for the outcome of an initial outbreak and must be truthfully accounted for in mathematical models of epidemiological spread.