The descriptive epizootiology of phocine distemper in the UK during 1988/1989

The 'time, place, individual' approach, widely used in characterising human epidemics, was applied to the 1988 phocine distemper virus (PDV) epizootic affecting North Sea seals. Estimates of time of death from 157 (69%) of the 228 dead seals necropsied in 1988 indicated that the number of carcasses which were found more than 14 days post mortem increased as the epizootic progressed. Although information provided by epizootic curves based on when and where carcasses were reported are affected by the accuracy of such data, the PDV epizootic curves were characteristic of a propagative epidemic. The individual characteristics of the carcasses provided more useful information. 1. Common seals were more susceptible than grey seals. 2. Males and older seals had a greater exposure to the virus than females and younger animals. 3. The likelihood of viral transmission was greater on land. Seasonal, sex and age related variation in haul-out behaviour affected transmission probabilities and rates. 4. The risk of infection for a susceptible individual during August was higher for seals in England and Northern Ireland than for those in Scotland. These findings illustrate the importance of population characteristics including behaviour and social organisation in determining the pattern and spread of wildlife epizootics.