A mechanistic model to describe the spread of phocid distemper virus

1. The 1988 epizootic among seals in N.W. Europe led to the death of more than half of the population. Several researchers have fitted data from the epidemic with the Kermack and McKendrick model for disease spread. 2. We argue that for animals living in herds or colonies, like seals, the mutual contact behaviour is such that this model ought to be applied with special care for the distinction between numbers and densities. This is shown by a mechanistic description of the contacts among seals, which leads us to a different formulation of the standard model. 3. This formulation is useful for the description of epidemics among all kinds of animals living in herds. 4. Further analysis shows that the survival of infected animals has a disproportionately great influence on the intensity of the epidemic. 5. Marine pollution may not only have contributed to the high death rates, but may have intensified the epizootic as well.