# Masterclass - Population aspects of parasite infection

#### Distribution of parasites within a population of horses

Given a field containing 10 horses, each exposed to infective larvae on the pasture, how many would you expect to be heavily infected with worms? Most people assume that they will all be infected. This is an incorrect assumption. Scientific data, and our own experience of testing large numbers of horses for parasites, indicates that parasites are unevenly distributed throughout the host population. This has been modelled mathematically and for most host-parasite relationships the model that fits best is the negative binomial distribution.

There are considerable practical consequences of this distribution:

• Within a group of horses, only a small proportion will be heavily infected. If you can recognise which ones, by using diagnostic tests, treatment efforts can be concentrated on these animals
• It is only the heavily infected animals which are likely to suffer from parasite-related disease. Identify these animals, and you can treat the parasites in order to prevent disease

A simplified way of thinking of this distribution is that 80% of the parasites live in 20% of the horses!