A further fascinating feature of parasite infection is that animals are not equally susceptible. Certain horses will repeatedly become infected with parasites, whereas others will never acquire high burdens even if they are not treated. Scientists are undecided about the reason for this phenomenon. It is consistent with immunity. Some animals develop a good immune response and others do not. It is also consistent with behaviour. Some animals may show behavioural differences that make them ingest higher levels of infective larvae.
There are very practical implications to this phenomenon. If the heavily infected animals, which are predisposed to infection can be identified with diagnostic tests, treatment efforts can be concentrated on these high risk animals. Also, these animals are the ones likely to develop high levels of infection in the future and therefore more likely to suffer from parasite related disease. Information from diagnostic tests has implications for the future management of individual animals.