Diseases as components of mammalian ecosystems: mayhem and subtlety

Abstract

Parasitic and infectious agents are found in all mammals in every ecosystem. Death of the mammalian host is unusual, and only occurs (i) if serious illness facilitates transmission, as in rabies, (ii) if the parasite does not depend on the affected host species for survival and can complete its life cycle after the host dies, or (iii) if the pathogen moves through host populations over wide geographic areas, over a long period of time. The high mortality rate occasioned by myxomatosis in Australia and Europe is an example of a pathogen occurring in a completely susceptible population for the first time. Coevolution of virus and host has resulted in a decrease in mortality. Some other diseases affect host populations by causing adverse effects on natality or by interfering with normal movement. Brucellosis in caribou has both effects. The effects of most diseases in populations of wild mammals are more subtle. Infectious and parasitic agents can have adverse effects on the host energy balance by reducing energy intake or increasing energy costs through higher body temperatures and metabolic rates. Diseases of mammals have certain practical considerations as well. Attempts to manage populations of wild mammals may inadvertently create situations where disease transmission is facilitated, as was the case with anthrax in Africa. Wild mammals harbor diseases that are transmissible to man and movement of these wild mammals in commerce can spread the disease, as has been the case with Hantaan virus. Also, ecological changes can favor increases in abundance of some animals that are reservoirs of human disease, as has been the case with Bolivian hemorrhagic fever and Lassa fever in Africa. Wild mammals and domestic mammals frequently share common diseases and these diseases may pose economic threats which can lead to campaigns to reduce numbers of wildlife reservoirs. Some of the challenges and opportunities associated with rapid ecologic changes are discussed.