As a result, wildlife disease patterns and trends are, to a substantial extent, indicators of environmental quality and changing host-agent interactions within the environment being evaluated. The types, distribution, and frequency of diseases causing major avian die-offs have changed greatly during the 20th century. Too little is known to assess the changes of most avian diseases that result in chronic attrition rather than major die-offs, or about those that affect reproductive success, reduce body condition, or affect survival in other indirect ways. Nevertheless, the changing patterns and trends in highly visible avian diseases provide notice of problems needing attention. Comprehensive assessments of causes of wild bird mortality, magnitude of losses, and geographic distribution of specific diseases are not possible from these data, although we can identify general relationships for waterfowl and some other species.
Common factors required for disease to occur.
Environmental factors greatly influence occurrence of disease by changing the amount and type of host-agent interactions. The most dramatic example of geographic expansion of a noninfectious indigenous disease is avian botulism, caused by the bacterium Clostridium botulinum . In 1914 a Bureau of Biological Survey researcher began investigating catastrophic die-offs that had begun in 1910 and in which millions of waterbirds along the Great Salt Lake, Utah, had died. Later studies revealed that avian botulism was responsible for those die-offs. The continued reporting of avian botulism die-offs since the early 1900's makes researchers suspect that much of the disease's geographic expansion is of recent origin.