DO WILDLIFE DISEASES REALLY MATTER? The waterfowl manager who wakes
up one morning to find ten thousand dead and dying birds in the
marsh would think so. Yet virtually every wild bird and mammal
harbors at least a few parasites seemingly without obvious adverse
consequences. Parasites, viruses, bacteria, and fungi are component
parts of the ecosystems in which wildlife are found, but do not
necessarily cause disease. Millennia of coevolution have engendered
a modus vivendi that assures the survival of both host and parasite
populations. Then why the ten thousand sick and dying birds?
Ecosystems are changing. Waterfowl are concentrated on shrinking
wetlands and remain there for longer periods of time, facilitating
bird-to-bird spread of the bacteria that cause avian cholera. Or
permitting the buildup of parasites in their hosts from a small,
relatively benign number to massive numbers that cause disease and
death. Water quality of wetlands changes, favoring the production
of deadly botulinum toxin by bacteria and its mobilization up the
food chain to waterfowl. New, totally artificial habitats are
created with unpredictable results. The extreme temperature,
salinity, and other conditions of the Salton Sea have created an
unusual ecosystem in which botulism occurs in fish and in birds
through biological cycles that are not yet understood. Wetland loss
in southern California leaves few alternative places for waterbirds
to go, so they are attracted to the Salton Sea. Behavior changes.
Mallard ducks take up residence on the ponds and lakes of city
parks and lose their migratory habits. They share these bodies of
water with exotic species, such as Muscovy ducks that have also
taken up residence there after introduction by people, setting the
scene for outbreaks of duck plague, and creating the risk of spread
to migratory waterfowl that also use these areas. Raccoons and
skunks become well adapted to urban life, bringing rabies and
canine distemper with them into the city. The environment changes
the physiology of wild animals. Human activity introduces into
wildlife habitats chemical compounds that adversely affect
physiological processes such as reproduction and immune
responsiveness. These compounds become incorporated into the
ecosystems, often becoming more concentrated as they move up food
chains. Their effects can influence wildlife populations. Some of
these endocrine-disrupting chemicals, such as chlorinated
hydrocarbons (DDE, PCBs), interfere with normal endocrine function
by mimicking natural hormones, with resulting eggshell thinning and
breakage. Effects of these chemical compounds on immune-system
responses to infectious and parasitic agents are less well
understood. What to do? Incorporating disease-prevention measures
into wildlife management practices requires more information than
is usually available. The information-gathering process must begin
in the field. Field biologists must monitor disease occurrence.
This Field Manual is a valuable aid in identifying the diseases
that are likely to be present, and in giving guidance on the
gathering and treatment of specimens needed to establish the
diagnosis in the laboratory. But the wildlife field biologist is in
a position to provide valuable information that goes beyond the
collection of samples from sick and dead individuals. Although
diseased individuals are the basic unit of surveillance, the
occurrence of disease must be put into ecological perspective. A
careful description of the ecological setting in which the disease
is occurring, and any changes that have occurred over time, are
ultimately as important as a careful description of the lesions
observed in the individual, if the epidemiology of that disease is
to be understood, and the disease prevented through sound
wildlife-management practices.
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