Epidemic of Crossover Diseases Threatens Biodiversity, Human Health

By Cat Lazaroff

ATHENS, Georgia, January 21, 2000 (ENS) - Newly discovered infectious diseases in wildlife may pose an increasing and significant threat to human health and to global biodiversity, a new international study reveals. The report urges wildlife and medical experts to look for evidence of crossover diseases that can jump from species to species - even to humans.


Mosquitos can carry infectious diseases like the West Nile virus from birds and other wildlife to humans (Photo courtesy Centers for Disease Control)
While emerging human diseases such as Ebola have grabbed headlines in recent years, similar diseases in wildlife have been understudied, and few regulations concerning exotic disease threats to wild animals or systems for surveillance are in place to prevent their spread.

"With a new wave of globalization on an unprecedented level, we don't even know what the greatest threats are in terms of emerging infectious diseases of wildlife," said Dr. Peter Daszak of the University of Georgia's Institute of Ecology and department of botany. "The problem has largely been ignored by policy makers and the threat that these wildlife pose to human, directly or indirectly, should be taken far more seriously."

Daszak’s report on the scope of the problem, cowritten with Dr. Andrew Cunningham of the Zoological Society of London and Dr. Alex Hyatt of the Australian Animal Health Laboratory, was published Thursday in the journal "Science."

Human history is filled with the catastrophic consequences of emerging infectious diseases. The introduction of smallpox, typhus and measles by the Spanish conquistadores in the 15th and 16th centuries resulted in 50 million deaths among native South Americans. Despite suspicions that disease may have caused similar effects on wildlife, systematic studies of emerging infectious diseases of wild animals and their effect on human populations have been few and far between.

That all changed with the discovery that wild animals can act as natural reservoirs for diseases that can be extremely virulent among humans. The influenza virus, for example, causes pandemics in humans following the periodic exchange of genes between the viruses of wild and domestic birds, pigs and humans.

The report by Daszak and his colleagues points out that many emerging infectious diseases of wildlife are associated with the spill-over of pathogens from domestic animals to wildlife populations; with the translocation of host or parasites by human intervention; and with events that have no human or domestic animal involvement, such as global warming or floods. Whatever the reason, these diseases have spread just as human diseases did.

"In the same way that Spanish conquistadores introduced smallpox and measles to the Americas, the movement of domestic and other animals during colonization introduced their own pathogens," said Daszak.


Brucellosis, a disease that causes spontaneous abortions and stillborn calves, can be transmitted from wild bison to domestic cattle (Photo courtesy National Academy of Sciences)
The first major method of animal disease transmission, "spill-over," refers to the spread of infectious agents from reservoir animal species - often domestic animals - to wildlife. Outbreaks of spill-over diseases represent a serious threat to wildlife and domestic animals, Daszak said.

For instance, a disease called brucellosis that causes pregnant animals to abort was probably co-introduced to America with cattle. So the presence of the disease in bison of Yellowstone National Park is considered a potential threat to domesticated cattle grazing at the park's boundaries. The problem has led to tension between conservationists and cattlemen, and the shooting by the Montanta Department of Livestock of bison that graze the same areas as domesticated herds, even though there is little evidence of cattle becoming infected, Daszak says.

The relocation of wildlife species - the second method of spreading emerging infectious diseases - occurs often in conservation efforts or for agriculture or hunting.

"The introduction of animals to new geographic regions and the co-introduction of their pathogens is a serious problem," said Daszak. "For example, avian malaria on Hawaii is thought to have caused the extinction of a number of native species and was originally introduced with exotic, alien birds."

The emergence of infectious diseases without overt human involvement is among the thornier issues facing conservationists. For instance, weather patterns can cause changes in the abundance of certain parasites that are deadly to some species of sheep.

Researchers are finding new diseases even in sites considered pristine. A newly discovered fungal disease has recently been identified as the cause of death in frogs, toads and salamanders of the Central American and Australian rain forests, areas scientists thought were beyond the reach of human environmental change.


A fungal infection may be contributing to the worldwide decline of frogs and other amphibians (Photo courtesy World Land Trust)
Whatever the source of infectious wildlife diseases, both human health and global biodiversity are being increasingly threatened, the authors argue.

Recent genetic analyses have shown that avian influenza can be transmitted directly from birds to humans. Potential non-human primate reservoirs for human immunodeficiency viruses - HIV-1 and HIV-2 - have also been found. Daszak said that natural reservoir hosts for such feared disease as Ebola have been more elusive, though bats and some small forest dwelling mammals have been tentatively implicated.

Researchers involved with human health have begun to search for new animal diseases as part of a strategy to control emerging disease threats to humans, but far too little is known about potential threats at this point, the authors say.

While potential human disease outbreaks can be linked to animal disease, just as troubling is the role of animal disease in the loss of global biodiversity. While there are numerous examples of disease emergence following the introduction of pathogens to a population, "there undoubtedly are many more that have not been identified as such."

The international movement of food crops, timber, agricultural materials and domesticated animals, as well as landfill wastes and ship ballast water, combine to cause global threats. The authors say that even such remote protected areas as the Galápagos Islands and Antarctica are not exempt.

deer ticks

Deer ticks help transmit Lyme disease from deer and other mammals to humans (Photo courtesy American Lyme Disease Foundation, Inc.)
Just how science can offset the effects of emerging infectious diseases in wildlife is still unclear, but the scope of the problem is becoming increasingly obvious, said Daszak. The problems involved have clear economic consequences. In 1994, 655 people who had potential contact with a single rabid kitten in a New Hampshire pet store were given post exposure treatment at a cost of $1.1 million, the authors point out. The costs of Lyme disease treatments spreak by ticks in the U.S. may be as much as $500 million a year.

"Current measures for the detection and control of emerging infectious diseases in humans and livestock are inadequate for the identification of similar threats in wildlife," said Daszak. "The conservation community has drawn up guidelines to prevent the release of animals carrying exotic pathogens to new areas, but these recommendations are now under used. We need an integrated approach, using traditional and cutting-edge techniques, to investigate outbreaks as they occur in wildlife."

Techniques now used to help control emerging diseases in humans and domestic animals - including satellite imaging, Internet news groups, outbreak investigations and large-scale vaccinations - may well work in certain populations of wild animals. Until then, these diseases in wildlife will likely continue to cause serious and growing problems for human health and biodiversity.