Tug-of-war

The ongoing avian influenza epidemic in Southeast Asia is one of many topical examples illustrating the convergence of human and animal health. The highly pathogenic flu strain has so far resulted in 48 human cases, 35 of them fatal. The World Health Organization worries the avian virus could eventually spark the first flu pandemic of the new millennium.

Last December, Dr. Lonnie King spoke to a gathering of some 60 public policy officials at the Senate Dirksen Office Building in Washington, D.C., about the looming threats of avian influenza and numerous other animal diseases capable of jumping the species barrier to infect people. The AVMA Governmental Relations Division hosted the seminar, the first of several presentations meant to establish the AVMA among members of Congress as the authoritative source on matters relating to veterinary medicine.

Veterinary medicine, Dr. King told them, has to be integrated into the nation's public health system if people are to be safe from these diseases, which are on the rise.

Epidemiologic eras

"That was the first time we saw a major increase in zoonotic diseases," Dr. King said. "That was the start of the era of zoonoses."

The second epidemiologic period got under way around the time of the Industrial Revolution in the late 18th and early 19th centuries. During this era, morbidity and mortality from infectious diseases dropped, vaccines became important in public health, and it looked as if heart disease, cancer, and other chronic diseases had replaced infectious diseases as the chief threats to public health.

But infectious diseases were not on the way out. In fact, they returned with a fury some 30 years ago, ushering in the current epidemiologic period, which is marked by a preponderance of zoonotic diseases.

Infectious organisms cause some 1,620 human diseases, 60 percent of which are zoonotic, Dr. King said. He added that, of the 35 major human disease outbreaks occurring over the past two-and-a-half decades, 75 percent were zoonotic. "Public health-wise," Dr. King said, "we weren't prepared for this epidemiologic era."  

The perfect microbial storm 

It's the same in the microbial world, the authors suggest, only the conditions necessary for new and emerging disease outbreaks are happening more frequently and are continuing to accelerate at an alarming rate.

For starters, microbes are a competitive lot. They adapt and evolve to survive. Many acquire the ability to move from host to host, regardless of species. They even develop immunities to the drugs designed to kill them. And in this tug-of-war between people and germs, the microbes are winning. "They adapt, they change, they win," Dr. King commented.

Microbes are unintentionally aided by human activities and natural occurrences. International travel, for instance, contributes to disease spread. Dr. King said that in 2003, more than 50 million people visited the United States, and 1.4 billion people traveled by air.

"You can traverse the globe faster than the incubation period for any disease," he said, adding that contaminated goods and animal products entering the country each year are potentially contagious. Just consider how the West Nile, monkeypox, and SARS viruses arrived in the Western Hemisphere within the recent past.

In addition, new diseases are appearing as people encounter wildlife by expanding into rainforests and other previously untouched ecosystems. Weather events such as El Nino can also create conditions ripe for diseases. Leptospirosis, hantaviruses, and Rift Valley fever are prime examples of diseases heavily influenced by weather.

Dr. King noted that if the average world temperature were to increase by three degrees, the zone in which malaria is spread would expand from 45 percent of the world's population to 60 percent.

Population increases are another factor, especially in developing countries where growth projections are highest. People living in poverty are especially at risk from food- and waterborne diseases, Dr. King said.

A host's susceptibility to disease is another key factor. Animal agriculture in the United States is second to none, said Dr. King, who, prior to joining Michigan State, ran the Agriculture Department's Animal and Plant Health Inspection Service. The United States is free of foot-and-mouth disease and several other devastating animal pathogens active elsewhere in the world.

Yet, disease-free status is a double-edged sword because food-producing animals in this country are especially vulnerable to exotic microbes such as FMD virus, as well as Rift Valley fever, Nipah virus, and other zoonotic diseases. Moreover, concentrated food animal operations could facilitate easy disease transmission from animal to animal.

Since 9-11, there has been a great deal of focus on bioterrorism and the vulnerability of U.S. agriculture. Last December, outgoing Health and Human Services Secretary Tommy Thompson admitted he worries "every single night" about terrorists attacking the food supply because it would be so easy.

Government funds have helped bolster the nation's public health system for combating infectious diseases, whether naturally occurring or deliberately introduced, Dr. King observed. But, like Secretary Thompson, Dr. King believes animal agriculture isn't much safer than before 9-11.

Terrorists aren't the only danger to animal agriculture; infected wildlife spread disease to domestic animal populations in what is known as "spillover." Not too long ago, it looked as if tuberculosis was going to be eradicated from cattle in the United States, Dr. King said. Yet, the bacteria found a new host in free-ranging white-tailed deer, which recently infected 33 cattle herds in Michigan. Now, TB appears to be staging something of a comeback in cattle. 

Animal health+public health=one medicine

Wild birds transmit the virus to domestic poultry, which go on to infect other commercial poultry. The highly pathogenic virus then jumps to humans, hogs, tigers, and other mammals. H5N1 has been circulating in Southeast Asia since 1997, and, like all influenza viruses, is constantly mutating. Health officials worry the virus will eventually acquire the ability to pass from person to person in a sustained manner.

Dr. King pointed out the impossibility of protecting people in Southeast Asia from the H5N1 virus, without understanding the avian disease or animal agriculture production practices in that region. "You can't disconnect public health and animal health," he said.

Public health systems meant to fend off infectious diseases through surveillance, preparation, and response all fall short if veterinarians are not part of the equation, Dr. King continued, adding that veterinary medicine is, in fact, a public health medicine. "Frankly, we're all involved in public health, even if you're a small animal practitioner," he said.

The "one medicine" approach recognizes that one way of protecting public health is keeping animals free of infectious diseases. Further underscoring the need for veterinarians in the public health sphere is the knowledge that 80 percent of the diseases most likely to be used in a biologic attack are zoonoses.

"You're just as likely to find these agents in a veterinary practice or animal health diagnostic lab as you are in a human hospital or physician's office," Dr. King said. "Thus, veterinary practitioners and lab workers must be better trained to recognize and report these conditions."

More needs to be done to integrate public and animal health systems. Fortunately, there seems to be a growing acceptance among government and public health staff that veterinarians are more than dog and cat doctors, Dr. King said. At least 75 veterinarians are on staff with the national Centers for Disease Control and Prevention, he noted.

Veterinarians must do their part, too. While the public is being well-served in the area of small animal medicine, veterinarians are underrepresented in food animal medicine, biomedical research, and ecosystem management. "There is a crying demand that we aren't meeting, and we in the profession need to do something," Dr. King said.

To win the war against microbes, Dr. King believes several things have to happen. Applied research must have a better understanding of zoonoses; interdisciplinary disease centers must be established in the United States, as is the case in the European Union; domestic and global infectious disease surveillance must include animal diseases; veterinary students must be trained for public health careers; and veterinarians must be more engaged in public policy to help make these changes reality.