Zoonotic diseases are pathogens that have jumped between humans and animals. Most of these come from wild animals. In 2001, a team of researchers from the University of Edinburgh found that of the 1415 species of infectious organisms “known to be pathogenic to humans,” 868 were zoonotic. Sometimes the name of a disease makes it obvious: cowpox virus, European bat lyssavirus 2, Baboon cytomegalovirus.
Zoonotic diseases are not a new phenomenon. Long before humans knew about viral diseases, they knew that mad dogs could pass on rabies. The bubonic plague was a zoonotic disease that was originally passed between rodents. And scientists have identified armadillos as a potential source of the bacterium that causes leprosy in humans. Recently emerged zoonotic diseases include SARS and H1N1.
“More than two new species of human virus are reported every year,” wrote researchers from the University of Edinburgh Centre for Infectious Diseases in a 2008 paper for the Proceedings of the Royal Society B. “Recently discovered viruses are even more likely to be associated with a non-human reservoir.”
Scientists tracking emerging diseases pay close attention to how pathogens are transmitted between individuals. A “Stage 1” pathogen is only passed between animals and does not affect human health. Bluetongue disease is an example of a stage 1 pathogen. After stage 1, epidemiologists get nervous. At stage 2, an animal pathogen can be transmitted from animal to human but does not have the ability to spread from human to human. At stage 3, an animal pathogen can be passed from human to human, but it does not last long in the human population. Ebola is a stage 3 pathogen. Ebola outbreaks originate in the non-human primate population and can cause devastating, but short-lived, outbreaks in human populations. Stage 4 pathogens are gung-ho for humans. Stage 4 diseases like yellow fever enter the human population from animals and cause extended outbreaks in humans.
“We do not pay much attention to Stage 1 pathogens until they become stage 2, 3 or 4,” said X. J. Meng, a professor of molecular virology at Virginia Tech.
That lack of attention is a problem if epidemiologists want to stop stage 1 pathogens from adapting to humans.
Meng spoke in March at the Farm Animal Integrated Research (FAIR) conference. The conference focused on the future of animal agriculture, and Meng argued that successful animal production requires disease control in animals and humans.
“Infectious disease is the second leading cause of death worldwide,” Meng said. He explained that the risk of zoonotic disease is greater in developing countries. “The veterinary services in those countries are usually very limited.”
Many diseases do not spread to humans; our physiology and immune systems fight them off. But when animal pathogens do infect humans, certain groups are particularly vulnerable. Infants, pregnant women and the elderly are “immunocompromised,” meaning it is harder for them to fight off infections. People with HIV or AIDS or those undergoing cancer treatments are also vulnerable.
Proximity to wild animals is also a risk. According to Meng, wild animals are the sources of 75 percent of zoonotic diseases. That was the case in 1999, when a group of pig farmers in Malaysia fell ill with Nipah virus. Scientists eventually identified a population of cat-like palm civets as the likely source of that strain of Nipah virus. The disease probably spread from the civets to the pigs to the pig farmers. This case reveals another factor in catching zoonotic diseases: when a disease finds a way to hit humans, animal producers are often the first ones affected.
“They share their whole environment with farm animals,” Meng said.
Meng said advances in animal science can minimize future zoonotic disease outbreaks.
In his talk at FAIR, Meng talked about his experience studying the hepatitis E virus [HEV]. In 1997, Meng discovered that swine were one reservoir of a strain of HEV that can affect humans. Though HEV does not have a high mortality rate, it can cause acute, severe liver disease in some people, particularly in pregnant women.
Meng discovered that swine and human HEV were related by analyzing the strains genetically. Since the discovery of swine HEV, Meng has used blood tests to track the prevalence of the disease.
“In some herds, up to 80 percent of the pigs are infected with the Hep E virus,” Meng said.
When humans get the swine strain of HEV, it is usually through contaminated water or food. Like with Nipah virus, humans close to infected animals are at the greatest risk. Meng has found that HEV is common in swine veterinarians in the United States.
By tracking the spread of swine HEV through genetic analysis and blood tests, scientists have identified at-risk populations.