University Park, Pa. — Continuing reports on the spread of the H1N1 virus, or swine flu, have served as reminders that animal health and human health are more closely linked than many people realize, according to researchers in Penn State's College of Agricultural Sciences.
"In fact, about half of the 1,500 diseases that can affect people are transmissible between animals and humans," said Bhushan Jayarao, extension veterinarian and professor of veterinary science.
The College of Agricultural Sciences is uniquely positioned to monitor zoonotic diseases — animal diseases that can infect humans — since college researchers deal with both animal and human health, as well as connections between the two. Using interdisciplinary approaches from molecular biology to population dynamics, scientists are continually gaining new insights into the behavior of zoonotic organisms.
Some commonly known zoonoses include rabies, Lyme disease, West Nile virus, Ebola virus and avian influenza. A zoonosis can be a bacterium, a virus or even a parasite, explained Jayarao. "These diseases are not new -- they have been around for several thousands of years."
Only in the last 100 years or so have zoonoses increased, in part because of dramatic advances in transportation that allow diseases to travel as quickly as people do. "In the 1850s, it probably took 365 days to circumvent the globe," he said. "Today it can be done in 36 hours."
Zoonotic diseases also have increased over the years because of human and animal populations living in closer proximity. Jayarao points to farming practices in Southeast Asia as an example.
"Farmers raise chickens, goats and pigs, and they dwell and raise their children in the same environment where these domesticated animals are interacting with each other," he said. "This is an ideal setting for a migratory bird to bring in a virus that settles into the local fowl, then moves to the farm animals and possibly to the humans. There is an active exchange of genetic material from different sources of viruses in one common area."
Ottar Bjornstad, professor of entomology and biology and co-director of Penn State's Center for Infectious Disease Dynamics, studies host-pathogen interactions and the population dynamics that influence the spread of infectious diseases. He explained that while some zoonoses, such as rabies, are spread through direct contact with an infected animal, others need a vector to carry the pathogen -- such as the mosquito with West Nile virus and the deer tick with Lyme disease.
In the case of Lyme disease, bacteria are vectored by the deer tick and infect various forest animals, including deer, mice and skunks. The bacteria move from mammal to mammal by first infecting ticks, which then move from one host to the next. Research has shown that the deer tick prefers some animals over others; it will thrive on deer, for example. But there are other host species that the bacteria themselves prefer.
"It's an interesting ecological balance," Bjornstad said. "You can have just enough of the hosts that are good for the ticks and just enough of the hosts that are good for the bacteria. Then you have a lot of ticks that are all infected with the bacteria, and that's when you get spillover into humans."
Some pathogens can make the species jump more easily than others, and Bjornstad and his colleagues explore all of the variables that affect an organism's ability to enter a new host, multiply and create an infection. For example, the pathway the pathogen takes to enter the host -- whether it be through food, air or water -- can affect its ability to thrive. Other variables include the resistance of the host and the creativity and adaptability of the pathogen.
H1N1 influenza is just one more example of a zoonosis that can "jump" from one species to another and may begin to act differently in a new host. Although the H1N1 virus causes relatively mild symptoms in pigs, when it combines with genes from human and avian influenzas -- as it seems to have done in the current outbreak -- it turns unpredictable and potentially dangerous.
"When a virus such as H1N1 influenza jumps from one animal species to another, it can become a different organism altogether," said Vivek Kapur, professor and head of veterinary and biomedical sciences. "It is too soon in this particular outbreak in humans to be sure where the virus or disease will end up."