On the rise
There are a number of reasons for the cause and spread of zoonotic diseases. According to Lisa Schloegel, amphibian decline specialist with the NY-based Wildlife Trust, the importation of wildlife is a major factor. “The international trade in live wildlife runs the risk of importing exotic, zoonoses that could have significant health implications for humans and animals,” she says.
But ensuring that animal-borne pathogens will not enter the U.S. through importation is difficult, she notes. “To date, the U.S. has minimal screening and quarantine protocols regarding the importation of animals that could harbor known or unknown pathogens.”
Jonathan Epstein, DVM, MPH, a senior research scientist at the NY-based Consortium for Conservation Medicine, acknowledges that the pet trade is an important factor. But he also says that while the U.S. may be lax in screening, there is virtually no testing at the point of origin.
Perhaps even more important than the importation of wildlife in the spread of zoonotic pathogens is the role international travel plays. “With the high rate of international travel and trade today, the opportunity for the emergence and intercontinental spread of a highly pathogenic zoonosis is significant,” says Schloegel. “SARS, for instance, spread rapidly from Asia to Europe, North America and South America infecting more than 8,000 people and killing over 700.”
Epstein explains how some of the most infamous infectious diseases have spread: “SARS spread to 26 different counties from China, mainly by travel. It emerged in China from bush food, and then spread to bats and to civets, which are commonly eaten by people in China.
“While we are not entirely sure how West Nile virus came into this country, it is thought to have come from a mosquito on an airplane. Avian flu continues to circulate in Asia among domestic and wild birds. But most cases come from domestic poultry.”
And anthrax is very common in Africa and the Middle East, he says. Yet he adds, “The bacteria that forms spores can be found in soils — even in Texas.”
Of course, there are real concerns that new zoonotic diseases can emerge with devastating consequences. “Health officials are currently concerned that a highly lethal influenza strain could emerge and rapidly spread through the human population,” Schloegel says.
Epstein says scientists also are keeping a watchful eye on the Nipah virus, which is found in India, and the Hendra virus in Australia. “Both are carried by fruit bats.” The concern is that the more opportunities there are for these viruses to jump from bats to people, the more likely they are to mutate, he explains.
Ongoing research
With the threat of zoonotic diseases causing worldwide pandemics, researchers in the U.S. are studying specific diseases and their animal hosts, says Epstein. These include mosquito-borne West Nile virus; rabbit fever; Lyme disease, which is carried first by white-footed mice, then by deer ticks; and the historically infamous plague, whose bacteria has been found in prairie dogs.
Vaccines can help contain an outbreak…. but … we need
to keep zoonotic diseases from spreading in the first place.
In addition, the discovery in some primates of new retroviruses related to HIV is prompting researchers to look more closely at genetic sequencing, he says and that research on rabies is ongoing at the Centers for Disease Control and Prevention (CDC).
Coincidentally, two years ago the CDC issued a warning that dengue fever was moving out of the tropics and could quickly establish a foothold in the U.S. Estimates put new cases in the U.S. at between 100 and 200 each year.
Although yet spread from person to person, dengue fever is transmitted by two types of mosquitoes: the yellow fever mosquito and the Asian tiger mosquito. The latter is now found in 36 states with a concentration in the Southeast and Texas.
Sometimes, research on zoonotic diseases can yield unexpected results, says Schloegel. “A group at Vanderbilt University, for instance, recently identified antimicrobial peptides present on the skin of certain amphibian species capable of inhibiting HIV infection in a laboratory setting. It is thought that further studies of such peptides could be used to develop a prophylactic for HIV.”
Prevention is key
Controlling an outbreak of an infectious zoonotic disease for which a vaccine has been developed normally leads public-health officials to call for mass inoculations. But as has been seen in the recent H1N1 outbreak, sometimes vaccines can be in short supply. “Vaccines can help contain an outbreak, but it does take time to create a vaccine. There is a debate going on in the vaccine community as to how best to grow viruses for a vaccine,” Epstein says. “We need to keep zoonotic diseases from spreading in the first place.” He notes it is essential that we first understand the risk factors and how pathogens are transmitted from animals to people.
Wildlife surveillance on a global scale is one way to detect diseases before they become a threat to animals and humans, he says, and — by analyzing emerging diseases in the field — “hot spot” maps can then be created. Even in those cases where pathogens have already been transmitted from animals to humans, it rests on the shoulders of physicians to consider all possibilities when attempting to treat their patients.
“Diagnostically, you work from the common to the rare,” Epstein says. “But their training is focused on what is common. Physicians have not traditionally been aware of zoonotic diseases.” Fortunately, he says, that is changing as the link between animal health and human health becomes more evident.
Global initiatives
The growing awareness of zoonotic diseases is leading to a spirit of cooperation. “The Global Viral Forecasting Initiative, Consortium for Conservation Medicine, and the Wildlife Trust all have collaborations and/or contacts within the CDC that do their best to increase awareness within the scientific community,” says Schloegel. “Conservation medicine is a new field that is aimed at bringing together researchers from the animal and public-health sectors. It has become apparent in recent times that collaboration among these groups is essential for both human and animal health.”
Scott Newman, DVM, PhD, Animal Health Officer in the Animal Production and Health Division of the Food and Agriculture Organization of the United Nations. As the EMPRES Wildlife Unit Coordinator — part of the Emergency Center for Transboundary Animal Diseases — his program focuses on understanding how diseases that are transmitted between livestock, wildlife, and humans and, ultimately, how to minimize impacts of these diseases on livelihoods, food safety and security, species conservation, and public health.
Over the last five to seven years, Newman explains, global efforts to study and combat zoonotic diseases have been based on the principles of “One Health,” the main thesis of which is that “most emerging diseases are a result of human actions, and multiple interrelated global factors drive these processes” and that you can “ensure the health of people by ensuring the health of the environment.”
While Newman actively collaborates with many organizations worldwide, some of his biggest challenges arise from coordinating efforts within countries. “If you look at highly pathogenic avian influenza or HPAI), SARS, Ebola, or pandemic influenza A (H1N1), these are good examples of diseases that cut across government sectors” he says. “In general, these diseases and others that have emerged in the last 10 to 20 years have impacted human health, livestock production, wildlife species, international trade, and tourism … often areas that are under the jurisdiction of different government ministries.”
As a result, those government ministries overseeing agriculture, health, forestry, and commerce need to cooperate. But Newman notes, “Each ministry has a slightly different mandate.” So, for example, if a pathogen affects livestock, wildlife, and people, it is necessary to for all three ministries to collaborate to effectively address the multiple communication angles, disease ecology, and epidemiology; but, frequently, ministries work within their own silos.” Still, Newman says significant improvements are being made, especially since the global community has had to combat H5N1 and HPAI.
“Ministries are starting to appreciate the value of working together, and they now recognize the complementary nature of their efforts.” In looking at these efforts globally, he says, “We are on the verge of a global movement toward a “One Health” approach. We are by no means there yet, but we are at a very interesting point historically.”
