Ehrlichia: New Tick Infection May Be Linked To Environmental Change
If the rising risks of Lyme disease, Anaplasmosis or Babesiosis weren't reasons enough to take extra precautions while outdoors this summer -- and to do a thorough tick check before going back inside -- researchers have now identified yet another unpleasant tick-transmitted disease.
A still unnamed, tick-borne bacterium appears to have transmitted ehrlichiosis to at least 25 people in Wisconsin and Minnesota, with more cases likely unaccounted for due to the flu-like symptoms common among other diseases that pass through ticks, according to a new paper published on Thursday in the New England Journal of Medicine.
The discovery builds on evidence of the increasing number of infectious diseases spread from wildlife to humans, researchers say. Bats are currently transmitting the deadly Hendra virus among horses and humans in Australia while rodents scatter hantavirus across the U.S. -- most recently taking lives in New Mexico and Washington State. The more familiar Lyme disease is also on the rise across much of the country, with Lyme-carrying deer ticks found for the first time on an island off the coast of northern Minnesota and cases of the disease rising seven-fold over the last decade in Maine.
Overall, an estimated 75 percent of emerging human diseases, including HIV, SARS and West Nile, come from non-human animal populations. According to some experts, changes to the climate and to the landscape -- through deforestation and development -- are at least partially to blame.
"This [Ehrlichia species] is but one example of a continuing trend of infectious disease emergence that affects both humans as well as wildlife," Pieter Johnson, an ecologist at the University of Colorado at Boulder, told The Huffington Post in an email. "Overall, environmental change is playing a very large role in disease emergence."
The result, he said, is significant problems for human health, the economy and wildlife conservation. "Emerging diseases and species extinctions are two of the biggest challenges facing the planet today -- understanding the connection between them is a major research priority," added Johnson. "The health of humans and the health of wildlife and domestic animals are intimately linked to one another."
A study published in a July issue of Science highlighted one key case in point: the rapid decline of large predators worldwide.
Richard Ostfeld of the Cary Institute of Ecosystem Studies in Millbrook, N.Y., explained that when we degrade or fragment habitats, or homogenize them through agriculture or livestock production, we tend to create ideal conditions for the culprits responsible for many diseases. These are usually the smaller and hardier species, such as the mice and deer that provide the means for ticks to contract and spread Lyme disease as well as the American robins that play a similar role alongside mosquitoes in the life cycle of the West Nile virus.
"The first types of species we lose are the predators and other larger-bodied creatures, because they usually require more space to maintain viable populations," Ostfeld, an expert in the link between biodiversity and infectious disease, told The Huffington Post. "The littler, more generalized species are the ones that persist in disturbed ecosystems."
Meanwhile, in the case of tick-borne infections, deforestation allows for the growth of tall grasses that appeal to the insects. Such landscape changes are often linked to rising human development, which means more humans living in closer proximity to more ticks. The likely end result: more disease.
Dr. Georgios Pappas, head of the Zoonoses Working Group of the International Society of Chemotherapy in Greece, noted a similar scenario in which new roads are altering the waterways in his country and creating stagnant water that attracts migratory birds with West Nile virus. Urban development has also been blamed for the spread of Hendra virus in Australia, according to another recent study.
"The diversity of animals or plants have evolved over millions of years, while we've done a lot of things that erode biodiversity in very short time frames -- decades, years, even months," Ostfeld said. "But we can do something about that with new policy and management schemes."
Johnson agreed: "We need to find new ways to treat and prevent emerging infections that go beyond chemical therapeutics."
In addition to the need to preserve biodiversity -- both directly through protecting habitats and indirectly by preventing pollution and invasive species -- a broader policy issue is also at play: climate change. Dr. Bobbi Pritt, lead researcher on the paper outlining the new Ehrlichia species, noted the role of changing weather patterns in the rise of infectious diseases.
"Ticks like warm and humid weather," Dr. Pritt of the Mayo Clinic in Rochester, Minn., told The Huffington Post, noting the increased presence of these climatic conditions in recent years.
Johnson also pointed to examples of climate change leading to shifts in patterns of infections, including increases in disease. However, he suggested that "generalities are still few and far between."
Of course, part of the explanation for the rising numbers of new diseases may also be that we now have more sophisticated techniques to identify those infectious agents. The species of illness-inducing bacterium identified in both ticks and humans in the Midwest could have been around for some time, noted Dr. Pritt. "But we may not have had the tools to detect it," she added.
As with other emerging diseases, such as the first discovery of Lyme disease more than 30 years ago in Connecticut, the new finding is likely just the beginning. While researchers are fairly confident that ticks are responsible for the transfer of the bacterium to humans, they still have a lot of unanswered questions. Many species can be involved in the disease's life cycle, noted Johnson, including numerous possible hosts, predators of those hosts and other species that can influence their interactions. "And all of these species will in turn be influenced by the pathogen and characteristics of the environment," he added.
Ostfeld emphasized the distinction between the initial emergence and the rise of a new disease.
"We have a tremendous fascination with disease emergence," he said. "It's like out of a science fiction movie: A new beastly, terrible microbe jumps into humans, making us bleed from all our orifices and drop dead within 48 hours."
"This is compelling to people, but actual emergence itself may not be nearly as important as what happens after it has become established in human populations," added Ostfeld, pointing to the more recent rapid increase in Lyme decades after its discovery. "In many cases, we're not going to be able to prevent an emergence. So once a disease emerges, we want to know what causes it to spread and what makes it get worse."
Perhaps this is where the plot twists and recasts humans as the villains.
"By entering an environment that has its own logic and hierarchy, humans behave as a virus," said Dr. Pappas. "Let us call, then, such emerging zoonotic infections a part of the environment's immune response against the human intruder."
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First Posted: 08/04/11 02:58 PM ET Updated: 10/04/11 06:12 AM ET