Research Update: Tick-borne Disease Vertebrate Reservoirs
An interdisciplinary team at Washington University in St. Louis has been keeping a wary eye on emerging tick-borne diseases in Missouri for the past 20 years. Team members include ecologists Brian F. Allan and Jonathan M. Chase, molecular biologists Robert E. Thach and Lisa S. Goessling, and physician Gregory A. Storch.
The team recently developed a sophisticated DNA assay, described in the March 2010 issue of Emerging Infectious Diseases, that allows them to identify which animal hosts are transmitting pathogens to ticks.
“This new technology is going to be the key to understanding the transmission of diseases from wildlife to humans by ticks,” Allan says.
“The question,” says Thach, Ph.D., professor of biology in Arts & Sciences and of biochemistry and molecular biophysics in the School of Medicine, “is where do infectious diseases come from?”
“Most seem to come from nature – they exist in other animals – and then make the leap from animals to people, Thach says.”
Assuming this model applies to the lone star tick diseases, what is their animal reservoir and why are they jumping?
Lone star ticks need blood meals to power their metamorphoses (they go through three stages: larva, nymph and adult) and egg laying.
They sometimes bite coyotes, foxes and other animals, but their favorite hosts are wild turkey and white-tailed deer.
Especially white-tailed deer, which seem to be playing a major role in maintaining large lone star tick populations and setting the stage for tick diseases to jump to people.
The first step in the assay is to pulverize the ticks to release the DNA, which is then amplified using a procedure called the polymerase chain reaction, or PCR. This provides enough DNA for identification.
Following amplification is a step called reverse line blot hybridization. Probes, which are short sequences of DNA unique to a bacterium or to a host animal, are deposited in lines on a membrane. The membrane is then rotated, and the products of the PCR step — tagged with a chemiluminescent (light-generating) dye — are laid down in lines perpendicular to the probe lines.
Wherever two lines cross, DNA from the tick sample mixes with probes for either bacterial or animal DNA. If the two match, the molecules will bond, or hybridize. When the membrane is later washed, tick-sample DNA that has not hybridized washes off. DNA that has hybridized sticks and shows up as a chemiluminescent spot on the membrane. Reading the spots, tells the scientists which bacteria the tick was carrying and which animal provided its last blood meal.
Assay results showed that most of the nymphal lone star ticks infected with E. chaffeensis fed upon a white-tailed deer in the larval life stage. “So deer are definitely a primary reservoir for this bacterium,” says Thach. “But we also found some kind of squirrel – which we have more recently identified as the common gray squirrel – and what appears to be some kind of rabbit.”
In general, the results suggest deer are probably “weakly competent reservoirs” for the tick diseases, meaning that ticks that bit deer stood only a small chance of picking up one of the pathogens. On the other hand, deer have huge “reservoir potential,” because there are so many of them.
The bottom line: a sprinkling of deer is ok; crowds of deer are a problem.
For the complete article: http://www.medicalnewstoday.com/articles/180589.php