1. Why are deer called the "reproductive hosts?" Is it simply because more ticks can host on deer than on smaller mammals, so there is more likelihood of mating? S.H., Massachusetts
TERC Answer: As you may know, bloodsucking arthropods like ticks, mosquitoes and fleas require a blood meal in order to produce eggs, a key step in reproduction. In studying vector-borne diseases, hosts that might provide blood for arthropod vectors to develop and reproduce are typically categorized as reservoir hosts if they contribute infectious pathogens along with their blood, or reproductive hosts if they contribute blood but no pathogens. In the blacklegged tick/Lyme disease ecological web, small rodents like mice and chipmunks, in particular, are key reservoir hosts. Immature stage ticks blood feeding on infected rodents can become infected, and later pass that infection when they blood feed again. Adult stage blacklegged ticks are unsuccessful in blood feeding on small rodents; in fact, a very small proportion of the population successfully blood-feeds on medium-sized mammals, either. While rodent-derived blood meals provide nourishment for immature tick stages to develop, those animals contribute little to the ultimate reproduction process in blacklegged ticks, which is growth of the egg stage. In contrast, adult stage blacklegged ticks commonly blood feed on deer, becoming engorged and obtaining all of the protein necessary for laying their clutch of 1,500-2,000 eggs. For various reasons (host density and distribution, grooming and development of tick resistance, tick control, etc), few other animals are as efficient in providing the population of adult blacklegged ticks with this all important blood meal. But that blood meal typically doesn't include pathogens, at least not pathogens that infect humans, and so we label deer as reproductive hosts in the blacklegged tick/Lyme disease system.
2. I saw deer in my yard; do ticks get their infections from deer?
TERC Answer: Blacklegged (deer) ticks do not get infected with Lyme disease, babesiosis, or granulocytic anaplasmosis by feeding on deer. Instead, white-footed mice, other small rodents, and some birds carry the infections and pass them to ticks that blood feed on them. Deer are important, however. They are the main reproductive host for the adult stage of Blacklegged ticks. Rule of thumb: no deer, likely no deer ticks. See deer, watch out, ticks may be present! Remember, deer typically are active at night, so even if you don't see them on your property, they may still be visiting. Look for signs, like browse on favorite plants, pellet piles or even hoof prints.
3. What role do mice play in the Lyme Disease cycle? L.P., Maryland
TERC Answer: Blacklegged (deer) ticks become infected with the bacterial agent causing Lyme disease [and other disease-causing agents] when they take their blood meal from small rodents, such as white-footed mice. Ticks use the blood meal to grow, and the infection just goes along for the ride-for example, if larval ticks feed on an infectious mouse, most become infected (80-90%), and once they transform (molt) into nymphal ticks , are able to transmit the infection at their next blood meal. This makes animals like mice the 'reservoir' of infection. Even though they are called "deer" ticks, the infections they transmit come from rodents. Prevention strategies that target ticks feeding on mice may help reduce risk of encountering infected ticks.
4. Could you tell me if there is any danger of contracting Lyme Disease by consuming deer meat from a deer infected by Lyme Disease? James C., North Kingstown, R.I.
TERC Answer: We’ve never heard of any issues. While deer all throughout the eastern and mid-western US certainly are bitten by many infected deer ticks, and are exposed to all of the agents transmitted by those ticks, curiously they do not themselves become infected, or at least not infected enough to pass the infection on, even to other ticks feeding on them. We use the term INFECTION RESERVOIR to describe animals that can pass an infection to a tick feeding on it, and deer have been shown time and time again to NOT be competent as reservoirs for Lyme. We published a study years ago that mostly explained the phenomenon—deer have a very aggressive innate immune system when it comes to the Lyme bacteria. Their blood serum contains molecules that are exceptionally good at killing the Lyme bacteria.
One last comment—deer are the most important REPRODUCTIVE HOST for deer ticks. In RI, on average every deer, every year feeds enough adult deer ticks to allow those ticks to create an estimated 450,000 new, larval ticks. Blame the ticks on the deer, blame the infections on small rodents that are INFECTION RESERVOIRS.