You may think that the transmission of Lyme disease is very simple – you get it from a deer tick bite. That information is essentially true, and if you just want to avoid Lyme disease, that’s all you really need to know. What goes on in the woods, however, is interesting, and it helps to explain why Lyme disease is spreading and becoming more common.
Lyme transmission starts with the White-footed mouse, a common forest creature that carries Borrelia burgdorferi, the organism responsible for Lyme disease. In the most common scenario, newly hatched larval Ixodes sp. ticks feed on the mice and become infected with the organism. The ticks then develop through a nymphal stage to an adult tick, feeding on other hosts as they get older.
At both the nymph and adult stage, ticks are found clinging to the tall grasses in open forests and forest edges, the same type of environment favored by White-tailed Deer. When a deer wanders by, the ticks climb on and take another blood meal. If it’s a human that wanders by, or a dog, a raccoon, or even a bird, the tick is not fussy – deer ticks will readily feed on all of these hosts as well as other warm blooded animals.
Borrelia burgdorferi gets passed on to the new host as the tick feeds. Many animals aren’t affected by the organism and their immune systems simply kill it. White-tailed deer and some bird species are among the lucky animals that don’t get Lyme disease. Humans, however, can suffer a long and debilitating illness.
Deer are responsible for supporting large numbers of ticks; one deer can have hundreds of ticks feeding on it at any one time. Anyone who has driven the roads of North Eastern North America at dusk and dawn in the summer months knows that the White-tailed deer population is booming like never before. Deer graze by the roadsides and wander into cities and gardens. Lots of deer means lots of ticks, and deer in human communities means Lyme transmission to humans.
Deer don’t spread Lyme disease over large geographical distances however – birds do that. When a deer tick bites a bird, two things can happen: if the bird is already infected, having been bitten by an infected tick somewhere else, the new tick becomes infected and can spread the organism to other hosts. If the tick is already infected and the bird is not, the organism gets passed the other way, and because ticks take days to finish their meal, both tick and bird can be many miles away from their starting point before the tick drops off. Both can now continue to Lyme transmission. Migrating birds are thought to have spread B. burgdorferi to many new locales.
A final factor to consider is global warming. Warmer climates have opened up new territory for deer ticks, and they are now surviving and reproducing in areas where they were previously unable to withstand cold winter temperatures. Thus, ticks carried in on migrating birds are more and more likely to survive the trip.