You may know that ticks get the Lyme bacteria, Borrelia Burgdorferi, from the white-footed mouse and then pass it on to us. However, do the mice themselves get Lyme Disease? Do they get sick? What about all the other animals that ticks bite, like chipmunks, squirrels, raccoons, foxes, coyotes, birds and the like? The list of animals that ticks bite is a long one. To make sense of the research, we need to break down this list. It is easier to understand how Lyme Disease exists in wildlife by talking about small, medium and large animals, and birds. The size of the animal seems to make more of a difference than the species when discussing how Lyme Disease affects wildlife.
How is Lyme Disease Transmitted?
Before discussing how Lyme is transmitted, we need to define two terms. The first is the term, host. A host is, as you would expect, someone who serves someone something. In the tick’s case, the host is the animal or bird who serves them their blood meal after the tick bites them. A tick needs a blood meal in each of the three stages of its life cycle, larvae, nymph, and adult to grow and move onto the next stage. The second term we need to define is a reservoir. Since ticks aren’t born with Lyme Disease, they need to get the bacteria from somewhere. The term reservoir refers to where the bacteria are present. Many animals can be reservoirs for the Lyme bacteria, Borrelia Burgdorferi. Some reservoirs have an abundance of bacteria in their blood. In other animal reservoirs, there may be very few Borrelia Burgdorferi bacteria. An animal with a large number of bacteria in their blood is referred to as a competent reservoir. This means they have enough bacteria to infect any ticks that bite them. Other animal reservoirs can be incompetent reservoirs, meaning they have so few bacteria present they will not infect the tick when it bites them.
White-footed mice, chipmunks, rats, squirrels, moles, voles and certain species of birds are competent reservoirs. These animals are often infected with Lyme Disease, as well as Anaplasmosis and Babesiosis. These small animals are the ones most responsible for transmitting Lyme Disease to larvae and nymph ticks. Since larvae and nymph ticks are often in the dens and burrows of these animals, they are often the young tick’s first blood meal. The small ticks in this life stage do not need a lot of blood for their first meal. Adult ticks need a larger animal for their larger blood meal. Although studies show these small animals are often infected with Lyme bacteria, there has been little to no research into how the disease affects them. Whether they get a rash or sick from the bacteria is unknown. One study that looked into the mortality of white-footed mice and showed 93% of them succumbed to predators, primarily weasels. Only one mouse death in the study was attributable to Borrelia Burgdorferi bacteria. From this study, we can hypothesize that most or all of these small animals can succumb to the infection, but it is not common. Is the lifecycle of the white-footed mouse so short or predation so high, that Lyme Disease doesn’t have a chance to take more of them? We just don’t know. Many studies have shown that birds are often infected with Lyme Disease and are responsible for its geographical spread. One NIH study found, “ticks have detrimental effects on their avian hosts even under natural infestation conditions…and may also present symptoms of infection, though these may be subtle.” Birds do seem to be affected physically by the Lyme bacteria; however, it is not acute or obvious. Lizards may also become infected with Lyme Disease but are incompetent reservoirs.
Medium Size Animals
Raccoons, groundhogs, rabbits, beavers, opossums, foxes, bobcats and coyotes rarely serve as reservoirs for Lyme Disease. Scientists also do not find many ticks on them when sampled. That doesn’t mean they don’t transmit other diseases. Rabbits transmit tularemia, rarely Lyme Disease. Beavers, like rabbits, transmit tularemia but are not identified with helping to spread Lyme Disease. Groundhogs transmit bronchopneumonia and hepatitis B primarily. Squirrels transmit tick fever, and their fleas can transmit plague. Coyotes are incompetent reservoirs of Lyme Disease. Foxes are also incompetent reservoirs of Lyme Disease. Foxes provide a benefit when it comes to Lyme Disease because they are predators of white-footed mice reducing their numbers. Even when the Lyme bacteria are found in these medium size animals, the bacteria present is minimal, making these animals incompetent reservoirs. No research has been done on how or why the Borrelia Burgdorferi bacteria present in them is so low. Whether some of these animals have less exposure to ticks carrying Lyme, or whether their immune system is prepared to prevent a significant infection, is not known. Some scientists and studies have hypothesized that these animals may have a “primitive immune system” but no scientific definition exists for that term. No studies exist to confirm or deny it.
Large Size Animals
The only wild large size animal in the tick lifecycle is the deer family. White tail Deer, as well as other members of the deer family, including elk, moose and caribou, are the blood meal host often chosen by adult ticks. First, several thousand ticks can attach to deer because of their size. This number of ticks makes it easy for males to locates the female ticks as ticks use deer to find a mate much like a local “pick-up” joint. A female tick will use a deer to consume a large blood meal, to lay a clutch of 2,000 to 18,000 eggs. Elk and moose carry dog ticks in the West, which are often infected with Ehrlichiosis and can be transmitted to humans. With the introduction of elk and moose in the Eastern US, no one knows the effect they may have in Lyme Disease and Ehrlichiosis transmission. All deer are incompetent reservoirs with low levels of Lyme bacteria in their blood, despite the number of ticks that bite them. Once again, scientists attribute the deer’s immunity to a primitive immune system or an ability to flush the bacteria from their blood. Based on numerous research papers, it is clear that the animals most responsible for transmitting Lyme Disease are small ones, especially white-footed mice, voles and other rodents. Research studies show that voles become important in the transmission of Lyme bacteria mostly in areas where few white-footed mice exist. Medium size and larger animals and birds spread the disease further geographically that any of the smaller animals because of their territorial ranges and their effectiveness in giving infected ticks a ride to new locations.
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Tracing the epidemiology of Lyme Disease is complex, as you can see. With so many vectors, hosts and reservoirs responsible for the disease, researchers are a long way off from knowing how the disease continues to spread. Rather than wait for science to provide a solution, there are effective solutions now such as barrier sprays for your yard. Contact Mosquito Squad of Greater Washington DC now to discuss our tick barrier spray and tick tube program that will protect your yard all season long.