By Hannah Foster
Upon hearing the words “Lyme disease,” most people think of two things: ticks and deer. Although these are certainly important aspects of the disease, ticks and deer are only two pieces of the puzzle. Lyme disease is actually extremely complicated, and its spread is affected by a wide range of organisms. Knowing the full story is important for individuals to protect themselves, and it’s necessary for scientists and epidemiologists to determine how to control the disease.
Lyme disease is the most common tick-borne disease in the United States, with around 300,000 cases per year, mainly in the Northeast. It causes flu-like symptoms, and, if left untreated, can result in serious chronic symptoms like arthritis, neurological problems, cardiac arrhythmia, and cognitive defects. The disease is caused by a bacterium called Borrelia burgdorferi, which is carried and transmitted to humans by the blacklegged tick (Ixodes scapularis), also known as the deer tick.
A primary strategy for controlling the spread of Lyme disease has been to decrease the deer population. Deer are one of the main hosts of the blacklegged tick, so it makes sense that reducing the deer population would also diminish the tick population, consequently limiting the spread of Lyme disease. The problem is, it’s more complicated than that, and other animals play a part in addition to the deer.
The blacklegged tick has a two-year life span that is divided into three main developmental stages: larva, nymph, and adult. A blacklegged tick requires a bloodmeal in every one of these stages, and each of these bloodmeals provides an opportunity for the tick to contract or spread B. burgdorferi.
Bloodmeal 1 (larvae): Tick may contract B. burgdorferi
Mother ticks cannot pass B. burgdorferi on to the 2,000 eggs they lay in the spring. Thus, when ticks hatch into larvae in the summer, they are un-infected. To move on to the next stage of life, a tick larva must find a source of blood for its first meal. The larva’s preferred meal at this stage comes from a common rodent called the white-footed mouse (Peromyscus leucopus), but it may also feed on other small mammals or birds. If the animal on which a larva feeds is infected with B. burgdorferi, the larva can become infected as well and will be able to transmit the disease to animals that it bites in the future. An important note is that humans and other large mammals are not considered carriers of B. burgdorferi like certain small animals. Levels of bacteria sufficient to infect ticks cannot be found in human or deer blood. Thus, since the tick cannot infect its eggs with B. burgdorferi, the bacteria depend on small animals like the white-footed mouse to act as reservoirs of the disease. It is only from these carriers that the disease can be picked up and spread by the tick vector.
Bloodmeal 2 (nymphs): Infected tick may spread disease to new host
After its bloodmeal, the larva falls off its host and molts into a nymph, which will remain dormant throughout the winter. In the spring, the nymph becomes active and must eat again. If the nymph was already infected during its first meal as a larva, it can now transmit B. burgdorferi to its new host. Usually, this second bloodmeal is also from the white-footed mouse or another small animal, but sometimes it’s from a human.
This nymphal stage is the most dangerous for humans. Most adult ticks are so large that they are quickly noticed and removed before B. burgdorferi can be spread because a tick usually must be attached to its host for at least 36 hours to spread the disease. Tick nymphs, on the other hand, are only about the size of a poppy seed, so they can easily go unnoticed by humans for days.
Bloodmeal 3 (adults): Infected tick may infect new host with B. burgdorferi
After taking its second bloodmeal in May, June, or July, nymphs molt into adult females and males. These ticks become active in the fall, which surprises many people who assume they’d be killed off by frosts. While adult ticks may be inactive when temperatures are freezing, they can become active and will seek a host when not frozen or covered by snow. Adult blacklegged ticks feed on larger mammals, most commonly on white-tail deer (Odocoileus virginianus). The tick mates on the deer and then the female drops off and lays her eggs. Deer are considered to be the main reproductive host for blacklegged ticks, but they can also bite humans and their pets. If they are infected with B. burgdorferi, they can transmit the disease to humans at this stage.
So How Do We Reduce Lyme Disease Cases?
The tick life cycle and populations are tightly intertwined with the spread of Lyme disease, and ticks need small mammals like white-footed mice to become infected. We also know that adult ticks prefer to feed on deer, so they play a role too. Culling deer herds has been shown to help, but an integrated approach is even better — one that involves removing some deer while treating the remaining ones with acaricides (pesticides that kill ticks). A device called the 4-Poster Deer Treatment Bait Station is one way of applying acaricides to deer.
Exterminating all of the white-footed mice would be neither practical, nor environmentally friendly. Plus, it is unlikely to be effective, since the ticks would simply find another host, such as chipmunks or birds. However, there are ways to treat the mice with acaricides as well. One method that can be utilized by homeowners involves providing nesting material for mice that contains an acaricide. When the mice build nests out of the material, they are exposed to the chemical and ticks that feed on them will die.
Another product lures mice into small boxes that brush them with insecticide. More creative methods of tick control, such as the use of microscopic roundworms or fungi to infect and kill ticks, are also under investigation.
On a smaller scale, there are steps individuals can take to protect themselves from Lyme disease:
– Be aware when entering rural areas that may have dense tick populations. Suburban backyards and even semi-urban parklands are just some of the more common settings where these ticks are found.
– In the summer, wear insect repellent in your shoes and clothing, especially from May through July. Even wearing treated light summer clothing can be effective and protective.
– In cooler weather, wear long pants and insect repellent. Tucking your pants into your socks will not win you any fashion awards, but it will offer extra protection because ticks tend to crawl up from the ground.
– Thoroughly check yourself for ticks when you come in from the outdoors.
– Remove any ticks as soon as possible.
– See a doctor if you develop a fever or a rash.
Hopefully, with increasing public awareness and more creative techniques to control tick populations, we will begin to see a decrease in Lyme disease cases. Until then, medical entomologists are invaluable for shedding light on this convoluted disease and for determining how best to fight it.
Hannah Foster is a PhD student in molecular and cellular biology at Harvard University and a freelance writer. She studies protein biochemistry in microbes, and enjoys writing about science and non-science alike. You can follow her on Twitter at @Foster_HR and read her blog about boxing as it pertains to life at theblowbyblow.com. She is also a frequent contributor to Harvard Science in the News Flash and to The Bitter Empire.