November 1 was a warm, sunny day in Vermont a few seasons ago. As I walked out of the woods at the LaPlatte River Natural Area in Shelburne, I looked down at my pant legs and saw them-ticks-a dozen or more on each leg. As I brushed the tenacious ticks away I thought, “Vermont-we’ve got a problem”. Lyme disease is the most common tick-borne disease in the country, accounting for over 30,000 new cases each year. In recent years Lyme disease cases in Vermont have skyrocketed. In 2013 nearly 900 probable or confirmed cases were reported to the Vermont Department of Health, placing Vermont first in the country in terms of the incidence of this disorder. Other states with a high incidence of Lyme disease included Maine and New Hampshire. What is going on here?
In North America Lyme disease is caused by a spirochete called Borrelia burgdorferi. This microorganism is transmitted by the bite of black-legged ticks (Ixodes scapularis), formerly known as deer ticks. These ticks are commonplace in our forests and are increasing, due in part to reforestation of the landscape and perhaps the effect of climate change. But this is not the whole story. Ixodes ticks have a complicated life cycle. These ticks do not live forever, thank God. Rather, they live for two years during which they undergo three successive molts, that is transformation to a larger size and different form. After hatching from an egg these stages of development are called larva, nymph, and adult. At each one of these stages the tick must ingest a blood meal from an appropriate host or else it dies. Many hosts can serve as blood donors-mice, chipmunks, deer, even birds, especially birds that nest or forage on the ground like sparrows, robins, thrushes, and ovenbirds. Once the ticks ingest a blood meal they drop off the host, hide in the leaf litter while they undergo a molt to the next stage of development, and lurk in the brush waiting to attach like a burr to their next victim. Ultimately, adult male and female ticks meet on a host and reproduce. White-tailed deer appear to be the preferred host for adult ticks. At this point the pregnant female, after ingesting a third blood meal, drops off the host and lays her eggs on the ground.
If the ticks are infected with B. burgdorferi they can transmit this spirochete to their blood donors. However, not all of their hosts are “competent” to serve as a reservoir for this infection in nature. To qualify as a reservoir three conditions must be met-availability, poor grooming behavior, and an ability to tolerate chronic infection with B. burgdorferi. The mammals that best fit these qualifications are white-footed mice, chipmunks, and shrews. White-footed mice, in particular, are the most abundant and competent reservoir host.
Interestingly enough, black-legged tick larvae are not infected at birth. Rather, they acquire Borrelia from a reservoir host during their first blood meal. Subsequently, the ticks are infected for life and can pass this infection on to other animals and to you. However, many animals, including deer, clear the infection rapidly and do not pass the spirochete on to ticks or to other creatures. Birds carry ticks, too, but efficiently remove them and kill them with their bills during the process of grooming. Birds also recover quickly from Borrelia burgdorferi infection and do not transmit the organism to other animals. Thus, sustaining Borrelia infections in nature involves a complicated process in which the infected reservoir population, and therefore the density of infected ticks, is regulated by the availability of food, the presence or absence of predators and competitors for that food, and the diversity and number of available hosts for black-legged ticks, only some of which serve as a reservoir for B. burgdorferi.
So what about you? If you are a Vermont birder who spends time in the woods you are at significant risk of acquiring Lyme disease from a tick bite. As soon as you enter the woods you intersect with the life cycle of the black-legged tick. That is, you have entered their environment. Both nymph and adult black-legged ticks are capable of transmitting B. burgdorferi to you, but the nymphs are more problematic because they are tiny and their bite may not be recognized. Adult ticks, in contrast, are larger and are often removed before they have attached and fed. The first nymphs appear in the spring, when the temperature exceeds 40 degrees F. Nymphs that have fed successfully drop off of their hosts and develop in the leaf litter into adults. The population of adult ticks peaks in the fall. Hence, the ticks that I found on my pant legs in November were adult black-legged ticks.
How likely is it that a nymph or adult black-legged tick in Vermont is, in fact, infected with B. burdorferi? The short answer is “very”. First, according to tick surveys conducted by the Vermont Department of Health, black-legged ticks are the most common ticks found in Vermont, accounting for over 75% of the ticks submitted for identification, although American dog ticks and Lone Star ticks were identified in the survey, as well. In 2013 Alan Geise from Lyndon State and Erica Berl from the Vermont Department of Health conducted a survey of ticks obtained from 12 sites in Vermont-six in the eastern part of the state and six in the western part. Both northern and southern counties were represented in this study. With the exception of the northeastern corner of the state infected ticks were discovered in every county studied. In aggregate 27% of the captured nymphs were infected with B. burgdorferi as were 62% of the adults. These percentages tended to be higher in the southern and western counties in Vermont. Once you know that the percentage of infected ticks is high, though, the exact number becomes less important, since the density of infected ticks in specific habitats varies tremendously even within a given county. Smaller forest fragments with abundant understory and leaf litter and fewer predators and competitors tend to support larger populations of white-footed mice and a higher density of infected ticks. So be careful out there!
So what to do to prevent Lyme disease assuming that you are unwilling to stay out of the woods? The Vermont Department of Health makes wise recommendations for prevention encapsulated in the slogan “Repel-Inspect-Remove”. Wear light clothing to more easily recognize attached ticks. Tuck your pant legs into your socks. Spray Permethrin, an acaricide, onto your clothes. Apply 25%-30% DEET to your exposed skin. Shortly after returning from your walk in the woods check your entire body for ticks in front of a mirror. Shower within a few hours of returning home to wash off unattached ticks that you may have missed and to remove DEET. You can even toss your clothes into the dryer for an hour at high heat. Dry heat kills ticks. Each of these measures contributes independently to the success of prevention, so the more methods you use, the greater the likelihood of success.
If you do find a tick attached to your skin remove it with tweezers by grasping its mouth parts at the level of the skin and lifting steadily. Don’t bend, fold, or mutilate the tick if you can avoid it. Wash the wound with soap and water to avoid secondary infection. Black-legged ticks must be attached to your skin for at least 24-36 hours before infection with B.burgdorferi is transmitted to you. The reason for this time lag is that the spirochete resides in the mid-gut of the tick and is tethered to the cells lining the gut. Once the tick begins to ingest blood, though, the organism undergoes a reorganization of the surface proteins responsible for adherence, releases its grip on the lining cells, and migrates into the tick's circulation, ultimately appearing in the saliva. It is tick saliva that transmits the organism to you.
A newly-attached tick often stimulates a local inflammatory response in the skin surrounding the attachment site. This is not a sign of Lyme disease. Also, during feeding ticks become engorged with blood and increase significantly in size. At this point after removing the tick, a single dose of an antibiotic such as doxycycline given within 72 hours is often effective in preventing Lyme disease. Consult your primary care physician for advice. Don’t forget your pets. Dogs and other domestic animals can get Lyme disease and can bring ticks into the house from outside. Check your pets for ticks as well as yourself.
What if, despite all of your efforts at prevention, you develop Lyme disease? Lyme disease peaks in June, July, and August, mirroring the appearance of tick nymphs, and evolves through several stages-early manifestations, early disseminated disease, and late complications. Over 70% of Lyme disease patients develop a characteristic skin rash expanding from the site of the tick bite called erythema migrans. This rash occurs 3-30 days after the tick bite and may or may not have a characteristic “bulls-eye” appearance. This rash may be accompanied by constitutional symptoms such as fever, muscle pain, and fatigue. A few weeks later other symptoms may develop such as a disseminated skin rash, stiff neck, headache, facial weakness (Bell’s palsy), or even cardiac symptoms such as palpitations. Weeks or months later asymmetrical arthritis occurs in about 30% of untreated patients, manifested by swelling of a large joint, typically one of the knees, with or without pain. Subtle cognitive disabilities may be present, as well. If the diagnosis of Lyme disease is established antibiotics can be effective in killing the spirochete. The earlier in the course of the disease that treatment is started the better.
A few other diseases are transmitted by black-legged ticks, although currently these diseases are rare or uncommon in Vermont. Anaplasmosis, a rickettsial disease manifested by flu-like symptoms and depression of white blood cells and platelets in the circulation, has been reported in Vermont mainly in Bennington and Rutland counties. Anaplasmosis responds to antibiotic therapy, but can cause severe illness if left untreated and occasionally can be fatal. The number of cases of Anaplsmosis reported in Vermont, while less than 100 cases per year, has been rising steadily.
Babesiosis is a parasitic disease like malaria and, like malaria, can cause severe hemolytic anemia. Babesiosis is transmitted by black-legged ticks. Only a single reported case of babesiosis that was acquired in the state (that is, not acquired by travel to other parts of the country where babesiosis is endemic) has been confirmed in Vermont.
Finally, Powassan-type virus infection can be transmitted by Ixodes ticks. This virus causes encephalitis that resembles West Nile virus infection. Only one case of Powassan virus infection has been confirmed in Vermont in 1999.
In conclusion Lyme disease is a multisystemic inflammatory disorder and is the most common tick-borne disease in the country. The incidence of Lyme disease has increased dramatically in Vermont, New Hampshire, and Maine during the past decade. Awareness of the risk of tick-borne diseases in Vermont is essential for individuals who spend a great deal of time outdoors. The risk of acquiring Lyme disease is best understood in the context of the life cycle of the black-legged tick and how you and your pets intersect with this life cycle. Effective means of preventing Lyme disease are available and should be used. Remember Repel-Inspect-Remove and be careful.
For further information visit the Vermont Department of Health website at http://healthvermont.gov and the CDC website at http://cdc.gov and search for Lyme disease. Here is a link to my 2015 presentation about Lyme disease in Vermont and its prevention.
Lyme Disease in Vermont Presentation by Bruce MacPherson (PDF)