What is Lyme Disease?
by , M.D. (June 1998)
Lyme disease (LD) is of concern to orienteers, because the disease is transmitted by tick bites. Ticks are endemic in outdoors terrain, and therefore we are at high risk for being bitten. Since Lyme disease can affect the skin, joints, heart, central nervous system, and other organ systems, we should know as much about its prevention, diagnosis, and treatment as possible to avoid these serious sequellae.
Lyme disease is an uncommon complication that may result from being bitten by the Western Black-legged tick, Ixodes pacificus. The causative agent is Borrelia burgdorferi, a type of spirochete-like bacterium that lives in the tick. If the tick that bites you is not the Ixodes type, then there is no risk of Lyme disease.
If the tick is of the Ixodes genus, don't panic. Only a small percentage of Ixodes ticks are infected. Also, removal of the tick within 24 hours of attachment has shown to reduce or prevent transmission of Lyme disease. If the tick is of the correct species and has been attached a long time, your doctor may recommend that the tick be tested for the agent that causes Lyme disease. If the tick test is negative, it is extremely unlikely that you were exposed to Lyme disease.
If the tick test is positive, this does not necessarily mean that you have Lyme disease. Even an infected tick does not always transmit the infection. If you develop an enlarging red rash (which may occur near the tick bite or elsewhere), fever, malaise, headache, muscle or joint pain, or other flu-like symptoms in the following days to weeks, see your physician about further serological testing and/or antibiotic treatment.
|Table of contents
Prevalence and Incidence
The prevalence of infected ticks is highest in New England and the middle Atlantic states, but they also are present in northern California and Oregon. In California, 172 cases of LD were reported in 1997, up 100% from the 86 cases reported in 1996. There were 16,461 cases reported nationwide in 1996, so California has a relatively few number of cases. However, the true incidence is probably higher because of under-reporting.
The good news is that the disease is only carried by a less common species of tick, and only a small percentage of these ticks are infected with the spirochete. There are three common ticks found in California. However, the only species that has found to be a vector for LD is Ixodes pacificus, the Western Black-legged tick. In a study performed by Robert Lane, an entomologist at UC Berkeley, of the ticks found in Tilden Park, only 26% were Ixodes, and 64% were Dermacentor occidentalis, the Pacific Coast tick, and few than 1% were the Dermacentor variabilis, the American Dog tick.
In addition, only 0 to 6% of adult ticks were found to be carriers, and 0 to 14% of nymphs were infected. The highest rates were found in places like Potter Valley in Mendocino County. Therefore, for every 1000 tick bites, only 260 will be by the Ixodes genus, and of those, less than 3 will be infected with Borrelia. Also, the tick must be attached to the human for at least 48 hours before the infection is transmitted. If the tick is removed within 24-48 hours and before it is engorged with blood, human infection will be very rare.
The Tick's Life Cycle
In the spring, tick eggs mature into larvae, which feed on mice. If the mouse is infected, the larva may pick up the spirochete. After one year, larvae develop into nymphs (1.0-1.2 mm in size, or about the size of a poppy seed). Larvae live in leaf litter in damp, well-shaded forests, and don't survive long on a sunny dry trail. Nymphs feed on the dusky-footed wood rat, deer, and other mammals including domestic pets and humans. Nymphs are most active in spring and summer. If the wood rat is a B. burgdorferi carrier, then the nymph may pick up the spirochete during its blood meal (or pick up more spirochetes if it was already infected as a larva).
The spirochetes multiply in the nymph midgut, but then may pass through the gut wall into the lymph stream and lodge in its salivary glands. The spirochete may then be transmitted to a human or other mammal via tick saliva. This process is believed to take at least 48 hours (11% infected at 48 hours, 25% at 72 hours, and 80% at 96 hours). Most of the blood meal is not ingested until the final day. Thus, the nymph remains small and hard to detect until just before detachment.
After another year, a nymph becomes an adult tick (2-3 mm in size). Adult ticks also feed on mammals and humans. The adult tick lays eggs and the cycle is repeated. Adult ticks live in brush and grassland and low vegetation, and most are active from fall to early spring. Adult ticks must feed for 24 hours to transmit Borrelia burgdorferi to their victim.
Erythema migrans (EM) is the classic rash of early Lyme disease. It develops in 60-80% of those who contract LD. It often occurs 7-10 days after the tick bite, but may occur days to a few months later. EM has a red annular (ringlike) border that gradually expands with partial central clearing (bull's eye rash). It may occur anywhere on the body. It is at least 5 cm in diameter, with the median size being 15 cm. The rash usually last 3-4 weeks if untreated, and one week if treated with antibiotics. Note that most patients will have an inflammatory reaction at the site of the tick bite cause by tick saliva. This rash does not expand, goes away in a few days, and is not EM.
Other acute symptoms include flu-like symptoms, such as fever, malaise, fatigue, headache, mild neck stiffness, enlarged lymph nodes, muscle and joint aches. These symptoms usually develop within one month after the tick bite, and often at the time when EM is present.
Late manifestations include arthritis, neurological and cardiac symptoms. The arthritis is typically recurrent (about every 3 months), brief (lasting weeks or months) episodes of pain, swelling and redness of one or a few joints. Virtually any neurological symptom can be caused by LD, including neuritis, meningitis, encephalitis, and facial palsy. Heart block can occur, as well as inflammation of the heart muscle.
There are a variety of serological tests that can be performed to see if you have been exposed to Borrelia burgdorferi. However, there is no single "gold standard" test that can give you early, reliable evidence of a recent infection with the Lyme spirochete. There is variable sensitivity (being able to pick up all of the people who are infected, i.e., it has a low rate of false negative tests) and specificity (only picking up those who are infected with the Lyme spirochete and not some other process, i.e., having a low rate of false positive tests) to these tests. These tests should be considered as an aid to clinical diagnosis, and if the clinical symptoms strongly suggest Lyme disease, then a positive test will be confirmatory. For most people, however, a serological test will not be necessary.
Early Lyme disease should be treated with oral antibiotics. There are many effective antibiotics, but treatment should be for 21 days. Because 99% of tick bites will not result in LD, prophylactic antibiotics should not be used. This is not only because of the cost, but because 80% of patients will develop minor drug reactions, and 6% life-threatening drug reactions. However, if you develop the rash Erythema migrans, you should take antibiotic treatment.
Awareness and knowledge about LD are perhaps the most important factors in its prevention. Wear a hat, long-sleeved shirt, and long pants. Use duct tape to seal your pants and gaiters to your shoes to prevent ticks from crawling up your legs. Insect repellents such as DEET can be used on your skin and permethrins on clothes. After an orienteering event, do a careful tick search. Have a friend examine your back and scalp.
If you find a tick, remove it by grasping its head with tweezers and applying gentle traction. Save the tick! Place it in a closed container such as a 35mm film canister with a cotton ball moistened with water.
- If an adult tick has been on you less than 24 hours, (or a nymph for less than 48 hours), and it is not engorged, you are at low risk for getting the disease, and no serological test and no antibiotic treatment is required.
- If the tick has been on you for longer than 24 (or 48 hours for a nymph) and is engorged, identify the tick species. Most hospital labs can do species identification. The Ixodes has black legs. If the tick is not Ixodes, there is no risk of disease, and no serological test and no antibiotic treatment is necessary.
- If the tick is Ixodes, have it tested to see whether it is infected with Borrelia burgdorferi.
- The Contra Costa Mosquito & Vector Control District, 155 Mason Circle, Concord, CA 94520, 925-685-9301, will do the test on live ticks only. At this time (January 2009), there is no charge.
- The Sonoma Public Health Laboratory, 3313 Chanate Road, Santa Rosa, CA 95404, 707-576-4711, will test live or dead ticks, but not dry ones. They charge $20.
- IgeneX, Inc., 797 San Antonio Road, Palo Alto, CA 94303, 800-832-3200. They will test live or dead and dry ticks. The cost is $49.00.
- BBI Clinical Laboratory, 75 North Mountain Road, New Britain, CT 06053, 800-225-1900 will test live or dead ticks for $39.00.
- If you develop Erythema migrans, you should be treated with antibiotics, even if you don't remember being bitten by a tick. Sometimes the tick is so small that it is not noticed.
- If you have been in an endemic area within 30 days of developing clinical symptoms of Lyme disease (flu like symptoms, arthritis, neurological symptoms, etc.), you should see your doctor and probably have one or more of the serological tests performed. If the clinical symptoms are suggestive of LD, and the serological tests are positive, you should be treated with antibiotics.
There was a lyme disease vaccine, Lymerex, on the market between 1998 and 2002 but the manufacturer, GlaxoSmithKline, withdrew it citing poor sales. At this time (January 2009), there is no vaccine against Lyme disease available.