Wednesday, June 13, 2007
Summertime is upon us here in New England, and you know what that means. Trips to the beaches of Cape Cod or the islands, backyard barbeques with friends and neighbors, and, as some of us know far too well, a sharp increase in bug bites. Mosquitoes might be the more frequent agitators, but around these parts they pose a much smaller medical risk than the other major arthropod: the tick population.
Of import, Lyme disease is carried by ticks, as well as other illnesses, and is especially prevalent along the east coast between Maryland and Boston. It is an interesting disease, as its discovery was one of the great victories of epidemiology. In 1975, in the small town of Lyme, Connecticut, there was an outbreak of juvenile rheumatoid arthritis, a fairly rare disease. This prompted the CDC to do further investigation, and they discovered that the arthritis was a result of a tick-borne agent, Borrelia burgdorferi. Here in Rhode Island there are about 80 cases per 100,000 people, Connecticut is at 133, New York about 30, and the highest incidence was Columbia County, NY, with 1,583 cases per 100,000. And it is becoming more prevalent; Connecticut recently noted that cases are underreported, and even across the Atlantic in England concerns of Lyme disease are mounting.
Borrelia burgdorferi is an interesting type of bacteria. Instead of having the typical oval-rod shape that we think of with bacteria, borrelia is a spirochete, and it looks almost exactly like it sounds it might. It likens a corkscrew, and is so thin that it is very hard to see with most standard microscope techniques. The spirochete has two membranes, and between these inner and outer membrane is a series of 7 to 11 endoflagella, which are basically like little tails that make the spirochete undulate and move around, somewhat akin to a snake.
With Lyme disease, the spriochete is carried in Ixodes scapularis, the common deer tick. The deer tick has an interesting life cycle. It feeds three times in its life: first as a larvae, on a white footed mouse where it gains the spirochete; then as a nymph it bites and infects a human or dog; and finally as an adult it bites its namesake, the deer.
Disease commonly presents with three stages. First, you present with erythema migrans, a bulls eye rash around the initial bite that is caused by the spirochetes moving within the skin centrifugally around the wound. You may also have generic flu symptoms, which don't tell much about the disease, or neurologic symptoms from the migration of the spirochetes. This stage lasts a few weeks and 50% don't recall a tick bite. The infection then goes underground, only to resurface weeks to months later. Here you commonly get meningitis, and an interesting neurological symptom called Bell's Palsy, where the infection irritates the facial nerve and half of your face becomes paralyzed and "droops." Patients can have heart complications that vary in severity; rarely they may also see eye involvement. This stage will also subside, and in another 6 months to a year the end-stage complications of the infection will manifest. These include frank arthritis, which is thought to be an autoimmune disease initiated because the spirochete may mimic a cellular antigen. The majority of patients will also have neurologic involvement, with difficulty concentrating, symptoms that may appear like multiple sclerosis or a progressive encephalitis, and an incapacitating fatigue.
The good news is that we can treat the illness rather effectively if we catch it early. Some of the late effects cannot be reversed, and the arthritis often remains because it is an autoimmune reaction that persists after the spirochete is gone. Moreover, the tick needs to bite for more than 24 hours to transmit a high enough dose of spirochetes to infect. If you find a tick on you, pull it out slowly but firmly with some tweezers perpendicular to your skin. Of course, if you are going to be out in the woods, prevention is the best cure, and using DEET or tucking your pants into your socks can help avoid the deer ticks in the first place. Above all, have some fun this summer, and don't let a bug bite make you miss a barbeque!