Lyme disease, also known as Lyme arthritis or Lyme borreliosis, is named after the town in Connecticut where it was initially described in the late 1970s. It is caused by a spirochetal bacterium, Borrelia burgdorferi, which is transmitted to humans and animals primarily by Ixodes scapularis in most parts of the U.S., and by I. pacificus in the western U.S. The primary reservoir host for B. burgdorferi in the east and midwest is the white-footed mouse, Peromyscus leucopus. Ticks are hatched from eggs as uninfected larvae, which feed upon infected mice, then molt and harden into infected nymphs. Nymphs are indiscriminant in their host range, and feed upon various mammals and birds, thereby spreading the infection among wildlife reservoirs which facilitate expansion of the geographic range of both the agent and the vector. Nymphs feed, molt and harden into adults, which feed preferentially upon white tailed deer (Otocoileus virginianus). Humans are at most risk of contracting Lyme disease through the nymphal stage of the tick, whereas dogs and livestock can acquire infection from adult ticks as well. Lyme borreliosis also occurs in other parts of the world, and is transmitted by ticks of the Ixodes perculcatus complex that interface with other reservoir hosts. In these areas, the disease is associated with infection with B. burgdorferi sensu stricto, B. afzelii, B. garinii, and other closely related Borreliae, belonging to a group known as B. burgdorferi sensu lato (s.l.). In Europe, ticks may carry all three types of Borrelia, and co-infections of humans have been documented. Throughout the world, ticks of the I. persulcatus complex carry and transmit not only B. burgdorferi s.l., but also a guild of other pathogens, including Babesia microti, Anaplasma (formerly Ehrlichia) phagocytophilum, tick-borne encephalitis viruses, and other agents. Co-infections of humans have been documented, and complicate diagnosis and treatment. Furthermore, “Lyme-like” disease, or “Southern Tick Associated Rash Illness” (STARI), is associated with EM that is indistinguishable from the EM of Lyme disease. STARI occurs in the southern U.S. in association with the lone star tick, Amblyoma americanum, and Borrelia lonestari, which is only distantly related to B. burgdorferi and more closely related to relapsing fever Borreliae.
Lyme disease is the most common arthropod-borne disease in the U.S., with approximately 25,000 cases reported by the CDC in 2002. These cases include those that fit within strict CDC case criteria, but the actual number of cases is generally believed to be at least ten-times higher. Certain areas of the country are hyper-endemic, including the northeast region, mid-Atlantic region, upper north central region, and some regions of northern coastal California, whereas other areas may be endemic, but environmental factors and life cycles of reservoir hosts and vectors may preclude a high risk (such as in California). The geographic distribution and prevalence of the vector and the agent are increasing, as are the number of human cases. Although Lyme disease is rarely fatal, it can be associated with a wide variety of symptoms that unfortunately mimic many other diseases, making definitive clinical diagnosis a challenge. Patients who live in non-endemic areas but acquire infection during visits to endemic areas, or patients from low-endemic areas may not be effectively diagnosed and treated during the early stage of infection, when successful treatment is most likely. Delayed diagnosis or late treatment may be associated with a high risk of treatment failure. Co-infection with other members of the tick-borne pathogen guild may also be responsible for treatment failure when treatment is specifically directed at B. burgdorferi.
The most diagnostic lesion of Lyme disease humans is an expanding “bull’s eye” skin rash, known as erythema migrans (EM), which is recognized in 70-80% of cases. EM occurs at the site of the tick bite, approximately 2 weeks after the tick has fed. This stage of infection is often associated with flu-like symptoms and regional lymphadenopathy. EM spontaneously resolves without antibiotic treatment, but infection persists and proceeds to dissemination to multiple organs. Secondary EM may arise in multiple sites following disseminated infection. Major syndromes during the disseminated phase of infection include cardiac arrhythmias, atrioventricular block and subtle myocarditis. More severe manifestations may occur in some patients, including myopericarditis, left ventricular dysfunction, cardiomyopathy, or fatal pancarditis. Intermittent oligoarticular arthritis (thus the name Lyme arthritis) occurs in about 60% of patients, involving a number of joints, especially the knee. About 10% of patients proceed to chronic arthritis after several intermittent attacks. Chronic arthritis may last several months or years, despite antibiotic therapy. This tends to be associated with patients of HLA DR4 haplotype. Peripheral neuropathy is common, and Lyme disease is the most common cause of facial nerve paralysis (Bell’s palsy) in endemic areas. Meningitis, neck stiffness, neurocognitive disorder, encephalitis, dementia, motor or sensory radiculoneuritis, ataxia, and myelitis have all been described, along with many other signs and symptoms. In utero transmission has been suspected. Treatment with oral antibiotics is generally effective when administered during early infection. Intravenous antibiotic therapy for longer periods of time may be required for chronic cases, but objective criteria for success are difficult, as symptoms improve slowly over a period of months. A small number of appropriately treated patients continue to have subjective symptoms of musculoskeletal pain, neurocognitive difficulties or fatigue that may last for years. This syndrome can be very disabling, and has been termed “chronic Lyme disease” or “post-Lyme disease syndrome.” There is continuing debate in the medical community over whether these patients have active Lyme disease (i.e. infection), or are manifesting sequelae of prior infection that has been eliminated.
Because of the indiscriminant host range of I. scapularis, Borrelia burgdorferi has been shown to infect a myriad and wide variety of wild mammals and birds within endemic areas, and readily infects a number of experimental animals. Experimental studies in rodents, dogs and non-human primates have confirmed that infection can be associated with disease in several tissues, especially heart and joints, and infection is typically persistent. A high percentage of dogs within endemic areas are seropositive, and likely to be infected, but only a small number of these animals manifest clinical signs, including transient fever, anorexia, oligoarticular arthritis and cardiac conduction defects. As in humans, disease is intermittent. Anterior cruciate rupture has been linked to Lyme arthritis in dogs. In addition, some dogs develop severe treatment-refractory and fatal glomerulonephritis, which is not seen in other species. The pathogenesis of kidney disease has not been determined, but it is suspected to have an immune complex etiology. It is not directly associated with infection of the kidney. Infection has been documented in cows and horses, with arthritis as the major feature, and cats are also likely to be infected. Other than dogs, very little is known about Lyme disease in domestic animals.
The most effective means of avoiding Lyme disease is prevention. When in Lyme disease-endemic areas, tick repellants may be used, but protective clothing is best (hats, long sleeve shirts, long pants tucked into socks, etc.). Close inspection of the body after being outdoors should be routinely practiced, with particular attention to armpits, crotch, belt and bra lines, and hair lines. Ticks attach and feed for 3 or more days, and Borreliae are not generally transmitted until 24-48 hours after tick attachment. Thus, prompt removal of ticks is effective at avoiding infection. The risk of infection after tick feeding is relatively low. The tick attachment site can be monitored for the appearance of EM over the next few weeks, at which time prompt treatment is advised. In contrast, if prophylactic treatment is desired, a single oral dose of 200 mg doxycyline within 72 hours of a tick bite has been found to be effective. Prompt treatment in the early EM-stage of infection with 3-4 weeks of oral doxycyline or amoxicillin is highly effective. If a patient is allergic to penicillin or cannot take tetracyclines, patients can be successfully treated with erythromycin or cefuroxime axetil. In later disease, particularly in patients with objective neurologic manifestations, intravenous ceftriaxone or penicillin for 4 weeks or more may be needed. Treatment failure may occur, requiring re-treatment. An effective recombinant outer surface protein A (OspA) vaccine has been developed for use in humans, but was withdrawn from the market. A whole bacterial bacterin has been aggressively marketed for use in dogs, but consumers must beware of the unique risk of dogs to immune complex glomerulonephritis that is associated with undefined antigens of B. burgdorferi. Because dogs with Lyme disease respond well to antibiotic therapy, if they manifest illness at all, the need for vaccination should be thoroughly considered, especially in areas in which Lyme disease is not hyper-endemic.