Lyme disease is a multisystemic, tick-vectored, zoonotic disease associated with infection by spirochetes of the genus Borrelia (Levine,1995).
The disease was first characterized in the mid-1970s during an investigation of an outbreak of arthritis near Old Lyme, Connecticut (Spach et al., 1993). Though the name Lyme disease was new, the syndrome was soon recognized to be similar to erythaema chronicum migrans and acrodermatitis chronica atrophicans, disease entities recognized in Europe as early as 1883 (Levine, 1995). Borrelia burgdorferi is the etiological agent of Lyme disease in the United States, while B. garinii and B. afzelii are also associated with borreliosis in Europe (Saint-Girons et al., 1994).
Lyme disease is the most reported vector-borne disease of humans in the United States. Lyme disease is common in children, with about one-fourth of all reported cases occurring in children < 14 years of age (Sood, 1999). During 1992–2006, a total of 248,074 cases of Lyme disease was reported to CDC by health departments in the 50 states, the District of Columbia, and US territories.
The annual count increased 101%, from 9908 cases in 1992 to 19,931 cases in 2006. During this 15-year period, 93% of cases were reported from 10 states (Connecticut, Delaware, Massachusetts, Maryland, Minnesota, New Jersey, New York, Pennsylvania, Rhode Island, and Wisconsin). Incidence was highest among children aged 5–14 years, and 53% of all reported cases occurred among males (Bacon et al., 2008). For comparison, the Borrelia infestation rate of Ixodes ricinus is 7%, with wide disparity between administrative districts. Prospective work in 1999–2000 established the estimated incidence of Lyme disease (9.4/100,000) and of neuroborreliosis (0.6/100,000) in France. Incidence was higher in certain regions, such as Alsace, with an estimated Lyme disease incidence at 86 to 200/100,000 inhabitants and neuroborreloisis at 10/100,000 (Blanc, 2009).
Infection with B. burgdorferi has been demonstrated in several mammalian and avian species (Barbour and Fish, 1993; Levine, 1995). However, as Lyme disease is not a reportable disease in animals, reliable incidence data are not currently available for wild and domestic species. The enzootic cycle of Lyme disease does not normally include humans or domestic animals (Mather et al., 1989). Rather, maintenance of B. burgdorferi in the environment is dependent on a wildlife reservoir and a transmission vector. B. burgdorferi is exceptional amongst Borrelia spp. in that it is capable of infecting a wide range of tick and vertebrate hosts. Thus, different animals and vectors are responsible for perpetuating B. burgdorferi in the different geographic regions in which Lyme disease is endemic. Several ticks of the genus Ixodes have been demonstrated to be competent in transmitting Lyme borreliosis. In the eastern and north-central United States I. scapularis is the main vector (more than 50% of the ticks can be found infected), while in the Pacific states transmission is primarily via I. pacificus (of which 1–6% are infected). In Europe I. ricinus is the main vector (Lane et al., 1991; Barbour and Fish, 1993). Transovarial passage of B. burgdorferi in Ixodes ticks is only 1–5% (Lane et al., 1991), suggesting that ticks act as vectors rather than as reservoirs for the spirochete. In the north-eastern United States, it has been shown that the white-footed mouse, Peromyscus leucopus, is the primary reservoir of B. burgdorferi (Mather et al., 1989; Levine, 1995). Mice generally become infected when fed upon by nymph stage I. scapularis, which acquire spirochetes during their larval feeding (Genchi, 1992). Adult I. scapularis feed primarily on white-tailed deer (Odocoileus virginianus). In northern California the life cycle of B. burgdorferi often involves two tick species. I. neotomae is responsible for maintaining the spirochete in the dusky-footed wood rat (Neotoma fuscipes), the primary reservoir in that region (Lane et al., 1991). However, the host range of I. neotomae is narrow, so I. pacificus is the vector responsible for spreading B. burgdorferi infection to other species including humans and dogs. All three stages of ticks can be found on humans and domestic animals (Kazmierczak and Sorhage, 1993). In Europe, Lyme borreliosis is mainly transmitted by I. ricinus (Genchi, 1992). The infection rate in these ticks varies from 4% to 40%.
A major concern of any zoonosis of the dog is the potential that contact with canines will increase the likelihood of humans contracting the disease. Studies have indicated that dogs are competent reservoirs of B. burgdorferi (Mather et al., 1994). Thus, naive ticks that feed on infected dogs are likely to become infected. Such ticks are then capable of infecting other vertebrates including humans. By introducing infected ticks into the human environment, dogs are capable of increasing dog owners’ exposure to B. burgdorferi. Estimates of the prevalence of antibody to B. burgdorferi in dog populations in Massachusetts offers a sensitive, reliable, and convenient measure of the potential risk to people of B. burgdorferi in the environment (Lindenmayer et al., 1991). Risk factors for canine seropositivity may directly or indirectly illuminate certain aspects of the epidemiology of human Lyme disease. Dogs infected with B. burgdorferi may manifest some of the clinical signs which are common in humans, including acute onset of recurrent lameness, fever, lethargy, and inappetance. Other symptoms that occur with lower frequency include generalized lymphadenopathy, CNS disorders, uveitis, renal lesions, and cardiac disease (Kazmierczak and Sorhage, 1993). In humans, Lyme disease manifests with a variety of dermatologic, rheumatologic, cardiac, and neurologic abnormalities (Steere, 1989; Levine, 1995). The CDC case definition of Lyme disease includes the characteristic erythema migrans (EM) lesion (> 5 cm in diameter) or laboratory confirmation of B. burgdorferi infection, and at least one of the objective clinical signs of the disease (CDC, 1990). Erythema migrans is virtually pathognomonic when it occurs, but is only detected in approximately 60% of patients (Steere, 1989). In the early stages of the disease, EM is often accompanied by muscle pain, headache, and fatigue. Leukocytosis, increased RBC sedimentation rate, and haematuria may also occur. In Europe, the manifestations of Lyme borreliosis are slightly different, with nervous-system involvement being more common, especially facial palsy, meningitis, and polyradiculoneuritis, and are more common in children (17–38% of cases) than in adults (Sood, 1999). Chronic arthritis has been reported in up to one-third of German children with Lyme arthritis (Sood, 1999). Diagnosis is mainly based on serological assays. A ‘two-step approach’ has been widely adopted and has improved the specificity of the diagnosis. It is based on an enzyme immunoassay or indirect immunofluorescence assay followed by an immunoblotting assay (Western blot). Detection of IgM is investigated in early stages of the infection. Borreliosis is resolved in most patients with a 10–21-day course of treatment with doxycycline (100mg × 2 per day) or amoxicillin (2g × 3 per day) orally (per os). Intravenous ceftriaxone (2g per day for 2–3 weeks) may be indicated if the infection is not detected in the early stages or appears refractory to initial treatment protocols (Steere, 1989; Levine, 1995). For reasons that are poorly understood, some individuals are unable to overcome B. burgdorferi infection. These patients may experience chronic peripheral nervous system and CNS abnormalities including depression, fatigue, sleep disorders, and memory loss for months to years following the initial infection (Barbour and Fish, 1993). Prevention of Lyme disease in humans and domestic animals relies largely on minimizing exposure to ticks. Tick-infested areas should be avoided whenever possible. When travelling in areas of high tick density, exposure can be minimized by wearing long-sleeved shirts and long pants tucked into one’s socks. Tick repellents and acaricides are available for human and animal use. Humans should inspect themselves and their pets regularly for ticks, and carefully remove them as soon as possible after contact (Barbour and Fish, 1993; Kazmierczak and Sorhage, 1993). Removal of ticks within 48h of attachment has been shown to significantly decrease the likelihood of transmission of B. burgdorferi from an infected tick (Shih and Spielman, 1993). In addition to vector control, the other method of preventing borreliosis in dogs and humans is by vaccination. There are currently various vaccines available for use in dogs in the United States including inactivated whole-organism bacterins, while the others are recombinant vaccines based on either OspA and/or OspC proteins (Levine, 1995). Concern about residual pathogenicity has precluded development of whole-cell vaccines for human use (Barbour and Fish, 1993).