Steadily increasing over the past two decades, Lyme Disease (LD) is now the most prevalent vector borne disease in the northern hemisphere accounting for over 90% of all vector borne diseases in the United States (Bacon et al., 2008). True annual United States LD incidence nears 300,000 cases (Kuehn, 2013). In Canada, LD became a reportable disease in 2009; since then reported cases have more than doubled from 128 in 2009 to over 500 in 2013 (Public Health Agency of Canada, 2014). Spurred by climate warming, projection models suggest that the range of the disease’s tick vector (Ixodes scapularis) will expand 46km/year, such that the proportion of the human population of Eastern Canada residing in a tick inhabited zone will increase from 18% (2010), to 80% in 2020 (Leighton et al., 2012). Presently LD is endemic in areas of British Columbia, Manitoba, Ontario, New Brunswick and Nova Scotia. Interestingly, LD incidence peaks in middle-aged adults aged 55-59, an occurrence uncommon for infectious diseases.
Lyme disease caused by infection with the bacterial spirochete Borrelia burgdorferi, is the most prevalent vector-borne disease in North America. The US Centers for Disease Control and Prevention state that reported incidence of this multisystem disease has increased steadily since its discovery over 30 years ago, with more than 30,000 new confirmed or probable cases in 2013. Persistent infection with B. burgdorferi can lead to severely debilitating clinical manifestations in humans including chronic arthritis, carditis, and neurological impairment. Lyme disease can usually be treated effectively with antibiotics during early stages of infection, however early clinical symptoms including fever, headache, and extreme fatigue are often ambiguous, leading to misdiagnosis and mistreatment. In addition, chronic Lyme disease is frequently refractory to long-term antibiotics, and animal studies are beginning to indicate the persistence of some form of live antibiotic-resistant B. burgdorferi after antibiotic treatment. Due to the difficulty in diagnosing early infection and the limited treatment options available for the chronic disease, prevention is a much more appropriate control measure for Lyme disease unless more decisively effective treatment measures are found. Unfortunately, there is currently no approved vaccine available for humans.
Borrelia burgdorferi pathogen and enzootic cycle
Ticks have a three-stage life cycle: larva, nymph and adult; one blood meal is taken per stage. Transovarial transmission does not occur in B. burgdorferi, however; the infection is passed after moulting between the three life cycles. Larva or nymph stage ticks acquire the bacteria by feeding on reservoir animals carrying the bacteria, namely mice, squirrels or birds. Additionally, adult ticks feed, as well as mate on larger animals such as deer allowing for the maintenance of tick and B. burgdorferi populations. Nymphs are primarily responsible for the majority of transmission to humans; humans are considered dead end hosts and do not form part of the enzootic cycle. Dogs are considered incidental hosts and also do not form part of the enzootic cycle.
B. burgdorferi is an obligate parasite, requiring either a tick vector or vertebrate host for survival. Lyme disease is enzootic in nature, with small mammals including the white-footed mouse (Peromyscus leucopus) serving as the primary reservoir for B. burgdorferi. Infection is transmitted between vertebrate hosts by hard-shelled tick vectors, with Ixodes scapularis and I. pacificus as the most significant vectors in North America. Uninfected larval ticks acquire infection during a blood meal from an infected host, resulting in colonization of the midgut by B. burgdorferi. Infected larval ticks then molt into nymphs, and can transmit B. burgdorferi to uninfected animals and humans during a second bloodmeal. Upon initiation of feeding, B. burgdorferi migrates to the tick salivary glands and is subsequently transmitted to the new host, completing the enzootic cycle. This horizontal transmission from tick to mammal and back again, is central to the maintenance of B. burgdorferi in the environment. An emerging philosophy for the control of vector-borne diseases involves a multi-faceted approach by targeting the host-pathogen, vector-pathogen, and vector-host interfaces. In order for this type of approach to be successful for the control of Lyme disease, a thorough understanding of the interactions between B. burgdorferi, the tick vector, and the vertebrate host is essential.