Tickborne Infections

Epidemiology

Although ticks are found throughout the world, individual species have evolved with requirements for specific hosts, habitats, and climates. Most tickborne diseases also have a specific geographic distribution ( Table 89.1 ). Several factors influence the incidence of tickborne infections. Human factors include the degree and duration of exposure to tick-infested habitats and host immunity. Tick-related factors include the prevalence of ticks and their usual hosts, infection rates, seasonal activity, and the relative likelihood of individual species to feed on humans. Climate change, including temperature, precipitation and seasonal weather patterns, also plays an important role in increasing the prevalence and transmission of tickborne disease. ^, Warming temperatures allow for the spread of tick vectors to new geographic areas, enhance tick abundance, and result in longer seasonal activity.

Not all diseases caused by ticks are infectious. Tick paralysis may result after exposure of the host to neurotoxic substances produced salivary glands of engorged female ticks, usually after attachment for at least 5 days. ^, This rare disease manifests as ataxia or acute symmetric ascending flaccid paralysis without sensory loss or pain. Symptoms may be confused with botulism or Guillain-Barré syndrome. Most cases occur in children, with girls affected more often than boys. There are no diagnostic tests for tick paralysis. The diagnosis is made by finding an embedded tick and watching for improvement after tick removal. Symptoms usually resolve within 24 hours. ^, Cases of tick paralysis have been reported worldwide, and 43 tick species have been implicated. In North America, most episodes occur in the western US and Canada and are associated with Dermacentor andersoni or Dermacentor variabilis. Cases cluster in April through June, when Dermacentor species ticks emerge from hibernation.

Pathogenesis

The tick life cycle includes three stages: larvae, nymph, and adult. Both the larva and the nymph must take a blood meal to progress from one stage to the next. Hard-bodied ticks feed slowly over several days and only once per stage. Soft-bodied ticks may take up to 10 short blood meals (i.e., minutes to hours) that involve multiple hosts. Ticks become infected with microorganisms when this blood meal is taken from an infected host. Some pathogens (e.g., Borrelia spp., Rickettsia spp., Francisella tularensis ) are also transmitted transovarially and transtadially (i.e., from larva to nymph to adult), allowing ticks to serve as reservoirs and vectors. Horizontal transmission when ticks are cofeeding on a host has been described but is rare.

Transmission of pathogens to humans usually occurs when the feeding site of an attached tick is contaminated with infected salivary secretions , regurgitated midgut contents, feces, or coxal fluid. The duration of tick attachment necessary for transmission varies widely. Transmission of bacterial agents usually requires more than 24 hours, although Anaplasma phagocytophilum can be transmitted more rapidly. Tick-borne encephalitis virus is transmitted within minutes of a tick bite, and early removal does not decrease transmission. Transmission occasionally occurs in the absence of attachment, as is seen when nonintact skin or conjunctiva are touched after crushing an infected tick.

Many of the pathogens described in this chapter are not transmitted exclusively by tick bites. Blood transfusions can be the source of A. phagocyophilum. Babesia species can be transmitted by blood products, organ transplantation, or transplacentally. Tick-borne encephalitis virus can be acquired by consumption of unpasteurized dairy products, and uncommon modes of transmission include the slaughter of viremic animals, blood transfusions, and from mother to child via breastmilk. Human-to-human transmission of Crimean-Congo fever has been described, with outbreaks in healthcare settings.