Vector of Disease
The different life cycle stages of deer ticks may serve as vectors of several pathogens. Some very relevant pathogens are listed below.
In 1982 a treponema-like spirochete lateron recognised as a new species and named Borrelia burgdorferi was first isolated from the midgut of adult Ixodes scapularis and suggested to be involved in the etiology of Lyme disease.
The spirochete is most likely transmitted from the tick to the host through the saliva, as these ticks salivate excessively during feeding.
Typically, immature I. scapularis acquire B. burgdorferi during their initial bloodmeal from infected reservoir hosts, primarily from white-footed mice. After molting, the subsequent life stage is transstadially infected.
Transstadially infected nymphs and adults can then transmit the spirochete to non-infected hosts.
Because nymphal feeding precedes larval feeding in a given year, the enzootic transmission of B. burgdorferi is highly efficient. Thus, the majority of mice become infected with Lyme disease spirochetes in the spring before serving as hosts to the larvae of a different population later that summer. Spirochetes overwinter in the fed larvae, in the unfed nymphs or in the host animal.
Transmission of Babesia microti in humans takes place primarily through direct tick-host contact, although transfusion-acquired human babesiosis cases have been documented on several occasions.
I. scapularis becomes infected with B. microti most likely while feeding on infective reservoir hosts. In the northeastern United States, this enzootic cycle is maintained principally between immature I. scapularis and their primary animal host, the white-footed mouse (P. leucopus). Larvae acquire the infection when feeding on B. microti-infected mice. Engorged larvae then overwinter and pass the parasites transstadially to the subsequent nymphal stage the following spring.
Research indicates that B. microti can survive in salivary glands of ticks for 9-10 months. Nymphs infected from the larval stage are able to transmit the infection to susceptible animals upon feeding.
Adults primarily feed on white-tailed deer (O. virginanus), which is not a competent reservoir for B. microti.
Apparently, B. microti parasites acquired by larval I. scapularis do not survive into the adult stage and only about 25% of the adults that derived the infection as nymphs become infectious. Transovarial transmission of the infection in I. scapularis has not been documented. Therefore, it appears that the role of adult I. scapularis in the transmission of B. microti infection is minimal.
When an infected tick attaches to a host, sporozoites are readily shed into the saliva and injected into the animal during the feeding process.
Human Anaplasmosis/Human Ehrlichiosis
Human granulocytic anaplasmosis (HGA), formerly known as human granulocytic ehrlichiosis (HGE) with its etological agent Anaplasma phagocytophilum is transmitted among others by I. scapularis. The infection occurs internationally, and areas of endemicity include the United States (northeastern and mid-Atlantic, Upper Midwest, and Pacific Northwest states), Europe, and Asia (China, Siberian Russia, and Korea) (Dumler et al., 2005). For the US I. scapularis is the vector in the eastern United States, while Ixodes pacificus is the vector in the western United States (Dumler et al., 2007).
Small mammals, such as white-footed mice (Peromyscus leucopus); dusky-footed wood rats (Neotoma fuscipes); or others, such as Apodemus, Microtus, or Clethrionymus species are likely reservoirs (Dumler et al., 2005).
A tick-borne encephalitis-like virus was first discovered in 1997 from I. scapularis in coastal New England (Telford et al., 1997). This virus, provisionally referred to as deer tick virus (DTV), was isolated and identified via reverse transcriptase PCR and direct sequencing of the products. DTV is similar to, but distinct from, Powassan virus and may represent a new subtype of Powassan virus.
- Dumler JS, Choi KS, Garcia-Garcia JC, et al.: Human granulocytic anaplasmosis and Anaplasma phagocytophilum. Emerg Infect Dis. 2005, 11, 1828–34
- Dumler JS, Madigan JE, Pusterla N, et al.: Ehrlichioses in humans: epidemiology, clinical presentation, diagnosis, and treatment. Clin Inf Dis. 2007, 45 (Suppl.), S45–51
- Telford SR 3rd, Armstrong PM, Katavolos P, et al.: A new tick-borne encephalitis-like virus infecting New England deer ticks, Ixodes dammini. Emerg Infect Dis. 1997, 3, 165-70