B burgdorferi in host animals and ticks

I, scapularis has a wide host range, with immature stages feeding on over 30 species of mammals, 49 species of birds, and several species of lizards (Magnarelli et al, 1986; Anderson, 1988). A few relatively common species, such as the white-footed mouse (Peromyscus leucopus), may feed the majority of individual ticks (Wilson, 1998). This generalist granivore (Nupp and Swihart, 1996) is common in woodlands of the eastern US. Its significance in the transmission cycle of tick-borne pathogens lies in its role as a reservoir host that is capable of becoming infected with B, burgdorferi, Anaplasma phagocytophilum, and Babesia microti from ticks that feed on it, maintaining the infections for at least several weeks, and transmitting the pathogens to new ticks it picks up in the environment (Piesman and Spielman, 1979). Although white-footed mice are considered the principal reservoir for B, burgdorferi in nature (Levine et al,, 1985; Donahue et al,, 1987), other animals can serve to infect ticks with this pathogen and may be important reservoirs in certain regions. These include medium-sized mammals such as raccoons (Procyon lotor) and skunks {Mephitis mephitis) (Fish and Daniels, 1990), as well as smaller mammals such as chipmunks (Mather et al,, 1989), voles, and gray squirrels (Hanincova et al,, 2006), which play an important role in Lyme disease ecology in residential areas. Birds are important as carriers of ticks and some, such as American robins (Turdus migratorius), serve as reservoirs of B, burgdorferi (Anderson et al,, 1986; Battaly et al,, 1987; Richter et al,, 2000). Birds are implicated in the emergence of Lyme disease in previously unaffected areas (Richter et al,, 2000).

Interestingly, although white-tailed deer can serve as hosts for both immature and adult I, scapularis, they do not appear to be competent reservoirs for B burgdorferi. In one study, only 1 percent of nymphal ticks derived from larvae collected from deer were infected - significantly less than the 23 percent infection rate of field-collected nymphal ticks in that geographic area (Telford et al,, 1988).

The B, burgdorferi infection rate of ticks varies considerably among stages. Transovarial transmission of B, burgdorferi, defined as infection of the maternal oocytes by pathogens, with passage to the progeny (Harwood and James, 1979), is not very efficient in either I, scapularis or the closely-related I, pacificus tick species (Magnarelli et al., 1987; Schoeler and Lane, 1993). Therefore, ticks in the larval stage typically have very low infection rates (< 1 percent) and are not considered epidemiological^ important vectors for B. burgdorferi.

Nymphal I. scapularis, on the other hand, have infection rates that usually average 20 to 25 percent in endemic areas, although there can be tremendous site-to-site variation in these rates (Maupin et al, 1991; Schwartz et al., 1997; Daniels et al., 1998). Adult I. scapularis have had an additional blood meal and are therefore afforded an increased opportunity to become infected with B. burgdorferi. Infection rates in adult ticks typically average 45 to 50 percent in endemic areas (Maupin et al., 1991; Schwartz et al, 1997).

The two-year lifecycle of I. scapularis in the northeastern US is an important factor in maintaining B. burgdorferi in nature. Host-seeking larvae are uninfected with B. burgdorferi, therefore they must feed on infected reservoir host animals to become infected and pass the spirochete transstadially to the nymphal stage. This is possible because the larvae of one population cohort feed after the nymphs of the previous cohort in any one year (Figure 5.4). Since approximately 25 percent of the nymphs in that previous cohort (with peak activity in June and July) are infected, they serve to infect hosts, such as white-footed mice, which then serve to infect the larvae that subsequently feed on them in August and September, and the spirochete is maintained through the enzootic cycle. This two-year lifecycle, and the resulting seasonal activity patterns of immature I. scapularis, are the keys to both the efficient transmission of B. burgdorferi to the next generation of ticks (Spielman et al., 1985; Wilson and Spielman, 1985) and the high risk of human exposure to spirochete-infected nymphs (Fish, 1993).

It should be noted, however, that since the two-year lifecycle may not occur in all parts of the range of I. scapularis, zoonotic transmission of B. burgdorferi may not always occur in this way. For example, in the southern US, larval activity may precede and/or occur along with that of nymphs, and thus amplification of the spirochete in I. scapularis is attenuated (Clark et al., 1998).

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