Am. J. Trop. Med. Hyg., 60(4), 1999, pp. 598–609
Copyright 1999 by The American Society of Tropical Medicine and Hygiene
LONGITUDINAL STUDY OF INFECTION WITH BORRELIA BURGDORFERI IN A
POPULATION OF PEROMYSCUS LEUCOPUS AT A LYME DISEASE-ENZOOTIC
SITE IN MARYLAND
ERIK K. HOFMEISTER, BARBARA A. ELLIS, GREGORY E. GLASS, AND JAMES E. CHILDS
Department of Molecular Microbiology and Immunology, Johns Hopkins University School of Hygiene and Public Health, Baltimore, Maryland
Abstract. The maintenance of Borrelia burgdorferi in a population of Peromyscus leucopus was investigated from 202 mark and recapture mice and 61 mice that were removed from a site in Baltimore County, Mar yland.
Borrelia burgdorferi infection was detected by culture and polymerase chain reaction (PCR) of ear tissue, and exposure to the spirochete was quantified by serology. Overall prevalence of B. burgdorferi, as determined by culture and PCR of ear tissue at first capture, was 25% in the longitudinal sample and 42% in the cross-sectional sample. Significantly more juvenile mice were captured in the longitudinal sample (18%) than in the cross-sectional sample (0%).
Among 36 captured juvenile mice, only one was infected with B. burgdorferi; this contributed to a significant trend for infection with B. burgdorferi with age. Recover y from infection with B. burgdorferi was not detected among 77 mice followed for an average of 160 days.
The incidence rate of infection with B. burgdorferi was 10 times greater in mice captured during two periods of high risk of exposure to nymphal Ixodes scapularis ticks compared with a period of low risk. Maintenance of B. burgdorferi in this population was dependent on indirect transmission of the organism from infected ticks to susceptible mice and development of chronic infection with the spirochete, which had no measurable effect on the sur vival of infected mice.
Since Lyme disease was first described in 1977,1 it has become the most commonly reported vector-borne disease in the United States.2 The disease, which is caused by the spirochete Borrelia burgdorferi,3,4 is transmitted to humans by the bite of an Ixodid tick: Ixodes scapularis in the eastern and northcentral parts of the United States (I. dammini5 as synonymized by Oliver and others6) and I. pacificus7, 8 in the western United States.
An enzootic cycle involving vertebrate hosts and ticks is critical for the maintenance of the spirochete in Lyme disease enzootic areas in the eastern and mid-central parts of the United States because the rate of transovarial transmission of the spirochete in I. scapularis is extremely low.9
Transovarial transmission of B. burgdorferi in I. pacificus has been demonstrated;10 however, on the west coast of the United States the relative importance of ticks and vertebrate animals in the maintenance of B. burgdorferi remains unclear.
Although B. burgdorferi has been isolated from a wide variety of mammalian and avian hosts in the eastern and northcentral parts of the United States,11,12 only white-footed mice (Peromyscus leucopus),13,14 chipmunks (Tamias striatus) and meadow voles (Microtus pennsylvanicus),15 skunks
(Mephitis mephitis) and raccoons (Procyon lotor),16 and Norway rats (Rattus norvegicus)17 have been shown to be competent reser voir hosts of B. burgdorferi in enzootic cycles of the organism in those areas.
White-footed mice, in comparison with meadow voles and chipmunks15 and skunks and raccoons,16 are generally recognized in these areas as the most important reser voir hosts of the organism.
In other areas of the United States in which B. burgdorferi is enzootic, other mammals and reptiles have been demonstrated to be competent reser voir hosts12,18–20 and may play an equally prominent role.
Field studies have implicated I. scapularis as the vector of indirect transmission of the spirochete to mice and other reservoir hosts.21
The rates of direct horizontal and vertical transmission of B. burgdorferi in the maintenance of the spirochete in a population of white-footed mice are unknown, but the rate of vertical transmission may be extremely low based on laborator y studies.22
The effects of B. burgdorferi infection on P. leucopus fitness and population dynamics remains unclear.
Despite the development of a specific immune response,23,24 field25,26 and laborator y studies22,27 have shown that white-footed mice develop chronic infection with B. burgdorferi.
Histologic lesions of carditis and arthritis28 and cystitis29 have been described in experimentally infected, immature white-footed mice; however, histologic lesions were not obser ved in mature mice similarly infected with the spirochete.28
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