Gordon A. Surgeoner

DURATION OF BORRELIA BURGDORFERI INFECTIVITY IN WHITE-FOOTED MICE FOR THE TICK VECTOR IXODES SCAPULARIS UNDER LABORATORY AND FIELD CONDITIONS IN ONTARIO

The duration of Borrelia burgdorferi infectivity in white-footed mice (Peromyscus leucopus) experimentally inoculated or infested with infected Ixodes scapularis nymphs was evaluated. Infectivity was assessed by infesting these mice with unfed I. scapularis larvae at 7, 21, 35 and 49 days post-inoculation (DPI) or post-infestation (PI). At 7 DPI, B. burgdorferi was transmitted from 18 of 24 syringe-inoculated mice and all three tick-infected mice to I. scapularis larvae which fed upon them. However, at 21, 35 and 49 DPI, significantly fewer mice were infective. Borrelia burgdorferi was isolated from tissues of 14 of 22 syringe-inoculated mice about 56 DPI, and from all three tick-infected mice. However, the level of agreement between xeno-diagnosis and bacterial culture was no greater than would be expected by chance alone. We also determined if B. burgdorferi infectivity of mice varied in relation to periods of tick feeding in the field. White-footed mice were trapped during April, July and August 1993 from two habitats on Long Point peninsula (Ontario, Canada), where B. burgdorferi is endemic. Mice from each habitat were infested with laboratory-reared I. scapularis larvae. Ticks from each mouse were subsequently examined by immunofluorescent assay for B. burgdorferi infection and mice were cultured for B. burgdorferi. None of 3577 I. scapularis larvae fed on 62 mice captured within the cottonwood dune habitat were infected with B. burgdorferi, although it was isolated from six of these mice. Within the maple forest habitat, 0/24, 8/21 (38%) and 1/21 (5%) mice transmitted B. burgdorferi to I. scapularis larvae during April, July and August, respectively. Most mice from the maple forest with B. burgdorferi-positive tissues (14/21) were collected during July, although the level of agreement between xenodiagnosis and tissue culture was poor. Because B. burgdorferi infectivity in mice appears to be of short duration, overwintered I. scapularis larvae and nymphs may have to feed upon infected hosts at the same time of year in order for a cycle of B. burgdorferi infection to be maintained on Long Point. Infected I. scapularis nymphs, rather than persistently infected vertebrate hosts, likely serve as the overwintering “reservoir” for B. burgdorferi on Long Point.

THE GROUNDHOG TICK IXODES COOKEI (ACARI: IXODIDAE): A POOR POTENTIAL VECTOR OF LYME BORRELIOSIS

Evidence for infection with the spirochete, Borrelia burgdorferi, was sought in Ixodes cookei and in groundhogs (Marmota monax) in southern Ontario, Canada, and ticks fed on experimentally inoculated hosts were examined for the spirochete. Borrelia burgdorferi was not detected by immunofluorescent examination of 110 larval, nymphal or adult I. cookei collected from the environment, or taken from humans and other animals. Three groundhogs inoculated with B. burgdorferi developed titers of 1:20 to 1:80 by the indirect immunofluorescent antibody test, but B. burgdorferi was not isolated from the spleens, kidneys, or urinary bladders of these animals. One of 30 wild groundhogs had an antibody titer of 1:20 to B. burgdorferi. Three (5%) of 59 I. cookei larvae fed on B. burgdorferi-infected hamsters became infected, in comparison with 23 (28%) of 82 I. dammini larvae fed on the same hosts. Borrelia burgdorferi was present in 5%, 16% and 4% of molted I. cookei nymphs fed on infected hamsters, rats or a groundhog, respectively; prevalences of infection in I. dammini fed on the same hosts were significantly (P < 0.05) higher (45%, 36% and 23%, respectively), as was the intensity of infection. A naive groundhog on which I. cookei nymphs from an infected cohort fed did not become infected with B. burgdorferi, but it is uncertain whether an infected tick engorged on the experimental host. Ixodes cookei seems to be an inefficient vector of B. burgdorferi, and is unlikely to be significant in nature. Groundhogs are potential wildlife reservoirs of B. burgdorferi, based on their capacity to transmit infection to I. dammini.

RESPONSE OF THE MEADOW VOLE (MICROTUS PENNSYLVANICUS) TO EXPERIMENTAL INOCULATION WITH BORRELIA BURGDORFERI

The response of the meadow vole (Microtus pennsylvanicus) to infection by experimental inoculation with Borrelia burgdorferi was evaluated. Forty-two adult voles were inoculated subcutaneously with 0.5 × 106 spirochetes. Sera taken during the 196 day trial were tested by indirect fluorescent antibody (IFA) assay for antibodies to B. burgdorferi. Tissues from animals which died during the trial, and from animals killed at 28, 112 and 196 days post-inoculation (DPI), respectively, were cultured in BSK-II medium for ≤6 weeks. They also were examined histologically for lesions and the presence of spirochetes. All inoculated animals developed antibodies by 14 DPI and maintained titers ≥1:10 for the duration of the trial. Spirochetes were isolated from ears, bladder, and spleen. Spirochetes also were identified by Bosma-Steiner silver stain or tissue IFA assay in sections of ears, bladder, kidney and heart. Infection as confirmed by re-isolation persisted for ≤111 days. No lesions were identified in association with the presence of spirochetes. No increase in mortality was observed in inoculated animals compared with controls. Sensitivity of the IFA test at a cut-off titer of 1:10 was 100% from ≥14 DPI, but at 1:20 reached a maxiumum of 97%. Specificity at 1:10 was 84% and at 1:20 was 97%. Use of antiserum to Microtus immunoglobulin (Ig) in a double-layered test provided no significant advantages over use of a commercial fluorescein-conjugated anti-mouse Ig in a single-layered IFA test.