Comparative vector competence of Dermacentor variabilis and Ixodes scapularis (Acari : Ixodidae) for the agent of human granulocytic ehrlichiosis

Vector competence of Ixodes scapularis Say and Dermacentor variabilis Say f or the agent of human granulocytic ehrlichiosis (HGE) was compared. Five wh ite-footed mice, Peromyscus leucopus Rafinesque, were inoculated intra-peri toneally with blood from a mouse infected with the agent of BCE. Approximat ely 100 I. scapularis and D. variabilis larvae were placed on each mouse an d allowed to feed to repletion. Fed larvae were collected. separated accord ing to species and allowed to molt to nymphs. Twenty-six per cent of I. sca pularis (34/131) and 11% of D. variabilis (11/96) tested positive for the a gent of HGE by polymerase chain reaction (PCR) after molting to nymphs. Pos itive I. scapularis nymphs transmitted the agent of HGE to P. leucopus mice in 5 of 5 trials. In contrast, the positive D, variabilis nymphs did not t ransmit the agent of HGE in any of 6 trials. In a 2nd experiment, 3 P. leuc opus mice were infested with I. scapularis nymphs that were infected with t he agent of HGE. All 3 mice became infected with the agent of HGE and appro ximate to 300 D, variabilis larvae were placed on each mouse and allowed to feed to repletion. Larvae were collected and allowed to molt to nymphs as before. Approximately 8% (5/60) of the nymphs became positive for the agent of HGE as determined by PCR Twenty-five of these nymphs were then placed o n each of 9 P. leucopus mice and allowed to feed to repletion. Evidence of transmission was not observed in any of 9 mice exposed to D. variabilis nym phs. These results demonstrate that although I, scapularis is a competent v ector of the agent of the HGE, D, variabilis is not.