Seasonal dynamics of Anaplasma phagocytophila in a rodent-tick (Ixodes trianguliceps) system, United Kingdom

Abstract

We investigated the reservoir role of European wild rodents for Anaplasma phagocytophila using polymerase chain reaction (PCR) analysis of blood collected from individually tagged rodents captured monthly over 2 years. The only tick species observed in the woodland study site was Ixodes trianguliceps, and ruminant reservoir hosts were not known to occur. A. phagocytophila infections were detected in both bank voles and wood mice but were restricted to periods of peak nymphal and adult tick activity. Most PCR-positive rodents were positive only once, suggesting that rodent infections are generally short-lived and that ticks rather than rodents may maintain the infection over winter. Bank voles were more likely to be PCR positive than wood mice, possibly because detectable infections are longer lived in bank voles. This study confirms that woodland rodents can maintain A. phagocytophila in Great Britain in the absence of other reservoir hosts and suggests that I. trianguliceps is a competent vector.

Figure 1

The mean (+/- SE) numbers of larval, nymphal, and adult Ixodes trianguliceps ticks counted per rodent at 4-week intervals, 1997–1998. Shaded areas of similar intensity indicate ticks of different instars that may have belonged to the same cohort, according to interstadial development times deduced by Randolph (21). Arrows indicate potential transmission cycles: bold arrows indicate potential transmission from infected nymphs to uninfected larvae by means of rodent infections. Fine arrows indicate potential transstadial transmission from infected engorged larvae to infected host-seeking nymphs. For clarity only one within-year (A) and one between-year (B) cycle involving nymphal and larval ticks are illustrated.

Figure 2

Prevalence of infection of Anaplasma phagocytophila (bar graphs +/- exact binomial errors) in blood samples collected from bank voles (graph marked a) and wood mice (graph marked b) compared to the mean monthly numbers of nymphal and adult Ixodes trianguliceps ticks counted per rodent at the time blood samples were collected (line graphs), 1997–1998.