Risk factors in habitats of the tick Ixodes ricinus influencing human exposure to Ehrlichia phagocytophila bacteria.

Ixodes ricinus L. (Acari: Ixodida) were sampled during 1996-99 in southern Scotland, on vegetation using cloth drags, on humans by removal from clothing and on roe deer (Capreolus capreolus L.) by searching legs of culled deer. Developmental microclimate was recorded by automatic recorders and questing microclimate by portable instruments during tick collections. Ticks and deer were examined for infection with Ehrlichia phagocytophila bacteria (Rickettsiales) using microscopy and polymerase chain reaction. This pathogen causes tick-borne fever of sheep in Europe and human granulocytic ehrlichiosis in North America, but in Europe human clinical ehrlichiosis due to E. phagocytophila has not been recorded despite serological evidence of exposure. Among three types of habitat, coniferous woodland was most infested with questing ticks (560 ticks/km of drag; mean numbers collected on long trousers: 24.3 larvae, 13.5 nymphs and 0.8 adult ticks/km walked), deciduous woodland had slightly lower infestation (426 ticks/km drag) and upland sheep pasture had much lower infestation (220 ticks/km drag). Of the three main vegetation types, bracken was least infested (360 ticks/km drag), ericas most (430 ticks/km drag) and grassland had intermediate infestation density (413 ticks/km drag). Questing and developmental microclimates were poor predictors of exposure within these habitats, except lower infestation of pastures was attributed to greater illumination there. Collectors who walked a total of 300 km through all habitats (taking 360 h in all seasons), wearing cotton trousers hanging outside rubber boots, were bitten by only four nymphs and 11 larvae of I. ricinus (but no adult ticks). There was a negative correlation between densities of deer and ticks collected, although presence of deer remains a major indicator of exposure. The proportion of infected ticks was fairly uniform at four sites studied. Overall prevalence of E. phagocytophila in I. ricinus was 3.3% in nymphs (40/1203) but only approximately 1.5% in adults of both sexes (although males do not bite). It was estimated that nymphs of I. ricinus gave 4.4% probability of one infected bite/person/year (for occupational exposure during this research) due to presence in all seasons and habitats, their human biting rate of 0.011 nymphs/h or 0.013 nymphs/km and widespread infection with E. phagocytophila. The frequency distribution of intensity of infection in ticks was approximately normal (mean 98 morulae/nymph infected), thus there is a high risk of receiving a high dose from any one infected tick bite.

Field evidence that roe deer (Capreolus capreolus) are a natural host for Ehrlichia phagocytophila.

Samples of blood, spleen and legs from 112 culled roe deer (Capreolus capreolus) were collected from nine sites widespread in the United Kingdom. The prevalence of infection with Ehrlichia phagocytophila was determined by serology and polymerase chain reaction. Means of 58% of 102 plasma or serum samples were seroreactive by IFA, 38% of 84 blood samples and 29% of 82 spleen samples were positive by PCR. Ticks on legs of 71 roe deer were Ixodes ricinus larvae, nymphs and adults and 83% of legs were infested. Numbers of ticks corresponded positively to the percentage of samples positive for E. phagocytophila by serology and PCR for different sampling sites. Ixodes ricinus nymphs collected from the vegetation at one site with infected deer were analysed for infection with E. phagocytophila by examination of Feulgen stained salivary glands. Of 135 nymphs 5% were infected. These results confirm that roe deer are commonly parasitized by both E. phagocytophila and its vector tick in such a way that it is likely to be an important natural mammalian reservoir of E. phagocytophila.

Natural prevalence of infection with Ehrlichia (Cytoecetes) phagocytophila of Ixodes ricinus ticks in Scotland.

Ixodes ricinus nymphs and adults were collected from vegetation and from sheep at four sites in Scotland typical of areas endemic for tick-borne fever in sheep caused by infection with Ehrlichia (Cytoecetes) phagocytophila (Rickettsiales). The great majority of ticks examined was from woodland sites adjacent to sheep farms where there was a high probability of them feeding on roe deer (Capreolus capreolus) in a non-domestic focus of infestation and infection. Ticks were examined for infection by five methods. Batches of ticks were examined either by feeding on susceptible sheep or by feeding on rabbits and then prepared as stabilate which was inoculated into susceptible sheep. The sheep were monitored for clinical signs of tick borne fever. Batches of ticks were examined by polymerase chain reaction for Ehrlichia phagocytophila. Salivary glands were dissected out and stained by the Feulgen method to detect Ehrlichia masses, and were examined by indirect fluorescent antibody test. Each of the methods detected infection in ticks and the prevalence of infection in nymphs with the various methods ranged from >0.25% to 2.0%. Small samples of adults examined by Feulgen staining of salivary glands indicated infection prevalences of 2.1% in males and 1.6% in females. It is considered that these low infection prevalences may be typical of natural foci of infection where deer could be a major host of ticks and E. phagocytophila.