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Field evidence for density-dependent facilitation amongst Ixodes ricinus ticks feeding on sheep

Published online by Cambridge University Press:  30 July 2002

N. H. OGDEN ,
A. N. J. CASEY ,
N. P. FRENCH ,
J. D. W. ADAMS  and
Z. WOLDEHIWET
N. H. OGDEN
Affiliation:
Department of Veterinary Preclinical Science, University of Liverpool, Faculty of Veterinary Science, Brownlow Hill and Crown Street, Liverpool L69 7ZJ, UK
A. N. J. CASEY
Affiliation:
Department of Veterinary Preclinical Science, University of Liverpool, Faculty of Veterinary Science, Brownlow Hill and Crown Street, Liverpool L69 7ZJ, UK
N. P. FRENCH
Affiliation:
Department of Veterinary Clinical Science and Animal Husbandry and
J. D. W. ADAMS
Affiliation:
Department of Veterinary Clinical Science and Animal Husbandry and
Z. WOLDEHIWET
Affiliation:
Department of Veterinary Pathology, University of Liverpool, Faculty of Veterinary Science, Leahurst, Neston, South Wirral CH64 7TE, UK
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Abstract

Engorged Ixodes ricinus nymphs collected from sheep resident in an upland UK field site were significantly lighter than nymphs that engorged on previously tick-naïve sheep, indicating that site-resident sheep continually exposed to ticks acquired anti-tick resistance. The weights of engorged nymphs that fed on naturally tick-resistant sheep increased significantly, however, when increasingly high numbers of adult female ticks fed on the sheep during seasonal peaks of tick activity. This relationship was unaffected by variations in nymph weight amongst individual sheep, between seasons and years, and potential effects of sheep infection with Ehrlichia phagocytophila; this suggests that high adult tick infestations may directly inhibit the expression of acquired anti-tick resistance by sheep. The length, width and weight of adult ticks and the scutum length of adult females were linearly related to their weight as an engorged nymph. The mean scutum length of adult female ticks feeding on sheep in the field site was greater than that of adult females obtained from engorged nymphs collected from sheep of the same site. This suggests that larger ticks have a survival advantage and that I. ricinus ticks exhibit density-dependent intraspecific facilitation at high infestation levels with potential consequences for the transmission of tick-borne diseases.

Keywords

sheepIxodes ricinustickresistancetransmissionBorrelia burgdorferiEhrlichia phagocytophila
Type
Research Article
Information
Parasitology , Volume 124 , Issue 2 , February 2002 , pp. 117 - 125
Copyright
2002 Cambridge University Press

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