Transmission of Anaplasma phagocytophilum to Ixodes ricinus ticks from sheep in the acute and post-acute phases of infection

Abstract

A total of 60 sheep were exposed to Anaplasma phagocytophilum infection on an enclosed area of Ixodes ricinus-infested pasture in North Wales, United Kingdom, and rapidly acquired acute A. phagocytophilum infections detectable by PCR and blood smear examination. Of the ticks that had engorged in the previous instar on infected sheep, 52% of adult ticks and 28% of nymphs were PCR positive; a significant, 10-fold increase in prevalence compared to that of ticks that engorged on sheep preinfection was observed (P = 0.015). The likelihood that ticks were PCR positive, after feeding on the sheep and molting to the next instar, increased marginally with increasing numbers of infected neutrophils per milliliter of blood of their sheep host (P = 0.068) and increased significantly when they were collected from sheep carrying higher numbers of adult female ticks (P = 0.017), but increasing numbers of feeding nymphs had a significant negative effect on transmission (P = 0.049). The numbers of circulating neutrophils and of infected neutrophils also varied significantly with the numbers of ticks feeding on the sheep when the blood was collected. Our study suggests that ruminants are efficient reservoirs of A. phagocytophilum during the acute and post-acute phases of infection. The risk of ruminant-derived infections may, however, be strongly affected by variations in tick densities, which may influence transmission from acutely infected animals via effects on the numbers of infected cells in the blood and possibly by within-skin modulation of infection.

FIG. 1.

Box plots showing medians, quartiles, and outlier values of the numbers of I. ricinus ticks counted on sheep during the study. Letters in boxes refer to the groups of sheep in the study, and the arrows indicate the time points at which these groups of sheep were introduced to the I. ricinus-infested study site.

FIG. 2.

Proportion of A. phagocytophilum-infected circulatory neutrophils (log₁₀ transformed) (A), numbers of infected cells per milliliter of blood (log₁₀ transformed) (B), and numbers of neutrophils per milliliter of blood (C) in blood samples from the study sheep before and after the sheep acquired A. phagocytophilum infection (as detected by PCR of blood samples). Box plots show the medians, quartiles, and outlier (circles) and extreme (stars) values.

FIG. 3.

Prevalence of A. phagocytophilum infection in adult ticks that molted from engorged nymphs (stippled bars [± exact binomial SE]) and in nymphal ticks that molted from engorged larvae (open bars [± exact binomial SE]) from sheep preinfection and in the first and subsequent weeks of infection.

FIG. 4.

Prevalence of infection in adult ticks (bar graphs [± exact binomial SE adjusted for intrasheep correlation]) that molted from nymphs that had engorged on sheep carrying different numbers of adult female ticks. The mean numbers of nymphal ticks carried by these sheep (± SE) are indicated by black squares. Heavy black error bars refer to counts of nymphs.