Antibiotic treatment of the tick vector Amblyomma americanum reduced reproductive fitness

Abstract

Background: The lone star tick Amblyomma americanum is a common pest and vector of infectious diseases for humans and other mammals in the southern and eastern United States. A Coxiella sp. bacterial endosymbiont was highly prevalent in both laboratory-reared and field-collected A. americanum. The Coxiella sp. was demonstrated in all stages of tick and in greatest densities in nymphs and adult females, while a Rickettsia sp. was less prevalent and in lower densities when present.

Methodology/principal findings: We manipulated the numbers of both bacterial species in laboratory-reared A. americanum by injecting engorged nymphs or engorged, mated females with single doses of an antibiotic (rifampin or tetracycline) or buffer alone. Burdens of the bacteria after molting or after oviposition were estimated by quantitative polymerase chain reaction with primers and probes specific for each bacterial species or, as an internal standard, the host tick. Post-molt adult ticks that had been treated with rifampin or tetracycline had lower numbers of the Coxiella sp. and Rickettsia sp. and generally weighed less than ticks that received buffer alone. Similarly, after oviposition, females treated previously with either antibiotic had lower burdens of both bacterial species in comparison to controls. Treatment of engorged females with either antibiotic was associated with prolonged time to oviposition, lower proportions of ticks that hatched, lower proportions of viable larvae among total larvae, and lower numbers of viable larvae per tick. These fitness estimators were associated with reduced numbers of the Coxiella sp. but not the Rickettsia sp.

Conclusion/significance: The findings indicate that the Coxiella sp. is a primary endosymbiont, perhaps provisioning the obligately hematophagous parasites with essential nutrients. The results also suggest that antibiotics could be incorporated into an integrated pest management plan for control of these and other tick vectors of disease.

Figure 1

Box-and-whisker plots of log₁₀ ratios of the copies of the endosymbiont Coxiella sp. fusA gene to copies of the host Amblyomma americanum MIF gene in different stages (larvae, nymphs, and female adults) and ages after molt of the host ticks. Gene copies per tick were estimated by quantitative polymerase chain reaction assay with gene specific primers and probes.

Figure 2

Proportion of A. americanum females without oviposition by day after treatment by injection with single doses of phosphate-buffer saline (PBS; n = 9), rifampin (n = 10), or tetracycline (n = 10). Time was censored from day 18.

Figure 3

Box-and-whisker plots of log₁₀ ratios of fusA gene of Coxiella sp. to MIF gene of A. americanum (upper panel) or log₁₀ ratios of gltA gene of Rickettsia sp. to MIF gene (lower panel) in post-oviposition ticks that were treated by injection with single doses of PBS (n = 9), rifampin (n = 10), or tetracycline (n = 10) as fed, mated females.

Figure 4

Association between post-oviposition burdens of the Coxiella sp., as estimated by fusA/MIF gene ratios, and reproductive fitness, as estimated by the ratio of viable larvae to non-viable larvae and unhatched eggs, in female A. americanum after injection of PBS (blue X) or rifampin (red circle). The ratios have been log-transformed. The blue and red ellipses correspond to bivariate standard deviations of the means of the x and y variables for each treatment group, respectively.