Infection with Babesia bovis alters metabolic rates of Rhipicephalus microplus ticks across life stages

Abstract

Background: Metabolic responses to infection differ based on arthropod and pathogen. Increased metabolic rates can result in faster depletion of energetic resources, and decreases may allow for energy conservation. Babesia bovis is a protozoan pathogen transmitted by the cattle fever tick, Rhipicephalus microplus. Adult female ticks acquire B. bovis by feeding on an infected animal. Babesia bovis undergoes development and invades the ovaries where it is transmitted transovarially to tick offspring. The effects of infection on R. microplus metabolic rate are not well studied.

Methods: We tested the hypothesis that R. microplus infected with B. bovis would have altered metabolic rates (volume of carbon dioxide [VCO₂]) across life stages using flow-through respirometry. Replete females from either an infected or naïve calf were measured across 3 days to determine differences in VCO₂. Hemolymph smears were used to categorize the number of B. bovis kinetes present in the hemolymph of replete females during egg oviposition. The VCO₂ for groups of their offspring were measured twice as eggs and once as larvae. The number of individuals and successfully hatched larvae in each group were enumerated at the end of the experiment to determine the average VCO₂ per individual.

Results: Infected replete females have decreased VCO₂ while their offspring have increased VCO₂ at the egg and larval stages. Interestingly, replete females had a 25% reduction in body mass compared to uninfected female tick controls. Uninfected larvae were twice as likely to hatch than larvae from infected replete female ticks.

Conclusions: VCO₂ varied between control and infected ticks depending on life stage. Infected replete females had lower VCO₂ and body mass while their offspring had higher VCO₂ than their control counterparts. Higher larval VCO₂ may promote earlier questing and a shorter lifespan. Changes in metabolic and hatch rates have implications that may promote disease spread.

Keywords: Arthropod vector; Cattle fever tick; Flow-through respirometry; Metabolic rate; Protozoan pathogen; Transovarial transmission.

Fig. 1

Raw CO₂ (ppm) output example from replete females (A) and larvae (B). Blue vertical lines indicate the start and stop of baseline and chamber readings. Replete females actively respired for the entire 30-min chamber readings. Eggs and larvae were measured in groups of individuals because CO₂ output was not detectable for a single tick

Fig. 2

Comparison of absolute VCO₂ between replete females that fed on either an uninfected calf or a calf infected with B. bovis. A Control ticks had higher overall VCO₂ than infected ticks. B Replete females were measured across 3 days. Control ticks had higher VCO₂ on days 2 and 3. Linear mixed-effect model results can be found in Table 2. C There were no significant differences between control (gray open triangles) and infected slopes (black closed circles; ANCOVA; F_1,111 = 1.32, P = 0.253). However, infected ticks had a significant positive linear relationship with body mass and VCO₂ (linear regression; F_1,71 = 22.3, P < 0.0001). Significant differences are denoted by * < 0.05, ** < 0.01, and *** P < 0.001. Extending lines represent the 25th to 75th percentiles of the data. Points beyond are considered outliers

Fig. 3

Replete females that fed on an infected calf weighed approximately 0.075 g less than control ticks (linear mixed-effect model; F_1,74 = 168, P < 0.0001). Significant differences are denoted by * < 0.05, ** < 0.01, and *** P < 0.001. Extending lines represent the 25th to 75th percentiles of the data. Points beyond are considered outliers

Fig. 4

Metabolic rates for R. microplus eggs (A) and larvae (B). VCO₂ was divided by the number of individuals in each chamber. Significant differences are denoted by * < 0.05, ** < 0.01, and *** P < 0.001. Linear mixed-effect model results can be found in Table 2. Extending lines represent the 25th to 75th percentiles of the data. Points beyond are considered outliers

Fig. 5

Percentage of larvae that successfully hatched. A More control offspring hatched compared to females that fed on an infected calf. Significant differences are denoted by * < 0.05, ** < 0.01, and *** P < 0.001. B Hatch percentages varied when offspring were separated based on the infection category of their corresponding maternal tick. Boxes with different letters are significantly different from each other (P < 0.05). Extending lines represent the 25th to 75th percentiles of the data. Points beyond are considered outliers