Effects of temperature and humidity on the mortality of the tick Dermacentor reticulatus in the UK

Student thesis: Master's ThesisMaster of Science (MSc)

Abstract

The disease canine babesiosis, transmitted by the tick Dermacentor reticulatus, has recently been found for the first time in the UK. This disease outbreak highlighted our lack of knowledge of the distribution and abundance of this tick species in the UK, which is believed to be expanding under the influence of climate change. However, given our limited understanding of the influence of microclimatic variables on its activity, it is difficult to predict its future spread to manage any potential risks. The research presented in this thesis aimed to quantify the effects of temperature and relative humidity on the mortality of D. reticulatus.
Adult ticks were collected between September 2019 and February 2020 from 4 locations in the UK: West Wales, Essex, North Devon, and South Devon. Different concentrations of potassium hydroxide were used to create a range of humidities inside sealed desiccator jars, each one kept in incubators at temperatures of 4°C, 15°C or 30°C. Five relative humidities were tested: 20%, 40%, 60%, 80% and 95%. Each desiccator contained three tubes of five ticks which were checked every 2 or 3 days for mortality over a period of 9 weeks (63 days).
Mortality increased significantly over time. The data show that tick survival rate was lower when the temperature was higher, the humidity was lower, and the longer the ticks had been exposed to these conditions in the desiccator. Integrating temperature and humidity to calculate a measure of saturation deficit showed that there was no effect on tick survival rate until day 20 and that where the saturation deficit value was lower, indicating a high air moisture content, the survival rate increased.
This study suggests that D. reticulatus is able to survive at relatively low temperatures, lower than those preferred by other species of ticks such as I. ricinus and I. hexagonus, and that an environmental humidity exceeding 80% RH is required for D. reticulatus ticks to prevent water loss via evaporation.
These results have implications for predictions about how D. reticulatus populations and tick-borne disease will be affected by climate change in the UK. It may be that D. reticulatus will spread northwards in the UK seeking cooler temperatures, perhaps more rapidly than other UK species such as I. hexagonus and I. ricinus which tolerate warmer environments. However, this will also be strongly affected by changes in precipitation. Higher rainfall will allow D. reticulatus survival even at elevated average temperature. As a result, pathogens transmitted by D. reticulatus will become more abundant and more widely established in areas of the UK they are not usually found in. Higher temperatures may also cause more successful incubation of pathogens with D. reticulatus, leading to a higher risk of disease.

Date of Award21 Jan 2021
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorRichard Wall (Supervisor) & Bryony O Sands (Supervisor)

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