Ticks, Diseases and Climate Change

Utrecht Centre for Tick-borne Diseases

Members of 44 institutions in 28 countries comprise the Integrated Consortium on Ticks and Tick-borne Diseases (ICTTD), and collaborate to study the effects of climate change on the global distribution of ticks and tick-borne diseases.

One of the aims of the consortium is to better understand the tick-host-pathogen interactions within diverse ecosystems wherein ticks survive and transmit disease. GIS-referenced data on ticks and pathogens are mapped and used to validate statistical models to predict climate-dependent changes.

Ticks transmit a greater variety of pathogenic micro-organisms than any other arthropod vector group and are among the most important vectors of diseases affecting humans and animals. Human tick-borne diseases have gained enormously in notoriety since the discovery of Lyme borreliosis (LB), now recognised as the most commonly reported tick-borne disease of humans in Europe and North America, accounting for tens of thousands of new cases yearly in both regions. Other zoonotic tick-borne illnesses of viral origin are characterised by encephalitis and haemorrhagic fevers and cause high mortality in humans.

In the (sub)tropics, ticks and diseases are a major constraint to animal production. Boophilus ticks transmit babesiosis and anaplasmosis to cattle, and severely affect their productivity, particularly of Bos taurus-type cattle.

Ecological changes

Ticks spend most of their life living off their hosts, where ambient temperature and humidity determine their geographic range and survival. Increasing temperatures with climate change may shorten their lifecycle, but increase their reproduction rate. Very high temperatures can reduce survival and mortality increases under drier conditions.

Climatic conditions dictate the dynamics of tick-borne diseases by affecting the distribution of ticks and their seasonal occurrence. Reliable predictive models are required to measure the direct effect of climatic change on the abundance of ticks and tick-borne diseases.

The ecology of tick-borne diseases is complex, as is the influence of climate on spatial and temporal variations in ticks and tick-borne diseases. The relative impact of climate is often difficult to discern from variations in other factors that are not directly climatic.

The LB ‘epidemic’ in Europe and North America has occurred because of an increase in abundance and range of infected Ixodes ticks, but is also the result of better surveillance and awareness. This epidemic has been associated with climate-independent land use changes, from farm land to woodlands, which resulted in increased abundance of hosts for ticks and the LB agent.

Climate change may affect not only tick survival, but have indirect effects on host ecology and abundance, causing emergence of tick-borne diseases in some regions, and their disappearance in other areas. Human behaviour - increased recreational activities and travel - have also contributed to an increased number of humans bitten by ticks. The LB epidemic shows how man-made ecological changes can result in disease emergence.

Predictive mapping

There are two approaches. First, where records of tick and tick-borne pathogen occurrence exist, statistical pattern matching of climatic and landscape variables can provide insight into projected distributions. Second, climate-driven simulation models of tick populations can be used to establish the geographic footprint of ticks and tick-borne pathogens, particularly where ticks are not currently established.

The geographic range of the Ixodes ticks, I. scapularis, (the vector of LB in North America) is expected to move further northwards into Canada based on a simulation model. Statistical models also suggest that this northward range expansion may be matched by northward movement of the southern limit of the tick’s range.

In Latin America, a mean increase of 2°C in temperature may contribute to dramatic changes in habitat suitability for the Boophilus tick, B.microplus, extending its distribution further south into Central Argentina, but disappearing elsewhere.

In southern and eastern Africa, an autochthonous species of the Boophilus ticks, B. decoloratus, retreats from vast areas increasingly occupied by B. microplus, a more dangerous disease carrier. It is not clear what the influence of climate change on this change is and will be.

A major challenge is the collection of field data on the occurrence of ticks now and in the future, to calibrate and validate suitable models, and to monitor anticipated changes in tick-borne disease risk with the final aim of making meaningful predictions.

ICTTD-3 is a Coordination Action supported by the European Union through project no. 510561 and is coordinated by the Utrecht Centre for Tick-borne Diseases, Faculty of Veterinary Medicine, Utrecht University, in The Netherlands.

THPbase

The project is a forum for genomics, biosystematics, molecular diagnostics, epidemiology and remote-sensing, and has developed an integrated database for ticks, hosts and pathogens (THPbase) for characterisation of ticks and diagnosis of pathogens.

Integrated Consortium on Ticks and Tick-borne Diseases Utrecht Centre for Tick-borne Diseases Utrecht University Professor Frans Jongejan For further information: Tel: + 31 6 20 97 11 10 E-mail: F.Jongejan@vet.uu.nl Web: www.icttd.nl