by Shannon O’Neill
The potential for a rapid increase of the geographical distribution of ticks and tick-borne pathogens with increasing temperatures is a major public health issue. Therefore, the relationships between the tick, pathogen, hosts, and each of their environments must be better understood in order to effectively manage future outbreaks. Climate change is often considered to be a driving force of increased tick-borne disease. However, the effects of climate on disease are difficult to distinguish from other potential causes. Ostfeld and Brunner (2015) specifically studied the Ixodes tick that spreads Lyme disease in an effort to discern why this tick and the pathogens it transmits have continued to increase with warmer temperatures. The researchers first identified environmental factors for the current tick distribution, then used these factors as a predictor of future suitable tick habitats with climatic changes. Finally, they looked at how various environmental factors sustain both tick populations and the pathogens they transmit.
Ticks’ longevity is correlated with high humidity and temperatures. However, even when conditions are not as favorable, ticks are able to adapt until more favorable conditions arise. For example, though low humidity can be lethal for a tick, it can inhabit a humid leaf or moist soil during such periods in order to survive. Such tactic, however will delay a tick from obtaining a blood meal and consequently from spreading a pathogen. This provides insight to the importance of humidity in tick transmission of disease. Furthermore, cumulative temperatures control the length of a tick’s developmental stages through its life cycle. Cold winters have been shown to limit tick distribution, as short-term exposure to extreme cold can be lethal for many tick populations. In conclusion, temperature and humidity seems to play an important role in tick distribution only at the most extreme level; whereas in areas that have more permissive conditions, the Ixodes can adapt so that these factors have little influence.
The effect of climate change on the Lyme disease pathogen is not as well understood. It has been hypothesized that temporal trends in Lyme disease are correlated to temporal trends in climate, suggesting that long-term climate changes would increase the public health burden of Lyme disease (Ostfeld and Brunner, 2015). Furthermore, one study found that in the northeastern United States, Lyme disease cases were positively correlated with precipitation occurring in May and June. More research needs to be initiated in the pathogen distribution in order to fully understand the effects of climate change and Lyme disease. Studies have found that some mosquito-borne pathogens replicate more rapidly in warmer temperatures, but this has yet to be found of tick-borne pathogens.
Finally, climate change can also increase the numbers of viable hosts, which can increase the spread of both the tick and the pathogen. As most models focus on the abiotic factors that affect tick and pathogen distribution, this fact has often been overlooked.
Looking forward, research should focus on how the abiotic factors identified affect the behavior, development and demographic process of ticks and their associated pathogens. Additionally, tick success rate at finding a viable host should be assessed in various climates in order to determine how effectively the disease can be distributed. In some cases, it has been shown that changing temperatures does not influence the emergence of nymphs, whereas larvae may be more dependent on temperatures. These various developmental stages need to be further explored in order to determine what controls tick emergence and activity. The potential for this rapid increase in Lyme disease is a large threat to the world, and therefore these relationships must be better researched and understood in order to create effect public health infrastructure to manage future outbreaks.
Ostfeld R.S., Brunner, J.L., 2015. Climate change and Ixodes tick-borne diseases of humans. Philosphical Transactions of the Royal Society of London B: Biological Sciences 370, 20140051. (http://rstb.royalsocietypublishing.org/content/370/1665/20140051.abstract)