One of the primary factors driving an increase in tick-borne diseases is climate change. Warming temperatures create more hospitable conditions for ticks and their animal hosts, thus creating more opportunities for ticks to spread diseases. In fact, the EPA uses Lyme disease as an indicator of climate change, highlighting the interconnection of human activity, the environment, and vector-borne diseases.
What Conditions Make Ticks Thrive?
Ticks thrive under certain environmental conditions. They need moisture in the air and ground; places to hide and quest such as the logs, fallen branches, brush, and leaf piles found in wooded areas; and, finally, plenty of animal hosts to feed on. This means that climatic factors such as temperature, precipitation, and humidity contribute both directly and indirectly to the habitats where ticks are best able to survive and reproduce. Changing climatic conditions correspond to changes in tick populations and activity.
Research shows that warming global temperatures contribute to:
- A longer tick season: Warmer, longer springs and summers mean longer periods of time with optimal conditions for survival for ticks and their hosts. In other words, ticks can remain active – reproducing, feeding, and spreading disease – for more months of the year than before.
- Ticks in new places: As temperatures rise, ticks can also survive in regions that were previously inhospitable (i.e., too cold or dry) to them and their hosts, such as rodents and deer.
- More tick-borne illness in endemic regions: Regions that were already tick hotspots see even more tick activity and thus more disease.
The Tick Life Cycle
According to IGeneX, which specializes in Lyme and Tick-Borne Disesase Testing, ticks depend on environmental conditions such as temperature, humidity, and the availability of animal hosts to survive and reproduce. To understand how this web of ecological factors contributes to higher incidence of tick-borne diseases, it helps to understand the tick life cycle. For the Eastern black-legged tick – the tick that spreads Lyme and Tick-Borne Relapsing Fever (TBRF) – this cycle consists of three stages: larva, nymph, and adult. In North America, it takes about two years to complete all three stages.
Most people know that summer is “tick season.” This is because ticks in all three stages are most active during the summer months. But let’s take a closer look at each stage:
- May of year 0: Adult ticks lay eggs, which become larvae.
- Summer of year 0: Larvae hatch and feed on small animals, including rodents such as the white-footed mouse, a so-called “reservoir host” for Borrelia – a group of bacteria that cause Lyme disease and TBRF. When these larvae feed, the disease-causing bacteria are transmitted from the animal hosts to the ticks.
- Winter of year 0: Temperatures drop and the larvae lie dormant.
- Early spring of year 1: The larvae hatched the previous year mold into the second stage, nymphs.
- Late spring/summer of year 1: The nymphs feed on larger animal hosts, including humans, transmitting disease-causing bacteria. After this bloody meal, the nymphs mold into adults.
- Winter of year 1: Nymphs that have fed and mold into adults typically overwinter – or lie dormant – in leaf litter near the ground. However, deer ticks can remain active as long as temperatures are above freezing. (This means that you can be bit by an infectious tick even during the winter.)
- Spring/summer of of year 2: Adults that survive winter emerge in spring of the following year to feed on large mammal hosts, most often deer. At this point, they lay eggs, completing the tick life cycle and reproducing a new generation of potentially disease-carrying ticks.
As you can see, both white-footed mice and deer are crucial to the tick life cycle. Warmer temperatures mean more hospitable conditions for mice and deer for longer periods and in larger geographical areas. More mice and deer, in turn, mean more food for disease-carrying ticks. In fact, one study predicts that climate change will cause cases of Lyme disease alone to increase by over 20 percent in the coming decades.
There are Other Ticks, Viruses, and Factors to Worry About
The Eastern black-legged tick, also known as the deer tick, doesn’t just spread Lyme disease and TBRF. It’s also responsible for Powassan virus, Babesiosis, and Anaplasmosis. What’s more, other types of ticks – such as the brown dog tick, lone star tick, and species of soft ticks – have distinct habitats, behavior, and preferred hosts, and spread distinct diseases. Incidences of all of these diseases are affected directly and indirectly by environmental factors including climate change.
Climate change isn’t the only environmental factor contributing to an increase in tick-borne diseases. Studies show that more direct human activities, namely deforestation and suburbanization, bring humans, tick hosts, and ticks in closer contact with one another. This leads not only to higher numbers of already common diseases (like Lyme) but also to the emergence of new diseases and/or increase of less common diseases.
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