Some of the diseases that plague humans are transmitted to us via ticks and insects. In North America, one of the most prevalent of these is Lyme disease, an infection caused by the bacteria (Borrelia burgdorferi) and borne by the deer tick (Ixodes scapularis). You might think that the growing incidence of this illness is due primarily to an increase in the deer population upon which the tick feeds. Not so, according to Taal Levi and his colleagues from the University of California, Santa Cruz and from the University of Bergen. It’s actually the red fox population that’s critical.
First of all, let’s delve into the tick life cycle (shown in the diagram below). Starting from an egg (far left), the tick first goes through a larval stage. At this developmental stage, the preferred blood source is a small mammal, such as a mouse. A year later, that larva has become a nymph, and its favorite host is also a mouse. Finally, the tick is all grown up and has one final meal as an adult. This time, it usually chooses a deer. If a human host replaces either the second mouse or the deer, that person can become infected with Lyme disease.
As you can see, Lyme disease can be transmitted either during the nymph or the adult stages (after the tick has had its first blood meal as a larva). However, notice that the adult stage occurs during the winter when people tend to be covered in multiple clothing layers. In contrast, the nymph stage takes place during optimal shorts and swimsuit weather. This means that people are far more likely to be infected by nymphs than by adults.
Once you get past a minimal threshold necessary to keep the adults laying eggs, increasing the deer population does not affect the abundance of the nymphs. On the other hand, dramatically increasing the number of small mammals has a huge affect on the nymph population. And here’s where the red foxes come in. As foxes have declined, small mammals have proliferated and with them have come infected tick nymphs.
So why has the fox population dwindled? It’s most likely due to the removal of wolves from most of North America. Thanks to the elimination of wolves, many areas have an overabundance of coyotes. These larger predators kill or compete with foxes but do not prey heavily on the small mammals that foxes prefer.