WHITE RIVER JUNCTION – “I admire its purity. A survivor… unclouded by conscience, remorse, or delusions of morality”
— Alien, 1979
Deer Tick (Ixodes scapularis) in Vermont
In Ridley Scott’s “Alien,” the fear of parasites living off humans was popularized in a story set on an uncharted moon 39 light years from Earth. That’s a long way to go to experience a phenomenon that’s alive and well in our own backyards. While not quite the same as the creatures in “Alien,” ticks readily parasitize warm-blooded hosts like deer, dogs, and humans.
Deer Ticks, arachnids infamous for spreading Lyme disease and anaplasmosis, are sorely misunderstood. Despite their namesake, Deer Ticks are not exclusively found on deer. In fact, White-tailed Deer (affectionately called “the bar” by some tick ecologists I know) are only involved in the tail end of the tick’s life cycle when they seek mates. At this stage, deer serve as the meeting ground for female ticks to find a mate and gorge themselves before detaching and laying their eggs.
Additionally, the pathogens Deer Ticks are so famous for carrying do not directly infect ticks; Lyme disease-carrying ticks are not sick themselves, but merely transport pathogens between sick animals and uninfected animals in their search for food.
The pathogens carried by deer ticks are actually found in more unassuming animals than White-tailed Deer: small mammals like mice, voles, and shrews. After a well-fed Deer Tick lays her eggs in late spring, the developing larvae emerge in summer to find their first blood meal. A fast, long-legged animal like a deer is hard to come by when you’re only as big as a grain of sand, so when you’re hungry for the first meal of your life, one of your best options is foraging rodents brushing their low-lying underbellies against the ground. And if you’re a tick-borne pathogen ready to hitchhike to a new host, a friendly parasite like a tick is your free ride. After their first blood meal, overwintering ticks only grow to the size of poppy seeds in their nymphal stage, and small rodents remain a solid source of food and pathogens.
When people contract Lyme disease, that strain of bacteria hitchhiked from a small mammal to human hosts, who historically have not been a Deer Tick’s “bar” of choice. After all, ticks are just animals looking for their next meal, not a source of evil with a vendetta against larger animals.
Evolutionarily, humans have no resistance to the bacteria that cause Lyme disease whereas many small mammals have partial resistance, enough to avoid being debilitated but not enough to clear the pathogen from their system. For their part, White-tailed Deer cannot transmit Lyme bacteria at all. Our novelty to this pathogen-tick cycle as accidental hosts for ticks makes us particularly vulnerable to the disease.
Best practices to protect against Lyme disease include checking for ticks (look closely—tick nymphs can be as small as a pencil tip!) and drying your clothes with high heat after walking through leaf litter and brush at the edges of forests.
Pia Carmen is on the staff of the Vermont Center for Ecostudies.
