They're a maligned group of creatures, and not entirely without reason. Ticks, fleas, lice and chigger mites fasten themselves to a host to feed – on blood, or, in some cases, dead skin, hair or feathers. In the process, they can be an irritant, and worse: they're second only to mosquitoes as vectors of disease.
You might think you could do without them, but there's no gainsaying their efficiency. These blood-suckers bring remarkable adaptations to their work.
Welcome to the weird, wild world of ectoparasites.
Snakes, spiders and scorpions, sure. But ticks in West Texas?
Chris Ritzi is a Sul Ross State University biologist who specializes in ectoparasites – parasites that live outside their hosts.
“Oh yes, we have many ticks in the Chihuahuan Desert,” Ritzi said. “We see ticks that are specialists on rodents. We see ticks that are specialists on rabbits – and there's generally no shortage of rabbits out here. Then you start looking at things like your reptiles, and you start finding ticks on the reptiles. There are ticks that feed on the snakes. There are ticks that feed on the birds. There are ticks all over the place.”
Ticks feed for two main reasons – to grow, and to reproduce. To complete their life cycle – to grow from larvae to nymphs and then adults – some ticks require multiple hosts. Our region is home to such a “three-host” tick. In year one, it parasitizes a deer mouse or other small rodent. It moves on to a ground squirrel or rabbit in year two. Next, it needs the blood of a cow or other large ungulate – or a human.
The adult female engorges to produce eggs. Sated, she retreats to a dark, safe place – one where her young are likely to find food.
Ticks are rarely life-threatening. But a small animal with the bad luck to be near an egg mass when hundreds of ravenous tick larvae emerge is in trouble.
“So sometimes you can come across a mouse that has been attacked by ticks,” Ritzi said, “and every single part of the body that has thin tissue that ticks could feed on has one, two, three, four ticks attached to it. And you go, 'Oh, you came out bad in the lottery!'”
Ticks are opportunists. Species that once fed on elk and bison here now parasitize cows. And ticks get around. Studying javelinas, one of Ritzi's grad students found a tick common to Uruguay. It had apparently hitched a ride on a migrating bird.
Lice are another ectoparasite. But unlike ticks, they're “obligates” – intimately bound to a specific host.
“Lice are extremely picky,” Ritzi said. “You'll see lice feeding on a particular species of gopher. You'll find lice feeding on a particular species of herbivore. Our javelina and our feral hogs have immensely large lice populations.”
The legs of lice are modified into claws – finely adapted to wrap around the hairs of their host. Outside its hairy environs, a louse, Ritzi said, is “like a turtle on its back.” There are lice that parasitize humans – hair, body and pubic lice – but as obligates, lice can't jump species. We have nothing to fear from javelina or gopher lice.
Fleas are another matter – they'll suck the blood of any creature near at hand. And they bring a different twist to the process. Fleas drop eggs to the ground as they feed. Then, they overeat – and excrete the blood-rich byproduct to the ground as well.
“If the host is randomly wandering the landscape that doesn't do you a lot of good,” Ritzi said. “But if you're host happens to be bedded down for the night in its nest or den, then you have a build-up of eggs, that hatch out as the larvae, and you have a build up of these blood-rich fecal pellets, and the larvae can feed on the food that mom's dropping off the host for them.”
Lyme disease diagnoses are on the rise in the United States. Fortunately, the ticks that transmit the disease are primarily woodland species, and risk here is low. But ectoparasite-borne diseases are present here – including Rocky Mountain Spotted Fever, and bubonic plague. Ritzi said that studying ectoparasites in West Texas can “help us know what diseases to be on the lookout for.”
There's an element of the macabre to the ectoparasites. But occasionally they can be beautiful.
Red and plush, velvet mites – or “Santa Claus bugs” – are a striking sight after a West Texas rain. The adults are hunters. But they're ectoparasites – as larvae, they suck the blood of grasshoppers and termites.
“They're gorgeous,” Ritzi said. “But when you get right down to it, they're larval stage is a blood-sucking parasite.”
