I got to look inside a lot of alligators a couple of weeks ago. One thing that surprised me was that there were no obvious parasites sitting in either their muscles or their guts. This is, believe me, not true of mammals.
So I got curious – where were the parasites in these animals? I asked around, and no one at the Rockefeller Wildlife Refuge had ever seen parasites inside alligator muscle or intestines, or more than a few big roundworms in their stomachs. But their lungs were another story entirely. Their lungs are full of tongue worms.
Tongue worms belong to a phylum of animals called pentastomids
. They’re all parasites, and their adult forms are only found in 
the respiratory tracts of carnivorous reptiles, birds, and mammals. They look a bit like segmented worms, but there are a few significant differences. For one, they’re covered in a hard cuticle that they have to shed when they grow, which suggests they’re much more closely related to arthropods than to earthworms. They also have five stubby lobes on their heads. The central lobe contains the tongue worm’s mouth, but each of other four bears a sharp hook to ram into its host’s lung tissue.
How do they get in there? Simple – their host eats ‘em. Eggs laid by adult female tongue worms get coughed up by their hosts and are either spit out in mucus or swallowed. 
Either way, they wind up outside. The eggs hatch after they’re swallowed by intermediate hosts like fish. Then the newborn tongue worms burrow through the host’s intestinal wall, encyst themselves in its liver, and wait for a predator like an alligator to decide that their host looks pretty tasty. As their former host gets digested, the tongue worm larvae break out of their cysts and burrow over to their new host’s lungs to mature.
The pentastomids secrete enzymes that dissolve tissue and eat blood, and they can cause a fair bit of damage, including lesions, hemorrhaging, and tissue necrosis. Humans aren’t a typical host, but we can get infected by eating pentastomid-infested meat or by drinking water contaminated by their eggs. Infected people don’t generally have any symptoms, even when they’re carrying hundreds of larvae, but one species – Linguatula serrata – can cause vomiting, throat pain, facial swelling and loss of hearing as the larvae migrate from the stomach to the nasal passages. Typically, the larvae die within a few weeks.
Sources:
Hazen, T. C. et al. 1978. The parasite fauna of the American Alligator (Alligator mississippianus) in South Carolina. J. Wildlife Diseases 14: 435 – 439.
SEM image from Zaman, V. and Keong, L. A. Handbook of Medical Parasitology, 2nd ed. Churchill Livingstone, London, 1989.
Larvae image from the University of Florida.
yay - obscure parasites! But I wonder what keeps the rest of the alligator free? very odd...
Posted by: Drhoz! | May 16, 2008 at 08:44 PM