Sunday, December 15, 2013

Behaviors, Diets, and Parasites in Antiquity: Hymenolepids in the Grain Bins

Over the course of this semester, I've come to truly appreciate the interplay of diet, behavior, and parasitism. As a biologist, I find it fascinating to think about how the parasitism of a population is affected by the behaviors and the diets of host species. As a budding archaeoparasitologist, I find it even more fascinating to look at how human diets and behaviors have played significant roles in the diversity and prevalence of parasites that have wormed their way into our bodies (and of course our hearts...mostly in a metaphorical sense...).

 One of the many parasites that is brought up in discussions of behavior, diet, and parasitism is a little tapeworm sometimes called the "dwarf tapeworm". It's name has been changed several times in the parasitological literature (Taenia nana, Hymenolepis fraterna, and my personal favorite, Vampirolepis nana), but is currently called Hymenolepis nana. Another closely related parasite that can be brought up in such discussions is Hymenolepis diminuta.

Like all tapeworms, these little dudes are classified as flatworms (Phylum Platyhelminthes) and belong to class Cestoda. They further belongs to the order Cyclophyllidae since they have four acetabula on their scoleces, hooked rostellae, and since they possess a single, compact postovarian vitelline gland. Other famous members of this order include the notorious taeniids (beef and pork tapeworms as well as Echinococcus sp.) and Dipylidium caninum, the double-pored dog tapeworm. H. nana and H. diminuta both fall within the family Hymenolepididae and are the only members of this family known to infect humans. Other members infect other mammals or birds instead. Most of the hymenolepidids require an arthropod as their intermediate host.

Life Cycles
The life cycles for these tapeworms are very similar. They begin with eggs being shed in the feces of an infected person or rat. These eggs are eaten by beetles, such as grain beetles (Tribolium spp.), and then hatch within the beetle's intestine. A cysticercoid with a tail develops within the beetle's hemocoel and waits to be eaten by the definitive host. A rat or human eats the beetle and the parasites are released in their new host's duodenum. From here, the parasites become oncospheres by shedding their tails and burrowing into the intestinal villi. The tapeworms absorb nutrients through their teguments as they grow and eventually little gravid proglottids snap off to release the tapeworms' eggs out of the host's body via defecation. With H. nana, the beetle is not a needed host, but is utilized from time to time. This species of tapeworm can actually infect definitive hosts via direct contact with contaminated feces.

Human Infection
We've already said that these guys can infect rats and humans alike, but we will just focus on humans for the sake of this post. (Sorry rats, another day!) As far as I can tell, there are rarely any major types of pathology related to infections with either of these tapeworms. It seems that the only real problems occur when a person is heavily infected with these which point the symptoms are similar to those for other tapeworm infections (e.g. abdominal pain, diarrhea, nausea, dizziness, anemia, etc.). When one is infected with either H. nana or H. diminuta, it is easy to cure with a dose of our old friend, Praziquantel. This drug is quick acting and does not typically require multiple doses.

Hymenolepis sp. in Archaeoparasitology
These tapeworms are found intermittently in coprolites from a variety of areas in the New World. Most cases are thought to have come from humans accidentally ingesting grain beetle gunk that got ground up when grains containing the beetles were being processed with stone tools. Since many rockselters where coprolites have been excavated make suitable habitat for small rodents, it is also possible that rats may have contaminated food sources or may have been a contaminated food source themselves. There is also the possibility that people were eating other kinds of beetles that housed one of these tapeworms. This could be especially true for H. diminuta, which has experimentally demonstrated that it can utilize over 90 different species of arthropods as intermediate hosts! It may sound weird today, but beetles and their grubs were great sources of protein for our ancestors. It would not be surprising to learn that they were eating infected beetles picked fresh from the vine or dug up with roots of tasty plants.

Archaeoparasitologists have demonstrated the presence of these tapeworms in coprolites from a number of archaeological sites. From Arizona, H. nana was found in Antelope House dating between 1175-1250AD. From nearby, Hymenolepis sp. was reported from Elden Pueblo, which dates from 1070-1250AD. From further south comes a report of Hymenolepis sp. from Santa Elina, Mato Grosso in Brazil that dates from 4000-2000BP. Such finds make it evident that these parasites have been opportunistically associated with humans for quite some time.

Moral of the Story
It is interesting to think of how behaviors such as no longer eating beetles on a regular basis or being perfectly content to crush grain beetles into our food have changed the type of parasites we as a society contract. Such simple changes in our diets and in how meticulous we've become in terms of food inspections have made cases of human infections with these parasites extremely rare occurrences in today's world.  It is amazing to think that not only what we eat, but what the things we eat are eating, can have an impact on our parasite burdens as a society. This dance between parasites, host behaviors, and dietary preferences is a wondrous one to behold. I'm hoping to soon get funding to further explore this balance along with how these things had effects on the development of the human immune system. I hope the grant proposal gets accepted...this would be quite an amazing dance to watch as the mysteries of our ancestors unfolds before my eyes. Here's to hoping the reviewers feel the same way!