Monday, November 22, 2010

The bizarre life of a parasite

I received an unusual sample last week that nicely illustrated the lengths some organisms go to survive.  A woman was puzzled by a number of hard flaky objects that mysteriously appeared in her bed sheets. The accompanying image shows the sample under 6X magnification.  The objects were hard, but crushable and 2-3 mm in length.

It's not uncommon for people to find objects presumed to be droppings or some other evidence of insect presence in a home.  Rodent droppings, American cockroach, silverfish and other insect droppings are not unusual to find indoors.  In addition, carpenter ants have the interesting habit of tossing insulation and other debris, along with dead insects and insect fragments, out of their nests.  Most often carpenter ant "frass" is found in windows and doorways, where carpenter ant "kick-holes" (garbage shoots) are commonly located.

My sample was none of these, however.  My first task was to determine whether the client had a dog or cat.  A quick phone call confirmed that the family had no cats, but did have a dog which frequently slept on the bed.

This answer clinched the diagnosis of "tapeworm proglottids". The dog tapeworm, Dipylidium caninum, lives and feeds as a parasite in the intestinal tract of dogs, cats and (rarely) humans.  These worms are long and flat and may reach lengths of up to 12 inches. The body of the tapeworm consists of segments called proglottids. As a tapeworm matures, the oldest proglottid segments detach from the main body of the tapeworm and wriggle from the anus of the infected animal. These fresh tapeworm segments move with a stretching and shrinking motion.  They are opaque or pinkish white, flat and rectangular, and initially can move short distances.  Eventually they dry into 1/16 inch-long, rice-shaped sacs as seen in the image.  These sacs contain viable tapeworm eggs, and are often seen attached to the hairs around the pets's anus, in feces, or in areas where the pet sleeps (in this case a human bed).

Here's where things get really bizarre.  Fleas are essential to the life cycle of the dog tapeworm.  Tapeworms use fleas to disperse from one host to another.

Flea larva. © M. Merchant.
Flea larvae are the least frequently seen life stage of the flea, but are always present in the soil (outdoors), flooring, carpet or pet bedding of an infested home.  Normally flea larvae scavenge unnoticed around pet loafing areas for the dried blood that always flakes off the fur and skin of a flea-infested host.  Since flea proglottids are likely to drop into these same locations, it's not uncommon for flea larvae to encounter and feed on them. Once a proglottid is nibbled on by a flea larva, the ingested tapeworm eggs hatch inside the flea's body.  In this way the flea larva (and eventually the adult flea) becomes infested with a life stage of the tapeworm that is capable of infecting warm-blooded hosts.

A cat or dog subsequently becomes infected with tapeworms when they ingest these infested fleas during grooming. Once released into the pet's digestive tract, the tapeworms begin to grow into mature adults that help themselves to a share of the unwitting pet's diet.

Finding tapeworm proglottids in a home does not indicate a threat to people (though small children have been known to become infested when they pick up and eat fleas!), but they are an indication of a tapeworm-infested pet. If you find tapeworm eggs in a home, recommend to the homeowner to take their pest to the vet for treatment with anti-parasite drugs.

The dog tapeworm has got to be one of the most unusual pest life cycles encountered in the indoor environment.  For more information about fleas and tapeworms see our online publication, Controlling Fleas.


Jay Jorns, A.C.E. said...

Can the hookworm have the same relationship with fleas as tapeworm? Can the tapeworm be transmitted to a flea host during blood feeding?

Mike Merchant, PhD said...

Jay, the quick answer to your question is no. Hookworm is a roundworm (helminth) parasite that is very rare these days in the U.S. Hookworms enter the environment through the feces of an infected person. The eggs can only hatch in warm, preferably sandy or loamy soil, in areas where rainfall exceeds 40 inches a year. People are usually infected through the skin by walking on feces-contaminated soil. With the nearly universal use of latrines and toilets in the U.S. the disease is practically gone here, though 600 million people are infected worldwide. The only insect connection that I know of is that cockroaches have been observed to carry hookworm eggs mechanically. They could possibly serve as vectors to disperse hookworm eggs where the pathogen is common in humans. Fleas are not known to be a factor.

Lou said...

The dog tapeworm is Dipylidium caninum. For some reason people have written the genus name as Diphylidium in error and it has appeared online with the incorrect spelling, too.

Mike Merchant, PhD said...

Thanks Loubugs. You're correct. I will change my misspellings.