The Weird, Wild World of Parasites Part 1

Parasitologists do not usually include bacteria, fungi, or viruses in what they study, but of course many bacteria and fungi, and all viruses, fit into the definition of parasites – “organisms” that benefit at the expense of other organisms. It is quite easy to get bogged down trying to find a universally acceptable definition of what a parasite is. Let’s simplify things and just proceed according to what is most commonly studied by parasitologists. The most typical definition of parasite includes such organisms as protists, flatworms, nematodes, and arthropods that live in or on other organisms. Humans are hosts to over a hundred different kinds of parasites (not all at the same time!). Just about any wild (or domestic) animal you encounter will have parasites. Even most people that live in quite sanitary environments have at least some parasites on or in them. We will emphasize the most common and/or harmful examples. Note that leeches are omitted as parasites; they are more-or-less temporary visitors that feed on blood but are not permanent residents. I guess the same could be said for ticks and fleas.

It has been only fairly recently that the importance of parasites in just about all aspects of life on Earth has been appreciated by evolutionary and ecological scientists. This fact still is not common knowledge among most other people, including many scientists. An evolutionary biologist at the University of Chicago named Leigh Van Valen has proposed an interesting notion about the significance of parasites. He calls it the Red Queen Hypothesis (named for the Red Queen that Alice met in the Garden of Live Flowers, where the queen said, “it takes all the running you can do, to keep in the same place.”) In this scenario parasites are a constant driving force in evolution. Sexual reproduction is thought to be vital in the on-going struggle between hosts and parasites – the sexual process creates completely new genetic combinations, some of which confer an advantage against potential parasites. Of course, the parasites often can catch up, inasmuch as they generally reproduce at a much faster pace, and, they too create new genetic combinations through sexual reproduction. A fascinating and excellent book about parasites is Parasite Rex by Carl Zimmer.

Katherine Hepburn checks out Humphrey Bogart’s leech collection Katherine Hepburn checks out Humphrey Bogart’s leech collection. (Some say leeches are not parasites, but they look like it to me.)

Blood-sucking ticks carry several rickettsial diseases such as Lyme disease and Rocky Mountain Spotted Fever. Rickettsias are very small prokaryotic organisms. Of course, one really killer bacterial disease, bubonic plague, is transmitted by fleas from rats. Lyme disease vector Rocky Mountain Spotted Fever vector A gang of fleas having dinner together.

Protists

The most deadly parasite for humans has been a protist (Plasmodium) that causes malaria. More people have died of malaria than any other cause. Even now malaria kills 1-2 million people each year. A fairly close relative, Toxoplasma, is much less dangerous. In fact, almost everyone has it living in them without any noticeable effects. It is only a problem to those with weak immune systems – fetuses and AIDS sufferers. Other protists shown here include Giardia, Cryptosporidium (which killed over 200 people in Milwaukee), and Trypanosoma (African sleeping sickness, Chagas disease). There are many others that could have been included. Many parasites, not just protists, have different parts of their life cycle in different kinds of hosts. As you go through the examples, note the different hosts involved.

Anopheles mosquito and Plasmodium (causative agent of malaria) in and among red blood cells.

Life cycle of Toxoplasma gondii (Toxoplasmosis)

Toxoplasma is found in many people without any symptoms – exceptions are fetuses and AIDS sufferers.

Giardia – commonly found in swimming pools; lacks mitochondria and has an amusing appearance. Causes intense diarrhea and is readily transmitted within households.

Cryptosporidium in human intestinal lining Cryptosporidium in human intestinal lining. This protist sickened thousands and killed 200 in Milwaukee in 1993. Cryptosporidium merozoites breaking out

Blood-sucking conenose (Triatoma) - vector for Chaga’s disease (Trypanosoma cruzi). It is now thought that Charles Darwin contracted this, causing him to be a near invalid most of his life.

Life cycle of Trypanosoma spp Life cycle of Trypanosoma spp. – cause of African Trypanosomiasis (sleeping sickness)

Tsetse fly (Glossina) Causative agent of African sleeping sickness (Trypanosoma, sp.)

Flatworms

Some flatworms are free-living animals such as the planaria. However, most flatworms are parasites such as flukes and the tapeworms. Most flukes and tapeworms have quite complex life histories with multiple hosts. In many tapeworms there is an alternation between herbivore and carnivore hosts. The herbivore gets the parasite by consuming plants on which fecal wastes bearing eggs or other stages have been deposited. Once in the herbivore host the parasite often forms cysts within muscles or other tissues; it is passed on when the host is eaten by a carnivore. In the carnivore the parasite forms the adult tapeworm that lives in the intestine. If a human ingests eggs, etc., the result can be devastating, inasmuch as the parasite can form cysts just about anywhere – heart, lung, brain, bones, etc. You can see the result on a subsequent slide showing hydatid cysts of Echinococcus, the dog tapeworm.

Life cycle of Fasciola hepatica (Sheep Liver Fluke)

Fasciola – 2 species of sheep liver fluke

Immature stages of Fasciola Fasciola miracidium (from eggs in water; penetrate snail host) Immature stages of Fasciola Fasciola cercaria (penetrate vertebrate host in water) Fasciola redia (emerge from sporocysts in snail)

Once a male finds a female he never lets her go – even if she drops dead. (live in large veins in abdomen) Cercaria (larva) of Schistosoma

Taenia solium – scolex (how it holds on in the gut) Tapeworm cyst in human brain Taenia solium – cysts in human eye (cysts can be just about anywhere in body)

Echinococcus – dog tapeworm. Actual size is only 3 to 6 mm in length.

Hydatid cysts (Echinococcus) in a monkey Each cyst is filled with thousands of small parasite units.

Hydatid cyst in human brain (left) Human kidney with hydatid cysts (arrows)