This article is the first of two on the subject of parasites of animals and people.
The study of parasites is called parasitology. It’s an important discipline because internal parasites cause death and disease whose treatment costs billions of dollars in animals each year. These parasites have highly evolved life cycles that make their elimination impossible. In addition, many internal parasites may affect people, with a potential for serious consequences.
The Centers for Disease Control and Prevention has an extensive website on parasites. A synopsis of a small portion of this information that relates to parasites of people and animals is as follows:
- 2 million people are infected with Giardia each year in the U.S.
- There is an estimated 1.5 million new Toxoplasma infections and 400–4,000 cases of congenital toxoplasmosis in the U.S. each year
- 1.26 million persons in this country have ocular involvement from the effects of toxoplasmosis—toxoplasmosis is the third-leading cause of deaths from food-borne illnesses (375+ deaths).
A parasite is an organism that lives on or in a host organism and gets its food from or at the expense of its host. There are three main classes of parasites that can cause disease in humans and animals. The internal parasites fall into two categories, and the external parasites fall into one category:
- Protozoa—Internal Parasite Protozoa are microscopic one-celled organisms that can be free living or parasitic. They are able to multiply in humans, which contributes to their survival and also permits serious infections to develop from just a single organism. Transmission of protozoa that live in the human intestine typically occurs by a fecal-oral route, for example, contaminated food or water, or person-to-person contact. Protozoa that live in the blood or tissue of humans are transmitted to humans by an arthropod vector, for example, through the bite of a mosquito or a sand fly. An example of a protozoan parasite that we encounter in people and animals is Giardia.
- Helminths—Internal Parasite Helminths, derived from the Greek word for worm, are large, multicellular organisms that are generally visible to the naked eye in their adult stages. When people think of worms, these are the types that come to mind. Like protozoa, helminths can be either free living or parasitic. In their adult form, helminths cannot multiply in humans. There are three main groups of helminths that are human parasites. Typical examples include roundworms, tapeworms, and hookworms.
- Ectoparasites—External Parasites Although the term ectoparasite can broadly include blood sucking arthropods such as mosquitoes, which are dependent on a blood meal from a human host for their survival, it’s also generally used more narrowly to refer to organisms such as ticks, fleas, lice, and mites that attach or burrow into the skin and remain there for relatively long periods of time, e.g., weeks or months.
In our next article, we will talk more about the internal parasites we see most commonly in animals in our practice. Until then, for more information on parasites, please click here to read more about parasites. In the next article, we’ll talk about two common internal parasites, Giardia and roundworms.