Protozoa
What are Protozoa?
Protozoa are a group of microscopic (need a microscope to see them) single-celled organisms, such as ameba, that mainly feed on bacteria, although some eat other protozoa and organic matter. While protozoa are many times larger than bacteria, you still need a microscope to see many of them, some protozoa can be seen without a microscope. A pinch of soil can contain thousands. In humans, protozoa usually cause disease.
Protozoa can be free-living or parasitic (living in or on other living organisms (known as the host) for their nutrients and life cycle) in nature. 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 to a human typically occurs by a fecal-oral route (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 (example - through the bite of a mosquito or sand fly).
The protozoan parasite Plasmodium causes malaria, a tropical disease that usually is transmitted to humans during the bite of the Anopheles mosquito. In ancient times, this disease was mentioned in Egyptian writings, called hieroglyphics, and was described in detail by the Greek physician Hippocrates. Malaria ravaged invaders from the Roman Empire. Though rare in the United States, malaria remains a serious public health threat worldwide.
- Malaria
World Health Organization (WHO) estimates between 300 million and 500 million new cases of malaria occur worldwide each year, causing more than 1 million deaths annually. In the United States, approximately 1,300 cases are reported annually. Most of the U.S. cases occur in people who had been infected while traveling abroad. Other cases occur in people bitten by infected mosquitoes in the United States.
Malaria Parasite
Another protozoan parasite, Toxoplasma gondii, causes toxoplasmosis in humans. This is an especially troublesome infection in pregnant women because of its effects on the fetus, and in people with HIV infection or other immune deficiency disorder.
Some protozoa, like plankton, live in water environments and serve as food for marine animals, such as some kinds of whales. Protozoa also can be found on land in decaying matter and in soil, but they must have a moist environment to survive. Termites wouldn't be able to do such a good job of digesting wood without these microorganisms in their guts.
A common way for some microbes to enter the body, especially when caring for young children, is through unintentionally passing feces from hand to mouth or the mouths of young children. Infant diarrhea is often spread in this way. Day care workers, for example, can pass diarrhea-causing rotavirus or Giardia lamblia (protozoa) from one baby to the next between diaper changes and other childcare practices. It is always important to wash your hands with hot, soapy water before handling foods and eating, and after using the toilet, diapering young children, and handling animals.
Is There Treatment for Protozoa Diseases?
Controlling parasitic diseases is a problem because there are no vaccines for any of them.
Diseases caused by protozoan parasites are among the leading causes of death and disease in tropical and subtropical regions of the world. Developing countries within these areas contain three-quarters of the world's population, and their people suffer the most from these diseases.
In many cases, controlling the insects that transmit these diseases is difficult because of pesticide resistance, concerns regarding environmental damage, and lack of adequate public health systems to apply existing insect-control methods. Thus, disease control relies heavily on the availability of medicines. Health care providers usually use antiparasitic medicines to treat protozoal infections. Unfortunately, there are very few medicines that fight protozoa, and some of those are either harmful to humans or are becoming ineffective.
The fight against the protozoan Plasmodium falciparum, the cause of the most deadly form of malaria, is a good example. This protozoan has become resistant to most of the medicines currently available to destroy it. A major focus of malaria research is on developing a vaccine to prevent people from getting the disease. In the meantime, many worldwide programs hope to eventually control malaria by keeping people from contact with infected mosquitoes or preventing infection if contact can't be avoided.
What Are Nonpathogenic Intestinal Protozoa?
References:
U.S. Department of Health and Human Services (HHS)
National Institutes of Health (NIH)
National Institute of Allergy and Infectious Diseases (NIAID)
NIH Publication No. 06-4914
January 2006
www.niaid.nih.gov/