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Pathogens cause disease - protozoa

On page 2, we saw that there are four main types of microbial infection. We have looked at bacteria, fungi and viruses. On this page, we will look at protozoa.

Picture 9. These Trypanosomona parasite can cause sleeping sickness in humans.
Courtesy: CDC, USA.

Protozoa

Protozoa are large, single-celled organinsms. Perhaps the most famous protozoa, the amoeba, can be up to 1 mm in size (gigantic for a single cell). The different types of protozoa show the same features: a cell membrane, cytoplasm, vacuole and nucleus. They are able to move by pushing their cytoplasm forward and get their food by engulfing smaller microbes or cells.

The protozoa that cause parasitic infections in humans are usually in the size range of 1-100 micrometres (millionths of a metre) and have two stages in their life cycle. Infection is usually spread by structures called cysts which are cells that have secreted a protective layer around them so that they can survive the journey from one host to the next. Once inside the new host, these cysts develop into the active protozoa which grows, reproduces and causes symptoms of disease in the new host. Common infections caused in humans are amoebic dysentery, sleeping sickness and malaria.

Picture 10. In many developing nations, water is scarce and needs to be collected from communal taps.
Courtesy: WHO/P.Virot©

Protozoic diseases

Amoebic dysentery, caused by the protozoa Entameba hystolytica, is estimated to infect 50 million people worldwide. It causes severe diarrhoea and in severe cases liver damage. Infection is spread when water is used for bathing and preparing food that is contaminated with sewage containing the cysts. The protozoa can be successfully treated with medicines but often re-infection occurs if the drinking water supply is not kept free from the Entameba.

Trypanosomiasis, or sleeping sickness, is only found in Africa and is transmited by the bite of a small insect called the tsetse fly. Several months later, the infected person starts to show symptoms including fever, rash, severe headaches and extreme tiredness. If not treated, the infection becomes more serious and eventually death can occur as the protozoa attacks the central nervous system.

Not all protozoa are harmful to humans. Modern sewage treatment works actually use some protozoa to help digest the raw sewage so that it can be dispersed safely into the sea.

On page 10, you can find out more about Malaria, a protazoic disease.