There are over 100,000 described living species of protists, and it is unclear how many undescribed species may exist. Many protists live in symbiotic relationships with other organisms and these relationships are often species specific. Our focus for the remainder of our discussion of the protists will be looking at examples of these important symbiotic relationships as well as the economic and ecological impact of protists.
The group of protists known as kinetoplastids live in symbiosis with host organisms, and a number of them are pathogenic. For example, the disease African trypanosomiasis, or African sleeping sickness, is caused by organisms from the genus Trypanosoma .
African Trypanosoma species are transmitted between mammalian hosts (including humans) by the tsetse fly, a biting insect that transfers the parasite from the blood of infected individuals to the blood of uninfected hosts. The symptoms of African sleeping sickness include fatigue and confusion, which results after Trypanosoma invades the central nervous system. If left untreated, Sleeping Sickness is almost always fatal. The economic burden of this disease in Africa has been estimated by the World Health Organization to be US$1.5 billion annually.
Another parasite from this same genus causes Chagas disease, or American trypanosomiasis, which is transmitted by insects known as "kissing bugs." Chagas disease, which occurs primarily in rural areas of Central and South America, affects between 8 and 11 million people and kills nearly 40,000 every year. It tends to be a chronic infection that causes acute symptoms (e.g., gastrointestinal discomfort and heart enlargement or other cardiac problems) between 10 and 20 years after infection.
There is some speculation that the debilitating fatigue and gastrointestinal distress that plagued Charles Darwin for the better part of his life after his return to England from South America was due to a chronic infection of Chagas disease.
Recently, it has been reported that the organism that causes Chagas disease is spreading and has been documented in the southern United States (along with the resurgence of other insect-borne infectious diseases such as Dengue fever).
Preventive measures for both African sleeping sickness and Chagas disease (as well as other insect-borne diseases) are largely focused on vector control, which ranges from insecticide-treated bednets (ITNs) to the "sterile insect technique" - a technology used to flood a wild population of insects with sterile individuals with the goal of reducing the insect population.
Please note: you do not need to know the details of the infection cycle of the Trypanosome-related diseases, but you should appreciate the complexity of the infection cycle. This complexity leads to complications in both diagnosing and treating the diseases.