Protist Pathogens in Humans

Plasmodium gallinaceum (Malaria) in mosquito gut; Credit: Wikimedia

Protists are a biologically diverse group of eukaryotic organisms. They are generally considered to be single celled and lack any of the specialized tissues associated with more complex groups of organisms, but these are their only defining features – without looking genetically. Their metabolism can be phototroph or organotroph, and the way they reproduce varies wildly depending on the specific species.

They might be difficult to define, but their role as pathogens is more certain. Some species will act as pathogens to plants, others to animals, but perhaps the most well-known are those in the Plasmodium genus, which can cause malaria in humans.

Malaria

In 2010 the World Health Organization estimated there were 219 million cases of malaria, with 660,000 of those resulting in death. Malaria is a serious condition that left untreated can cause severe symptoms, such as: fever, vomiting and jaundice, eventually leading to death – this is especially apparent in undeveloped countries where treatment options are limited, or non-existent.

The female of the Anopheles species of mosquito is responsible for the spread of malaria. The mosquito has the ability to carry the specific Plasmodium without being harmed itself. The Protist also remains unharmed, so has a chance to multiply enough that it will be present in the saliva when the mosquito bites into a human to feed on their blood.

When in the human bloodstream the Protist, now called a ‘sporozoite’ as it has a host, gets transported through normal circulation into the liver. Once in the liver the organism migrates into liver cells where it multiplies into the thousands over a period of 8-30 days. These liver cells eventually burst, releasing all of the Plasmodium back into the bloodstream, where they infect red blood cells. The red blood cells that now hold the organisms also get to a point of bursting, and thus, the infection spreads and becomes more uncontrollable.

The Plasmodium avoids triggering an immune response that would eradicate it initially by hiding in the cell membranes of the liver cells, and then by adding an adhesive protein to red blood cells which makes them stick to smaller circulatory vessels – at this stage the immune system can remove infected red blood cells, but too many are infected for this to work effectively.

African trypanosomiasis (Sleeping Sickness)

Caused by the protist Trypanosoma brucei, African sleeping sickness affects about 10,000 individuals annually, predominantly in sub-Saharan Africa. This particular protist is transmitted to the host by the bite of an infected tsetse fly. While treatable, without intervention sleeping sickness will lead to death.

Initial symptoms will start to occur between 1 and 3 weeks of being bitten by a tsetse fly and include headache, fever, joint pain, itching, fatigue and swollen lymph nodes. As the infection progresses increasing confusion, insomnia and uncontrollable daytime sleepiness as well as multiple neurological symptoms will occur.

It's important to note that tourists to Africa are generally not considered to be at risk for infection since the infected tsetse flies are found exclusively in rural areas. Only individuals with prolonged and repeated exposure to woodland areas are really at risk.

Trichomoniasis (Trich)

The previous examples of protist infections occurred mostly tropical locations, but it's important to note that we're not exempt from these parasites in colder climates such as the United States. Trichomoniasis is a prime example in that almost 4 million people have the disease here. It's caused by the protist Trichomonas vaginalis, and luckily is quite curable.

Trichomoniasis is an STI (sexually transmitted infection) and so the parasite is passed from an infected individual to their partner during sexual intercourse. It commonly infects the vagina, vulva and urethra in women and usually just the urethra in men. Although it is one of the most common STI's, many infected persons show little to no symptoms. The ones that do usually have irritation and swelling, discomfort during urination and sex, and possibly a discharge from the penis or vagina.

Left untreated trichomoniasis is known to increase the risk of spreading and contracting other STIs. Pregnant women run the risk of premature and low birth weight babies. Condom use drastically reduces the risk of transmission although not entirely.

Treating Protist Infections

Malaria and other protist infections, such as African sleeping sickness, can be difficult to treat because the organism hides until it has multiplied exponentially, and causes much larger problems.

The most effective treatment for malaria is artemisinin, in combination with other drugs which help to prevent a resistance building up. This drug combination works by directly affecting the organisms, but are also known to have severe side effects.