Cardiovascular and Lymphatic System Diseases

Protozoan Cardiovascular Diseases

Serious protozoan blood infections include the insect-transmitted Chagas disease, caused by Trypanosoma cruzi, and malaria, caused by the apicomplexans of the genus Plasmodium.

Another example of a blood-borne illness is Chagas disease, which is caused by the protozoan Trypanosoma cruzi. T. cruzi are transmitted to humans via the bites of Triatominae insects, where they bore through epithelial tissue to feed on blood and lymph. There are three main phases of Chagas disease: acute, intermediate, and chronic. Depending on how well an infected person develops an effective immune response, any of these stages can range from asymptomatic to potentially life-threatening. The outcome of infection at each phase is also influenced by the effectiveness of treatment. Acute Chagas disease has flu-like symptoms, including fever, fatigue, muscle aches, and headache. The characteristic symptom is called Romaña's sign, a localized swelling around the eye. Severe chronic Chagas disease occurs in 20–30% of Chagas cases, and the infected person can eventually develop heart rhythm abnormalities and endocarditis. There are no vaccines against Chagas disease, but antiparasitic drugs, such as benznidazole and nifurtimox, are effective.

Malaria is caused by protozoa of the genus Plasmodium. Plasmodium species belong to the phylum Apicomplexa. An apicomplexan is a single-celled parasitic protozoan with a specialized set of organelles, vacuoles, and microtubules called the apical complex. Apicomplexans have a special organelle, the apicoplast, that aids in host penetration and metabolic functions such as fatty acid synthesis. Apicomplexans also contain an apical complex composed of organelles (including conoid, apical polar ring, rhoptries, and micronemes), vacuoles, and microtubules. The apical complex is critical for invasion of host cells by Plasmodium and other apicomplexan parasites, such as Toxoplasma gondii and Cryptosporidium parvum. Five species of Plasmodium are capable of infecting humans, each of which has different disease severity and is endemic on different continents. Plasmodium falciparum has the highest mortality rate because of its ability to spread to nervous tissue and develop into cerebral malaria, a life-threatening condition. Malaria is spread specifically by female Anopheles mosquitoes, which inject the parasites into the bloodstream when they take a blood meal. The protozoan multiplies in red blood cells, which eventually burst to release new infective protozoans. The death of red blood cells and spread of the pathogen causes severe paroxysms and sudden onset of fever and chills on a strict schedule, causing symptoms every two or three days. There are a few different antimalarial drugs that can be used based on the severity of the infection, but resistance to the historically available medications, like chloroquinine, is rising quickly. Researchers are now investigating apicoplasts, the non-photosynthetic plasmids found in apicomplexans, as potential targets for new, more effective chemical therapies against malaria. This is because plasmodia and other apicomplexans are unable to survive outside of host cells and cannot invade new host cells when their apicoplasts are damaged.

The malaria parasite life cycle involves two hosts. While feeding, the infected female Anopheles mosquito passes sporozoites into the human host. These sporozoites travel to the liver where they become schizonts. These body forms then grow and rupture to release another body form called a merozoite. After replicating in the liver, the parasites undergo asexual reproduction in the red blood cells and become another body form called trophozoites. The ring stage trophozoites mature into schizonts, which rupture releasing merozoites. These merozoites that matured in red blood cells can infect other red blood cells where they develop into gametes. Plasmodium gametes are transferred in blood to a mosquito that feeds on an infected person. While in the mosquito’s stomach, the gametes become zygotes. The zygotes are able to move and penetrate the wall of the mosquito where they develop into oocysts. The oocysts grow, rupture, and release sporozoites, which make their way to the mosquito’s salivary glands. Inoculation of the sporozoites into a new human host keeps the life cycle going.
Malaria is a protozoan disease that is spread through the bite of a mosquito. It affects the nervous system of the host and causes severe chills and fever.