1) Blood Parasite Schistosoma Species
The Schistosoma species of blood parasites are responsible for the disease called schistosomiasis. There are three species in the Schistosoma family that are known to infect humans: Schistosoma haematobium, S. japonicum, and S. mansoni.
Two other species, more localized geographically, are S. mekongi and S. intercalatum. Schistosoma parasites begin their lives as eggs that are excreted from a host in feces or urine. Once the eggs hatch, the parasites will swim in the water until they infect a snail as their new host. They pass through several life stages within the snail and are again excreted as parasites that can penetrate the skin of the newest host, humans.
Schitosoma migrate through several tissues and pass through several life stages until they make their permanent home in the veins. These blood parasites may occupy the veins around with the large or small intestine, and they are capable of migrating between the two sites. Once in the veins, the female parasites will begin to lay eggs that will gradually progress from the intestines to the bladder until they are subsequently excreted with the fees or urine. Schistosoma parasites average seven to twenty millimeters in length.
These blood parasites may be found in the waters in certain parts of South America, Africa, the Caribbean, Asia, and the Middle East. Therefore, human contact with infected water is necessary for contact with Schistosoma. Animals such as dogs, cats, rodents, pigs, horses and goats may serve as intermediate hosts for this parasite.
2) Trypanosoma Brucei
There are two subspecies of Trypanosoma brucei that can distinctly different illnesses in humans. Trypanosoma brucei gambiense is responsible for West African sleeping sickness and Trypanosoma brucei rhodesiense causes East African sleeping sickness. The tsetse fly is critical to the development of these blood parasites. The life cycle begins with an infected tsetse fly that then bites a mammalian host.
With each bite, the fly injects metacyclic trypomastigotes into the skin tissue of the host. This passes the parasites into the lymphatic system of the host where they can then pass into the bloodstream. Once inside the host, Trypanosoma pass through several life stages, moving to various sites on the body, and infecting other blood fluids, such as lymph fluid and spinal fluid. If this mammal where then bitten by another tsetse fly, it would pass the blood parasites on to that fly and can allow the process to begin again. It takes approximately three weeks before an infected fly can become an infecting fly.
Trypanosoma brucei gambiense is found in certain parts of West and Central Africa. Whereas, Trypanosoma brucei rhodesiense is found in East and Southeast Africa.
3) Plasmodium Species
Blood parasites of the genus Plasmodium are responsible for the disease we know as malaria. Of the one hundred fifty six named species of Plasmodium, only four are known to infect humans. These are P. falciparum, P. vivax , P. ovale and P. malariae.
These blood parasites utilizes two separate hosts to assist with the life stage development. The cycle begins with an infected mosquito of the genus Anopheles. The mosquito injects sporozoites of the parasite into a human host as it simultaneously draws blood for its meal. These sporozoites then continue to infect liver cells and pass through several life stages in the liver. (Incidentally, these parasites can remain dormant in the liver, reinfecting the host weeks or even years after the initial infection).
The parasites multiply while occupying the liver and then pass into the red blood cells. Once in the blood, the parasites will set about causing the damage that is ultimately responsible for the symptoms of malaria. They can then also be passed on to a mosquito during a blood meal, beginning the cycle anew.
4) Babesia Species
Blood parasites of the genus Babesia, are responsible for the illness known as babesiosis. There are over one hundred species of this parasite that have been recorded, however only a few are known to infect humans. In most cases of human infection, Babesia microti and Babesia divergens are the source parasites.
Similar to other blood parasites, the life cycle of Babesia species involves two hosts. Babesia species typically infect a tick that then in turn bites and infects a human host. The parasites continue to multiply and pass through life stages in the human host until they enter the blood stream. It is in this location that they are responsible for the clinical symptoms of babesiosis.
Humans will not usually re-infect ticks. However, it is possible for humans to transmit this type of parasite to other humans through blood-blood contact (Such as transfusions.) This family of blood parasites is found worldwide in locations such as Europe, the United States, Africa, Australia, and Asia. In areas that are prone to malaria epidemics, babesiosis may often be misdiagnosed for this condition. This has led to inaccurate information about the occurrence of Babesia in African and Asian nations.
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