Common Diseases in Human Beings - Protozoan Diseases

Protozoan diseases:

About 15 genera of protozoans live as parasites within the human body and cause diseases.

1) Amoebiasis:

• Amoebiasis also called amoebic dysentery or amoebic colitis is caused by Entamoeba histolytica, which lives in the human large intestine and feeds on mucus and bacteria.
• The infective stage of this parasite is the trophozoite, which penetrates the walls of the host intestine (colon) and secretes histolytic enzymes causing ulceration, bleeding, abdominal pain, and stools with excess mucus.
• Symptoms of amoebiasis can range from diarrhoea to dysentery with blood and mucus in the stool.
• House flies (Musca domestica) act as a carrier for transmitting the parasite from contaminated faeces and water.
  

  Entamoeba histolytica

2) African sleeping sickness:

• African sleeping sickness is caused by the Trypanosoma species.
• Trypanosoma is generally transmitted by the blood-sucking Tsetse flies.
• Three species of Trypanosoma cause sleeping sickness in men.
  1) T. gambiense is transmitted by Glossina palpalis (Tsetse fly) and causes Gambian or Central African sleeping sickness.
  

  Trypanosoma gambiense

  2) T. rhodesiense is transmitted by Glossina morsitans causing Rhodesian or East African sleeping sickness.
  3) T. cruzi is transmitted by a bug called Triatoma megista and causes Chagas disease or American trypanosomiasis.

3) Kala-azar:

• Kala-azar or visceral leishmaniasis is caused by Leishmania donovani, which is transmitted by the vector Phlebotomus (sand fly).
• Infection may occur in the endothelial cells, bone marrow, liver, lymph glands and blood vessels of the spleen.
• Symptoms of Kala-azar are weight loss, anaemia, fever, and enlargement of the spleen and liver.

4) Malaria: 

• Malaria is caused by different types of Plasmodium species such as Plasmodium vivax, Plasmodium ovale, Plasmodium malariae, and Plasmodium falciparum.
  

  Types of malaria

• Plasmodium lives in the RBC of human in its mature condition it is called a trophozoite.
• It is transmitted from one person to another by the bite of the infected female Anopheles mosquito.
• Of these, malignant malaria caused by Plasmodium falciparum is the most serious one and can even be fatal.

The life cycle of Plasmodium:

• Plasmodium vivax is a digenic parasite, involving two hosts, a man as the secondary host and a female Anopheles mosquito as the primary host.
• The life cycle of Plasmodium involves three phases namely schizogony, gamogony, and sporogony.
• The parasite first enters the human blood stream through the bite of an infected female Anopheles mosquito. As it feeds, the mosquito injects the saliva containing the sporozoites.
• The sporozoite within the blood stream immediately enters the hepatic cells of the liver. Further in the liver they undergo multiple asexual fission (schizogony) and produce merozoites. After being released from liver cells, the merozoites penetrate the RBC’s.
• Inside the RBC, the merozoite begins to develop as unicellular trophozoites.
• The trophozoite grows in size and a central vacuole develops pushing them to one side of cytoplasm and becomes the signet ring stage.
• The trophozoite nucleus then divides asexually to produce the schizont.
• The large schizont shows yellowish-brown pigmented granules called Schuffners granules.
• The schizont divides and produces mononucleated merozoites.
• Eventually, the erythrocyte lyses, releasing the merozoites and haemozoin toxin into the blood stream to infect other erythrocytes.
• Lysis of red blood cells results in cycles of fever and other symptoms.
• This erythrocytic stage is cyclic and repeats itself approximately every 48 to 72 hours or longer depending on the species of Plasmodium involved.
• The sudden release of merozoites triggers an attack on the RBCs. Occasionally, merozoites differentiate into macrogametocytes and microgametocytes.
• When these are ingested by a mosquito, they develop into male and female gametes respectively.
• In the mosquito's gut, the infected erythrocytes lyse and male and female gametes fertilize to form a diploid zygote called ookinete. The ookinete migrates to the mosquito's gut wall and develops into an oocyte. The oocyte undergoes meiosis by a process called sporogony to form sporozoites. These sporozoites migrate to the salivary glands of the mosquito.
• The cycle is now completed and when the mosquito bites another human host, the sporozoites are injected and the cycle begins a new.
• The pathological changes caused by malaria affect not only the erythrocytes but also the spleen and other visceral organs.
• The incubation period of malaria is about 12 days.
• The early symptoms of malaria are headache, nausea, and muscular pain. The classic symptoms first develop with the synchronized release of merozoites, haemozoin toxin and erythrocyte debris into the blood stream resulting in malarial paroxysms – shivering chills, high fever followed by sweating. Fever and chills are caused partly by malarial toxins that induce macrophages to release tumour necrosis factor (TNF-α) and interleukin.

Prevention:

1. It is possible to break the transmission cycle by killing the insect vector. Mosquitoes lay their eggs in water. Larvae hatch and develop in water but breathe air by moving to the surface. Oil can be sprayed over the water surface, to make it impossible for mosquito larvae and pupae to breathe.
2. Ponds, drainage ditches, and other permanent bodies of water can be stocked with fishes such as Gambusia which feed on mosquito larvae. Preparations containing Bacillus thuringiensis can be sprayed to kill the mosquito larvae since it is not toxic to other forms of life.
3. The best protection against malaria is to avoid being bitten by mosquito. People are advised to use mosquito nets, wire gauging of windows and doors to prevent mosquito bites.
4. In the 1950’s the World Health Organisation (WHO) introduced the Malaria eradication programme. This programme was not successful due to the resistance of Plasmodium to the drugs used to treat it and resistance of mosquitoes to DDT and other insecticides.

Do You Know?

A malaria vaccine is used to prevent malaria. The only approved vaccine as of 2015 is RTS,S (Mosquirix). It requires four injections and has relatively low efficacy (26-50%). Due to this low efficacy, WHO does not recommend the use of RTS,S vaccine in babies between 6 and 12 weeks of age.