Protozoa are now given their own order in the living kingdom though they were at one time thought to be minute animals as they were motile and were not green like plants - they now have a distinct kingdom under the classification scheme for all life on earth. Many protozoa are parasites on man and cause from mild to extremely dangerous diseases - some of them are parasitic on man and form the simplest of all the internal parasites because of their minute size.

The cellular organization of a protozoan is vastly more complex and the size is much bigger even though like bacteria, all protozoa are one-celled organisms-the entire organism is composed only of a single cell. The type of environments that most protozoa live in is very cosmopolitan and their distribution is ubiquitous, they are found in almost every watery environment and in damp places - this includes ponds, rivers, lakes and fish tanks, ditches etc.

The majority of protozoa do not cause harm to animals or people, and are considered free-living, which is to say that they do not require a host for living and multiplication. However, there are several protozoan's which are parasitic, among these parasitic organisms, some can bring on extremely serious health related problems to individuals unlucky enough to be infected by the protozoan.

The first pathogenic protozoan to be identified was far back in 1875 a protozoan. The parasitic protozoan was a type of amoeba called Entamoeba histolytica, which is also known as the dysentery causing amoeba - it is very common in occurrence and affects people around the world. One of the first and perhaps one of the greatest early microscopist and cell biologists, Anton van Leeuwenhoek was the one to notice a species of protozoan gliding about in his own feces that he examined.

The tiny microscopic life form was of the Giardia genus and from since Leeuwenhoek's discovery, the parasites such as the Giardia lamblia and the protozoan species Cryptosporidium, have been connected to the cause of painful intestinal infections along with outbreaks of severe diarrhea around the world. Water that has been polluted by human or animal feces normally contains these two parasitic protozoans, and drinking such contaminated water is the usual cause of disease.


A parasitic protozoan, known as Babesia microti is responsible for outbreaks of this disease. The parasite enters the red blood cells of various animals it infects and causes the typical symptoms so characteristic of the condition.

The main reservoir for the disease are rodents, the mode of transmission of the disease is through the bit of ticks, perhaps of similar species which are involved in the transmission of Lyme disease - thus ticks are the main vectors for the disease and spread it out from the rodent hosts to other animals including man by biting. A common disease called Texas fever - sometimes also known as cattle tick fever which affects cattle is caused by a protozoan related to this species.

The condition produces symptoms in the affected individuals, all of these symptoms seem to be related or similar to the symptoms seen in patients affected by malaria-which is also caused by a protozoan.

The physical symptoms include persistent tiredness and physical fatigue lasting seven days after initial exposure to the parasite, this initial period is followed by a period where the patient develops sudden chills, along with other conditions like fever, a severe headache and muscle soreness in the entire body. Some affected individuals often progress to anemia and kidney failure though the majority of symptoms are mild and the condition mostly resolves itself in weeks.

Fatalities in certain cases have however been known to occur, this is true with particular respect to patients who are of in the older age groups. No specific medications exists for the condition though symptomatic treatment using the same medications used to treat malaria have been tried out with some success - prevention is the best policy as far as this disease is concerned and incidences are rare.

Chagas' disease

A protozoan disease of the topics-Chagas' disease is a serious health issue in Latin America. The protozoan organism known scientifically as Trypanosoma cruzi is responsible for the conditions, the disease is widespread in Central and South America-where it is a public health concern.

The vector for this disease is an insect known as kissing bugs, sometimes known as the "cone-nosed bug", people living in areas with marginal housing infrastructure which is infested with this insect are likely to contract Chagas' disease-family. The insect vector is of the insect family Reduviidae, the subfamily Triatominae, and the genus Triatoma-this insect is again very common in South America.

The opossums and armadillos and also infected humans serve as reservoirs for the protozoan, the insect which is a blood sucker transmits the parasite from such sources to uninfected animals and people - infections spread usually in the same family among humans - the insect passes the parasite from host to host as it sucks blood and leaves waste matter on new hosts.

After a bug has fed on an infected mammal, it will transmit the protozoan which has already passed down its intestine with the waste it leaves on the skin of the uninfected host animal or human.

When this area is involuntarily scratched by the human, the waste in the insect bite area contaminates the wound and repeated scratching bruises the skin-the protozoan is also often passed through the eyes-when the person rubs the waste, the protozoan enters the body using the broken area of the skin. The tropical disease was identified in 1912, by the Brazilian physician Carlos Chagas-hence the name Chagas' disease.

The appearance of a small red sore on the skin or the development of swollen and reddened, inflamed eyes are the most common early symptoms of the disease in man.

After several weeks, the sore in the affected area of skin appears to heal and gives off a crusty scab which is soon lost, this leaves behind a small, dark scar in the area-if the person is lucky. In other cases, the initial symptoms may be succeeded by the appearance of fevers, the development of swelling in the lymph glands and nodes, and the development of skin rashes over the entire body.

The severe forms of Chagas' disease can involve the heart and symptoms such as chest pains and a sudden shortness of breath become common. Convulsions may also come, if the disease affects the nervous system-the entire body may be involved in continuous convulsions in such cases. Fatalities can result in some cases of the condition-it remains a health risk.

Giardiasis (Beaver fever)

This fever is a human parasitic disease which is the most common forms of parasitic disorder in parts of the United States and in other areas of North America-in particular the western regions of Canada.

The cause of the fever is a tiny parasitic protozoan organism which is called Giardia lamblia. The parasitic organism stays in the inner surface of the human gut, and is particularly found in large numbers in the small intestine of affected individuals. The common physical symptom aside from an increase in the temperature of the body is a persistent infectious diarrhea.

The usual source for the spread of the parasite is through the consumption of contaminated food and water. If strict and proper sanitary conditions are not maintained, many childcare centers and nursing homes become areas from which outbreaks of giardiasis frequently occur-thus children are one of the first to fall victim to the parasite.

The mode of transmission of the parasite in the majority of all infective cases is through the entry of parasitic cysts into the digestive tract along with the contaminated food or water-such sources usually contains fecal wastes from humans and other animal vectors.

About three weeks following initial infection from the parasite, chronic diarrhea begins to affect the individuals afflicted by giardiasis - this is the usual time frame for the appearance of this symptom in the majority of patients. In addition to this symptom, the affected individuals also develop symptoms of nausea along with other complaints such as a persistent and severe abdominal discomfort.

The digestive system is disturbed during prolonged periods of infection, the end result may be sudden weight loss in the affected individual-this is due to the very poor absorption of the digested food from the intestinal region. The individual may remain infected for several years at a stretch, though in certain cases affected individuals begin to show spontaneous recovery from the symptoms.

The major reservoir of the protozoan parasite in the wild was earlier believed to be the beavers-this was believed to be true for many years until recent evidence showed otherwise. The domesticated cattle and sheep are now known to be the major reservoirs for the disease according evidence seen in the recent studies.

It is now believed that the beavers, similar to humans, may probably be a secondary host and not a reservoir for the parasite. Animals like the beavers, the voles, the muskrats and kangaroo rats have all been shown to have the ability to carry this parasite - they may all act as hosts similar to humans.

Sleeping sickness (African trypanosomiasis)

A small fish like protozoan parasite causes the dangerous disease known as sleeping sickness - this deadly disease is widespread in certain areas of Africa. The protozoan is a trypanosome-a type of nematode worm, this worm can swims around in the blood of its victims by lashing its whip like tail or more accurately its flagellum. The infamous tsetse fly is responsible for the transmission of this lethal disease.

The fly bites and sucks blood, once the parasite enters the body. The person slowly losses consciousness and death ensues if the parasite manages to invade the regions of the spinal cord and the brain of its victims. The chances of dying from the disease is very high once a person is infected, and sleeping sickness is known to wipe out the entire domestic meat supply in an area - as this parasite also infects livestock such as cattle.

Fatalities of both human and animal victims are high in areas where the disease is endemic and prevention is the best option against the disease. The habitat of the tsetse fly is widespread in tropical Africa and some scientist and health experts feel that with rising temperatures due to global warming, the possibility of this habitat greatly expanding exists.

In such a scenario, the tsetse fly as well as the parasite has a cosmopolitan area to infect. This is not the only worst case scenario for one insect vector; it is generally believed that global warming might expand the habitat range of other insect vectors as well-leading to the spread of more tropical parasites to other regions of the world.


The parasitic protozoan leishmania is responsible for this debilitating and often lethal disease-which is prevalent in some tropical and sub tropical regions of the world. A tiny blood-sucking sand fly is responsible for the spread of this deadly parasitic protozoan-this fly is the only vector for the parasite. The condition known as visceral leishmaniasis, otherwise known as kala-azar is the most lethal stage in the progression of the infection.

In endemic areas, the disease is capable of wiping out two thirds of the population of some severely affected villages - this is true especially in rural areas in the tropics where medical care is poor or entirely unavailable. The presence of a persistent fever, a dark gray skin, the presence of severe anemia are some of the first physical symptoms of kala azar, in addition, the spleen and the liver become grossly enlarged, and this results in an unsightly distention of the abdominal region.

The disease can turn lethal very quickly if the symptoms are left untreated for any long period of time, kala azar is usually dealt with using medications which contain the heavy metal antimony-such drugs are the only effective cure against the parasite. The white blood cells in the body are the area in which the leishmania protozoan actually resides within the body-these are the protective cells that should have swallowed up the protozoa when infection began. The white blood cells ingest the protozoan but are not able to destroy or digest it.

The result is that the leishmania protozoa thrives inside the very white blood cells on which the body depends for its own protection. The white blood cells in which the protozoan reside is destroyed over time, similar to a disorder like HIV, this naturally wreaks immense havoc on the immune system of the afflicted person. Opportunistic and deadly bacterial infections such as pneumonia or dysentery overcome the body's weak and ineffective defenses as the white blood cells die in large numbers due to the protozoan. If this goes on, the person dies from the symptoms.


The real meaning of the word malaria is simply "bad air" This bad air was supposed to have made people very ill with an intermittent fever according to the ancient Greeks, when they first came across the disease. Indeed, the ancient Greek physician Hippocrates supposed that it was caused by the coming of miasmas upon the person-these are the deadly mists, which supposedly disturbed the careful balance existing in the body's four humors-which is the ancient medical belief for the blood, the phlegm, the black bile, and the yellow bile.

On the other hand, an imbalance in the yin and yang within the body was supposed to have caused paroxysmal fever-namely malaria-according to the ancient Chinese medical system and the disharmony between these two opposing male and female life forces within the body was attributed to be the cause of the condition.

Several centuries back the great taxonomist and biologist Carolus Linnaeus made a hypothesis that malaria may be caused by the presence of tiny suspended particles of clay in the drinking water, this suspended particulate matter is supposed to have led to the clogging of blood vessels, and an impaired circulation was supposed to have brought on the disease.

All of these were theories on the cause of malaria made before the advent of the microscope and at the very beginning of modern science, it was not until 1898, that the true causative agent of malaria would be identified - the British doctor Ross would correctly attribute it to the bite of an infected mosquito and he would go on to identify the lethal pathogen as a type of protozoan.

Only the bite of the female mosquitoes of the anopheles species is responsible for the transmission of malaria in humans and the few other mammals that can be infected. The female anopheles is the only mosquitoes which are vectors of the human malaria; male mosquitoes do not bite and cannot spread the disease.

When the female anopheles mosquitoes bites its victim, it sucks blood through a puncture hole in the skin of the individual and at the same time will inject thousands of threadlike protozoan larvae called sporozoites - these larvae are of the plasmodium species of protozoan's. Once they are in the body of the victim, the sporozoites will travel via the blood stream straight to the liver, where each one of the larvae will begin to infect and live in a liver cell of its own.

Within these liver cells, the sporozoites round up and begin dividing at a rapid rate, multiplying repeatedly to produce up to forty thousand spores per individual larvae. Two weeks pass as the production and multiplication of spores in the liver cell continues, there are absolutely no signs or symptoms of the illness during this window period and the person is healthy. Here however, the first seeds of full blown malaria are being planted to begin the invasion of the body.

The bursting of the liver cell in which the spores have formed triggers the first clinical signs and is the starting stage of the attack on the body - the person has heavy sweating and develops a high fever at this stage. The bursting liver cells now begin releasing the numerous spores directly into the bloodstream of the victim.

At this stage, each individual spore now invades a red blood cell where it will reside for sometime, slowly using up the cell's hemoglobin content and increasing in size. The spore then divides or shatters into many smaller fragments - this is an asexual means of producing thousands of other spores-the scientific name of the spores at this stage is merozoites.

Finally the red blood cell wills also bursts, and this will release all the merozoites present within and these then invade new blood cells in the neighborhood. This cyclic effect and the repeated infection and rupturing of red blood cells will finally bring on a chronic malarial condition and may result in the death of the person.

The rupturing and invasion of the red blood cells is a very well timed affair and there exists an amazing synchrony to the entire infection process within the bloodstream of the victim. The underlying causes for the simultaneous rupturing of millions of infected red blood cells to release their merozoites are still not fully understood and needs to be studied. The worst bouts of the shakes and shivering are due to this well timed, and en-masse rupturing of the red blood cells within the body.

The final process of transformation of these merozoites into male or female gametocytes also begins to occur following several rounds of infection of blood cells and their subsequent rupturing - these gametocytes are the reproductive cells of the plasmodium parasite and at this stage of infection, the human phase of the parasitic life cycle can be deemed to have ended.

If the patient is lucky, he or she would have survived this stage-however, he or she is infective at this stage because if a female anopheles mosquito sucks this malaria infected blood, it will also take in millions of the parasitic gametocyte cells. The gametocytes of the protozoan will undergo further development into eggs and sperms within the body of the female mosquito, starting the next stage in their new host and fertilization of these gametes will occur within the body of the insect.

The cycle is completed when the fertilized eggs of the parasite grow into the threadlike sporozoites within the body of the mosquito and these then make their way to the mosquito's salivary glands, ready for transfer to a new human host. At this particular time, the female mosquito becomes a deadly vector for the malarial parasite and her bite will transfer the new sporozoites to start a new cycle within another human host.

A very interesting fact is that people with the sickle-cell anemia disease and even those who suffer from its milder form, or those who have the sickle-cell shape trait in the red blood cells, are likely to have much greater protection against malaria offer. Sickle cell anemia deforms the red blood cells of the body and this changed shaped of the cells, directly inhibits and prevents the protozoa from residing within the cell.

This is a evolutionary response to malaria as the people suffering from the sickle-cell anemia tend to be largely concentrated in regions of the world with the highest rates of infection from malaria-the sickle cell shape of red blood cells is a genetic defect common only in human populations exposed year round to malarial disease.

So, how exactly is malaria treated? And what kinds of weapons are employed against the disease. Drugs and anti-malarial medications are some of the tools with which doctors and scientists treat malaria directly - the disease can also be controlled by regulating the population of the insect vector.

The quote "Treat the patient, not the mosquito" was often used by Robert Koch, the brilliant microbiologist who won fame with the identification of tuberculosis. The only anti-malarial drug which was available in his day-which was the late nineteenth century was the plant derived drug known as quinine.

The anti-malarial drug quinine, is a medical preparation derived from the bark of the cinchona tree, it had been used in traditional anti-malarial remedies by the natives of South America for centuries. As of today, it is still used as an excellent anti-malarial medication in many parts of the world. High dosages of the drug are unfortunately toxic, and it can cause deafness when used in the high doses needed completely remove the plasmodium parasite from the body.

Till less harmful derivatives were obtained in the 1930s, its use was extensive and universal - it has now been substituted by less harmful derivatives synthesized from the same plant. The compound chloroquine is the main synthesized medication used to treat malaria these days. However, as a drug of choice in the treatment of malaria, the drug chloroquine has been largely displaced by another anti-malarial mefloquine in most countries around the world.

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