Protozoa are single-celled organisms.  They are the cause of more sickness, death, mutilation, and debilitation in the world than any other group of disease-causing organisms.  As eukayotes, protozoa have a membrane-bound nucleus with well-defined chromosomes.  They also have other cellular organelles that are characteristic of eukaryotes.

Trichomonas vaginalis

This is a flagellate venereal disease that is found worldwide.  Males carry the infection, but are usually asymptomatic.  Females are also often asymptomatic. However, vaginal infections may cause inflammation, discomfort, and discharge. Secondary bacterial infections may occur along with trichomonas vaginalis.

The Trichomonas vaginalis organism feeds on bacteria and white blood cells. It can live outside the body in bath water, on toilet seats, or on washcloths. Diagnosis is by finding the parasite in discharges or by culture.  The drug of choice for treating this condition is metronidazole.

Trypanosoma (T. cruzi, T. gambiense, etc.)

These organisms are transmitted by an animal vector.  They parasitize (they live in) insects and have secondary vertebrate hosts, such as cattle and humans.  Most trypanosomes live in the blood and tissue fluids.  Some important species, such as T. cruzi, live inside cells.  Most are transmitted by blood-feeding invertebrates, such as flies.

African sleeping sickness is transmitted by the tsetse fly and is caused by T. rhodesiense or T. gambiense.   These flies live in the heart of Africa, between 15 N and 20 S latitude. In some regions up to 30% of the populace is infected with trypanosomes.

In the vertebrate host, these organisms live in the blood, lymph nodes, spleen, and cerebrospinal fluid.  The trypanosomes do not actually invade or live in cells.  Instead, they inhabit spaces in connective tissue in various organs.

During an infection, the lymph nodes become swollen and congested, especially in the neck, groin and legs. Periods of fever accompany early stages of the infection.

Successive parasite populations change immunologically.  This means that they disguise themselves to avoid being attacked and destroyed by the immune system.  These successive populations cause increased swelling, pain, headaches, weakness, and cramps.

When the central nervous system becomes involved, the classic symptoms of sleeping-sickness are induced.  Apathy, mental dullness, disturbances in coordination, tremors, paralysis, and convulsions usually follow.

As the infection progresses, sleepiness increases.  People often fall asleep while eating or standing.  Finally, coma and death occur.

Arsenical drugs are used in treatment, but toxicity is common. Serious complications can involve the nervous system, liver and heart.  The trypanosomes can also become tolerant to these types of drugs.  Suramin, pentamidine, and Berenil have been used successfully in early stages of the disease.

There is no vaccine because the parasites periodically change surface antigens--the markers that identify them as foreign to the immune system.

Chagas disease

Chagas disease is common in Mexico and South America. It is thought that Charles Darwin suffered from chronic Chagus disease following his trip to South America. It is caused by T. cruzi, and is characterized by an acute phase in which several symptoms occur:  anemia, loss of strength, nervous disorders, chills, muscle and bone pain, and varying degrees of heart failure.  Death may occur 3-4 weeks after infection.

A chronic form of Chagas disease is seen usually in adults.  The chronic form is characterized by dysfunctions in the central and peripheral nervous system.  Many patients die of heart failure due to an enlargement of the heart and subsequent lack of cardiac muscle tone.  Others suffer heart damage because the parasite damages the nervous supply to the heart.  The heart muscle then atrophies (the muscle shrinks).  The atrophy leads to congestive heart failure, because the muscle cannot pump blood efficiently. The intestine may also become immobile and enlarged due to the degradation of the nervous supply to that organ.

Nifurtimox, available in the US, kills the parasite when it is in the blood.  However, there is no known treatment for the intracellular stages of the parasite.  Treatment of parasite-induced heart failure is an attempt to control the symptoms.  Intestinal problems usually require surgical repair.

Leishmaniasis

Leishmaniasis is caused by Leishmania donovani and is transmitted by sand flies.  Leishmaniasis has a wide distribution that extends from the Mediterranean to Middle Asia, to southern Russia and China.  It is also known as kala-azar (black fever) or Assam fever.

The disease usually begins slowly with a low-grade fever and malaise.  This is followed by progressive wasting and anemia, protrusion of the abdomen from an enlarged liver and spleen, and finally by death in 2-3 years. In some forms, the parasite attacks the skin.  In these cases, the parasite causes severe lesions that may persist in some forms or spontaneously heal in others.

Diagnosis is by finding the parasite in smears from skin lesions or in bone marrow, spleen, liver, or blood smears. Treatment consists of an injection of antimonial compounds, good nursing care, and nutritional supplementation.

Giardia

Giardia lamblia is the most common flagellate in the human digestive system. It is highly contagious and can be found almost anywhere that people are also found.  It occurs in the highest concentrations in warm climates.  Epidemics occur sporadically, and waterborne outbreaks have been reported in the US. In some geographic locations, giardiasis has become a traveler's diarrhea like ammoebic dystentery.  Children are especially susceptible to infection.

The disease occurs when cysts are ingested, usually by drinking contaminated water.  The protozoans attach to the surface of the intestine and feed on the mucous secretions of the intestine.  A heavy infection can hinder the body's ability to take up nutrients.  Symptoms usually appear two weeks after exposure and include:  diarrhea, dehydration, stomach cramps, gas, and weight loss.

Prevention of giardiasis depends on good sanitation. Standard concentrations of chlorine for water treatment do not kill the infective form of giardia. In emergencies, the parasite can be killed by using 2-4 drops of household bleach in 1 liter of water, then by letting the water sit for 1 hour before drinking.

The disease is diagnosed by finding cysts in fecal smears. Treatment is typically with metronidazole (Flagyl®) and usually produces a cure within a few days. Alternatives include furazolidone, paromomycin, or bacitracin.

Amoebic dysentery

Amoebic dysentery has been a great debilitator in times of war throughout the ages.  Modern tourists often find they have picked up amoebic dysentery (popularly called "Montezuma's Revenge") along with their souvenirs. It is found throughout the world, and is caused by the protozoan Entamoeba histolytica.

The protozoan is transmitted by fecal cysts in contaminated food or water.  Once ingested, the organism secretes enzymes (special chemicals) that digest in the intestinal lining.  This leads to a perforated colon and peritonitis, which is an inflammation of the lining of the abdominal wall.  These complications can be fatal.  The liver and the lungs may also become hosts to the parasites.

Host responses vary, so symptoms also vary.  Factors that affect the host's response include the physical and emotional condition of the host when his or her immune system is challenged.  Usually, the disease develops slowly, with intermittent diarrhea, cramps, vomiting, and a general bad feeling.  In severe cases, dehydration and anemia result from the loss of fluids and blood.

Diagnosis is made by finding the organism in stool samples. Treatment is a two-fold attack.  Antibiotics are used to kill the organism in the body, and an agent is used to kill the microorganisms throughout the intestine.  Several antibiotics are used, including metronidazole (the drug of choice), tetracycline, and chloroquine.

Amoebic meningoencephalitis

Amoebic meningoencephalitis is caused primarily by Naegleria fowleri.  This is normally a free-living ameoba.  The parasite gains entry into its host via the nasal passages when the person swims in infected waters. It colonizes the nasal epithelium, then travels down nerve tracts to the brain.  Once in the brain, the infection causes headaches, fever, stiff neck, nausea, drowsiness, convulsions, and coma.  Death occurs within 10 days, usually on the 5th or 6th day. The incubation period is 2-3 days to 2 weeks. No person-to-person transmission has been reported.

Balantidaisis

Balantidaisis is a form of dysentery.  It is caused by the only ciliated protozoan parasite of humans (Balantidium coli). This parasite is found worldwide.  There is a slight correlation between the incidence of the disease and the presence of hogs.

The balantidaisis organisms feed on epithelium and red blood cells in the colon and can cause ulcers. Diagnosis is by finding the parasite in feces.

Toxoplasmosis

Toxoplasmosis is caused by ingesting infective cysts of the parasite Toxoplasma gondii. The intestine is affected first, followed by the lymph nodes, which swell and become painful.  Other symptoms include fever, headache, muscle pain, and anemia. Toxoplasmosis may also infect the eye, causing blindness.

In congenital toxoplasmosis, a mother infected at the time of conception or during pregnancy, transmits the infection to the developing fetus. The parasite infects the fetus by crossing the placental barrier.  Most neonatal infections are without symptoms.  Some cause fetal death or birth defects.  Fetal infection may result in spontaneous abortion or stillbirth.

Toxoplasmosis is acquired by eating raw or undercooked meat, and through contact with an infected feline, such as the household cat.  Any cat, no matter how protected, well-fed, or well-cared for may pass the infective organism in excrement unless they are kept from hunting and eating the kill. Even strictly kept house cats may become infected through cockroach or fly hunting as these are transport hosts. Cats pass the organism for several days after becoming infected.

A woman who might be or is pregnant should not clean cat litter boxes or handle intestinal messes (vomit, hairballs, etc). Infection may also be acquired from gardening or ingesting unwashed, contaminated fruits and vegetables. Children's sandboxes should be covered to prevent contact with excretions from infected cats.

Diagnosis of toxoplasmosis is by demonstration of the organism in biopsy.  More commonly, diagnosis is by demonstration of a specific antibody (protein marker in your blood) for the organism.

Treatment is usually with a combination of pyrimethamine and sulfonamides. Possible side effects of these drugs include low numbers of platelets and or white blood cells.  These side effects can be avoided by taking supplements, particularly of folic acid.

Cryptosporidium parvum

This protozoan is present in most surface water and is an obligate intracellular parasite of many species.  Those species from mammals are generally infective for other mammals. C. parvum seems to lack strict host specificity and is thought to transmit from calves to mice in nature. Humans become infected usually via the ingestion of infected organisms in drinking water.

The infective stage of the organism, the oocyst, is about 3 um in diameter. Through contamination of the environment, food, or water, a suitable host ingests oocysts. In the gastrointestinal or respiratory tracts of the host, sporozoites exit from the oocyst and parasitize epithelial cells. The infectious dose is thought to be less than 10 organisms and presumably one organism can initiate an infection. Watery diarrhea, cramps, and headache characterize Cryprosporidiosis in its most frequent intestinal form. The lungs and trachea can be affected as well, producing low-grade fever and coughing.

It is most dangerous to children and immunocompromised patients, such as AIDS patients, and people undergoing cancer chemotherapy. Healthy individuals normally have shorter duration of symptoms (<20 days) and spontaneous complete recovery. Immunodeficient individuals, especially AIDS patients, may have the disease for life, with diarrhea and pulmonary symptoms contributing to death.

There are no vaccines available for prevention of cryptosporidiosis in either animals or humans. Human infections have been reported on six continents, in developed and less developed countries and in urban and rural areas. In the U.S., large outbreaks have been associated with contaminated water supplies. Some experts estimate that 10 million cases of cryptosporidiosis occur per year. Several large waterborne outbreaks in municipal water sources have occurred since 1987 in Georgia, Texas, Oregon and Wisconsin.

Malaria

Malaria, the world's most prevalent vector-borne disease, is caused by the Plasmodium species and has been known since the time of the Egyptians. Historical records show that descriptions of an intermittant fever are reported following the flooding of the Nile. In early American history, people living in coastal marsh areas would have separate inland living quarters for the summer season, if they were fortunate. Today some 1,000 cases of malaria are reported annually in the US while 500 million people are infected worldwide each year and 2-3 million die.

The parasite requires two hosts: a mosquito and a vertebrate. When a mosquito feeds on a human, the parasites are injected into the bite wound. The parasite undergoes some development prior to infecting red blood cells.  In red blood cells, the parasite digests hemoglobin (the protein that carries oxygen in the blood), and leaves behind indigestible portions and waste materials in the blood cell. When the blood cell ruptures, the released waste causes some of the symptoms associated with malaria.

Malaria symptoms occur in cycles. Symptoms include chills, fever, and anemia.  A typical attack is characterized by severe chills, followed by fever. The teeth chatter, and the victim may shiver intensely.  Nausea and vomiting are common.  Nausea and/or vomiting is followed by an intense feeling of heat and mild delirium. The victim may sleep after an episode and until the next cycle.

Host defenses are critical in limiting the infection.  Exposure to the parasite may allow some hosts to develop some protective immunity. Relapses are possible, even years after the initial infection.

Uncomplicated attacks of malaria can be treated with chloroquine.  One chloroquine-resistant strain of malaria is treated with mefloquine and quinine.  The resistant strain is more likely to be found in select areas, but most malaria is responsive to chloroquine. Travelers heading for endemic areas must take prophylactive chloroquine or some other preventative medication.

A new and hopeful development is the clinical testing of a multi-gene DNA vaccine intended to prevent infection by the malaria parasite Plasmodium falciparum (Pf). The vaccine incorporates five genes that are designed to cause production of Pf immunogens and trigger an immune response against the malaria parasite in the sporozoite and liver stages of its life cycle. The vaccine has been mildly successful (50-65% efficacy) in field trials, but the protection doesn't last longer than two months. Numerous other approaches to a malaria vaccine are under investigation by various research groups.

Sickle-Cell Anemia

Sickle cell anemia results from an alteration in the gene for hemoglobin that is protective against malaria. When an individual has two copies of the gene (homozygous) the gene is deadly.

However, in the heterozygous condition (one copy) the gene is protective.  In this case, the mutation of some of the hemoglobin interferes with the normal growth and development of the parasite within the red blood cell.  People with the gene for sickle-cell anemia have a lower incidence of malaria and less-severe cases of malaria than other members of the population.