The Protozoan Parasites

OVERVIEW OF PARASITOLOGY

Reading assignment: Please look up all of these in your textbook.

THE PROTOZOAN PARASITES

1. Sarcodina - The AMOEBA
AMOEBIC DYSENTERY - Caused by Entamoeba histolytica world wide if CYSTS get into food or water. Approximately 5% of US population are ASSYMPTOMATIC CARRIERS of this organism. Spread is by ORAL-FECAL ROUTE. Initial infection is in large intestine with attack on epithelium. There is severe dysentery with numerous small stools containing blood, mucus, and necrotic intestinal epithelium. There is acute pain and tenderness and fever. May break through and invade the liver where it may set up AMOEBIC LIVER ABCESS.
Naegleria and Acanthamoeba are free living fresh water amebas which have been known to cause AMEBIC MENINGITIS. Infections are rare but interesting since most cases seem to result from swimming or diving in warm fresh water lakes and ponds although sometimes swimming pools have been implicated also. The organisms apparently come in through the nose and penetrate through the cribiform plate into the base of the brain. The clinical course is rapid and death usually follows 4-5 days after onset of symptoms of fever, headache, blocked nose, altered taste and smell, stiff neck and Kernig's sign.

2. Zoomastiga - The FLAGELLATES

Giardia lamblia is transmitted through food and water which have been contaminated by human or animal feces containing CYSTS. About 7% of the US population are healthy carriers and a number of wild animals also serve as carriers. Fresh water streams in Hawaii have been vehicles of infection. The organism multiplies in the small intestine and causes MALABSORPTION SYNDROME with impaired absorption of vitamin B12 and fats. The failure to absorb the vitamin may eventually lead to anemia. Other symptoms however are more dramatic - flatulence, abdominal distention, steatorrhea with foul-smelling bulky stools, anorexia, nausea and eventual weight loss.
Trichomonas vaginalis is a frequent cause of vaginitis in women whose normal vaginal flora and/or normal vaginal pH have been disturbed. It appears that many women and men can be assymptomatic carriers of this organism. Symptoms of trichomoniasis include tender vaginal walls, frothy, creamy greenish-yellow discharge, itchy and burning sensation.

3. Ciliophora - The CILIATES

Balantidium coli is the only ciliate known to infect humans. This organism infects the large intestine and infection is transmitted by the oral-fecal route.

4. Apicomplexa - protozoa with complex mouth parts.
MALARIA - This disease which infects more than 300,000,000 people each year causing about 4,000,000 deaths is the number 1 infectious disease on earth. Of the four types of malaria, the type caused by Plasmodium falciparum is the most severe and causes almost all of the fatalities. This organism can infect all of the erythrocytes and once infected the RBC's tend to get sticky and obstruct small capillaries throughout the body. These obstructions lead to severe complications in the brain, kidneys, heart, lungs and gastrointestinal tract. CEREBRAL MALARIA describes the infected, swollen congested brain. BLACKWATER FEVER, a complication of quinine treatment, describes the passing of hemolyzed blood through the kidneys.
Malaria is transmitted through the bite of the female anopheles mosquito which has infectious sporozoites in her salivary glands. (See the description of the life cycle in Tortora et. al. and in Beishir). The cycle of fevers or malarial paroxysms is a hallmark of malaria and it is correlated with the synchronized rupture of infected RBC's. It occurs every 48 hours in P. falciparum infections.

TOXOPLASMOSIS - The organism Toxoplasma gondii is widely dispersed in domestic cats and humans are often infected by cysts in cat feces. Food animals such as pigs, cattle and sheep can also be infected and humans are sometimes infected by eating undercooked meats. Infections in healthy humans are generally without consequence, however there are two situations in which infections are serious.:

Infection during pregnancy may lead to severe fetal damage involving the brain, viscera and muscles.

Infection of individuals with immunodeficiencies such as AIDS may lead to serious brain and eye lesions.

CRYPTOSPORIDOSIS - Cryptosporidium parvum has only been known as a human pathogen since the late 1970's. This organism can cause life-threatening diarrhea in individuals with AIDS. People with normal immune systems will suffer from mild diarrhea. Increasingly this organism is being seen as a contaminant of city water systems. This may be so because the cysts of this organism are resistant to chlorination. Some cities have started filtering water to get around this problem.