There are many parasites that commonly affect humans. Among these parasites are the protozoa that share the Earth's water, soil and food supply with the human population. Some may not be able to cause disease if you are healthy, while others can do harm if they are able to reach a particular organ in your body.
Entamoeba Histolytica
Donald Heyneman, Ph.D., writes in "Jawetz, Melnick, & Adelberg's Medical Microbiology," that Entamoeba causes disease if it manages to invade the intestines. This parasite has enzymes that it uses to create what are described as "flask-shaped" lesions in the large intestines. It will spread downwards to infect the sigmoid colon and rectum, or migrate up to infect the small intestines. There can be blood and pus in bowel movements. And it can also cause liver infections, either abscesses or hepatitis (inflammation of the liver).
Giardia Lamblia
Unlike Entamoeba and its enzymes, Giardia has a disk which it uses to suck and attach onto the wall of the small intestines. According to Richard Pearson, M.D., professor of medicine and pathology at the University of Virginia School of Medicine, many people who have Giardia do not have any symptoms. If they do have symptoms, they will usually have cramps, abdominal discomfort and a watery diarrhea that smells very bad.
Naegleria Fowleri
Naegleria is a protozoan which lives in the soil. It usually affects children who swim in water that is contaminated with soil containing Naegleria. The parasite enters the nose, goes through the ethmoid bone and enters the brain. It can cause a severe headache, nausea and a high fever, due to a disease called primary amoebic meningoencephalitis (inflammation of the brain and its meninges membrane). Diagnosis must be done quickly, as the disease causes death in less than one week.
Cryptosporidium Parvum
This parasite will only cause a mild and short case of diarrhea in people who are healthy. But if the immune system is impaired--for example, due to steroid use, chemotherapy, advanced/very young age or AIDS--Cryptosporidium parvum will cause severe and prolonged diarrhea.
Toxoplasma Gondii
Dr. Pearson notes that 20 to 40 percent of adults have antibodies against Toxoplasma gondii, so it is a very common parasite. But people are only at risk of developing disease if their immune system is impaired. (A fetus in the womb of an infected mother, who does not have antibodies, is also at risk.) Healthy people will usually not have any symptoms, but may perhaps have fever and muscle aches. If their immune system is impaired, they can have seizures, vision problems, inflammation of their brain (encephalitis) and brain lesions.
References
- "Jawetz, Melnick, & Adelberg's Medical Microbiology"; George Brooks, M.D., Karen Carroll, M.D., et al.; 2007
- The Merck Manual: Giardiasis
- The Merck Manual: Toxoplasmosis
- The Merck Manual: Extraintestinal Protozoa
- The Merck Manual: Intestinal Protozoa
