By Omar Amin, PhD
Parasitology Center
Scottsdale, Arizona
Many of us have heard about illnesses caused by giardia and other parasites, but we tend to overlook connections between these microbes and digestive disease. According to the Centers for Disease Control, microscopic parasites probably cause more than 90 percent of all parasitic infections in the United States. Many doctors believe that we may be seriously underestimating parasites as contributors to disease. And then there are those that dismiss parasites as contributors to adverse health effects in the US. Worldwide, however, infectious diarrhea due to cholera, amoebas, giardia, and blastocystis, among others, is the second leading cause of death, fatal to at least 3 million people a year (Amin, 2005b). The burden of human infections with parasites worldwide is summarized below (Amin, 2017).
| Disease | Human infections | Annual deaths |
| Malaria | 489 million | 1 – 2 million |
| All worms | 4.5 billion | |
| Ascaris | 1.0 billion | 20 thousand |
| Hookworms | 900 million | 50 – 60 thousand |
| Whipworms | 750 million | |
| Filarial worms | 657 million | 20 – 50+ thousand |
| Schistosomes | 200 million | 0.5 – 1.0 million |
Are Parasites Just Tropical Diseases?
The medical profession views the parasite problem as an exception in America, a rarity. Most doctors and patients don’t usually think of parasites as a common cause of illness. We assume we’ve eradicated these problems with modern sanitation and water treatment. But research shows that parasitic infections are common, and the incidence is increasing. In many cases these infections underlie familiar digestive illness and other conditions as well (Amin, 1997, 1998, 2002, 2005a, b, 2007, 2011, 2017, 2021 and Amin and Amin, 2013).
Symptoms of intestinal infection are not isolated or unusual. Opportunities for exposure to and transmission of parasitic infection increase as overseas travel and immigration expand. Parasites are also transmitted in food processed through mass methods of farming, food manufacturing, and shipping from sources all over the world. Water treatment in huge urban systems is unable to totally eliminate contamination and periodically makes it worse.
There are parasites for every single tissue of the human body, once they gain access. An intestinal parasite has to gain access via the oral cavity with contaminated food or drink if it is to cause infection. Other portals of entry are essential for the proper selection of testing and control methods. Nine ways by which humans can contract parasitic infections (Amin, 2017, in part) are briefly summarized below.
Drinking water. Some of the most common microscopic human parasites (Protozoa) are transmitted via drinking water contaminated with fecal material from infected persons. This simple cycle occurs in water from running streams as well as from tap water in homes in large US and Canadian cities served by surface water treatment plants. Parasites transmitted in this manner include Cryptosporidium and Giardia. Cryptosporidium caused illness in more than 400,000 people in Milwaukee in 1993. More than 4,000 were hospitalized, and more than 100 died. Cryptosporidium is found in the public water systems and reservoirs of many American cities.
Skin contact with contaminated running water. This is the only method of infection available to certain parasites such as the schistosomes, some of the deadliest fluke (trematode) parasites of mankind. Swimmers’ itch is another form of schistosome infections in the United States. After emerging from the snail host, the infective larvae (cercariae) penetrate the skin of a person (swimmer, agricultural worker, washer women, children playing, etc.) and migrate in the human body ending up as adults in blood vessels of the hepatic portal system. To get infected, one has to be exposed in an endemic area, e.g. Africa, China, Mexico, Puerto Rico. At PCI, we have identified eggs in fecal samples from an isolated area near a stream in California nearby where a population of Vietnamese immigrants have settled.
Food. Food intake is perhaps one of the most common routes of transmission of parasitic infections caused by microscopic (Protozoa) and macroscopic (worm, helminth) parasites alike. For example, Blastocystis and the cysts of the amoebas (both are protozoans) are infective when swallowed with contaminated food via the fecal-oral route. This can occur in a household setting or a restaurant. Similarly, the ingestion of the eggs of the human roundworm, Ascaris, readily occurs when fresh vegetables, ex., lettuce, grown in farms fertilized with infected-infective human waste, are eaten without proper washing. Cyclospora, a parasite in the news, is tracked as a new or emerging pathogen; sometimes it is transmitted on imported fruit, especially on Guatemalan strawberries and raspberries, even though it maintains an endemic transmission cycle in the USA.
Insects. Most blood-sucking insects are capable of transmitting infectious agents via their bite as they attempt to feed on human blood. In the US, ticks transmit Lyme disease, Rocky Mountain spotted fever, relapsing fever, Colorado tick fever, babesiosis, and rabbit fever. Fleas transmit plague and endemic typhus. Mosquitoes transmit malaria and dog heartworm. Triatoma (kissing) bugs transmit Chagas disease, and head lice can transmit epidemic typhus. If a person has had a history of a recent insect bite in any temperate or tropical part of the world, his/her blood should be tested for parasites. Insect-borne pathogens normally cause no harm to their natural (reservoir) hosts, ex., rodents, but become highly pathogenic in humans (their unnatural hosts).
Air. Air-borne viruses, bacteria, and fungi are usually eliminated with the feces (occasionally orally) of a natural reservoir (usually wildlife) host but infect humans upon accidental inhalation. Examples in North America include histoplasmosis, Valley fever, and Hanta virus. These diseases are associated with bat guano, dust, and rodent feces, respectively.
Pets. Despite what you may have been told, dogs, among other pets, are not man’s best friend, parasitologically speaking. Dogs carry an intestinal tapeworm, Echinococcus, whose eggs spread all over their fur from the anal orifice during grooming. Unhealthy human contact with infected dogs, e.g., by kissing, brings the eggs into the human intestine which they penetrate as larvae and encyst in the body cavity, e.g., the liver or even the brain, as the life-threatening hydatid cysts. Other worm parasites (helminths) are also readily transmitted from pets and other animals to man. Most notable are the beef and swine tapeworms, Taenia, by the consumption of beef and ham contaminated with larvae of these tapeworms.
People. Close human-to-human contact is conducive to transmission of quite an assortment of sexually transmitted diseases, including AIDS and herpes as well as other viruses causing cold, flu, and COVID-19. Eating food in a restaurant or at home that may have been contaminated with Taenia eggs or Entamoeba cysts from the servers’ fecal through improper sanitary practices will surely produce infections with cysticercosis (appearing as lumps in the body or nerve organs) or amebiasis (causing severe gastrointestinal distress, etc.), respectively. A recent inspection of an expensive restaurant in Los Angeles showed that 100% of all employees (not just servers) had fecal matter under their nails.
Soil. Certain roundworm (nematode) parasites spend their transitional stages between one host and another as immature larvae in warm moist soil. Walking bare-footed or sitting on such fecal-contaminated “seeded” soil in a wooded area or by a lake side, etc. will invite the larvae of hookworms or Strongyloides to penetrate the exposed skin and migrate in the body to finally become adults in the intestinal tract where the damage is done.
Common Parasites in North America and Their Health Implications
The most common parasites identified from North American patients at our center are listed below in order of their prevalence from high to low:
- Protozoa:Blastocystis hominis, Cryptosporidiuin parvum, Entamoeba histolytica, Entamoeba hartmanni, Entamoeba coli, Chilomastix mesnili, Endolimax nana, Giardia lamblia, Cyclospora cayetanensis
- Helminths: Ascaris lumbricoides (human roundworm), Strongyloides stercoralis (threadworm), Ancylostoma duodenale/Necator americanus (hookworms), Enterobius vermicularis (pinworm), Trichuris trichiura (whipworm)
Article continues on next page:
Causes of Infection, Symptoms, Risks, Managing Infections, Damage to The Body
