Parasites are organisms that live on or in a host organism and get their food from or at the expense of the host. There are three main classes of parasites that can live in humans: protozoa, helminths, and ectoparasites.
Protozoa
Protozoa are single-celled organisms that can live and multiply inside the human body. The most common protozoan parasites in the U.S. population are:
- Giardia lamblia – Causes giardiasis, an intestinal infection marked by diarrhea, gas, greasy stools, nausea and weight loss.
- Cryptosporidium – Causes cryptosporidiosis, an intestinal infection that can lead to watery diarrhea, stomach cramps, nausea and weight loss.
- Toxoplasma gondii – Causes toxoplasmosis, which often has no symptoms but can cause damage to the brain and eyes in people with weakened immune systems.
- Entamoeba histolytica – Causes amoebic dysentery, which results in stomach pain, diarrhea and bloody stools.
Giardia lamblia is the most frequently identified intestinal parasite in the United States and infects an estimated 1.2 million people each year. Cryptosporidium infects 748,000 people annually, while E. histolytica infects about 74,000. Toxoplasma gondii has infected about 22.5% of the U.S. population.
Helminths
Helminths are parasitic worms that feed on a living host to gain nourishment and protection. The most common helminth infections in the U.S. include:
- Pinworms – The most common helminth infection in the U.S. Pinworms cause itching around the anus or vagina and restless sleep.
- Hookworms – Occur mostly in the southeastern U.S. and can cause anemia and protein deficiency.
- Roundworms – Can infect people who eat uncooked meat or food contaminated by feces. Cause nonspecific GI issues.
- Whipworms – Can cause diarrhea, rectal prolapse and chronic inflammation of the colon.
It’s estimated that between 2-4 million Americans are infected with pinworms. Around 576 million people globally are infected with hookworms, with most cases occurring in Sub-Saharan Africa, Latin America and Southeast Asia. Infection numbers for roundworms and whipworms in the U.S. population are not well documented.
Ectoparasites
Ectoparasites live on the surface of the host rather than inside the body. Examples of ectoparasites include:
- Lice – Feed on blood and cause itching and sores on the skin.
- Fleas – Can transmit diseases like plague and typhus in rare cases.
- Ticks – Carry diseases like Lyme disease, ehrlichiosis, Rocky Mountain spotted fever.
- Mites – Cause scabies and mange in humans and animals.
Head lice are the most common ectoparasite infection in the United States, with an estimated 6-12 million infestations occurring each year among children 3-11 years old. Meanwhile, the number of reported cases of flea-borne plague in the U.S. has declined from around 100 per year in the early 1900s to an average of 7 cases per year from 2000-2012. Over 300,000 Americans are diagnosed with Lyme disease each year.
Overall Prevalence
Given the prevalence data available for specific parasites, we can estimate that parasitic infections affect millions of Americans each year. However, the overall percentage of the U.S. population with parasitic infections at any given time is not well documented.
One extensive study on the prevalence of intestinal parasites was conducted by the Centers for Disease Control and Prevention between 1988-1994. Stool samples from over 45,000 Americans nationwide were analyzed. The results showed that:
- 4% tested positive for pathogens like Giardia lamblia, Cryptosporidium, and Entamoeba histolytica.
- Less than 1% were infected with helminths like tapeworms, hookworms, and roundworms.
Based on a U.S. population around 300 million today, we can very roughly extrapolate these rates to estimate that:
- Around 12 million Americans may be infected with protozoan parasites
- Less than 3 million may have helminth infections
However, these are very rough estimates and the actual percentage of Americans with parasites could be lower or higher. Limitations of these numbers include:
- Prevalence likely varies considerably between different states, rural vs urban areas, socioeconomic status and other demographic factors.
- Many parasitic infections have no symptoms and go undiagnosed.
- Ectoparasitic infections like lice and scabies were not included in this study data.
- The data is now over 20 years old; infection rates may have changed.
Risk Factors
While parasitic infections can affect anyone, certain groups have higher risk factors including:
- International travelers
- Immigrants and refugees
- People with compromised immune systems
- Young children
- Pet owners
- People who handle animals
- People exposed to unsanitary conditions
- People who eat raw or undercooked meat/fish
- People who swallow water while swimming
- People living in warm, humid climates
- People living in crowded conditions
- Outdoor enthusiasts exposed to wildlife and vectors
Prevention
Many parasitic infections can be avoided by following good hygiene and safety practices such as:
- Washing hands frequently, especially before eating and after using the bathroom
- Washing all fruits and vegetables thoroughly
- Avoiding drinking or swimming in contaminated water
- Cooking meats thoroughly and freezing for 1-2 weeks to kill parasites
- Avoiding contact with potentially infected surfaces
- Practicing safe sex
Travelers should take extra precautions when visiting tropical areas. Anti-parasitic medications may also help protect against infections like malaria. Prompt treatment of any infections can help prevent parasite transmission and re-infection.
Conclusion
Parasitic infections are relatively common in the United States, with millions of cases occurring annually. However, the overall percentage of Americans infected at any given time is difficult to quantify exactly given limitations in data. Reported prevalence rates range from 4-12 million cases for specific protozoa and helminth infections. Groups at highest risk include travelers, children, and people with compromised immunity. Following good hygiene and safety practices can help lower infection risk.