Most Common Intestinal Parasites in the United States
The most common intestinal parasites in the United States are Giardia lamblia (7.2%), followed by Cryptosporidium, Blastocystis hominis (2.6%), and Entamoeba histolytica/E. dispar (0.9%), with pinworm being the most prevalent in children specifically. 1, 2
Prevalence Data from National Surveillance
According to the 2017 IDSA guidelines, giardiasis, cryptosporidiosis, cyclosporiasis, and trichinellosis are nationally notifiable conditions in the United States 3. However, pinworm—despite being the most common intestinal parasite in children—is not a notifiable infection 1.
Specific Prevalence Rankings:
Protozoal parasites dominate:
- Giardia lamblia: 7.2% - Most common overall, with prevalence increasing broadly since 1979, particularly in Midwest and Northwest states 2, 4
- Cryptosporidium parvum: Second most common - Reported from 25 states across the country 2
- Blastocystis hominis: 2.6% - Prevalence appears to be increasing in recent years, accounting for 23% of infected cases in one study 2
- Entamoeba histolytica/E. dispar: 0.9% 2
Helminthic parasites are less common:
- Hookworm: 1.5%
- Trichuris trichiura: 1.2%
- Ascaris lumbricoides: 0.8% 2
Important Clinical Context
Seasonal patterns matter: Giardia identifications peak during summer and fall months, especially in northern states and the Midwest, with prevalence lowest (22-27%) in winter and highest (36-43%) between July and October 2, 5.
Geographic considerations: Zoonotic parasites like Toxocara and Toxoplasma are more frequent in southern states, in African Americans, and in populations with lower socioeconomic status 5.
High-Risk Populations
The 2017 IDSA guidelines emphasize that immunocompromised patients with AIDS and persistent diarrhea require additional testing for Cryptosporidium, Cyclospora, Cystoisospora, microsporidia, Mycobacterium avium complex, and CMV 3. These patients experience more severe or prolonged illness with parasitic infections 3.
Travelers with diarrhea lasting ≥14 days should be evaluated for intestinal parasitic infections, with giardiasis being the most common cause of persistent traveler's diarrhea (approximately 10% of cases) 3.
Clinical Pitfalls
One critical caveat: approximately 300,000 people in the United States have Trypanosoma cruzi infection, with local vector-borne transmission documented in southern states 5. This is often overlooked but represents a significant endemic parasitic burden.
The overall positivity rate for intestinal parasites in stool specimens ranges from 19.7-20% nationally, indicating that intestinal parasitism should not be overlooked as a cause of gastrointestinal illness in the United States 4.