The 3 Different Types of Parasites That Infect Humans: Part 1 of Parasitic Infections

When people think of a parasitic infection in humans, many imagine parasites as just problems in developing countries. But the reality is that they're common throughout the United States. 

In my practice, many patients with chronic gastrointestinal (GI) symptoms are referred to me. Many of these patients have previously been diagnosed with Small Intestinal Bacterial Overgrowth (SIBO) or Small Intestinal Fungal Overgrowth (SIFO). Typically, they’re worked up for and treated for these conditions, but their symptoms still come back. 

I’ve found that these patients often have concurrent parasitic infections in their colon (large intestine) that need to be worked up and addressed prior to their SIBO or their SIFO completely clearing. 

Earlier this year, The Guardian reported on this very thing: dangerous parasitic infections in a place called the Mississippi Delta.¹ The Mississippi Delta is subject to poor sanitation and drinking water, which contributes to the spread of many harmful intestinal parasitic infections in humans. Many states are affected by the water in the Mississippi Delta including:

  • Minnesota

  • Wisconsin

  • Iowa

  • Illinois

  • Missouri

  • Kentucky

  • Tennessee

  • Arkansas

  • Mississippi

  • Louisiana

We're also seeing more cases of parasites spreading through contaminated food, especially fresh produce.²

But parasitic infections don’t just affect places like the Mississippi Delta–they impact people all over the United States. The most recent data show that about 12 million Americans have parasitic infections.³

So in this blog, we'll discuss the different types of parasites found in the United States that infect humans, the symptoms they cause, and how these infections spread. Knowing the signs of these infections can help you get the right treatment and avoid long-term health problems.

What is a Parasite?

A parasite is an organism that lives in or on another organism to survive. The organism a parasite lives off of is called a host organism, which typically includes humans and animals. 

Unlike bacteria or viruses, parasites live off of a host organism without immediately killing them. However, parasites can cause long-term damage to the host.⁴

There are all types of parasites including ones that can only be seen with a microscope and ones that can be seen with the naked eye. But even though parasites are small, the impact they have on host organisms, especially humans, is significant.

What Are the Types of Parasites That Infect Humans?

There are three main types of parasites that cause disease in humans. They include:

  • Protozoan parasites

  • Helminths 

  • Ectoparasites 

Each type of parasite causes a different form of infection, but I want to focus mainly on the parasites that infect the stomach and intestines--as they're becoming commonly detected in the United States.

1. Protozoan Parasitic Infections

Protozoan parasites are single-celled organisms that multiply within their host. They often infect the intestines, blood, or other tissues in the body. When they're in humans, they can cause both acute and chronic illness. 

Giardiasis

This infection is caused by the parasite Giardiasis spp. which affects the gastrointestinal tract (stomach and intestines). It's spread by contaminated food or water. This could include drinking stream or lake water or eating raw produce washed with contaminated water.

Giardiasis affects about 7% of people in developed countries like the United States, and it's the most common cause of parasitic diarrhea. Symptoms of an acute infection include:⁵

  • Diarrhea

  • Abdominal cramps

  • Nausea

  • Gas

  • Bloating

You can also have a chronic giardiasis infection which typically causes:

  • Poor absorption of water and nutrients

  • Diarrhea

  • Fatty stools that look bulky, pale, foul-smelling, and oily (steatorrhea)

  • Weight loss

Cryptosporidiosis

Cryptosporidiosis is an infection most commonly caused in humans by these two species of cryptosporidium: Cryptosporidium parvum and Cryptosporidium hominis. Both parasites are primarily spread through water, but they can also be spread through the stool and by breathing in infected particles.

Over the last 10 years, there's been a 47.2% increase in cases of Cryptosporidiosis, making it one of the main causes of parasitic disease in humans.⁶

Cryptosporidiosis affects the small intestine. In people with healthy immune systems, it typically presents as diarrhea that resolves on its own. However, in people with compromised immune systems, it can cause:⁵

  • Chronic diarrhea that doesn't go away without treatment

  • Inflammation of the gallbladder (cholecystitis)

  • Inflammation and scarring of the ducts of the liver (sclerosing cholangitis)

  • Inflammation of the pancreas (pancreatitis)

Blastocystosis

This parasitic infection is caused by Blastocystis spp, which was previously called Blastocystis hominis. It's the most common protozoan found in human stool, and it affects about 20% of people in developed countries like the United States.⁵

The main way that Blastocystis spp. spreads is the fecal-oral route, which means that infected stool particles are consumed either through water or food. Blastocystosis affects the colon (large intestine) and causes symptoms including:⁵

  • Diarrhea

  • Abdominal pain

  • Gas

  • Bloating

  • Nausea

  • Anal itching

  • Itching of the palms of hands and soles of feet (palmoplantar pruritus)

  • Hives (Urticaria)

People with Blastocystosis may also have no symptoms at all. 

Cyclosporiasis 

Cyclosporiasis is an infection caused by the parasite Cyclospora cayetanensis. This illness is the second most common parasitic infection of the intestines in developed countries like the United States. The infection is caused by consuming food and water contaminated with infected feces. This typically happens when eating unwashed produce. 

Cyclosporiasis affects the small intestine, and infected patients usually presents with:⁵

  • Large amounts of watery diarrhea

  • Abdominal pain

  • Increased gas

  • Nausea

  • Fatigue

  • Weight loss

Symptoms sometimes go away within a few days in people with healthy immune systems. However, in people with compromised immune systems, symptoms can be persistent.

Cystoisosporiasis

Cystoisosporiasis is a parasitic infection caused by Cystoisospora belli. This infection is less common in developed countries like the United States, but it still happens. Cystoisosporiasis commonly affects those with compromised immune systems or people living in areas with poor sanitation or institutions. 

This parasite is spread by consuming contaminated stool particles (fecal-oral route), and it affects the small intestine. People with this infection usually present with:

  • Diarrhea that is non-bloody and watery

  • Fatigue

  • Abdominal cramps

In people with healthy immune systems, symptoms usually go away in seven to 10 days. However, in people with compromised immune systems, the diarrhea can be chronic and lead to:

  • Dehydration

  • Electrolyte abnormalities like low potassium

  • Weight loss

Immunocompromised people may also have fatty stools that look bulky, pale, foul-smelling, and oily (steatorrhea). Cases of gallbladder inflammation (cholecystitis) and high levels of a white blood cell called eosinophils have also been reported.⁵

Amebiasis / Amebic Dysentery

This last protozoan parasitic infection is caused by Entamoeba histolytica.  While it's the second most common parasitic disease in the world, it's not as common in countries like the United States. Instead, it's much more common in developing countries with poor sanitation and contaminated water. 

The main way that amebiasis spreads is by consuming contaminated food or water (the fecal-oral route), and this infection affects the colon (large intestine).

About 90% of people with amebiasis don't have symptoms. But in people who have symptoms, they typically present with dysentery--infected diarrhea with mucus and blood. Infected people may also present with symptoms like abdominal pain and mild diarrhea that's watery and non-bloody. 

Certain factors increase the risk of amebiasis being severe, including:⁵

  • Alcoholism

  • Steroid use

  • A compromised immune system

  • Cancer

  • Poor nutrition

  • Young age

  • Pregnancy

2. Helminth Parasitic Infections

The next parasites to review are Helminths. Helminths are parasitic worms that can usually be seen with the naked eye in their adult form. These worms infect the small intestine, large intestine, and sometimes organs like the lungs or brain. 

Helminths grow inside humans and lay eggs which create new helminths inside humans. 

Enterobiasis

Enterobiasis is an infection caused by the human pinworm or seat worm (Enterobius vermicularis). This infection affects about 40 million people in the United States, making it the most common worm infection in America. 

The infection starts in the small intestine and then moves into the colon (large intestine) and appendix. The most common way that Enterobiasis spreads is the fecal-oral route. However, you can also get Enterobiasis by coming into contact with pinworm eggs which can be found on contaminated:

  • Bedding

  • Linen

  • Pajamas

  • Carpets

  • Curtains

  • Cat and dog fur

  • Furniture

While some patients with Enterobiasis won't have symptoms, the most common symptom is anal itching. 

Ascariasis

The parasitic infection Ascariasis is caused by the roundworm Ascaris lumbricoides. This roundworm is the most common parasitic worm infection in the world and also the largest roundworm that infects humans. Ascariasis is seen in places with poor sanitation, making it quite uncommon in the United States. 

Ascariasis is spread through the fecal-oral route and affects the small intestine. Most people with Ascariasis don't have symptoms. If infected people do have symptoms, they include:

  • Abdominal pain

  • Bloating

  • Loss of appetite

  • Nausea

  • Vomiting

  • Mild diarrhea

A severe infection may cause nutritional deficiencies and even spread to the lungs. Large worms can even cause blockages in the gastrointestinal tract leading to conditions like: 

  • Bowel obstructions

  • Bowel perforation

  • Inflammation of the appendix (appendicitis)

  • Inflammation of the gallbladder (cholecystitis)

  • Inflammation of the ducts of the liver (cholangitis)

  • Inflammation of the pancreas (pancreatitis)

Trichuriasis

Trichuriasis is a parasitic worm infection caused by the roundworm Trichuris trichiura. This type of worm is also known as the human whipworm, and it's the third most common intestinal worm infection that's spread through the soil.

While less common, cases of Trichuriasis do happen in the United States--specifically the southeastern parts. Trichuriasis is more common in areas where there's poor sanitation and contaminated soil. 

When Trichuriasis first starts, the small intestine is affected. But as the infection progresses, the worm moves into parts of the colon (large intestine).

The symptoms of Trichuriasis depend on how bad the infection is. People with a mild infection typically have no symptoms. While more severe infections can cause symptoms like:

  • Abdominal pain

  • Diarrhea with mucus and blood (trichuris dysentery)

  • Rectal prolapse: a condition where the rectum moves out of its normal positioning

Ancylostomiasis

This last worm infection to discuss is caused by the hookworms Ancylostoma duodenale, Necator americanus, and Ancylostoma ceylanicum. Ancylostomiasis is commonly seen in countries with poor sanitation, including the United States. 

Ancylostomiasis affects the small intestine and is spread by either walking barefoot on contaminated soil or consuming contaminated food.

Similar to the other intestinal worm infections, people with mild infections don’t always have symptoms. However, people with more severe infections can have symptoms like:

  • Fatigue (usually due to iron deficiency)

  • Abdominal pain

  • Bloating

  • Diarrhea

These worms can also spread to the lungs with severe infection.

3. Ectoparasitic Infections

The last class of parasites I want to briefly review is called ectoparasites. Ectoparasites are different from protozoons and helminths because instead of living inside of the host, they live on or just under the skin. 

Even though they just live on the skin, they can cause allergic reactions, skin irritation, and spread infectious diseases. Most ectoparasite infections spread through close person-to-person contact, sharing clothing or bedding, or from infested pets. 

The types of Ectoparasites include:⁷

  • Lice: Mainly lives on the skin, and certain types live in head and body hair. 

  • Mites: These ectoparasites burrow into the skin and cause scabies. 

  • Fleas: These are small jumping ectoparasites that typically infect animals but can spread to humans.

  • Ticks: While these don't live in the skin, they're vectors (carriers) of many different infectious diseases.  

In general, ectoparasite infections cause symptoms including:

  • Itching of the skin

  • Red skin rash or raised bumps

  • Allergic reactions

  • Sores from scratching the skin

Sometimes, people can develop a bacterial infection from open sores. 

How Long Do Parasitic Infections Last?

The duration of a parasitic infection depends on the type of parasite involved. Some parasitic infections resolve within days while others can last for years if not properly diagnosed and treated. 

Recognizing the signs and symptoms of parasites and getting treatment as soon as possible is essential to avoid chronic illness and sometimes complications.

Why are Parasitic Infections Missed?

Parasitic infections are missed as a diagnosis for several reasons.

They’re missed mostly because of bias. Providers are simply not asking enough questions and not looking for it because we were all taught in medical school that parasitic infections are very rare in the United States. 

But this is simply not true. 

And it’s also missed because we don’t have good enough standard testing that will catch these infections. 

There’s no way to test the GI track directly so we’re relying on what sloughs off and comes out of the intestines to test. This is a problem because it’s not always going to produce a sample of an organism for us to find. 

Despite testing not being exact, it’s what we have for now, and it’s important to test for these infections with any chronic and especially any unresolving inflammatory gastrointestinal disorder. I’ve found that many of my patients have parasitic infections that, once cleared with proper treatment, were able resolve their long-standing symptoms.

Diagnosis and Tests for Parasitic Infections

Parasitic infections don't just affect developing countries--they're present in the United States, and certain types are becoming more common.

Next month, we'll review how the different parasitic infections are diagnosed. But if you suspect you have an infection from a parasite, contact our office today to discuss this further and determine if you need testing.