Parasitic Infections in the United States

Many Americans believe that parasitic infections are rare in the United States (US). However, the Centers for Disease Control (CDC) reports that native, imported, and animal-transmitted parasitic infections affect millions of Americans each year. 

Sources of parasitic infection among Americans include pets, food, water, blood, and insects. With the increase in globalization through travel and trade and warming climate trends, there is a growing demand for qualified healthcare providers to identify parasites and clinically diagnose their diseases. This requires knowledge of parasitic life cycles, transmission, and treatment.

This article focuses on the parasitic infections that are most common in the US. It will also review treatment strategies, common challenges in managing parasitic infections, and prevention guidelines.

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Understanding Parasitic Infections

Parasitic infections are conditions or illnesses caused by parasites living within and reproducing within the body.  A parasite is a creature that lives on or in another host organism and acquires food from or at the host's expense. This provides parasites with the nutrients they need for survival.

Ectoparasites, protozoa, and helminths are the three predominant classes of parasites known to cause common parasitic infections and illnesses in humans. 

• Ectoparasites: 
  • Ectoparasites are a type of parasite that lives on the exterior of the host. 
  • These include fleas, lice, mites, and ticks.  
  • Ectoparasites are vectors that carry infections through blood, affecting animals and humans.  
• Helminths:
  • Helminths are multi-celled parasitic worms that infect the gastrointestinal tract.
  • In adulthood, they are visible to the naked eye and range from 1 mm to 1 m in length.
  • Helminths include roundworms and flatworms.  Most helminths cannot replicate in the host’s body but can mature and lay eggs there.  Adult helminths leave the host through the stool, but the eggs remain in the intestine, creating an opportunity for spreading to others who come into contact with infected stool. They commonly spread through contaminated food, water, and surfaces.
  • Helminths are categorized as round worms (Nematodes), tape worms (Cestodes), and flukes (Trematodes).
• Protozoa:
  • Protozoa are single-cell parasites that can infect the gastrointestinal tract, the blood, or bodily tissues.
  • A microscope is necessary to see them.
  • Protozoa also spread when you ingest trace amounts of stool or soil that contain their eggs, most often through water, fruit, and vegetables.
  • Protozoa are different from helminths because they are much smaller, comprise single cells, and can replicate within the host. Tens of thousands of protozoan species are classified according to their locomotion type.
  • The most common types of protozoa include: 
    • Amoebas -  amoebas employ “false feet,” or pseudopods, for mobility (Ex: Entamoeba histolytica)
    • Ciliates - ciliates are defined by their delicate and short cilia, hair-like structures used for locomotion and gathering food (Ex: Balantidium coli)
    • Flagellates - with whip-like flagella, allowing them to assess their surroundings and move around. (Ex: Giardia intestinalis or Trichomonas vaginalis )
    • Sporozoans - sporozoans are non-flagellated, non-ciliated, and non-amoeboid protozoa. They are unable to move in their adult state. (Ex: Plasmodium and Cryptosporidium)

Common Parasites in Humans in the United States

Below is an explanation of the most common parasites in the US.

• Giardia: 
  • Giardia is the most common intestinal parasite in the US.
  • It is caused by the flagellated protozoan G duodenalis. 
  • Giardia symptoms include diarrhea, abdominal cramps, nausea, gas, and bloating.
  • People, especially children, are most likely to get sick with giardia if they are in close contact with someone who is infected with it. 
  • It is more common in areas with limited access to safe drinking water and in people who drink untreated water from shallow wells, rivers, lakes, and springs.
• Toxoplasma:
  • Toxoplasmosis is caused by Toxoplasma gondii. 
  • This is most commonly contracted from consuming undercooked meat or from cat feces. 
  • The parasite can pass to a baby during pregnancy and cause birth defects or miscarriage.
  • Toxoplasma is unique because most people infected with the parasite do not have symptoms.
  • People who experience symptoms are most often infants or immunocompromised.  

• Ascaris:
  • Ascariasis is a roundworm infection. Adult worms can be more than a foot long.
  • Ascaris is one of the most common worm infections worldwide. It is present in the United States but less common than in tropical and subtropical regions.
  • If the infection is mild, most people with this type of parasitic infection have no symptoms. However, symptoms can be severe with a heavy infection.
  • Once ascariasis eggs have been swallowed, larvae hatch in the small intestine and migrate through the bloodstream to the lungs. This can cause signs and symptoms such as a chronic cough, wheezing, and shortness of breath.
• Hookworms:
  • Hookworms get their name from the hook-like shape of their heads. 
  • Adult worms and larvae live in a person’s small intestines. 
  • Hookworm infection occurs when people walk barefoot on or consume eggs from stool-contaminated soil.
  • Some people infected with hookworm have no symptoms, while others experience itching and a localized rash. These skin symptoms occur where the larvae enter the skin.
  • People with a more severe infection may experience abdominal pain, diarrhea, loss of appetite, weight loss, and anemia.
• Pinworms:
  • Pinworms are the most common worm infection in the United States, affecting about 40 million Americans.
  • Pinworm infection is called Enterobiasis.
  • They are most common in school-aged children but can affect people of any age.
  • The worms live in the intestines and rectums of infected people.
  • Female pinworms lay eggs around the anus, causing rectal itching, vaginal itching in females, and sleeplessness.

Epidemiology of Parasitic Infections in the United States

Despite previously more common infections such as malaria being successfully eliminated in the US, other parasitic infections remain endemic, creating a notable public health concern. Numerous parasitic infections are also imported each year. Intestinal parasitic infections are more common in developing countries, but cases are also increasing in developed countries due to international travel, migration, and the globalization of food.

The prevalence of parasitic infection varies from one part of the country to another due to many factors, including environment, geography, and social conditions. 

• Parasitic infections are more common in warmer climates and areas with crowded living conditions, poor sanitation, or insufficient water supply. 
• They are also more common in individuals with lower incomes and less personal hygiene and food preparation education.

Clinical Presentation and Diagnosis

Some parasitic infections cause no symptoms, while others are associated with many symptoms, some quite serious. Symptoms include:

• Gastrointestinal symptoms, including changes in stool, diarrhea, nausea, and vomiting, are some of the most commonly associated with parasitic infections.
• Systemic symptoms include fatigue, fever, muscle/joint pain, and teeth grinding. 
• Skin symptoms such as general itching, rectal itching, rashes, and hives. 

Many different types of testing are employed to diagnose parasitic disease. Often, a physician orders tests based on the symptoms and signs present in each individual, which can vary.  

• A stool exam, an ova and parasite test (O&P), is commonly ordered when parasite infection is suspected. The CDC recommends that at least three stool samples, collected on separate days, be examined. This test uses the naked eye and microscopy to assess for eggs, larvae, and adult parasites.
• Blood testing can also be used to detect some parasitic infections. Two different types of blood tests are used: serology testing and blood smears. 
  • Serology tests look for antibodies to parasites made by the body when it is fighting the infection. For example, a serology test may be used to identify toxoplasma-specific antibodies.  
  • A blood smear is a test used to look for parasites under a microscope that can be found in the blood. Examples of parasites found in the blood are babesiosis, a tick-borne illness, and malaria.  
• Molecular testing: Some protozoan infections are easier to identify via polymerase chain reaction methodology (PCR). This method uses diagnostic primers and tests DNA preparations extracted from fecal samples. PCR is often used when microscopic detection is difficult, especially if there is a low number of parasites in the stool sample. 
• Imaging studies can also be employed when diagnosing parasitic infections. These include X-ray, Computerized Tomography scan (CT), and Magnetic Resonance Imaging (MRI) scan. Imaging is used to look for lesions in the organs that some parasitic diseases may cause.

Treatment and Management of Parasitic Infections

The following are standard treatments and management strategies for parasitic infections:

Medications

A variety of medications are used to treat parasitic infections. The drug of choice varies with the parasite and the severity of the disease. Antiparasitic medications stop parasite growth, kill or paralyze them, or kill their eggs.  

• Examples of commonly employed antiparasitic medications are Pyrantel, Mebendazole, Ivermectin, Chloroquine, or Metronidazole.  Topical ointments or shampoos may also be prescribed for infection involving the skin. 

Functional medicine treatments may involve herbal preparations, including plant compounds with antiparasitic activity, rather than prescription medications. Wormwood, oregano, black walnut, garlic, and clove are examples of herbs with antiparasitic activity. 

While many antiparasitic drugs and herbs are fairly well tolerated, side effects vary and can include gastrointestinal disturbance, itching, rash, sleep disturbance, headache, and liver toxicity. 

Lifestyle Factors

Several lifestyle strategies can be employed to limit the risk of parasitic infection; these include: 

• Frequent hand washing, especially before meals
• Routine bathing with hot water and soap
• Ensuring clean and properly prepared food and water sources before consuming
• Protection from insects with appropriate clothing or bug spray
• Safe sex practices to limit the risk of trichomoniasis
• Check pets regularly for insects and follow recommended antiparasitic practices

In addition, functional medicine recommendations to limit the risk of parasitic infection include incorporating foods with protective properties, such as raw garlic and honey, into the nutrition plan. Research also supports using vitamin C, probiotics, zinc, and enzymes to protect against parasites.  

Challenges and Considerations for Practitioners

Diagnosing parasitic infection can be challenging due to the wide variety of symptoms that can be present, including symptoms that overlap with a variety of chronic health concerns. This alone can lead to the diagnosis being easily overlooked for many years. 

Another challenge when addressing parasitic infections is the growing resistance to drug and herbal treatment. Efforts to ensure accurate parasite diagnosis and employ efficacious treatment for a therapeutic duration are critical in curbing parasites' growing resistance to our treatment agents. 

Special consideration should be given to testing and treating immunocompromised patients, children, and pregnant women for parasites. These groups are at higher risk for more severe disease. 

Prevention and Control of Parasitic Infections

In the US, the CDC’s Division of Parasitic Diseases and Malaria works to limit the spread of parasitic diseases by improving the prevention, diagnosis, and treatment of parasitic infections. 

With the rise in international travel, warming climates, and poverty in some regions of the country, it is essential to continue revising data-driven diagnosis and treatment guidelines for healthcare providers. Equally essential is continuing to provide education and community outreach to the populations at the highest risk for parasitic infection. 

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Key Takeaways

• With the increase in globalization through travel and trade and warming climate trends, parasite infections are increasing in the United States.
• There is a growing demand for qualified healthcare providers who can identify parasites and clinically diagnose their diseases here in the United States.
• Ectoparasites, protozoa, and helminths are the three predominant classes of parasites known to cause common parasitic infections and illnesses in humans. 
• Infections with giardia, toxoplasma, ascaris, hookworm, and pinworm are the most common intestinal parasites in the United States.
• The prevalence of parasitic infection varies from one part of the country to another due to many factors, including environment, geography, and social conditions. 
• Parasitic infections are more common in warmer climates and areas with crowded living conditions, poor sanitation, or insufficient water supply.
• Many different types of testing are employed to diagnose parasitic disease. These include microscopy, blood testing, molecular techniques, and imaging.
• Antiparasitic medications and herbs arrest parasite growth, killing or paralyzing them, or killing their eggs.
• Several lifestyle strategies involving hygiene and proper food and drink preparation can significantly limit the risk of parasitic infection. 
• Special consideration should be given to testing and treating immunocompromised patients, children, and pregnant women for parasites. These groups are at higher risk for more severe disease. 
• Managing parasitic infection will require more research and development to meet the demands of our complex, changing environment.