Parasitic Infections

What are parasitic infections?

Parasitic infections are diseases caused by parasites, organisms that depend on a host to survive and spread. They can infect humans, animals, and plants, causing anything from mild symptoms to life-threatening illness.

These infections are common in tropical and subtropical regions, with malaria among the deadliest. They also occur in the United States, where common examples include trichomoniasis, cryptosporidiosis, giardiasis, and toxoplasmosis.

Common types of parasites

Parasites that infect humans fall into three main groups:

  • Protozoa: Single-celled organisms that multiply within hosts.

    • Plasmodium (malaria)
    • Giardia lamblia (giardiasis)
    • Entamoeba histolytica (amoebiasis)
    • Cryptosporidium (cryptosporidiosis)
    • Trypanosoma (sleeping sickness, Chagas’ disease)
  • Helminths (worms): Multicellular organisms often visible as adults.

    • Nematodes (roundworms) such as Ascaris lumbricoides
    • Cestodes (tapeworms) such as Taenia species
    • Trematodes (flukes) such as Schistosoma species
  • Ectoparasites: Live on the host surface.

    • Lice
    • Fleas
    • Ticks
    • Mites (e.g., scabies)

How parasitic infections occur

Exposure routes include:

  • Contaminated food or water (eggs, cysts, larvae)
  • Poor sanitation and hygiene (fecal–oral spread, barefoot contact with soil)
  • Insect bites (mosquitoes, sandflies, blackflies)
  • Person-to-person contact (skin contact or shared personal items)
  • Contact with infected animals (Toxoplasma from cats)
  • Travel to endemic regions
  • Weakened immune systems (HIV/AIDS, transplant recipients)

What does not cause parasitic infections

They are not spread by casual contact, food, water, or respiratory droplets. Direct blood or tissue exposure is required.

Symptoms

Symptoms vary by parasite but may include:

  • Gastrointestinal: Abdominal pain, diarrhea, nausea, bloating, weight loss
  • Skin: Rashes, itching, sores, redness, lumps
  • Neurological: Headache, seizures, confusion
  • Respiratory: Cough, chest pain, shortness of breath
  • Visual: Eye pain, redness, blurred vision
  • Fever, night sweats, muscle/joint pain, swollen lymph nodes

Seek medical care for persistent or severe symptoms.

Diagnosis

Diagnosis involves:

Clinical evaluation

  • History (symptoms, travel, exposures, occupation)
  • Physical exam (vitals, skin, abdominal, neurological)

Laboratory tests

  • Stool analysis (ova and parasite exam)
  • Blood smears (malaria, filariasis)
  • Serology (antibody/antigen detection)
  • Imaging (ultrasound, CT, MRI for cysts or lesions)
  • Endoscopy/colonoscopy if stool tests are inconclusive

Treatment

Depends on the parasite type and infection severity:

Antiprotozoal medications

  • Metronidazole, tinidazole (giardiasis, amoebiasis, trichomoniasis)
  • Chloroquine (malaria)
  • Pentamidine (leishmaniasis, sleeping sickness)
  • Nitazoxanide (cryptosporidiosis, giardiasis)
  • Pyrimethamine (toxoplasmosis)

Antihelminthic drugs

  • Albendazole, mebendazole (roundworms, hookworms)
  • Praziquantel (tapeworms)
  • Ivermectin (scabies, various worms)
  • Pyrantel pamoate (pinworm, roundworm)

Surgery may be needed to remove cysts or drain abscesses.

Prevention

  • Wash hands with soap and water regularly
  • Drink safe or boiled water
  • Cook meat thoroughly
  • Wash and dry personal items in hot cycles
  • Use insect repellent and protective clothing
  • Check yourself and pets for ticks and lice after outdoor activities

Sources

  • About Parasites. CDC. Accessed Oct. 7, 2024.
  • Approach to Parasitic Infections. Merck Manual. Accessed Oct. 7, 2024.
  • Cummings RD, et al. Parasitic Infections. In: Essentials of Glycobiology. 4th ed. 2022.
  • Campbell S, Soman-Faulkner K. Antiparasitic Drugs. StatPearls. Accessed Oct. 7, 2024.
  • Transmission of parasitic disease. CDC. Accessed Oct. 7, 2024.
  • Braseth AL, Elliott DE, Ince MN. Gastrointestinal Parasitic Infections. Gastroenterol Clin North Am. 2021.