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: protozoa, helminths, and ectoparasites.
Protozoa
Protozoa are microscopic, one-celled organisms that can live and multiply inside the human body. The main types of protozoa that infect humans as parasites are:
Giardia
– Giardia intestinalis (also known as Giardia lamblia or Giardia duodenalis) is one of the most common intestinal parasites.
– It lives in the intestine and is passed in feces.
– It causes a diarrheal illness called giardiasis.
– Giardia is found on surfaces or in soil, food, or water that has been contaminated with feces from infected humans or animals.
Cryptosporidium
– Cryptosporidium is a genus of protozoa that can cause gastroenteritis in humans. The most common species is Cryptosporidium hominis and Cryptosporidium parvum.
– Cryptosporidium infects the epithelium of the digestive tract and is spread through ingesting contaminated food or water or contact with infected persons or animals.
– It causes watery diarrhea, stomach cramps, nausea, and fever.
Entamoeba histolytica
– Entamoeba histolytica is a species of amoeba that can be a human parasite. It causes amoebiasis and amoebic dysentery.
– It is transmitted by ingesting water or food contaminated with feces containing the parasite.
– E. histolytica invades the intestinal lining and can spread to other tissues such as the liver, lungs, and brain.
Toxoplasma gondii
– Toxoplasma gondii is a common parasitic protozoan that can infect most warm-blooded animals, including humans.
– T. gondii is found in cat feces, contaminated food and water, and in rare cases through transfusions or organ transplants.
– It causes the disease toxoplasmosis, which is generally mild in healthy adults but can cause more serious illness in people with weakened immune systems and pregnant women.
Helminths
Helminths are worm-like parasites that survive by feeding on a living host. The major groups that infect humans are:
Flatworms
– Flatworms (platyhelminths) like tapeworms, liver flukes, and lung flukes have flat bodies.
– Tapeworms attach themselves to the lining of the small intestine and can reach over 15 meters long. They absorb nutrients from partially digested food.
– Flukes like liver flukes and lung flukes enter the body through skin and migrate to various organs.
Roundworms
– Roundworms (nematodes) have long, round bodies.
– Some types that infect humans include pinworms, whipworms, hookworms, and threadworms that live in the intestine and absorb nutrients.
– Filarial worms live in the lymphatic system and bloodstream.
Thorny-headed worms
– Thorny-headed worms (or spiny-headed worms) have a retractable proboscis covered in spines that they use to burrow into tissue.
– Two species, Acanthocheilonema and Gnathostoma, can infect humans and migrate through the body, causing damage to organs and tissues.
Ectoparasites
Ectoparasites are parasites that live on the external surface of the host’s body. Common human ectoparasites include:
Lice
– Head lice, body lice, and pubic lice feed on blood and live in the hair and clothes. They cause irritating itching and rashes where they bite.
– Lice are spread by close personal contact and are most common in children.
Fleas
– Fleas like the human flea, cat flea, and rat flea feed on blood. Their bites cause red spots with a central red dot.
– They can transmit diseases like plague and typhus to humans. Fleas spread easily between animals and humans.
Ticks
– Ticks like the deer tick, dog tick, and sheep tick are blood-sucking external parasites.
– They are vectors for transmitting diseases like Lyme disease, Rocky Mountain spotted fever, tick-borne encephalitis, and babesiosis.
– Ticks live in grass and woodlands and crawl onto animals or humans that brush against vegetation.
Mites
– Microscopic mites like scabies mites burrow under the skin and lay eggs, causing intense itching and rashes. They spread through skin-to-skin contact.
– Chiggers, a type of mite larva, inject enzymes into the skin that digest cell tissue, causing small red welts.
– Demodex mites live in hair follicles and are normal skin fauna in humans.
Bed bugs
– Bed bugs feed on blood at night, leaving small itchy bite marks. They hide in mattresses and furniture during the day.
– Bed bug infestations have resurged in recent years, requiring professional pest control. They spread easily in living areas.
| Parasite Type | Examples | Key Characteristics |
|---|---|---|
| Protozoa | Giardia, Cryptosporidium, Entamoeba histolytica, Toxoplasma gondii | – Microscopic single-celled organisms – Live in intestine, blood, tissues – Cause diarrheal, intestinal diseases |
| Helminths | Tapeworms, flukes, roundworms, thorny-headed worms | – Worm-like parasites – Absorb nutrients from host – Damage tissues and organs |
| Ectoparasites | Lice, fleas, ticks, mites, bed bugs | – Live on external surface of host – Bite and feed on blood – Vector infectious diseases |
Transmission of Parasitic Infections
Parasites can be transmitted in several ways:
Fecal-oral route
Many protozoa like Giardia and helminths like roundworms are transmitted via the fecal-oral route:
– Ingestion of food or water contaminated with human or animal feces containing parasite cysts or eggs
– Poor hygiene practices that enable transmission from feces to mouth
– Common in areas with poor sanitation and unsafe drinking water
Vector transmission
Some parasites rely on insect vectors to transfer them between hosts:
– Mosquitos transmit malaria (Plasmodium) and filarial parasites
– Ticks transmit Babesia, Borrelia that causes Lyme disease
– Triatomine bugs transmit Trypanosoma cruzi that causes Chagas disease
Skin contact
Ectoparasites like scabies mites directly spread through skin contact with infected persons or items:
– Close personal contact allows transfer of mites
– Shared clothing, towels, bedding enables spread
– Common in crowded living conditions, prisons, childcare centers
Foodborne transmission
Consuming raw or undercooked meat and fish can transmit parasitic infections:
– Taenia solium tapeworm cysts in pork
– Trichinella roundworm larvae in undercooked meat
– Anisakis roundworm larvae in raw fish
Mother to child transmission
Some parasites can cross the placenta and infect the fetus or spread through breastfeeding:
– Toxoplasma gondii
– Trypanosoma cruzi
– Plasmodium species that cause malaria
Signs and Symptoms
Different parasites cause varying symptoms and health effects:
Intestinal parasites
Intestinal protozoa and helminths often cause:
– Diarrhea, nausea, vomiting
– Abdominal pain and cramping
– Fatigue and malnutrition from nutrient absorption disruption
Systemic parasites
Parasites residing in blood, tissues, or organs can produce:
– Fever, chills
– Cough, wheezing, breathing difficulty
– Skin lesions, rashes, itching
– Joint pain and swelling
– Neurological issues like headaches, seizures
Ectoparasites
Parasites living on the external surfaces and skin typically cause:
– Itching, stinging, redness at bite sites
– Rashes or skin inflammation from secretions and waste products
– Allergic reactions to bites in sensitive individuals
Asymptomatic parasitism
Sometimes parasitic infections produce no obvious symptoms, especially in healthy adults and in mild or early infections. But asymptomatic carriers can still transmit parasites to others.
Diagnosis
Diagnosing a parasitic infection relies on:
Microscopic identification
– Stool samples viewed under a microscope can detect protozoan trophozoites and cysts, eggs and larvae of helminths.
– Blood smears examined for parasites like Plasmodium and trypanosomes.
– Skin scrapings or nail clippings to visually identify ectoparasites.
Antigen detection
– Immunoassay tests detect parasite antigen in stool, blood, urine, or other body fluids.
– More sensitive than microscopy alone. Detects giardia, cryptosporidium, and entamoeba histolytica.
Molecular methods
– Polymerase chain reaction (PCR) amplifies parasite DNA for accurate identification.
– Can detect parasites missed by other methods and distinguish species.
Imaging scans
– Ultrasounds, CT, or MRI scans help visualize parasites in tissues, organs, and body cavities.
– Useful for cysts, granulomas, worm masses, foreign bodies.
Exposure history
– Clinical history of potential parasite exposure through travel, contaminated food/water, or animal contact.
– Occupational exposure risks in certain jobs like agriculture and sanitation.
Treatment
Treatment options for parasitic infections include:
Antiparasitic drugs
– Anthelmintics like albendazole, ivermectin, and praziquantel that kill worms and flukes
– Antiprotozoals like metronidazole, tinidazole, and nitazoxanide against protozoa
– Topical pediculicides like permethrin to kill lice
– Scabicides like topical sulfur formulations against scabies
Antibiotics
– Antibiotics for secondary bacterial infections and sepsis prevention
– Doxycycline to eliminate some tick-borne parasites
Antimalarials
– Chloroquine, artemether, quinine for Plasmodium species
– Primaquine to treat dormant liver stage hypnozoites
Surgery
– Endoscopic or open surgery to remove parasitic mass lesions
– Drainage of cysts or abscesses that develop from parasite infiltration
– Debridement of necrotic tissue damaged by parasites
Supportive care
– Fluid and electrolyte replacement for dehydration
– Nutritional support
– Treatment of anemia and other blood disorders
Prevention
Preventing parasitic infections involves:
Sanitation and hygiene
– Wash hands before handling food and eating
– Wash fruits and vegetables
– Drink treated water instead of raw water sources
– Proper human and animal waste disposal
Vector control
– Use insect repellent and protective clothing
– Control mosquito, tick, and insect breeding areas
– Indoor residual spraying against vectors
Food safety
– Thoroughly cook meats and fish
– Prevent cross-contamination in kitchen
– Avoid raw or unpasteurized foods
General precautions
– Avoid direct contact with potentially contaminated soil
– Use gloves when gardening or working outdoors
– Do not drink or swim in water that may contain parasites
– Regularly check pets and livestock for parasites
Conclusion
Parasitic diseases remain a major public health issue globally. Protozoa, helminths, and ectoparasites can all take up residence in the human body and cause significant morbidity and even mortality. Contamination of food and water supplies combined with poor sanitation enables these pathogens to spread readily in the population. Advancements in diagnostic techniques now allow more sensitive detection of parasites. But the foundation of parasitic infection control relies on improved public health infrastructure, disease surveillance, access to treatment, and education on prevention methods. With a multipronged approach, the burden of human parasitism could be drastically reduced, especially in developing regions. This in turn would decrease detrimental health impacts and improve wellbeing and productivity across affected communities.
