A parasite is a living organism (maybe a bug, bacteria, fungi, or protozoa) that lives upon and derives nutrients directly from a host (another living organism) without giving any benefit to the host. Without a host, a parasite cannot live, grow and multiply. Therefore, a parasite normally doesn’t kill the host, but it can spread diseases, and some of these can be fatal. Parasites can be unicellular or multicellular and are often smaller than the host.
Hookworm, bed bugs, lice, honey fungus, Entamoeba, etc. are a few examples of parasites.
Types of parasites
Based on their location, parasites may be classified as:
Organisms that inhabit the surface of the body of the host without gaining access to internal tissues are called ectoparasites. For example, ticks, fleas, lice, and mites attach or burrow into the skin of humans.
Ectoparasites can survive on the host’s skin and cause skin irritations. Infection can range from asymptomatic to moderate itching. The infection by these parasites is called an infestation.
Some common ectoparasites of humans are:
Scabies (Sarcoptes scabiei)
Scabies mite burrows into the upper layer of the skin. They can intense itching and a pimple-like skin rash. Scabies outbreaks can occur in crowded living facilities such as nursing homes, dorm rooms, and prisons.
Bed bug (Cimex lectularius)
Bed bugs are parasitic insects that feed solely on the blood of people and animals while they sleep. Bed bug infestations usually occur in apartments, shelters, rooming houses, hotels, cruise ships, buses, trains, and dorm rooms.
Fleas are ectoparasites of humans. Their bites cause itching and irritation. They also serve as vectors for transmitting diseases like cat scratch disease, flea-borne typhus, and plague.
Ticks: Ticks are tiny blood-sucking ectoparasites. Common ticks are black-legged tick (Ixodes scapularis), lone star tick (amblyomma americanum), American dog tick (Dermacentor variabilis, D. similis), brown dog tick (Rhipicephalus sanguineus), groundhog tick (Ixodes cookei), gulf coast tick (Amblyomma maculatum), Rocky Mountain wood tick (Dermacentor andersoni), soft tick (Ornithodoros spp.), and western black-legged tick (Ixodes pacificus).
Common tick-borne diseases are Rocky Mountain spotted fever, Lyme disease, ehrlichiosis, babesiosis, and anaplasmosis. Ticks of the genus Ixodes transmit three infectious diseases: Lyme disease, ehrlichiosis, and babesiosis, whereas ticks of the genus Dermacentor transmit tularemia, Rocky Mountain spotted fever, and also Ehrlichiosis.
Human Lice: Human lice survive by feeding on human blood. Close person-to-person contact is needed for the transmission of lice infestations (pediculosis and pthiriasis). Three types of lice are found in the human body. They are
- Head louse (Pediculus humanus capitis)
- Body or clothes louse (Pediculus humanus corporis). Body lice also serve as vector for the transmission of epidemic typhus.
- “Crab” louse or pubic louse (Pthirus pubis)
Ectoparasites are important vectors for transmitting diseases such as spotted fever, typhus, ehrlichiosis, etc.
Organisms that live within the host’s body (in the blood, tissues, body cavities, and other organs) are called endoparasites. Invasion by the endoparasite is called an infection. In humans, all pathogenic protozoan and helminthic parasites are endoparasites. For example, roundworm, hookworm, amoeba, etc.
Endoparasites can be further subdivided into the following types based on their relation to the host.
- Obligate parasites: Parasites that complete a phase of their lifecycle in the host; therefore, without a host, they cannot complete their life cycle. For example, Plasmodium (malarial parasite), Toxoplasma gondii etc.
- Facultative parasites: Parasites that under favorable conditions, may live either a parasitic life or free-living life. Examples include Acanthamoeba, Naegleria fowleri etc
- Accidental parasites: Parasites that infect an unusual host. For example, Echinococcus granulosus is common in dogs but may infect humans accidentally.
- Aberrant parasites or wandering parasites: Parasites that infect a host where they cannot live or develop further (e.g., Toxocara in humans).
Source of infection of parasites
Humans may acquire parasites from various sources, such as contaminated soil and water, food, and contact with
- Contaminated soil and water: Soil polluted with human excreta may contain eggs of the parasites (such as hookworm, Ascaris species, Strongyloides species, and Trichuris species) and is an important source of infection. Similarly, infection may occur by drinking water contaminated with human excreta-containing cysts of E. histolytica or Giardia lamblia.
- Raw or undercooked meat: Raw beef containing the larvae of Cysticercus bovis and pork containing Cysticercus cellulosae are examples where undercooked meat acts as a source of infection.
- Other sources of infection:
- Aquatic source: Freshwater fishes (source of Diphyllobrothium latum), crab or crayfishes (source of Paragonimus westermanii), aquatic plants such as watercress (source of Fasciola hepatica)
- Bloodsucking insects: Mosquitoes (source of Plasmodium, Wuchereria bancrofti), tsetse fly (source of Trypanosoma spp), sandflies (Leishmania spp)
- Domestic animals: Animals such as cats (source of T.gondii) and Dog (source of E.granulosus), could also be the source of infection of parasites
- Man: The human itself is the host to several parasites and can excrete infective stages (eggs and larvae) and become the source of parasites such as E.histolytica, G.lamblia, E.vermucularis, etc
Modes of Transmission
The infective stages of various parasites may be transmitted from one host to another in the following ways:
- The oral or feco-oral route is the most common mode of transmission of the parasites. Infection is transmitted orally by ingestion of food, water, or vegetables contaminated with feces containing the infective stages of the parasite, soiled fingers, etc. (e.g., cysts of E. histolytica, and ova of Ascaris lumbricoides)
- Penetration of the skin and mucous membranes: Infection is transmitted by the penetration of the larval forms of the parasite through unbroken skin (e.g., filariform larva of Strongyloides stercoralis and hookworm can penetrate through the skin of an individual walking bare-footed over fecally contaminated soil), or by the introduction of the parasites through insect bite of bloodsucking insect vectors (e.g., Plasmodium species, Leishmania species, and Wuchereria bancrofti)
- Physical and sexual contact: Parasites such as E. gingivalis is transmitted from person to person by kissing or drinking from contaminated drinking utensils. Parasite such as Trichomonas vaginalis is transmitted by sexual contact. Others such as Entamoeba, Giardia, and Enterobius are also transmitted rarely by sexual contact among homosexuals.
- Vertical transmission: Infections with Toxoplasma gondii, Plasmodium spp., and Trypanosoma cruzi may be transmitted transplacentally from an infected mother to a fetus.
- Blood transfusion: Certain parasites like Plasmodium species, Babesia species, Toxoplasma species, Leishmania species, and Trypanosoma species can be transmitted through the transfusion of blood or blood products.
- Autoinfection: Few intestinal parasites may be transmitted to the same person by contaminated hand (external autoinfection) or reverse peristalsis (internal autoinfection). It is observed in Cryptosporidium parvum, Taenia solium, Enterobius vermicularis, Strongyloides stercoralis, and Hymenolepis nana.
List of Parasitic Diseases and Vectors
|Name of the Disease||Etiological agent (s)||Vector(s)|
|African Sleeping Sickness (African trypanosomiasis)||Trypanosoma brucei||Tsetse fly (Glossina species)|
|American Trypanosomiasis (Chagas Disease)||Trypanosoma cruzi||Various species of triatomine bugs|
|Ascariasis (intestinal roundworm infection)||Ascaris lumbricoides|
|Babesiosis||Babesia species||Ixodes scapularis ticks|
|Cysticercosis and Neurocysticercosis||Taenia solium|
|Echinococcosis (Hydatid Disease)||Echinocococcus granulosus|
|Elephantiasis (Filariasis, Lymphatic Filariasis)||Wuchereria bancrofti||Mosquitoes of the genus, Anopheles, Culex, Aedes and Mansonia|
|Enterobiasis (pinworm infection)||Enterobius vermicularis|
|Fascioliasis (Fasciola Infection)||Fasciola hepatica (liver fluke)|
|Guinea Worm Disease (Dracunculiasis)||Dracunculus medinensis|
|Hookworm infection||Ancylostoma duodenale, A. ceylanicum, and Necator americanus|
|Kala-azar (Leishmaniasis, Leishmania Infection)||Leishmania donovani||Sand fly (Phlebotomus papatasi and Phlebotomus argentipes)|
|Loiasis (Loa loa Infection)||Loa loa||Deerflies (mango flies or mangrove flies) of the genus Chrysops.|
|Malaria||Plasmodium species (vivax, ovale, malariae and falciparum)||Infected female Anopheles mosquito|
|Onchocerciasis (River Blindness)||Onchocerca volvulus||Blackflies of the genus Simulium|
|Trichinellosis (Trichinosis)||Trichinella species|
|Whipworm Infection (Trichuriasis, Trichuris Infection)||Trichuris trichiura|
Prevention of parasitic infections
- Washing hands regularly, especially after handling uncooked food or feces.
- Consumption of boiled water and properly cooked food
- Avoiding walking barefooted in soil, and enclosed spaces such as caves.
- Practicing safe sex, using a condom.
- Sastry A.S. & Bhat S. (2014) Essentials of Medical Parasitology. Jaypee Brothers Medical Publishers (P) Ltd
- Gracia, L.S. (2016). Diagnostic Medical Parasitology. ASM Press.