A parasite is a living organism (maybe a bug, bacteria, fungi, protozoa) that lives upon and derives nutrients directly from a host (another living organism), without giving any benefit to the host. In absence of 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 as compared to the host.
Hookworm, bed bugs, lice, honey fungus, Entamoeba, etc are a few examples of parasites
Types of parasites
On the basis of their location, parasites may be classified as:
- Ectoparasite: Organisms that inhabit the surface of the body of the host without penetrating into the tissues are called ectoparasites. For example, human lice, fleas, ticks, mites, etc. They are important vectors transmitting the pathogenic microbes. The infection by these parasites is called an infestation.
- Endoparasite: Organisms that live within the body of the host (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 examples 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 some 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:
- Oral or feco-oral route: It 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 transfusion of blood or blood products.
- Autoinfection: Few intestinal parasites may be transmitted to the same person by contaminated hand (external autoinfection) or by reverse peristalsis (internal autoinfection). It is observed in Cryptosporidium parvum, Taenia solium, Enterobius vermicularis, Strongyloides stercoralis, and Hymenolepis nana.
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, 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.