The Concept of Parasitism and Its Theories
The term «parasite» originates from the Greek words meaning "eating beside another": *para* — beside, *sitos* — food. Parasites include all viruses, most bacteria, a number of fungi, and numerous animals; exceptions are cnidarians and sponges. There are about 50 definitions of parasitism, which can be grouped into four main concepts.
- Ecological concept defines parasitism primarily by one organism living inside another, but this also encompasses mutualism and commensalism.
- Immunological concept considers the host's ability to recognize the parasite as a foreign antigen; the drawback is its similarity to immune responses against tumors and allergens.
- Metabolic concept characterizes parasitism as the dependence of one organism's metabolism on that of another; it fails to account for the parasite's own homeostatic mechanisms.
- Pathophysiological concept focuses on the harm inflicted on the host as the primary criterion; it does not differentiate between effects at the organismal and population levels.
A comprehensive definition of parasitism emerges only from the combination of all four concepts.
Classifications of Parasites and Hosts
Parasites are classified according to several criteria:
- By the nature of their relationship with the host: true parasites (Ascaris, pork tapeworm) and false parasites (housefly larvae); obligate (tapeworms) and facultative (Strongyloides); hyperparasites — parasites of parasites.
- By localization: ectoparasites (on the integument) and endoparasites (inside the host).
- By duration of contact: temporary (for feeding only) and stationary.
- By distribution: ubiquitous (worldwide) and tropical (Schistosoma, Brugia).
- By biological and epidemiological features: soil-transmitted helminths (geohelminths), biohelminths (via an intermediate host), and contact-transmitted helminths.
Hosts are categorized by the parasite's developmental stage into definitive (where sexual reproduction of the parasite occurs), intermediate (larval stages and asexual reproduction), accessory (second intermediate), and reservoir (accumulation of infective stages) hosts. Based on developmental conditions, we distinguish obligate, facultative, and potential hosts.
Pathways of Parasite Entry into the Body
- Sexual — Trichomonas vaginalis.
- Contact-household (fomites) — Sarcoptes scabiei (itch mite).
- Via mother's milk — Ascaris larvae.
- Transfusion — Trypanosoma, Plasmodium species.
- Transplacental — Toxoplasma gondii, Plasmodium species.
- Percutaneous — fluke larvae through the skin.
- Airborne (respiratory) — viruses, bacteria.
- Alimentary (fecal-oral) — the primary route for helminth eggs and larvae.
- Vector-borne — via blood-sucking arthropods through inoculation (via proboscis) or contamination (rubbing vector feces into the wound).
Human Protozoan Parasites
Sarcodina. Entamoeba histolytica is the causative agent of amebiasis. It exists as cysts (with four nuclei) and trophozoites in three forms: small vegetative, large vegetative (erythrophagocytic), and tissue forms. Transmission is alimentary; the large and tissue forms are pathogenic and can invade the liver and brain.
Flagellates. Trichomonas vaginalis possesses five flagella and a supporting axostyle, is sexually transmitted, and does not form cysts. Lamblia intestinalis (Giardia) features an adhesive disk and inhabits the duodenum. Leishmania alternates between flagellated (promastigote) and non-flagellated (amastigote) forms; the vector is the sandfly Phlebotomus. Trypanosomes exhibit trypomastigote, epimastigote, and amastigote stages; African trypanosomiasis is transmitted by the tsetse fly, while American trypanosomiasis is transmitted by triatomine bugs (contamination).
Ciliates. Balantidium coli is the only human parasite among ciliates, residing in the large intestine; pigs are the source of infection.
Apicomplexans. The malaria parasite undergoes tissue and erythrocytic schizogony in humans (intermediate host) and sporogony in the female Anopheles mosquito (definitive host). In Toxoplasma gondii, felids are the definitive hosts; tachyzoites and bradyzoites in pseudocysts are formed in the intermediate host.
Flatworms (Platyhelminthes)
Class Trematoda (Flukes). Liver, lancet, lung, cat, and blood flukes (schistosomes). They possess a blind-ending gut, a protonephridial excretory system, and typically a hermaphroditic reproductive system. The life cycle includes: egg → miracidium → sporocyst → redia → cercaria → metacercaria → marita. The first intermediate host is a freshwater mollusk, and the second is usually a fish.
Class Cestoda (Tapeworms). Pork, beef, dwarf tapeworms, broad and dog tapeworms, Echinococcus, Alveococcus. The digestive system is absent; nutrient absorption occurs across the entire body surface. Life cycle: egg → oncosphere → cysticercus (bladder worm) → adult worm. Differences between broad tapeworms (diphyllobothriids) and true tapeworms (taeniids):
- Attachment organs: broad tapeworms have bothria (sucking grooves), whereas taeniids have muscular suckers.
- Uterus: broad tapeworms have an open-type uterus, while taeniids have a closed uterus (eggs are shed with proglottids).
- Proglottid shape: broad tapeworms have rectangular segments (broader than long), while taeniids have square segments.
- The life cycle of broad tapeworms is obligatorily linked to an aquatic environment, whereas taeniids are not.
Roundworms (Nematodes) and Associated Diseases
Phylum Nematoda (Ascaris, pinworm, guinea worm, Onchocerca, whipworm) is characterized by a primary body cavity filled with pressurized fluid. The digestive tract is complete (mouth to anus), the nervous system features a circumoral nerve ring, and the excretory system is protonephridial. Nematodes are dioecious with pronounced sexual dimorphism; spermatozoa lack flagella and move via pseudopodia. The life cycle is simple: egg → larva (with migration through the bloodstream and lymphatics) → adult.
Any parasite exerts toxic and mechanical effects on the host. Prevention is non-specific: thermal processing of food and water, hand and surface hygiene, and adherence to sanitary standards. Diagnosis relies on serology (detection of antigens in blood) and microscopy (identifying pathogens and eggs in feces). Treatment utilizes metronidazole, sulfonamides, and anthelmintics, such as praziquantel and its analogs.
Parasitism also has positive aspects: continuous activation of the host's immune system, regulation of animal population sizes, acting as a driver of natural selection and evolutionary progress, and the participation of parasites in food webs as secondary and tertiary consumers.
Key points
- Parasitism is described by the synthesis of ecological, immunological, metabolic, and pathophysiological concepts.
- Parasites are classified by their relationship type, localization, duration of contact, and epidemiology; hosts are categorized as definitive, intermediate, accessory, and reservoir.
- There are nine pathways of parasite entry: ranging from alimentary and sexual to vector-borne via inoculation and contamination.
- Medically significant protozoa include Entamoeba histolytica, Giardia, Trichomonas, Leishmania, Trypanosoma, Balantidium coli, Plasmodium, and Toxoplasma.
- The life cycle of flukes includes miracidium, sporocyst, redia, cercaria, and metacercaria stages, involving a mollusk and a fish.
- Tapeworms lack a digestive system; key developmental stages are the oncosphere and the metacestode (finna).
- Roundworms are dioecious, possess a primary body cavity, and have a complete gut.
- Diagnosis of parasitic infections relies on serology and microscopy, while treatment is based on metronidazole and praziquantel.