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General parasitology

Last updated: August 10, 2021

Summarytoggle arrow icon

Parasites are symbiotic organisms that live on or in a larger organism (host) in order to feed, develop, and/or multiply, causing harm to the host in the process. Parasitism is distinguished from commensalism, in which the symbiont benefits from the relationship without harming or benefiting the host in return, and mutualism, in which the symbiont and the host equally benefit from the relationship. A definitive host is a host in whom a parasite reaches its adult form and/or undergoes a sexual phase of development; an intermediate host is a host in whom a parasite passes one or more phases of its asexual development. Intermediate hosts often function as vectors that carry the parasite from one host to the next, but parasites themselves can also function as vectors for viruses, bacteria, and other parasites. Parasites can be classified by size as microparasites (unicellular) or macroparasites (multicellular); by the class of organism as protozoans, helminths, and arthropods; by dependency on a host for survival as obligate (require a definitive host and may go through one or more intermediate hosts to complete their life cycle) or facultative (do not rely on a host to complete their life cycle but may adopt parasitic activity); and by their relation to the host as endoparasites, which “infect” the host and live inside their body (mostly protozoa and helminths), and ectoparasites, which “infest” the host and attach to, burrow into, or temporarily feed off a host's integumentary system (mostly arthropod macroparasites). Arthropod infestations are clinically significant in their own right, but play a far greater role in the transmission of other pathogens (e.g., Plasmodium by mosquitoes and Borrelia by ticks).

Symbiosis

  • Definition: a long-term interaction between dissimilar species living together in biological association
  • Depending on the degree of dependence and the advantages or disadvantages derived from the relationship, symbiosis can be classified as:
    • Parasitism
      • A relationship between a symbiont (parasite) and its host, in which the parasite benefits from the relationship and the host is harmed, possibly even causing death
      • Examples: tapeworms, head lice, Plasmodium spp.
    • Commensalism
      • A relationship between a symbiont (commensal) and its host, in which the commensal benefits from the relationship and the host derives neither benefit nor harm
      • Some organisms can be commensal under certain conditions (e.g., as intestinal or skin microbiota in a healthy individual) but become pathogenic if these conditions change (e.g., host immune deficiency, colonization of other sites, changes in temperature or humidity).
      • Example: E. Coli, which is part of the physiological intestinal flora, but can cause infections when it colonizes, for example, the urinary tract
    • Mutualism
      • A relationship between a symbiont and its host that benefits both parties
      • Like commensals, mutualism can become pathogenic under certain conditions.
      • Example: symbiosis between humans and intestinal Enterobacter agglomerans, in which the bacteria benefit from the nutrient supply and the human benefits from the menaquinones (vitamin K) that the bacteria produce

Classification of parasites [1][2]

Parasites can be categorized by location, dependence on host for survival, size, and type of organism.

  • Location
    • Ectoparasites: live and/or feed on the integumentary system of the host (e.g., ticks, mosquitoes)
    • Endoparasites: live inside the host (e.g., helminths)
  • Class of organism
  • Host dependence
    • Obligate parasites: cannot complete their life cycle without a host (e.g., Plasmodium, Toxoplasma)
    • Facultative parasites: do not require a host to complete their life cycle and adopt parasitic behavior only if necessary or opportunity presents itself (e.g., Naegleria fowleri)
  • Size
    • Macroparasite: multicellular organisms
    • Microparasite: unicellular organisms

Life cycle [1]

The life-cycle of obligate parasites can be divided into a parasitic stage, during which the organism lives in or feeds on the host, and a free-living stage, during which it lives outside the host and is not dependent on the host. Transmission generally occurs during the free-living stage.

  • Microparasites (protozoa):
    • The life cycles of microparasites vary greatly from species to species, but for most clinically relevant protozoans a simple division into two stages applies:
  • Helminths
  • Arthropods
    • Ovum
    • Larva
    • Nymph/pupa
    • Adult
    • Some arthropods adopt parasitic behavior only during specific phases of development (e.g., adult female mosquitoes, who require a blood meal to produce eggs, or ticks, who require a blood meal to transition from one stage of development to the next)

Diagnostic and infective stage [3]

  • For clinical purposes, the life cycle of endoparasites can be divided into two stages:
    • Infective stage: the stage during which the parasite assumes a form in which it can invade its host.
    • Diagnostic stage: the stage during which the parasite can be detected using the naked eye or laboratory methods. The diagnostic stage typically coincides with the stage during which the parasite leaves the host (via stool, urine, or sputum) to proliferate.
  • Protozoans are typically infective while encysted and noninfective while in trophozoite form, while helminths are typically infective in the oval or larval form and noninfective as adults. However, the diagnostic stage varies greatly between the species of both classes.

Hosts and vectors

Host

A host is an organism that harbors a smaller parasitic, commensal, or mutualistic organism. The life cycle of any obligate parasite involves at least one host. The parasite uses a host for nutrition, protection, and/or conditions for reproduction.

  • Reservoir host
  • Transport host: a host that mechanically transfers a parasite to another host (e.g., rodents that transport certain helminths to cats and dogs)
  • Dead-end host: a host that can be affected by a parasite but cannot act as a vector (e.g., humans for the West Nile virus)
  • Intermediate host: a host in which the parasite undergoes one or several asexual phases of its development (e.g., humans for Plasmodium)
  • Definitive host: a host in which a parasite undergoes development into its adult form and, in some cases, undergoes the sexual phase of its development (e.g., humans for Wuchereria bancrofti)

Vector

Vectors are organisms that can carry a pathogen and transmit it to another organism. They can be intermediate or transport hosts.

Pathological effects of parasites

Overview [4]
Characteristics Endoparasites Ectoparasites
Microparasites (protozoa) Macroparasites
Helminths Arthropods
Pathogens/vectors
  • Arachnids (i.e., ticks, mites)
  • Insects (e.g., fleas, lice, mosquitoes, tsetse flies)
Life cycle
  • Trophozoite (active, noninfective, cannot survive outside host)
  • Cysts (dormant, infective, proliferative, can survive outside host)
  • Ovum (typically infective)
  • Larvae (typically infective)
  • Adult (typically noninfective)
Blood changes

Definition

Protozoa are microscopic, single-celled nucleated organisms with complex life cycles, involving multiple stages and forms of development. Protozoa can cause a variety of gastrointestinal, visceral, hematological, neurological, genitourinary, and ocular diseases.

Classification

Clinically significant protozoa can be classified into four groups, depending on their means of movement.

  • Mastigophora (flagellates)
  • Sarcodina (amoeba)
    • Means of motion: pseudopodia formed by cell protoplasm
    • Examples: Entamoeba, Acanthamoeba, Naegleria
  • Ciliophora (ciliates)
    • Means of motion: cilia
    • Example: Balantidium
  • Sporozoa
    • Means of motion: gliding motility
    • Possess an apical complex that allows them to enter the host cells
    • Examples: Plasmodium, Toxoplasma, Babesia

Overview [1][5]

  • Population at risk
    • Travelers to the endemic regions
    • Individuals with close contact to persons with a recent history of living in endemic regions (e.g., migrant workers, recently immigrated individuals)
    • Individuals in crowded conditions
    • Individuals at potential risk of fecal-oral zoonotic transmission (e.g., via contaminated soil, sandboxes, or close contact with animals)
    • Individuals living in crowded conditions (e.g., residential institutions, barracks, correctional facilities, orphanages)
  • Transmission
    • Fecal-oral (waterborne, foodborne, hand-to-mouth)
    • Intestinal protozoa may also be transmitted sexually to the insertive partner during anal intercourse.
  • Diagnostics
    • Stool analysis
    • Immunoassay to detect parasite antigens
Overview of clinically significant intestinal protozoa
Feature Entamoeba histolytica Giardia lamblia Cryptosporidium parvum
Distribution
  • Subtropical and tropical regions
  • Common in resource-limited countries with poor sanitation (e.g., Central and South America, Africa, South Asia)
  • Subtropical and tropical regions
  • North-American mountain regions
  • Worldwide
Hosts
  • Humans and other mammals
Morphology
  • Has several asexual (merozoite, trophozoite, schizont) and sexual (microgametocyte, macrogametocyte) forms
  • Oocysts
Disease
Features of diarrhea
  • Loose stools with bright-red blood, pus, and mucus
  • Pain during defecation, tenesmus
  • Fatty, foul-smelling stools
Features in special populations
Treatment

All tissue and blood protozoa require a vector for transmission.

Sporozoa

Overview of clinically significant Sporozoa
Characteristics Plasmodium spp. Babesia spp. [6]
P. vivax P. ovale P. malariae P. falciparum
Distribution
  • Tropics and subtropics
    • West and Central Africa (e.g., Guinea, Ghana, Nigeria)
    • South and Southeast Asia (e.g., India, Indonesia)
    • Latin America (e.g., Brazil, Columbia)
  • Northeast, Northwest, and upper Midwest of the US
Factors affecting risk of infection and disease
  • Individuals with asplenia are at a higher risk of severe disease.
Vector (definitive host)
  • Female Anopheles mosquito
Intermediate hosts
  • Humans and other mammals
Morphology [1] Trophozoite
  • Resembles a diamond ring
  • Immature: a ring with a single chromatin dot (occupies ∼ ⅓ of an RBC)
  • Mature: ameboid ring
  • Immature: resembles immature P. vivax, but larger
  • Mature: ameboid ring
  • Immature: resembles immature P. vivax, but smaller (occupies ∼ 1/6 of an RBC)
  • Mature: band-like structure that crosses the RBC
  • Immature: a ring with one or two chromatin dots (headphone sign)
  • Mature: heavy ring, visible only in severe infection
Schizont/merozoite
  • Immature: contains multiple chromatin bodies
  • Mature: consists of 6–24 merozoites, which are round dark bodies that occupy most of the cell
  • More merozoites than in other species
  • Visible only in severe infection
Gametocytes
  • Crescent-shaped
  • N/A
  • Indistinguishable from other sexual stages via light microscopy
Persistence in liver
  • Yes
  • No
Disease
Clinical features
Fever spikes
  • Every 48 hours
  • Every 72 hours
  • Irregular
  • Irregular
  • Fever may persist without spikes
Diagnostics
Treatment
  • For more information about the treatment of different forms of malaria, including drug-resistant forms, see “Treatment” in “Malaria.”

Hemoflagellates [1][5][7]

All hemoflagellates have similar morphological stages.

  • Trypomastigote
    • Elongated S- or C-shape
    • Undulating membrane runs along one side of the body
    • Flagellum at the anterior end
  • Amastigote: round or oval with a peripherally located nucleus
  • Epimastigote: resembles trypomastigotes, but smaller and with undulating membrane only along half of one side of the body
  • Promastigote: resembles epimastigote without the undulating membrane
Overview of clinically significant Hemoflagellates
Characteristics Trypanosoma brucei Trypanosoma cruzi

Leishmania

Species and subspecies
  • N/A
  • L. mexicana
  • L. braziliensis
  • L. guyanensis
  • L. panamensis
Distribution
  • Central and South America
  • Africa
  • India
  • Southwest and Central Asia
  • Central America
  • Central and South America
Vector (definitive host)
  • Phlebotomine sandflies
Intermediate hosts
  • Humans, cattle, antelope
  • Humans and other mammals
Infective stage
Disease
Clinical features
Diagnostics
Treatment

Overview of protozoa affecting CNS and eyes
Feature Naegleria fowleri [8] Acanthamoeba spp. [9][10] Toxoplasma gondii
Distribution
  • Found in bodies of warm freshwater (e.g., lakes, rivers, hot springs)
  • Worldwide
  • Predominantly found in bodies of standing water (e.g., sea water, swimming pools)
  • Worldwide; preferably areas with hot, humid climate
Affected population
  • Typically children and young adults
  • History of swimming in hot springs
Transmission
  • Oral
  • Airborne
  • Water-borne
  • Fecal-oral
  • Vertical
Hosts
  • Final host: felines
  • Intermediate hosts: humans, birds, cattle
Morphology
Pathophysiology
  • Inhalation or entry via skin/mucosal lesions → hematological dissemination → CNS invasion
  • Ingestion of oocysts → release of sporozoites in the gastrointestinal tract → invasion of intestinal epithelium transformation into trophozoites tissue invasion
  • Can colonize the placenta, cross the placental barrier, and infect the fetus
Disease
Diagnostics
Treatment

Trichomonas vaginalis

Overview of antiprotozoal agents
Indications Mechanism of action [11][12] CNS penetration Adverse effects
Pentamidine
  • No
Suramin
Nifurtimox
  • Not fully understood; suggested to induce oxidative stress reactions
  • Yes
Melarsoprol
  • A prodrug that contains arsenic
  • Taken up by aminopurine transporters (e.g., P2) of T. brucei metabolism into melarsen oxide → lysis of T. brucei through an unknown mechanism
Eflornithine
Miltefosine
Paromomycin
  • No
Diloxanide
Iodoquinol

Sodium stibogluconate

  • No data

Introduction

Helminths, i.e., parasitic worms, are a group of macroparasites encompassing a variety of species that can infect their hosts in three different ways: ingestion of eggs or larvae (e.g., via contaminated food and water or fecal-oral route), direct penetration of the skin, and via the bite of vectors (e.g., certain species of flies and mosquitoes). Most helminth species colonize the gastrointestinal tract of their hosts, provoking symptoms such as abdominal pain, nausea, and diarrhea. The larvae of certain helminth species, such as those of the Ascaris and Ancylostoma genus, migrate from the intestines via the portal vein to the lungs, potentially causing asthma-like symptoms (e.g., dry cough, wheezing). Other species, such as Taenia solium, are capable of colonizing other human tissue, such as the brain or the liver, which can lead to life-threatening complications (e.g., neurocysticercosis). Diagnosis of helminth infection is made primarily via evidence of eosinophilia in the blood and direct detection of worms, eggs, or larvae in stool samples. Serum IgE levels are often elevated. Treatment consists of anthelmintic agents, such as albendazole or praziquantel. For helminth infection prevention, see “Food and water safety” in the article on food poisoning.

Classification

Trematodes (flukes) are small, flat, oval worms with two suckers (one located at the mouth and the other ventrally) and a blind-ending gut. Most species are hermaphroditic, but some also form separate male and female adults.

Overview of trematode infections
Disease Pathogen Mode of transmission Clinical features Diagnosis Treatment
Schistosomiasis
  • Penetration of the skin by a larvae
  • Freshwater snails are intermediate hosts
Clonorchiasis
  • Consumption of raw/undercooked freshwater fish

Paragonimisias

  • Consumption of raw/undercooked seafood
  • Utilization of contaminated cooking utensils
Fascioliasis
  • Consumption of contaminated freshwater plants
  • Triclabendazole
  • ERCP in case of biliary obstruction

Cestodes (tapeworms) are long, flat, ribbon-like worms composed of numerous segments and a single scolex at the head with which they anchor themselves to the intestine. Since they do not have a digestive tract, all nutrients are absorbed through the tegument. Cestodes are hermaphroditic (they contain both male and female organs).

Overview of cestode infections
Disease Pathogen Mode of transmission Clinical features Diagnosis Treatment
Taeniasis

Intestinal taeniasis

  • Consumption of undercooked beef or pork
  • Mostly asymptomatic
  • GI symptoms
    • Abdominal pain
    • Nausea, vomiting
    • Weight loss
Cysticercosis
  • Fecal-oral
Diphyllobothriasis
  • Consumption of raw or undercooked freshwater fish
Echinococcosis
  • Fecal-oral
  • Hoofed animals (e.g., sheep) are intermediate hosts
Hymenolepisis
  • Ingestion of eggs or cysticercoids
  • Eggs in stool

Nematodes (roundworms) are long, thin, unsegmented, tube-like worms with a longitudinal digestive tract opening at both ends. Adult worms form separate sexes, with the males usually being smaller than the females. Filarial Nematodes are thread-like nematodes. They are transmitted by arthropod vectors.

Nematodes (roundworms)
Disease Pathogen Mode of transmission Clinical features Diagnosis Treatment
Ascariasis
  • Fecal-oral
  • Pulmonary symptoms
  • GI symptoms
    • Abdominal discomfort
    • Nausea
    • Intestinal or bile duct obstruction
Enterobiasis (pinworm)
Trichuriasis
  • Eggs in stool
Toxocariasis
Trichinellosis
  • Consumption of undercooked meat (pork)
  • Fecal-oral (rare)
Hookworm (ancylostomiasis, necatoriasis)
  • Penetration of the skin by a larvae
Strongyloidiasis
Filariasis Loiasis
  • Bite of mango fly, deer fly, or horse fly
Onchocerciasis
  • Female blackfly bite
Lymphatic filariasis
  • Female mosquito bite

eating a Toxic TrEAT: Toxocara, Trichiniella, Enterobius, Ascaris, and Trichiuris are transmitted by ingestion.

SANd on your Shins, Ancles, and Neck: Strongyloides, Ancylostoma, and Necator penetrate the skin while walking on sand.

The OWL bites: Onchocera, Loa loa, and Wucheria are transmitted by bites.

Overview of antihelminthic agents
Drug Indications Mechanism of action Adverse effects

Bendazoles (e.g., albendazole, mebendazole) [17][18]

Pyrantel pamoate [19]
Praziquantel [20]
  • Causes vacuolization of the schistosome integument and increases cell membrane permeability to calcium, resulting in paralysis, dislodgement, and death
  • A single dose is sufficient for most helminths.
Ivermectin [21]
Diethylcarbamazine [22]

Most important antihelminthics (Pyrantel pamoate, Praziquantel, Ivermectin, Mebendazole, and Diethylcarbamazine:Pesky Parasites Inevitably Meet their Doom.

General characteristics

  • Overview
    • Arthropods are a group of ectoparasites that comprises arachnids and insects.
    • Feed on the blood, skin cells and oils, and/or organic debris of their hosts
    • Infestation is a disease in its own right, but the role of ectoparasites as vectors for other diseases is generally more clinically significant.
  • Morphological features
    • Common features include a chitin exoskeleton and segmented bodies with paired appendages (e.g., legs, wings, antennae)
    • Life cycle involves several stages and may involve molting and metamorphosis.
      • Most arthropods undergo a transformation from larva to an adult organism through several life stages characterized by distinct morphology.
        • No metamorphosis: egg → adult
        • Simple metamorphosis: egg → nymph adult
        • Complete metamorphosis: egg → larva → pupa/nymph adult

Ticks [1]

Ticks play a clinically significant role as vectors for pathogenic bacteria. The only clinically significant direct effect of tick bites is tick paralysis, a rare response to neurotoxins in the tick's saliva characterized by tingling and numbness throughout the body. See “Lyme disease” for a full discussion of tick-borne diseases.

Overview of clinically significant tick species
Species Distribution Associated pathogen Vector-borne diseases
Ixodes spp. I. scapularis
  • Eastern US
  • Southeastern Canada
I. pacificus
  • Pacific coast of the US
  • Western coast of North America
I. ricinus
  • Europe, northern Africa, and Russia
I. persculatus
  • Central and Eastern Asia
Dermacentor spp.
  • Cosmopolitan distribution
Ornithodoros spp.
  • Western US
  • Spain, Portugal
  • Parts of Ukraine and Turkey
Amblyomma americanum
  • Southeastern and Eastern US

Mites [1]

Overview of clinically significant mite species
Species Associated condition Pathophysiology
Parasitic Sarcoptes scabiei
  • Females burrow into the skin to lay eggs.
  • Larvae emerge after hatching and live on the skin, molting into a nymph stage before maturing into adult mites.
Nonparasitic Dermatophagoides farinae (house dust mite)
Demodex spp. [23]

Lice [1]

For more information on the various species of lice, see “Lice infestation.”

Fleas [1]

  • Examples
    • Xenopsylla cheopis
    • Tunga penetrans (jigger flea)
  • Life cycle
    • Usually involves 4 stages of development: egg → larva → pupa → adult
    • After adult fleas have found a host and have taken a blood meal, they mate and lay eggs
    • Hatching of eggs takes 1–10 day
    • Larvae feed on blood and flea feces (“flea dirt”) and will spin a cocoon within 5–20 days to enter the pupa stage
    • After several days/weeks, adult fleas are ready to emerge from the cocoon following the detection of a host (e.g., via movement or body heat)
  • Associated conditions

Flies [1]

Parasitic flies are clinically relevant mainly as vectors for pathogens, which they transmit during a blood meal.

  • Main types of parasitic flies
    • Tsetse flies (e.g., Glossina spp.)
    • Sandflies (e.g., Phlebotomus spp.)
    • Black flies (e.g., Simulium spp.)
    • Deer flies (e.g., Chrysops spp.)
    • Botflies (e.g., Dermatobia hominis)
  • Associated conditions
    • Flies serve as vectors for a range of pathogens (see the table below).
    • Myiasis: infestation of human tissue by fly larvae
      • Female fly lays eggs onto a hematophagous insect, which acts as intermediate host, or onto the host directly (e.g., into uncovered wounds). → The larvae hatch and penetrate the skin, digging tunnels into the host's subcutaneous tissue. → Once mature, the larvae drop from host to pupate.
      • D. hominis is the only species of fly that routinely parasitizes humans, causing myiasis.
      • However, other species of the family Oestridae (botflies) as well as the families Calliphoridae (blowflies) and Sarcophagidae (fleshflies) may infest humans if the opportunity presents itself (e.g., uncovered wounds).
      • Obligatory myiasis is most common in species found in Central and South America and sub-Saharan Africa, where humans are most commonly affected.
      • Treatment involves the removal of larvae and surgical debridement of the wound.
      • Complications: Bacterial infection, potentially leading to sepsis, is common.
Overview of clinically significant fly species

Type

Distribution

Associated pathogen

Disease
Tsetse fly
  • Africa
  • African tripanosomiasis
Sandfly
  • Asia
  • South America
  • Central America
  • Africa
  • Mediterranean
Black fly
  • Africa
  • Central and South America
Deer fly
  • Africa
Bot fly
  • Dermatobia hominis: southern Mexico to Uruguay
  • N/A

Heteroptera

Overview of ectoparasiticides
Drug Indications Mechanism of action Application

Adverse effects

Permethrin
  • Blocks the deactivation of Na+ channels → uncontrollable depolarization of neuronal membranes → seizures and ultimately paralysis and death of the parasite
  • Topical application (in severe/recalcitrant cases: PLUS ivermectin PO)
Malathion
Dimeticone
  • Penetrates the respiratory system via the spiracles and suffocates the louse.
  • Topical
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