Ten simplest human parasites

Many protozoa live in the human body. Many of them are pathogenic. Our story is about ten of them, the most-most. The review is based on both historical and recent publications.

Balantidium is the largest protozoan parasite

Largest. BalantidiumBalantidium coli

The largest protozoan is a human parasite, and the only ciliate in this company. Its dimensions vary from 30 to 150 microns in length and from 25 to 120 microns in width. For comparison: the length of the malarial plasmodium in the largest stage is about 15 microns, and several times less than the balantidium of the intestinal cells, among which the infusoria lives. An elephant in a china shop.

Distributedwherever there are pigs - its main carriers. Usually lives in the submucosa of the colon, although in humans it also occurs in the pulmonary epithelium. It feeds onB. colibacteria, food particles, fragments of the host's epithelium. In animals, the infection is asymptomatic. People can develop severe diarrhea with bloody, slimy discharge (balantidiasis), sometimes ulcers form in the walls of the colon. It is rare to die of balantidiasis, but it causes chronic exhaustion.

People are infected through dirty water or foods containing cysts. The infection rate in humans does not exceed 1%, while pigs can be infected all over the world.

Treatedwith antibiotics, no reports of drug resistance of this ciliate have been reported yet.

Discoveredby the Swedish scientist Malstem in 1857. Today, balantidiasis is associated with tropical and subtropical areas, poverty and poor hygiene.

Oral amoeba

The very first. Oral amoebaEntamoeba gingivalis

The first parasitic amoeba found in humans. The description of amoebas was published in 1849 in the oldest scientific journal. Found amoeba in dental plaque, hence the name from the Latin gingivae - gums.

Livesin the mouth of almost all people with aching teeth or sore gums, inhabits gum pockets and plaque. It feeds on epithelial cells, leukocytes, microbes, and in case of erythrocytes. It is rare in people with a healthy oral cavity.

This small protozoan, 10–35 µm in size, does not come out into the environment and does not form cysts; it is transmitted to another host by kissing, through dirty dishes or contaminated food. E. gingivalisis ​​considered an exclusively human parasite, but is sometimes found in captive cats, dogs, horses and monkeys.

In the early twentieth century,E. gingivaliswas described as the causative agent of periodontal disease, as it is always present in inflamed dental cells. However, its pathogenicity has not been proven.

The drugsthat affect this amoeba are unknown.

Dysentery amoeba

The most pervasive. Dysentery amoebaEntamoeba histolytica

This intestinal parasite with blood penetrates the tissues of the liver, lungs, kidneys, brain, heart, spleen, genitals. Eats what will get: food particles, bacteria, red blood cells, leukocytes and epithelial cells.

Distributedeverywhere, especially in the tropics. Usually, people become infected by swallowing a cyst.

In temperate countries, the amoeba tends to remain in the intestinal lumen and the infection is asymptomatic. In the tropics and subtropics, the pathological process often begins:E. histolyticaattack the walls. The reasons for the transition to the pathogenic form are still unclear, but several molecular mechanisms of what is happening have already been described. So, it is clear that amoebas secrete lysing substances, break through the mucus and kill cells. Apparently, the amoeba can destroy the host cell in two ways: by triggering apoptosis in it or simply by chewing off pieces. The first method was considered the only one for a long time. By the way, the mechanism of cellular suicide with a record speed - in minutes - has not been identified. The second method was described quite recently, the authors called it trogocytosis from the Greek "three" - to gnaw. It is noteworthy that the cell-biting amoebas abandon their prey as soon as it dies. Others can phagocytose dead cells entirely. It is assumed that biting and devouring cells differ in the pattern of gene expression.

Now the ability of amoeba to penetrate into the bloodstream, liver and other organs is associated with trohocytosis.

Amoebiasis is a deadly disease, about 100 thousand people die from infection withE. histolyticaevery year.

The dysentery amoeba has a non-pathogenic twin,E. dispar, so microscopy is not enough to diagnose the disease.

To curemust be destroyed as mobileE. histolyticaand cysts.

DescribedE. histolyticaand determined its pathogenic nature in 1875 in a patient with diarrhea. The Latin name for amoeba was given in 1903 by the German zoologist Fritz Schaudin.Histolyticameans tissue destructive. In 1906, the scientist died from an amoebic intestinal abscess.

Intestinal lamblia

The most common. Intestinal lambliaGiardia lamblia (G. intestinalis)

Giardia, the most common intestinal parasite, is ubiquitous. 3-7% of people in developed countries and 20-30% in developing countries are infected. That is about 300 million people.

The parasites livein the duodenum and bile ducts of the host, where they either swim, working with flagella, or attach to the epithelium with the help of a sticky disk located on the underside of the cell. For 1 cm2, the epithelium adheres to a million lamblia. They damage the villi, which interferes with the absorption of nutrients, causing mucosal inflammation and diarrhea. If the disease affects the bile ducts, it is accompanied by jaundice.

Giardiasis is a disease of dirty hands, water and food. The life cycle of a protozoan is simple: in the intestine there is an active form, and at the exit with fecal masses, stable cysts. To become infected, it is enough to swallow a dozen cysts, which in the intestines will again turn into an active form.

The main secretof the ubiquity of lamblia in the variability of surface proteins. The human body fights against lamblia with antibodies and, in principle, is able to develop immunity. But people living in the same area and drinking the same water become infected again and again by the descendants of their own parasites. Why? Because during the transition from the active phase to the cyst and vice versa, lamblia changes the proteins to which antibodies are produced - variant-specific surface proteins. There are about 190 variants of these proteins in the genome, but only one is always present on the surface of an individual parasite; the translation of the rest is interrupted by the RNA interference mechanism. And the change happens about once every ten generations.

It is treatedwith an antiprotozoal agent with antibacterial activity. The disease goes away in a week, but if the bile ducts are infected, relapses are possible for many years. Cysts are fought by iodizing water.

DiscoveredGiardia lambliain 1859 by Czech scientist Vilém Lambl. Since then, the simplest has changed several names and the current one has received in honor of the discoverer and French parasitologist Alfred Giar, who did not describe lamblia.

And the first sketch of Giardia was made by Anthony van Leeuwenhoek, who found it in his own upset chair. It was in 1681.

By the way, Giardia is also very evolutionarily ancient, it comes almost directly from the ancestor of all eukaryotes.

Vaginal Trichomonas

The most intimate. Trichomonas vaginalisTrichomonas vaginalis.

The simplest, which is sexually transmitted. It lives in the vagina, and in men - in the urethra, epididymis and prostate gland, it is transmitted sexually or through wet washcloths. Babies can become infected by passing through the birth canal.T. vaginalishas 4 flagella at the anterior end and a relatively short undulating membrane; if necessary, it releases pseudopods. The maximum size of Trichomonas is 32 by 12 microns.

Trichomonas is morewidespreadthan the causative agents of chlamydia, gonorrhea and syphilis combined. It affects about 10% of women, and possibly more, and 1% of men. The latter figure is unreliable because it is more difficult to detect the parasite in men.

T. vaginalisfeeds on microorganisms, including lactic acid bacteria of the vaginal microflora, which maintain an acidic environment, and thus creates an optimal pH for itself above 4. 9.

Trichomonas destroys mucosal cells, causing inflammation. About 15% of infected women complain of symptoms.

It is treatedwith an antibacterial drug. As a preventive measure, regular douching with diluted vinegar is recommended.

Describedin 1836 by the French bacteriologist Alfred Donne. The scientist did not understand that there was a pathogenic parasite in front of him, but he determined the size, appearance and type of movement of the simplest.

Trypanosome - causative agent of sleeping sickness

The most deadly. The causative agent of sleeping sicknessTrypanosoma brucei

The causative agent of African sleeping sickness is the deadliest protozoan. A person infected with it dies without treatment. Trypanosoma is an elongated flagellate 15–40 µm long. There are two subspecies that are outwardly indistinguishable. Disease caused byT. brucei gambiense, lasts 2-4 years.T. brucei rhodesienseis ​​a more virulent, transient pathogen from which they die after a few months or weeks.

Distributedin Africa, between the 15th parallels of the Southern and Northern Hemispheres, in the natural range of the carrier - blood-sucking insects of the genusGlossina(tsetse fly). Of the 31 species of flies, 11 are dangerous for humans. Sleeping sickness affects the population of 37 countries south of the Sahara at 9 million km2. Up to 20 thousand people fall ill every year. Now there are about 500 thousand patients, 60 million live at risk.

From the fly intestineT. bruceienters the human bloodstream, from there it enters the cerebrospinal fluid and affects the nervous system. The disease begins with fever and inflammation of the lymph glands, followed by lethargy, drowsiness, muscle paralysis, exhaustion and irreversible coma.

The parasite's lethality is associated with its ability to cross the blood-brain barrier. The molecular mechanisms are not fully understood, but it is known that when it enters the brain, the parasite secretes cysteine ​​proteases and also uses some host proteins. In the central nervous system, on the other hand, the trypanosome takes shelter from immune factors.

The first description of sleeping sickness in the upper reaches of the Niger was made by the Arab scholar Ibn Khaldun (1332-1406). At the beginning of the 19th century, Europeans were already well aware of the initial sign of the disease - swelling of the lymph nodes on the back of the neck (a symptom of Winterbottom), and slave traders paid special attention to it.

DiscoveredT. bruceiScottish microbiologist David Bruce, after whom she is named, and in 1903 he first established the connection between trypanosome, tsetse fly and sleeping sickness.

Treatmentdepends on the stage of the disease and medications cause severe side effects. The parasite has a high antigenic variability, so it is impossible to create a vaccine.


The most extravagant. LeishmaniaLeishmania donovani

Leishmanias have earned the title of the most extravagant parasites, because they live and reproduce in macrophages - cells designed to destroy parasites.L. donovaniis ​​the most dangerous of them. It causes visceral leishmaniasis, colloquially dumdum fever, or kala azar, from which almost all patients die without treatment. But the survivors acquire long-term immunity.

There are three subspecies of the parasite.L. donovani infantum(Mediterranean and Central Asia) mainly affects children, dogs are often its reservoir.L. donovani donovani(India and Bangladesh) is dangerous for adults and the elderly, has no natural reservoirs. The AmericanL. donovani chagasi(Central and South America) can live in the blood of dogs.

L. donovani- flagellate no more than 6 microns in length. People become infected after being bitten by mosquitoes of the genusPhlebotomus, sometimes through sexual contact, babies - passing through the birth canal. Once in the blood,L. donovanipenetrate into the macrophages, which carry the parasite through the internal organs. Reproducing in macrophages, the parasite destroys them. The molecular mechanism of survival in macrophages is rather complex.

Symptoms of the disease- fever, enlarged liver and spleen, anemia and leukopenia, which contribute to a secondary bacterial infection. Every year 500 thousand people fall ill with visceral leishmaniasis and about 40 thousand die.

Treatmentheavy - intravenous administration of antimony preparations and blood transfusion.

Taxonomic affiliationL. donovaniwas determined in 1903 by the famous malaria researcher and Nobel laureate Ronald Ross. It owes its generic name to William Leishman, and the specific name to Charles Donovan, who in the same 1903 independently discovered protozoan cells in the spleen of patients who died from kala azar, one in London, the other in Madras.


The most difficult life cycle.Babesia spp.

Babesias, in addition to multistage asexual reproduction in mammalian erythrocytes and sexual mites in the intestines of the genusIxodes, complicated their development by transovarial transmission. From the intestines of a female mite, protozoan sporozoites penetrate the ovaries and infect embryos. When the mite larvae hatch, babesia pass into their salivary glands and, with the first bite, enter the vertebrate's blood.

DistributedBabesia in America, Europe and Asia. Their natural reservoir is rodents, dogs and cattle. A person is infected with several types: B. microti, B. divergens, B. duncaniandB. venatorum.

Symptoms of babesiosis are similar to malaria - recurrent fever, hemolytic anemia, enlarged spleen and liver. Most people recover spontaneously, but babesiosis is fatal for patients with weakened immune systems.

Treatment methodsare still under development, while antibiotics are prescribed and, in severe cases, blood transfusions.

Babesia was described by the Romanian microbiologist Victor Babes (1888), who discovered it in sick cows and sheep. He decided that he was dealing with a pathogenic bacterium he namedHaematococcus bovis. Babesia was long considered an animal pathogen until it was discovered in 1957 in a Yugoslav shepherd who died of B. divergens infection.


The most influential. The causative agent of toxoplasmosisToxoplasma gondii

T. gondiiis ​​the most powerful parasite as it controls the behavior of intermediate hosts.

Distributedeverywhere, unevenly distributed. In France, for example, 84% of the population is infected, in the United Kingdom - 22%.

The life cycle of Toxoplasma consists of two stages: asexual occurs in the body of any warm-blooded, sexual reproduction is possible only in the epithelial cells of the cat's intestine. ToT. gondiicould complete development, the cat must eat an infected rodent. By increasing the likelihood of this event,T. gondiiblocks the rodents' natural fear of cat urine smell and makes it attractive by targeting a group of neurons in the amygdala. How she does it is unknown. One of the supposed mechanisms of action is a local immune response to infection. It alters cytokine levels, which in turn raises levels of neuromodulators such as dopamine. Toxoplasma also affects human behavior, which is manifested even at the population level. So, in countries with a high level of toxoplasmosis, neuroticism and a desire to avoid uncertain, new situations are more common. It is possible that infection withT. gondiican lead to cultural changes.

Infectionin humans is often asymptomatic, but with weakened immunity, it destroys cells of the liver, lungs, brain, retina, causing acute or chronic toxoplasmosis. The course of infection depends on the virulence of the strain, the state of the host's immune system and its age - older people are less susceptible toT. gondii.

Treattoxoplasmosis with antiprotozoal drugs.

Describedin 1908 in desert rodents. This honor belongs to the staff of the Pasteur Institute in Tunisia Charles Nicolas and Luis Manso.

Malaria plasmodium

Most pathogenic. PlasmodiumPlasmodium spp.

Plasmodium malaria is the most pathogenic parasite in humans. The number of patients with malaria can reach 300-500 million, and the death rate during epidemics - 2 million. The disease still claims three times more lives than armed conflicts.

Five types of Plasmodium cause malaria in humans:Plasmodium vivax, P. falciparum, P. malariae, P. ovaleandP. knowlesi, which also affects macaques.

Distributedin the range of vectors - mosquitoesAnopheles, which need a temperature of 16–34 ° C and a relative humidity of more than 60%.

Comparison of the genome of the most virulent of the plasmodia,P. falciparum, with the gorilla plasmodia suggests that humans were infected by its ancestor from these monkeys. The emergence of this form of Plasmodium is associated with the emergence of agriculture in Africa, which led to an increase in population density and the development of irrigation systems.

Sexual reproduction of plasmodia occurs in the intestines of mosquitoes, and in the human body it is an intracellular parasite that lives and reproduces in hepatocytes and erythrocytes until the cells burst. 1 ml of patient's blood contains 1 - 50 thousand parasites.

The disease manifests itself as inflammation, recurrent fever and anemia, in case of pregnancy it is dangerous for the mother and fetus. Erythrocytes infected withP. falciparumclog the capillaries, and in severe cases ischemia of internal organs and tissues develops.

Treatmentrequires a combination of several drugs and depends on the specific pathogen. Plasmodia become resistant to drugs.