Ten simplest human parasites

Many protozoa live in the human body. Many of them are pathogenic. Our history is about ten of them, at most. The review is composed of 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 Plasmodium falciparum in the largest stage is about 15 microns and several times smaller than the balantidia of the intestinal cells in which the ciliate lives. Elephant in a china shop.

Distributedwherever there are pigs - its main carriers. It usually lives in the submucosal layer of the colon, although in humans it is also found in the lung epithelium. It feeds onB. colibacteria, food particles, fragments of the epithelium of the host. In animals, the infection is asymptomatic. People can develop severe diarrhea with bloody, mucous secretions (balantidiasis), sometimes ulcers form in the walls of the colon. It rarely dies from balantidiasis, but causes chronic exhaustion.

People become infected through dirty water or food containing cysts. The rate of infection in humans does not exceed 1%, while pigs can be infected worldwide.

Treatedwith antibiotics, no drug resistance has been reported for this lash.

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

Oral amoeba

The first. Oral amoebaEntamoeba gingivalis

The first parasitic amoeba found in humans. The description of amoebae 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 toothache or gum pain, inhabits gum pockets and plaque. It sometimes feeds on epithelial cells, leukocytes, microbes and erythrocytes. Rare in people with a healthy mouth.

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

In the early twentieth centuryE. gingivaliswas described as a cause of periodontitis, as it is always present in inflamed tooth cells. However, its pathogenicity has not been proven.

The drugsthat affect this amoeba are unknown.

Dysenteric amoeba

The most common. Dysenteric amoebaEntamoeba histolytica

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

Widespreadeverywhere, 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. histolyticaattacks 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 amoebae secrete lysing substances, pierce mucus and kill cells. Apparently, the amoeba can destroy the host cell in two ways: by triggering apoptosis in it or simply by chewing pieces. The first method has long been considered the only one. Incidentally, the mechanism of cellular suicide at a record rate - in minutes - has not been established. The second method has been described recently, the authors call it trogocytosis from the Greek "three" - to bite. Remarkably, biting cell amoebae abandon their prey as soon as it dies. Others can phagocytose completely dead cells. It is assumed that the biting and ingesting cells differ in the pattern of gene expression.

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

Amebiasis is a deadly disease, with about 100, 000 people dying from E. histolytica infection each year.

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

To be curedmust be destroyed as mobileE. histolyticaand cysts.

DescribedE. histolyticaand determined its pathogenic nature in 1875 in a patient with diarrhea. The Latin name of the amoeba was given in 1903 by the German zoologist Fritz Schaudin.Histolyticameans destructive tissue. In 1906, the scientist died of 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's about 300 million people.

Parasites livein the duodenum and bile ducts of the host, where they are carried by working with flagella, then attached to the epithelium using a sticky disc located on the underside of the cell. At 1 cm2the epithelium adheres to a million lamblia. They damage the lint, which interferes with the absorption of nutrients, causing inflammation of the mucous membranes 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 the simplest 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 intestine will again become active.

The main secretof the ubiquity of lamblia in the variability of surface proteins. The human body fights against lamblia with antibodies and is generally able to develop immunity. But people living in the same area and drinking the same water become infected again and again by the offspring of their own parasites. Why? Because during the transition from the active phase to the cyst and vice versa, the 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 a single parasite; translation of the remainder is interrupted by the RNA interference mechanism. And change happens about once every ten generations.

Treatedwith an antiprotozoal agent with antibacterial activity. The disease passes after a week, but if the bile ducts are infected, relapses are possible for many years. Cysts fight water iodization.

DiscoveredGiardia lambliain 1859 by the Czech scientist Willem Lambl. Since then, the simplest has changed several names and the current one was named in honor of the discoverer and French parasitologist Alfred Giard, who does not describe giardia.

And Giardia's first sketch 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, coming almost directly from the ancestor of all eukaryotes.

Vaginal Trichomonas

The most intimate. Trichomonas vaginalisTrichomonas vaginalis.

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

Trichomonas is more commonthan the causes of chlamydia, gonorrhea and syphilis combined. It affects about 10% of women, and probably more, and 1% of men. The latter figure is unreliable, as 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 themselves above 4, 9.

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

Treatedwith an antibacterial drug. As a preventive measure, regular pouring of diluted vinegar is recommended.

Describedin 1836 by the French bacteriologist Alfred Don. 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 - the cause of sleeping sickness

The deadliest. 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. The trypanosome is an elongated scourge 15-40 µm long. There are two subspecies that do not look different. 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 vector - blood-sucking insects of the genusGlossina(tsetse fly). Of the 31 species of flies, 11 are dangerous to humans. Sleeping sickness affects the population of 37 sub-Saharan Africa 9 million km2. Up to 20, 000 people get sick every year. There are now about 500, 000 patients, 60 million living at risk.

From the intestines of the flyT. 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 lethality of the parasite is related to 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, trypanosomes are hidden by 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). In the early 19th century, Europeans were already well aware of the initial sign of the disease - swelling of the lymph nodes in the back of the head (a symptom of Winterbottom) and slave traders paid special attention to it.

DiscoveredT. bruceiScottish microbiologist David Bruce, after whom it was named, and in 1903 he first established the link between trypanosome, tsetse fly and sleepwalkerdisease.

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


The most extravagant. LeishmaniaLeishmania donovani

Leishmanias have won the title of the most extravagant parasites because they live and multiply in macrophages - cells designed to kill parasites.L. donovaniis ​​the most dangerous of them. It causes visceral leishmaniasis, colloquial dummy fever or kala azar, from which almost all patients die without treatment. But 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 the elderly and elderly, there are no natural reservoirs. The AmericanL. donovani chagasi(Central and South America) can live in the blood of dogs.

L. donovani- a whip with a length of not more than 6 microns. 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. donovanienters macrophages, which carry the parasite through the internal organs. By multiplying in macrophages, the parasite destroys them. The molecular mechanism of survival in macrophages is quite complex.

Symptoms of the disease- fever, enlarged liver and spleen, anemia and leukopenia, which contribute to secondary bacterial infection. Every year, 500, 000 people get visceral leishmaniasis and about 40, 000 die.

Treatmentsevere - intravenous antimony preparations and blood transfusion.

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


The most difficult life cycle. Babesia spp.

Babesia, in addition to the multistage asexual multiplication in mammalian erythrocytes and sex mites in the gut of the genusIxodes, complicate its development by transovarian transmission. From the intestines of female mites, protozoan sporozoites enter the ovaries and infect embryos. When the larvae of the mites hatch, the babesia passes into their salivary glands and with the first bite enters the blood of the vertebrates.

WidespreadBabesia in America, Europe and Asia. Their natural reservoirs are rodents, dogs and cattle. Man is infected with several species: B. microti, B. divergens, B. duncaniandB. venatorum.

The symptoms of babesiosis resemble malaria - recurrent fever, hemolytic anemia, enlarged spleen and liver. Most people recover spontaneously, but babesiosis is fatal for patients with a weakened immune system.

Treatmentsare still being developed while antibiotics and, in severe cases, blood transfusions are being prescribed.

Babesia was described by the Romanian microbiologist Victor Babes (1888), who discovered it in sick cows and sheep. He decided he was dealing with a pathogenic bacterium he namedHaematococcus bovis. Babesia was long considered an animal pathogen until it was found 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 UK - 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 intestines. ToT. gondiican complete its development, the cat must eat an infected rodent. By increasing the likelihood of this event,T. gondiiblocks rodents' natural fear of smelling cat urine and makes it attractive by directing it to a group of neurons in the amygdala. How he does it is unknown. One of the putative mechanisms of action is a local immune response to infection. It alters cytokine levels, which in turn increases levels of neuromodulators such as dopamine. Toxoplasma also affects human behavior, which occurs even at the population level. So in countries with high levels of toxoplasmosis, neuroticism and a desire to avoid insecurity, new situations are more common. Infection withT. gondiimay lead to cultural changes.

Infectionin humans is often asymptomatic, but when the immune system is weakened, it destroys the cells of the liver, lungs, brain, retina, causing acute or chronic toxoplasmosis. The course of the infection depends on the virulence of the strain, the state of the host's immune system and its age - the elderly 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 Nicholas and Luis Manso.

Plasmodium falciparum

Most pathogenic. Plasmodium malariaPlasmodium spp.

Plasmodium malaria is the most pathogenic parasite in humans. The number of patients with malaria can reach 300-500 million, and mortality during epidemics - 2 million. The disease still takes three times as many lives as armed conflict.

Five species 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 relative humidity above 60%.

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

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

The disease manifests itself as inflammation, recurrent fever and anemia, in case of pregnancy is dangerous for the mother and fetus. P. erythrocytes infected with P. falciparum clog 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 drug resistant.