Many protozoa live in the human body. Many of them are pathogenic. Our story is about ten of them, the largest. The review is based on both historical and recent publications.
The 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 malarial plasmodium in the largest stage is about 15 microns, and several times shorter than the balantidium of intestinal cells, among which ciliates live. An elephant in a china shop.
Distributedwherever there are pigs - its main vectors. It usually lives in the submucosa of the colon, although in humans it also occurs in the lung epithelium. It feeds on bacteriaB. coli, food particles, fragments of the host's epithelium. In animals, the infection is asymptomatic. People can develop severe diarrhea with a bloody, slimy discharge (balantidiasis), sometimes ulcers form in the colon walls. It is rare to die of balantidosis, but it causes chronic exhaustion.
People get infected by dirty water or food containing cysts. The infection rate in humans does not exceed 1%, while pigs can be infected all over the world.
Treatedwith antibiotics, no drug resistance reports have yet been reported for this ciliate.
Discoveredby the Swedish scientist Malstem in 1857. Today balantidosis is associated with tropical and subtropical areas, poverty and poor sanitation.
The 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 any person with sore 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 it, 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 20th 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 drugsaffecting this amoeba are unknown.
The most pervasive. Ameba dysenteryEntamoeba histolytica
This intestinal parasite with blood penetrates the tissues of the liver, lungs, kidneys, brain, heart, spleen, genitals. Eat what it gets: food particles, bacteria, red blood cells, leukocytes, and skin 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. histolyticaattacks the walls. The reasons for the transition to the pathogenic form are not yet clear, but several molecular mechanisms of what is happening have already been described. Thus, it is clear that amoebas secrete lysing substances, break through mucus and kill cells. Apparently, the amoeba can destroy the host cell in two ways: by triggering apoptosis in it or by simply chewing on pieces. The first method was considered the only one for a long time. By the way, the mechanism of cellular suicide with record speed - within minutes - has not been identified. The second method was described quite recently, the authors called it trogocytosis from the Greek "three" - gnaw. Interestingly, the amoebas that bite cells abandon their prey as soon as it dies. Others can completely engulf dead cells. Biting and devouring cells are assumed to differ in the pattern of gene expression.
Now the amoeba's ability to enter the bloodstream, liver and other organs is associated with troocytosis.
Amoebiasis is a fatal disease, with around 100, 000 people dying each year fromE. histolyticainfection.
The dysentery amoeba has a non-pathogenic twin,E. dispar, so microscopy is not sufficient to diagnose the disease.
To curemust be destroyed as cellE. histolyticaand cyst.
He describedE. histolyticaand determined its pathogenic nature in 1875 in a patient with diarrhea. The Latin name for the amoeba was given in 1903 by the German zoologist Fritz Schaudin.Histolyticameans destructive to the tissues. In 1906, the scientist died of an amoebic intestinal abscess.
The most common. Intestinal lambliaGiardia lamblia (G. intestinalis)
Giardia, the most common intestinal parasite, is omnipresent. 3-7% of people in developed countries and 20-30% in developing countries are infected. This is about 300 million people.
Parasites livein the duodenum and bile ducts of the host, where they float, working with flagella, then attach themselves to the epithelium with the help of a sticky disc located on the undersidecell. For 1 cm2, the epithelium adheres to one million lamblias. They damage the villi, which interfere with the absorption of nutrients, causing inflammation of the mucosa 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 get infected, it is enough to swallow a dozen cysts, which in the intestine will again turn into an active form.
The main secretof the ubiquity of lamblia in the variability of surface proteins. The human body fights lamblia with antibodies and, in principle, is able to develop immunity. But people who live in the same area and drink the same water are continually infected with the descendants of their own parasites. Because? 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 ever present on the surface of a single parasite; the translation of the remainder is interrupted by the RNA interference mechanism. And the change happens about once every ten generations.
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. The cysts are fought with iodized water.
DiscoveryGiardia lambliain 1859 by the Czech scientist Vilém Lambl. Since then, the simplest has changed several names, and the current one was received in honor of the French discoverer and parasitologist Alfred Giar, who did not describe lamblia.
And the first sketch of Giardia was done by Anthony van Leeuwenhoek, who found him in his distraught chair. It was 1681.
By the way, Giardia is also very evolutionarily ancient, deriving almost directly from the ancestor of all eukaryotes.
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 wipes. Babies can become infected by passing through the birth canal.T. vaginalishas 4 flagella at the anterior end and a relatively short wavy membrane; if necessary it releases pseudopodia. The maximum size of Trichomonas is 32 x 12 microns.
Trichomonas is morecommonthan the causative agents of combined chlamydia, gonorrhea and syphilis. 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 humans.
T. vaginalisfeeds on microorganisms, including lactic bacteria of the vaginal microflora, which maintain an acidic environment and thus create an optimal pH for itself above 4. 9.
Trichomonas destroys mucosal cells, causing inflammation. About 15% of infected women complain of symptoms.
Is treatedwith an antibacterial drug. As a preventive measure, it is recommended to wash regularly with diluted vinegar.
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.
The most lethal. The causative agent of sleeping sicknessTrypanosoma brucei
The causative agent of African sleeping sickness is the deadliest protozoan. An infected person dies without treatment. The trypanosome is an elongated flagellate 15–40 µm long. There are two subspecies which are externally 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 fifteenth parallels of the southern and northern hemispheres, in the natural radius 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 countries south of the Sahara at 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 intestine 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, wasting and irreversible coma.
The lethality of the parasite is associated with its ability to cross the blood-brain barrier. The molecular mechanisms are not entirely clear, 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 repairs itself from immune factors.
The first description of sleeping sickness in the upper part of Niger was left 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 (Winterbottom symptom), and slave traders paid special attention to it.
DiscoveryT. bruceiThe Scottish microbiologist David Bruce, from whom it takes its name, and in 1903 established for the first time the connection between trypanosome, tsetse flyand 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 make a vaccine.
The most extravagant. LeishmaniaLeishmania donovani
Leishmania 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 or kala azar fever, from which almost all patients die without treatment. But the survivors gain 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. AmericanL. donovani chagasi(Central and South America) can live in the blood of dogs.
L. donovani- flagellates not exceeding 6 microns in length. People become infected after being bitten by mosquitoes likePhlebotomus, sometimes through sexual contact, babies - they pass through the birth canal. Once in the blood,L. donovanipenetrates macrophages, which carry the parasite through internal organs. By reproducing 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 fall ill with visceral leishmaniasis and about 40, 000 die.
Heavy treatment- intravenous administration of antimony preparations and blood transfusions.
Taxonomic affiliationL. donovaniwas defined 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 dead kala azar patients, 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 sex mites in the intestine of the genusIxodes, have complicated their development by transovarial transmission. From the intestine of a female tick, the sporozoites of the protozoa penetrate the ovaries and infect the embryos. When the mite larvae hatch, babesia passes into their salivary glands and, at the first bite, enters the vertebrate blood.
DistributedBabesia in America, Europe and Asia. Their natural reservoir are rodents, dogs and cattle. A person is infected with several types: B. microti, B. divergens, B. duncaniandB. venatorum.
The symptoms of babesiosis are similar to malaria: relapsing fever, haemolytic 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 and, in severe cases, blood transfusions are prescribed.
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 which he namedHaematococcus bovis. Babesia was long considered an animal pathogen until it was discovered in 1957 in a Yugoslavian 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 blood, sexual reproduction is possible only in the epithelial cells of the cat's intestine. ForT. gondiicould complete development, the cat has to eat an infected rodent. By increasing the likelihood of this occurrence,T. gondiiblocks the rodents' natural fear of cat urine odor and makes it attractive by targeting a group of neurons in the amygdala. How it does this is unknown. One of the purported mechanisms of action is a local immune response to infection. It alters the levels of cytokines, which in turn increases the levels of neuromodulators such as dopamine. Toxoplasma also affects human behavior, which also manifests itself at the population level. Thus, in countries with a high level of toxoplasmosis, neuroticism and a desire to avoid uncertainty, new situations are more common. It is possible thatT. gondiiinfection could lead to cultural changes.
Infectionin humans is often asymptomatic, but with weakened immunity, it destroys cells in 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 - older people are less susceptible toT. gondii.
Treatstoxoplasmosis with antiprotozoal drugs.
Describedin 1908 in desert rodents. This honor goes to the staff of the Pasteur Institute in Tunisia Charles Nicolas and Luis Manso.
More pathogenic. Plasmodium malariaPlasmodium spp.
Plasmodium malaria is the most pathogenic parasite in humans. The number of malaria patients can reach 300-500 million, and the death rate during epidemics - 2 million. The disease still causes 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 affectsthe macaques.
Distributedin the range of vectors - mosquitoesAnopheles, which require a temperature of 16–34 ° C and relative humidity above 60%.
Comparison of the more virulent plasmodia genome,P. falciparum, with the gorilla plasmodia 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.
The 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 the patient's blood contains 1 - 50 thousand parasites.
The disease manifests itself as inflammation, periodic fever and anemia, in case of pregnancy it is dangerous for the mother and the fetus. Red blood cells 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.