What are human parasitic protozoa?

In the world of parasites dangerous to humans, the second place in frequency of occurrence after helminths is occupied by the simplest unicellular organisms. They do not belong to either plants or animals, they exist only at the expense of their owners. The simplest parasite of a person, fills his body, tests the strength of immunity and always inevitably causes disease (protozoonosis), which often has a severe course and leaves serious consequences for health. The extent of unicellular "unlawful" influence on the human body depends on its type. All of them are the object of medical parasitology, more precisely, protozoology.

The simplest unicellular organism

General information

There are about 1500 species of protozoa, most of which are parasites. This is an impressive figure compared to the three hundred known worms.

During long centuries of evolution, protozoa, such as worms, have developed their own survival system in the human body. When unfavorable conditions arise, they discard the vehicle and turn into cysts - round cells with a dense outer skin that protects against immunity and antibiotics.

The reverse transformation to the active form occurs after the cessation of drug action or when the body's immune defenses weaken. Adult parasites and cysts can live deep in tissues, where there is no access to cellular immunity mechanisms and drugs. They are also able to live in the blood, often parasitizing in the internal organs of the human body and its cavities. Distinctive features of protozoan parasites from multicellular organisms:

  • a cell that functions as a single organism due to a set of organelles that perform all the necessary functions;
  • microscopic dimensions;
  • a complex life cycle, consisting of different stages in form and function;
  • individual stages of the parasite cycle can occur in different host bodies or in the same organism, but in different tissues;
  • the ability to move with the help of special devices (flagella, pseudopodia or growths in the form of legs);
  • the possibility of transformation into a cyst;
  • Different types of amoeba
  • speed of reproduction - for most protozoan parasites, simple longitudinal cell division in half is characteristic.

Human protozoan parasites use simple inorganic materials from the host's digestive tract to feed. They breathe oxygen dissolved in blood and other body fluids, or they are anaerobes (for example, Giardia).

Effects on the human body

Not all unicellular organisms that exist in the world can cause harm to humans. Which of the protozoa is not a parasite for humans? For example, all amoebas (except the causative agents of dysentery), some types of Trichomonas (except Trichomonas vaginalis). True parasites among protozoa in host organisms act quite aggressively. From their point of introduction, they penetrate into the bloodstream and are sent to various tissues and organs, where they settle and reproduce in the protoplasm of cells (for example, Toxoplasma reproduces in phagocytes). As a result, the affected cell swells, the nucleus inside is pushed to the periphery, and the organelle dies. In the place of the former cell, a pseudocyst is formed - this is a formation with parasites inside. Then such a pseudocyst is destroyed, the parasites are in the blood, the cycle repeats, and they can move from one organ to another. As a result, inflammation develops in the affected tissue, connective tissue grows (cirrhosis, fibrosis). Often there is degeneration of cells damaged by parasites with the replacement of genetic material, which leads to the appearance of benign neoplasms (polyps, cysts), which over time can turn into malignant tumors.


All human parasitic protozoa, in addition to common characteristics, have differences related to the structure, the way they enter the body and the diseases they cause. Features of the main types of unicellular parasites for better perception are presented as a table:

A class of protozoa

Parasite and disease names

Route of infection, vector

Clinical manifestation

Invasive stage of the parasite



cutaneous and visceral leishmaniasis

Infection occurs through mosquito bites, the carrier can be humans and animals (dogs), the carrier is a mosquito of the genus Phlebotomus.

Cutaneous leishmaniasis is characterized by skin lesions with ulcer formation and focal necrosis. Visceral leishmaniasis is an acute inflammatory process in internal organs with the formation of ulcers and bleeding.

Promastigote (mobile form of the parasite)



Fecal-oral route of infection - dirty hands, contaminated food and water. Carriers - humans and warm-blooded animals; flies, cockroaches can be carriers

Inflammation of the small intestine according to the type of enteritis, allergic reactions (urticaria, asthma), nervous disorders (sadness, depression)

Giardia cysts



Only sexual transmission between people, newborn infection is possible - communication between mother and child during childbirth

Vulvovaginitis, cystitis in women. Asymptomatic men, there may be manifestations of urethritis. In a chronic course, complications are possible - oncology, infertility

Any stage of development


trypanosomiasis - African sleeping sickness, Chagas protozoonosis (American trypanosomiasis)

Insect bites or blood transfusions from sick people. Carriers can be humans and animals. Carrier - for sleeping sickness tsetse fly, for Chagas disease, triatomic bug

Damage to the lymph nodes, liver and spleen tissue, severe pathology of the nervous system, can be fatal

Motile trypanosomal form




Fecal-oral route of infection - dirty hands, raw milk and meat; in the womb from mother to child when a woman is infected during pregnancy. Source of infection - domestic cats

Asymptomatic course in most cases with a latent and chronic form of the disease. Acute course - in weak patients, newborns with intrauterine infection. Dangerous diseases of pregnant women - miscarriage or birth of children with defects of the nervous system and eyes


Plasmodium malaria 4 species,


Insect bites, blood transfusions, intrauterine infections in the fetus. The carrier of the infection is a human, the carrier is a mosquito of the genus Anopheles

Fever attacks with chills, possible bleeding, severe damage to the kidneys, nervous system, until death

sporozoite stage



diabetic dysentery (balantidiasis)

Fecal-oral route of infection - contaminated water, unwashed vegetables, undercooked pork. Carriers - pigs, less often dogs, vectors - two-winged insects

Inflammation of the mucous membrane in the large intestine, which is accompanied by the formation of ulcers, which is indicated by diarrhea with impurities of blood and mucus, death is possible.


Sarcode parasites

amoebic dysentery,


Fecal-oral route of infection - unwashed hands, contaminated food and water. The source of invasion is humans, the carriers are cockroaches and flies.

The intestinal form of amoebiasis is manifested by hemocolitis (diarrhea with blood) and vomiting. The form of extraintestinal amebiasis is characterized by acute pathology of the liver, heart, lung tissue and other organs. May be an asymptomatic carrier


Significance of the Problem

More often than others, Giardia, Toxoplasma, amoeba and Trichomonas are found in humans. The most dangerous is the symbiosis of protozoan parasites that act in pairs, for example, Toxoplasma and Trichomonas of several types. The third in this tandem is often chlamydia. The symbiosis of this parasite can cause the development of chronic pathologies of the respiratory system, cardiovascular system, organs of vision, teeth, etc.

It is relatively easy to contract diseases caused by protozoa, especially in endemic areas, as well as in the case of non-compliance with the rules of personal hygiene and food culture, but it is difficult to treat them due to the impossibility of using antibiotics and the low effectiveness of pharmacological agents.

The symptoms of protozoonosis differ at different stages of the life cycle of protozoa, their severity depends on the state of the human immune system, on the level of infection and localization of the parasite in the body. Diseases caused by protozoa often have non-specific manifestations, which complicates the process of diagnosis and complicates their course. Scientists have discovered that parasites can cause the development of human diseases such as:

  • mastopathy, uterine fibroids in women;
  • liver cirrhosis;
  • Parkinson's disease, ankylosing spondylitis;
  • encephalomyelitis, meningitis;
  • amyloid damage to the kidneys and joints;
  • osteochondrosis;
  • sexual disorders such as impotence, etc.

It has been established that many severe and even incurable diseases are based on the origin of parasites, so the simplest must be treated seriously and carefully.