Blood Flukes (Schistosomes): Life Cycle, Development, Types And Characteristics

2023 Author: Riley Dean | [email protected]. Last modified: 2023-11-27 09:15

Page content

• Types of blood flukes
• Structure and habitat
• Blood flukes: life cycle
• Infection routes
• Infection symptoms
• You can defeat parasites!

Blood flukes, of many species, are the causative agents of schistosomiasis, a group of diseases that ranks second after malaria in the number of deaths. Suffice it to say that up to 200 thousand people die from schistosomiasis in Africa every year.

Severe schistosomiasis requires surgery.

Vaccines against these parasites have not yet been invented, therefore preventive measures play a huge role.

Types of blood flukes

Blood flukes, or schistosomes, are a fluke class called trematodes. These parasitic flatworms are found primarily in the tropics and subtropics; to date, about 5 thousand of their varieties have been identified.

Blood flukes cause diseases that are combined into a group of infectious diseases - schistosomiasis.

Three types of Schistosoma are most commonly found in humans:

• intestinal;
• urogenital (genitourinary);
• Japanese, resembling intestinal, but much more active and contagious.

The localization of these species is different, and the name is due to the organ in which they parasitize, or the region where this species was found (Mekong, Malay, Guinean, etc.). According to this principle, blood and pulmonary flukes also got their names.

Blood flukes differ from other types of helminths by a striking feature: they are dioecious, and for the first six months they lead a separate existence, after which the females unite with males, and they unite in the literal sense of the word.

Although the cord-like body of females is longer (up to 20 mm) than the wider one in males (up to 10-15 mm), they are located in a special groove, the so-called gynecophoric canal, located on the male abdominal cavity; though not completely - their body parts are outside. Mating takes place there.

The male, with the help of a developed abdominal sucker, attaches to the walls of blood vessels and not only feeds on the host's blood himself, but also feeds his pair.

In such a “touching duet”, these worms live their entire life, which in the human body averages about 8 years, although there are cases when the life expectancy of blood flukes reached 30 and even 40 years.

Structure and habitat

The photo clearly shows how the female is located in the body of the male.

Blood flukes have a special structure, in addition to the gynecophore canal: their digestive system consists of the anterior section (mouth) and the esophagus with two branches passing into the middle section.

In this case, the anus is absent, that is, the digestive system ends blindly, and waste products are excreted through the bladder and excretory opening.

Blood flukes have well-developed muscles, but rather weak sense organs, although the larvae distinguish light and strive for it, on which one of the methods for diagnosing schistosomiasis is based. The type of exchange they have can be both oxygen and anoxic.

As already mentioned, blood flukes are widespread in regions with tropical and subtropical climates: Africa, Latin America and the Caribbean, the Middle East, China, and Southeast Asia.

Statistics show that blood fluke, the causative agent of schistosomiasis, is found in more than 200 million people in countries with a high risk of epidemics, and about 700 million more may be infected.

In Europe and North America, schistosomiasis is diagnosed primarily in people who have visited these regions.

Blood flukes: life cycle

The life cycle of blood flukes includes several stages. Localization in the human body of sexually mature individuals, connected in pairs, as described above, depends on the type of blood fluke: in intestinal schistosomiasis, these are the veins of the abdominal cavity, in urogenital, small veins of the small pelvis, etc.

Females lay daily more than 300 eggs with a sharp tip or thorns, with the help of which they penetrate from the blood vessels into the walls of the intestines, bladder and other organs. This can cause ulceration and inflammation.

In order for the eggs to develop further, they need to get into the water. They leave the human body along with feces, and in a fresh water body they are transformed into miracidia - the larvae of the first stage. Miracidia need an intermediate host, which becomes freshwater molluscs, mainly snails.

In sporocysts, redia develop, which are the second parasitic generation of blood flukes.

They rupture the sporocyst and begin to feed on the tissues of the mollusc. From redia, cercariae develop, leaving their body and again falling into a reservoir, where they are looking for their final owner. Cercariae are quite mobile due to a long tail with a swimming membrane.

The final owner for them is a person or an animal (cattle, rodents, dogs, etc.). As you can see, blood flukes have a rather complex and long development cycle, the result of which is the appearance of cercariae that can penetrate into the human body through the skin and mucous membranes, and clothes are not able to protect against their penetration.

Having penetrated the human body, cercariae shed their tail and turn into larvae of the last stage - schistosomula.

Through the vessels of the circulatory and lymphatic systems, the schistosomules of the systems enter the heart (right ventricle), then into the lungs and liver, where they mature and from there spread to the mesenteric venules, intestines or pelvic organs.

After 30-40 days, the female blood fluke begins to lay eggs, and the life cycle of the parasite repeats. Depending on the localization of the parasites, various types of schistosomiasis are distinguished.

Infection routes

As can be seen from the development cycle, infection with blood flukes, or rather, their larval stage - cercariae - occurs in fresh water: when bathing, washing and / or using it for domestic needs.

For local rice farmers, infection often occurs in rice paddies flooded with water, where a breeding environment for snails, and hence the larvae of blood flukes, is favorable.

In regions with a high risk of infection by blood flukes, it is strongly recommended not to drink unboiled water and to minimize all contact with untreated water, including bathing.

Tourists often underestimate the danger of infection with these parasites and do not attach importance to the symptoms that appear, however, the consequences can be very serious, so preventive measures must be strictly observed.

Infection symptoms

The penetration of blood fluke larvae into the human body causes the initial acute stage of schistosomiasis, caused by the migration of schistosomules (larvae of the last stage) along the blood and lymphatic vessels and their toxic-allergic effect.

This is usually accompanied by:

• the appearance of small swelling and rashes on the skin, resembling hives;
• headaches;
• muscle and / or joint pain;
• cough, sometimes with blood clots;
• fever, chills, excessive sweating.

In the chronic stage, the destructive action of parasites is predominantly mechanical in nature: the spines of the eggs violate the integrity of the walls of the intestine, bladder and other organs, causing the appearance of ulcers and growths of a polypous nature.

In a mild form, there is a minor health disorder, in particular, increased fatigue, but performance remains.

With moderate severity of the disease, anemia, diarrhea develop, due to an increase in the size of the liver and spleen, severe pain occurs in the right side; the temperature rises; with the urogenital form, urination is disturbed.

Severe forms lead to serious damage to the infected organs, and in the most extreme cases - to death (for example, when parasite eggs enter the pulmonary circulation, which leads to blockage of the pulmonary vessels).

Do not forget that tissue changes occur around each egg, leading to a secondary infection.

Take a test for schistosomes

Symptoms Answer Itching and skin rashes Yes Not Temperature Yes Not General weakness Yes Not Sleep disturbances Yes Not Muscle pain Yes Not Nausea Yes Not Blood in urine Yes Not Diarrhea Yes Not Dry cough Yes Not Convulsions Yes Not

There are cases when eggs were localized in the central nervous system, which led at first to headaches, then disorientation, amnesia and, ultimately, to a coma. If the eggs are localized in the heart, then this will cause expansion and then arrest of the right ventricle.

The variety of schistosomiasis requires careful instrumental and laboratory diagnostics, the methods of which are sufficiently developed.

When the first symptoms of infection with blood flukes appear, it is necessary to consult a doctor in order to diagnose the disease and begin timely treatment, which, as a rule, gives a good prognosis.