Schistosomiasis - routes of infection, forms, prevalence, symptoms, treatment, prevention

Schistomiasis (schistosomiasis, bilharzia) is a group of tropical trematodes, which are characterized in the acute stage by toxic-allergic reactions, in the chronic stage by damage to the intestines or genitourinary system.

Schistosomes

These helminths are dioecious trematodes. The body of sexually mature individuals is elongated, cylindrical, in males it is thickened, in females it is thread-like. The dimensions of the male are: length 10-17 mm, thickness 1 mm, females - length 12-20, thickness 0.25 mm.

Schistosomes feed on blood. Males and females parasitize in pairs. In this case, the female is located in a longitudinal, slit-like canal on the male’s body.

Types of schistosomes pathogenic to humans

Currently, the following types of these helminths have been studied:

• Sch. haematobium is the causative agent of genitourinary schistosomiasis;
• Sch. mansoni - the causative agent of intestinal schistosomiasis, or Manson's schistosomiasis;
• Sch. japonicum is the causative agent of Japanese schistosomiasis, which occurs primarily with intestinal damage;
• Sch. intercalatum – intercalate schistosomiasis, which occurs predominantly with intestinal damage;
• Sch. mekongi – Mekong schistosomiasis, close to Sch. Japonicum.

These types of helminths parasitize the venous system of the genitourinary organs and intestines of humans, as well as some mammals, where females release eggs. The eggs have a short lateral spine, which facilitates their penetration into the wall of the blood vessel.

Life cycle

When it enters the water, a miracidium covered with cilia emerges from the egg, which penetrates the intermediate host - a mollusk. In it, the development and reproduction of larval forms of schistosomes occurs with the formation of cycaria - larvae with a tail forked at the end. They leave the body of the intermediate host and again end up in the water. Upon contact with a person or animal, they actively penetrate his body through mucous membranes or skin.

Young helminths migrate through the blood vessels and reach the abdominal cavity, where they grow and develop until puberty. They reach it on average after 1 month.

The lifespan of adult schistosomes is 5-8 years, some individuals live 20-30 years. Each female helminth produces 300-3000 eggs per day, depending on the species.

Routes of infection

The main source of invasion is the patient, who secretes schistosome eggs. In the genitourinary form, the role of animals in infection is minimal. In the intestinal form, animals make a slightly greater contribution to the contamination of water bodies, and in Japanese and intercalate schistosomiasis, the role of animals is significant.

Intermediate hosts of parasites are mollusks

• Bulinus and Biomphalaria in Africa,
• Oncomelania in the Far East.

People usually become infected through the skin

• while swimming,
• washing clothes,
• work on irrigated fields and
• when drinking water from open reservoirs.

Infection with Japanese schistosomiasis occurs through contact with grass on which infected shellfish crawl.

Prevalence of the disease

The distribution of schistosomiasis extends along the equator between 38° north and 35° south latitude. This helminthiasis is registered in 73 countries of the world.

Urogenital schistosomiasis is common in countries such as:

• in Africa - Egypt, Sudan, Uganda, Congo, Zimbabwe, Angola, South Africa, Cameroon;
• in Asia - Iraq, Syria, Saudi Arabia, Israel, Yemen, Iran, India;
• in Australia;
• on the islands - Cyprus, Mauritius, Madagascar.

Intestinal schistosomiasis is common:

• in Africa - in Egypt, Sudan (on the east coast from the island of Zanzibar to the Zambezi River), Zambia, Tanzania, the Central African Republic, Congo, Cameroon and others;
• in South America - in Venezuela, Guiana, Brazil, on the islands of Haiti, Puerto Rico.

Japanese schistosomiasis is registered:

in Asian countries - in Indonesia, China, Malaysia, the Philippines, Japan.

Mekong schistosomiasis is found in Cambodia, Laos, and Thailand.

Sch. Intercalatum - in Central and Southern Africa, namely in Zaire, Gabon, Cameroon, Chad.

The disease is most common in age groups 10-20 years old and does not tend to heal on its own. However, over time, immunity is formed and re-infection does not occur.

What harm do schistosomes cause to health?

At the early stage of migration of larvae, the body is poisoned by their metabolic products and decay of schistosomulae. In addition, they damage tissue.

In the chronic stage, the pathological process is caused by

• the movement of eggs through the walls of blood vessels,
• Bladder,
• intestines and
• carrying them into the lungs and liver.

Inflammatory-allergic granulomas are formed around the eggs, followed by fibrosis. The proliferation of fibrous tissue around the portal vein network leads to obstruction of blood flow in its system, the development of portal hypertension, and splenomegaly. In this case, the death of patients often occurs due to bleeding from varicose veins of the esophagus.

Similar processes in the lungs lead to an increase in pressure in the pulmonary circulation, which contributes to the development of the pulmonary heart.

Tumors of the genitourinary system and colon are often found in foci of schistosomiasis.

Symptoms

The incubation period is 4-16 weeks.

The following stages of the disease are distinguished:

• early or acute - penetration and migration of helminth larvae;
• chronic - parasitization of adult parasites.

Early stage

The duration of this period is 7-14 days. It occurs in the same way for all types of schistosomiasis - asymptomatic or with minor manifestations.

During initial infection, the disease does not manifest itself, but during repeated infection, signs such as:

• itching,
• tingling sensation in places where parasites have invaded,
• local erythema,
• papular rash,
• symptoms of intoxication (general weakness, fever, headache),
• muscle pain,
• joint pain.

During the period of schistosome migration, especially when they pass through the lungs,

• cough with sputum,
• the liver, spleen, and lymph nodes enlarge.

A general blood test reveals:

• leukocytosis,
• eosinophilia,
• increased ESR.

Chronic stage

It begins 4-5 weeks after infection, when the parasites complete puberty and intensive egg laying begins.

All symptoms of the previous stage begin to appear more clearly

• fever,
• joint pain,
• cough,
• enlarged liver and spleen,
• join them
• nausea,
• vomit,
• diarrhea.

A general blood test shows high eosinophilia,

α₂- and ϒ-globulinemia is a manifestation of an allergic reaction to the metabolic products of mature parasites and their eggs.

Severe clinical manifestations of schistosomiasis are observed in 4-12% of patients.

In the vast majority of infected people, the disease occurs in the form of compensated invasion: either asymptomatic or with mild nonspecific symptoms.

Urogenital schistosomiasis (ICD-10 code - B6.0)

A clear typical sign appears - blood in the urine, and it appears at the end of urination.

Urination becomes frequent and painful. Secondary inflammation develops, which leads to destruction of bladder tissue - ulceration of the mucous membrane. Inflammation spreads up the ureters to the kidneys.

At a later stage, the outflow of urine is disrupted and hydronephrosis develops. In this case, the disease can be complicated by the formation of stones in the bladder,

• in men - damage to the seminal vesicles and prostate gland;
• in women - papillomatosis of the vagina and uterus.

When schistosomes are carried into the lungs, hypertrophy of the right ventricle of the heart develops, shortness of breath, palpitations, dull pain in the heart area, cough, and hemoptysis occur.

When eggs enter the liver, granulomatous hepatitis develops. Infestation Sch. Haematobium creates the preconditions for the development of primary bladder cancer.

Intestinal schistosomiasis (Manson, ICD-10 code - B65.1)

The pathogen lives in small vessels flowing into the inferior mesenteric vein. The eggs laid by the female pass through the walls of the venules, the intestinal wall and are released into the external environment with feces.

With this form the following symptoms appear:

• stomach ache,
• loss of appetite,
• weight loss,
• frequent stools,
• mucus and blood in the stool.

With severe infection, complications such as:

• mesenteric fibrosis,
• haemorrhoids,
• rectal prolapse,
• polyposis,
• intestinal ulceration,
• liver fibrosis.

Intercalate schistosomiasis (ICD-10 code - B-65.8)

The most benign disease of the entire group of intestinal schistosomiasis, the symptoms are usually mild.

Japanese schistosomiasis (Katayama disease, ICD-10 code - B65.2)

This is the most severe helminthiasis of this group of invasions. Its pathogen is localized in the mesenteric and hemorrhoidal veins. These helminths have increased reproductive rates and produce the largest number of eggs. The eggs accumulate in large masses and become calcified.

In Japanese schistosomiasis, liver fibrosis often develops with the formation of fluid in the abdominal cavity (ascites) and edema of the lower extremities.

Extensive damage to the intestines, including obstruction, and the introduction of eggs into the nervous system with subsequent paresis, paralysis, encephalitis, meningoencephalitis, and brain tumors (persistent headache, vomiting, visual disturbances) are often observed.

Mekong schistosomiasis (ICD-10 code B6-8)

The symptoms are the same as in Japanese.

Diagnostics

Due to the fact that schistosomiasis is often asymptomatic, in different forms and with diverse manifestations, a set of methods is used in their diagnosis:

• anamnestic,
• epidemiological,
• parasitological,
• immunological,
• instrumental.

History and epidemiological data are especially important for diagnosis when infected with schistosomes of one sex, when parasite eggs are not laid and cannot be detected either in urine or feces.

Parasitological diagnosis is based on the detection of eggs in urine and feces.

The preferred time for urine collection is between 10 a.m. and 2 p.m.

Sometimes it is recommended to examine urine after physical activity or massage of the prostate gland.

Immunological diagnosis is important in the early stages of diseases, during the period of migration and maturation of helminths and in the late stage, when most of the eggs settle in the tissues and do not get out.

An intradermal test is used using extracts from schistosomes as an antigen.

instrumental methods are reliable :

• cystoscopy (for genitourinary schistosomiasis) - allows you to identify infection, identify complications, and monitor the dynamics of recovery after chemotherapy;
• biopsy of the mucous membrane of the bladder - allows you to detect live and calcified helminth eggs;
• sigmoidoscopy or colonoscopy - for intestinal schistosomiasis, it allows to identify disorders of the mucosa characteristic of these helminths (“sandy tubercles” and “sand spots”), and eggs are found during a biopsy of the mucosa;
• angiography and laparoscopy - to examine the portal vein of the liver;
• Ultrasound of the abdominal and pelvic organs.

Treatment

Complex therapy must be carried out, which is carried out in a hospital.

The drug of choice is praziquantel .

Along with the antiparasitic drug, pathogenetic and symptomatic treatment is necessary.

In the initial stage of the disease, ointments containing prednisolone and diphenhydramine are used to reduce skin itching.

In the second and third stages, antihistamines are used to reduce allergic reactions; in severe cases, glucocorticosteroid drugs are used.

To monitor the effectiveness of specific treatment, repeated parasitological studies are used over several months.

Prevention

Measures to prevent the disease are:

• destruction of shellfish,
• timely treatment of patients,
• protection of water and soil from contamination by feces,
• compliance with sanitary standards,
• when staying in areas of schistosomiasis, it is not recommended to swim or wash clothes in small freshwater bodies of water, or to walk barefoot where shellfish are found,
• drink clean water.