🦠 Parasitic infections: everything about types, routes of infection, symptoms and protecting the body

Parasitic diseases are a broad group of infections caused by organisms that live at the expense of their host. According to World Health Organization estimates, more than one and a half billion people on the planet are infected with various parasites. Despite advances in medicine and improved sanitary conditions, parasitic diseases remain a pressing problem in every country, including economically developed ones. Parasites can live unnoticed in the body for months and years, gradually undermining health, weakening immunity and triggering chronic diseases.

What parasites are and how they are built

Parasites are organisms that live inside or on the surface of another organism (the host) and feed at its expense. Unlike bacteria and viruses, most parasites are multicellular creatures with a complex structure and life cycle. They are superbly adapted to life inside the human body: they can hide from the immune system, release substances that suppress defensive reactions, and are highly fertile — some species lay up to 200,000 eggs a day.

Parasites can live in almost any organ: the intestines, liver, lungs, muscles, brain, skin and subcutaneous tissue. Their size ranges from microscopic (Giardia — about 15 micrometres) to several metres (the broad fish tapeworm — up to 15 metres).

Classification of human parasites

Helminths (worms)

Roundworms (nematodes) are the most numerous group. Ascarids are large worms up to 40 cm long that live in the small intestine. Their larvae migrate through the lungs, liver and blood vessels before settling in the intestine. Pinworms are small worms (up to 1 cm) that cause enterobiasis, the most common parasitic infection in children. The telltale sign is severe itching around the anus, especially at night. The whipworm attaches to the wall of the large intestine and feeds on blood, causing chronic anaemia. Trichinella are particularly dangerous parasites whose larvae penetrate muscle tissue and form capsules, causing severe pain.

Tapeworms (cestodes) have a flat, ribbon-like body made up of many segments. The beef tapeworm reaches 10–12 metres and can live in the intestine for more than 20 years. The pork tapeworm is dangerous because its larvae (cysticerci) can penetrate the brain, eyes and muscles — a condition called cysticercosis. Echinococcus is a tiny worm, but its larvae form large cysts in the liver and lungs that can grow to the size of a child's head. The broad fish tapeworm is transmitted through raw freshwater fish and can absorb all the vitamin B12 from the host's food, causing severe anaemia.

Flukes (trematodes) are flatworms with suckers that they use to attach to tissues. The liver fluke lives in the bile ducts, causing opisthorchiasis, a serious disease common in regions where raw river fish is eaten. The cat liver fluke can trigger bile duct cancer in long-standing infection.

Protozoa (protozoan parasites)

Giardia are single-celled organisms that affect the small intestine and the biliary tract. Giardiasis is especially common among children and can cause chronic digestive disorders and allergic reactions. Toxoplasma is an intracellular parasite whose main host is the cat. For people with normal immunity, toxoplasmosis is usually not dangerous, but it poses a serious threat to pregnant women: infection can lead to severe foetal abnormalities. Amoebas are protozoa that can cause severe amoebic colitis with bloody diarrhoea and the formation of liver abscesses.

Routes of parasitic infection

Understanding the routes of transmission is the key to effective prevention. The faecal-oral route is the most common: parasite eggs enter the mouth from unwashed hands, contaminated vegetables, fruit, herbs and water. A single ascarid egg remains viable in soil for up to 7 years. The food-borne route involves eating contaminated food: undercooked or raw meat (beef and pork tapeworms, Trichinella), raw or lightly salted fish (Opisthorchis, the broad fish tapeworm, Anisakis). The contact route is relevant for pinworms: the eggs get onto underwear, toys and door handles and are easily passed from person to person. The vector-borne route — through insect bites — is typical of tropical parasitic diseases (malaria, leishmaniasis).

Pets are an important source of infection. Dogs and cats can carry Toxocara, Echinococcus, Toxoplasma and other parasites. Through contact with infected pets, parasite eggs get onto the hands and then into the mouth. Children can become infected on playgrounds through soil and sand contaminated with animal faeces.

Symptoms of infection by body system

The clinical picture of parasitic infections is extremely varied and depends on the type of parasite, where it lives and the severity of the infection.

Gastrointestinal tract: abdominal pain (often around the navel), bloating and excess gas, nausea, irregular stools (alternating diarrhoea and constipation), reduced or altered appetite, unexplained weight loss or gain, itching around the anus (with pinworms). Children may grind their teeth in their sleep (bruxism).

Skin and allergic reactions: chronic rashes (hives, eczema), itchy skin for no apparent reason, angioedema, persistent allergic reactions that do not respond to standard treatment. These manifestations are linked to the fact that the waste products of parasites are powerful allergens.

Nervous system: chronic fatigue and weakness, irritability, anxiety, sleep disturbances (especially with pinworms — restlessness at night), poorer memory and concentration, headaches. When the brain is affected (neurocysticercosis, toxocariasis) — seizures and visual disturbances.

Blood and immune system: anaemia (in ascariasis, hookworm disease, diphyllobothriasis), eosinophilia — an elevated level of eosinophils in the blood (a characteristic marker of parasitic infection), frequent colds and infections due to weakened immunity.

Liver and biliary tract: pain and heaviness under the right ribs, a bitter taste in the mouth, nausea after fatty food, an enlarged liver (in opisthorchiasis, fascioliasis, echinococcosis).

Chronic consequences of parasitic infections

Long-term parasitism has serious consequences for the body. Chronic intoxication with parasite waste products suppresses the immune system and creates favourable conditions for other infections and autoimmune processes to develop. Deficiency of nutrients — vitamins, trace elements, proteins — leads to exhaustion of the body, developmental delay in children and impaired blood formation.

Mechanical damage to tissues by parasites can cause chronic inflammation which, according to some data, increases the risk of cancer. For example, opisthorchiasis is recognised as a risk factor for cholangiocarcinoma (bile duct cancer), and schistosomiasis for bladder cancer. Intestinal blockage by a tangle of ascarids, rupture of an echinococcal cyst and damage to the brain by cysticerci are examples of acute, life-threatening complications.

Modern diagnosis

Diagnosing parasitic infections can be a difficult task, as many of them masquerade as other diseases. The classic method — microscopic examination of stool for helminth eggs — needs to be repeated several times (at least three times at intervals of 3–5 days), since eggs may not be shed constantly. A perianal scraping for enterobiasis detects pinworm eggs.

Serological methods (ELISA — enzyme-linked immunosorbent assay) detect antibodies to specific parasites in the blood. This method is particularly informative for diagnosing tissue parasitic diseases (toxocariasis, echinococcosis, trichinellosis), when the parasite is outside the intestine and its eggs are not shed in the stool. The PCR method (polymerase chain reaction) detects the parasite's DNA and is highly sensitive.

Imaging methods — ultrasound, CT, MRI — are used when tissue forms of parasitic disease are suspected: echinococcal cysts in the liver and lungs, cysticerci in the brain, abscesses in amoebiasis. A complete blood count showing characteristic eosinophilia may be the first signal pointing to a parasitic cause of the complaints.

Parasitic infections in children

Children become infected with parasites much more often than adults. This is due to their still-developing hygiene skills, their tendency to put hands and objects in their mouths, and close contact in groups of children. Enterobiasis is the most common — up to 20–30% of preschool children are infected with pinworms. In second place are giardiasis and ascariasis.

Parasitic infections in children can show up as delayed physical and mental development, poorer school performance, frequent respiratory infections, persistent allergic manifestations, poor appetite and low body weight. Restlessness at night, teeth grinding and bedwetting are symptoms that may also be linked to a worm infestation. It is important to carry out regular preventive check-ups of children and to teach them hygiene rules from an early age.

Prevention measures

The prevention of parasitic infections is based on simple but strict rules:

• Hand hygiene: wash hands thoroughly with soap before eating, after using the toilet, after contact with animals, after working in the garden or vegetable plot, and after playing outdoors.
• Food handling: wash vegetables, fruit, herbs and berries under running water and, where possible, pour boiling water over them. Fry or boil meat until fully cooked (internal temperature of at least 70 °C). Fish, especially freshwater fish, should be sufficiently heat-treated or deep-frozen (−20 °C for at least 7 days).
• Drinking water: drink only boiled or filtered water. Avoid swallowing water when swimming in open water.
• Pet care: deworm pets regularly (every 3–6 months), wash your hands after contact with them, keep animals away from food and dishes, and clean up their droppings promptly.
• Household hygiene: regular wet cleaning, washing bed linen and underwear at a high temperature, keeping the toilet and bathroom clean.

Precautions when travelling

Trips to Southeast Asia, Africa, and Central and South America carry an increased risk of infection with exotic parasites. It is advisable to consult an infectious disease specialist before travelling. While travelling, drink only bottled water, avoid raw and undercooked local dishes, use insect repellents, wear closed shoes (some parasites penetrate through the skin of the feet) and do not swim in freshwater bodies in tropical countries. After returning, if any unusual symptoms appear, see a doctor and mention the trip.

Myths about parasites

"Parasites are only a problem in poor countries" — this is a misconception. Pinworms, Giardia, Toxocara and other parasites are found in every country in the world. "Garlic and herbal teas drive out all worms" — folk remedies have no proven antiparasitic effectiveness and cannot replace medical treatment. "If the stool test is negative, there are no parasites" — a single test detects parasites in only 30–50% of cases; repeated testing and additional diagnostic methods are needed. "You should take deworming tablets for prevention" — taking anthelmintic drugs without medical indication is not recommended because of their toxicity to the liver; preventive treatment is justified only in certain risk groups and when prescribed by a doctor.

Parasitic infections are a serious but preventable group of diseases. Following hygiene rules, paying attention to the quality of food and water, having regular check-ups and seeing a specialist promptly when suspicious symptoms appear are reliable protection of health for the whole family.