Table of contents:
- Characteristics of biohelminths
- Roundworms - geohelminths
- The mechanism of infection by helminths
Biohelminths are parasitic worms that have a final and intermediate host. Helminths are the general name for parasites from the genus of worms, the vital activity of which is provided by various animals and humans. Adult biohelminths live in a final host, which can be both humans and animals, and at the larval stage, the biohelminth's habitat is an intermediate host.
The content of the article:
- 1 Biohelminths
2 Characteristics of biohelminths
- 2.1 Diphyllobothrium latum (broad tapeworm)
- 2.2 Ligula intestinalis (lamb)
- 2.3 H. nana (pygmy tapeworm)
- 2.4 H. diminuta (rat tapeworm)
- 2.5 Taeniarhynchus saginatus, bovine tapeworm
- 2.6 Pork tapeworm (T. solium)
- 3 Roundworms - geohelminths
- 4 The mechanism of infection by helminths
Geohelminths have only a final host. Their larvae mature in the external environment, more often in the soil (geo - earth).
The digestive system consists of the anterior (mouth, pharynx, esophagus) and the middle (intestine). There is no anal opening. Undigested food debris is passed through the mouth. Tapeworms do not have a digestive system.
Excretory system of the protonephridial type. It begins with star-shaped terminal cells. From the cells, metabolic products enter the excretory canals, which flow into the common excretory canal and are released outside. The cilia of terminal cells help the movement of metabolic products.
The nervous system consists of nerve nodes in the front of the body. Nerve trunks extend from the nodes.
The digestive system has an anterior (mouth, pharynx, esophagus) and middle sections (two intestines) that end blindly.
The excretory and nervous systems have a structure typical of all flatworms.
Flukes are hermaphrodites (with the exception of schistosomes). The female reproductive system consists of an ovary, oviduct, vitelline cells, uterus, ootype, calf (gland) Melis, seminal receptacle. Male - from two testes, vas deferens, ejaculatory canal, cirrus.
Characteristics of biohelminths
The class Tapeworms (Cestoda) has about 3000 species. All parasites without exception. They parasitize most often in the small intestine of chordates. Length - from 0.5 mm to 30 m. The body is flat, ribbon-like, consists of a scolex, a neck and a strobila. The scolex carries the organs of fixation - hooks, proboscis, suction cups, bothria, botridia, etc.
The cervix is the segment budding zone. The strobila is made up of many segments called proglottids. Only in a few species does the body have visible dismemberment. The musculocutaneous sac corresponds to that of flukes, but in contrast to them, the submerged epithelium is equipped with microscopic villi - microtrichia.
The digestive system is absent, absorption occurs through the entire surface of the body. The excretory system of the protonephridial type, the most developed are two channels on the sides of the body, connected in each segment by a transverse channel. The reproductive system is hermaphrodite type. Each segment contains one or two sets of gonads.
The male reproductive system is represented by vesicular testes (from 1 to 1200), the vas deferens extending from them, which in turn flow into a large canal that opens into the cirrus. The female reproductive system consists of an ovary (usually paired), ootype, vitelline and uterus.
The ootype is connected to the female genital opening by the vaginal canal, which has an extension - the seminal receptacle. The genital openings of both the male and female reproductive systems open into the genital cloaca, located on the side of the segment, or on its flat side.
Echinococcal bladder - consists of several layers, the inner one of which is germinal, that is, capable of budding both scolexes and daughter bubbles, and those, in turn, are grandchildren. Bubbles form in the parenchymal organs.
The alveococcal bladder is of a cellular structure. Each cell contains several scolexes. In the lungs and liver of mammals.
Parasitization of tapeworms in the small intestine leads to various pathological changes in the host organism. Significant competition for food leads to a sharp weight loss of the owner. In addition, the worm absorbs vitamin B12 and vitamin deficiency occurs. Helminth also releases toxins. Anemia, nausea, and pain in the intestines are also characteristic. People can develop parasitic psychosis.
The following genera and species pose the greatest danger to human and animal health.
Diphyllobothrium latum (broad tapeworm)
Systematic position: order Pseudophyllidea, family Diphyllobothriidae.
Strobila up to 20 m, scolex is armed with two suction slits - bothria. The segments are very wide - up to 15 mm and short - 6-8 mm. The uterus is a highly convoluted tube that looks like an irregular spot in the central part of the segment. Eggs (68-71 microns long and 45 microns wide).
It parasitizes humans and fish-eating mammals. Eggs are deposited either in the intestines, or they are secreted together with the segments. At the same time, fragments of strobila with a length of 2-4 to 60 cm are periodically isolated. When a large specimen of helminth is infested, an infected person can secrete up to 2 million eggs in 1 g of feces. For further development, the egg must enter fresh water, where after 3-5 weeks the coracidium larva emerges from it.
This larva is swallowed by the first intermediate host - the diaptomus or cyclops crustacean, in which a procercoid forms after 1–2 weeks. If an infected crustacean is eaten by a fish (an additional host), then within a month a larva develops from a procercoid - a plerocercoid, which is invasive.
If this fish is eaten by a pike, pike perch or other predatory fish, then the plerocercoid passes into the muscles and internal organs of this predator (reservoir host). Large pikes can contain up to several tens or hundreds of plerocercoids.
Human infection occurs most often when eating raw pike caviar. Symptoms are nausea, dizziness, abdominal pain, anemia, discomfort on the tongue while taking medications, sour and salty foods, then the papillae of the tongue atrophy, and the edge of the tongue seems varnished. There is a decrease in the production of hydrochloric acid by the stomach, disruption of the heart, vitamin deficiency. With an invasion of several helminths or one large, intestinal blockage may occur.
Foci of diphyllobothriasis exist in the basins of large rivers. So, in Ladoga and Peipsi lakes, as well as in the Vuoksa system, most pikes are infected. At the beginning of the 20th century, 10% of the population in St. Petersburg was ill. Now the average incidence in Russia is 11.7 per 100,000 population, in the Leningrad region - 15.6; in Karelia - 35.5; Komi - 49%; In the Nenets Autonomous District - 258.5; Evenk Autonomous Okrug - 534.9. More than 90% of patients are adults.
Ligula intestinalis (lamb)
Systematic position: order Pseudophyllidea, family Ligulidae.
The belt-like strobilus reaches 1 m. The Scolex is very poorly developed. The segments are very short, their boundaries are not clearly marked.
Testes and vitellines are numerous. The uterus is in a central position.
In an adult state, it lives in the intestines of fish-eating birds for a very short time - from 2-5 days to 2-4 weeks. Eggs in the water develop and coracidia emerge from them in 1-3 weeks. They are swallowed by cyclops crustaceans - the first intermediate hosts. In cyclops, a procercoid is formed after 2 weeks.
If a fish eats an infected cyclop, then in its body the procercoid turns into a plerocercoid, which develops within 1-3 years and grows up to 1 m long. During this time, many parts of the reproductive system appear in the larva. The infected fish is inactive, emaciated, weakened, its abdomen is strongly swollen, it swims poorly and becomes easy prey for gulls, terns and other birds.
Sometimes the body wall breaks, and the ligula is released into the water. In some small water bodies of the Leningrad Region, crucians are infected by 90-95%.
In economically valuable fish, the larva of L. intestinalis (lamb) causes a dangerous and widespread disease - ligulosis. Prevention of ligulosis is carried out by scaring away fish-eating birds and maximum catching of diseased fish. Ligules are absolutely safe for humans. A closely related species, Digramma interrupta, is very similar to ligula.
Strobila 0.2 cm - 1 m. The Scolex has 4 suction cups and a proboscis armed with hooks. In different representatives the hooks and proboscis are developed to varying degrees. Semennikov 1 -3, a well-developed seminal receptacle. The ovary is large. Parasites of mammals and birds cause hymenolepiasis, proceeding as enteritis.
H. nana (pygmy tapeworm)
It is often found in humans, less often in mice and rats. The strobila is 2 cm long. The scolex has a proboscis with a rim of hooks. Three testicles in a hermaphroditic segment are arranged in a row in the lower part of the segment. The uterus is saccular. Eggs are oval, colorless, 40-50 microns in size. Inside the oncosphere is contained, from the poles of which 2 pairs of long wriggling processes extend. It develops most often without an intermediate host (sometimes an insect wedges into the cycle).
Oncospheres immediately emerge from the laid eggs, which penetrate into the villi of the intestine, where the further development of the larvae - cysticercoids - occurs. The larva matures within 102 hours. After that, the cysticercoid falls into the intestinal lumen and a new tapeworm grows.
With this disease, superinvasion occurs quite often, in which the number of parasites is in the tens of thousands.
Children are sick more often, however, by the age of 12-14, self-healing occurs, and re-infection occurs very rarely. The incidence is 0.1-0.4 per 100,000. The basis of preventive measures is adherence to the rules of personal hygiene.
H. diminuta (rat tapeworm)
Parasitizes in synanthropic rodents and sometimes in humans. Strobilus up to 60 cm long. The proboscis on the scolex is underdeveloped. In the hermaphroditic segment, the ovary and vitelline occupy a central position; one testis is located on one side of the ovary (the genital opening opens on the same side), and two on the other. The uterus is saccular. Eggs are rounded with a double-contour shell, 60-70 microns in diameter, containing an oncosphere. Intermediate hosts are mainly insects (pests of stocks), in which cysticercoids are formed. A person becomes infected by eating baked goods prepared in violation of technology from flour containing infested insects. Infection of a person in one third of cases does not give pronounced symptoms. The main symptoms are: abdominal pain, nausea, unstable stools, general weakness, dizziness. The acidity in the stomach also often decreases. When the rat tapeworm is parasitized, nervous seizures can occur.
Hymenolepidids often parasitize in the intestines of waterfowl. Drepanidotaenia lanceolata is of veterinary importance. The strobila is up to 23 cm in size. The Scolex is equipped with 4 suction cups and a proboscis with 8 hooks. Sex holes are located on one side of the strobila.
Three testes lie in one line. Eggs are oval, up to 106 µm in length and up to 46 µm in width.
Intermediate hosts are cyclops and diaptomuses, in which the oncosphere penetrates into the body cavity and, after 11-12 days, turns into a cysticercoid. After another 3 weeks, the larva becomes invasive. Ducks and geese become infected by swallowing crustaceans with food. In the body of a bird, the helminth reaches sexual maturity in 2-3 weeks. If a Cysticercoid-infested Cyclops accidentally eats a mollusk, then the helminth larva retains its invasiveness. The sick bird is emaciated, the feces are liquid. Paralysis of the legs is common.
Taeniarhynchus saginatus, bovine tapeworm
Systematic position: order Cyclophyllidea, suborder Taeniata, family Taeniidae, subfamily Teniinae.
The powerful strobila (sagina - obesity) reaches 10-14 m in length. The scolex is equipped with 4 powerful suction cups. In the hermaphroditic segment, the testes are small, numerous, the ovary is bifurcated. In a mature segment, the central canal of the uterus has 13 to 35 lateral branches on each side. The muscular system is highly developed. Eggs are oval, 30-40 microns in size. They are covered with a thick membrane with radial striation and contain a mature oncosphere.
Only man serves as the final owner of the bovine tapeworm. Mature segments can independently crawl out of a person's anus for 6-11 pieces per day, but more often they come out with feces. They crawl for a long time, scattering eggs. Thus, grass and hay are polluted. Eggs remain viable for a long time. The intermediate host is cattle, swallowing eggs with food. The oncosphere comes out of them in the intestine, which is carried into the muscles with the blood flow. There, after 5-6 months, cysticercus (Finns) are formed.
A person becomes infected by eating poorly processed Finnish meat. Causes human disease teniarinhoz (in Russia - 0.2-0.4 per 100,000 population). Symptoms of teniarhynchiasis: diarrhea, nausea, vomiting, loss of appetite, flatulence, abdominal pain, salivation. Irritability and restless sleep are noted.
Anemia is also observed. Women get sick more often than men by almost 40%. The crawling out of mature proglottids greatly depresses the psyche of patients. Tapeworm larvae (Finns) parasitize cattle and cause bovis cysticercosis (finnosis), which is usually asymptomatic. This cattle disease in Europe is found in 0.3-0.4% of slaughtered animals, and in the countries of East Africa - in 30-80%.
Pork tapeworm (T. solium)
Strobila up to 3 m long. Scolex with 4 suction cups and double hooks. The hermaphroditic segment is similar in structure to that of the bovine tapeworm and is distinguished by the presence of a third (additional) ovarian lobule. In a mature segment, the central canal of the uterus forms 4-10 branches on each side. A mature segment contains up to 100 thousand eggs, which in morphology are indistinguishable from the eggs of a bovine tapeworm (Fig. 14).
The ultimate master is only human. The intermediate host is a wild boar and a domestic pig, in whose muscles cysticercus (Finns) are formed. The heart is especially often affected. Human infection occurs when eating slightly salted or raw pork meat and especially lard, which contain viable finns. Larvae remain in meat for a long time - in the freezer of the refrigerator for up to 2 weeks. The tape stage causes teniasis in humans. Mature proglottids never spontaneously crawl out.
Symptoms are the same as with teniarinhoses. Once more than 100 tapeworms with a total length of 128 m were expelled from a person. The larval phase causes cellulosic cysticercosis in pigs, which, as a rule, is asymptomatic. When the finca enters the human intestine, the head is turned out of the bladder and attaches to the intestinal mucosa.
When the eggs of the parasite enter the human stomach (for example, when swallowing or reverse intestinal peristalsis), a person can become an intermediate host: the oncosphere emerges from the egg and is brought into different organs with blood flow, where finns are formed.
Damage to the brain and eyes is especially dangerous. The damage to the brain by the larvae is accompanied by headaches, vomiting, memory loss and nervous seizures. With eye damage, there is a complete or partial loss of vision. Human cysticercosis is treated either conservatively (with praziquantel) or operatively.
Roundworms - geohelminths
In these worms, eggs or larvae necessarily develop in the surface layers of the soil when oxygen is available and sufficient moisture is available. With the exception of pinworms, all helminths are more common in regions with hot and humid climates, which provide more opportunities for larvae and eggs to develop in the soil. Geohelminths are not found in the Arctic and southern subpolar regions. Males and females of helminths are easily distinguishable: males of most species have a rear end of the body bent to the ventral side or spirally twisted, while in females it is straight.
Geohelminths live in the intestinal lumen and reproduce with eggs, which are excreted in the feces and develop further in the soil. They either become invasive after a certain time, or larvae develop from them, leading a free lifestyle for some time and later becoming invasive.
Geohelminths that infect humans cannot parasitize animals. Accordingly, the nematodes caused by these parasites are anthroponotic diseases. Most of geohelminths are infected by ingesting eggs or larvae with food contaminated with soil.
Some of the geohelminths, getting into the human digestive system, quickly reach sexual maturity and begin to multiply in the intestines, without migrating through the host's body. The larvae of others, before reaching puberty, necessarily move through the blood vessels and the respiratory system and only then develop in the intestines.
The mechanism of infection by helminths
The ways of penetration of parasites are varied, but with helminthiases common in the territory of the Russian Federation, only the fecal-oral transmission mechanism is actually implemented. In this case, infection through the mouth, as a rule, causes the subsequent localization of the parasite in the intestine. The fecal-oral mechanism is realized when eating the meat of vertebrates and invertebrates, which are intermediate hosts of helminths (teniosis, trichinosis, diphyllobothriasis, opisthorchiasis, paragonimiasis).
Transmission factors are food products or sometimes water accidentally contaminated with propagative stages (eggs, larvae) of helminths (ascariasis, trichocephallosis, etc.). Finally, the pathogen can be introduced into the mouth and through contaminated hands or environmental objects, which is especially observed in enterobiasis and hymenolepiasis. It should be borne in mind that oral penetration of the helminth is also realized with those invasions in which a person is not the final owner.
Geohelminthiases are invasions, the causative agents of which develop without the participation of an intermediate host.
Eggs or geohelminth larvae released from the body develop to the invasive stage in the soil. The eggs of the pathogens of contagious helminthiases are excreted from the human body completely mature, that is, they are immediately infectious to humans.
They admit the possibility of a suppressive effect of actinomycetes and bacteria isolated from the soil of irrigated fields on the development of ascaris eggs. The soil also serves as a substrate for many ectoparasites, vectors of vector-borne diseases (ticks, fleas, mosquitoes, flies, horseflies). Some species of ticks, beetles, flea larvae, eaters, ants are intermediate hosts for helminths. Many insects live in the soil - pests of agriculture and forestry, gardens and vegetable gardens.
Find out more:
- Helminths in humans: signs, symptoms and treatment
- Helminth worms. What worms live in a person. Types of parasites