1st year DDS - Practical class 3

General characteristics of Cestoda (tapeworms)

additional characters:
• Tapeworms exhibit a great variety of size.
• The body consist of a scolex, a neck and proglottids (immature proglottids, mature proglottids and gravid proglottids).
• Scolex is equipped with elements for attachment to the intestinal wall of the definitive host (bothria = suctorial grooves, or four suckers with or without of hooks
• The digestive tract is absent.
• The body wall is covered by a tegument by use of which tapeworms absorb nutrition.
• All species are hermaphroditic.
• All the species of Cestoda are heteroxenous (with exception of Hymenolepis nana).

Diphyllobothrium latum
(the fish tapeworm)

Kind of parasite: heteroxenous and polyxenous.

Definitive host(s): human and fish-eating mammals.

Intermediate host(s): 1st – copepods, 2st – freshwater or anadromous fish.

Infective stage: plerocercoid (worm-like larva in muscles of fish).

Transmission by ingestion of raw or under-cooked fish containing plerocercoids.

Site of infection: lumen of the small intestine.

Diagnostic method(s):

• microscopic identification of eggs or - less frequent - mature proglottids in stool specimens;

• molecular methods (NAAT - Nuclear Acid Amplification Test).

Geographical distribution: endemic in northern Europe (Scandinavia, Russia), Asia (Korea, China, Japan), the Arctic and North America, Brasil.

Remarks:
The parasite absorbs nutritives first of all rich in vitamin B12, and as a result the infection can have pathological states (B12 vitamin deficiency) for some infected patients.

Prevention consist:

• in protection of freshwater lakes from contamination with untreated sewage;

• in health education aimed at changing the eating habits;

• treatment of infected patients.

Taenia saginata
(the beef tapeworm)

Kind of parasite: heteroxenous and monoxenous.

Definitive host(s): human.

Intermediate host(s): cattle.

Infective stage: cysticercus larva in the muscles of intermediate host.

Transmission by ingestion of raw or poorly cooked beef containing cysticerci.

Site of infection: lumen of the small intestine.

Diagnostic method(s):

• microscopic identification of gravid proglottids (they have 15-30 lateral branches of uterus on each side) in stool;

Geographical distribution: cosmopolitan. Infections caused by the parasites are connected with dietary practice (comsumption of raw beef). Infection are more frequently detected on Sub-Saharan areas in Africa (Ethiopia), in Northern Mexico, in Argentina, in Asia and in middle Europe.

Remarks:
• determination of Taenia species is impossible if solely based on microscopic detection of eggs, because all Taenia species produce the eggs that are morphologically identical;
• prevention consist in treatment of infected individuals, protection of cattle from contamination with human feaces, inspection of beef for cysticerci and refrigeration or cooking of beef.

Taenia solium
(the pork tapeworm)

Kind of parasite: heteroxenous and monoxenous.

Definitive host(s): human.

Intermediate host(s): pig and sometimes human.

Infective stage: cysticercus larvae (if human is the definitive host); eggs (if human is the intermediate host).

Transmission 

• the definitive hosts (humans) are infected by eating raw or poorly cooked pork containing cysticerci;
• human cysticercosis may occur after the ingestion of eggs (exo-autoinfection is possible) or by reverse peristalsis (endo-autoinfection).

Site of infection:

• the small intestine for adult stage (human is the definitive host);

• every organs and tissues for cysticercus larvae (human is the intermediat host).

Diagnostic method(s):

• microscopic identification of gravid proglotids (they have 7-13 lateral branches of uterus on each side) in stool;
• the diagnosis of cysticercosis is made by radiological examination (X-ray, USG, CT, MRI) and biopsy examination;
• serological methods (enzyme-linked immune-transfer blot)

Geographical distribution: cosmopolitan. Infections are more frequent detected in human population of America continent (Brazil,  Mexico), Eastern Europe, Southeast Asia (China, the Indian subcontinent), and Sub-Saharan areas in Africa.

Remarks:
• determination of Taenia species is impossible if solely based on microscopic detection of eggs, because all Taenia species produce the eggs that are morphologically identical;
• prevention consist in treatment of infected individuals, protection of pigs from contamination with human feaces, inspection of pork for cysticerci and refrigeration or cooking of pork;
• in the prevention of human cysticercosis early detection and elimination of the infection with adult tapeworm and a high level of personal hygiene (autoinfection is possible) would be of crucial importance.

Hymenolepis nana
(dwarf tapeworm)

Kind of parasite: homoxenous and monoxenous.

Definitive host(s): human.

Intermediate host(s): human.

Infective stage: egg.

Transmission by ingestion eggs of the parasite.

Site of infection: lumen the small intestine for adult stage (human is the definitive host), and villi for the larval stages (cysticercoids, human play a role of intermediate host).

Diagnostic method(s):

• microscopic identification of eggs in stool.

Geographical distribution: the parasite has cosmopolitan distribution, and infects mostly children as a result of low level of personal hygiene (poor neighborhoods, institutional settings). From the same reason infection caused by the parasite can be detected also among patients of high-security (locked) wards of psychiatric hospitals.

Remarks:
• the gravid proglottids often rupture within the intestine and infestive eggs may pass in the feaces or liberated oncospheres may repeat the cyclic development (autoinfection and endoautoinfection in human’s organism may occur);
• prevention consist in treatment of infected individuals and proper personal hygiene.

Echinococcus spp.

Two main similar species from Echinococcus genus were described during theseminar and practical class: E. granulosus and E. multilocularis.

Kind of parasite: heteroxenous and polyxenous.

Definitive host(s): all carnivorous; the main definitive host for E. granulosus is dog whereas for E. multilocularis are wolfs and foxes.

Intermediate host(s): herbivorous and omnivorous mammals, and rodents; human is accidental host.

Infective stage: eggs or gravid proglottids with the eggs.

Transmission by ingestion of the eggs of parasites (unsatisfactory level of personal hygiene).

Site of infection: to the most common sites of infection belong: liver, lungs and brain.

Diagnostic method(s):

• immunological tests;
• radiological (USG, MRI and CT) examination;
• fine-needle biopsy of the hydatid cyst and examination of the hydatid cyst content for the presence of numerous protoscolices (hydatid sand).

Geographical distribution:

• E. granulosus – cosmopolitan;
• E. multilocularis – northern hemispere: Europe, Asia, North America.

Remarks:
• E. granulosus - the incidence of human hydatidosis (i.e. the occurrence of hydatid cysts) depends on the association of man with infected dogs. Hydatidosis constitutes a very serious medical problems in sheep- or cattle raising areas. Humans are infected when they ingest eggs, usually as a result of contact with dogs (which are not under the veterinary service). The parasite circulates between domestic animals (anthropogenic cycle) and wild animals (sylvatic cycle).
• E. multilocularis – this tapeworm causes alveolar echinococcosis in humans. The tapeworm circulates primary among wild animals but the rodents are responsible for transmission of parasite into anthropogenic cycle.