1st year DDS - Practical class 4

General characteristics of Nematoda

• The common name of the nematodes - round worms.
• Nematodes are elongated, tapered at both ends.
• The nonsegmented body is covered with a noncellular cuticle that is secreted by underlying hypodermis. The cuticle is shed four-times during ontogeny.
• The digestive system of female is complete with mouth, esophagus, intestine and the anus near the posterior end.
• The digestive system of male is complete with mouth, esophagus, intestine and cloaca.
• Most nematodes are dioecious and show sexual dimorphism.
• The general life cycle involves following stages: egg – larvae - adult stages (female and male).
• Parasitic nematodes have simple or - rarely - complex life cycles.

Ascaris lumbricoides
(the fish tapeworm)

Kind of parasite: homoxenous and monoxenous.

Host: human.

Infective stage: fully embryonated egg (infective egg).

Transmission:

• the hosts acquire the infection by the ingestion of infective eggs (containing developed larvae);
• eggs are transmitted by hand contaminated with soil or ingestion of raw vegetables fertilized with human faeces;
• the larvae hatch in duodenum, enter venuls and migrate in the circulatory system via liver and right heart to the lungs, reach the trachea and pharynx and after swallowing the larvae maturate in the small intestine.

Site of infection: lumen of the small intestine.

Diagnostic method(s):

• microscopic identification of eggs (fertilized or unfertilized) in stool specimens;

• direct visualization of adults on endoscopy (gastroscopy).

Geographical distribution: cosmopolitan.

Remarks:
Ascaris lumbricoides is a geohelminth - part of the life cycle takes please in moist, warm soil where eggs become fully embryonated.

Trichuris trichiura
(whipworm)

Kind of parasite: homoxenous and monoxenous.

Host: human.

Infective stage: fully embryonated egg (infective egg).

Transmission by the ingestion of infective eggs (containing developed larvae). After ingestion of infective eggs, larvae enter the Lieberkühn’s crypts and after a short development, reenter the intestinal lumen, migrate to the large intestine and maturate.

Site of infection: the large intestine.

Diagnostic method(s):

• microscopic identification of eggs in stool specimens;

• direct visualization of adults on endoscopy (colono- or rectoscopy).

Geographical distribution: cosmopolitan.

Remarks:
• the larvae do not migrate in blood of host organism;
• T. trichiura - is a geohelminth.

Enterobius vermicularis
(pinworm)

Kind of parasite: homoxenous and monoxenous.

Host: human.

Infective stage: eggs or larvae.

Transmission 

• ingestion - the hosts acquire the infection by the ingestion of infective eggs (containing developed larvae);
• inhalation of dust with infective eggs;
• retroinfection - juvenile larvae hatched on the skin re-enter the intestine through the anus.

Site of infection: the large intestine.

Diagnostic method(s):

• the infection is diagnosed by microscopic examination of swabs for eggs deposited on the perianal skin;
• the eggs are sampled by using cellophane strip fixed on a glass applicator stick or by tongue depressor covered with a transparent adhesive tape.

Geographical distribution: cosmopolitan.

Remarks:
• Females of E. vermicularis actively migrate down the colon and outside by the anus at night, where the eggs are deposited on the perianal skin, embryonated eggs complete the development within 6 hours and therefore retroinfection is possible.

• In female patients, during retroinfection, the larvae that hatch on the perianal skin may enter occasionally the vagina instead of the rectum.

Trichinella spp.

Kind of parasite: homoxenous and polyxenous.

Host(s): carnivorous and omnivorous animals (mammals, birds, reptiles) and human.

Infective stage: the muscle larvae (ML).

Transmission: by ingestion of infected and raw meat.

Site of infection: adult stages live in the small intestine and the larval stages live in the skeletal muscles.

Diagnostic method(s):

• serological methods;

• muscle biopsy , and next trichinoscopy and artificial digestion in 1% solution of HCl-pepsin.

Geographical distribution: Trichinella genus has cosmopolitan distribution but the species are characteristic for defined areas. Trichinella genus involves: T. spiralis, T. nativa, T. britovi, T. pseudospiralis, T. nelsoni, T. murrelli, T. papuae, T. zimbabwensis and T. patagoniensis.

Remarks:
There are two main cycles that maintain the infection: the sylvatic and the synanthropic.
• The sylvatic cycle, which also includes other Trichinella species (T. nativa, T. nelsoni, T. britovi, T. murrelli, T. zimbabwensis, T. pseudospiralis and T. papuae), primarily involves carnivores and is generally maintained by the eating of meat, including scavenging and cannibalism. This cycle plays a role in human infection in some regions; in the Arctic bear and walrus meat, in Africa bush pigs, and in the USA, northern Asia, and Europe bear meat and wild boar constitute sources of human infection;
• The synanthropic cycle, which involves the cycling of the parasite in pig with occasional offshoots to man, rats or dogs, is primarily man-related. Pigs become infected by eating garbage (pork scraps, swine offal, wild animal carcasses) or rats; infection can also spread from pig to pig when they nip off and eat each other's tails, a common practice in crowded piggeries. Infected pork is the main source of human trichinellosis. Recently, outbreaks of trichinellosis in man associated with the consumption of raw horseflesh have been reported;
• Prevention of trichinellosis consists in:
- eliminating the source of infection mainly in pigs and the destruction of the parasite in pork;
- prohibition of the feeding of raw garbage to pig should significantly reduce trichinellosis in pigs;
- inspection of pork or wild animal meat and cooking or deep-freezing any pork or other meat products are useful methods of prevention (the smoking, curing, and drying of meat are not reliable methods for the prevention of trichinellosis);
• Species identification is possible only by molecular examination.

Wuchereria bancrofti

Kind of parasite: heteroxenous and monoxenous.

Definitive host(s): human.

Intermediate host: biting insects (Culex sp., Anopheles sp., Aëdes sp.) which are the vectors of the parasite.

Infective stage: infective larvae.

Transmission by vector - the infective larvae enter the skin of human through the bite site.

Site of infection: adult stages live in the lymphatic vessels and lymph nodes; the larval stages (microfilariae) circulate in the blood.
Diagnostic method(s):
• identification of microfilariae by microscopic examination from the blood smears (during the night);

• serological methods;
• antigen detection (an immunoassay);
• molecular diagnosis using PCR is available for W. bancrofti.

Geographical distribution: endemic; W. bancrofti is encountered in tropical areas worldwide.

Remarks: the sheathed microfilariae have a nocturnal periodicity and occur in the peripheral blood of infected humans at night. Wuchereriosis is also known as elephantiasis.

Loa loa

Kind of parasite: heteroxenous and monoxenous.

Definitive host(s): human.

Intermediate host: biting insects - deerflies (Chrysops) which are the vectors of the parasite.

Infective stage: infective larvae.

Transmission by vector - the infective larvae enter the skin of human through the bite site.

Site of infection: adult stages live and migrate in the subcutaneous and deep connective tissues; the larval stages (microfilariae) circulate in the blood.

Diagnostic method(s):
• identification of microfilariae by microscopic examination from the blood smears (during the day);
• serological methods;

• antigen detection (an immunoassay);
• identification of the adults during subcutaneous biopsies or the adults removal from the eye.

Geographical distribution: endemic (Africa).

Remarks: the sheathed microfilariae have a diurnal periodicity and occur in the peripheral blood of infected humans during the day.

Onchocerca volvulus

Kind of parasite: heteroxenous and monoxenous.

Definitive host(s): human.

Intermediate host: biting insects - blackflies (Simulium) which are the vectors of the parasite.

Infective stage: infective larvae.

Transmission by vector - the infective larvae enter the skin of human through the bite site.

Site of infection: adult worms often locate under the skin where they become encapsulated, due to the host’s reaction, forming prominent nodules (the so-called onchocercomas); microfilariae can be usually found in the skin (rarely in blood, sputum or urine).

Diagnostic method(s):
• identification of microfilarial stages from bloodless skin snips;
• antigen detection (an immunoassay);
• identification of the adults from tissue samples collected during nodulectomies.

Geographical distribution: endemic (tropical Africa, Yemen, Saudi Arabia and Central and South America).

Remarks: onchocercosis is also known as river blindness; about 50 million people are infected, of whom approximately 1 million become blind.