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Subject 11 (Labours, 1st year DDS)


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Helminths

in general, it is a group of a number of phyla. From a medical point of view among all the helminths some important phyla can be distinguished:
Platyhelminthes - with classes Trematoda (flukes) and Cestoda (tapeworms), and
Nematoda (nematodes).

Trichinella spp.

Kind of parasite: homoxenous and polyxenous.

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

Infective stage: the muscle larvae.

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 (trichinoscopy and artificial digestion in1% 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.

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., AŽdes 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;
• 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 during the night.

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;
• 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.

 Author: Piotr Nowosad date: 2020-05-19  print    back  
 
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