Human helminths infections, Biology tutorial


The term Helminths basically derived from the Greek which means 'worms' have outbreak humans since before the period of our most primitive recorded history. The eggs of intestinal helminths can be obtained in the mummified feces of the humans dating back thousands of years, and we can identify most of the characteristic clinical traits of helminth infections from the earliest writings of Hippocrates, Egyptian medical papyri and the Bible. These similar helminthiases markedly modified the course of modern 20th century world history, particularly in China throughout the Cold War, if the schistosome was recognized as 'the blood-fluke which saved Formosa' as acute schistosomiasis sickened Mao's troops and aborted their amphibious assault of Taiwan (historically recognized as Formosa) just long adequate for American ships to enter the Straits of Taiwan.

There are three main groups of helminths having members which have man as their main hosts, these being the Digenean Flukes, the Tapeworms (or Cestodes) and the Roundworms (or Nematodes). 

Nematode Infections:

Enterobius vermicularis - Pinworm, Threadworm

E. Vermicularis infection, a very common nematode infection, specifically in temperate regions like Western Europe and North America and mainly in children.  It has been anticipated that the annual incidence of infection is around 200 million, this probably being a conservative figure. Samples of Caucasian children in the U.S.A. and Canada have illustrated incidence of infection from 30% to 80%, with the same levels in Europe. 

Ascaris lumbricoides - The Large Human Roundworm

The incidence rates for this parasite are much high having > 1500 million cases of infection yearly, of which around 210 million cases are symptomatic. 

Trichuris trichiuria - The Large Human Roundworm

In this, the incidence rates for the parasite are as well very high, with estimates of around 1300 million cases of infection yearly, of which >133 million cases are symptomatic.

The Hookworms:

These are illustrated by two parasites, Necator americanus in the tropics and sub tropics globally and the S. E. states of the U.S.A. and Ancylostoma duodenale, again having a global distribution in the tropics and sub-tropics as well as the Mediterranean area. There are > 1200 million cases of hookworm infection yearly, of which around 100 million cases are symptomatic. 

Lymphatic filariasis - Elephantiasis

This disease is mainly caused by two parasites, Wuchereria bancrofti having an annual rate of infection of around 106 million cases and Brugia malayi having an annual rate of infection of around 12.5 million. The net number of people infected with other kinds of lymphatic filarial worms is much smaller, at around 1.5 million cases. Such lymphatic filarial worms, all along with the associated filarial parasite Onchocerca volvulus, are strange among the nematodes in that they build up with and are transmitted through insect vector intermediate hosts.

Onchocerca volvulus - River Blindness:

In this, the incidence rates for parasite are not as high as some of the earlier explained parasites, having an annual rate of infection of around 18 million, though, due to the extreme pathology related with this parasite, often with all adult members of affected villages losing their sight, all along by severe skin conditions, that is, the infection is significant. 

Dracunculus medinensis - Guinea Worm:

In this, the incidence rates for the parasite are much lower having an estimated annual rate of infection of around 100 000. This is much lower than in the recent past, when up to 50 million people were infected. This reduction in incidence describes how successful helminth control programs can be efficient in decreasing the disease caused by such organisms.

Other significant nematode infections comprise; Trichinella spiralis, Strongyloides stercoralis, and a number of much uncommon infections. Nematodes which generally infect other animals might still cause disease in the man. These comprise Toxocara canis and a number of nematodes causing anisakiasis. 

Digenean Trematode Infections:

Schistosomiasis - Bilharzia

Schistosomiasis is mainly caused through Schistosoma mansoni, S.haematobium, S.intercalatum, S. japonicum and S.mekongi. This disease is the most significant human helminthiasis in terms of mortality and morbidity. The numbers of people infected are lower than those of most of the nematode infections having an estimated annual incidence of infection of > 200 million cases. In terms of active disease though, the parasite is much more significant having an estimated annual mortality rate of around 1 million deaths directly due to infection by these parasites.

 Opisthorchis sinensis - The Chinese Liver Fluke 

This is as well a very significant Trematode infection having an estimated annual incidence of infection of around 20 - 30 million cases, mainly in the Far East, in China, Japan, Taiwan and South East Asia. 

Paragonimus spp. - The Lung Fluke

This fluke causes a pulmonary disease, that is, the adult parasites living in the lungs of their definitive hosts (example: man). There are a number of various species of this parasite; the most well documented being P. westermani in the Far East. It might though, be locally much common, with up to 40 to 50% of the population infected.

There are a number of other Digenean Trematode infections. These comprise different Echinostome infections and also a number of other flukes. Moreover, there are a number of such parasites which generally infect domesticated animals, however as well causes well known human infections as well. These comprise Fasciola hepatica and Dicrocoelium dendriticum. 

Cestode (Tapeworm) Infections:

Taenia saginata - The Beef Tapeworm:

This only causes much limited pathology in man; however the yearly incidence of infection is high, at an expected 50 million cases. 

Taenia solium - The Pork Tapeworm:

This consists of a similar estimated annual incidence of infection of around 50 million cases. Though, in this case the effects might be more severe, due to the added risk of contracting infection having the larval metacestode (cysticercosis).

This might encompass extreme effects in terms of the pathology related with infection having an estimated yearly mortality rate of around 50,000 deaths. For the cestodes, such annual incidence rates depend on detection of infection by the adult parasite. This is accomplished by examination of faeces, urine or sputum for parasite eggs. Diagnosis of infection having larval metacestode parasites, like Echinococcus sp. is much difficult, due to the lack of non-invasive diagnostic methods. It is in effect, very difficult to approximate annual rates of infection, even although such metacestode might be much significant pathogens.

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