General Parasitology - Human Helminth Infections
This page aims to introduce the student to the diversity of helminth infections in man, and even more importantly, to the numbers of individuals that harbour these infections in all regions of the world. There are three major groups of helminths containing members that have man as their main hosts, these being the Digenean Flukes, the Tapeworms (Cestodes), and the Roundworms (Nematodes). These are dealt with in greater detail on specific pages in this web site.
Enterobius vermicularis - Pinworm, Threadworm.
An extremely common nematode infection, particularly in temperate areas such as Western Europe and North America, (it being relatively rare in the tropics) and particularly in children. It has been estimated that the annual incidence of infection is over 200 million, this probably being a conservative figure. Samples of caucasian children in the U. S. A. and Canada have shown incidences of infection of fro 30% to 80%, with similar levels in Europe.
Ascaris lumbricoides - The Large Human Roundworm.
Again the incidence rates for this parasite are very high with > 1500 million cases of infection annualy, of which ~210 million of these cases are symptomatic (* but see below).
Trichuris trichuria - The Large Human Roundworm.
The incidence rates for this parasite are also very high, with estimates of ~1300 million cases of infection annualy, of which >133 million of these cases are symptomatic (* again but see below).
These are represented by two parasites, Necator americanus in the tropics and sub tropics worlwide and the S. E. states of the U. S. A., and Ancylostoma duodenale, again with a worldwide distribution in the tropics and sub tropics as well as the Mediterranean region. In the case of these parasites there are > 1200 million cases of hookworm infection annualy, of which ~100 million of these cases are symptomatic (* again, but see below).
Lymphatic filariasis - Elephatiasis
This disease is caused principaly by two parasites, Wuchereria bancrofti with an annual rate of infection of ~106 million cases, and Brugia malayi with an annual rate of infection of ~12.5 million. The total number of people infected with other types of lymphatic filarial worms is much smaller, at ~1.5 million cases. These lymphatic filarial worms, (along with the related filarial parasite Onchocerca volvulus, are unusual among the nematodes in that they deveope with, and are transmitted by insect vector intermediate hosts.
Onchocerca volvulus - River Blindness.
The incidence rates for this parasite are not as high as some of the previously described parasites, with an annual rate of infection of ~18 million, but due to the extreme pathology associated with this parasite, often with all adult members of affected villages losing their sight, along with severe skin conditions.
Dracanculus medinensis - Guinea Worm.
The incidence rates for this parasite are much lower, with an estimated annual rate of infection of ~100 000. This is much lower than in the recent past, when up to 50 million people were infected, and this reduction illustrates how successfull helminth control programs can be effective in reducing the disease caused by these organisms.
Other important nematode infections include Trichinella spiralis, Strongyloides stercoralis, and a number of more rare infections. Nematodes that normally infect other animals may still cause disease in man. These include Toxocara canis and a number of nematodes causing Anisakiasis.
Digenean Trematode Infections
Schistosomiasis - Bilharzia.
This disease is the focus of this parasitology web site, and in terms of morbidity and mortality is the most important human helminthiasis. The numbers infected are lower than those of many of the nematode infections, with an estimated annual incidence of infection of > 200 million cases. In terms of active disease however the parasite is much more important, with an estimated annual mortality rate of ~1 million deaths directly due to infection with these parasites.
Opisthorchis sinensis - The Chinese Liver Fluke
This is also a very important trematode infection, with an estimated annual incidence of infection of ~20 - 30 million cases, mostly in the Far East, in Japan, China, Taiwan and South East Asia.
Paragonimus sp. - The Lung Fluke
This fluke causes a pulmonary disease, the adult parasites living in the lungs of their definitive hosts (e.g. man). There are a number of different species of this parasite, the most well documented being P. westermani in the Far East. Here it may be locally very common, with up to 40 to 50% of the population infected.
There are a number of other digenean trematode infections. These include various Echinostome infections as well as a number of other flukes, described in a seperate Human fluke page within this site. In addition there are a number of these parasites that usually infect domesticated animals, but are also cause well known human infections as well. These include Fasciola hepatica and Dicrocoelium dendriticum.
Cestode (Tapeworm) Infections
Taenia saginata - The Beef Tapeworm
This only causes very limited pathology in man, but the anual incidence of infection is high, at an estimated 50 million cases.
Taenia solium - The Pork Tapeworm
This has a similar estimated annual incidence of infection of ~50 million cases. However in this case the consequences may be more severe, due to the added risk of contracting infection with the larval metacestode, (cysticercosis). This may have extreme consequences in terms of the pathology associated with infection, with an estimated annual mortality rate of ~50 000 deaths.
For the Cestodes these annual incidence rates are based on detection of infection with the adult parasite. This is achieved by examination of faeces, urine or sputum for parasite eggs . Diagnosis of infection with larval metacestode parasites, such as Echinococcus sp. is very difficult, due to the lack of non invasive diagnostic techniques. It is in consequence very difficult to estimate annual rates of infection, even though these metacestodes may be very important pathogens.