NEMATODE INFECTIONS

Enterobius vermicularis - Pinworm, Threadworm.

     E. vermicularis infection, a widespread nematode infection, particularly in temperate areas such as Western Europe and North America, ( 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 prevalence of disease from 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 infections annually, of which about 210 million cases are symptomatic.

  Trichuris trichiuria - The Large Human Roundworm.

The incidence rates for this parasite are also very high, with estimates of about
1300 million cases of infection annually, of which >133 million cases are symptomatic.

The Hookworms.

These are represented by two parasites, Necator americanus in the tropics and subtropics worldwide 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. There are > 1200 million cases of hookworm infection annually, of which about 100 million cases are symptomatic.

Lymphatic filariasis - Elephantiasis

This disease is caused principally by two parasites, Wuchereria bancrofti with an annual rate of infection of about 106 million cases, and Brugia malayi with a yearly rate of infection of approximately 12.5 million. The total number of people infected with other types of lymphatic filarial worms is much smaller, at about 1.5 million cases. These lymphatic filarial worms, along with the related filarial parasite Onchocerca volvulus, are unusual among the nematodes in that they develop 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 about 18 million, however, 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, the infection is significant.

Dracunculus medinensis - Guinea Worm

The incidence rates for this parasite are much lower, with an estimated annual rate of infection of about 100 000. This is much lower than in the recent past when up to 50 million people were infected. This reduction in incidence illustrates how successful helminth control programs can be useful in reducing the disease caused by these organisms.


Other significant nematode infections include; Trichinella spiralis, Strongyloides stercoralis, and several more rare infections. Nematodes that usually infect other animals may still cause disease in man. These include Toxocara canis and several nematodes causing anisakiasis.

Digenean Trematode Infections 

Schistosomiasis - Bilharzia.

Schistosomiasis is caused by Schistosoma mansoni, S. haematobium, S. intercalated, S. japonicum, and S. mekongi. This disease is the most important human helminthiasis in terms of morbidity and mortality. The numbers of people 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
the mortality rate of about 1 million deaths directly due to infection with these parasites.

Opisthorchis Sinensis - The Chinese Liver Fluke

This is also a significant trematode infection, with an estimated annual incidence of infection of about 20 - 30 million cases, mostly in the Far East, in Japan, China, Taiwan, and South East Asia.

Paragonimus spp. - The Lung Fluke

This fluke causes pulmonary disease, the adult parasites living in the lungs of their definitive hosts (e.g., man). There are several different species of this parasite, the most well documented being P. westermani in the Far East. It may, however, be locally widespread, with up to 40 to 50% of the population infected.


There are several other digenean trematode infections. These include various
Echinostome infections as well as several other flukes. Also, there are
a number of these parasites that usually infect domesticated animals, but 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 annual 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 about 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 about 50,000 deaths. For the cestodes, these annual incidence rates are based on the detection of infection with the adult parasite. This is achieved by examination of feces, 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 noninvasive diagnostic techniques. It is in consequence very difficult to estimate annual rates of infection, even though these metacestodes may be very important pathogens.

SUMMARY

Parasitic helminths are grouped into nematodes, cestodes, and trematodes. The nematodes are the most diversified groups. Parasitic helminths infect a wide range of hosts, ranging between man, domestic animals, and wild animals. Morbidity is often high leading to the death of several thousand of people.