Impact of Roundworms on Human Society

 

First and foremost, roundworms are extremely important to our environment and without them the world would be quite different.  However, there is a distinct reason that most of what is currently known about nematodes pertains to parasitic species. Parasitic forms of nematodes provide direct impact on humans and human society in a variety of ways.  Just about every documented species of animal has one or more parasitic nematodes.  

        Plant parasitic nematodes cause much damage to plants and, most importantly, to agricultural crops.  Some sources including George O. Poinar’s The Natural History of Nematodes indicate that nematode plant parasites can ruin as much as 15 % of each year’s agricultural crop in the United States.  Most plants can tolerate these parasites and their damage because they have achieved a state of equilibrium as the host.  When this balance is upset, large scale damage to crops is seen.  In order to illustrate the overwhelming effect of plant parasitic nematodes in terms we can understand, it is estimated that 200 million dollars worth of wheat crops are lost every year in Australia alone to roundworms (Hodda 2004).  One suggested solution for this problem is the application of nematicides like Vydate 10G (oxamyl) to the soil in between crop rows in order to lessen nematode feeding activity early on ("In-furrow Guide..." 52).  Recently, trickle irrigation and liquid forms of nematicide have proved most effective (Abel 56).

Damage to a canola crop due to plant parasitic nematodes.  Picture obtained from M. Hodda; http://www.ento.csiro.au/science/nematodes/introduction.html

 

        Roundworms that inhabit animals and most importantly humans as parasites can be extremely detrimental to the health of their hosts.  Often, host immunity is altered by proteins secreted by the parasite, allowing the parasite to thrive inside of the host (Harcus et al. 39).  According to the Raven and Johnson Biology textbook, there are around fifty species of nematodes that are normal human parasites (Raven et al. 746).

 

Hookworms- These roundworms are found in warm regions and burrow through skin they come in contact with.  Their hook shaped anterior ends allow them to latch onto the intestinal wall of their hosts and feed on blood.  They routinely suck more blood from the intestinal wall than they need and this can lead to anemia in the host (Raven et al. 746).

 

Pinworms (Enterobius)- These nematodes are a less serious parasite that inhabit the intestinal tract of humans in the same way that hookworms do.  The often cause irritation and disturbances in sleep (Kucik 4).  Despite being very widespread and affecting as much as 30% of children and 16% of adults, pinworms can be easily controlled by drugs (Raven et al. 746).

 

Ascaris (Intestinal Roundworm)-  These nematodes are found exclusively in humans and are present mostly in unsanitary areas without modern plumbing.  Around one in six people worldwide are infected by Ascaris (Raven et al. 746).  A study conducted in Cameroon focused on school children and found that 65.5 % were infected with Ascaris parasitic roundworms.  Ascaris eggs are spread through feces, and, when ingested, hatch and bore through the intestinal wall.  They move to the heart, the lungs, and finally out the breathing passages where they are swallowed, thus perpetuating the cycle.  Females lay hundreds of thousands of eggs each day and can reach nearly 30 centimeters in length (Raven et al. 746).

 

This is an example of a parasitic roundworm, Ascaris, in a young child.  Image with permission of Dr. Tietjen at Bellarmine University from http://cas.bellarmine.edu/tietjen/images/phylum_nematoda.htm

 

Trichinella-  This nematode is responsible for the most serious roundworm-caused disease, which is known as trichinosis.  They live in pigs’ intestines and produce young that make their way to muscle tissue and form tough cysts.  Interestingly, nurse cells that aid in parasite growth in the cyst contain two different types of cytoplasm ("Microbiology..." 74).  When uncooked pork is ingested by humans, the roundworms cause trichinosis, which can sometimes be fatal.  This parasite is also found in bears.  In the United States it is suspected that almost two and a half percent of the population has are infected with trichinosis (Raven et al. 746).

                    

 Left: Male and Female Trichinella roundworms.  Right:  Trichinella roundworms encysted in pork muscle tissue.  Images courtesy of BIODIDAC.  http://biodidac.bio.uottawa.ca/thumbnails/catquery.htm?Kingdom=Animalia&phylum=Nematoda

Filaria-  This type of roundworm causes serious problems in the tropics where it can lead to filiarisis, a condition that affects some 250 million people across the globe.  In extreme cases, filiarisis can lead to elephantiasis, which is a grotesque swelling of the legs and other extremities due to worms blocking the lymphatic system.  In areas that are abundantly populated with these nematodes, settlement is all but impossible (Raven et al. 746).  In developing countries like Sri Lanka where filariasis is most common, morbidity control clinics play a key role in disseminating information about treatment and prevention (Chandrasena et al. 6).

 

Elephantiasis resulting from a filarial parasitic nematode.  Image with permission of Dr. Tietjen at Bellarmine University from http://cas.bellarmine.edu/tietjen/images/phylum_nematoda.htm

        According to T.L Heitman and his colleagues, studies conducted on mice have shown that several factors affect the transmission and abundance of parasitic nematodes.  Specifically, caloric intake and energy play a huge role in a host's ability to combat parasites and utilize immunity.  Hosts that are deficient in protein and other essentials for good health are much more susceptible to parasitic nematodes, because they are less able to fight their transmission (Heitman et al. 1767).

        As a result of all the study of nematode parasites, considerable progress has been made in preventing infections and curing them when they occur.  For example, universal insecticides known as Bacillus thuringiensis (Bt) crystal proteins have recently been found to affect many free-living nematodes as well as a species of parasites in rats.  This kind of crystal protein is expected to prove effective in controlling most parasitic nematodes in vertebrates (Wei et al. 2760).

 

 

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Evolution & Phylogenetic Relationships

Features Shared by Nematodes with Related Groups

Features Unique to Nematodes

World Distribution of Roundworms

Nematode Habitat Use

Energy/Modes of Nutrition

Ecological Roles of Nematodes

Impacts on Human Society

References Page