The Good, The Bad, The Nematode

Free-living nematode from marine sediments.

Speaking of worms (see previous post), your body might be serving as housing for some right now.  The likelihood of this, of course, is related to factors such as the region in which you live, your diet, travels, hygiene and health care.  Still, there is a group of invertebrates that includes many parasitic species for which humans serve as hosts.  They are called roundworms, or nematodes.


Agreement on the number of species of nematodes is lacking.  A quick search on-line will reveal estimates from 12,000 to more than 20,000 species.  It also reveals an understanding among invertebrate taxonomists that the actual species diversity of the group is substantially higher.  Meldal et al. (2007) cite estimates of 0.1 to 1 million species; well, that helps.  If we remain incredibly conservative, we can still say that the species-level diversity of these animals is more than double that of the mammals.
Most species of nematodes are free-living and inhabit sediments in terrestrial, freshwater and marine environments.  In these sediments, nematodes can be extraordinarily abundant.  One study of forest soils in different areas in China found these worms to be present in densities ranging from several hundred to several thousand individuals per 100 grams of soil (Zhang et al., 2012).  Very similar estimates exist for sediments in other reaches of the world.


In a TED Talk in 2007, the famous naturalist and ant biologist E. O. Wilson said:
"Consider the nematode roundworm, the most abundant of all animals.  Four out of five animals on Earth are nematode worms -- if all solid materials except nematode worms were to be eliminated, you could still see the ghostly outline of most of it [Earth] in nematode worms.  About 16,000 species of nematode worms have been discovered and diagnosed by scientists; there could be hundreds of thousands of them, even millions, still unknown."

It seems that he was drawing from the a statement made by biologist Nathan Cobb in 1914:
In short, if all the matter in the universe except nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes.  The location of towns would be decipherable, since for every massing of human beings there would be a corresponding massing of certain nematodes.  Trees would still stand in ghostly rows representing our streets and highways.  The location of various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites."

Yowsa.  Can we just agree on something simple: Wherever we might be, we're surrounded by nematodes worms.  What are they doing?  As you might guess, their life cycles vary considerably.  Some nematodes feed upon plant tissues.  Some specialize on fungal cells.  Other nematodes consume bacteria.  Their activity in sediments is enormously important on a global scale when it comes to the transfer of energy and the cycling of elements through these environments.  In fact, the degree to which they modify the chemistry of sediments makes the Nematoda some of the "most valuable players" when it comes to our planet's animal fauna.

A specimen of Trichinella spiralis.

The role of nematodes as parasites probably does not hold the same weight, but they do impact a wide array of other species.  Those species of nematodes that infect plants include some groups that affect our crops.  So, I'm sure you are wondering which species pay visits to the human body.

Trichinella spiralis encysted in skeletal muscle.

Let's start with Trichinella spiralis.  There's a reason you want to cook meat such as pork well.  The larvae of this roundworm bore through the intestinal wall of a host, such as a pig, and are then carries via the circulatory and lymphatic systems to striated muscle.  These larvae then encyst in the muscle.  Should a human consume muscle with viable cysts, the larvae are released in the stomach and then mature in the small intestine in a period of 1 to 2 weeks.  The larvae then enter the person's lymph or blood vessels and encyst in muscle cells.  Of course, that's not the end of the story; the infection comes with less-than-desirable symptoms and health consequences. 

The "filarial" nematode parasites might disturb you a bit more.  Multiple species of these "thread-like" nematodes in the superfamily Filaroidea affect humans.  Consider the species Wuchereria bancrofti.  This species, and a couple of others, cause a disease known as lymphatic filariasis.  Humans become infected when an infected mosquito takes a blood meal and introduces larvae into the bloodstream, where they migrate to lymph nodes, most commonly in the legs and genital regions.  The larvae continue to develop over the course of a year and then, as adults, reproduce in the lymph vessels.  The female can produce thousands of the early-stage larvae, called microfilariae, which are released into the bloodstream.  Their presence in the bloodstream ensures that they can be ingested by any mosquito that should feed on the infected human, beginning the cycle anew.  Having an abundance of parasitic worms in your lymphatic vessels is not something that the body takes to kindly.
  

Two Haitian women with elephantiasis.

The presence of these nematodes can lead to marked inflammation of the lymph nodes and vessels and obstruction of these pathways.  The consequences range for severe fevers and chills to "elephantiasis."  The image provided here is one of the more kind.  In men, the filariasis often affects the legs and scrotum.  

A nematode infection beneath the cornea of the human eye.

If that is not enough, imagine having a subcutaneous nematode infection in you eye.  That's exactly where nematodes such as Loa loa and Onchocerca volvulus like to migrate.  These nematodes can cause blindness.  Just avoid being bitten by the African flies that are their vectors.

A lovely pile of Ascaris lumbricoides.

Better known nematodes are the intestinal parasites of the genus Ascaris.  The species A. lumbricoides causes a disease called ascariasis.  We can get these worms by eating food or drink that has been contaminated by fecal material from an infected individual.  The eggs ingested via the contaminated food then hatch in the small intestine, with larval forms burrowing through the intestinal wall and entering the bloodstream where they migrate to the lungs.  The larvae manage to work their way into the alveoli (air sacs) and into the trachea ("wind pipe"), where they are coughed up and then swallowed for a second time.  This brings some back to the gut, where they develop into adult worms.  Those people infected might not show any symptoms, depending on the number of worms, but they can cause malnutrition, respiratory illness and other complications (among them, death).

Whipworms, each about 4 cm in length.

Given improvements in technology and practice in the areas of food safety and sanitation, you are not likely to experience the infections mentioned above.  The human pinworm (Enterobius vermicularis), however, is one for which you might be a candidate.  Humans, most often children, ingest this nematode's eggs via contaminated foods or hands.  The larvae hatch in the small intestine and migrate to the colon.  Adults then lay eggs around the anus and are passed with feces.

Whipworm (Trichuris trichiura) has a very similar life cycle.  The spread of this parasite is facilitated when people defecate outside and/or when human feces is used as fertilizer.  Eggs reside in the soil and can contaminate water or foods.  Whipworm might infect up to 800 million people worldwide, most commonly in tropical countries; that's more than twice the population of the United States.

I am going to leave hookworm, yet another human-loving nematode, for you to investigate separately.  If you have a pet dog, you might opt to avoid letting him or her "kiss" your face.  Now that I have mentioned dogs, you will also want to be sure to give your pet canine a regular dose of heartworm medication.  It is not some scam on the part of veterinarians; the dog heartworm, Dirofilaria immitis, is nasty business.  As a biologist, I have seen my fair share of preserved specimens.  One of the most impressive collections was the heart of a German shepherd that was literally bursting with adult heartworms.

Of course, nematodes do infect other invertebrates as well.  I came across the interesting story of the Myrmeconema neotropicum, a roundworm that so far is only known to infect a particular species of South American ant.  When infected, the posterior-most section of the ant's abdomen enlarges and develops an appearance that resembles the red berries commonly found in the tropical forest it inhabits.  The infected ants, with their now bright red abdomens, crawl to an elevated position.  The thought is that fruit-eating birds confuse the infected ants for berries and consume them.  Researchers are examining the possibility that the birds then spread the nematode parasite via their egg-laden droppings.

From the free-living masses that writhe through sediments to the tiny ant-infesting nematodes to the 9-meter-long Placentonema gigantissima that resides in the placenta of the sperm whale, nematodes have radiated into every niche imaginable.  You probably will not see one at all on your next visit to the museum, but you might wonder if someone standing next to you is carrying a few.