The filarial nematodes are a group of worms which
are found in the blood and tissue of the host. Normally, filarial nematodes are reliant upon an arthropod vector
(eg. a biting insect) to complete the life-cycle and transmit
them from person to person. The larval filarial nematodes are known as microfilariae
(MY-crow-fill-AIR-ee-ee) and are found in the blood or in the tissue beneath the skin.
This facilitates transmission by the insect when it comes for its next feed. The microfilariae of
each species are characteristic and blood films may be used to diagnose the infection. The adults
may be found in a variety of different tissues. The two main culprits for elephantiasis are
Wuchereria bancrofti (wook-er-RARE-ee-ah ban-CROFT-ee) and Brugia
malayi (BREW-shee-ah mah-LAY-eye). These worms are transmitted by mosquitos
and are found throughout the South-Pacific and Southeast Asia, respectively. Wuchereria
is particularly interesting, as its microfilariae show nocturnal periodicity - the larval worms move
to the capillaries at the surface of the skin at night time, which is the time when their mosquito
vector is most active. No-one knows for sure how the worms know it's night time, although it
probably has something to do with changes in chemical signals from the host.
Filarial Nematodes
Elephantiasis-Producing Filarial Nematodes
The adults of some filarial nematodes lodge in the lymph nodes which sit at the top of the limbs.
The presence of the worm, plus the reaction of the host's tissue to it, causes a blockage in the
lymphatic channels, resulting in fluid retention in the limbs and extremities. If left for long
enough, the body produces tissue in the swollen extremities, producing oversized arms, legs, and
even genitals. This condition is known as elephantiasis (elephant-EYE-ah-siss). This is
not to be confused with the condition from which John Merrick "The Elephant Man" suffered.
This condition was called elephantitis, which was easier to say than neurofibromatosis (it has
recently be suggested that Merrick suffered from neither elephantiasis nor elephantitis, but a rarer
congenital condition called proteus syndrome).
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There are a number of treatments available for these forms of filariasis, although it is best to catch it before elephantiasis sets in. New therapies (eg. drug therapy combined with massage and physiotherapy) has been recently shown to be capable of reversing all but the most severe cases of elephantiasis.
Aside from the elephantiasis producing filarial nematodes,
there are some species which prove to be a little tougher to find. Onchocerca
volvulus (on-kah-CER-kah VOL-view-luss) is a nematode found in Africa. The
adults form "tumours" of tissue under the skin, and the larvae are found in the top layers of the
dermis, rather than the peripheral blood. Adult worms will wander around the body before they
settle down, and one of the places where they may causes problems is in the eye.
Onchocerca may be seen migrating across the eye (by both the victim and outside
observers) and if left untreated, this may lead to permanent damage and blindness.
Onchocerca is transmitted by the bite of the blackfy, Simulium
(sy-MULE-ee-um), a small insect whose larvae need fast flowing water to survive
(if placed in standing water, they will continue to work against the non-existent current and pull
themselves out of the water !). Therefore, cases of Onchocerca tend to be found next to
bodies of moving water like rivers and streams, giving the name to the common manifestation
of this infection : river blindness. Before treatment programs began, there were some villages in
endemic regions in which there was not one person past the age of puberty who could see - the
children of the village spent their days leading the adults around.
Another worm that likes to wander is Loa loa (LOW-ah LOW-ah). In fact,
Loa loa never settles down, creating transitory calabar swellings which last for a couple
of days as it burrows through the subcutaneous tissues. Loa loa is another worm which
may wander across the eye. It is mostly restricted to Africa, and is transmitted by the bite of the
blood sucking fly Chrysops (CRY-sops).
Go into any veterinary clinic in the northern parts of Australia and you will be assured to find
at least one bottle with a dog's heart afflicted by heartworm. In fact, 15-20 years ago, most of the
local dogs who weren't killed in accidents were likely to die from having this worm. Dirofilaria
immitis (DIE-row-fill-AIR-ee-ah IM- it-iss) is a filarial worm transmitted by
mosquitos. The microfilariae are found in the peripheral blood, while the adults are found in the
chambers of the heart. Eventually, the strain of pumping blood past the worms becomes too
much and cardiac failure ensues. Treating the infection was frequently dangerous - if the drugs
(which used to contain arsenic) didn't kill the dog, then strokes caused by floating pieces of the
dead worm could also cause problems. Heartworm is now rarely seen, thanks to mosquito control
programs and the widespread use of preventative drugs. If a human is bitten by a mosquito
carrying dog heartworm, the worms cannot continue their life- cycle, but frequently become
lodged in the lungs. The body lays down tissue to surround and kill the worm and, although the
mass is not dangerous in itself, it may resemble lung cancer on an X-ray. People have had large
portions of their lungs removed only to find out that the malignancy was a harmless worm.
Images of the following Filarial Nematodes are contained in WormLearn
: How Safe is Australian Pork ?
Trichinella spiralis has yet to be found in Australian pigs - even the feral pigs - and
so Australia is believed to be free of this parasite. There is a related species, Trichinella
pseudospiralis, which cycles between carnivorous marsupials and carrion eating birds. There
has only been one reported case of T. pseudospiralis in humans.
There are a number of religions which prohibit the consumption of pork. Whilst there are
undoubtably good theological reasons for this, there are also sound parasitological reasons. One
of these is Trichinella
spiralis (try-kin-ELL-ah spih-RAH-liss), a nematode found in the flesh of pigs
and a range of other omnivorous and carnivorous animals. The adults of Trichinella are
small (1-2mm) and live in the small intestine. The females lay live larvae which penetrate
through the walls of the intestine and enter the blood stream. Eventually they settle in the muscle
and form dormant cysts. If the flesh of that animal is eaten by another, the larval worms will
break out of the muscle and set up home in the intestine of the new host. In humans, because we
are unlikely to be eaten by other animals, the life-cycle comes to a bit of a dead end - the body
eventually kills the encysted worms and lays down calcium salts in their place. This is like
having tiny grains of sand rubbing about between your muscles and can cause long term muscle
pain. Trichinosis is difficult to treat because the worms are in the tissue (most worming drugs
stay in the intestine - they are not absorbed by our bodies), but prevention can be achieved by
ensuring that the pork is properly cooked.
Parasitology and the History of Medicine
Parasitologists have a theory regarding the origins of the recognised symbol for medicine and
surgery - the caduceus. In biblical times (and before), infection by Guinea Worm was widespread
throughout the Mediterranean and Middle East. It is likely that the surgeons and other health
professionals were kept busy removing this particular worm. Since the majority of people in
those days were illiterate, it would make sense that a professional advertised their business by
supplying a picture of what they did - cobblers showed images of shoes, smiths a forge, and
surgeons had pictures of worms being wrapped around sticks. Well, it's a good story anyway.
Of all the nasty tropical diseases, none strikes fear into the hearts and minds of those who
heard about it as the Guinea Worm, Dracunculis medinensis (drack-UN- cue-liss
med-ee-NEN-siss). Guinea worm is found throughout Africa the Mediterranean and the
subcontinent, and is currently the focus of the World Health Organisation's eradication program,
after their success with smallpox. The adults females of Dracunculis grow to a whopping 1m in
length, although they are quite thin. Humans contract the infection by eating water contaminated
with minute crustaceans (water fleas) infected by the
larval worms. The larvae penetrate through the wall of the gut and wander out into the abdominal
cavity, where they start to grow. Usually, they will grow down and extremity (commonly a leg).
When the female reaches full size, she produces an ulcer on the end of the extremity (eg. foot or
ankle) and sticks here head out through it, laying her larvae directly into the next body of water
the sufferer walks through. In fact, the life-cycle of this worm is perfectly timed so that worms
contracted in one rainy season will be mature and producing larvae by the next rainy season. The
name Dracunculis literally means "fiery serpent" or "dragon" - the inflammation caused
by the presence of the worm is excruciating. The most effective way of treating the infection is
the ancient method of removal - start winding the worm onto a stick nice and slowly (about half
a turn per day). Any faster raises the risk of the worm breaking and causing a massive
inflammatory reaction. If the does manage to kill the worm, it will lay down insoluble calcium
salts in its place, leaving a body not unlike an umbrella spine made of chalk in the soft tissues
of the limb.
Images of
Trichinella spiralis are contained in WormLearn
In evolutionary terms, parasitic species learn to get along with their hosts over time. This is
in the best interests of a parasite, as some literally cannot survive without their hosts. A parasite
which kills its host before the parasite is ready to reproduce is likely to become extinct very
quickly. A dead host is of absolutely no use to a parasite.
Therefore it follows that parasites which haven't had the chance to get used to fooling our
immune system might not know how to behave when they enter our bodies. The great majority
of unfamiliar parasites simply cannot survive in our bodies, however, the few that do can cause
significant problems. A disease in humans caused by a parasite which has another animal as its
normal host is called a zoonosis
(zo-oh-KNOW-siss).
Dogs and cats have their own species of
hookworms and roundworms which have similar life-cycles
to the human species. If a human swallows the eggs of the dog or cat roundworm,
Toxocara sp. (TOX-oh-CAR-ah), the larval nematodes get lost while wandering
through the body. As the worm wanders the host lays down inflammatory tissue in its path, which
may damage the function of the organ through which the worm is migrating. This condition is
known as visceral larva migrans. If the worm migrates through sensitive parts of the body (eg.
the eye or brain), serious damage may occur. If humans come into contact with the larvae of the
dog hookworm Ancylostoma caninum (an-sill-OSS-tome-ah kay-NINE-um), the
larvae will try to penetrate through our skin, but get lost halfway through. Instead of entering the
blood, the worm starts moving sideways, leaving a trail like a roadmap on the skin. This is known
as cutaneous larva migrans. Both of these conditions are good reasons to ensure that pet dogs are
regularly wormed, particularly if children are around. As the common name suggests,
Angiostrongylus cantonensis (AN-jee-oh-STRON-jih-luss
CAN-ton-NEN-siss) is normally a parasite of rats. The worm cycles between snails and rats,
spending time in both the brain and the lung of the latter. Humans can contract the infection by
accidentally eating snails (watch that salad !) or even eating salad vegetables over which a
heavily infected snail has crawled. The worms get as far as the human brain, where they stop and
generate a meningitis. In dogs, the effects may be more serious : the worm may stop anywhere
along the spinal cord, and where it does decides the amount of paralysis for the animal
involved. Anisakis (an-iss-ARK-iss) is usually found in large marine mammals (whales,
dolphins, seals, etc), who contract the infection from eating large ocean dwelling fish. If humans
eat the undercooked flesh of these fish (sashimi anyone ?), the worms can cause considerable
irritation and inflammation of the stomach and intestine. It has recently been shown that the
reaction is so strong that after an initial exposure people may suffer allergic reactions to the
presence of the dead worm in cooked fish.
Wuchereria bancrofti
Onchocerca volvulus
Dirofilaria immitis
Guinea Worm
Zoonotic nematodes
Visceral and Cutaneous Larva Migrans
The Rat Lungworm
Anisakis
Angiostrongylus cantonensis
Capillaria hepatica
This page Copyright 1997 Dr Peter Darben.
Last Updated 31.1.2002
