Many nematodes are parasitic, that is they need other creatures or plants as hosts for survival.
Nearly every main animal group has a parasitic nematode. Some parasites are harmful while some
hardly have any serious effect on the host.
Typically, nematodes compete with their host for nutrients. They can proliferate into numbers
that block the flow of nutrients and fluids. Nematodes can block the lymphatic system with can
cause dangerous swellings.
Because of the wide variety of parasitic nematodes, we will only explore the following examples to give you a brief look.
Intestinal parasites are one of the most common world-wide infections. Hookworm is an intestinal
parasite of humans. They are from the order Strongiloidae.
Both are found in tropical and subtropical climates.
They also have a rather complex life cycle as depicted below.
Two common species of hookworm include:
Found in southern Europe, northern Africa, northern Asia and parts of South America 1
Found in southeastern parts of the United States 1
"Eggs are passed in the stool (1), and under favorable conditions (moisture, warmth, shade),
larvae hatch in 1 to 2 days. The released rhabditiform larvae grow in the feces and/or the
soil (2), and after 5 to 10 days (and two molts) they become filariform (third stage) larvae
that are infective (3). These infective larvae can survive 3 to 4 weeks in favorable environmental
conditions. On contact with the human host, the larvae penetrate the skin and are carried through
the veins to the heart and then to the lungs. They penetrate into the pulmonary alveoli, ascend the
bronchial tree to the pharynx, and are swallowed (4). The larvae reach the small intestine, where
they reside and mature into adults. Adult worms live in the lumen of the small intestine, where they
attach to the intestinal wall with resultant blood loss by the host (5). Most adult worms are
eliminated in 1 to 2 years, but longevity records can reach several years. Some A. duodenale larvae,
following penetration of the host skin, can become dormant (in the intestine or muscle). In addition,
infection by A. duodenale may probably also occur by the oral and transmammary route.
N. americanus, however, requires a transpulmonary migration phase." 2
Infection is due to having direct contact with or accidentally swallowing contaminated soil.
This would entail walking or playing barefoot in the contaminated area. Itching and a rash
can occur on the foot at the site of entry. Children are often at high risks because they
like to play in the dirt. Hookworm cannot be transmitted from person to person. Transmission
of hookworm requires development of the larvae in the soil. Once the hookworm is in the human
host and in the intestines, its hooks latch onto the wall of the small intestines so that it
can feed on the hostís blood. Thus earning them the nickname, bloodsucking nematodes.
Light infection can have no real symptoms. Heavy infections can cause anemia, abdominal pain,
diarrhea, loss of appetite, and weight loss 1. Side effects of these causes include tiredness,
difficulty in breathing, enlargement of the heart, and irregular heartbeat. Heavy hookworm
infections in young children can lead to stunted growth and mental development. This occurs
because the heavy infections can affect their bodies at that critical growing age. Such heavy
infections can be serious to a personís health. The development of anemia and protein deficiency
caused by such blood loss is very serious.
Hookworm infection can be diagnosed by the presence of hookworm eggs in a stool sample. Treatment
is available. "In countries where hookworm is common and reinfection is likely, light infections
are often not treated. In the United States, hookworm infections are generally treated for 1 to 3
days with albendazole (this drug is approved by the FDA, but considered investigational for this
Plant parasitic nematodes are obligate parasites, meaning that they must feed on plant tissue at
some point in their life to complete their life cycle. Their feeding structures are a bit different
so that they are more efficient for feeding on plant material. There are generally two different
categories of plant parasitic nematodes: Ectoparasites, who feed from the outside of the plant
tissue or endoparasites, who enter the plant tissue to feed and thrive. 1
Plant parasitic nematodes can damage plants in a number of ways. Their feeding can cause open wounds
on the plant, exposing it to other pathogens. They can also create vascular tissue damage that affects
the transport of water and nutrients to the plant. Both can cause great harm to the
plantís health. Some can even produce metabolites that will kill the host tissue. Either way, the
plantís energy is being spent on supporting the nematode and not itself. 2
Meloidogyne is the scientific name for the root-knot nematode and are considered to be endoparasites.
Meloidogyne can be found world-wide. The root-knot nematodes are different from most plant nematode
because their female become swollen and lose their worm shape (vermiform). Generally speaking,
the root-knot nematodes change the root structures to form small knots on the infected root, hence
earning their nickname: root-knot nematode.
Meloidogyne enter the roots of plants as juveniles and settle down in the root cells to make home.
They swell up as they continue development. The hormones that they give off create the swollen plant
areas called galls or root knots. Males leave the root when they reach adulthood while the females
stay to fatten up. The female Meloidogyne lay her eggs in the soil and thus the cycle begins again.
The image to the right displays a classic example of an infected carrot root by Meloidogyne.
As you can see, its root system is weak, stunted, and deformed. In cases as such, the damage
is permanent, rendering plants like potato tubers and carrot taproots inedible. 2
Corn is one crop that Meloidogyne targets. "There are at least six Meloidogyne species which affect
corn roots" 3. Corn infection can include stunting, chlorosis, root galls, and proliferation
of fibrous roots. 3
Some control is available through chemical fumigants and granular nematicides.
Nematodes that reside in insects are called entomopathogenic. The results often kill or damage
the development of the insect host, which can be useful as a biological control agent. The nematode
order Mermithida parasitizes invertebrates, particularly insects. The Mermithida adult is
free-living but it is their larval stage that infects the host insect. This order of nematode has
seen to be valuable as a biological control agent for blackflies and mosquitoes. 4
The species Romanomermis culicivorax has been the source for these laboratory studies on beneficial
entomopathogenics. This species is naturally found in the southern part of the United States.
Currently, all information about the use of mermithids for the control of insects has been tested
by studies with R. culicivorax.
Usually, the mermithid method of parasitizing involves entry into the larval insect host as a
preparasitic stage. The mode of entry is through cuticular penetration, which paralyzes the host
temporarily as the nematode sets up home. After entering the host, they being to mature into the
juvenile stage. The juvenile gets its nourishment from the host for development. When the
mermithid is ready to leave and morph into the adult stage, they emerge by rupturing a hole in the
hostís cuticle (usually through the intersegmental membranes). This rupturing often kill the
insect host, but some adult insects can survive the parasite emergence. 4
Mermithids seem to be the ideal biological control agent because they target specific host insects,
they end up killing the host, and they are reasonably easy to control and grow in the laboratory
setting. Most of all, they do not seem to present any environmental threat and are relatively safe
for nontarget organisms (including humans). 5
As ideal as this entomopathogenic, Mermithida sounds, it has yet to prove the whole theory as a good
biological control agent out in the field. Studies in the laboratory and in some field tests have shown
promising results but further testing is still needed.