Nematodes (Roundworms): Classification and Characteristics

For instance, Baylisascaris procyonis infects raccoons, B. columnaris infects skunks, B. melis infects badgers, B. devosi infects martens and fishes. Also B. schroederi infects giant pandas, B. transfuga infects bears, and B. tasmaniensis infects Tasmanian devils, native cats, and quolls.

Baylisaascaris spp. infection occurs when the eggs are ingested directly or the larvae are ingested in a small mammal (intermediate host). When an animal swallows the eggs, the larvae hatch in the intestine, invading the intestinal wall. If in a definitive host, they develop for some weeks and later enter the intestinal lumen where they mature, mate, and produce eggs.

These eggs are then carried out in the fecal stream. For example, an infected raccoon ( the female worm produces about 179 000 eggs in a day) may shed millions of eggs in a day in their feces. The young raccoon may ingest infective eggs and become infected. Whereas older raccoons get infected from third-stage larvae in intermediate (mostly rodents) hosts.

On the other hand,  If the larvae are in a paratenic host they will break into the bloodstream and enter several organs. Most especially, the central nervous system causing great damage. The body tries to respond to the attack by attempting to kill it. In order to escape, the larvae move rapidly in search of the liver, spinal cord, eyes, or brain. At times the larvae can be found in the lungs, heart, and other organs. Later on, the larvae die and the body reabsorbs them.

The animal will not show symptoms of disease If the larvae do not cause any significant damage to important organs of the animal. But if the larvae destroy part of the brain of the host and cause behavioral changes, the host becomes easy prey, bringing the larvae into the new host’s intestine. In small species like mice, it may only take 1 or 2 larvae in the brain of the mice to cause death.

Haemonchus contortus

This nematode is one of the most common pathogens in sheep. It is also known as the barber’s pole worm. The barber’s worm infection is called haemonchosuis. Haemonchus feed on blood and causes anemia, resultant edema, hypoproteinemia, and sometimes death. They are usually found in the abomasums of ruminant animals. Haemonchosis causes huge economic loss for farmers globally. Most especially for farmers in warmer climates.

The animals get infected from ingesting highly infested pastures that contain numerous third-stage larvae. The young sheep ( lamb) are usually susceptible. When the ruminant animals graze, they ingest the Haemonchus larvae on grasses that enter the abomasum. There they may lie inactive within the gastric glands. Once they develop into an adult, they enter into the abomasal surface and attach themselves with a buccal tooth. Then pass eggs in the feces, however,  completing its life cycle.

Anthelmintics are used to treat and prevent this nematode infection and other roundworm infections. Although the resistance of this parasitic nematode against this chemical is growing. However, some ruminant breeds like the  N’Dama cattle and the West African dwarf goat are more resistant to the parasite than other breeds.

Entomopathogenic nematodes

This group of nematodes parasitizes insects.  This roundworm is soft-bodied and non-segmented. They are endoparasitic as they live parasitically inside the infected insect host. Entomopathogenic nematodes are highly diverse, specialized, and complex. They infect many soil insects like the larval form of butterflies, moths, beetles, and flies. Also the adult forms of grasshoppers, beetles, and crickets. They locate their host insects in response to carbon dioxide, other chemical cues, and vibration.

Some entomopathogenic nematodes are used in the biological control of harmful insects and these are the species most studied. Members of Heterorhabditidae and Steinernematidae are such examples. They are the only insect parasitic nematodes that possess an optimal balance of biological control features. The species in these two families (Heterorhabditidae and Steinernematidae) fit perfectly into integrated pest management programs. Most especially because they are seen to be non-toxic to humans.

The juvenile stage of the entomopathogenic nematodes penetrates the host insects through the mouth, spiracles, or anus. In some other species of insects, it penetrates through the intersegmental membranes of the cuticle. After penetrating it enters into the hemocoel. The juvenile stage then releases cells of their symbiotic bacteria from their intestine into the hemocoel of the insects. The bacteria eventually multiply in the hemolymph of the insects causing death within 24-48 hrs. The insect cadaver is usually red if killed by Heterorhabditids and then brown if killed by Steinernematids. The color is due to the pigments produced by the monoculture of the mutualistic bacteria that grows in the host.

Even after the death of the host insects, these nematodes continue to feed on the tissue of the host insects. Hence they mature and reproduce. One or more generations of the nematode may be produced within the host cadaver depending on the resources available. A large number of the infective juvenile produced are then released into the environment to continue their life cycle and infect other insects.

However, reproduction in Steinernematid nematodes and Heterorhabditid nematodes differs. The infective juvenile of Heterorhabditid nematodes eventually become hermaphroditic adults. Although individuals of the next generation produce both females and males. Whereas In Steinernematid nematodes, males and females produce all generations.

Myrmeconema neotropicum

Myrmeconema neotropicum is a tetradonematid nematode parasite known to infect a South American ant (Cephalotes atratus). The South American ant is the only host currently known to be infected by this new species of nematode. The ant has a black abdomen but upon infection, the gaster of the abdomen becomes red, resembling one of the several red berries found in a tropical forest. This nematode induces fruit mimicry.

This nematode’s life cycle begins when a bird eats an infected ant. The nematode eggs are defecated out upon passing through the bird’s digestive system. The defecated eggs are then picked up and fed to the ant larvae( immature ant) by the ant. Once the eggs are inside the larvae’s gut, they migrate to the gaster. It is in the gaster of the ant that they will mature fully.

The mature nematodes begin to reproduce inside the gaster, once the ant larvae pupate. The males of the nematode expire soon after mating whereas the females hold the eggs within themselves. When the ant grows into a young adult, you can easily see the red embryo because the gaster begins to become translucent. The infected ant however travels outside resembling small redberries. Hence being that red berries are bird’s favorite food, the ants are confused for a berry and eaten by birds. Thus starting the whole cycle of Myrrmeconema over again.

Examples of Nematodes in Plants

The nematodes that are parasites of plants are called Eelworms.

Aphelenchoides (Foliar nematodes)

Aphelenchoides are plant-feeding nematodes and are known as Foliar nematodes. They are fungivorus and some species are known to be facultative plant parasites feeding on live plant tissues.

The foliar nematodes infest the aerial portions of the plants, unlike other plant-parasitic nematodes that dwell strictly in soil and roots. The damage they cause to the plants can reduce the yield in food crops and spoil the marketability and appearance of the plants.

Foliar nematodes infest varieties of plants: Clubmosses, ferns, liverworts, angiosperms, gymnosperms, monocots, and dicots.

Globodera ( Potato cyst nematodes)

Globodera rostochiensis is a plant pathogenic nematode that infects tomatoes and potatoes. This nematode is commonly known as the golden eelworm, yellow potato cyst nematode, or golden nematode.

The nematode infects plants that belong to the family; Solanaceae. They form cysts on the root of the plants. The cysts are yellow in color and are composed of dead nematodes which are usually formed for the female egg’s protection.

When the golden nematode infects the plant, it causes poor growth, wilting, and chlorosis. Further heavy infestation of the nematode can result in water stress, reduced root systems, and deficiency of nutrients. Whereas an indirect infestation may cause an increased vulnerability to fungal infections and premature senescence in the crop.

However, an infestation may take many years to develop and can likely go between 5-7 years unnoticed. The plants may not show unique symptoms during an infestation, hence identifying the parasite is done through soil sample testing. After detecting the nematode, it may take as long as 30 years for the parasite to be effectively and completely eradicated.

Heterodera ( Soybean cyst nematode)

Heterodera glycines is a common nematode known to infest soybean and is known as the soybean cyst nematode. It is a major soybean pest in semi-arid areas of the USA and other places like Brazil, Argentina, Colombia, Egypt, China, Iran,  Japan, Iran,  Korea, Paraguay, and Indonesia.

The eggs of this nematode are usually retained in a cyst formed from the dead female’s cuticle. The infective juvenile in the second stage is found within plant roots and in soil. The mature female is a semi-endoparasite of plant roots and the adult males may be found in the soil.

The yellow dwarf disease of soybean is caused by H. glycines. The infestation causes stunted growth in the plant and can be combined with chlorosis. The infected plants may have yellowed foliage, reduced Rhizobium nodulation, and few lateral roots. The crop yield eventually gets reduced and in worst cases, the plants die.

Meloidogyne (Root-knot nematodes)

Meloidogyne commonly called the root-knot nematode is a pathogenic nematode that attacks the roots of various shrubs, trees, and herbaceous plants. They strive in soil and areas with short winters or hot climates. A lot of plants are susceptible to root-knot nematode infection. These nematodes cause an estimated 5% of global crop loss.

The nematode larvae infect the roots of plants and cause the development of root-knot galls. These galls drain the plant’s nutrients and photosynthate. Young plants infected by the root-knot nematode may die and the infection causes reduced yield in matures plants. Plants that are affected appear discolored, lack vigor, and wilt under stress. There is stunted growth in the plants whose foliage turns yellow and wilts regardless of the adequate soil moisture.

As egg masses, these nematodes survive in soil and in plant debris as infective juveniles. They can be carried by irrigation water to crops. The root-knot nematode at second-stage juvenile infects the plants. This juvenile settles down in the young roots to feed. They later reach maturity after 1-2 months. The female adults are about 0.5-1.0 mm in length and are pear-shaped and translucent. However, they can be found within the galled tissue. Usually, they attach themselves to a gelatinous sac that contains lots of eggs.

The development of the parasite in some cases is parthenogenic. In others, before eggs are laid, the tiny worm-like male mates with the female. The first stage juvenile then develops within the eggs. Then, the juveniles break free at the second stage and either migrate inside the root or escape into the soil to commence feeding some other place. It’s been observed that these nematodes in dry conditions survive not more than a few weeks but can survive for about 3 months in moist soil.

Nacobbus

Nacobbus is a plant-parasitic nematode that attacks crops like tomatoes, sugarbeet, potatoes, and quinoa. There are 3 species of the genus Nacobbus: N. aberrans, N. dorsalis, N. bolivianus. These nematodes are prevalent in South and North America. N. abberans is a common pest of potatoes in South America and of sugarbeet in North America while N. dorsalis is somewhat rare and seen mainly in some locations in California.

The juvenile Naccobus is a migratory endoparasite that penetrates the roots tips or axial roots of the plants. This penetration causes slight swelling on the roots at their axis and tip. Hence inducing the formation of enlarged cells and root galls.

However, the swellings on sugarbeets may extend over a huge portion of the root axis, and the potato roots after being invaded by the juvenile may exhibit lesions with discolored tissues.

Pratylenchus (Lesion nematodes)

Pratylenchus is commonly known as lesion nematodes. They are plant-parasitic and are known for the root-lesion disease in most crops like soybeans, corn, banana, wheat, and potatoes. However, Pratylenchus is mostly found in corn. These nematodes are migratory endoparasites. They move around, feed, and reproduce in the root of the host plants, feeding mainly on the cortex of the host root.

In many species of Pratylenchus, males are rare. However, the female root-lesion nematode lays single eggs in the root of the plant or in the soil. They usually reproduce through parthenogenesis, which involves laying eggs without fertilization.

Necrotic lesions of the host plant roots are a major symptom of the disease caused by the nematode. A secondary infection can also occur as the lesion may serve as an entrance for pathogenic fungi and bacteria to cause another infection.

The infected plant becomes stunted in growth. Also, the crop becomes chlorotic and wilted. In most cases, the crop dies. Hence the root-lesion nematode causes a significant yield loss.

Trichodorus obtusus

Trichodorus obtusus is a plant-parasitic nematode commonly called stubby root nematodes. They are named so because they feed and cause a stunted and stubby appearing root system in the host plant. The stubby-root nematode is an ectoparasitic nematode that feeds on the meristem cells of the plant root tip as their bodies remain in the soil.

These nematodes attack turf grasses and other plants like bermudagrass, zoysiagrass, tomato, Sideoats grama, eucalyptus, Saw palmetto, potato, Kentucky bluegrass, rhododendron, big bluestem, seashore paspalum, sorghum-sudangrass, sweetbay magnolia, little leaf linden, and St. Augustine grass.

Stubby root nematodes are characterized by possessing a six-layer cuticle and are unique among other nematodes for having an onchiostyle, a curved solid stylet, or spear which they use for feeding. They make use of the onchiostyle like a dagger to puncture holes in plant cells whereas other plant-parasitic nematodes possess straight, hollow stylets.

After puncturing the plant cells, the nematode then secrets salivary material from its mouth into the punctured plant cell. This salivary material hardens and forms a feeding tube. The nematode then uses the feeding tube as a straw to withdraw and ingest the cell contents of the host plant. After feeding on an individual cell, the nematode moves on to other cells to feed.

They leave the old feeding tube behind and form new ones in each cell that they feed on. The stubby root nematode eventually causes stunted roots and also transmits viruses. The Common viruses these nematodes transmit are tobravirus, pea early-browning virus, tobacco rattle virus, and pepper ringspot virus.

Trichodorus obtusus have to mate in order to produce offsprings. However, the female Stubby root nematodes lay eggs after mating. The eggs remain in the soil until they hatch. The eggs hatch as second-stage juveniles. As they must feed on plants to reproduce and survive. When they locate a root and commence feeding, the juvenile nematode will molt 3 times before becoming an egg-laying adult nematode.

The nematode damages the root system of its host plants making it prone to environmental stresses. This can lead to an increase in the use of water and fertilizer inputs. It also makes the plants less competitive with weeds leading to an increase in the use of herbicides.

For instance, the damage caused by Trichodorus obtusus on turf grasses usually occurs in irregularly shaped patches within a given area. The turfgrass may wilt in those areas, thin out, and eventually, die if drought occurs. The symptoms however are more severe in sandy soil than heavier soils.

Xiphinema  (Dagger nematode)

These nematodes parasitize plants and are also known as dagger nematodes. They are ectoparasites and all stages (except eggs) of these nematodes attack and feed on the host plant’s root. Major species of this nematodes include X.italiae, X.americanum, X.diversicaudatum, X.pachtaicum, and X.index. They are characterized by their long bodies and stylet.

The stylet is usually long enough to reach the plant’s vascular tissue. As the body of the dagger nematode stays in the soil outside the plant root, it inserts its long stylet deep into the root. The stylet penetrates the plant tissues and punctures cell walls. As they feed, they secrete enzymes to digest the plant cell content. These enzymes digest and destroy the root cells which cause a malformation of the root tissues. Eventually, the root cells collapse.

Different members of this nematode group through penetration have been known to cause a moderately large amount of damage to the plant roots, resulting in gall formation in some species. Xiphinema attacks a large variety of plants like grape, strawberry, hops, nectarine, oak, raspberry, grapevine, carrot, peach, cherry, rose, and soybean.

Species of Xiphinema are sensitive to soil moisture and temperature changes. Hence they migrate away from desiccating conditions in topsoil. Therefore, most of them can survive and live deep in the soil.

Dagger nematodes are vectors of nepoviruses. Most of these nematodes during feeding transfer viruses to plants. Some of the viruses they transfer are the Tomato ringspot virus, cherry rasp leaf virus, and tobacco ringspot virus. They feed on the host plant’s roots and spread viral mosaic and wilting diseases, which can cause economic damage and death. Controlling viral disease in susceptible crops is a major challenge. However, studies have shown that biofumigation and crop rotation are effective Control measures to an extent.

Xiphinema has 6 life cycle stages. The females lay their eggs singly in thin water layers in the soil. There are 3 or 4 molts after the first stage juvenile emerges from the egg. With each molt, they increase in size until they become adults. While the vector-capable juveniles feed on the virus-infected plants, they mature into adults. Also, they acquire plant pathogenic viruses that form a lining in the pharynx-stylet tube. During feeding, the viruses are then injected into the root tissues. The feeding at the meristematic root tip causes damage. It destroys the root cells and causes a reduction in the root volume. Also, causing an above-ground effect of stunted growth of the crop and patchy fields.

Bursaphelenchus

Bursaphelenchus is a genus of nematodes that cause severe wilt diseases in a number of tree species. There are two species of this roundworm; the pinewood nematode (B. xylophilus) and the red ring nematode (B. cocophilus). However, these nematodes have built a symbiotic relationship with some insects such as weevils and beetles.

They invade these insects and are transported from a diseased tree to a healthy tree. There are two common tree wilt diseases caused by these nematodes (i.e the pine wilt and coconut red ring disease). They are significant pests of pine trees and coconut palms.

The pine wilt disease is caused by B. xylophilus and beetles of the genus Monochamus are vectors of the disease.

B. cocophilus causes the coconut red ring disease and has two vectors- the sugarcane weevil Metamasius sp and the palm weevil Rhynchophorus sp. 

These nematodes inhabit decaying wood or soil. They feed on fungal hyphae growing on the wood or feed on the wood itself. When they reduce the fungal load on the wood, they are said to be beneficial.  However, as they consume the tissue of the plant, they can make the tree die.