Secernentea (Secernenteans)
Secernentea
(Secernenteans)
Phylum Nematoda
Class Secernentea
Number of families 60+
Thumbnail description
Almost exclusively terrestrial nematodes, generally parasitic of plants and invertebrate and vertebrate animals; they are bilaterally symmetrical and non-segmented
Evolution and systematics
Nematoda, the phylum above the class Secernentea, has left very few fossil remains. The earliest fossils that contained nematode structures were found in Eocene strata (the era from about 55–38 million years ago). More authenticated fossils are of nematodes preserved in amber within such items as fossilized shark muscles and mammals frozen in permafrost. The fossil record is too fragmented to explain much about nematode origins, so most conclusions about phylogeny are based on observations of living species. It is hypothesized that nematodes originated during the Precambrian era, what was the Proterozoic period (about one billion years ago).
Earlier in the classification process Chitwood separated hematodes into two main classes, the Phasmidia (now Secernentea) and Aphasmidia (now Adenophorea). Controversies still exist, but for the most part, scientists, such as A. R. Maggenti, who helped to develop the classifications under this system, treat nematodes as a separate phylum with two classes, Adenophorea and Secernentea, which have been divided based on molecular and morphological characteristics. These two classes are primarily separated (along with other important criteria) with respect to whether they possess phasmids (as in Secernentea) or do not possess phasmids (as in Adenophorea). The total number of species of secernenteans is estimated at about 8,000 worldwide, with scientific surveys suggesting that an enormous number of species has yet-to-be discovered. There are six orders and the number of families ranges from 60 to 89.
Physical characteristics
Secernenteans vary greatly in size from microscopic to several feet long. The largest known secernentean, which is up to 30 ft (9 m) in length, lives in the placentas of female sperm whales. The body of secernenteans consists of a flexible cylinder that tapers at both ends, with a pointed tail and a blunt head. They are considered non-segmented pseudocoelomates; that is, creatures possessing a three-layered body that has a fluid-filled body cavity (pseudocoelom) between the endoderm and the mesoderm (the innermost and middle tissue layers).
A flexible but tough collagenous cuticle surrounds the body with a system of grooves across the body from head to tail, which protects them internally. The non-cellular cuticle varies from four to two layers and is almost always transversely striated. Laterally for most of the body length, the cuticle is generally modified into a wing area that is marked by longitudinal ridges; generally, this region is only slightly above the normal body contour. However, in some parasitic forms, it may extend out a distance equal to the body's diameter. The cellular hypodermis is the subcuticular layer that secretes the cuticle.
The sensory system contains phasmids, which are a pair of bilateral cuticular, glandular organs, situated laterally in the caudal (posterior to the anus) region and opening to the surface by a slit or pore. Also known as precaudal glands, phasmids are unique to the secernenteans, in which their function is believed to be sensory. At the other end are pore-like amphid apertures, which are a pair of glandular chemosensory organs situated dorso-laterally in the cephalic (head or anterior) region and opening through the cuticle. Although usually pore-like, in isolated instances the aperture can be an oval or a cleft. The apertures show little variation throughout the secernenteans. The amphids are always labial (located on the lips). The external amphidial aperture is usually less well developed than in adenophorean worms.
Somatic and cephalic setae, which are elongated structures jointed with the cuticle, are rare. When present, the cephalic sensilla are located on the labial region, and they are pore-like or papilliform. In males, there may be caudal setae. In females, somatic setae are absent. Generally, sixteen sensilla are present in the shape of two circles (an inner circle of six, and an outer circle of 10). In some parasitic groups, the number of cephalic sensilla may be reduced. Deirids, pairs of pore-like sensilla that usually protrude above the surface of the cuticle, are usually present on the cervical region near the level of the nerve ring.
Secernenteans contain no hypodermal glands, but the hypodermal cells of the hypodermis (a thin tissue layer beneath the cuticle that thickens to form the dorsal, ventral, and two lateral hypodermal chords, and extends the length of the body) are usually multinucleate (syncytial: more than two nuclei), but may also be uninucleate (cellular: one nuclei). These divide the muscle cells into four fields. A layer of longitudinal muscles underlies the hypodermis.
Bursae, or caudal alae, are sublateral projections generated by a longitudinal widening of the cuticle. It is common for them to be present within male secernenteans, each looking like a fluid-filled body sac. The thin cuticle extensions are located on both sides of the anus, specifically on either side of (or sometimes surrounding) the cloaca (the urogenital opening) of males. The well-developed excretory system is in the shape of an H or U. It is a simple tubular system that is located in one or both of the lateral hypodermal chords, and embedded between the three cell bodies in the hypodermal chord. The system opens ventromedially by way of a cuticularized duct. The rectal part of the gland system is usually present. They have no caudal glands.
The muscular esophagus or pharynx varies in configuration, but the majority of secernenteans have three esophageal glands: two that are subventral and one that is dorsal. The subventral glands open into the posterior metacarpus. The dorsal gland opens anteriorly into the procorpus or the anterior metacarpus. Its basic structure is corpus (the anterior part is cylindrical), with the basal (bottom) region sometimes swollen in the shape of a bulb. The glands empty their contents into the esophagus to aid in digestion. The tail is the region between the anus and the back tip of the body.
Distribution
Secernenteans are distributed worldwide in terrestrial environments, and only rarely in regions containing water habitats.
Habitat
Secernenteans show considerable diversity in their habitats, including free-living microbotrophs, plant parasites, invertebrate parasites, and vertebrate parasites. They are rarely found in marine and freshwater habitats, being almost exclusively terrestrial. They are mostly parasites, living on or within their plant or animal hosts. When free living, they often live in soil.
Behavior
This class encompasses most of the plant, invertebrate, and vertebrate parasites in the Nematoda phylum. Within the class, there are more than 3,000 animal parasites and 2,000 plant parasites. Their lifestyle is classified primarily as parasitic, but there are some free-living species. When parasitic or a combination of parasitic and free-living, secernentean behavior primarily revolves around their hosts. Each species has developed particular ways to infect their hosts, adapting their behaviors to best suit their needs while living off their hosts.
Feeding ecology and diet
They feed mostly on bacteria, fungi, and soil organisms, as well as on the nutrients such as blood, body fluid, intestinal contents, and mucus found within their hosts. Secernentean worms generally have some form of stylet, a hard, sharp spear that is used for feeding. Muscles move the stylet in and out, allowing them to puncture cells. Once opened, the worm will empty the contents of the cell.
Reproductive biology
The female ovary contains germ cells that give rise to eggs. Fertilization, most of the time by male sperm, takes place in the uterus and eggs are released through the vagina. Males have one testis (monarchic), when testes are present. Most species produce males and females (that is, dioecious), but some species only produce females, in which both male and female structures are contained in the same individual. In those few cases the species are hermaphroditic. Most eggs are about the same size and shape. Males produce sperm in the testes, which are shaped similar to female ovaries. Sperm accumulate in the seminal vesicle and exit through the anus. Males have accessory copulatory organs, called spicules. During mating, a spicule becomes rigid so it can be inserted into the vagina in order to form a passageway for the sperm. Male bursae, usually numbering four, are used to clasp the female during copulation. Males possess paired preanal supplement glands (or genital papillae) in two sublateral rows on the ventral side of the body. The glands are used for secretion and attachment, and function during copulation.
The life cycle is generally a straight cycle going from fertilized egg, through four juvenile (often called larval) stages involving a set number of cell divisions, and into adulthood. Secernenteans generally produce offspring through internal sexual reproduction. Fertilization occurs in the female's uterus, where the zygote is then placed inside a tough shell. Development from egg to adult is generally similar for all species, although many differences exist to the norm (often times brought about by environmental conditions and special types of life, as two examples). Usually a young juvenile hatches from the egg. It usually resembles the adult except in size and in the maturity of its sex organs. Each of the four larval stages (usually referred to as L1, L2, L3, and L4) is separated from one another by the complete shedding of the outer layer (what is called ecdysis, or molting in other animals), including the lining of the mouth and rectum. There is no increase in the number of cells after hatching, with growth coming exclusively from an increase in the size of the original cells. Development in successive stages is gradual over all, however organs themselves can develop at rapid rates.
Conservation status
No secernenteans are listed on the 2002 IUCN Red List of Threatened Species. They are found worldwide and frequently occur in very great densities.
Significance to humans
As crops are cultivated to feed the world's population, secernenteans become more numerous as they feed on agricultural plants. Parasitic secernenteans cause yield losses and they may join with other soil-living organisms such as bacteria, fungi, and viruses to advance disease development in plants. They also can cause loss of nutrients and water into the plant, thus increasing the plant's susceptibility to other dangers. Secernenteans, when infecting humans, can cause various diseases and, in some circumstances, death to the human host. When infected animals are used as food or kept as pets for humans, these parasites can cause and transmit various diseases and, often, death to the hosts. On the other hand, free-living forms of secernenteans can be good indication of biodiversity, and important to the health and survival of humans. They help to cycle carbon and nitrogen and to breakdown organic matter in the soil environment.
Species accounts
List of Species
Maw-wormThreadworm
Canine heartworm
African river blindness nematode
Dog hookworm
Rat lungworm
Barber's pole worm
Citrus spine nematode
Cod worm
Maw-worm
Ascaris lumbricoides
order
Ascaridida
family
Ascarididae
taxonomy
Ascaris's lumbricoides Linnaeus, 1758, Homo sapiens, Europe.
other common names
English: Large roundworm.
physical characteristics
One of the largest and most common parasites, it measures 8–18 in (20–46 cm) in length by 0.12–0.24 in (3–6 mm) in width for females; males generally more slender and shorter at 6–12 in (15–30 cm) in length by 0.08–0.16 in (2–4 mm) in width. Bodies are cylindrically shaped, with a head that is slightly narrower. Males have a curved tail with two spicules, but no copulatory bursa; females have a vulva approximately one-third of the length of the body down from the head, and a blunt tail. Characterized with smooth, finely striated cuticle; lip region is well developed and separated from the cervical region. The mouth has three lips, each equipped with small papillae. Internally, they have a cylindrical esophagus opening into a flattened ribbon-like intestine. Female ovaries can be 3.3 ft (1 m) in length). The eggs have thick shells consisting of a transparent inner shell covered in a warty, albuminous coat, and are sticky to the touch. The size of the eggs are 0.00197–0.00295 in (50–75 µm) in length by 0.00158–0.00197 in (40–50 µm) in width. The excretory system empties every three minutes or so.
distribution
Cosmopolitan. In 1986 they caused over 64 million infections worldwide. (Specific distribution map not available.)
habitat
Most common in warm, moist climates or in regions with temperate or tropical climates. They live in the small intestines of humans and pigs.
behavior
Have a direct and simple lifestyle, with no intermediate hosts, unlike many parasites. Adults live in the lumen of the small intestines and eggs are passed in the feces. The eggs hatch in the small intestine. Once they grow into an adult worm and mate, they become as large as 12 in (31 cm) in length by 2 in (4 cm) in width.
feeding ecology and diet
Feeds on the semi-digested contents of the gut, although there is some evidence that it can bite the intestinal mucous membrane and feed on blood and tissue fluids.
reproductive biology
Females can produce 200,000 to 2 million eggs per day, and 70 million in a year with very developed ovaries. The ovaries can contain 27 million eggs at one time. The fertilized eggs are broadly oval, and the shell is thickened, tuberculate and measure about 0.0098 in (250 µm) in length by 0.00059 in (15 µm) in width. About 2–3 weeks after passage in the feces and with ideal environmental conditions, the eggs contain an infective juvenile, and humans are infected when they ingest such infective eggs. Adults can live in the small intestine for six months or longer. In the intestine, eggs are only embryonated mass of cells, with further differentiation occurring outside the host. Eggs can stay alive in the soil for many years if conditions are adequate. The cycle from egg ingestion to new egg production takes approximately two months.
conservation status
Not listed by the IUCN.
significance to humans
About a sixth of the world's population suffers from ascariasis, a maw-worm infection often called "large roundworm infection." An estimated 25% of the people in developing countries are believed to be infected. In the United States, occurrence is uncommon with only four million people infected, mostly in the rural southeast. Infection occurs from ingestion of raw food such as fruit or vegetables. Infection with the species is rarely fatal, but death may occur because of mechanical intestinal obstruction.
Threadworm
Strongyloides stercoralis
order
Rhabditida
family
Strongyloididae
taxonomy
Anguilla stercoralis (Bavay in Normond, 1876), In Toulon, France, on host Homo sapiens, but host from Cochin China.
other common names
English: Dwarf threadworm.
physical characteristics
An important parasite of humans, primates, dogs, and other animals, being one of the smallest to inhabit the human body. Females measure from 0.079–0.098 in (2.0–2.5 mm) in length; males slightly smaller. Size of the eggs is about 0.00158 in (40 µm) in length by 0.00118 in (30 µm) in width. Both sexes have small buccal cavities and a long esophagus. Females have a cylindrical pharynx with no posterior bulb swelling. Both free-living male and female worms have a prominent rhabdiform pharynx.
distribution
Occur all over the world, but exist in large numbers in Southeast Asia and South America, and in Eastern Europe and in the Mediterranean region. In the United States, they are rare, although are more prevalent in the rural southeast. (Specific distribution map not available.)
habitat
Found primarily in temperate climates, and to a lesser degree in tropical climates. Female worms are found in the superficial tissues of the small intestines of vertebrates, mainly humans and dogs. Males are believed not to be parasitic, but free living in the soil.
behavior
Heterogonic lifecycle, i.e., a parasitic generation is interspersed with a free-living one. There can be multiple cycles of each phase. Females are more invasive than males. This parasite generally replicates within its definitive hosts. Females lay partially embryonated eggs that are released into the host's submocosa or the intestinal lumen. Once in the external environment (after passing through the host's feces), the larvae either remain as infective stages or develop to free-living adults.
feeding ecology and diet
As parasites, they feed off of their hosts, taking nutrients, mostly mucosa from the intestine, from various locations within the body as they travel through the circulatory system, lungs, stomach, and small intestines. As free living, they feed on organic matter, bacteria, and other nutrients in the soil.
reproductive biology
Females parthenogenetic; males exist, but are only rarely found, and then only in feces. Free-living males and females mate, produce more larvae, and some of these larvae will develop into free-living larvae, while others will develop into parasitic juveniles.
conservation status
Not listed by the IUCN.
significance to humans
Septicemia is caused by the infection of this worm into the human body. More than 70 million people are infected, mostly in the temperate regions of the world.
Canine heartworm
Dirofilaria immitis
order
Spirurida
family
Onchocercidae
taxonomy
Filaria immitis (Leidy, 1856), Canis familiaris, United States.
other common names
English: Dog heartworm.
physical characteristics
Females are 9-12 in (25-30 cm) in length by only about 0.13 in (5 mm) in width, while males are about half the size of females with a length of 5-6 in (12-16 cm). They have somatic (coelomyarian) muscles. (Somatic muscles provide the shape to individual muscle cells thus, supplying a portion of the hydrostatic skeleton in nematodes). Specifically, coelomyarian muscles (a type of muscle cell arrangement) make a protoplasmic zone that bulges into the pseudocel and fibrillar zone, which extends up the sides of the cell.) The average diameter of the filaments are 0.007-0.017 µm. The cuticle, which is the elastic covering of the body and all body openings, consists of a thin polysaccharide-rich layer. It does not possess longitudinal ridges.
distribution
Widespread throughout the world and exists anywhere mosquitoes live. (Specific distribution map not available.)
habitat
Primarily live in the tropics, subtropics, and some temperate areas. They are found in dogs, cats, foxes, wolves, and other wild carnivores as well as in sea lions and humans. Within the host, adults live in the right ventricle and the adjacent blood vessels from the posterior vena cava, hepatic vein, and anterior vena cava to the pulmonary artery.
behavior
Usually infest the heart of its hosts through about 30 species of mosquitoes. Adults live in the peripheral branches of the pulmonary arteries and produce large numbers of microfilaria that circulate throughout the bloodstream. They usually infect dogs, but also cats, ferrets, and seals. These hosts are the definitive hosts, while mosquitoes are the intermediate hosts. Lifecycle begins when a dog with circulating microfilaria is bitten by a mosquito. They are passed into the bloodstream where they remain active for up to one year or more, but are incapable of further development until ingested by a mosquito. Microfilaria matures into infective larvae inside the mosquito within about 14 days. When the infective mosquito bites a host, larvae are injected into the host's skin and begin to mature into adults in the subcutaneous tissues, muscles, and fatty tissues; they develop to 0.98–4.33 in (25–110 mm) in length. They arrive in the right lower chamber of the heart at 2–4 months. After infection, an additional four months are required for the worms to reach maturity; microfilariae first appear in the peripheral blood circulation about eight months after infection. Adults may live and continue to produce microfilariae for several years. Adults live in the right ventricle and the adjacent blood vessels from the posterior vena cava, hepatic vein, and anterior vena cava to the pulmonary artery.
feeding ecology and diet
Live off nutrients of their hosts, primarily through the blood (mostly pulmonary arteries) in and around the heart and lungs.
reproductive biology
Females produce large numbers of microfilaria after about six months, which circulate throughout the bloodstream. Up to 5,000 microfilariae are shed into the host's bloodstream each day, and can remain alive and infective in a host's bloodstream for up to three years.
conservation status
Not listed by the IUCN.
significance to humans
One dog may be infected with 25–50 canine heartworms. In heavy infestations, with 50–100 worms, a dilation of the heart is apparent, as well as pathological problems with lungs, liver, and kidney. Pharmacueticals or surgery are used to remove the worms from the infected host.
African river blindness nematode
Onchocerca volvulus
order
Spirurida
family
Onchocercidae
taxonomy
Filiria volvulus (Lenckart in Mason, 1893), originally Filaria Homo sapiens, West Africa.
other common names
English: River blindness nematode.
physical characteristics
Filarial parasites of primates, primarily humans. They have an adult length of 0.8–27.6 in (2–70 cm) with females measuring 13.0–27.6 in (33–70 cm) in length by 0.011–0.016 in (270–400 µm) in width; males measure 0.8–1.6 in (2–4 cm) in length by 0.00512–0.00827 in (130–210 µm) in width. Microfilariae measure 0.00866–0.0142 in (220–360 µm) in length by 0.000197–0.000354 in (5–9 µm) in width, are unsheathed, and have a lifespan of almost two years. The epicuticle is folded separately from the underlying cuticle, with the adult male epicuticle showing a honeycomb-like pattern. The intestinal cells of adult females are very thick. The body is slender and blunt at both ends. Lips and a buccal capsule are absent, and two circles of four papillae each surround the mouth. The esophagus is not conspicuously divided. The female vulva is behind the posterior end of the esophagus. The male tail is curled and lacks alae; it bears four pairs of adanal and six or eight pairs of postanal papillae. The microfilariae are unsheathed.
distribution
Distributed throughout the world, including central Africa, Central America, northern South America, and Mexico. (Specific distribution map not available.)
habitat
Live primarily in the tropics and subtropics near fast flowing rivers where the Simuliam black fly breeds. (They are normally transmitted by the flies' bites.) They accumulate in raised nodules found under the skin and in the lymphatic system of connective tissues of the human host. Also found occasionally in peripheral blood, urine, and sputum. They can also enter the eye, leading to the formation of lesions and cataracts.
behavior
Complex lifecycle begins when an infected female takes a blood-meal from a human host. The larvae enter the host's subcutaneous tissues and slowly mature into adults in one year. Adults can live for 15 years, and many males and females can live together. The microfilariae reach about 11.8 in (300 mm) in length by 0.03 in (0.8 mm) in diameter, and are sheath-less with sharply pointed, curved tails. Many thousands of microfilariae migrate in the subcutaneous tissue. When the infected host is bitten by another female fly, microfilariae are transferred from the host to the black fly where they develop into infective larvae, and the lifecycle continues.
feeding ecology and diet
Live off the nutrients located in such places as subcutaneous connective tissues, peripheral blood, urine, sputum, and skin found within the host.
reproductive biology
Females may produce 1,000 microfilariae per day, which are already hatched when they are born (unsheathed). After mating eggs inside the female develop into microfilariae ovivipariously, which leave the worm one by one.
conservation status
Not listed by the IUCN.
significance to humans
Causes onchocersiasis, which has infected an estimated 18 million people worldwide (mostly in Central and South America and sub-Saharan Africa). It has also caused more than 270,000 cases of bilateral blindness and more than one million cases of visual impairment. It rarely causes death, and is the second most common cause of infectious blindness. The severity of this disease has far reaching economic consequences but, fortunately, advancements have been recently made in reducing the disease. Controlling black flies is the prime way to control the disease. As a result, this species has almost been eliminated from some locations.
Dog hookworm
Ancylostoma caninum
order
Strongylida
family
Ancylostomatidae
taxonomy
Sclerostoma caninum (Ercolani, 1859), originally Sclerostoma Canis familiaris, Europe.
other common names
English: Creeping eruption (when found in humans).
physical characteristics
The most widespread of the hookworm species, they target dogs and other canids. Male adults are 0.43–0.55 in (1.1–1.4 cm) in length, with a copulatory bursa, two large lateral lobes, and two equal filiform spicules; females 0.5–0.8 in (1.3–1.9 cm) in length, with no vulvular flap. Adults have an anterior end that is bent dorsally, a buccal capsule that is deep and supported by thick cuticle, three pairs of teeth (three on each side of the ventral margin) that are located at the anterior stoma, and a strongyliform esophagus. They are often red in color because of the blood in their gut; otherwise, they are gray in color. Their heads "hook" into the small intestines of the host, where they begin to eat away at the tissue and suck blood. Eggs are 0.00209–0.00272 in (53–69 µm) by 0.00142–0.00209 in (36–53 µm), ovoid-shaped, and thin-shelled embryos. The first-stage larvae have a mouth tube, bulbed rhabditiform esophagus, and straight tail. No genital rudiment is visible.
distribution
Found in eastern Asia, Central and South America, and Australia. (Specific distribution map not available.)
habitat
Generally live in temperate forests and rainforests, temperate grasslands, tropical deciduous forest, tropical rainforests, and tropical scrub forests. Within the host, they live within the small intestines of dogs and other canids, and can also live in foxes and cats. They can also live under the skin of humans.
behavior
Intermediate hosts are generally not present, however, paratenic hosts (hosts that act as transfer hosts where the parasite does not develop) are normally encountered. The first eggs appear 60–75 days after the initial exposure. On reaching the small intestine, they proceed to develop to adult males and females. They suck the blood and tissues; both the plasma and corpuscles undergo at least partial digestion. In some cases, some larvae may go dormant, but can be reactivated later by unknown means.
feeding ecology and diet
As parasites, feeds off its hosts. In the free-living larvae stage, they feed on organic matter.
reproductive biology
Dioecious; following copulation, females lay eggs in the intestines of the host. Females lay 7,000–30,000 eggs per day. The embryonated eggs are carried out in the host's feces. Eggs develop in the soil/feces under favorable environmental conditions of moisture, oxygen, and temperature. About three weeks later, the juveniles are unsheathed, non-feeding, and infective. When they reach the small intestine of the host, they molt a final time and develop to maturity in about five weeks.
conservation status
Not listed by the IUCN.
significance to humans
In humans, the infection of the species is called "creeping eruption," and causes a severe rash. This species causes disease primarily in puppies; in its severest form, the disease is life threatening to puppies and sometimes to adult dogs.
Rat lungworm
Angiostrongylus cantonensis
order
Strongylida
family
Angiostrongylidae
taxonomy
Pulmonema catonesis (Chen, 1935), Dominican Republic, originally Pulmonema raltucnorvegicus, Canton, China.
other common names
None known.
physical characteristics
Moderate sized, adult males measure 0.79–0.87 in (20–22 mm) in length by 0.0126–0.0165 in (320–420 µm) in width, and adult females measure 0.87–1.34 in (22–34 mm) in length by 0.0134–0.0221 in (340–560 µm) in width.
distribution
Distribution can only be identified by the disease caused by this species, which has been reported in Asia (Philippines, Indonesia, Malaisia, Thailand, Vietnam, Taiwan, Hong-Kong, Japan); Oceania (Pacific Islands [Tahiti, New Caledonia], Papua New Guinea, Australia); Cuba, Puerto Rico, Hawaii; United States; and Africa (Madagascar). (Specific distribution map not available.)
habitat
They utilize a wide variety of invertebrate intermediate hosts where adults live primarily in the blood vessels of the lungs of the host.
behavior
Adults live within the blood vessels of the lungs of the rodent host. Females produce eggs that hatch in the lungs and then attach to the terminal branches of the pulmonary arteries. First-stage juveniles enter the respiratory tract and, from there, the juveniles move up the trachea, where they are then swallowed and later passed in the host's feces; juveniles can be detected in feces 40–60 days after infection. Requires an intermediate host (usually snails, but it can be found in almost any invertebrate such as oysters, slugs, and crabs) to complete its lifecycle. The rodent or human host is infected when it ingests an intermediate host (such as snails and slugs) containing infective juveniles. The infective juveniles develop to adults through two stages in 2–3 weeks. Adults enter the pulmonary arteries and the lungs where they become mature; they eventually enter the rodent host's brain. Adults then migrate back to the host's lungs via the venous circulation. In human hosts, the parasites enter the brain, but do not develop further and die.
feeding ecology and diet
Live off nutrients of their hosts, specifically around the lungs (and pulmonary arteries) and brains of rodent hosts, and usually only around the lungs of human hosts, but also in the blood vessels between the lungs and brain.
reproductive biology
After mating, oviposition takes place in the lungs. Eggs are then coughed up, swallowed, and are subsequently passed in the host's feces. Hatched larvae develop in about two weeks to the infective third stage.
conservation status
Not listed by the IUCN.
significance to humans
Especially dangerous to humans because it ruptures vessels in the brain. The presence of juveniles in the blood vessels, meninges, or tissue of the human brain can result in symptoms such as headache, fever, paralysis, neurological disorders, and even coma and death. The appearance of worms is often associated with eosinophilia, and is the primary cause of eosinophilic meningoenciphalitis, a disease that occurs when humans eat infected, raw snails.
Barber's pole worm
Haemonchus contortus
order
Strongylida
family
Trichostrongylidae
taxonomy
Strongylus contours (Rudolphi, 1803), originally Strongylus ovis-aries (?), Europe.
other common names
English: Barber pole worm, sheep stomach worm, wire worm.
physical characteristics
A stomach parasitic roundworm found inside ruminants such as sheep, goats, cattle, and wild ruminants. Males have a length of 0.7–0.8 in (18–21 mm); females 0.7–1.2 in (18–30 mm). Females possess white uteri and ovaries that spiral around their red blood-filled intestine, which gives a twisted ("barber pole") appearance. The small buccal capsule contains a curved dorsal tooth. There are two distinctive lateral spike-like cervical papillae near the connection of the first and second quarters of the esophagus. The male bursa has long lateral lobes and slender rays with a flap-like dorsal lobe, which is located asymmetrically near the bases of the left lateral lobe. Spicules are 0.018–0.020 in (450–500 µm) in length, each with a terminal barb, and the gubernaculums is navicular (that is, its structure is considered to be shaped like a small boat). Usually, an anterior thumb-like flap covers the vulva, and may be reduced to a mere knob in some worms. The oval eggs are 0.00276–0.00335 in (70–85 µm) in length by 0.0016–0.00173 in (41–44 µm) in width.
distribution
Distributed in the Arctic and immediately adjacent temperate regions of Europe, North Africa, and Asia, north of the tropics. (Specific distribution map not available.)
habitat
Found in coastal and high rainfall areas, especially in areas where hosts such as goats and sheep are plentiful. They are found in many terrestrial habitats, including tundra, taiga, temperate forest, and rainforest, temperate grassland, chaparral, tropical deciduous and scrub forests, tropical savanna and grasslands, and mountains. They live inside the abomasums (fourth stomach) of ruminants. Egg development is limited to areas and seasons where pastures are moist during warm months. (They are more prevalent in warm, moist regions than in cold, dry ones.) However, juveniles can survive for some time, particularly during cool conditions, and can infect hosts even without favorable periods of development. Pastures that remain green over the summer, perennial pastures (especially with kikuyu grass), irrigated pastures, and areas near creeks, troughs, and seepage points are preferred.
behavior
Do not require an intermediate host. The first three juvenile stages are free-living. After the infective stage is reached, the organisms move to an area that will optimize its chances of being eaten by a host. The organisms have a short lifecycle (90 days) and must find a host quickly after it has completed its first stages of growth.
feeding ecology and diet
As a parasitic species, they feed on their hosts. They parasitize the abdomen or stomach of its host, using a single dorsal tooth to make cutting movements in the host tissues. A secreted anti-coagulant allows them to feed on blood, but cell contents and other fluids are also consumed. Adults in the fourth stage of life are able to form a clot and feed from it.
reproductive biology
Following internal fertilization, females lay eggs that pass out in the feces of the host. Each female can deposit 5,000–10,000 eggs per day. The first microscopic larvae hatch 14–17 hours after being passed through the feces; in 3–5 days the organism will have the ability to infect a host. First- and second-stage juveniles feed on bacteria. Third stage juveniles retain the second stage cuticle as a sheath; they do not feed and are infective for the vertebrate host. In a sheep's gut, larvae develop to adults in about three weeks. Mating of adults occurs and egg production commences. The eggs hatch in soil or water. Infections by third stage juveniles may also occur through the skin. Enormous numbers of juveniles may accumulate on heavily grazed pastures. However, many die during low temperatures.
conservation status
Not listed by the IUCN.
significance to humans
Cause haemonchosis, which is related to the degree of blood loss. Large numbers of worms can accumulate very rapidly, causing host deaths often without warning, especially in young animals. Vaccination of sheep and goats (de-worming) is used for control. When the worms move through the soil, they help to aerate the soil, which helps to control erosion and keeps the soil from clumping and hardening.
Citrus spine nematode
Criconema civellae
order
Tylenchida
family
Criconematidae
taxonomy
Criconema civellae (Steiner, 1949), Greenhouse, Beltsville, Maryland, United States. Latest name: Crossonema civellae, Menta and Raski, 1971.
other common names
None known.
physical characteristics
Females are of length 0.0114–0.0339 in (0.29–0.86 mm), with 40–93 annules. Contains spear muscles that are well developed and large compared to other species. The muscles are 0.000591–0.00394 in (15–100 µm) in length. The labial cap is not easily distinguished from the overall body contour, except for a narrowing that leads to the oral disk. The esophagus is distinctive, the corpus is typical for the family, and the post-corpus is very distinctive. The isthmus and glandular regions merge into an almost cylindrical form that is slightly expanded at the end. This general region is narrower than the corpus, and equal to or shorter than the corpus in length as measured from the spear knobs. The external cuticle is ornamented with annulations. The annules are round—not lobed—and with a continuous edge of fine, blunt spines on most of the body. The spines are generally simple in shape, except in the posterior region where they are bifurcate, knobbed, or clubbed. The lip area is elevated (with the lip generally pointed), and the sub-median lobes are absent. The slylet is 0.00244–0.00453 in (0.06–0.12 mm) in length. The vulva is 3–16 annules from the terminus.
distribution
Not known. (Specific distribution map not available.)
habitat
Migratory.
feeding ecology and diet
As parasites, they live off the nutrients found on their hosts.
behavior
Plant parasites, specifically, ectoparasites, living on the outside of the plant host.
reproductive biology
Males are degenerate and incapable of breeding.
conservation status
Not listed by the IUCN.
significance to humans
None known.
Cod worm
Phocanema decipiens
order
Ascaridida
family
Anisakidae
taxonomy
Ascaris's decipens (Krabbe, 1878), originally Cristophora cristata, Greenland coast. Latest name: Pseudoterranova decipens.
other common names
English: Seal worm.
physical characteristics
Often found in cod, but are also found in many other species of fish. In the larval stage, they are 0.20–2.28 in (5–58 mm) in length by 0.012–0.047 in (0.3–1.2 mm) in width, and yellowish, reddish, or brownish in color. They have well-developed and distinct lips. The excretory system is elongated and cord-like, while the adult esophagus is cylindrical in shape.
distribution
Located in the Atlantic Ocean. (Specific distribution map not available.)
habitat
Found in the guts or flesh of fish such as cod. In their final hosts, they are found mostly in gray seals and other similar animals.
behavior
When inside fish, they are usually found tightly coiled in the flesh and guts of fish. They are often found in considerable numbers, particularly in the belly flaps of fish, where they can remain for extended periods of time encased in a sack-like membrane produced by the fish tissue. Adults also live in the stomach of gray seals and other similar creatures. Eggs of the parasite pass into the waters with the mammal's excreta, and when the eggs hatch, the microscopic larvae must invade a new host in order to develop. Small shrimp-like crustaceans, euphausiids (often called krill), and other parasitic crustaceans eat the larval worms. When a fish eats these infested crustaceans, the larval worms are released into its stomach. They then bore through the stomach wall and eventually become encased in the guts or in the flesh of the host fish. The lifecycle is completed when a suitable marine mammal eats an infested fish. The incidence of infection in fish generally increases with length, weight, and age of the fish host.
feeding ecology and diet
As a parasitic species, they live off of nutrients of their hosts, primarily from the guts and tissues of fishes.
reproductive biology
Females have ovaries and uteri, while males have copulatory spicules.
conservation status
Not listed by the IUCN.
significance to humans
The cause of human illness in countries where there is ingestion of raw or lightly cured fish. The disease is called anisakiasis and can be easily prevented because larvae are killed in only one minute at a temperature of 140°F (60°C) or higher. In the wrong host the worms get the wrong signal and migrate through the tissues of its host, causing hemmoraging and bacterial infections. This kind of migration is known as "visceral larval migranes."
Resources
Books
The Audubon Society Encyclopedia of Animal Life. New York: Clarkson N. Potter, 1982.
Bird, Alan F. The Structure of Nematodes. New York: Academic Press, 1971.
Chitwood, B. G., and M. B. Chitwood. Introduction to Nematology. Baltimore: University Park Press, 1950.
Croll, Neil A., and Bernard E. Matthews. Biology of Nematodes. Glasgow and London: Blackie and Son Limited, 1977.
Levin, Simon Asher, ed. Encyclopedia of Biodiversity. San Diego: Academic Press, 2001.
Mehlhorn, Heinz, ed. Encyclopedic Reference of Parasitology: Diseases, Treatment, Therapy. 2nd edition. New York: Springer, 2001.
Maggenti, Armand. General Nematology. New York: Springer-Verlag, 1981.
Malakhov, V. V. (translated by George V. Bentz, edited by W. Duane Hope). Nematodes: Structure, Development, Classification, and Phylogeny. Washington, DC, and London: Smithsonian Institution Press, 1994.
The New Larousse Encyclopedia of Animal Life. New York: Bonanza Books, 1981.
Parker, Sybil P., ed. Synopsis and Classification of Living Organisms. New York: McGraw-Hill Book Company, 1982.
Poinar, George O., Jr. The Natural History of Nematodes. Englewood Cliffs, NJ: Prentice-Hall, Inc., 1983.
Stone, A. R., H. M. Platt, and L. F. Khalil, eds. Concepts in Nematode Systematics. Special volume no. 22. London: Academic Press, 1983.
Wharton, David A. A Functional Biology of Nematodes. London: Croom Helm, 1986.
William Arthur Atkins