Nematodes – Morphology, Classification, Examples

What are Nematodes?

• Nematodes are a diverse group of organisms known for their elongated, cylindrical bodies and unsegmented structure. These worm-like creatures are found in various environments, with many species inhabiting soil and water. They are also recognized for their thread-like shape, a feature reflected in their name, derived from the Greek word “nema,” meaning “thread.”
• Unlike trematodes and cestodes, which are entirely parasitic, most nematodes are free-living. Many of these species play crucial roles in ecological processes, while others exist as parasites in plants and animals. Nematodes that parasitize plants are of significant economic importance, particularly in agriculture, as they can cause damage to crops. In the animal kingdom, nematodes are found parasitizing a wide range of invertebrates and vertebrates, including humans.
• Of the helminths (worm-like parasites), nematodes represent the largest group of parasitic species affecting humans. It is estimated that there are about 500,000 species of nematodes, some of which are responsible for various health issues. Therefore, understanding nematodes is important not only from a biological standpoint but also for managing their impact on agriculture and human health.

Definition of Nematodes

Nematodes are elongated, unsegmented, thread-like worms, found in various environments such as soil and water. They can be free-living or parasitic, affecting plants, animals, and humans.

General Characteristics of Nematodes

Nematodes, often referred to as roundworms, exhibit distinct morphological and physiological features that set them apart from other organisms. Their bodies are bilaterally symmetrical and elongated, with a triradiate symmetry present at the anterior end. These features contribute to their adaptability across a range of environments. Below are the key characteristics of nematodes:

• Shape and Size: Nematodes are cylindrical or filariform in shape. They vary significantly in size, ranging from microscopic species like Strongyloides stercoralis, which measure about a millimeter, to larger species such as Dracunculus medinensis, which can reach up to a meter in length. Males are generally smaller than females, with their posterior end typically curved or coiled ventrally.
• Body Structure: Their bodies are covered by a tough outer cuticle, which can be smooth, striated, bossed, or spiny depending on the species. Beneath the cuticle lies the hypodermis, followed by a somatic muscular layer that enables movement through sinuous flexion of the body. The body cavity, known as a pseudocoel, suspends the internal organs and provides structural support.
• Digestive System: Nematodes possess a complete digestive system. The mouth, located at the anterior, connects to the esophagus, which varies in shape and structure across species. The intestine, lined with columnar cells, processes nutrients and leads to the rectum, where waste is expelled through the anus. In males, the rectum also serves as a shared passage for both digestive waste and reproductive materials, opening into the cloaca.
• Excretory and Nervous Systems: Nematodes have relatively simple excretory and nervous systems. Their excretory system helps regulate internal balance, while their nervous system coordinates basic movements and responses to environmental stimuli.
• Reproductive System: Nematodes are diecious, meaning sexes are separate. Males have a reproductive system composed of a single tubule, differentiated into the testis, vas deferens, seminal vesicle, and ejaculatory duct. Additionally, males possess specialized copulatory structures such as spicules or a bursa, or both, which aid in reproduction. In females, the reproductive system includes an ovary, oviduct, seminal receptacle, uterus, and vagina. Female nematodes can be oviparous, producing eggs, or viviparous, giving birth to live larvae. Some species are ovoviviparous, meaning they lay eggs containing larvae that hatch immediately.

Morphology of Nematodes

• Body Structure:
  • Nematodes possess a long, cylindrical, and unsegmented body that tapers at both ends, contributing to their worm-like appearance. This body shape is vital for movement through soil or aquatic substrates.
  • They are bilaterally symmetrical, which is a common trait in many animal phyla, indicating that their body can be divided into mirrored halves along a central plane.
  • The epidermis is either cellular or syncytial, meaning it can consist of distinct cells or a multinucleate layer without cellular boundaries separating the nuclei.
• Tissue Organization:
  • Nematodes are triploblastic, having three primary germ layers: ectoderm, mesoderm, and endoderm. This tissue complexity allows for more specialized structures and organ systems compared to simpler organisms.
  • The body cavity in nematodes is termed a pseudocoel, which differs from the true coelom found in other phyla like annelids. This pseudocoel is not lined by mesoderm but is filled with a fluid-filled matrix called parenchyma, providing both structural support and facilitating nutrient transport.
  • Their body organization is distinctly arranged into organ systems, underscoring their biological complexity.
• Musculature:
  • Nematodes have a unique muscular arrangement composed of only four bands of longitudinal muscle fibers. This configuration enables them to exhibit a whip-like movement, essential for locomotion through their habitats.
• Cuticle:
  • The body is covered with a thick, flexible, multi-layered collagenous cuticle, which serves several purposes, including protection against desiccation and environmental stresses. The cuticle is often adorned with setae (hair-like structures), spines, or annulations (ring-like structures), enhancing their ability to navigate various environments.
• Reproductive System:
  • Nematodes exhibit a tubular gonadal structure, with males and females having distinct reproductive systems. In males, the genital duct enters the cloaca, while females possess individual openings for each genital duct.
  • A notable characteristic is the absence of asexual reproduction; nematodes reproduce sexually through internal fertilization. They can undergo cross-fertilization or self-fertilization, depending on the species.
  • Interestingly, nematodes do not produce amoeboid sperm cells; instead, their sperm has unique structural adaptations suited for fertilization.
• Nervous System:
  • The nervous system of nematodes is relatively underdeveloped compared to other more complex organisms. It consists of a circumpharyngeal nerve ring and a longitudinal nerve cord, which allows for basic motor functions and responses to environmental stimuli.
• Digestive System:
  • Nematodes possess a complete digestive system featuring a distinct mouth and anus, allowing for efficient food processing. The muscular pharynx lacks cilia, which differentiates it from other worm-like organisms that often use cilia for feeding.
• Developmental Patterns:
  • They undergo direct development, meaning they develop into adults without a larval stage, although some species may involve intermediate hosts. This developmental strategy is essential for their survival in various ecological niches.

Life Cycle of Nematodes

The life cycle of nematodes is a complex process that typically involves four larval stages followed by an adult stage. Each stage plays a critical role in the organism’s development, with the nematode shedding its cuticle, or outer layer, as it progresses from one stage to the next. Below is a detailed breakdown of the general life cycle of nematodes:

• Developmental Stages: Nematodes undergo four distinct larval stages (L1 to L4) before reaching adulthood. After hatching from the egg, the nematode progresses through these stages, molting its cuticle between each stage. This process, known as ecdysis, allows for growth and adaptation as the nematode prepares for its next developmental phase.
• Host Requirements: Humans serve as the optimal hosts for many nematode species. Most nematodes complete their entire life cycle within a single host. However, exceptions exist, such as species from the superfamilies Filarioidea and Dracunculoidea. In these cases, two hosts are required: an insect vector or Cyclops (a type of aquatic crustacean) serves as an intermediate host for the larval stages before transmission to a definitive host like humans.
• Localization and Transmission: Many nematodes localize within the host’s intestinal tract. Their eggs are passed out of the host’s body through feces. Once in the environment, these eggs must undergo developmental changes before they can infect a new host. These changes ensure that the nematode larvae are in the correct stage of development for infecting the next host they encounter.

Modes of Infection of Nematodes

Nematodes can infect their hosts through various modes, which are crucial for their survival, transmission, and propagation within different environments. These modes of infection are highly adaptive and involve distinct mechanisms depending on the species and life stage of the nematode. Below is a detailed breakdown of the primary modes of infection for nematodes:

1. Ingestion of Eggs:
   • Certain nematodes infect humans through the ingestion of eggs. These eggs are usually found in contaminated soil, water, or food. When consumed, they hatch within the host’s digestive tract and begin their development.
   • Examples include:
     • Ascaris lumbricoides (causes ascariasis)
     • Enterobius vermicularis (causes pinworm infection)
     • Trichuris trichiura (causes trichuriasis)
2. Ingestion of Larvae in Intermediate Hosts:
   • Some nematodes require an intermediate host to develop their larvae. Humans become infected by ingesting these larvae when they consume contaminated water or tissue from the intermediate host.
   • Example:
     • Dracunculus medinensis (causes Guinea worm disease), where the larval stage is transmitted through infected copepods (tiny water crustaceans).
3. Ingestion of Encysted Larvae in Muscle Tissue:
   • Certain nematodes infect through the consumption of meat containing larvae encysted within muscle tissue. When humans eat undercooked or raw infected meat, the larvae are released and begin their cycle in the host.
   • Example:
     • Trichinella spiralis (causes trichinosis), which is transmitted through the ingestion of infected pork or wild game.
4. Skin Penetration:
   • Some nematodes have the ability to directly penetrate the skin, usually when humans come into contact with contaminated soil. The larvae actively invade the skin and travel through the body to the intestine, where they mature into adults.
   • Examples include:
     • Ancylostoma duodenale (causes hookworm infection)
     • Necator americanus (also a hookworm species)
     • Strongyloides stercoralis (causes strongyloidiasis)
5. Transmission by Blood-Sucking Insects:
   • Certain species of nematodes rely on blood-sucking insects to transmit their larvae. These insects act as vectors, transmitting the infective larvae into the human bloodstream during feeding.
   • Example:
     • Filariae (causes diseases like lymphatic filariasis), which are transmitted by mosquitoes.
6. Inhalation of Dust Containing Eggs:
   • In some cases, nematode eggs can become airborne in dust particles, and humans may become infected by inhaling these contaminated particles. Once inhaled, the eggs reach the digestive system, where they hatch and mature.
   • Examples include:
     • Ascaris lumbricoides
     • Enterobius vermicularis

Feeding Habits of Nematodes

Their feeding strategies can be broadly classified into several groups, each with distinct adaptations and ecological functions.

• Herbivores:
  • This group primarily consists of plant-parasitic nematodes, particularly those from the order Tylenchida, along with some from Aphelenchida and Dorylaimida.
  • Herbivorous nematodes possess a specialized mouthpart known as a needlelike stylet, which they use to puncture plant cell walls, facilitating the extraction of nutrients.
  • There are two main types of herbivorous nematodes:
    • Ectoparasites: These nematodes remain in the soil and feed on the surfaces of plant roots, often causing damage to the plants.
    • Endoparasites: These nematodes invade the roots, living and feeding within the plant tissues, which can lead to significant agricultural damage.
• Bacterivores:
  • Bacterivorous nematodes are primarily free-living organisms that feed exclusively on bacteria, which are abundant in soil ecosystems.
  • They possess a hollow tube-like mouth or stoma, designed specifically for the ingestion of bacterial cells.
  • This group includes many members of the order Rhabditida and is crucial for the decomposition of organic matter, thus contributing to nutrient cycling in the soil.
• Fungivores:
  • Fungivorous nematodes feed on fungi and typically use a stylet to puncture and consume fungal hyphae.
  • Many representatives of this group belong to the order Aphelenchida and are integral to the decomposition process, similar to bacterivores.
  • By feeding on fungi, these nematodes help break down organic material, making nutrients available to other organisms in the soil.
• Predators:
  • Predatory nematodes consume other nematodes and small soil animals, playing a vital role in controlling populations within the soil ecosystem.
  • These nematodes do not specialize in their prey; instead, they feed indiscriminately on various free-living and plant-parasitic nematodes.
  • The order Mononchida comprises exclusively predatory nematodes, while some predators can also be found in Dorylaimida and other orders. Although they are less common than other feeding types, they contribute to maintaining balance within soil communities.
• Omnivores:
  • Some nematodes exhibit omnivorous feeding habits, consuming a mix of organic material, including bacteria, fungi, and sometimes small invertebrates.
  • For example, certain nematodes from the order Dorylaimida may ingest both fungal spores and bacteria, demonstrating their versatile dietary preferences.
  • This ability to consume multiple food sources allows omnivorous nematodes to adapt to varying environmental conditions.
• Unknown Feeders:
  • Due to limited research, the feeding habits of some free-living nematodes remain unidentified.
  • Their microscopic size complicates the study of their feeding behaviors, making it challenging to determine whether they feed on decaying plant roots or fungi.
  • Some nematodes displaying ambiguous feeding behaviors may be categorized as plant associates, indicating the need for further research to elucidate their roles in ecosystems.

Classification of Nematodes

The classification of nematodes, also known as roundworms, is a complex process due to the vast diversity of forms and structures found within the phylum Nematoda. Currently, approximately 15,000 nematode species are identified, and this classification is crucial for understanding their ecological roles, evolutionary relationships, and potential impacts on human health and agriculture. The primary classification of nematodes is based on specific morphological features, particularly the presence or absence of sensory structures called phasmids.

• Phylum Nematoda:
  • Nematodes are categorized into two main classes: Aphasmidia and Phasmidia, based on the presence of phasmids.
• Class Aphasmidia:
  • This class comprises nematodes that lack phasmids, yet most species possess caudal adhesive glands and epidermal glands.
  • Amphids, which are chemosensory organs, are typically postliberal in shape and can appear pouch-like, tube-like, or pore-like.
  • Excretory systems are characterized by renette cells, which do not include collecting tubules.
  • The majority of species within this class inhabit marine environments and include both free-living and parasitic types. They can also be found in terrestrial and freshwater ecosystems.
  • Orders within Aphasmidia:
    • Dorylaimida:
      • Known for their smooth, bristle-free cuticle.
      • Mostly terrestrial; characterized by a protruding spear in the buccal cavity and 6-10 labial papillae.
      • Example: Trichodoris.
    • Chromadorida:
      • Features a cuticle that is either ringed or smooth without bristles.
      • Possesses a posterior bulb in the pharynx; can be free-living or marine.
      • Example: Paracanthonchus.
    • Monohysterida:
      • Exhibits a smooth, ringed, and bristly cuticle with circular amphids.
      • Found in marine, freshwater, or terrestrial environments.
      • Example: Monohystera.
    • Enoplida:
      • Primarily marine, characterized by bristles on the cuticle and cyanthi-form amphids.
      • Example: Anticoma.
    • Araeolaimida:
      • Possesses a smooth cuticle with potential bristles and labial papillae.
      • Features four cephalic bristles and inwardly spiraled amphids.
      • Examples: Wilsonema, Plectus, and Odontophora.
    • Desmoscolecida:
      • Distinguished by a prominent ring of bristles and four sensory bristles at the anterior end.
      • The amphids are crescent or pump-shaped; they inhabit marine environments.
      • Examples: Epsilonema, Desmoscolex, and Greefiella.
    • Mermithida:
      • These nematodes have a smooth, bristle-free cuticle and exhibit 16 labial papillae.
      • They possess reduced or cyanthi-form amphids and display a larval parasitic stage invertebrate hosts while being free-living as adults.
      • Examples: Mermis, Agamermis, and Paramermis.
• Class Phasmidia:
  • This class predominantly consists of parasitic nematodes characterized by the presence of phasmids, which are pouch-like sensory structures.
  • The excretory system includes paired lateral canals.
  • Orders within Phasmidia:
    • Strongylida:
      • Known as lipless vertebrate parasites with established buccal capsules and no pharyngeal bulb.
      • Example: Strongylus.
    • Ascaridida:
      • These are oviparous, stout nematodes that inhabit vertebrate intestines; they may or may not have a posterior bulb in the pharynx.
      • Example: Ascaris.
    • Trichuroida:
      • Commonly referred to as whipworms, characterized by a thin pharynx and lack of lips.
      • Example: Trichuris.
    • Spirurida:
      • Thread-like nematodes with medium to large sizes, exhibiting sexual dimorphism with males being larger.
      • They typically have two lateral lips and a bulbless pharynx, with significant diversity among parasites.
      • Examples include Spirura, Wuchereria bancrofti (Filaria), Loa loa (Eye worm), and Brugia.
    • Camallanida:
      • Thread-like in appearance, with larger females compared to males.
      • They often lack lips and can either have a large or absent buccal capsule.
      • Notable examples include Camallanus, Procamallanus, and Dracunculus medinensis (Guinea-worm).
    • Rhabditida:
      • Exhibits a smooth and ringed cuticle with sensory bristles and a posterior bulb in the pharynx.
      • Includes both free-living and parasitic species.
      • Examples: Heterodera, Bunonema, and Rhabditis.
    • Oxyurida:
      • Ranging from small to medium in size, males possess copulatory spicules while females have a long, pointed tail.
      • These nematodes often parasitize invertebrates and vertebrates.
      • Examples: Oxyuris and Enterobius vermicularis (Pinworm).

Larva Migrans

Larva migrans is a condition in which nematode larvae migrate through human tissues in an abnormal or arrested manner. This typically occurs when a human is infected with a nematode species that is parasitic to non-human hosts, preventing the larvae from completing their normal life cycle. As a result, the larvae wander through tissues, causing inflammation and other symptoms. This phenomenon is classified into two main types: cutaneous larva migrans and visceral larva migrans, depending on whether the migration occurs in the skin or in deeper tissues.

• Larval migration often occurs when non-human nematode species infect humans. Since humans are an atypical host, the larvae fail to develop into adult worms and remain in an arrested migratory state.
• In some cases, human parasitic nematodes may also exhibit abnormal migration when infecting immune individuals. The immune system prevents the larvae from following their normal developmental path, leading to larval arrest.
• Cutaneous Larva Migrans (CLM):
  • This form occurs when nematode larvae migrate through the skin. It is commonly referred to as “creeping eruption” due to the characteristic serpentine appearance of the tracks formed by migrating larvae under the skin.
  • These infections are caused by animal hookworms, typically from dogs or cats. The larvae hatch from eggs in feces and develop in contaminated soil. When humans come into contact with this soil, the larvae penetrate the skin, causing erythematous, itchy, and inflamed tracks along their migration path.
  • Common causes include:
    • Ancylostoma braziliense (hookworm of dogs and cats)
    • Ancylostoma caninum (dog hookworm, particularly in Australia)
    • Uncinaria stenocephala (European dog hookworm)
    • Bunostomum phlebotomum (cattle hookworm)
    • Baylisascaris procyonis (raccoon roundworm)
• Visceral Larva Migrans (VLM):
  • VLM occurs when nematode larvae migrate through deeper tissues or organs, leading to eosinophilic inflammation. This form of larva migrans is more systemic than CLM and can affect multiple organs, including the liver, lungs, and even the central nervous system.
  • In VLM, the larvae do not develop into adult worms but instead cause damage through their wandering behavior within human tissues.
  • Common nematodes causing VLM include:
    • Toxocara canis (dog roundworm), the most common cause of VLM
    • Toxocara cati (cat roundworm)
    • Ascaris suum (pig roundworm)
    • Angiostrongylus cantonensis (rat lungworm)
    • Gnathostoma spinigerum (found in fish and amphibians)
    • Anisakis simplex (found in marine fish)

Cutaneous Larva Migrans

Cutaneous larva migrans, commonly referred to as creeping eruption, is a skin condition caused by the penetration of nematode larvae through the skin. This condition is primarily associated with non-human species of hookworms, which typically infect dogs and cats, leading to a distinctive serpentine rash as the larvae migrate through the epidermis.

• Etiology:
  • The condition is most commonly attributed to Ancylostoma braziliense and Ancylostoma caninum, both of which are non-human hookworms.
  • Parasite eggs are excreted in the feces of infected animals, contaminating the soil. Under suitable environmental conditions, these eggs hatch into larvae.
  • Infection occurs when humans come into contact with soil contaminated by the feces of infected animals. The larvae can then penetrate human skin, initiating the infection.
• Pathogenesis:
  • Upon skin contact, the larvae breach the outer layers of skin. They can migrate through the epidermis for a period ranging from a few days to several months.
  • In normal hosts, such as dogs and cats, the larvae can reach deeper layers of the skin and subsequently enter the bloodstream. Once in the intestine, they mature into adult worms, reproduce, and continue the life cycle by excreting more eggs.
  • However, in human hosts—who serve as accidental hosts—the larvae cannot penetrate the basement membrane to invade the dermis. This limitation confines the infection to the outer layers of the skin, leading to the characteristic symptoms of cutaneous larva migrans.
• Clinical Features:
  • Patients typically experience intense itching and the development of papules that evolve into serpiginous tunnels within the epidermis. The movement of the larvae causes the lesions to appear to creep along the skin, hence the term “creeping eruption.”
  • Secondary bacterial infections may occur due to scratching the lesions.
  • In addition to the primary causative agents, transient creeping eruptions can sometimes arise from infections with the human hookworm, Necator americanus. Other conditions like gnathostomiasis and sparganosis can also present with similar larval migration symptoms, resulting in deeper lesions.
  • Strongyloides stercoralis can cause a particularly rapid-moving lesion in immunocompromised individuals, a phenomenon known as larva currens.
  • Other types of ectopic infections, such as those caused by Fasciola or Paragonimus, can lead to creeping lesions on the abdominal wall.
• Diagnosis:
  • Eosinophilia, an increase in eosinophils in the blood, is rarely observed and typically occurs only when Loeffler’s syndrome develops, which is associated with pulmonary infiltrates.
  • Currently, serological tests for diagnosing cutaneous larva migrans are not available.
  • Biopsy results usually yield few findings of larvae in the skin lesions.
  • Therefore, the diagnosis relies primarily on the clinical features presented, including the distinctive appearance of the lesions.
• Treatment:
  • Thiabendazole has proven effective in treating cutaneous larva migrans. For cases where there are only a few lesions, freezing the advancing edge of the eruption with ethyl chloride can also be an effective treatment strategy.

Visceral Larva Migrans

Visceral larva migrans (VLM) is a condition characterized by the migration of larvae from non-human species of nematodes through the body, primarily following oral ingestion. This condition is notably associated with specific parasites that can cause significant health issues in humans, particularly children who may inadvertently consume contaminated soil.

• Etiology:
  • The most frequent causative agent of visceral larva migrans is the dog ascarid, Toxocara canis. The cat ascarid, Toxocara cati, can also lead to VLM but is less common.
  • Other contributing parasites include Anisakis, large ascarid nematodes typically found in marine animals, Gnathostoma spinigerum, and Angiostrongylus cantonensis.
  • Although rare, human nematodes such as Ascaris lumbricoides and Strongyloides stercoralis can produce VLM when they migrate to ectopic sites within the host.
• Pathogenesis:
  • Infection occurs when infective eggs from contaminated soil, typically from dog and cat feces, are ingested. Upon reaching the small intestine, the eggs hatch, and the larvae penetrate the intestinal wall.
  • Following penetration, the larvae migrate through the bloodstream to various organs, including the liver, lungs, brain, and eyes.
  • Unlike in their natural hosts, the larvae do not mature into adult worms in humans; instead, they cause granulomatous lesions. These lesions result in localized tissue damage and inflammation, which can lead to various clinical manifestations.
• Clinical Features:
  • The clinical manifestations of visceral larva migrans vary depending on the affected organs and the severity and duration of the infection.
  • Children are particularly susceptible due to their tendency to ingest contaminated dirt. Common symptoms include:
    • Fever
    • Hepatomegaly (enlarged liver)
    • Pneumonitis (inflammation of lung tissue)
    • Hyperglobulinemia (elevated levels of globulins in the blood)
    • Pica (compulsive eating of non-nutritive substances)
  • Neurological disturbances, referred to as neural larva migrans, can occur, along with ocular complications known as ophthalmic larva migrans, which may lead to serious vision impairment.
  • Laboratory findings often reveal marked leukocytosis (increased white blood cell count) and persistently high eosinophilia, indicating a response to the parasitic infection.
• Diagnosis:
  • Diagnosis of visceral larva migrans can be achieved through various serological tests, including passive hemagglutination, bentonite flocculation, microprecipitation, and more specifically, enzyme-linked immunosorbent assay (ELISA), which is particularly effective for detecting Toxocara infections.
• Treatment:
  • The primary treatment for visceral larva migrans involves the use of diethylcarbamazine (DEC), administered at a dosage of 100 mg three times a day for three weeks, which effectively kills the larvae and halts disease progression.
  • Thiabendazole may also be used as an alternative treatment option.
  • In cases of severe inflammation or allergic reactions, corticosteroids such as prednisolone may be administered either topically or systemically to reduce symptoms.
• Prophylaxis:
  • Preventive measures focus on controlling the contamination of soil, particularly in areas frequented by children. Regular deworming of household pets can significantly reduce the risk of Toxocara infection and subsequent environmental contamination.

Examples of Nematodes

Here are some examples of notable nematodes:

• Human Parasitic Nematodes:
  • Ascaris lumbricoides: This large intestinal roundworm causes ascariasis and is transmitted through the ingestion of contaminated food or water. It can lead to malnutrition and intestinal blockage.
  • Enterobius vermicularis: Also known as the pinworm, this nematode is a common intestinal parasite, particularly in children. It causes enterobiasis, characterized by anal itching and discomfort.
  • Ancylostoma duodenale and Necator americanus: These are hookworms that cause ancylostomiasis. They penetrate the skin and can lead to anemia and protein deficiency due to blood loss.
  • Strongyloides stercoralis: This threadworm can cause strongyloidiasis, particularly in immunocompromised individuals. It can lead to severe gastrointestinal and systemic symptoms.
  • Toxocara canis and Toxocara cati: These are the dog and cat roundworms, respectively. They can cause visceral larva migrans in humans when their eggs are ingested.
• Animal Parasitic Nematodes:
  • Dirofilaria immitis: Commonly known as heartworm, this nematode affects dogs and can lead to serious cardiovascular disease.
  • Haemonchus contortus: A significant parasite in sheep and goats, this nematode affects the abomasum and can cause severe anemia.
  • Trichinella spiralis: This nematode causes trichinosis in humans, often transmitted through the consumption of undercooked pork containing encysted larvae.
• Free-Living Nematodes:
  • Caenorhabditis elegans: A model organism widely used in biological research, particularly in genetics and developmental biology. It is a free-living nematode found in soil and decomposing organic matter.
  • Rhabditis spp.: These are also free-living nematodes found in soil and decaying plant material.
• Plant-Parasitic Nematodes:
  • Meloidogyne spp.: Commonly known as root-knot nematodes, they infect plant roots, causing significant agricultural damage and crop loss.
  • Heterodera glycines: Known as soybean cyst nematode, it is a major pest of soybean crops and can severely impact yields.
• Marine Nematodes:
  • Plectus spp.: Found in marine sediments, these nematodes play essential roles in the nutrient cycling of marine ecosystems.
  • Halicephalobus mephisto: A deep-sea nematode known for its unique adaptations to extreme environments.