Parasitology: Nematodes

Nematodes

General Characteristics

• Nematodes, also known as "roundworms," possess cylindrical and bilaterally symmetrical bodies.

• They have a complete digestive system, featuring both a mouth and an anus.

• Nematodes possess a pseudocoelom, which is a fluid-filled cavity.

• They lack both a cardiovascular and respiratory system.

• Nematodes have a well-developed reproductive tract.

Classification of Helminths

• Nematodes are classified under the phylum Aschelminthes.

• Other helminths include Platyhelminthes with classes Trematoda and Cestoda.

• Nematodes primarily involve the intestinal system of the host, while Trematoda and Cestoda affect blood and tissues.

• Examples of Nematode species include:

  • Enterobius vermicularis

  • Ascaris lumbricoides

  • Trichuris trichiura

  • Ancylostoma duodenale

  • Necator americanus

  • Strongyloides stercoralis

  • Trichinella spiralis

  • Wuchereria bancrofti

  • Dracunculus medinensis

  • Brugia sp.

  • Loa loa

  • Oncocerca volvulus

  • Mansonella sp.

Reproduction and Infection

• Both male and female worms exist.

• Males are typically smaller than females.

• Humans are considered the definitive host for many nematodes.

• Reproduction occurs through fertilized eggs or larvae.

• Mechanism of infection by female nematode:

  1. Ingestion of eggs.

  2. Skin penetration from the soil.

  3. Transmission via vectors.

Ascaris lumbricoides (Large Intestinal Roundworm)

• Classified by Linnaeus in the 1700s.

• Affects one billion people worldwide.

• Common source: using human feces as fertilizer.

• Eggs are found in "coprolites" (stony casts in human feces).

• Female worms can produce 200,000 eggs per day.

Morphology of Adults

• Large worms, reaching up to 35 cm in length (males 10-31 cm, females 22-35 cm).

• Sometimes excreted in feces and may protrude.

• Terminal mouth with three lips and sensory papillae.

• Adults reside in the mucosa of the small intestine.

• Produce pepsin inhibitor 3 to resist digestion.

• Produces phosphorylcholine to suppress lymphocyte proliferation.

• Female worms have paired reproductive organs.

• Males have a curved ventral posterior end with 2 spicules.

Eggs of Ascaris lumbricoides

• In fertile eggs are longer, ranging from $80$ to $90$ um long.

• They appear disorganized with no visible structures.

• They have a thin shell and irregular mammilated coating.

• If the mammilated covering is missing, they are considered decorticated.

• Fertile eggs are broad and oval, ranging from $45-75$ um long and $35-45$ um in breadth.

• They have a thick outer transparent shell with a bile-stained mammilated coating with hyaline.

• They feature a delicate vitelline layer and lipoidal inner membrane.

• Decorticated eggs lack the mammilated coating.

Infective Stage

• The infective stage is a fully embryonated egg.

• Acquisition occurs through ingestion.

• Eggs hatch in the lumen of the small intestine.

• Larvae migrate to the cecum and initial part of the colon (large intestine).

• They then enter venules and travel to the liver via the portal vein, then to the heart, pulmonary vessels, and air sacs of the lungs.

• Larvae then proceed to the larynx, pharynx, and are reswallowed to the GI tract, potentially leading to Loffler’s syndrome.

Life Cycle

1. Eggs are ingested.

2. Larvae hatch in the small intestine.

3. Larvae migrate through the bloodstream to the lungs.

4. Larvae are coughed up and swallowed.

5. Maturation occurs in the small intestine.

6. Adults reside in the small intestine.

7. Eggs are passed in feces.

Pathogenesis

• Some infections are asymptomatic with low worm burden.

• Heavy worm burden infections cause symptoms such as abdominal pain, intestinal obstruction, and intestinal perforation.

• Migrating worms can cause biliary tract occlusion, appendicitis, nasopharyngeal expulsion.

• Eosinophilia is prominent during larval migration.

• The most frequent complaint is abdominal pain.

• Malabsorption may occur.

• Excessive biting of the adult worms for food may cause irritation.

Diagnosis

• Microscopic identification of eggs in stool samples.

• Methods include direct wet mount smear and Kato-Katz technique (quantitative).

• Gastric or pulmonary aspirate (migration).

• Sputum (migration phase).

• Molecular methods.

Treatment

• Antihelminthic drugs are used.

• Common medications include Mebendazole or Albendazole (Benzimidazoles) and Piperazine.

• "Mass drug administrations" are also implemented.

Epidemiology

• One of the most common human helminthic infections globally.

• Highest prevalence in tropical and subtropical regions.

• Generally rare or absent in developed countries.

• Common in impoverished regions.

• Approximately 2,000 deaths annually in heavily infected rural areas globally.

• Highest prevalence in East Asia and the Pacific.

• Main factors contributing to prevalence:

  • High population density.

  • Use of night soil as fertilizer.

  • Illiteracy.

  • Poor sanitation.

  • Poor community hygiene.

Prevention and Control

• Surveillance and monitoring.

• Strong Soil-Transmitted Helminth (STH) control programs by community health departments.

• Mass drug administration programs (DOH).

• Availability of sanitary facilities.

• Proper handwashing.

• Community promotion of hygiene.

• Avoidance of using human feces as fertilizer.

Enterobius vermicularis (Pinworm)

• Causes Enterobiasis or Oxyuriasis.

• The most common helminth parasite in temperate regions.

• Not a serious threat to overall health; causes discomfort through anal irritation.

• Children are more susceptible due to frequent hand-to-mouth contact.

• Perianal itching or “Pruritus ani” is the most characteristic symptom.

Morphology of Adults

• The female worm is larger than the male.

• They have cuticular alar expansions at the anterior end and a prominent posterior esophageal bulb.

• Adult female size: 8-13 um x 0.4 mm.

• Adult male size: 2-5 um x 0.1-0.2 mm, with a curved tail and single spicule.

• Males usually die after copulation.

• Found in the cecum.

• Gravid (pregnant) females migrate during nighttime to exit the anus and deposit eggs in the perianal skin.

Eggs of Enterobius vermicularis

• Oval but flattened on one side, described as "D-shaped Ova."

• Asymmetrical with a size of $50-60$ um x $20-30$ um.

• Thick shell containing an outer triple albuminous covering for mechanical protection, and an inner embryonic lipoidal membrane for chemical protection.

• Larvae visible inside the egg.

Life Cycle

1. Eggs are deposited on perianal folds.

2. Embryonated eggs are ingested by a human.

3. Larvae hatch in the small intestine.

4. Adults reside in the lumen of the cecum.

5. Gravid females migrate to the perianal region at night to lay eggs.

Infective Stage

• The infective stage is the embryonated egg.

• Eggs become fully embryonated in 6 hours and contain the 3rd stage larva.

• Resistant to disinfectants.

• In moist conditions, can survive up to 13 days.

• Survive well in high humidity and moderate temperatures.

• Can be inhaled after constant scratching (Autoinfection).

Pathogenesis

• The route of infection is via the mouth and respiratory system.

• Some infections are asymptomatic.

• Pruritus ani (itchy anus) is a common symptom.

• In children: irritability, nervousness, disruptive behavior.

• Skin breaks can lead to bacterial infections.

• Rare cases: loss of appetite, inflamed vagina in females, appendicitis, intestinal obstruction.

Diagnosis

• Graham’s scotch adhesive tape swab or Perianal cellulose tape swab.

• Biopsies can be used in some cases.

Treatment

• Mebendazole 100 mg PO single dose.

• Or Albendazole 400 mg PO single dose.

• Treatment of the entire household is recommended.

Epidemiology

• Temperate and tropical regions.

• High prevalence in developed and developing countries.

• Cannot be controlled by sanitation.

• Risk factors include thumb sucking, overcrowding, nail-biting, and lack of education on worms.

• Prevalence of about 29% in the Philippines.

Prevention and Control

• Personal cleanliness.

• Personal hygiene.

• Fingernails should be cut short.

• Proper handwashing.

• Use of showers and bathtubs.

• Linen should be washed thoroughly with hot water and soap/detergents.

• Vacuum cleaning.

• Mass drug administration as public health advocacy in elementary schools.

Trichuris trichiura (Whipworm)

• Causes trichuriasis.

• Resembles a "whip" in shape.

• Soil-transmitted helminth.

• Third most common roundworm of humans.

• Common in tropical countries with poor sanitation practices.

• More common in children.

• Commonly affects the large intestine of humans.

Morphology of Adults

• Long and slender anterior portion with thicker, wider handles at the posterior end.

• Narrow esophagus resembling beads.

• Single set of reproductive organs.

• Male: coiled tail, 30-50 mm x 35-50 mm with a single spicule.

• Female: rounded blunt posterior end, 30-45 um in length.

• Lay 3,000 to 10,000 eggs / day.

• Inhabit the cecum and colon.

• Larvae penetrate the intestinal villi and produce Pore-forming protein 🡪 TT47.

Eggs of Trichuris trichiura

• The infective stage.

• Lemon or football-shaped, also described as barrel-shaped.

• Translucent prominent bipolar mucogelatinous plugs at each terminal end.

• Covered by a smooth thick shell.

• More susceptible to desiccation (moisture removal).

• Larvae penetrate the intestinal villi after escaping.

• No heart-lung migration (unlike Ascaris).

Life Cycle

1. Unembryonated eggs are passed in feces.

2. Eggs undergo a 2-cell stage.

3. Embryonated eggs are ingested.

4. Larvae hatch in the small intestine.

5. Adults reside in the cecum.

Pathogenesis

• Ingestion of eggs via contaminated water.

• Soil contact or eating vegetables from contaminated soil.

• Close contact during gardening, farming, and placing dirty fingers in the mouth.

• Trichuris dysentery syndrome:

  • Chronic dysentery.

  • Rectal Prolapse.

  • Blood-streaked diarrhea.

  • Abdominal pain.

  • Tenderness.

  • Blood loss / Anemia.

  • Poor appetite, wasting.

• No Lung or Heart Migration.

Diagnosis

• Usually diagnosed only in heavy infections.

• Direct Fecal Smear.

• Kato thick / Kato Katz smear (calculate egg by quantitation).

• Concentration techniques.

• FLOTAC.

Treatment

• Mebendazole 100 mg 2x a day for 3 days.

• Others: Albendazole.

• Mass Drug Administration.

Prevention and Control

• Biannual MDA.

• Provisions of a safe water supply.

• Proper sanitation.

• Hygiene Education.

• Soil control.

Strongyloides Stercoralis (Threadworm)

• Has a free-living "rhabditiform" stage and a parasitic "filariform" stage.

• A major cause of strongyloidiasis in humans.

• Produces mild, short-lived cutaneous infections in humans.

• Distributed in tropical and subtropical countries.

• Common in summer months.

Morphology of Worms

• First stage “Rhabditiform” (L1) Larva:

  • Usually found in stool.

  • Eggs embryonate and hatch in the small intestine with only a thin shell.

  • Found in soil, 180-30 um long.

  • Short buccal canal.

  • Esophagus spanning 1/3 of body length.

  • Pyiform posterior bulb.

• Second stage “Rhabditiform” (L2) Larva:

  • Longer, smaller esophagus:intestine ratio.

• Female “parasitic/filaiform” (L3) Adult:

  • 2. 0 x 0.4 mm, colorless, semi-transparent.

  • The infective stage, with finely striated cuticle and a notched tail.

  • Esophagus to intestine ratio = 1:1.

  • Found in the soil.

  • Invades the host (humans) by skin penetration causing autoinfection.

• Free-Living Adult Stage:

  • Found in the host or human soil.

  • Females are long, slender; 1.0 mm and measure 2.0 to 3.0 mm in length.

  • With muscular double-walled esophagus. Indirect cycle having the free-living forms.

Life Cycle

• Can have both a free-living cycle and a parasitic cycle.

• Free-living cycle involves development in the soil, while the parasitic cycle involves infecting a host.

• Autoinfection can occur where rhabditiform larvae in the large intestine become filariform, penetrate intestinal mucosa (or perianal skin), and migrate to other organs.

Pathogenesis

• Three stages of infection:

  1. Skin Invasion Stage

  2. Migration Stage

  3. Penetration Stage

• Light infection = no intestinal symptoms.

• Moderate infection = diarrhea with constipation.

• Heavy infection = intermittent diarrhea, “Cochin China Diarrhea”, numerous bloody stools.

• Skin Invasion:

  • Erythema, pruritic elevated hemorrhagic papules.

• Migration:

  • Lungs are destroyed results to Lobar pneumonia, hemorrhage.

  • Cough, tracheal irritation, “Bronchitis-like”.

• Penetration:

  • Adults found in intestinal mucosa from the stomach to the rectum.

  • Greater numbers found in the duodenum and ileum.

• Chronic Infection – asymptomatic, vomiting, diarrhea, constipation, borborygmi may be seen

• Complications: Edema, loss of appetite, anemia, intestinal obstruction, malabsorption, GI Bleeding.

Diagnosis

• Microscopy (rhabditiform larva) in stool

• Biopsy specimens, wet mounts, concentration techniques.

• Duodenal fluid, Entero test, Harada-Mori culture, Baerman

Treatment

• Albendazole.

• Thiabendazole.

• Ivermectin = for hyperinfection.

Prevention and Control

• Proper sanitation.

• Feces disposal.

• Avoid walking barefoot.

Hookworms

• Cause intestinal hookworm disease, affecting 460-480 million people globally.

• Characterized by the "hook-like" shape of the head.

• Live in the small intestines as blood-sucking nematodes.

• Soil-transmitted helminth.

• More commonly found in tropical and subtropical countries.

• Two most common species:

  • Necator americanus

  • Ancylostoma duodenale

Taxonomical Classification

• Kingdom: Animalia

• Phylum: Nematode

• Class: Secernentea

• Order: Strongiloidae

• Family: Ancylostomatidae

• Genus: Necator/Ancylostoma

• Species: A. duodenale and N. Americanus

Epidemiology

• Estimated to affect 900 million people worldwide.

• Hookworm disease is common throughout the tropics and subtropics region of the world.

• Do not live long in clay, dry, hard-packed soils, or where temperatures are freezing or higher than $45$°C.

• Thrive between $25$°C and $35$°C in shady, sandy, or loamy soil for their larval development.

• Larvae can survive for up to 6 weeks in moist, shady, sandy, or loamy soil.

• Spread of the parasite in populations that do not wear shoes.

Introduction

• Hookworm is the second most common human helminthic infection after ascariasis.

• Two species of hookworms are human parasites:

  • Ancylostoma duodenale:

    • Predominant in S. Europe, N. Africa, western Asia, northern China, Japan, and the west coast of America.

  • Necator americanus:

    • Predominant in sub-Saharan Africa, south Asia, and the Pacific area and endemic in warm, moist tropical areas where people defecate in the soil.

• Common name - Old world hookworm

• Habitat - Small intestine

• Definitive host - Human

• Route of infection - Filariform larvae penetrate the skin of human

• Infective stage - Third stage larvae (filariform)

• Diagnostic stage - Eggs in Stool

• Disease - Hookworm infection, Ancylostomiasis

• Life span: A duodenale (one year) and N. americanus (three to five years)

Morphology

• Ancylostoma duodenale:

  • Females are 10-13 mm in length by 0.6 mm in breadth.

  • Males are 8-11 mm by 0.4 mm breadth.

  • Posterior end has an umbrella-shaped bursa with riblike rays in male, but in female the posterior end is straight.

  • Two pairs of curved teeth on the ventral wall of its buccal capsule.

  • Cylindrical, greyish white, and slightly curved with the anterior end bent slightly in the same direction of the body curve.

• Necator americanus:

  • Females are 9-11 mm in length by 0.4 mm in diameter.

  • Males are 7-9 mm by 0.3 mm.

  • Smaller than A. duodenale.

  • A pair of semilunar cutting plates on the ventral wall of the buccal capsule.

• Eggs of hookworm:

  • Indistinguishable between A. Duodenale and N. amircana.

  • Shape: oval with Clear space between segmented ovum & egg shell

  • Size: 60 x 40 μm.

  • Shell: thin egg shell, colorless and transparent containing 4-8 blastomeres

• Rhabditiform Larva (L1, L2):

  • Size: 250 x 15 μm, long, cylindrical shape with a long buccal cavity, and a bulbous-shaped esophagus.

• Filariform Larva (L3):

  • Infective stage larva is the filariform larva with a size ranging from 600-700 x 20-25 μm.

  • Long and slender, shaped larva

Differences Between A.duodenale and N. americana

Mode of Transmission

• Penetration of skin.

• Ingestion of filariform larvae.

• Breast milk from mother to infants.

• Transplacental transmission.

• Most common Sites of skin penetration: Thin skin between toes, dorsum of the feet, and inner side of the soles

Life Cycle

• Adult worms live in the small intestine and females lay 5-10000 eggs a day over 5 years .

• Eggs are passed with the feces

• Larvae develop outside the body and molt twice

• The filariform or L3 larvae move to the surface in search for a host

• If they come into contact with the host they penetrate the skin, enter blood vessels, and leave the circulatory system into the alveoli

• The larvae move up the trachea into the esophagus, are swallowed, and finally reach the intestine, where they molt twice more before they reach maturity

Signs and Symptoms

• Skin Infection: "Ground itch" (pruritic papule or vesicle).

• Lung Infection: Pneumonia, Dry cough, wheezing.

• Ingestion: Throat soreness, hoarseness, nausea, vomiting.

• GI Infection: Bloody stool from former attachment sites, abdominal pain, abdominal discomfort, anemia, etc

Pathology

• Tissue damage and symptoms are caused by migrating larvae and adult worms.

• Cutaneous phase:

  • Lesions in the skin due to the filariform larva penetrating the skin - maculopapules, erythema, and heavy itching.

• Pulmonary phase:

  • Lesions in the lungs - Bronchitis & bronchopneumonia: Larva carried in the circulation.

• Intestinal phase:

  • Hooked to the intestinal mucosal wall: --abdominal pain, nausea, diarrhea, Gastrointestinal bleeding, epigastric pain

Clinical Diagnosis

• Atrophic glossitis found with hypochromic microcytic anemia, caused by heavy infection of hookworm TPatient with atrophic glossitis also shows fingernail deformity (koilonichia)

Laboratory Diagnosis

• Direct Methods - direct stool smear for eggs or sometimes adult parasites

• Stool culture by Harada-Mori method.

• Blood examination:

  • Low erythrocyte count and haemoglobin level.

  • Increased number of eosinophil count.

• Imaging Methods - chest x ray to look for interstitial thickening or diffused nodular mottling

Treatment

• Hookworm infection is treated with albendazole, mebendazole, pyrantel pamoate

Filarial Worms

Classification

A. Lymphatic filariasis:

• Wuchereria bancrofti (Bancroftian filariasis)

• Brugia malayi (Malayan filariasis)

• Brugia timori

B. Subcutaneous filariasis:

• Loa loa (Calabar swelling/Fugitive swelling)

• Onchocerca volvulus (River blindness, dermatitis)

• Mansonella streptocerca (Skin diseases)

C. Serous cavity filariasis:

• Mansonella ozzardi (Non-pathogenic)

• Mansonella perstans (Non-pathogenic)

D. Zoonotic filariasis:

• Dirofilaria immitis

• Dirofilaria repens

• Brugia pahangi

• Brugia beaveri

• Brugia leporis

Global Scenario

• Population at risk: 1.2 Billion

• No. of countries: > 80

• Mf carriers: 76 Million

• Diseased: 44 Million

• Hydrocele: 27 Million

• Lymphoedema: 16 Million

• TPE: 1 Million

Epidemiology

• Mapping done to see LF Distribution In Endemic Countries

Transmission & Incubation Period

• Lymphatic Filariasis is transmitted by the bite of Infected mosquito which harbors L3 larva.

• Clinical Incubation period: 8-16 months

Clinical Manifestations

• Manifestations are 2 types

  1. Lymphatic Filariasis (Presence of Adult worms)

  2. Occult Filariasis (Immuno hyper responsiveness)

Stages in Lymphatic Filariasis

• There are 4 stages:

  1. Asymptomatic amicrofilariaemic stage

  2. Asymptomatic microfilariaemic stage

  3. Stage of Acute manifestation

  4. Stage of Obstructive (Chronic) lesions

Wuchereria bancrofti (Filarial worm)

• Definitive host: Man

• Intermediate host: Female Culex, Aedes or Anopheles mosquito

• Infective form: Third stage larva

• Mode of transmission: Inoculation - bite of mosquito.

• Site of localization: Lymphatics/lymph nodes of man

• Geographical distribution: India, China, Far East, Africa, South & Central America

Diagnosis

• Classical Filariasis vs Occult Filariasis

• Demonstration of microfilarae in the peripheral blood by Thick blood smear or membrane filtration method

• Immuno Chromatographic Test (ICT): Antigen detection assay can be done by Card test and through ELISA. Circulating Filarial Antigen detection is regarded as “Gold Standard” for diagnosing Wuchereria bancrofti infection

• Ultrasonography can locate and visualize the movements of living adult worms of W.b. in the scrotal lymphatics of asymptomatic males with microfilaraemia

• Haematology Increase in eosinophil count

Treatment

• Diethylcarbamazine (DEC) for microfilaria

Brugia malayi

• Malayan Filariasis

Loa loa

• African eye worm.

• Calabar swelling.

• Fugitive swelling.

• Adult: 3-7cm long.

• Treatment: Dec. Surgical removal

Onchocerca volvulus

• River blindness, convoluted or blinding filaria, adult reaches 50cm long

Capillaria philippinensis

Defining characteristics

• Gradual caudal and cephalic transition

• Eggs have two plugs with pits

Pathogenesis

• Capillariasis

• IH: fish

Diagnosis

• Direct smear

• Wet mount

• Stool concentration methods

• ELISA (coproantigen detection)

Treatment

• Electrolyte replacement

• Mebendazole 2x a day 200mg x 20 days

TRICHINELLA SPIRALIS

Defining characteristics

• Common name: Trichina worm

• Tissue nematode

• Cause: Trichinosis

Habitat

• Mucosa of small intestine of pig, rat, bear, & man

• Striated muscles- Encysted larvae

• No free-living forms

Morphology

• Thin & pointed anterior half of the body

• Pair of Claspers at the posterior end

Pathogenesis

• Larvae are released in the stomach and mature into adults over 1-2 weeks in small bowel causing: Irritation and mild abdominal cramping or even diarrhea

• Larvae migrate, penetrate striated muscle, reside in nurse cells, and encyst causing: Muscle pain, fever, periorbital edema, eosinophilia, occasional CNS or cardiac damage

• Encysted larvae ingested in undercooked pork, boar, horse, or bear

Diagnosis

• HISTORY Eating pork 2 weeks earlier with Recent -gasteroenteritis or BLOOD EXAMINATION showing Eosinophilia levels(20-95%)