► Nematodes are vermiform, cylindrical and
taper towards their anterior and posterior
ends. The body has a complete digestive
tract and covered with a thick cuticle.
► Adult Nematodes are dioecious and male
worms are typically smaller than the
females and have a typically coiled tail.
► Almost all nematode parasites, humans serve as
the optimum host where they pass their entire
life cycles. For some, however, an intermediate
host is required.
► Humans may serve as the definitive host to the
adult males(Ascaris) or as both an intermediate
and definitive host ( Trichinella).
► Despite their diversity and complexity, all
nematodes generally have 3 stages in their
lifecycle: egg, larva and adult.
Eggs may hatch either within the host or external
environment, where the first stage develops inside the
egg and hatches under suitable conditions and the larva
During the larval stage , the nematodes undergo a series
of four molts, thus referred to as the first, second, third
and the fourth stage, called the Rhabditiform larva,
filariform larva or microfilaria.
The infective stage can be the filariform stage (Hookworm) or the very first stage of the egg itself containing
the larva (Ascaris).
As adults, these live within the human body, but can
exist in the soil as well (Strongyloides).
Unlike protozoan parasites, the adult helminthes are incapable of
increasing their numbers within their definitive host. Thus the
severity of the clinical illness is related to the total number of worms
acquired over a period of time.
Adult nematodes cause damage by mechanical disruption and
toxicity. These may involve erosion, bleeding, inflammatory reaction
and proliferation of tissues.
The primary pathogenic mechanism for a larva appears to be the
invasion and subsequent migration in tissues of the host causing
immunological reactions with allergic or anaphylactic reactions to
fever, lymphadenopathy and transient pneumonias and nocturnal
paroxysms of asthma..
► Ascaris lumbricoides:
► Largest and most common of the intestinal
heminthes parasitizing humans.
► Reside in the Intestinal tract and cause
Ascariasis, producing embarrasment,
discomfort, malnutrition, anemia and occassional
► Cylindrical, fusiform body, light brown to pink.
► Mouth opening at the anterior end with three lips
with fine teeth at the lumen edge.
► Male slightly smaller than female with a curved
tail. Matured female can pass upto 200,000 eggs
► Fertilized eggs enclose a embryo cell and
elleptical in shape with a rough, albuminous
coating over their chitinous shells. Unfertilized
eggs have a thinner outer proteinic membrane.
Definitive hosts: Humans.
Intermediate hosts: None.
Adults live in small intestine and absorb nutrition from semi-digested
food – intense muscular activity – copulate – unsegmented ovum
deposited into intestinal lumen – passed in the feces – eggs
embryonate in the soil for 3 weeks – become infectious – enters
human body on the ingestion of eggs – larva hatch – penetrate the
intestinal mucosa – invade portal veinules – carried to the liver – as
still small can squeeze through capillaries and exit to the hepatic
vein – to the heart – to the lungs – as the larva have increased in
size become too big to wriggle out of the pulmonary capillaries rupture into the alveolar spaces – coughed up and subsequently
swallowed – regain their position in the upper small intestines.
► Pathogenicity and clinical manifestations:
► Both migrating larva and adult worms can
cause pathological changes and
symptoms due to mechanical damage and
► Migrating Larva:
► Larval migration of A.lumbricoides through the
lungs lead to vasrying degrees of pneumonitis
► The migrating larva continuously produce
immunogenic substances that cause immune
and allergic reactions to develop, including
bronchial asthma, transient eosinophillic
pulmonary infiltrates (Loeffler’s syndrome) and
II) Adult worm:
Worm burden small : asymptomatic.
Heavier worm load : Clinical manifestations result…
A) Malnutrition and growth retardation: presence of
Ascaris hinders absorption of fat, protein and
carbohydrates and deficiencies of Vitamins A, B2 and C.
Children frequently infected and show growth retardation
and severe malnutrition.
B) Allergic reaction: Ascaris allergen (a glycoprotein) most potent
allergen of parasitic origin. Clinical manifestations include urticaria,
itch, conjunctivitis and angioneurotic edema.
Symptoms of Ascariasis may be severe due to the characteristic
migratory activities of the adult worm – accelerated by peppery food,
rise in temperature (fever), anaesthesia, improper drugs etc… - can
migrate to bile duct, appendix, pancreatic duct causing obstruction
and inflammation of the organ.
Biliary ascariasis most common – abdominal pain, gall-stones,
gallbladder rupture, peritonitis or liver abscess.
Can cause mechanical obstruction of the ileum ( entangled worm
forming a bolus).
Ascaris may, unusually, crawl out of the patients mouth, nose, ear
or genitourinary organ.
► Lab diagnosis:
► Characteristic eggs ( oval to ellipsoidal
with irregular surface) in the stool
specimen. Occasional adult worm can
also be seen.
► Larva and eosinophils in the sputum,
during the pulmonary phase.
► Treatment and prevention:
► Treatment effective: Mebendazole,
Albendazole and pyrantel pamoate.
► Prevention: Proper disposal of feces.
Also known as Whipworm.
Habitats human cecum causes Trichuriasis.
Anterior part of the body thin and thread like and
constitute a single column of secretory cells (serves as
the oesophagus) and the posterior part is bulbous and
fleshy containing the intestines and reproductory organs
– giving a whiplike appearance.
Male little smaller and shorter than female, with a tightly
► Eggs: Females lay 3,000-10,000 eggs –
characteristic barrel shaped – distinctive
brown shell with a translucent knob at
Only one host : Man.
The adults reside in the colon attached to the colonic
mucosa – gravid female releases eggs – pass out with
the feces – reach soil – mature in 3-5weeks – become
embryonated and infectious- transmission is fecal-oral
route by playing children especially – or food
contaminated ( grown in sewage water) – ingested eggs
hatch in the duodenum – grow and mature for a month
and then move to the cecum as adults.
► Pathogenesis and clinical symptoms:
► Attachment of the adult worms to the colonic
mucosa and subsequent feeding produce
localized ulcerations and hemorrhage (0,005 ml
► Ulcers provide enteric bacteria with a portal of
entry- results in sustained bacteremia.
► IgE mediated immunity present but ineffective to
cause appreciable worm expulsion.
► Light worm load : Asymptomatic.
► Moderate worm loads : damages the intestinal
mucosa, induces nausea, abdominal pain,
diarrhea and stunted growth.
► Children with >800 worm load – entire colonic
mucosa parasitized – significant mucosal
damage – blood loss and anemia.
► Prolapse of the colonic or rectal mucosa through
the anus, particularly with strain at defecation or
► Lab diagnosis:
► Characteristic eggs in feces.
► Treatment and Prevention:
► Treatment of choice: Albendazole,
► Prevention: Improvement of sanitary
facilities and health education.
Ancylostoma duodenale (old world hookworm) and
Necator americanus (new world hookworm):
Both are hookworms. Cause Hookworm infection.
Indistinguishable from eggs and larval stages. Eggs are
small, may show a visible embryonic cleavage ( 2-,4-, or
Difference in adult worms for morphological differences.
Ancylostoma attach to the walls of the small intestine
with their teeth and Necator with their cutting plates.
Rhabdititform larva : immature, newly hatched hookworm
– actively feeding – presence of a long buccal cavity.
Filariform larva: non-feeding, infective larva emerges
from the rhabditiform larva – completes second molt –
hookworm filariform larva has a distinctly pointed tail.
Adult: pinkish, with head turned opposite to their curved
body, giving appearance of hook.
Males typically smaller.
Buccal capsule of A.duodenale has actual teeth,
N.americanus has a pair of cutting plates.
Life cycle: Both life cycles are similar.
Eggs passed out in feces at 4-, 8-cell stage development
– reach soil – hatch within 48 hours – release
rhabditiform larva – feeds on debris and bacteria - molt
to become infective filariform larva – penetrate skin
through unprotected feet – migrates to the lymphatics
and blood system – larva reaches lungs via blood –
penetrate capillaries and enter alveoli – migration
continues into bronchioles – coughed up to the pharynx –
swallowed and deposited in the intestines – mature to
Females lay 10,000 – 20,000 eggs per day, passed out
through feces – reach soil.
Pathogenesis and clinical manifestations:
Adult worms live for years within the intestines, leaving
bleeding points at old sites of attachment.
Adult A.duodenale extracts 0.2 ml blood/worm/day and
N.americanus 0.03ml/worm/day. The accumulated blood
loss is enormous.
Humoral antibody response and immediate
hypersensitivity (IgE mediated eosinophilia) reactions but
may not be effective.
► Worm burden small: Asymptomatic. A diet rich in
iron, protein and vitamins help maintain
► Hookworm disease:
► Repeatedly infected individuals may develop
intense allergic itching at the site of hookworm
penetration, known as Ground-itch.
► Larval migration manifests various symptoms
like sore throat, bloody sputum, wheezing,
headache, mild pneumonia with cough.
Intestinal phase symptoms depend on the number of
A) Chronic infections ( <500 eggs/gram of feces) : light
worm burden – vague GI symptoms, slight anemia and
weight loss or weakness.
B) Acute infection ( >5000 eggs/gram of feces): diarrhea,
anorexia, edema, pain, enteritis and epigastric
discomfort. Feeding hookworms may also lead to
microcytic hypochromic iron deficiency, weakness and
In children, may lead to heart failure or Kwashiorkar.
Mental, sexual, physical retardation can also result with
► Lab diagnosis:
► Characteristic eggs in stool specimen.
► Occult blood is frequent and with
► Treatment of choice: Mebendazole,
Albendazole, pyrantel pamoate.
► Prevention: Proper disposal of sewage
and proper footwear.
► Enterobius vermicularis:
► Also known as Pinworm or seatworm.
► Parasitic only to humans.
► Causes pinworm infection or Enterobiasis.
► Adults are small, white, spindle-shaped
and threadlike. Females have a wing like
expansion of the body wall, at the anterior
end called Alae. Males are smaller and
possess a curved tail.
► Eggs are ovoid with a colorless, thick shell
covering the larva. Embyonated eggs
infective to humans.
Only one host: Man.
Sexually mature worms inhabit the intestines and can spread to adjacent
regions of both small and large intestines like cecum, appendix, rectum,
colon etc…- adhere to the mucosa and feed on epithelial cells and bacteria.
Males die on copulation and the gravid females migrate to the perianal
regions. Lower temperature and aerobic conditions stimulate the laying of
eggs and they perish as well.
Each egg contains an immature larva and the third stage larva completes
development inside the egg.
Infection and reinfection occur – picked up from the bed-clothes, or finger
nails after scratching perianal region – or even inhaled.
Retroinfections can also occur when third stage larva hatch from peri-anally
located eggs and enter the hosts intestine through the anus.
Ingested eggs hatch shortly after reaching the duodenum – escape – molt
and reach adulthood before residing in the colon.
Pathogenesis and clinical symptoms:
Not highly pathogenic as does not cause significant damage to the
colonic mucosa and the allergic toxicity of pinworms is yet to be
Itching and irritation due to the gravid females migration around the
Heavy infections in children may cause sleeplessness, weight loss,
hyperactivity, teeth grinding, abdominal pain and vomiting.
Gravid females may migrate up the female genital tract causing
vaginitis, endometritis and granuloma in the uterus and fallopian
tubes – may also reach appendix, peritoneal cavity or urinary
► Lab diagnosis:
► Demonstration of small white thread-like
worms in the under garments of children.
► Eggs rarely found in feces – strip of
cellophane around the perianal area
touching the skin and observe under
microscope. Negative results for 7 days
consecutively indicates infection free.
► Treatment of choice: Mebendazole or
pyrantel pamoate kills worms in the colon,
but not the eggs. Hence, retreatment in 2
weeks time may be necessary. Reinfec
tion is common.
► Prevention: No definite prevention known.