Class Nematoda The Roundworms
Introduction:
Nematodes comprise the group of organisms containing the
largest number of helminth parasites of humans. They are
unsegmented, bilaterally symmetrical, and exhibit great
variation in their life cycles. Generally, they are long-lived (130+ years).
 Both free-living and parasitic forms - some can have both
free-living and parasitic stages in their life cycle.
 Vary greatly in size - from a few millimeters to over a meter.
 Male worms - frequently have a curved or coiled posterior
end with copulatory spicules; Some species exhibit a
copulatory bursa.
Class Nematoda The Roundworms
The adult anterior - may have hooks, teeth, or cutting plates in
the buccal cavity. These are used for attachment.
 Body is complex - the outer body surface is a cuticle, there
are muscle layers underneath.
 Internal organs - include a complex nerve cord, a welldeveloped digestive system and complete reproductive
organs. Males have testes, vas deferens, seminal vesicle
and an ejaculatory duct. Females have ovaries, oviduct,
seminal receptacle, uterus and vagina.
 Reproductive capacity - proportional to complexity of life
cycle.
Class Nematoda The Roundworms




Humans are definitive hosts.
Arthropods may serve as intermediate hosts
and/or vectors. Many nematodes require no
intermediate host.
The adult female produces fertilized eggs, or
larvae which may be infective to new host.
Eggs may be immediately infective after
ingestion by humans.
Class Nematoda The Roundworms



Eggs or larvae require a period of development
in the environment to become infective.
Eggs or larvae are transmitted to a new host by
an insect.
Developing larvae go through a series of 4 molts
with the third stage, the filariform larva, most
often being the infective stage.
Class Nematoda The Roundworms



Infection with roundworms can be by ingestion of
infective eggs or larvae, by larval penetration of skin, or
by transmission of larvae through insect bite.
About one-half of the nematodes parasitic for man are
intestinal, the others are found in various tissues.
Pathogenicity of intestinal nematodes may be due to
larval migration through tissue, piercing of intestinal
wall, blood loss, or allergic reactions to secretions of
adults or larvae.
Class Nematoda The Roundworms
Terminology:
rd
 Filariform larvae - the 3 or infective stage; Long,
thread-like; Designed for penetration.
 Rhabditiform larvae - characterized by the
presence of a muscular esophagus and bulbular
pharynx. The worms leaving the egg are termed
“rhabditiform” larvae.
Class Nematoda The Roundworms



Egg - characteristic of the genus. Size & shape are
relatively consistent.
Larvae - undergo several molts (third stage usually
the infective stage).
Adult - varies in size from genus to genus; Range
from less than 1 mm to over one meter.
Class Nematoda The Roundworms
General life cycle of intestinal nematodes:
 Humans ingest infective eggs. Hookworm and
Strongyloides stercoralis are exceptions. In these,
filariform larvae penetrate the skin to gain entry.
 Larvae hatch in intestine.
 Male and female adults develop in the intestine. With
Ascaris lumbricoides, hookworms, and Strongyloides
stercoralis larvae penetrate the intestinal mucosa and
initiate a heart - lung cycle enroute to the intestinal tract
to mature to adults.
Class Nematoda The Roundworms



Fertilized eggs are produced.
Diagnostic stages, eggs or larvae, exit the host
in its feces.
Larvae develop within the egg in warm, moist
soil (except for Strongyloides stercoralis, whose
eggs hatch in the intestine, with larvae passing
in the feces).
Class Nematoda The Roundworms
Enterobius vermicularis - the pinworm.







Most common helminth infection in the U.S.A.
Transmission is direct, person-to-person.
Egg is infective immediately or within hours of
being shed by the female.
Common worldwide but more prevalent in
temperate climates.
Higher prevalence in Caucasians than in
Negroes.
It is a group infection especially common
among children. Very often associated with
low sanitation and hygiene.
Humans are the only known host. Dogs and
cats are not infected.
Class Nematoda The Roundworms
Life cycle:

Eggs are ingested, hatch in intestine,
larvae mature to adults.

Gravid females migrate to the perianal
area at night to lay eggs.

Eggs develop to infective stage within 4-6
hours. Eggs can survive for extended
periods in cool, moist environment.

Eggs are found rarely in fecal samples;
Release is most often external to the
intestines. Dying worms may release
eggs in the bowel.
Class Nematoda The Roundworms

Adults - female: creamy white, ~ 8-13 mm
long, with sharply pointed tails; Wing-like
flaps (cervical alae) at head end; Male:
small (2-5 mm) with strongly curved
posterior.

Eggs - 50 to 60 x 20 to 32 microns, broadly
oval, and flattened on one side.
Compressed laterally; Normally are
embryonated (contain a larva).
Class Nematoda The Roundworms
Diagnosis:
 Recovery and identification of
eggs or adults from the perianal
region utilizing the cellophane
tape preparation.
 Specimens must be collected the
first thing in the morning upon
waking, especially before bathing
or bowel movements.
 Eggs are rarely found in fecal
samples because release is
usually external to the intestines.
Class Nematoda The Roundworms
Major pathology and symptoms:

One third of all cases are asymptomatic.

Infections rarely cause serious lesions.

Symptoms associated with the migration
of the female out of the anus to lay her
eggs include: perianal itching, nausea or
vomiting, loss of sleep, irritability,
irritation of the intestinal mucosa, and
vulval irritation in females due to
migrating worms entering the vagina
instead of the re-entering anus.
Class Nematoda The Roundworms
Trichuris trichiura - the whipworm.
Life cycle:
 Infective, fully embryonated eggs are
ingested, larvae hatch in small intestine,
penetrate and develop in the intestinal
villi, return to lumen and migrate to the
area of the cecum.
 Larvae mature and live in the colon.
Worms embed their anterior portion (as
much as two-thirds of the worm) into the
mucosa.
Class Nematoda The Roundworms
Life Cycle: (continued)
 Barrel-shaped eggs are released
into the stool.
 Eggs must undergo
development in the soil for
approximately 10 days to 3
weeks before they become
infective.
 The worm’s life span is
estimated to be 4 - 8 years.
Class Nematoda The Roundworms
Morphology:
 Adults - females: 35 to 50 mm
long, anterior two-thirds is
long and threadlike, expanding
into a broader posterior;
males: 30 to 45 mm long,
similar to female but exhibiting
a strong curvature of tail.
 Eggs - 50 to 55 x 22 to 25
microns, barrel-shaped, with
clear polar plug at each end.
Class Nematoda The Roundworms

Diagnosis - recovery and identification
of eggs in the feces.
Major pathology and symptoms:
 Slight infections - usually asymptomatic.
 Heavy infections - surface of colon is
matted with worms which causes
bloody or mucous diarrhea.
 weight loss and weakness - infections
with 200 or more worms in children may
cause a chronic dysentery, profound
anemia and growth retardation.
Class Nematoda The Roundworms
Major pathology and symptoms:
(continued)
 Abdominal pain and tenderness.
 increased peristalsis and rectal
prolapse, especially in children.
Class Nematoda The Roundworms
Distribution:

In the U.S.A., prevalent in the warm, humid
climate of the southeastern states. Commonly
found in refugees from tropical areas.

The third most common intestinal helminth
infection.

Higher prevalence in warm countries and areas
of poor sanitation, especially in countries which
utilize "night soil" for fertilizer.

Common among children and in the
institutionalized mentally retarded.

Commonly seen along with Ascaris lumbricoides
because of the similar mode of infection.
Class Nematoda The Roundworms
Ascaris lumbricoides - Large Intestinal Roundworm
Life cycle: (complex, involves a heart-lung cycle)

Humans ingest embryonated eggs containing
infective larvae.
 Larvae hatch from the eggs in the small intestine,
penetrate the intestine wall, enter the
bloodstream, migrate to the liver, travel to the
lung via the blood stream.

Larvae break out of lung capillaries into alveoli,
travel to the bronchioles, and are coughed up to
the pharynx. They are swallowed and return to
the intestine. Two molts to 4th stage larvae take
place in alveoli.
Class Nematoda The Roundworms
Life cycle: (continued)

Larvae mature to adults in the small intestine.

Worms do not attach to the intestinal wall, but
maintain their position by constant movement.
Worms have a life span of approximately 1
year.

Undeveloped eggs are passed in the feces.
These eggs develop in soil and are infective
after two weeks to one month. The egg shell is
very thick and resistant to environmental
changes.

Eggs can remain infective for up to 5 years if
protected from direct sunlight and desiccation.
Class Nematoda The Roundworms
Morphology:
 Adults - males are 15 to 30 cm long,
with strongly curved tails; females are
20 to 35 cm long, with straight tails.
 Eggs - one female produces 200,000
per day. The egg has an outer shell
membrane which is heavily mamillated.
This layer is sometimes rubbed off in
passage down the fecal stream.
Infertile eggs often appear longer, and
thinner shelled.
Class Nematoda The Roundworms
Diagnosis Identification of eggs and/or adults in fecal samples.
Major pathology and symptoms:

Pneumonia associated with migration of larvae in
the lungs.

Obstruction of the intestines, appendix, or
common bile duct.

Vomiting and abdominal pain.

May cause malnutrition in children with heavy
infections or poor diet.

Some infections are asymptomatic.
Class Nematoda The Roundworms
Distribution - Worldwide, similar to
T. trichiura; Eggs thrive best in
soil with dense shade, heavy rain
and warm climate.
General Information:
 Ascaris is the largest intestinal
nematode.
 The second most common
helminth infection in the U.S.
Class Nematoda The Roundworms



Mixed infections with T. trichiura are
common. If both are present, treat the
Ascaris infection first, since it is the more
likely of the two to migrate.
Non-specific treatment or administration of
anesthesia can cause worms to migrate,
including penetrating the intestinal wall;
forcing through the pyloric and cardiac
valves of the stomach, thus entering the
esophagus; or crawling into the common
bile duct.
Adult worms may rarely be recovered from
the anus, mouth, throat or nose.
Class Nematoda The Roundworms
Necator americanus - The New World hookworm
Ancylostoma duodenale - The Old World hookworm
Female
Male
Hookworm egg
Life cycle:

Eggs shed in soil hatch within 48 hours,
becoming rhabditiform larvae (1st & 2nd stages).

After ~ 7 days, worms stop feeding and molt,
transforming from the rhabditiform larvae to
infective filariform larvae.

Infections are acquired when the filariform larvae
penetrates the skin of a human.
Class Nematoda The Roundworms
Hookworm rhabditiform larva
Hookworm filariform larva
Life cycle: (continued)

Larvae enter the lymphatic system or
bloodstream, and travel to the lungs. After
maturating in the lungs, they migrate up the
trachea to be swallowed and reach the small
intestine, where they mature to adults.

Immature adults attach to the intestinal mucosa
by means of their stout mouth parts and suck
blood and tissue juices of the host.

About five weeks after infection, the worms have
undergone a final molt to become sexually
mature adults. Fertilization occurs, and the
females begin to release eggs. Worm life span is
about 1 year.
Class Nematoda The Roundworms
Hookworm rhabditiform larva
Hookworm filariform larva
Hookworm egg
Morphology:
 Rhabditiform larvae - long buccal
cavity, indistinct genital primordium.
Filariform larvae lose oral structures &
have sharp pointed tails.
 Adults - males: 7 to 11 mm long with a
copulatory bursa; females: 8 to 15 mm
long.
 Eggs - 55 to 70 x 35 to 40 microns; very
thin shell; usually seen in the 8 - 32 stage
of cleavage.
Class Nematoda The Roundworms
Diagnosis:
 Recovery and identification of eggs
(rarely larvae) in the feces.
 Cannot differentiate Hookworm species
by egg appearance.
 To determine if a significant infection is
present, count the number of eggs on a
direct smear of the unconcentrated
specimen. Five eggs per smear
indicates a light infection; 20 or more
eggs is clinically significant; 100 or
more is indicative of a very heavy
infection.
Class Nematoda The Roundworms
Major pathology and symptoms:






Serpent-like tunneling at site of penetration may
occur (cutaneous larva migrans).
During migration through the lungs, patients may
experience a sore throat and / or bloody sputum.
Heavy intestinal infections may result in enteritis,
anemia, weakness, and loss of strength due to the
anemia.
Chronic infections may experience anemia,
weakness, weight loss and gastro-intestinal
symptoms.
Nutritional and disease factors are commonly
seen in endemic areas. Children may exhibit
stunted growth and intellectual development.
Blood loss can be up to 100 milliliters/day.
Class Nematoda The Roundworms
Distribution:



Ancylostoma duodenale adults
Necator americanus adults
A. duodenale - Europe and south America
N. americanus - North America and Africa
Moist, warm regions of the world where the
skin frequently contacts the soil is optimal
for infection, especially in areas of poor
sanitation.
Class Nematoda The Roundworms
Strongyloides stercoralis - The Threadworm
Strongyloides stercoralis rhabditiform larva
Strongyloides stercoralis filariform larva
Life cycle: (very complex)

Infective third stage filariform larvae
penetrate skin, enter the lymphatics or
bloodstream.

Larvae migrate to the lungs, break out of
lung capillaries into alveoli.

After maturation, larvae travel to the
pharynx, are swallowed, and return to the
intestine.

Larvae mature to adults and attach to the
mucosa of the small intestine.
Class Nematoda The Roundworms
Strongyloides stercoralis rhabditiform larva
Strongyloides stercoralis filariform larva
Life cycle: (Continued)

Parthogenetic Females only - no parasitic
males. Females are capable of unisexual
reproduction, no fertilization required.
Produce viable eggs.

Eggs hatch in mucosa.

Larvae: Are passed in feces, live in the soil,
mature into a free-living adult males and
females, which produce eggs; Rhabditiform
larvae feed in soil and develop into
infective stage larvae which penetrate the
skin; First stage larvae develop into
infective stage larvae in the intestine
(autoinfection).
Class Nematoda The Roundworms
Morphology:



Buccal cavity of rhabditiform larva

Notch in tail of filariform larva
Rhabditiform larvae - short buccal cavity;
large, prominent genital primordium.
Filariform larvae - tail has a notch in it, in
contrast with the filariform larva of
hookworms.
Must be able to differentiate these from
hookworm larvae.
Eggs hatch in the intestine (not usually
passed in stool specimens). Eggs resemble
hookworm eggs, but are embryonated.
Class Nematoda The Roundworms
Diagnosis:
 Recovery and identification of
larvae in the feces.
 Recovery and identification of
eggs in duodenal drainage.
Class Nematoda The Roundworms
Major pathology and symptoms:

Skin – allergic reactions; raised, itchy, red
blotches at the site of larval penetration.

Lungs – pneumonia.

Intestinal - abdominal pain, diarrhea,
vomiting, weight loss, anemia,
eosinophilia. Light infections usually
asymptomatic; Heavy infection - bowel
becomes edematous and congested.

Death occurs in immunosuppressed
patients due to heavy autoinfection.
Class Nematoda The Roundworms
Strongyloides stercoralis - The Threadworm (Continued)



Distribution - worldwide, similar to hookworm.
While hookworm infection dies out over a period of
years after the patient has moved from an endemic area,
strongyloidiasis may persist for years, due to
autoinfection (internal infection).
In cases with severe diarrhea, Strongyloides eggs may
be present in stool specimens. These must be
differentiated from hookworm eggs. Strongyloides eggs
contain well-developed larvae. Hookworm eggs do not
have well developed larvae until passed from the body
and mature for one to two weeks in the soil.
Class Nematoda The Roundworms
Blood and Tissue-Dwelling Nematodes
Trichinella spiralis – trichinosis



Due to meat inspection programs, not very
prevalent in U.S.A. When seen, it is usually
due to home butchering and meat preparation.
Outbreaks most commonly associated with
pork.
Trichinella spiralis is a parasite of carnivorous
mammals. It is common in rats and in swine
fed uncooked garbage and slaughterhouse
scraps.
Human infections occur most often as a result
of consumption of raw or undercooked pork.
Class Nematoda The Roundworms
Trichinella spiralis – trichinosis
Life cycle:
 Infective stage larvae are ingested in
meat products.
 Tissue is digested, larvae are freed in the
intestine. They mature into adult males
and females.
 Female in mucosa releases larvae.
These disseminate throughout the body
via the bloodstream.
 Larvae encyst in striated muscle.
Class Nematoda The Roundworms
Trichinella spiralis - trichinosis
Morphology - females are 3.5 mm long;
males measure 1.5 mm long; larvae measure
100 microns long.
Diagnosis - Identification of encysted larvae
in muscle biopsy. Serology becomes
positive 3 to 4 weeks after infection. A
history of eating undercooked pork, deer,
walrus or bear is indicative whenever
appropriate symptoms appear.
Class Nematoda The Roundworms
Trichinella spiralis – trichinosis
Major pathology and symptoms:

Fever, muscle pain, bilateral periorbital edema,
and increased eosinophil count

Intestinal phase – small intestine edema and
inflammation, nausea, vomiting, abdominal
pain, diarrhea, headache, and fever.

Migrational phase - high fever (104 degrees),
blurred vision, edema, cough and pleural
pains.

Muscle phase – acute, local inflammation with
edema and pain.
Class Nematoda The Roundworms
Trichinella spiralis – trichinosis
Distribution:
 Cosmopolitan among meat-eating
populations, highest incidence is
reported from China, common in
Spain, France, Italy, and Yugoslavia.
 Prevalence in U.S.A. is about 4%
based on autopsy studies.
 Only about 100 cases are recognized
and reported per year in the U.S.A.
Class Nematoda The Roundworms
Blister containing Dracunculus medinensis
Adult Dracunculus emerging from broken blister
Dracunculus medinensis – The Guinea Worm
Overview:

An important parasite in the Middle East,
central India and Pakistan. Also found in
Africa in the Sudan and scattered through
central equatorial regions, and on its west
coast.

It is believed to no longer occur in the
Western Hemisphere.

Sometimes classified with the filarial
worms, but Dracunculus is not a true
filaria.
Class Nematoda The Roundworms
Dracunculus medinensis – The Guinea Worm
Life cycle:

Infective stage exists in a water flea (copepod –
the intermediate host).

Humans become infected by drinking water
containing the infected copepod.

Larvae penetrate the digestive tract to enter the
deep connective tissues where they mature in
about 1 year.

Females migrate to the subcutaneous tissue
(usually the skin of the extremities).

Females release larvae which leave the human
through ruptured blisters on the skin.

The larvae enter the water and are ingested by
copepods.
Class Nematoda The Roundworms
Dracunculus medinensis – The Guinea Worm
Morphology –

Males measure 40mm in length.

Females measure 800mm in length.
Diagnosis 
Visual observation of skin blister. The worm’s
serpentine presence beneath skin can be
seen.

Induce release of larvae from the skin ulcer by
applying cold water.
Class Nematoda The Roundworms
Dracunculus medinensis – The Guinea Worm
Major pathology and symptoms –

Mild allergic symptoms such as urticaria
during the migration phase.
 A papule develops into a blister with
localized erythrema and tenderness.

Generalized symptoms include nausea,
vomiting, diarrhea, and possibly asthma
attacks.

Additional complications include
secondary bacterial infections, permanent
damage to joints.
Distribution - Middle East and Africa.
Class Nematoda The Roundworms
The Filarial Worms





The Filariae are long thread-like nematodes. Eight species inhabit
portions of the human subcutaneous tissues and lymphatic system.
Adults of all species are parasites of vertebrate hosts.
Female worms produce eggs. The eggs modify, becoming elongated
and worm-like in appearance and adapting to life within the vascular
system.
Modified eggs, referred to as microfilariae, are capable of living a
long time in the vertebrate host, but cannot develop further until
ingested by an intermediate host and vector, an insect.
Microfilariae transform into infective larvae in the insect and are
deposited in the next host when the insect takes a blood meal.
Class Nematoda The Roundworms
The Filarial Worms
General life cycle:
 Human infection is acquired when infective
larvae enter the skin at the arthropod’s feeding
site.
 Larval migration and development takes place
in tissue.
 Adults are in various tissues (according to
species). They mature and produce
microfilariae.
Class Nematoda The Roundworms
The Filarial Worms
Wuchereria bancrofti microfilaria in blood smear
Wuchereria bancrofti:
“Bancroft's Filariasis.” A blood & lymphatic
dweller. The infection often results in
elephantiasis.

Vectors - Culex, Aedes, & Anopheles
mosquitoes.

Diagnosis - Detection and identification of
microfilaria in stained blood smears. Exhibits
a marked circadian migration, best seen at
night after 10 P.M.

Morphology - Microfilariae are sheathed, and
the nuclear column does not extend to tip of
tail.
Class Nematoda The Roundworms
The Filarial Worms
Wuchereria bancrofti:
Major pathology and symptoms –
 Swelling, due to allergic reaction
occurring around adult worms,
produces obstruction & elephantiasis.
Each individual reacts differently.
Very few develop elephantiasis, but in
some this is extensive.
Class Nematoda The Roundworms
The Filarial Worms
Brugia malayi:

Brugia malayi microfilariae in blood smear


“Malayan filariasis.” A blood & lymphatic dweller.
The infection can cause elephantiasis, but is not
as disfiguring or common as with Wuchereria
bancrofti.
Vectors - Mansonia, Anopheles & Aedes
mosquitoes.
Diagnosis - Detection and identification of
microfilaria in stained blood smears.
Morphology - Microfilariae are sheathed, nuclear
column extends to tip of tail with two nuclei near
end of tail, one in a swelling just short of tail’s
end, the other in the end of the tail.
Class Nematoda The Roundworms
The Filarial Worms
Brugia malayi:
Major pathology and symptoms  Swelling, due to allergic reaction
occurring around adult worms,
produces obstruction & elephantiasis.
Each individual reacts differently. More
often asymptomatic than infections
with W. bancrofti.
Class Nematoda The Roundworms
The Filarial Worms
Onchocerca volvulus:
The “blinding filaria.” Infections involve the
dermis and subcutaneous tissues, where
adults gather within nodules.

Vector - Simulium flies (blackfly, or buffalo
gnat).

Diagnosis - microfilariae are found in skin
scrapings from around nodules.

Morphology - Microfilariae not sheathed;
found only in skin, not in the blood stream.
Class Nematoda The Roundworms
The Filarial Worms
Onchocerca volvulus:
Major pathology and symptoms Characterized by fibrotic nodules which
encapsulate adults, usually on the trunk in
Africa, and on the head in central America.
A progressive, allergic skin rash develops.
Blindness occurs due to the presence of
microfilariae in ocular structures. This
parasite is a major cause of blindness in
Africa. Control is difficult because
Simulium flies breed in running water.
Class Nematoda The Roundworms
The Filarial Worms
Loa loa:
The “eyeworm.” Infections involve the dermis
and subcutaneous tissues (Calabar swellings).

Vector - Crysops (mango fly), a large fly with
biting mouthparts.

Diagnosis - Usually made from clinical
symptoms, but if laboratory confirmation is
required, blood should be drawn between 11
am & 1 pm.

Diagnosis - Microfilariae are sheathed, nuclear
column extends to tip of tail.
Class Nematoda The Roundworms
The Filarial Worms
Loa loa:
Major pathology and symptoms Infections cause a localized
subcutaneous edema, particularly
around the eye, because of larval
migration and death in capillaries.
Living adults cause no inflammation;
dying adults induce granulomatous
reactions.