Nematodes
Are surrounded by a secreted multilayered
cuticula, have only longitudinal muscles which
are organized in quadrants and a ‘pressurized’
pseudocoel
A simple linear intestine made up of columnar
cells with a muscular pharynx that pumps in
food against the internal pressure
A linear reproductive tract along which germ line
cells go through meiosis and mature into
gametes (amoeboid sperm, eggs surrounded by
an impermeable shell).
Nematodes
Believe in order and fate. They develop to a
defined number of cells and follow a highly
deterministic developmental program.
Four larval stages and adult, molt at each
developmental step. The L3 stage can
developmentally arrest and ‘hang in’ waiting for
better times or the next host.
Different life cycle stages can differ dramatically
in their metabolism (aerobic larvae, anaerobic
adults)
Nematodes or round
worms 2
GI Nematodes: Trichuris, Ascaris
& Hookworms
Worm expulsion and the Th2 immune
response
The hygiene hypothesis of allergy
Trichinella & the first modern public
health campaign
The importance of gastro-intestinal
(GI) nematode infections
Roughly halve of all
people are currently
infected with GInematodes
Infection is generally
associated with low
mortality, but there
can be considerable
morbidity
The importance of GI
nematode infectioins
Parasite
Prevalence
(millions)
Morbidity Mortality
(millions) (thousands)
Ascaris
1450
350
60
Hookworms
1300
150
65
Trichuris
1050
220
10
Children are especially at risk to suffer from
GI nematode related morbidity.
The WHO helminth control
program for school aged children
Children are continuously
exposed to infection
Children grow rapidly and
are especially susceptible
to infection and the
associate morbidity
Chronic helminth infection
seems to have a negative
impact on cognitive
development
10 cents annually per child
covers drug cost
The program is heavily
based on drug treatment
Trichuris trichuris (human whip worm)
Adults live in cecum
where the female worm
produces 20,000 eggs
per day which are shed
with the feces
Embryonation occurs
within 3 weeks in soil
Infection is through
ingestion of eggs, larvae
hatch and penetrate the
mucosa and tunnel it
Larvae grow and molt 4
times until mature
Trichuris
Trichuris vulpes (dog whip worm in
the intestine of a heavily infected dog)
Adults remain embedded with
their thin front end in the
mucosa
Infections with less than 10
worms are asymptomatic
Moderate to heavy infections
cause abdominal pain,
diarrhea, weight loss
Heavily infected children may
have profuse bloody diarrhea,
cramps, tenesmus and
urgency which may result in
rectal prolapse
Adults can live for years so
worm burden can build up
Trichuris
Diagnosis by demonstration
of characteristic eggs
(“champagne corks”)
In moderate infections eggs
might be hard to find and
concentration techniques
might be required
Treatment with mebendazole
Sanitary disposal of human
feces prevents disease
Ascaris lumbricoides -- the
human round worm
Large intestinal nematodes
(Ascaris lumbricoides
infects humans, A. suum
infects pigs, the species are
almost indistinguishable,
but have clear host
specificity)
Males are 15-31 cm and
have incurved posterior
end, females are 20-49 cm
The anterior end of both
sexes shows three lips
One billion people infected
Ascaris
Fully embryonated eggs are
swallowed and L2 hatches in
the stomach and penetrate
stomach or duodenal mucosa
Larvae enter blood stream and
leave through alveoli into lung
Larvae molt several times in
the lungs L3/L4 move up and
get swallowed
2-3 months after infection the
adult worms start laying eggs
(200,000 daily)
Eggs are shed with the feces
and embryonate within 2-3
weeks
Ascaris
Infection depends on fecal
contamination of food, water or
soil
Eggs are sensitive to sun light
but otherwise extraordinarily
resistant (ascarosides - special
glycolipids secreted by the
embryo)
Fertilized eggs are shorter and
rounder than unfertilized
Ascaris
Occasional pulmonary symptoms
Intestinal phase mostly asymptomatic,
but worms can lead to a developmental
slow down in children
Dangerous complications are mostly
observed in children under 10
Volvulus, a mass of knotted worms
obstructing the intestine
Penetration of the bile duct and liver by
adult worms
Penetration of the intestinal wall,
followed by peritonitis
Wandering and obstruction can be an
unwanted side effect linked to certain
medications
Acylostoma duodenale & Necator
americanus -- human hookworms
Small nematodes (1-1.5
cm)
Head is slightly bend
(hook) and the ‘mouth’
carries characteristic
teeth (Ancylostoma) or
plates (Necator, note that
these are not real teeth
but cuticular formations
of the ‘buccal capsule’)
The posterior end of the
male worm is elaborated
into a copulatory bursa
Hookworms
Ancylostoma is found in
Europe around the
Mediterranean, on the West
coast of South America and
in parts of China and India
Necator is found over much
of the western hemisphere,
Africa and South East Asia
More than a billion people
infected
Hookworms
Infectious L3
Adult worms live in the small intestine
and female 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 lavae move up the trachea into the
esophagus, are swallowed and finally
reach the intestine, where they molt
twice more before they reach maturity
Hookworms
Skin penetration and
associated secondary
bacterial infection can
result in “ground itch”
Pulmonary phase is
usually asymptomatic
Intestinal phase: worms
attach to the mucosa
and feed on blood.
Worms continuously
move to new places
exacerbating bleeding
Hookworms
The main concern with hook
worm disease is blood loss
0.03 ml (N.a.) to 0.26 ml (A.d) per
worm, up to 200 ml per day in
heavy infections
Chronic heavy infection results
in anemia and iron deficiency
Together with malnutrition
infection can severely stunt
growth and development in
children
Anemia leads to weakness and
fatigue in adults
Antihelminthic treatment with
mebendazole
Creeping eruption or
cutaneous larva migrans
Humans occasionally get
infected by L3 larvae of
dog and cat hookworms
(e.g. Ancylostoma
caninum or brazilienzis
again by skin
penetration)
The larvae can not
establish a productive
infection in humans, but
wander about in the
subcoutaneous tissue,
causing significant
inflammation and painful
swelling
Responds well to
treatment
Hookworms
Acylostoma was commonly
found in coal mines
throughout Europe
Heavy infection rates due to
‘perfect’ conditions for
transmission in mines could
result in severe anemia and
pathology
At the turn of the 19th
century, severe hookworm
disease was an officially
recognized occupational
hazard in German coal mines
and miners with the disease
were eligible for workman’s
compensation
Hookworms
Hookworms were wide spread in
the Southern USA (read Jimmy
Carter’s boyhood biography for
detail on hookworm and other
infectious diseases in rural
Georgia)
Hookworm control programs were
a big part of the public health
campaign in the South during and
right after World War II
1972 12% of school children from
rural costal Georgia tested positive
for hookworm infection
Nurse brings hookworm medicine to a rural Alabama family in 1939
(National Library of Congress photo LC-USF34- 051377-D)
The immune response and
GI nematodes
Nematode infections are usually
chronic and have high rates of
reinfection
This suggests that the worms have
developed effective ways to a) avoid
getting killed by the host and b) they
are not inducing overly strong
inflammatory responses, which could
kill the host
However there are data from human
epidemiological studies which
indicate that a certain degree of
protective immunity might develop (at
least in some diseases)
The many variables in the study
populations make it hard to develop
sound models.
Mouse models to study GI
nematode immunity
The use of a mouse model
allows control over many
factors variable in human
studies
The mouse immune
system is very well studied
and many reagents are
available
A huge collection of so
called knock-out mice with
defined defects in specific
aspects of their immune
system can be used/
Lymphocytes are required for
worm expulsion
Mice are infected with Trichuris
muris eggs
After 11 days many worms can be
detected, but after 33 days all
adult worms are cleared
SCID mice lack T and B cells and
can not expel worms
Reconstitution of SCID mice with
naïve lymphocytes from a normal
mouse restores ability to control
infections
This approach of ‘taking it apart
and then putting it back together’
has been very powerful to dissect
the immune system and its
responses to various pathogens
A quick review of Dr. Tarleton’s
immunology intro
Worm expulsion requires T-cells,
more specifically a certain flavor of T helper cells
T-helper cells can develop
into two flavors (TH1 or 2)
The immune system and its cells make
important choices
The cells of the immune system talk to
each other through messenger
proteins called cytokines
T-cells both make and respond to
many cytokines
Stimulation with different cytokines
leads to the development of two types
of T-helper cells specialized for
orchestrating two very different
immune responses
Th1 and Th2 strongly down-regulate
each other
This polarization has important
consequences for the downstream
response and can spell life or death
TH1 cells promote cytotoxicity and
destruction of intracellular pathogens
TH1 cells orchestrate a
response directed to inhibit
intracellular pathogens like
viruses, certain bacteria
(e.g. TB), or certain
protozoan parasites
(Leishmania - see later
lecture)
They secret Interferon
gamma which activates
intracellular killing
mechanisms
They activate macrophages
and cytotoxic T-cells
TH2 cells promote neutralizing
antibodies and mast cell activity
TH2 cells suppress the
activation of macrophages and
activate eosinophiles and
mastcells
TH2 cells promote a strong
antibody response based on
neutralizing IgGs and IgEs
A TH2 response is most
effective to combat extracellular
pathogens
The key to expulsion is
mounting the “right” response
Certain inbred mouse strains mount a protective response to
Trichuris muris resulting in expulsion (resistant), others develop
chronic infections with worm persisting in the gut (susceptible)
Susceptible mice develop a strong immune response, but it’s a
Th1 response
If however these susceptible mice are treated to artificially shift
to a Th2 response they clear the infections
The immune system is not hard wired but makes important
choices which can control the outcome of an infection
Physiological changes induced
by anti-worm immunity
Recent work show strong effects of the antiworm immune response on the gut
physiology
Instead of directly killing the worms these
physiologial changes appear to render the
intestine inhospitable for the parasite
Increase in number of goblet cells (these
are the cells in the intestinal epithelium
which produce mucus - strong red stain in
the histology image)
Increase in mucin secretion
Increase in intestinal mobility
Increase in water influx into the intestinal
lumen (some of these seem to be directly
induced by cytokines and immune cells like
mast cells)
Worm expulsion
It is associated with a type 2 (TH2) response
The effector mechanisms include immunological
and physiological processes
Many worms have found ways to suppress or
modulate the immune system
This allows them to dodge an effective immune
response and/or prevent the potential demise of
their mammalian host due to massive inflammation
Immune modulation by worms can
be detrimental or beneficial
Numerous recent epidemiological studies show that
certain vaccines are less effective in children that
are infected with worms than those that have been
cured using drugs (these are mostly vaccines that
require a robust TH1 response)
This is backed up by many studies in mice that use
vaccination or co-infection
On the other hand worm infection can dampen
autoimmune diseases and allergies (diseases due
to an ‘over-active’ immune system)
The Hygiene Hypothesis
There has been a considerable increase in the diagnosis
of autoimmune diseases and allergies over the second
half of the 20th century
Prevalence of allergies in urban areas appears higher
than in rural environments
Environmental factors like pollution, nutrition etc. can be
important for specific allergies but have shown little
consistent overall association with allergies and
autoimmunity
Childhood infections though show strong negative
correlation with both autoimmune disease and allergies
Inverse correlation of type I diabetes
and chronic infectious diseases
Red delineates areas which harbour six or more of the low mortality neglected diseases (filariasis, leprosy, onchocerciasis, schistosomiasis, soiltransmitted helminths, and trachoma). Yellow delineates areas where there are relatively high incidences of T1D (> 8 per 100 000/year). Non
coloured areas delineate where T1D < 8 per 100 000/year and where the ‘neglected diseases’ are not endemic.From: Zaccone et al. Parasite
Immunol. 2006 28:515–523.
Inverse correlation of
allergies with infections
Among kids in
various studies in
different areas of the
world around 30%
have antibodies
against dust mite
allergen (suggesting
they all are exposed)
But whereas asthma
is found in 12% of
kids from Europe
and Australia, only
3% have asthma in
Gambia and Nigeria
The Hygiene Hypothesis
High pathogen burden
stimulates the immune
system to develop a
robust regulatory
network that keeps
inflammation in check
Worms set up long
lasting chronic
infections, they induce
strong TH2 responses
and promote regulation
of this process
Understanding this
process in detail could
point to new allergy
interventions
The Hygiene Hypothesis:
Treatment by worm infection?
Several clinical trials using
Trichuris suis have been
conducted to treat human
Crohn’s disease or ulcerative
colitis (autoimmune inflammation
of the intestine)
T. suis does not productively
infect humans yet modulates the
immune response
Most studies show clinical
improvement for a significant
fraction of the patients
The Hygiene Hypothesis:
Treatment by worm infection?
One example (double blind study with 54
patients suffering from acute ulcerative
colitis): The patients received either a
placebo or 2500 T. suis ova every 2 weeks
for 12 weeks.
43.3% of the patients given T. suis
improved compared with those given
placebo (16.7%).
The study also included a 12 week
crossover limb where patients originally on
placebo where switched to T. suis and
those on T. suis were switched to placebo.
In the crossover limb, 56.3% of the patients
given T. suis improved compared with
13.3% of patients given placebo (see Elliot
et al. (2007) International Journal for
Parasitology 37: 457-464 for detail)
Don’t try this at home!
Trichinella spiralis
Trichinosis is caused by
infection with Trichinella
spiralis
(additional morphologically
indistinguishable species
have now been described for
sylvatic cycles)
Infection of all hosts occurs
through larvae encysted in
muscle tissue (carnivorism,
salvage and or cannibalism
is needed to maintain
transmission)
Trichinella
The larva is freed from
its nurse cell, and enters
the mucosa of the small
intestine
Larvae grow, molt 4 time
and young adults
copulate within 32 hours
of infection
The females give birth to
live L1 larvae while
tunneling the epithelium
Females die after 4-16
weeks
Trichinella
The larvae enter blood
vessels and are carried
throughout the body
When they pass skeletal
muscle (they prefer
highly active muscles)
they leave the vessels
and penetrate the muscle
and the muscle cells
Trichinella
The larvae enters a muscle cell
and lives as an intracellular
parasite developing in the
cytoplasm of the host cell
The larvae manipulates the host
cell to its needs (probably by
secretion of suitable effector
proteins, the molecular
mechanism is not well
understood)
The end product is a nurse cell
A fine net of blood vessels forms
around the nurse cell
(angiogenesis)
Trichinella
The host cell looses its
myofilaments and several
additional subcellular
changes occur
Both host cell and worm
are enclosed by a
collagen capsule
(collagen mRNA has
been detected in nurse
cell, but some authors
suggest the capsule is
secreted by surrounding
fibroblasts
Trichinella
Strong dose dependence
of symptoms (>1200
larvae can be lethal)
Activity of the females
can cause inflammatory
reaction in the mucosa,
leading to diarrhea and
abdominal pain
Parasitized muscle
exhibits intense
inflammation, cyst start to
calcify in humans after 6
months
Trichinella
Muscle invasion stage begins in the
second week after infection
Fever and perorbital edema are followed
by myalgia (muscle pain) and weakness
Characteristic splinter hemorrhages can
be found under finger nails
Fever and chills can persist for weeks
Headache is common and dizziness may
develop
Muscle swelling, aching and tenderness
occurs often
Deaths are rare and due to myocarditis
(inflammation of the heart muscle),
encephalitis and pneumonia (larvae in
the diaphragm)
Trichinella control was one of the first
effective modern public health measures
Trichinella was discovered in 1835 and
linked to human disease 1860
Several large outbreaks in Germany
resulted in the formation of an expert
commission in Berlin which included
Rudolf Virchow one of the co-discoverers
of the Trichinella life cycle
Virchow suggested mandatory inspection
of all meat at the slaughterhouses a
practice that was introduced in Prussia in
1877
This has greatly reduced trichinellosis as
human health problem
Trichinella
Domestic infection
through pigs has become
rare in the US and
Europe
Meat control, rat control,
prompt removal of dead
animals, limit access of
wildlife to pigs
Almost all case now are
sylvatic transmission
from game
Aniskiasis
Humans are accidental host of
several nematodes which reach
maturity in marine mammals
Infection through consumption of
uncooked fish
Larvae often penetrate gastric and
intestinal mucosa causing
pronounced inflammation which
can mimic Crohn disease
Rapid onset of symptoms often
within hours
Diagnosis by endoscopy, surgical
or endoscopic removal of larvae in
cases with severe symptoms