What makes a microorganism or
virus a significant human pathogen?
• Ability to effectively cause infection i.e. genes
expressed which facilitate entry into body
• Ability to effectively replicate in the body and evade
immune response
• Ability to effectively exit body in a form which can be
transmitted directly or indirectly to a new human host
• Ability to produce gene products which cause
pathological effects such as toxins and/or
superantigens
Pathogenic Organisms are Cleared by the Immune
System in Progressively More Specific Ways as Infection
Progresses
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CL0NAL SELECTION
1. REMOVE ANTIBODIES
AGAINST SELF
2. SELECT B CELLS WHICH
HAVE INITIAL AFFINITY FOR
ANTIGEN
3. SELECT B CELLS WITH
HIGHER AND HIGHER
AFFINITY FOR ANTIGEN
4. SELECT B CELLS WHICH
PRODUCE SPECIFIC TYPE
OF ANTIBODY
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Expression of
the Antibody
Molecule as a
Surface
Antigen Allows
Clonal
Selection to
Occur
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VDJ rearrangement is the first step in generating
antibody which has affinity for antigen
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VDJ recombination
mediated by RAG
recombinase brings
together V,D and J
segments to form a
functional coding
region while
generating significant
diversity
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Mutation in 3 hypervariable
regions of variable chains
allows antibody with higher
affinity to be produced
Clonal selection leads to
mutliplication of high affinity
clones
Single base
changes occur at
a rate of 10-3 to
10-5 per base pair
per cell
generation in B
cells undergoing
somatic
hypermutation
AID=activation induced
cytidine deaminase
UNG=Uracil N
glycosylase
(removes uracil
from DNA)
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Specific regions of variable regions are
targets of somatic mutation
Somatic mutation continues until high
affinity antibodies are selected
Constant region of antibody molecules controls
function of antibody
DNA recombination used to switch antibody type
RECOMBINATION IN HEAVY CHAIN GENES USED FOR
CLASS SWITCHING
INITIAL ANTIB0DYCu IgM ANTIBODY SECRETED
and Cd IgD ANTIBODY ON CELL SURFACE
PRODUCED BY ALTERNATIVE SPLICING
LATER ANTIBODY Cg--IgG or Ca IgA or Ce IgE -SECRETED
IgG ANTIBODIES CAUSE EFFICIENT
DESTRUCTION OF BACTERIA
BECAUSE THEY BIND TO Fc
RECEPTOR AND PROMOTE
OPSONIZATION AND
PHAGOCYTOSIS
The Cells of the Immune System Derive
from a Common Stem Cell: they
Cooperate to Carry Out the Immune
Response
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T-LYMPHOCYTES:
Can play a role in stimulating B cells to produce
antibody or activating macrophages (Helper T
cells--CD4+)
OR
Can act directly to kill a cell expressing a foreign
antigen (Killer T cells--CD8+)
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Killer T cells (CD8+) React to a Viral
Antigen Presented by Class I MHC and
Causes Virus Infected Cells to be Killed
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A T cell recognizes target antigens
through the T cell receptor (TCR)
The T cell
receptor
resembles a
membrane
bound
immunoglobulin
molecule
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Structure of the T-cell receptor
The T-cell receptor heterodimer is composed of two transmembrane glycoprotein chains, a and b. The extracellular portion
of each chain consists of two domains, resembling
immunoglobulin V and C domains, respectively. Both chains
have carbohydrate side chains attached to each domain. A short
segment, analogous to an immunoglobulin hinge region,
connects the immunoglobulin-like domains to the membrane and
contains the cysteine residue that forms the interchain disulfide
bond.
T-cell receptor a
and b chain gene
rearrangement
and expression
The TCR a and b chain genes are composed of discrete segments that are joined by somatic
recombination during development of the T cell. For the a chain a Va gene segment rearranges to a
Ja gene segment to create a functional V-region exon. Transcription and splicing of the VJa exon to
Ca generates the mRNA that is translated to yield the T-cell receptor a chain protein.
For the b chain the variable domain is encoded in three gene segments, Vb, Db, and Jb.
Rearrangement of these gene segments generates a functional VDJb V-region exon that is
transcribed and spliced to join to Cb; the resulting mRNA is translated to yield the T-cell receptor b
chain. The a and b chains pair soon after their biosynthesis to yield the a : b T-cell receptor
heterodimer.
A killer T-cell must receive
its initial encounter with
antigen accompanied by a
costimulating signal from a
professional antigen
presenting cell (APC)
which is stimulated by the
innate immune system to
present a second
stimulatory signal to the T
cell
Once the killer T cell is
educated by the APC, it is
capable of killing cells
which present the foreign
antigen without a second
signal
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Class I MHC Peptide-Binding Site-Peptide and Class I MHC are the target
of TCR in CD8 (Killer) T cells
Class I MHC Peptide-Binding Site-Peptide and Class I MHC are the target
of TCR in CD8 (Killer) T cells
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The a1and a2 domains come together to form a groove in
which peptides are displayed. The two views shown reveal
that the peptide is surrounded on three sides by a b sheet
and two a helices, but it is accessible from the top of the
structure
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Peptides are bound to MHC class I molecules by their
ends. MHC class I molecules interact with the back-bone of a
bound peptide through a series of hydrogen bonds and ionic
interactions at each end of the peptide.
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Peptides bind to MHC molecules through structurally related anchor residues.
Peptides eluted from two different MHC class I molecules are shown. The anchor
residues (green) differ for peptides that bind different alleles of MHC class I molecules
but are similar for all peptides that bind to the same MHC molecule. The upper and lower
panels show peptides that bind to two different alleles of MHC class I molecules. The
anchor residues that bind a particular MHC molecule need not be identical, but are
always related (for example, phenylalanine (F) and tyrosine (Y) are both aromatic amino
acids, whereas valine (V), leucine (L), and isoleucine (I) are all large hydro-phobic amino
acids). Peptides also bind to MHC class I molecules through their amino (blue) and
carboxy (red) termini.
The Recognition of an Antigen by CD8+ T
cells is Influenced by the Allele at the MHC
Locus which Presents the Antigen
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C4+Helper T Cells Stimulate B Cells to Produce Antibodies
TH1 and helper T cells recognize antigen presented
by MHC class II molecules
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On recognition of their specific
antigen on infected
macrophages, TH1 cells activate
the macrophage, leading to the
destruction of the intracellular
bacteria.
When helper T cells recognize
antigen on B cells, they
activate these cells to
proliferate and differentiate into
antibody-producing plasma
cells
Antigen that binds to the Bcell antigen receptor signals
B cells and is, at the same
time, internalized and
processed into peptides that
activate armed helper T
cells.
Signals from the bound
antigen and from the helper
T cell induce the B cell to
proliferate and differentiate
into a plasma cell secreting
specific antibody
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Armed helper T cells stimulate the proliferation and then
the differentiation of antigen-binding B cells
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The specific interaction of an antigen-binding B cell with an
armed helper T cell leads to the expression of the B-cell
stimulatory molecule CD40 ligand (CD40L) on the helper T-cell
surface and to the secretion of the B-cell stimulatory cytokines
IL-4, IL-5, and IL-6, which drive the proliferation and
differentiation of the B cell into antibody-secreting plasma
cells.
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Peptides that bind MHC class II molecules are variable in length and their anchor
residues lie at various distances from the ends of the peptide. The sequences of a
set of peptides that bind to the mouse MHC class II Ak allele are shown in the upper
panel. All contain the same core sequence but differ in length. In the lower panel,
different peptides binding to the human MHC class II allele HLA-DR3 are shown. The
lengths of these peptides can vary, and so by convention the first anchor residue is
denoted as residue 1. Note that all of the peptides share a negatively charged residue
(aspartic acid (D) or glutamic acid (E)) in the P4 position (blue) and tend to have a
hydrophobic residue (for example, tyrosine (Y), leucine (L), proline (P), phenylalanine
(F)) in the P9 position (green).
Constant region of antibody molecules
control function of antibody
DNA recombination used to switch
antibody type
IgG ANTIBODIES CAUSE EFFICIENT
DESTRUCTION OF BACTERIA
BECAUSE THEY BIND TO Fc
RECEPTOR AND PROMOTE
OPSONIZATION AND
PHAGOCYTOSIS
Bacterial Capsules Can Protect Against Macrophage
and Complement Mediated Lysis
How Some Pathogens Evade
Macrophage Phagocytosis
•
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Some pathogens such as Pseudomonas,
enteropathogenic E. coli (EPEC), and Yersinia
can block phagocytic uptake into macrophages.
By translocating specialized type III effector
proteins into the macrophage cell, these
pathogens interrupt the signal transduction
cascades necessary for actin rearrangement
and thereby prevent phagocytosis. Other
pathogens that use a type IV secretion system
(such as Brucella and Legionella), or type III
secretion systems (such as Salmonella, and
probably Chlamydia), can induce their own
uptake into macrophages by stimulating actin
rearrangements proximal to attached bacteria,
and then secrete additional effectors into the
host cell to modify the trafficking of the invasion
vacuole in order to prevent fusion with
lysosomes. Vacuoles containing Legionella and
Brucella, for example, then acquire markers of
the endoplasmic reticulum, thereby creating a
protective niche for replication within cells.
COMPLEMENT SYSTEM PLAYS AN IMPORTANT
ROLE IN RESPONSE TO PATHOGENS
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The first protein in the classical
pathway of complement activation
is C1, which is a complex of C1q,
C1r, and C1s. C1q is composed of six
identical subunits with globular heads
and long collagen-like tails. The tails
combine to bind to two molecules
each of C1r and C1s, forming the C1
complex C1q:C1r2:C1s2. The heads
can bind to the constant regions of
immunoglobulin molecules or directly
to the pathogen surface, causing a
conformational change in C1r, which
then cleaves and activates the C1s
zymogen.
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The constant regions of IgM and some IgGs contain a binding
site for C1q. Binding of C1q activates C1s and C1r. Activated C1s
(a serine protease) cleaves two serum proteins
C4 is cleaved into a large fragment C4b, which binds covalently to
sugar residues on cell-surface glycoproteins, and a smaller,
inactive, fragment C4a which diffuses away.
C2 is cleaved into C2b, which binds noncovalently to a site on
C4b, leaving a smaller, inactive, fragment of C2a which diffuses
away. The complex of C4b•2b catalyzes the cleavage of C3.
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C3 is the most abundant protein of the complement system (~1.2 mg/ml).
Because of its abundance and ability to activate itself it greatly magnifies the
response. C4b•2b cuts C3 into two major fragments--C3b and C3a. C3b,
binds covalently to glycoproteins scattered across the bacterial cell surface.
Macrophages and neutrophils have receptors for C3b and can bind the C3bcoated cell or particle preparatory to phagocytosis.
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C3a, a small fragment of C3, is released into the
surrounding fluids. It can bind to receptors on basophils
and mast cells triggering them to release histamine and
other contents of their vacuoles. When C3a is released
locally at the site of infection it causes an inflammatory
response attracting more immune system cells to the site of
the infection. However, C3a, if released throughout the
body (systemic release) can cause anaphylaxis.
Complement component C5 is cleaved when captured by a
C3b molecule that is part of a C5 convertase complex
C5 convertases are formed when C3b
binds either C4b,2b to form C4b,2b,3b, or
C3b,Bb to form C3b2,Bb.
C5 binds to the C3b in these complexes.
C5 is cleaved by the active enzyme C2b
or Bb to form C5b and the inflammatory
mediator C5a which works like C3a. The
production of C5b initiates the membrane
attack complex.
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The Membrane Attack Complex: Lysis Bacteria By Creating Pores
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C5b triggers the assembly of a complex of one molecule each of C6, C7, and C8, in
that order. C7 and C8 undergo conformational changes that expose hydrophobic
domains that insert into the membrane. This complex causes moderate membrane
damage in its own right, and also serves to induce the polymerization of C9, again with
the exposure of a hydrophobic site. Up to 16 molecules of C9 are then added to the
assembly to generate a channel of 100 Å diameter in the membrane. This channel
disrupts the bacterial cell membrane, killing the bacterium. The electron micrographs
show erythrocyte membranes with membrane-attack complexes in two orientations,
end on and side on.
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The Iditarod: Commemorating the 1925
Emergency Delivery of Diphtheria Serum
to Nome, Alaska
The First Signs of an Epidemic
Nome, Alaska in 1925
Doctor
Curtis
Welch
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It was a normal mid-January afternoon in Nome. Doctor Curtis Welch, physician and director
of the US Public Health Service, was doing paperwork in his office at the Merchants and
Miners Bank of Alaska building. An Innuit man came into the office asking the doctor to
come quickly, his two children were very sick. Dr. Welch raced to the Sand Spit Innuit
settlement, west of the Snake River on the fringes of Nome. The children's temperatures
were dangerously high, and their breathing was labored and shallow. Dr. Welch asked the
mother how the children had become ill and what their symptoms had been. She replied that
they had been sick for about three days. She thought it was a bad cold because their throats
had become red and sore. Dr. Welch tried to examine their throats, but they could not open
their mouths far enough for him to do so. He tried to comfort the mother and then returned to
his office.Dr. Welch had wished many times that he had access to a good laboratory where
he could send specimens for analysis. It was very strange. Children don't die of sore throats,
but the two Innuit children were dying. At one point he considered diphtheria but it was
highly unlikely. He hadn't seen a case in northern Alaska in twenty years. Despite the
doctor's efforts the Innuit children died the following day.
http://www.lucidca
fe.com/library/idita
rod.html
Recognizing
diptheria and the
need for antitoxin
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A few days later, on January 21, Dr. Welch was called to the home of a family in
Nome to examine their six-year-old son. The child had been sick for two days
with a sore throat. Dr. Welch examined the boy's throat and recognized
immediately the dirty white patches of the diphtheria membrane. The doctor
realized the terrible implications of this diagnosis. Diphtheria, left unchecked,
would spread with devastating speed. Dr. Welch met at once with Nome's mayor
and city council. He told them of the imminent epidemic and stressed that some
way must be found to get the diphtheria antitoxin to Nome within 2 weeks. The
serum would check the spread of the disease and would help those already
infected. His main concern was with the native population that had little or no
immunity to diseases from the outside. A flu epidemic in 1919 had wiped out
entire Innuit villages.
http://www.lucidcafe.co
m/library/iditarod.html
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•
Diptheria antitoxin
reaches Nome by
Dogsled in Five
Days
There was widespread relief when it was discovered that the Alaska Railroad Hospital in
Anchorage had 300,000 units of the life saving serum. It was not much, but it would be
enough to stem the tide of the epidemic. It was decided to transport the serum by train from
Anchorage to Nenana, a town on the Tanana River 220 miles north of Anchorage, and then
by a relay of dog teams over the 674 miles between Nenana and Nome. The last leg reached
Nome on Monday, February 2 at 5:30 in the morning. Dr. Welch was awakened by a
persistent knocking on his front door. When he opened it he found an exhausted Gunnar
Kaasen, the musher of the final leg of the relay. Kaasen handed him a twenty pound, fur-andcanvas-covered package containing the 300,000 units of serum. In the street were his 13
dogs harnessed to a sled, their heads and bushy tails hanging almost to the ground. They
had covered the last fifty-three miles of the epic relay in seven and a half hours. These dogs,
and the teams that preceded them, had traversed 674 ice-and-snow covered miles in less
than five days. They delivered to Dr. Welch the life-saving serum that within a week would
break the back of the diphtheria epidemic.
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Diphtheria
• Diphtheria is still common in many other parts of the
world, including the Caribbean and Latin America.
During the last few years, large epidemics of
diphtheria have occurred in the former Soviet
republics. Outbreaks have also been reported in
Algeria, China, and Ecuador. The majority of cases in
many of these epidemics have been in adults and
adolescents.
http://www.astdhpphe.org/infect/dip.html
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Corynebacterium diphtheriae
Diphtheria
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http://gsbs.utmb.edu/microbook/ch032.htm
Diphtheria
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http://gsbs.utmb.edu/microbook/ch032.htm
BACTERIAL EXOTOXINS CAN INCLUDE
A POLYPEPTIDE (B) DESIGNED FOR
CELL ENTRY AND A SECOND
POLYPEPTIDE (A) WHICH CAUSES
TOXICITY TO HUMAN CELL
TOXIN CAN ENTER CELL THROUGH
ENDOCYTOSIS OR DIRECTLY
DIPHTHERIA TOXIN: TARGET RIBOSOMAL
PROTEIN OF HUMAN CELL--MOST SERIOUS
TOXIC EFFECT, CARDIOTOXICITY
DIPHTHERIA TOXIN INACTIVATES PROTEIN
SYNTHESIS BY ADP RIBOSYLATION OF
TRANSLATION FACTOR EF-2 AT A SPECIFIC
MODIFIED HISTIDINE RESIDUE
Bacterial Toxins
• Endotoxins-bacterial
lipopolysaccharides
• Exotoxins--specific polypeptides
produced by bacteria which cause toxic
effects
BACTERIAL EXOTOXIN GENES ARE
OFTEN ACQUIRED BY GENE TRANSFER
PATHOGENIC E. COLI
• ENTERTOXIGENIC (ETEC) (secretory diarrhea)
• ENTEROPATHOGENIC (Malabsorptive diarrhea) (EPEC)
• ENTEROHEMORAGGHIC (Malabsorptive diarrhea and
dysentery) (EHEC) (E. coli 0157)
All three types are major causes of infant deaths worldwide
MECHANISM OF ACTION ST AND LT TOXINS IN ETEC
INTERFERENCE WITH cAMP or cGMP METABOLISM LEADS
TO LOSS OF CONTROL OVER WATER FLOW AND
INTESTINAL WATER LOSS
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Cholera
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The profuse diarrhea and subsequent fluid and electrolyte loss
associated with infection with enterotoxigenic Escherichia coli and
Vibrio cholerae arise following binding of the luminal epithelium of the
intestine by the toxins Etx and Ctx, respectively
(a) Binding to the cell is mediated by the ability of the B subunit of the
toxin to interact and crosslink with ganglioside GM1 receptors.
(b) Crosslinking triggers vesicular uptake and
(c) Trafficking, a process that is also influenced by the presence of an
RDEL (or KDEL in the case of Ctx) sequence on the A2 fragment
(d) Enzymatic cleavage allows the A1 fragment to enter the cytosol,
where
(e) it catalyses adenosine diphosphate (ADP) ribosylation of GSa, a
membrane G protein that regulates the activation of adenylate cyclase.
(f) Once activated, adenylate cyclase accelerates the conversion of
adenosine triphosphate (ATP) to cyclic adenosine monophosphate
(cAMP), which subsequently induces phosphorylation of protein kinase
A (PKA) and leads to the opening of the cystic fibrosis transmembrane
regulator (CFTR) chloride ion (Cl–) channel.
(g) Efflux of Cl– from the cell results in a simultaneous osmotic shift of
water from the cell into the intestinal lumen, accompanied by cell
death. This results in the watery diarrhea characteristic of infection by
V. cholerae and E. coli
DELIVERY OF EPEC TOXIC PROTEIN TO HUMAN INTESTINAL
CELLS BY A TYPE III SECRETION SYSTEM
EPEC PILI BIND TO INTESTINAL EPITHELAL CELLS; CHANNEL IS
FORMED MAKING DIRECT CONNECTION BETWEEN EPEC CYTOPLASM
AND INTESTINAL CELL CYTOPLASM
TOXIC PROTEIN INJECTED WITHOUT EXPOSURE TO
HUMAN IMMUNE SYSTEM
Effects of EPEC infection on host intestinal epithelial cells
•
•
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EPEC initially adheres to the host cell by its bundleforming pili, which also mediate bacterial
aggregation.
Following initial attachment, EPEC secretes several
virulence factors by a type III-secretion system.
Signal transduction events occur within the host,
including activation of phospholipase C (PLC) and
protein kinase C (PKC), inositol triphosphate (IP3)
fluxes, and Ca2+ release from internal stores.
The bacterium intimately adheres to the cell by
secreting its own receptor, Tir, into the host and
binding to it with its outer membrane ligand, intimin.
Intimin can also bind beta1-integrins.
Several cytoskeletal proteins are recruited to the site
of EPEC attachment, including actin, alpha-actinin,
talin, and ezrin.
Cytoskeletal rearrangements occur following Tirintimin binding, resulting in the formation of a
pedestal-like structure upon which the pathogen
resides.
EHEC
• Shiga like toxin circulates through
bloodstream, enters kidney and causes
kidney damage
Pedestal Formation by EPEC and EHEC
occurs by different mechanisms
•
•
•
•
•
•
EPEC Tir becomes tyrosine-phosphorylated in
the host-cell plasma membrane and binds the
adaptor protein Nck.
Nck recruits N-WASP or a WIPﾐN-WASP
complex to trigger activation of the Arp2/3
complex, which leads to actin assembly.
EHEC Tir localizes to the plasma membrane, but
is not tyrosine phosphorylated.
Other EHEC proteins (X) in addition to Tir are
translocated into host cells.
This combination of Tir and other factors
promotes recruitment and activation of N-WASP
by an unidentified mechanism (designated with a
question mark).
N-WASP then stimulates Arp2/3-based actin
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The Immune System is Active
in the Intestine
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• M cells and dendritic
cells are specialized
cells which collect
antigen
• Some pathogens
have systems which
allow them to invade
M cells and pass
through into the
body
PATHOGENICITY OF SALMONELLA
Salmonella invasion into host epithelial cells
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•
Salmonella secrete virulence proteins, including
SopE and SptP, into host cells by the type IIIsecretion system.
•
SopE functions as a guanidine exchange factor
for small GTP-binding proteins, probably
mediating the exchange of GDP for GTP on a
Rho subfamily member, CDC42.
•
SptP is a tyrosine phosphatase required for
invasion, probably by disrupting the
cytoskeleton.
•
Invasion also stimulates phospholipase C (PLC)
activity, leading to inositol triphosphate (IP3) and
Ca2+ fluxes, which in turn may be involved in
cytoskeletal rearrangements leading to
membrane ruffling and Salmonella
internalization.
Shigella-mediated cytoskeletal rearrangements
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The outer membrane protein, IcsA, is
sufficient to drive actin-based motility of
Shigella.
IcsA directly binds two proteins, vinculin
and neural-Wiskott-Aldrich Syndrome
protein (N-WASP).
Vinculin undergoes proteolysis within the
host cell upon Shigella infection,
producing a 90-kDa fragment that can
bind to IcsA and to the vasodilatorstimulated phosphoprotein (VASP).
VASP in turn can recruit profilin to the
bacterial surface, which can provide actin
for tail construction.
N-WASP binding of IcsA can also recruit
profilin to the bacterial surface and may
be another means of obtaining
monomeric actin for tail formation and
subsequent bacterial motility.
Viral Gastroenteritis
• It is thought that viruses are responsible for up to 3/4 of all infective
diarrheas.
• Viral gastroenteritis is the second most common viral illness after
upper respiratory tract infection.
• In developing countries, viral gastroenteritis is a major killer of infants
who are undernourished. Rotaviruses are responsible for half a million
deaths a year.
• Many different types of viruses are found in the gut but only some are
associated with gastroenteritis.
Rotavirus Particle
(Courtesy of Linda Stannard, University of Cape Town, S.A.)
Rotaviruses
• Naked double stranded RNA viruses, 80 nm in diameter.
• Also found in other mammals and birds, causing diarrhea.
• Causes disease in all age groups but most severe symptoms
neonates and young children.
in
• Asymptomatic infections common in adults and older children.
Symptomatic infections again common in people over 60.
Rotaviruses
• Accounts for 50-80% of all cases of viral gastroenteritis.
• Usually endemic, but responsible for occasional outbreaks.
• 80% of the US population have antibody against rotavirus by age of 3.
• 2.5 million cases in children under 5 per year in US; 70,000
hospitalizations and 20-70 deaths per year in US
• Up to 30% mortality rate in malnourished children
• Responsible for up to half a million deaths per year, worldwide
Rotaviruses
• More frequent during the winter.
• Fecal-oral spread.
• 24-48 hr incubation period followed by an abrupt onset of vomiting
and diarrhea, a low grade fever may be present.
• Diagnosed by electron microscopy or by the detection of rotavirus
antigens in feces by ELISA or other assays.
Rotaviruses
• 1998--RotaShield vaccine introduced
• Increase in frequency of intussusception, a blockage or twisting of the
intestine
• 20 deaths attributed to vaccine in first 1,000,000 children vaccinated
• Rotashield withdrawn
• 2006-RotaTeq vaccine approved by FDA for US use after large scale
testing--no increase in intussusception reported
Adenovirus Particle
(Courtesy of Linda Stannard, University of Cape Town, S.A.)
Enteric Adenoviruses
•
Naked DNA viruses, 75 nm in diameter.
•
Fastidious enteric adenovirus types 40 and 41 are
gastroenteritis.
•
Associated with cases of endemic gastroenteritis, usually in young children
and neonates. Can cause occasional outbreaks.
•
Possibly the second most common viral cause of gastroenteritis (7-15% of all
endemic cases).
•
Similar disease to rotaviruses
•
Most people have antibodies against enteric adenoviruses by the age of
three.
•
Diagnosed by electron microscopy or by the detection of adenovirus antigens
in faeces by ELISA or other assays.
associated with
Astrovirus Particles
(Source: ICTV database)
Astroviruses
•
•
Small
RNA viruses, named because of star-shaped surface
morphology, 28 nm in diameter.
Associated with cases of endemic gastroenteritis, usually in young
children and neonates. Can cause occasional outbreaks.
•
Responsible for up to 10% of cases of gastroenteritis.
•
Similar disease to rota and adenoviruses.
•
Most people have antibodies by the age of three.
•
Diagnosed by electron microscopy only, often very difficult because of
small size.
Calicivirus Particles
(Source: ICTV database)
Caliciviruses
•
•
•
•
•
Small RNA viruses, characteristic surface morphology consisting of
hollows. particles 35 nm in diameter.
Associated mainly with epidemic outbreaks of gastroenteritis, although
occasionally responsible for endemic cases.
Like Norwalk type viruses, vomiting is the prominent feature of
disease.
Majority of children have antibodies against caliciviruses by the age of
three.
Diagnosed by electron microscopy only, often difficult to diagnose
because of small size.
Norwalk-like Virus Particles
(Source: ICTV database)
Norwalk-like Viruses
•
Small RNA viruses, with ragged surface, 35 nm in diameter, now
classified as caliciviruses.
•
Always associated with epidemic outbreaks of gastroenteritis, adults more
commonly affected than children.
•
Associated with consumption of shellfish and other contaminated foods.
Aerosol spread possible as well as faecal-oral spread.
•
Also named "winter vomiting disease", with vomiting being
prominent symptom, diarrhea usually mild.
•
Antibodies acquired later in life, in the US, only 50% of adults are
seropositive by the age of 50.
•
Diagnosis is made by electron microscopy and by PCR.
the
Control of Infectious Disease
• Sanitation and other environmental
controls
• Innate defenses of the body
• Vaccination
• Anti-infective therapy
Sanitation as a Defense
Against Infectious Disease
• Once germ theory of disease was
understood --sanitary engineering
became a key line of defense against
infectious disease
Goals of Sanitary Engineering
• Remove wastes safely from
environment
• Eliminate possible sources of
contamination by infectious agents
• Provide safe water, food and air