Lecture 29 Viral Hepatitis- Part 1
Reading Assignments: (1) Text Chapters 42, Chapter 57, p. 524. (2) Gilligan, P.H., M.L. Smiley, and D.S. Shapiro. 1997. Cases 30, 31, and 32. In: Cases in Medical Microbiology and Infectious Diseases (2nd ed.), American Society for Microbiology, Washington, D.C.
1. A PRIMER ON LIVER ANATOMY AND LIVER AND BILIARY TREE DISEASE
A. Basic Anatomy of the Liver
1. Where is the Liver Located?
Located in the right upper quadrant of the abdomen in the peritoneal space just below
the right diaphragm and under the rib cage. It weighs approx. 1400 g in the
adult and is covered by a fibrous capsule.
2. How Does Blood Flow Through the Liver?
The liver receives nearly 25% of the blood from the heart (approx. 1500 ml of
blood/min) from two sources:
a. venous flow from branches of the hepatic portal vein; and
b. arterial flow from branches of the hepatic artery.
flow exits via the central veins. The central veins come together to form hepatic
veins which drain into the inferior vena cava. Within the liver sinusoids, there are
large gaps between endothelial cells which allows close contact between plasma
and the liver cells (which are called hepatocytes). Anchored to the endothelial cells of
the liver sinusoids are many Kupffer cells- the fixed macrophages of the RES
system .
3. How is the Liver Organized?
a. The liver is organized in lobules of hepatic plates (organized around
individual central veins).
b. The hepatocytes are arranged in plates of single cells lying in a
cage of supporting cells.
c. The bile canaliculi are fine tubular canals that run between hepatocytes,
usually occurring singly between each adjacent pair of cells, and forming a
3-dimensional network of polyhedral meshes, with a single cell in each
mesh. Bile (see below), is made by the hepatocytes and drains into the bile
canaliculi and then into the bile ducts (see below). If the normal function
of the hepatocytes is disrupted (as occurs with the disease hepatitis), the flow
of bile is interrupted (cholestasis, see below).
B. Functions of the Liver
1. Formation and secretion of bile (see below).
2. Nutrient and Vitamin Metabolism
Glucose and other sugars, amino acids, lipids, fatty acids, cholesterol,
lipoproteins, fat-soluble vitamins, water soluble vitamins
3. Inactivation of Various Substances
Toxins, drugs, steroids, other hormones
4. Synthesis of Plasma Proteins
Acute phase proteins, albumin, clotting factors, steroid-binding and other
hormone-binding proteins
5. Immunity
Kupffer cells- part of the RES system
C. Bile and Bilirubin
BILE
1. What is bile and what is its purpose?
Bile is a fluid secreted by the liver which is poured into the small intestine via the
bile ducts. Important constituents of bile: conjugated bile salts (made from
cholesterol) , bile pigments (including bilirubin) , cholesterol, and electrolytes.
Bile alkalizes the intestinal contents, and conjugated bile salts reduce surface
tension and with phospholipids and monglycerides emulsify fats in preparation for
their digestion and absorption in the small intestine. When bile is excluded from
the intestine, up to 50% of ingested fat appears in the feces.
2. Where is bile made and where is it stored?
Bile is synthesized by the liver heptocytes, collected by the bile canaliculi, and
drained into hepatic bile ducts, the common bile duct, and then into the duodenum. Between meals, the opening into the duodenum is closed, and the bile flows into the gallbladder, where it is stored. When food enters the mouth, the sphincter around the opening relaxes, the gall bladder contracts,
and bile enters the duodenum.
3. How is bile recirculated in the body?
From 90 to 95% of the bile salts are reabsorbed from the small intestine and are
transported back to the liver in the portal vein and excreted into the bile again
(enterohepatic circulation). The small amount of bile that is lost in the stool is
replaced by synthesis in the liver. (The entire pool recycles twice per meal and
6-8 times per day)
4. How can disease of the gallbladder occur?
Any condition that leads to cholestasis can lead to an infection of
the gallbladder - cholecystitis.
Cholestasis – stoppage or suppression of the flow of bile:
1. extrahepatic causes - due to obstruction from a stone, stricture, or
neoplasm.
2. intrahepatic causes - due to liver cell disease, e.g. hepatitis.
Cholecystitis – inflammation of the gall bladder.
SEE TEXT P. 524-525.
Infections of the gallbladder (cholecystitis) are usually the result
of gallstone formation. When stones form in the gallbladder or become wedged in the cystic duct, the normal drainage of bile is impaired. This can result in infection, edema and compression of the local blood supply, which in turn can cause acute disease (including gas gangrene) in the gallbladder (See Reading Assignment (2) Case Study 30 – emphysematous (gas-forming) cholecyctitis due to C. perfringens and Text CASE 2, p. 524- cholecystitis due to E. coli and enterococci. Infections in the gallbladder can progress to bacteremia (See CASE 2, above),sepsis, and liver abscess formation.
Gallbladder infections can occur without the presence of gallstones if an enteric pathogen ascends from the intestines via the common bile duct and the cystic duct into the gallbladder. Common bacterial pathogens responsible for such infections include Salmonella species (remember Typhoid Mary???) and Campylobacter species.
BILIRUBIN
4. What is bilirubin and how does it end up being excreted in bile and ultimately
into the feces?
Bilirubin is made during the catabolism of hemoglobin. When old red blood
cells are destroyed by the fixed macrophages of the RES, the globin portion of
the hemoglobin molecule is split off, and the heme is converted to biliverdin.
(The iron in the heme is totally recycled.) Most of the biliverdin is then converted
to bilirubin and is transported in the blood bound to albumin. From the
bloodstream it is taken up by liver hepatocytes and conjugated to form a water
soluble pigment (bilirubin diglucuronide) which is excreted into the bile
canaliculi and ultimately into the small intestine as part of bile.
(An interesting note: The intestinal bacteria convert bilirubin into urobilinogen, and it is this brown pigment that is excreted in the stool.)
5. What does bilirubin have to do with the symptom of jaundice (icterus) seen in
patients with viral hepatitis?
During viral hepatitis, the hepatocytes are unable to take up and process bilirubin in
the normal fashion (described above) and excessive concentrations build up in the
blood (hyperbilirubemia)- leading to jaundice.
Jaundice - (Fr. Jaune – yellow) a syndrome characterized by hyperbilirubinemia
and deposition of bile pigments in the skin, mucous membranes and sclera (the
tough white outer coat of the eyeball) with resulting yellow appearance of the
patient; called also icterus.
Patients with jaundice are said to be icteric.
D. Capacity of the Liver for Regeneration
When hepatocytes are lost to disease (such as hepatitis) or if the liver is surgically
resectioned, poorly understood mechanisms stimulate the proliferation of the remaining
hepatocytes. More than 50% of the liver must be damaged or destroyed before liver
function fails. Regeneration of liver cells is rapid, but fibrous repair, when infection
persists, can lead to scar tissue.
E. Types of Liver Disease
1. Hepatitis
a. Definition – inflammation and damage to the liver
b. Symptoms – vary from malaise, anorexia and nausea to acute life-
threatening liver failure (rare). Associated with elevations of liver enzymes
such as alanine aminotransferase (ALT) and aspartate amino transferase
(AST).
2. Cirrhosis – liver disease characterized pathologically by loss of the normal
microscopic lobular architecture, and replacement with scar tissue.
Cirrhosis is the result of progressive liver injury. In cirrhosis, the liver becomes hard,
shrunken, and nodular and displays impaired function and diminished reserve due to a
decreased amount of functioning liver tissue. Blood flow is altered so that blood in the
hepatic portal vein is diverted around the liver rather than being filtered through the
liver. This results in a heightened sensitivity to noxious substances absorbed from the
GI tract (causing encephalopathy) , an increased risk of massive gastointestinal
bleeding, and malabsorption of fat in the stool. These changes are generally
irreversible (2).
E. The Liver as a Site for Viral Replication (see Text p. 391-392)
Pathogens (viral, bacterial, or protozoal) cross from the bloodstream into the liver
hepatocytes (Review Lectures 5 and 6). The regenerative capacity of the liver hepatocytes is
important for the host to recover from hepatitis, but it also provides new hepatocytes for the
pathogens to multiply in.
F. Immune Defenses of the Liver (nonspecific and specific)
1. Circulatory and lymphatic networks give excellent access to the immune system.
2. Kupffer cells aid in clearance pathogens from the bloodstream, induce inflammation,
and act as antigen presenting cells.
3. Extracellular pathogens – humoral immunity (neutralization by antibodies)
4. Intracellular pathogens – cellular mediated immunity – killed by TC cells
2. INTRODUCTION TO HEPATITIS VIRUSES
Many viruses can infect the liver and cause disease including (yellow fever virus, Lassa virus, and
several of the herpesviruses (HSV, EBV, and CMV), but five known viruses display marked tropism for
hepatocytes, and only cause hepatitis. (A 6th virus- provisionally designated hepatitis G
is currently uncharacterized.)
A. Hepatitis A virus – commonly known as infectious hepatitis is a picornavirus which is spread
by the fecal-oral route. It has an incubation period of approx. 1 mo., after which icteric
symptoms start abruptly. It does not cause chronic liver disease and rarely causes fatal
disease.
B. Similar to Hepatitis A, Hepatitis E virus (called enteric non A, non B) is also an enteric virus
(provisionally classified as a calcivirus) that is spread via the fecal-oral route. It has a short
incubation period like HAV and disease begins abruptly. Like Hepatitis A, there is no chronic
disease or carrier state.
C. Hepatitis B, previously known as serum hepatitis is caused by a hepadnavirus with a DNA
genome. It is spread parenterally by blood or needles, by sexual contact, or perinatally. It has
a mean incubation period of approximately 3 months, after which icteric symptoms start
insidiously. It is followed by chronic hepatitis in 5-10% of patients, and is causally associated
with primary hepatocellular carcinoma.
Previously, any undefined cause of viral hepatitis that was not Hepatitis A or B was called
NANB. That designation covered what we now know as Hepatits C, G and E viruses.
D. Hepatitis C virus is spread by the same routes as Hepatitis B and can also cause chronic
disease. It is a flavivirus with an RNA genome and it has a somewhat different clinical course.
The new virus, Hepatitis G is also a flavivirus but it can only be identified by PCR techniques.
E. Hepatitis D, or delta hepatitis is unique in that it requires actively replicating BV as a “helper
virus” and occurs only in patients who have active HBV infection. HBV provides an envelope
for HDV RNA and its antigen or antigens. Delta agent exacerbates the symptoms caused by
hepatitis B virus.
Specific laboratory tests for hepatitis A and B viruses available for years; now tests for the
others are becoming available. Liscensed vaccines only for hepatitis A and B.
3. ENTERIC-TRANSMITTED VIRAL HEPATITIS- Hepatitis A (HAV) and E (HEV)
HAV and HEV share a number of physical and biological characteristics:
A. Both are nonenveloped, RNA viruses with icosahedral capsids. They both
have single-stranded genomes of + polarity. (Since they belong to different
families, there are differences in their structure, genome replication, and their
molecular mechanisms of translation and RNA replication.)
Since HAV and HEV do not have lipid envelopes, they are resistant to
inactivation by acids (including the acid in the stomach), detergents (including bile in the small intestines), and temperatures as high as 60C. They can survive in fresh and salt water for many months.
B. Both have a similar incubation period, an abrupt onset and typically cause acute, self-limited
infections. They are not associated with chronic hepatitis or hepatocellular carcinoma.
C. Epidemiology
1. Fecal-oral transmission. Both HAV and HEV replicate primarily in the liver and
reach the intestinal tract following secretion from the hepatocytes into biliary canaliculi
and passage through the bile ducts. Their resistance to bile allows them to be
transmitted this way.
2. Because these viruses have a direct route to the outside- they are associated with
explosive outbreaks of hepatitis (not seen with the other hepatitis viruses).
3. Both are associated with common-source outbreaks -sources of infection include
contaminated water, food, and dirty hands.
How does the virus get into the water???
How does the virus get into food???
How does the virus get onto hands???
a. Important sources of the virus: shellfish, especially clams, oysters, and
mussels. (Filter feeders -concentrate the virus particles even from dilute
solutions.
Ex. Epidemic of HAV occurred in Shanghai, China, in 1988 in
which 300,000 people were infected with the virus as the result
of eating clams obtained from a polluted river.
b. Daycare settings – a major source of infection. Can spread among
the children and then to parents.
c. Related to poor hygienic conditions and overcrowding.
1. Developing countries – infected as children and have mild
illness.
2. Developed countries – infections occur later in life.
d. Infections often associated with travel to developing countries.
e. Pregnant women – high mortality associated with HEV.
4. Both are associated with asymptomatic shedding since contagious period occurs
before symptoms.
4. HEPATITIS A VIRUS (causes 40% of cases of acute hepatitis)
(See Reading Assignment (2) Cases 31 and 32)
A. Structure
Typical picornavirus; 27nm, naked icosahedral capsid surrounding a + sense genome. HAV
genome has Vpg protein attached to the 5 end and a poly A tail attached to the 3 end.
There is only one serotype of HAV.
B. Replication – like other picornaviruses (review this replication cycle). Tissue Tropism – HAV
interacts specifically with a receptors expressed on liver cells and a few other cell types.
Unlike other picornaviruses, HAV is not cytolytic and is released by exocytosis.
C. Pathogenesis and Immunity
1. Ingested and enters the bloodstream through the oropharynx or the epithelial lining of
the intestines to reach the hepatocytes of the liver. The virus can be visualized inside
Kupffer cells and the hepatocytes.
2. HAV replicates slowly in the hepatocytes without producing CPE.
3. Virus produced in the hepatocytes is released into the bile and from there into the
stool. Virus is shed in the stool about 10 days before symptoms appear or antibody
can be detected. Thus asymptomatic persons can spread the infection!
4. Interferon limits initial viral replication, but cytotoxic T cells are required to lyse
infected cells. Antibody, complement, and ADCC also facilitate clearance of the virus
and contribute to immunopathology.
5. Jaundice (icterus) resulting from liver damage, occurs when antibody is detected and
cell-mediated immune responses to the virus occur.
6. Liver pathology indistinguishable from Hepatitis B infections. Most likely caused by
immunopathology not virus-induced cytopathology. Unlike HBV, HAV cannot initiate
chronic infections and it is not associated with liver cancer. Immunity to reinfection is
life-long.
D. Epidemiology (see above)
E. Clinical Syndrome
1. Abrupt onset (15-50 days after exposure)
2 Initial symptoms fever, fatigue, nausea, loss of appetite, and abdominal pain.
3. Symptoms intensify for 4-6 days before jaundice appears (seen in 2/3 adults but only
1-2/10 children).
4. After jaundice appears, symptoms begin to recede.
5. Viral shedding occurs before symptoms begin.
6. Complete recovery – 99% of the time.
F. Laboratory Diagnosis
1. Clinical symptoms
2. Identification of a known infected source
3. Elevation in liver enzymes
4. Specific serological tests.
a. Acute infection – find anti-HAV IgM antibodies (measured by
ELISA)
b. Convalescent – find anti-HAV IgG antibodies
1. Prevention – interrupt the fecal oral spread of the virus. Avoid contaminated food and
water. Proper handwashing in day care centers, mental hospitals, and other care
facilities. Chlorination of water kills the virus.
2. Passive immunization with immune serum globulin given before or early in the
incubation period (less than 2 weeks after exposure) is 80-90% effective in preventing
clinical illness.
3. Inactivated HAV vaccine – recommended as prophylaxis against hepatitis A in high-
risk groups – international travelers, military personnel, male homosexuals, and those
in close contact with an active hepatitis A case. Also suggested for daycare workers,
health care workers, closed-institution occupants and patients with chronic liver
disease, and food handlers.