Page 201 - Textbook of Pathology, 6th Edition

illness and lymphadenopathy. Various forms of influenza and an apprehension of pandemic has sent alarm bells all 185
virus infections have occurred as an outbreak at different over world for quarantine.
times, sometimes with alarming morbidity and mortality in PATHOGENESIS. SARS is caused by influenza type A/
the world. Seasonal flu vaccine is administered to population H5N1 respiratory virus, also called SARS-associated
at high risk in these countries.
coronaviruses (SARS-CoV). Though it is not fatal for wild
ETIOLOGIC AGENT. Influenza virus is a single-stranded birds, it can kill poultry birds and people. Humans acquire CHAPTER 7
RNA virus belonging to coronaviruses. Depending upon its infection through contaminated nasal, respiratory and faecal
antigenic characteristics of the nucleoprotein and matrix, 3 material from infected birds. An individual who has human
distinct types are known: A, B and C. Out of these, influenza flu and also gets infected with bird flu, then the hybrid virus
type A is responsible for most serious and severe forms of so produced is highly contagious and causes lethal disease.
outbreaks in human beings while types B and C cause a milder No person-to-person transmission has been reported so far
form of illness. Type A influenza virus is further subtyped but epidemiologists fear that if it did occur it will be a global
based on its 2 viral surface features: epidemic. Humans do not have immune protection against
avian viruses.
Haemagglutinin (H): H antigen elicits host immune
response by antibodies and determines the future LABORATORY DIAGNOSIS. Following abnormalities in
protection against influenza A viruses. There are 16 distinct laboratory tests are noted:
H subtypes of type A influenza viruses. 1. Almost normal-to-low TLC with lymphopaenia in about
half the cases, mostly due to fall in CD4+ T cells. Infectious and Parasitic Diseases
Neuraminidase (N): Antibody response against N antigen
limits the spread of viral infection and is responsible for 2. Thrombocytopaenia.
reduction of infection. N antigen of influenza A exists in 9 3. Elevated liver enzymes: aminotransferases, creatine kinase
subtypes. and LDH.
Thus, the subtypes of influenza A viruses are 4. Virus isolation by reverse transcriptase-PCR on
designated by denoting serial subtype numbers of H and respiratory sample, plasma, urine or stool.
N antigens as H1N1, H2N2 etc. 5. Tissue culture.
6. Detection of serum antibodies by ELISA or
Influenza A viruses infect human beings, birds, pigs and immunofluorescence.
horses. In view of a high antigenic variation in H and N
components, influenza A viruses are responsible for many CLINICOPATHOLOGICAL FEATURES. Typically, the
known epidemics and pandemics in history and in present disease begins with influenza-like features such as fever,
times. Major antigenic variation in H or N antigens is called cough, dyspnoea, sore throat, muscle aches and eye
antigenic shift while minor variation is termed antigenic drift. infection. Soon, the patient develops viral pneumonia
In general, population at high risk are immunosuppressed evident on X-ray chest and acute respiratory distress (hence
patients, elderly individuals and infants. the term SARS), and terminally kidney failure.
Two of the known subtypes of influenza A viruses which There is apprehension of an epidemic of SARS if the avian
have affected the mankind in recent times and have attracted virus mutates and gains the ability to cause person-to-person
a lot of attention of the media and the WHO are as under: infection. Since currently vaccine is yet being developed, the
Avian influenza virus A/H5N1 commonly called available measures are directed at prevention of infection
“bird flu”. such as by culling (killing of the infected poultry birds) and
Swine influenza virus A/H1N1 commonly called isolation of infected case.
“swine flu”.
These two entities are briefly discussed below.
Swine Flu (Influenza A/H1N1)
Bird Flu ((Influenza A/H5N1) H1N1 influenza type A flu which appeared last in 1977-78 as
a mild form of pandemic has reappeared in April 2009 as an
H5N1 subtype of the influenza type A virus infection causes
severe acute respiratory syndrome (SARS) which is the outbreak in Mexico but is rapidly spreading elsewhere.
human form of bird flu or avian influenza with having Presently, the disease has already spread to 39 countries
similar symptomatology. Every year, there have been including US. In view of rising number of cases, with about
outbreaks in poultry birds in different parts of the world 10,000 confirmed cases and about 100 deaths by end-May 2009
resulting in slaughtering of millions of infected chickens attributed to swine flu from all over the world, the WHO has
every year. Human outbreak of the disease called SARS alerted that it may become a worldwide flu pandemic.
reemerged in December 2003 in southern China, Hong Kong PATHOGENESIS. H1N1 influenza type A virus is primarily
and Vietnam and then spread to other countries in Asia, an infection in pigs with low mortality in them. Human beings
Europe and America. Since then, every year there have been acquire infection by direct contact with infected pigs.
seasonal outbreaks in the human form of the disease in high However, further transmission of H1N1 flu occurs by person-
winter and has so far affected 15 countries and taken a toll to-person contact such as by coughing, sneezing etc but it is
of over 250 lives. Its rapidly downhill and fatal clinical course not known to occur from eating pork.