Orthomyxoviridae & Paramyxoviridae
Classification ORTHOMYXOVIRIDAE Influenza viruses Type A Type B Type C Family: Genus: Types:
“myxo” refers to interaction with mucins (glycoproteins) • Different from paramyxoviruses : -segmented genome • - smaller (average 110 nm in diameter against 150 nm).
An envelopedviruse, helicalsymmetrycapsid, segmentedlinear RNA genome Nucleocapsid: Nucleoprotein (7 or 8 RNA segments) Internal antigens Matrix protein (M) Lipid bilayer Haemaglutinin (HA) Surface antigens Neuraminidase (NA) 80 to 120 nm
Each hemagglutinin spike is made up of three entwined molecules while each neuraminidase is comprised of four entwined molecules. On the surface of the virus are M2 proteins. Inside the lipid envelope, there are eight RNA gene segments called RNPs(RNA molecule+ Nucleoprotein+ Polymerases). Ball shaped M1 proteins: as cushions for the RNPs inside.
Surface glycoproteins Haemagglutinin H or HA responsible for pathogenicity of the virus allows virus to adhere to endothelial cells in the respiratory tract main determinant of immunity Neuraminidase N or NA allows release of newly formed viruses within host determinant of disease severity
Antibody against the hemagglutininneutralizes the infectivity of the virus and prevents disease. Ab against neuraminidase only reduces disease.
Influenza subgroups Influenza A highly infective infects many species causes widespread epidemics Influenza B found only in humans capable of producing severe disease causes regional epidemics Influenza C causes mild disease humans are natural hosts, but isolates also found in pigs does not cause epidemics
Reassortment of segments of the genome RNA • Influenza viruses, especially type A show changes in the antigenicity of their hemagglutinin and neuraminidase proteins. epidemics. • Influenza viruses antigenes • Group-specific (internal ribonucleoprotein) antigenes. • Type-specific (surface N and H) antigens.
Many species of animal (eg. Birds, swine, and hourses) have their own influenza A viruses. These animal viruses are probably the source of the new antigenic types. Antigenic shift: Major changes based on reassortment of genome pieces. Occurs every 10-11 years Antigenic drift: Minor changes based on mutation occurs every year.
Antigenic shift appears to result from genetic recombination of human with animal or bird ,providing major antigenic change.This can cause a major epidemic or pandemic involving most or all age groups.
Epidemics and pandemics occur when the antigenicity of the virus has changed sufficiently that the preexisting immunity of many people is no longer effective.
Various combinations of RNA segments can result in a new subtype of virus (known as antigenic shift It is even possible to include RNA strands from birds, swine, and human influenza viruses into one virus if a cell becomes infected with all three types of influenza.
Occurrence of influenza A viruses Influenza A viruses 16 HA types 9 NA types Species affected humans, pigs, horses, birds, marine mammals In humans 3 HA types (H1, H2, H3) 3 NA types (N1, N2, N8) In birds all HA types all NA types
Influenza viruses nomenclature For example: A / Beijing / 32 / 92 (H3N2) A virus type, here A Beijing place where the strain was isolated 32 strain number 92 year of first isolation H3N2 subtypes H3 and N2 virus sub type, here H3N2
Pathogenesis After the virus is inhaled, the neuraminidase degrades the protective mucus layer, allowing the virus to gain access to the cells of the upper and lower respiratory tract. Viremia rarely occurs, but there is necrosis of the superficial layers of respiratory epithelium.
Immunity Circulating IgG against the virus occurs after infection, but offers little protection. Secretory IgA in the respiratory tract is protective.
Clinical findings • Incubation period: 24-48 hours • Symptoms: fever, myalgias, headache, cough develop suddenly. • The symptoms resolve spontaneously in 4-7 days but sometimes is complicated with secondary infections. • Rey’s syndrome (Encephalopathy and liver degeneration life-threatening complication in children) following some viral infections, particularly influenza B and chikenpox, if they have been given Asprin to reduce the fever.
COMPLICATIONS • Pneumonia • Respiratory failure • Convulsion (muscles contract and relax rapidly and repeatedly, resulting in an uncontrolled shaking of the body)
When to Seek Emergency Medical Care • has difficulty breathing or chest pain • has purple or blue discoloration of the lips • is vomiting and unable to keep liquids down • has signs of dehydration such as dizziness when standing, absence of urination, or in infants, a lack of tears when they cry • has seizures (for example, uncontrolled convulsions) • is less responsive than normal
RISK GROUPS • Persons with certain chronic medical condition • School children • Travelers to some high risk places • Border workers • Health care workers or public health workers
PREVENTION • Prevention in swine or other animal hosts. • Prevention of transmission to humans. • Prevention of its spread among humans.
Frequent washing of hands with soap and water Prevention of its spread among humans.
Use of towel while sneezing
Lab diagnosis • Virus isolation (by throat washing) with cell culture. Then flurescent-antibody staining of the infected cells by using antisera to influenza A and B. • A rise in antibody titer of at least 4-fold in serum samples using hemmagglutination inhibition or complement fixation. • PCR reactions
TREATMENT • supportive care is required. Bed Rest • Keep the sick person in a room separate from the common areas of the house. • Antibiotics (to treat this disease, do help prevent bacterial pneumonia and other secondary infections.) • Viral agent is used in severe infections. (Zanamivir orTamiflu is recommended by C.D.C.)
VACCINE • The current trivalent influenza vaccine is likely to provide protection against the new 2009 H1N1 strain.