Influenza Virus

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True influenza is an acute infectious disease caused by a member of the orthomyxovirus family: influenza virus A, B or, to a much lesser extent, influenza virus C (figure 3). However, the term 'flu' is often used for any febrile respiratory illness with systemic symptoms that may be caused be a myriad of bacterial or viral agents as well as influenza.

Influenza outbreaks usually occur in the winter in temperate climates. In the United States, the 'flu season usually starts in October or November and is at its height from December to March

Disease potential

Major outbreaks of influenza are associated with influenza virus type A or B. Infection with type B influenza is usually milder than type A. Type C virus is associated with minor symptoms.

Proteins

The internal antigens (M1 and NP proteins ) are the type-specific proteins (type-specific antigens) and are used to determine whether a particular virus is A, B or C. The M1 proteins of all members of each group show cross reactivity. The NP proteins of all members of each group also show cross reactivity.

The external antigens (HA and NA) show more variation and are the subtype and strain-specific antigens. These are used to determine the particular strain of influenza A responsible for an outbreak.

PATHOGENESIS AND DISEASE

Spread

The virus is spread person to person via small particle aerosols (less than 10μm) which can get into respiratory tract. The incubation period is short, about 18 to 72 hours.

Virus concentration in nasal and tracheal secretions remains high for 24 to 48 hours after symptoms start and may last longer in children. Titers are usually high and so there are enough infectious particles in a small droplet to start a new infection.

Site of infection

Influenza virus infects the epithelial cells of the respiratory tract. The cells die, in part due to the direct effects of the virus on the cell, and also possibly due to the effects of interferon. Cell death at later times may also result from the actions of cytotoxic T-cells. As a result, the efficiency of ciliary clearance is reduced, leading to impaired function of the mucus elevator; thus there is reduced clearance of infectious agents from the respiratory tract. Gaps in the protective epithelium provide other pathogens with access to other cells; however, viremia is very rare.

Recovery

Interferon may play a role by decreasing virus production. Many of the symptoms of uncomplicated influenza (muscle aches, fatigue, fever) are associated with the efficient induction of interferon. The cell-mediated immune response is important in viral clearance. The antibody response is usually not significant until after virus has been cleared. Repair of the respiratory epithelium begins rapidly, but may take some time to complete.

INTERFERON

Interferon response to an acute virus infection Interferons play an important role in the first line of defense against viral infections. They are part of the non-specific immune system and are induced at an early stage in viral infection – before the specific immune system has had time to respond.

Interferons are made by cells in response to an appropriate stimulus, and are then released into the surrounding medium; they then bind to receptors on target cells and induce transcription of approximately 20-30 genes in the target cells, and this results in an anti-viral state in the target cells.

TYPES OF INTERFERON:

TYPE I interferon:

Interferon-alpha (leukocyte interferon) is produced by virus-infected leukocytes, etc

Interferon-beta (fibroblast interferon) is produced by virus-infected fibroblasts, or virus-infected epithelial cells, etc

Interferon-a (a family of about 20 related proteins) and interferon-b are particularly potent as antiviral agents. They are not expressed in normal cells, but viral infection of a cell causes interferons to be made and released from the cell (that cell will usually die as a result of the infection). The interferon binds to target cells and induces an antiviral state. Both DNA and RNA viruses induce interferon but RNA viruses tend to induce higher levels. It seems that double-stranded RNA produced during viral infection may be an important inducing agent. Other stimuli will also cause these interferons to be made: e.g. exogenous double-stranded RNA, lipopolysaccharide, other components of certain bacteria.

TYPE II inteferon

Interferon-gamma (immune interferon) is produced by certain activated T-cells and NK cells.

Interferon-gamma is made in response to antigen (including viral antigens) or mitogen stimulation of lymphocytes.

Effects of interferon INTERFERON-alpha AND INTERFERON-beta (TYPE I INTERFERONS)

These interferons induce about 20-30 proteins, and the function of many of these is not fully understood. However, three of the proteins that appear to play an important role in the induction of the anti-viral state have been intensively studied. Expression of one of these proteins (2’5’ oligo A synthase) results in activation of the second of these proteins (a ribonuclease) which can break down mRNA, and expression of the third protein (a protein kinase) results in inhibition of the initiation step of protein synthesis. These activities target viral protein synthesis, but would also result in inhibition of host protein synthesis. Thus it is important that these proteins are only made and activated when needed.

Interferon treatment only induces synthesis of the inactive form of these proteins in the target cell. Activation requires that double-stranded RNA is present either to directly activate the protein (2’5’ oligo A synthase, the interferon-induced protein kinase), or because there is a requirement for 2’5’oligo A (ribonuclease L) which can only be made once 2’5’ oligo A synthase is activated. Thus these potentially toxic pathways are only activated in the interferon-treated cell if double-stranded RNA is made, this will usually only happen if virus infection actually occurs. The activation of these proteins in an infected cell may result in the death of the cell, but at least the progess of the infection is prevented.

OTHER EFFECTS OF INTERFERONS

The pathway described above is by no means the only way that interferons protect cells against viruses and other pathogens.

All three interferons increase expression of class I MHC molecules and thus promote recognition by cytotoxic T cells. All three interferons can activate NK cells which can then kill virus-infected cells.

Interferon-gamma increases expression of class II MHC molecules on antigen-presenting cells and thus promotes presentation of antigens to helper T cells. Interferon-gamma can also activate the ability of macrophages to resist viral infection (intrinsic antiviral activity) and to kill other cells if they are infected (extrinsic antiviral activity).

Interferons have many other effects on gene expression, not all of which are understood.

THERAPEUTIC USES OF INTERFERONS

Interferons-alpha and -beta have been used to treat various viral infections. One currently approved use for various types of interferon-a is in the treatment of certain cases of acute and chronic hepatitis C and chronic hepatitis B.

Interferon-gamma has been used to treat a variety of disease in which macrophage activation might play an important role in recovery, eg. lepromatous leprosy, leishmaniasis, toxoplasmosis.

Since interferons have anti-proliferative effects, they have also been used to treat certain tumors such as melanoma and Kaposi’s sarcoma.

SIDE EFFECTS OF INTERFERONS

Common side effects of interferons:

fever, malaise, fatigue, muscle pains

High levels of interferons can cause kidney, liver, bone marrow and heart toxicity.

VIRAL DEFENSES AGAINST THE NON-SPECIFIC AND SPECIFIC IMMUNE SYSTEMS

Not surprisingly, some viruses have developed defenses against the interferon-induced antiviral response and other aspects of the immune defense system. For example, viruses may code for proteins which block interferon binding to cells, inhibit the action of the interferon-induced protein kinase, inhibit NK function, interfere with cell surface expression of MHC, block complement activation, prevent the host cell committing apoptosis, etc.

Protection

A humoral antibody response is the main source of protection. IgG and IgA are important in protection against reinfection. Antibody to the HA protein is most important since this can neutralize the virus and prevent the virus initiating the infection. Neutralization frequently involves blocking of the binding of the virus to host cells and may work at other steps involved in the entry and uncoating of the virus. Antibody to the NA protein has some protective effect since it seems to slow the spread of the virus. IgG persists longer than IgA and so plays a more important role in long term immunity.

Clinical findings

The disease is usually most severe in very young children and the elderly. Children may have no antibodies and the small diameter of components of the respiratory tract in the very young also means that inflammation and swelling can lead to blockage of parts of respiratory tract, sinus system or Eustachian tubes. In the elderly, influenza is often severe because they frequently have an underlying decreased effectiveness of the immune system and/or chronic obstructive pulmonary disease or chronic cardiac disease.

CDC surveys show that each year about 114,000 people in the U.S. are hospitalized and about 36,000 people die because of the flu. Flu and pneumonia together constitute the sixth leading cause of deaths in the United States. Most flu fatalities are 65 years and older. Children younger than 2 years old are as likely as those over 65 to have to be hospitalized because of the flu. The 1918 Spanish flu outbreak killed more than 500,000 people in the United States and more than 20 million worldwide. The 1968-69 "Hong Kong flu" outbreak led to more than 34,000 deaths in the United States.

Symptoms and complications

1. Uncomplicated influenza

Fever (38 - 40 degrees C)

Myalgias, headache

Ocular symptoms - photophobia, tears, ache

Dry cough, nasal discharge

2. Pulmonary complications, sequelae:

Croup (acute laryngotracheobronchitis) in young children - symptoms include cough (like a barking seal), difficulty breathing, stridor (crowing sound during inspiration)

Primary influenza virus pneumonia

Secondary bacterial infection: Often involves Streptococcus pneumoniae, Staphylococcus aureus, Hemophilus influenzae

Build up of fluids and lack of mucociliary clearance provides good breeding grounds for bacteria.
Complications often occur in patients with underlying chronic obstructive pulmonary or heart disease. The underlying problems may not have been recognized prior to the influenza infection.

3. Non-pulmonary complications:

Myositis (rare, more likely to be seen in children after type B infection)

Cardiac complications

Encephalopathy - Increased surveillance of hospital patients less than 21 years of age in the state of Michigan in the United States during the 2002 - 2003 flu season revealed eight cases of influenza-associated encephalopathy. Two of these patients (aged two and five years) died. Similar complications of influenza have been reported from Japan. Even when not fatal, encephalopathy can have serious sequelae and this emphasizes the importance of vaccination. Neither of the Michigan fatalities had been vaccinated.

Reye's syndrome - The effects on the liver and brain are particularly serious. This is rare, but approximately 40% of cases are fatal. The origin of Reye's syndrome is unclear but seems to follow certain viral infections such as influenza or chicken pox (varicella zoster/herpes zoster), especially if they are in the young and especially if they have been treated with aspirin. Aspirin is contraindicated for childhood or adolescent fevers because it is a risk factor in the development Reye's syndrome. Acetaminophen is apparently not associated with Reye's syndrome.

Guillain-Barré syndrome (CNS) - The cause of this syndrome is mysterious. Recent vaccines do not seem to increase the risk of developing this.

The major causes of influenza-associated death are bacterial pneumonia and cardiac failure. Ninety per cent of deaths are in people over 65 years of age.

DIAGNOSIS

Firm diagnosis is by means of virus isolation and serology. The virus can be isolated from the nose or a throat swab. This is used to infect cells in culture (or eggs). Hemadsorption may be used to detect infected cells. Recently, rapid tests that can be used in a physician's office have been approved. Provisional diagnosis is often made clinically, based on knowledge of a current outbreak of influenza combined with appropriate clinical symptoms (fever, cough, runny nose, malaise).

EPIDEMIOLOGY

HA (hemagglutinin) protein

The HA protein is involved in attachment and membrane fusion in the endosome of the infected cell. The receptor binding site on the virus is in a pocket that is not exposed to the immune system. The antigenic domains are on the surface. These can be altered and the virus can thus avoid a humoral response without affecting its ability to bind to the receptor.

NA (neuraminidase) protein

The neuraminidase protein digests sialic acid (neuraminic acid) - which most cells have on their surface. Since sialic acid is part of the virus receptor, when the virus binds to the cell, it will be internalized (endocytosed). By late in infection, the sialic acid will have been removed from the infected cell surface by the neuraminidase making it is easier for the progeny virions to diffuse away once they exit the cell. Neuraminidase is also involved in penetration of the mucus layer in the respiratory tract.

Antigenic drift

Antigenic drift is due to mutation. Antibodies to the HA protein are the most important in protection, although those to NA also play a role. Both proteins undergo antigenic drift (i.e. accumulate mutations) and after a few years may have accumulated sufficient changes that an individual immune to the original strain is not immune to the drifted one. Antigenic drift results in sporadic outbreaks and limited epidemics.

Antigenic shift

Antigenic shift is due to reassortment. In the case of influenza A, antigenic shift periodically occurs. Apparently "new" HA and/or NA are found in the circulating viral strains. There is little immunity (particularly if both proteins change, or if new HA is present) and an epidemic/pandemic is seen.

Where does a "new" HA and/or NA come from? All thirteen HA and nine NA types circulate in ducks, some also circulate in other animals. It appears that some animal, somewhere, becomes infected with both a human and an animal virus, and that one of the reassortants contains genes for human internal components but a new HA and/or NA segment from the animal virus. If this virus can infect humans, it will have the same internal components as the current human virus, but new envelope components resulting in little immunity in the population. Influenza A strains/subtypes are therefore classified according to the nature of the HA and NA proteins. It is possible that we do not see such a shift in influenza B because there is no animal reservoir for this virus.

SURVEILLANCE

A measure of the severity of influenza in any one year is the excess of deaths due to pneumonia or influenza compared to the seasonally adjusted norm.

The World Health Organization (WHO) maintains constant surveillance of influenza outbreaks world wide and has a series of 'sentinel' labs to look at what is happening in the circulating virus population. The CDC does the same in the United States and co-operates with WHO.

Usually the most important influenza virus is influenza A, but in some recent years influenza B has played an important role. In recent years H1N1 and H3N2 have often co-circulated; the proportions of each can change dramatically from year to year.

PREVENTION

Vaccines

A new vaccine is formulated annually with the types and strains of influenza predicted to be the major problems for that year (predictions are based on worldwide monitoring of influenza). The vaccine is multivalent and the current one is to two strains of influenza A and one of influenza B. The vaccine given to adults at present is an inactivated preparation of egg-grown virus. It is contraindicated for those with allergies to eggs. It has a short lived protective effect and so is usually given in the fall so that protection is high in December/January - the usual peak months for flu in the northern hemisphere. It needs to be given every year since besides the short lived nature of the protection, the most effective strains for the vaccine will change due to drift or shift. Only certain formulations of the vaccine are approved for young children. Previously, a subunit vaccine was recommended.

In 2003, a live, attenuated (much less pathogenic than wild-type virus) vaccine (marketed as FluMist) was approved for use in the United States. It is only approved for healthy individuals (those not at risk for complications from influenza infection) from five to forty nine years of age. It is given nasally and should provide mucosal, humoral and cell-mediated immunity. In this new vaccine, the vaccine virus is a cold-adapted strain which can grow in the upper respiratory tract where it is cooler, but grows poorly in the lower respiratory tract. It is attenuated due to multiple changes in the various genome segments. Reassortment is used to generate viruses which have six gene segments from the attenuated virus and the HA and NA coding segments from the virus which is likely to be a problem in the up-coming influenza season. A reassortant is generated for each strain expected to be a problem. Since this is a live vaccine, given intranasally as a spray, it generates an IgA response and an IgM/G response. FluMist vaccine virus is also grown on eggs and so is contraindicated for people with an egg allergy. Since this is a live viral vaccine, it is also contraindicated for children and young adolescents on any therapy containing aspirin due to the potential risk of Reye's syndrome.

The CDC recommends: “Physicians should administer influenza vaccine to any person who wishes to reduce the likelihood of becoming ill with influenza (the vaccine can be administered to children as young as 6 months). Persons who provide essential community services should be considered for vaccination to minimize disruption of essential activities during influenza outbreaks. Students or other persons in institutional settings (e.g., those who reside in dormitories) should be encouraged to receive vaccine to minimize the disruption of routine activities during epidemics.”

2004-05 Antiviral Medications Usage Guidelines

CDC is issuing interim recommendations for the use of antiviral medications during the 2004-05 season. Local availability of these medications may vary from community to community, which could impact how these medications should be used.

CDC encourages the use of amantadine or rimantadine for chemoprophylaxis and use of oseltamivir or zanamivir for treatment as supplies allow, in part to minimize the development of adamantane resistance among circulating influenza viruses.

People who are at high risk of serious complications from influenza may benefit most from antiviral medications. Therefore, in general, people who fall into these high risk groups should be given priority for use of influenza antiviral medications:
Treatment

Any person experiencing a potentially life-threatening influenza-related illness should be treated with antiviral medications.
Any person at high risk for serious complications of influenza and who is within the first 2 days of illness onset should be treated with antiviral medications. (Pregnant women should consult their primary provider regarding use of influenza antiviral medications.)
Antiviral Use in Children: Rimantadine is approved for prophylaxis of influenza among children aged >1 year and for treatment and prophylaxis of influenza among adults. Although rimantadine is approved only for prophylaxis of influenza among children, certain specialists in the management of influenza consider it appropriate for treatment of influenza among children. Also available for treatment of children are amantadine (children aged >1 year), oseltamivir (children aged >1 year), or zanamivir (children aged >7 years).

Chemoprophylaxis

All persons who live or work in institutions caring for people at high risk of serious complications of influenza infection should be given antiviral medications in the event of an institutional outbreak. This includes nursing homes, hospitals, and other facilities caring for persons with immunosuppressive conditions, such as HIV/AIDS. When vaccine is available, vaccinated staff require chemoprophylaxis only for the 2-week period following vaccination. Vaccinated and unvaccinated residents should receive chemoprophylaxis for the duration of institutional outbreak activity. Rapid tests or other influenza tests should be used to confirm influenza as the cause of outbreaks as soon as possible. However, treatment and chemoprophylaxis should be initiated if influenza is strongly suspected and test results are not yet available. Other outbreak control efforts such as cohorting of infected persons, and the practice of respiratory hygiene and other measures also should be implemented. For further information on detection and control of influenza outbreaks in acute care facilities, see Detection and Control of Influenza Outbreaks in Acute Care Facilities.
All persons at high risk of serious influenza complications should be given antiviral medications if they are likely to be exposed to others infected with influenza. For example, when a high-risk person is part of a family or household in which someone else has been diagnosed with influenza, the exposed high-risk person should be given chemoprophylaxis for 7 days.
Antiviral medications can be considered in other situations when the available supply of such medications is locally adequate.

Chemoprophylaxis of persons in communities where influenza viruses are circulating, which typically lasts for 6-8 weeks:
Persons at high risk of serious complications who are not able to get vaccinated.
Persons at high risk of serious complications who have been vaccinated but have not had time to mount an immune response to the vaccine. In adults, chemoprophylaxis should occur for a period of 2 weeks after vaccination. In children aged <9 years, chemoprophylaxis should occur for 6 weeks after the first dose, or 2 weeks after the second dose, depending on whether the child is scheduled to receive one or two doses of vaccine.
Persons with immunosuppressive conditions who are not expected to mount an adequate antibody response to influenza vaccine.
Health-care workers with direct patient care responsibilities who are not able to obtain vaccine.

Treatment of infected adults and children aged >1 year who do not have conditions placing them at high risk for serious complications secondary to influenza infection.

Where the supplies of both influenza vaccine and influenza antiviral medications may not be sufficient to meet demand, CDC does not recommend the use of influenza antiviral medications for chemoprophylaxis of non-high risk persons in the community.

Chemotherapy

Rimantadine and amantadine block virus entry across the endosome and also interfere with virus release (see anti-viral chemotherapy section). They are good prophylactic agents for influenza A, but there are some problems in taking them on a long term basis. They may be given as protective agents during an outbreak, especially to those at severe risk and key personnel. They may also be given at the time of vaccination for a few weeks, until the humoral response has time to develop. (There is some evidence that these drugs can help prevent more serious complications if given early in infection.)

Two neuraminidase inhibitors have recently been approved by the FDA (zanamivir [Relenza] and oseltamivir). They are active against influenza A and influenza B. These drugs can reduce the duration of uncomplicated influenza (by approximately 1day). Oseltamavir is approved for prophylaxis as well as treatment. At the moment, Zanamivir is only approved for treatment but trials indicate it is probably as effective as oseltamivir in prophylaxis.

As yet there are no clear data on the ability of any of the these drugs to reduce serious complications when used to treat influenza (as contrasted with when they are used prophylactically).

You should check with the CDC MMWR Recommendations and Reports for Influenza for concerns such as dosage, side effects, and the annual update of recommendations.

Other treatment

The best treatments are rest, liquids, anti-febrile agents (not aspirin in the young or adolescent, since Reye's disease is a potential problem). Be aware of and treat complications appropriately