Infection with the flu virus H7N9 is normally confined to poultry birds - but it was first found to have infected a human in March 2013, going on to cause 132 infections in people during that spring, and leading to 44 deaths.1,2
Because humans have not developed immunity to these subtypes of flu virus - which are continually evolving - the infections they cause us can be severe and life-threatening.3
Another avian flu virus has crossed over from bird populations to spread among humans - it was the H5N1 bird flu that first broke out in 1997.3 It caused severe respiratory illness and death, and continues to circulate around poultry in parts of Asia and north-east Africa.4
Whenever a new type of bird flu infection is reported in a person, fears are soon raised that it could be deadly, too. The concerns come in three stages:
- Does the new virus subtype cross easily from birds to humans?
- How bad is the illness when the virus infects a person?
- Will the virus spread quickly and widely among human populations?
As soon as new reports emerge, researchers and governments swing into action to start identifying and tracking the new flu virus, the aim being to maximize the chances of containing any potential epidemic. An international body is at the center of this effort, called the Global Early Warning System (GLEWS), which coordinates disease surveillance work by bodies including the World Health Organization (WHO).
What are the symptoms of human infection with H7N9 flu?
Any flu infection causes fever and malaise but with H7N9 the lung disease is severe and life-threatening.
Flu infection in people is usually caused by very common seasonal infections with viruses to which we have developed some immunity, and this produces classic symptoms such as fever that are debilitating but that do not cause life-threatening illness in healthy people, and begin to pass within days.5
Human infection with H7N9 flu virus, however, causes serious respiratory symptoms and can lead to death - the most common symptom is severe pneumonia.6
During the early stage of H7N9 infection, most cases have shown the following symptoms, nearly all of which are familiar for flu:6
- Cough that produces sputum
- Wheeze (whistling sound during breathing, a sign of breathing problems)
- Myalgia (muscle pain/aches)
- General malaise.
The later stage of severe illnesses caused by H7N9 has been characterized by the following serious problems:6
- Pneumonia in both lungs
- Rapid progression to acute respiratory distress syndrome (ARDS)
- Multi-organ dysfunction (impaired liver or renal function)
- Septic shock (medical emergency caused by blood infection)
- Rhabdomyolysis (muscle breakdown)
- Encephalopathy (brain disease).
Critically ill patients also generally show lymphopenia (reduced numbers of lymphocytes, white blood cells) and thrombocytopenia (reduced thrombocyte, platelet, count).6
Flu virus types
All flu viruses are members of the family called Orthomyxoviridae. The viruses that affect us are grouped into influenza A or B, and the avian viruses that have infected humans belong to the influenza A genera.7
Flu A viruses are further subtyped H and N - there are 16 H subtypes and nine N subtypes. The particular H and N variant is what we see when viruses are named H7N9, H5N1, and so on.7
Influenza is evolving
The flu virus is continually evolving - even the common flu virus that affects large numbers of people seasonally is slightly different every year, which is why the vaccine against it must continually be tweaked to match.
The proteins that denote the flu subtype - H for HA or hemagglutinin, and N for NA or neuraminidase - are the parts of the virus that evolve.3
HA is the protein that enables the influenza virus to enter host cells, while the NA protein facilitates viral exit or 'shedding', and therefore its spread.3
These proteins are the same parts of the influenza virus that are recognized by the human immune system, so even a slight change in one of these proteins means that the virus successfully 'knocks us for six' each time we are newly infected. More abrupt changes in the virus proteins, meanwhile, take our immunity by greater surprise, enabling a new variant to cause serious illness because our immune system has not been presented with the subtype previously.3,8
Epidemics and pandemics
The H7N9 outbreak has so far been confined to people working closely with poultry in China.
The difference between epidemics and pandemics of flu infection is in the extent and severity of the spread. Pandemics are global infections of a newly emerged virus type, whereas epidemics happen every year - the typical seasonal infection.3,8
Pandemics are far less frequent but are more harmful for the people infected. Sometimes, they are extremely harmful - the Spanish flu pandemic of almost a century ago, in 1918, was caused by H1N1 and it is thought that it infected half the world's human population and caused between 40 and 50 million deaths, mostly among healthy 20- to 40-year-olds.8
Can H7N9 flu infection be treated?
Existing antiviral drugs used against the common seasonal flu virus are also given to people infected with the H7N9 avian flu.6 The class of drugs, known as neuraminidase inhibitors, are recommended for use as soon as possible in people suspected of infection with H7N9. The WHO guidance is that the drugs should be given while awaiting laboratory-tested confirmation of the infection, and not held off for the results.9
Two neuraminidase inhibitor drugs are recommended in the treatment of infection with the H7N9 avian flu virus:
- Zanamivir (Relenza, made by GlaxoSmithKline)
- Oseltamivir (Tamiflu, made by Roche).
These drugs are not effective in all people infected with H7N9, and even when they are effective - particularly when administered at the earliest opportunity - they reduce the severity of the illness rather than cure it.6 New drugs are being developed against influenza in general and may prove to be effective against H7N9, too.
Is there a vaccine against H7N9 flu?
Developing a vaccine against the H7N9 virus is difficult because of its molecular characteristics.6 Nonetheless, bodies in the US, UK and Japan have used WHO-recommended virus isolates to help in the development of vaccine candidates for future preparedness against potential pandemics.10
Successfully developing a vaccine against the new bird flu subtype would enable governments to stockpile products in readiness for use against a pandemic - the US government already maintains a stockpile of vaccines against the H5N1 infection.11
No fully-developed vaccine is yet available against the H7N9 virus, although at least four biopharmaceutical companies have candidates that are being tested in animal and human trials.6 The best promise seems to be in the development of a type of vaccine based on the use of virus-like particles - as opposed to traditional vaccines, which use the actual virus in an inactivated or attenuated form.
Written by Markus MacGill