Influenza, also called the “flu”, is a viral disease that affects humans and many animals. It most commonly causes a fever and respiratory problems. There are three groups of influenza viruses, called type A, B, or C. Influenza type A is the primary group affecting both humans and animals. Influenza A viruses also have many different sub-types.
Most only infect a single species (e.g., birds or pigs), while other sub-types can affect more than one species (e.g., birds, pigs and humans). Influenza A viruses can undergo rapid genetic changes (antigenic shift) which then allows them to infect new species of animals.
Influenza A is the most virulent type and is commonly associated with human disease. Between 1918 and 1919 flu is thought to have killed over 50 million people (6 times as many as died as a consequence of the World War I).
This global pandemic possibly infected 50% of the world’s population and up to 20% died. It was caused by an unusually pathogenic strain of influenza A virus. Other global influenza pandemics have been recorded through history and they seem to occur every 10 to 40 years. Each year, many countries, including the UK, experience seasonal influenza epidemics that affect hundreds of thousands of people.
What causes flu?
The influenza virus particle – virion – is usually spherical in shape and carries its genetic material on eight pieces of single stranded RNA known as segments. Each segment carries genes that encode for proteins that the virus needs in order to replicate inside the infected host cell. The genome is protected by a membrane envelope. Protruding from the virus envelope are hundreds of copies of two different varieties of viral glycoprotein spikes.
Approximately 80% of the spikes are haemagglutinin (HA) and the remaining 20% are neuraminidase (NA). The HA and NA surface proteins are involved in viral attachment and entry to host cells. They are also the main part of the virus recognized by our immune system as foreign, and most of the antibodies we make after infection are against these antigens.
What animals get influenza?
Birds, pigs, horses, ferrets, dogs, and cats can all be infected with various strains of the influenza virus. Influenza in birds is often referred to as avian influenza, in pigs as swine influenza, in horses as equine influenza and so on. Influenza in humans is often referred to as the seasonal flu. Waterfowl are important reservoirs for many subtypes of influenza.
The flu virus is extremely contagious and is transmitted from person to person by droplets expelled when sneezing and coughing. It can also be transmitted by direct contact, for example by touching virus-contaminated surfaces such as door handles and then touching the eyes or nose. Good hygiene practices, such as correct hand washing, are very important in preventing infection.
Most of the symptoms are in fact caused by the body’s immune response. These include: high fever, chills, severe fatigue, headaches, muscular aches and pains, non-productive cough and sore throat When the NA protein of influenza breaks down the mucus lining the epithelial cells, this also leaves the cells more susceptible to infection by other pathogens, such as bacteria like Staphylococcus aureus and Haemophilus influenzae.
Most people recover from flu within 1-2 weeks but more serious illness, and even death, can result from secondary infections e.g. bacterial pneumonia. This is more likely in infants, the elderly, people who are immuno-compromised and those with chronic lung disease such as emphysema.
There are two ways of tackling the disease: treating the symptoms and attacking the virus. The symptoms can be alleviated with drugs to reduce fever and pain such as paracetamol. Antiviral drugs can be effective against influenza but must be administered within the first 2 days of symptoms appearing.
Drugs such as oseltamivir (Tamiflu®) attack the virus by inhibiting the enzyme neuraminidase and therefore prevent the virus from infecting cells. Antibiotics are not used to treat influenza because they work by inhibiting bacterial cell wall production and protein synthesis and block cell metabolism and have no effect on viruses. They can be used to treat secondary bacterial infections.
Antibodies that recognize the HA and NA surface proteins of the influenza virus can protect us from infection by stopping the virus reaching the host cell surface. This means that people will not be infected by the same strain of influenza after they recover from flu. Giving a small dose of a crippled strain of influenza virus or an injection of purified HA protein as a vaccine, can stimulate the immune system to make antibodies to HA to protect us from flu.
Cold or flu?
The terms cold and flu are often used interchangeably but they are in fact distinct diseases caused by very different viruses. A cold is caused by several groups of viruses; the most common is rhinovirus and usually lasts between 2 and 4 days. Symptoms include runny nose, sneezing and mild fatigue. Flu is much more serious than a cold. Symptoms may last for a few weeks and the illness can be much more debilitating.