Influenza virus: a short introduction

This is excerpted from the ebook “Influenza Virus. Introduction to a Killer”, which is available here for US$9.99 .

Influenza: the disease

Influenza: a disease and a virus

Influenza as a disease in humans has been known for centuries; however, its cause was only discovered in the early 20th century: this was the group of viruses now known as Influenza virus types A, B and C.

There are several influenza viruses circulating in humans at any one time; these cause “seasonal flu”, which is usually a mild disease because most people have some degree of immunity.

Influenza pandemics, however, are caused by novel viruses – which are generally derived from animals, and usually originate in birds.  Here, the disease can be much more severe.

Influenza viruses have caused some of the biggest and yet some of the most insidious disease outbreaks to have hit humankind: from 1918 to 1920, the “Spanish Flu” pandemic killed more than 60 million people across the world; subsequent pandemics in 1957, 1968 and 1977 killed millions more, and the count is still unclear on the 2009 pandemic. However, in any given year more than 400 000 people probably die of so-called “seasonal flu” – yet universal vaccination against it is still a dream.

What is Influenza?

What is Influenza?

The Centers for Disease Control and Prevention in the USA define influenza as

“…a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and lungs. It can cause mild to severe illness, and at times can lead to death.”

The disease is transmitted mainly via droplets of respiratory secretions: these result from sneezing or coughing, which blows out a fine cloud of droplets or aerosol from the upper airways of infected people.  Breathing in or inhalation of these droplets – which can happen from 2 metres away – or transfer of droplets by hand from a contaminated surface to the mouth, is enough to cause infection. 

The virus initially infects cells of the upper airway, or the respiratory epithelium.  Spread to lower parts of the respiratory system, such as into the lung, depends upon the particular virus, and whether or not the individual is partially immune.

  • Fever or chills
  • Cough
  • Sore throat
  • Rhinitis, or runny nose
  • Muscle or body aches, headaches
  • Tiredness, “fuzzy head”
  • Vomiting and/or diarrhoea (more common in children than adults).

The average incubation period, or time from infection to disease, is about 48 hours.  Full recovery can take a month, although about two weeks is more common in seasonal flu.  People can pass on the virus before they show symptoms, and each infected person on average infects another 1.4 people.

While flu may be mild enough that it is hardly noticed, severe disease can also occur – especially in the elderly, the very young, heavy smokers, people who are chronically ill from other causes – and immunocompromised individuals.

While the virus can cause pneumonia directly due to damaging lung tissue, as happened in the “Spanish Flu” pandemic, severe illness with pneumonia is more usually due to secondary bacterial infections – which can be treated with antibiotics, unlike the viral pneumonia

Seasonal flu, or the disease caused by viruses circulating in the population, typically has an “attack rate” of between 5-15% of the population in annual epidemics.  Case fatality rates, or deaths among those infected, are usually between 0.1 – 0.3%. However,  pandemic flu – caused by new strains which arise spontaneously, and to which people are not immune – can attack from 25-50%, and kill 5% of those infected.  Seasonal flu also mainly infects children – because older people are often immune – but mainly causes severe disease and death in the elderly: up to 90% of victims are usually 65 or older

Conversely, pandemic strains may affect a different set of age groups: for example, the Spanish Flu affected mainly healthy young adults.

Seasonal influenza is typically a disease of the autumn and winter seasons in temperate zones – meaning October – March in the northern hemisphere, and April – August in the southern.  The CDC FluView graph shown here clearly illustrates the cyclical nature of seasonal flu, tracked in the USA over a 5 year period.  However, the exact timing is not reliable, and epidemics may peak as early as October in the north, or April in the south, or as late as the end of the season.

Tropical zones have a different epidemic profile:

here the virus may circulate year-round, typically with a peak during the one or two rainy seasons.  Because of demographic reasons incidence is severely under-reported: however, in a seasonal outbreak in Madagascar in 2002, there were more than 27 000 cases reported in 3 months, with over 800 deaths for a case-fatality rate of around 3%.  A WHO coordinated investigation of this outbreak found that there were severe health consequences in poorly nourished populations with limited access to adequate health care.

Why is influenza seasonal?

Many reasons have been invoked over the years to explain this, ranging from temperature, humidity, school schedules, increased indoor crowding during winter or rainy seasons, and even variations in host immunity due to lack of vitamin D or melatonin.  However, the same reasons cannot be given for both the increase in influenza incidence in temperate climates with the onset of winter, and the rainy season peaks in tropical regions, given the very different environmental conditions prevailing.

A recent study set out to systematically determine the interactions between relative humidity, and salt and mucus and protein content of droplets containing live flu virus, on the viability of the virus – and came up with conclusions that could explain the temperate / tropical transmission differences.

Essentially, their explanation for temperate region seasonality is that there is low relative humidity indoors in winter due to heating: this leads to increased survival of virus due to drying of particles – influenza A viruses are stabilised by being dried in the presence of salts, mucus and proteins – and leads to aerosols persisting longer in the interior environment due to smaller size, and being propagated further, meaning most transmission would be by this route.  Increased time spent indoors and increased indoor crowding due to the climate would obviously increase transmission rates under these conditions. 

Tropical environments present a very different picture: here, high temperatures would accelerate virion decay, which would tend to decrease any transmission.  However, in rainy seasons, temperatures drop and relative humidity increases to nearly 100% – conditions conducive to survival of large drops, which settle out quickly onto surfaces, where the virus remains viable.  Thus, transmission could be mainly by surface contact.  The same social factors apply as for temperate climates, with frequent rain leading to more time indoors and more crowding – and a greater opportunity for transmission.

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