Do viruses really cause pneumonia?
Well, the simple answer to this is yes, viruses do indeed cause pneumonia. There are approximately 20 cases of pneumonia per 1,000 population per year but of which only 6-7 per 1,000 population per year are viral. However, some pneumonias are in fact a mixture of bacterial and viral infection; 1-3 of these viral pneumonias have a bacterial co-pathogen e.g. Streptococcus pneumoniae, Haemophilus influenzae, Mycoplasma pneumoniae. It’s commonly believed that the virus causes damage to the lung allowing the bacterium to take advantage but could it be vice versa… the research is ongoing to look at this!
So the annual incidence of viral pneumonia is 6-7 per 1,000 population per year. Compare this to the fact that every adult gets 2-3 viral upper respiratory tract infections per year (3,000 per 1,000 population per year) and you realise that what most people are calling their new trendy “viral pneumonia” is actually a bad upper respiratory tract infection… but “I’ve got an upper respiratory tract infection” does #tag so well. Viral pneumonia does happen; it’s just not that common!
The most common causes of viral pneumonia are:
- Influenza Virus (7.1%) – the most severe form of viral pneumonia causing seasonal epidemics with a high mortality
- Rhinovirus (12.7%) – the most commonly isolated virus but usually a co-pathogen rather than the only infecting microorganism
- Parainfluenza Virus (3.4%) – causes severe infections in immunosuppressed patients, especially those with haematological malignancies or post bone marrow transplantation
- Coronavirus (2.8%) – usually a co-pathogen but severe infections can occur including Severe Acute respiratory Syndrome (SARS) and Middle-Eastern Respiratory Syndrome (MERS)
- Adenovirus (2.2%) – causes severe infections in immunosuppressed patients, especially those with haematological malignancies or post bone marrow transplantation
- Respiratory Syncytial Virus (RSV) (1.2%) – common seasonal epidemics which can be severe; especially bad in infants in whom it causes bronchiolitis
- Human Metapneumovirus (hMPV) (0.9%) - usually a co-pathogen rather than the only infecting microorganism
How do viruses cause pneumonia?
Until the advent of molecular detection of microorganisms it was thought that the lung was sterile. However, we now know that the lung has its own normal flora (microbiome) consisting of anaerobic bacteria such as Prevotella spp. and Veilonella spp. as well as microaerophilic Streptococcus spp. (microaerophilic bacteria prefer a slightly lower oxygen level than aerobic bacteria, which is all rather weird being that the lungs are full of O2!). This normal flora is kept in check, and at low numbers, by the normal host lung immunological defence mechanisms.
Pneumonia occurs when:
- A microorganism of particularly high virulence enters the lung (usually by microaspiration) e.g. Influenza Virus, RSV
- There is a large inoculum of microorganism; viruses are not part of the normal lung flora and so large inoculums of any of these microorganisms can cause pneumonia
- There is a defect in the normal host defence of the lung which allows other viruses or bacteria to cause infection e.g. smoking damaging alveolar white blood cells, Rhinovirus, hMPV or Coronavirus
How do you diagnose viral pneumonia?
Firstly you need to diagnose pneumonia. Pneumonia is diagnosed based on a combination of symptoms and signs as well as imaging.
The symptoms and signs of pneumonia are:
- Cough PLUS one other lower respiratory tract symptom (shortness of breath, purulent sputum, chest pain)
- New focal chest signs on examination (reduced chest movement, dull percussion, bronchial breathing, increased tactile vocal fremitus and vocal resonance)
- One systemic symptom (fever, sweats, shivers, aches and pains)
- No other explanation
In primary care, pneumonia is diagnosed based on these symptoms and signs above; in hospital the diagnosis needs to be confirmed by the presence of new infiltrates on a chest x-ray. However it is impossible to tell the difference between viral and bacterial pneumonia on a chest x-ray as the x-ray is just showing inflammation, not the microorganism.
Once the diagnosis of pneumonia is made you need to find a virus in order to say it is viral pneumonia.
There are two main ways of “finding” viruses:
- Enzyme-linked Immunoassays (EIA) – serum tests looking for the immune response (usually IgM antibodies) against the specific viruses
- Polymerase Chain reaction (PCR) – on sputum looking for the genetic material of the viruses themselves
PCR is the best way of detecting viruses as IgM is notorious for giving false positive results. PCR is very specific (it proves the presence of the virus) but can be a little insensitive (it can give a negative result even though the virus is there); specificity of >98% and sensitivity of >90%. The main drawbacks to PCR are that it is relatively expensive (>£100 per test for a multi-target PCR), it requires a good sputum sample from the lung not the upper respiratory tract and it will detect all viruses or co-pathogens even if they are only in the upper respiratory tract (15% of people can have viruses in their upper respiratory tracts at any one time).
How is viral pneumonia treated?
The only commonly used antivirals for pneumonia are targeted against Influenza Virus (Oseltamivir and Zanamivir). For all of the other viruses treatment is usually supportive with respiratory support, adequate hydration and antipyretics. Immunocompromised patients are occasionally treated with antivirals but these are usually last ditch efforts as toxicity can be more of a problem and the benefits are small e.g. RSV can be treated with Ribavirin (teratogenic) and Adenovirus can be treated with Cidofovir (bone marrow toxicity).
As there is no specific treatment for most viral pneumonias it might seem wasteful to do a £100 PCR to find a virus, however the value would lie in being able to stop antibacterials that might have side effects as well as potentially being able to discharge patients earlier and save hospital bed days.
Is bacterial pneumonia worse than viral pneumonia?
Pneumonia is pneumonia is pneumonia… (however “Old-monia” is apparently worse for those around the infected person!) There doesn’t appear to be much difference in outcome from either bacterial pneumonia or viral pneumonia; the mortality is the same and is related to the infections’ severity not cause.
So have the people I heard talking about their “viral pneumonia” really had pneumonia? Probably not, it’s not that common and it is more likely that they have had an upper respiratory tract infection. I’m not trying to belittle people’s illnesses but in my experience people do have a habit of exaggerating how unwell they were, maybe to justify time off work or away from the gym, it’s just human nature. We should remember getting over a bout of illness takes time; allow yourself the time without feeling guilty. I signed off for a week a few weeks ago as I had “Old-monia”, my wife wouldn’t take my temperature so I couldn’t have a fever and therefore I couldn’t really be ill! I think those that are really unwell tend to play down their illnesses, either to try and appear healthier than they actually are or perhaps it’s just that “stiff upper lip” of the British… either way that’s just as much human nature as exaggeration.
Perhaps the real answer is to get a better pair of headphones and try not to hear what others are saying in the gym… or better yet skip the gym and stay at home in the warmth… now there’s an idea, let’s look up what box set I “need” to watch… Text arrives… “are we going to the gym tonight!?” Rats!!!