Although nearly all of the influenza circulating in the the US and Canada right now is apparently the novel H1N1 `swine’ flu, it would be incorrect to say that if you have a flu-like illness, you must have the pandemic flu.
Last week’s numbers from the CDC’s FluView, which I’ve plugged into the above spreadsheet so as to generate a graph, show that more than 72% – almost 3/4ths – of all virus samples tested from people who had flu-like symptoms came back negative for influenza.
Although testing may miss some cases (samples degrade, viral shedding at the time of sampling may have been low, etc), it is pretty obvious that a lot of flu-like illnesses are caused by something other than influenza.
While we tend to talk most about influenza, the truth is, there are a plethora of pulmonary pathogens out there, and telling them apart isn’t easy to do. The signs and symptoms of all of these viruses can be pretty similar.
In fact, laboratory PCR testing is just about the only reliable way to sort them out. Rapid tests – commonly found in doctor’s offices - are very unreliable, with sensitivities running only about 50%.
Which is why doctors generally refer to ILIs, or Influenza-like Illnesses (or sometimes ARI Acute Respiratory Infection), when making a clinical diagnosis.
But it is worth noting that most ILI’s are not caused by the influenza virus.
This is something that Tom Nolan mentioned earlier this week in his blog on the BMJ. He presented a chart, similar to the one I created above, except that it breaks down some of the `other causes’ of ILI’s.
You’ll notice, however, that the bulk remain `UNKNOWN CAUSES’.
While it is estimated that, in a normal year, between 7% and 10% of Americans catch the `flu’, far more of us will endure some other kind of respiratory infection.
If it isn’t flu, you no doubt are wondering what else might it be?
Some of the `usual suspects’ would include metapneumovirus, parainfluenzavirus, respiratory syncytial virus (RSV), one of the myriad Rhinoviruses (Common cold), and adenovirus.
Testing isn’t generally done because of the costs involved, and because knowing the etiology doesn’t really affect treatment. Bed rest, fever reducers, and plenty of fluids is the usual regimen.
A recent study published in the Journal of Medical Microbiology gives us some idea of just how varied these viruses can be:
J Med Microbiol 58 (2009), 408-413; DOI: 10.1099/jmm.0.006098-0
Influenza viruses cause significant morbidity and mortality in adults each winter. At the same time, other respiratory viruses circulate and cause respiratory illness with influenza-like symptoms. Human respiratory syncytial virus (HRSV), human parainfluenza viruses (HPIV) and human metapneumovirus have all been associated with morbidity and mortality in adults, including nosocomial infections.
This study evaluated 154 respiratory specimens collected from adults with influenza-like/acute respiratory illness (ILI) seen at the Edward Hines Jr VA Hospital, Hines, IL, USA, during two successive winters, 1998–1999 and 1999–2000.
The samples were tested for ten viruses in two nested multiplex RT-PCRs. One to three respiratory viruses were detected in 68 % of the samples. As expected, influenza A virus (FLU-A) infections were most common (50 % of the samples), followed by HRSV-A (16 %).
Surprisingly, HPIV-4 infections (5.8 %) were the third most prevalent. Mixed infections were also relatively common (11 %). When present, HPIV infections were approximately three times more likely to be included in a mixed infection than FLU-A or HRSV.
Mixed infections and HPIV-4 are likely to be missed using rapid diagnostic tests. This study confirms that ILI in adults and the elderly can be caused by HRSV and HPIVs, including HPIV-4, which co-circulate with FLU-A.
Interestingly, more than 10% of those tested had two or more concurrent viral infections.
During the height of `flu season’, only between 25%-30% of the samples tested by the CDC usually test positive for influenza. During the summer, that percentage will drop to the low single digits.
The reason for bringing all of this up – besides the fact that it is interesting – is really two-fold.
First, since many of these milder ILI’s are commonly perceived by the public as being the `flu’, many people have a false perception of what having real influenza can be like.
And second, it is entirely possible that some of the people who suspect that they’ve already had the pandemic virus over the summer or last spring – and would therefore be immune – really had one of the other commonly circulating respiratory viruses.
Which means that they may not have acquired the protective antibodies they think they have.
Two things to consider, now that the H1N1 vaccine is becoming more widely available.