Photo Credit CDC
Although influenza is a hot topic right now (avian, human seasonal, even `mystery’), not every influenza-like-illness (ILI) out there is caused by an influenza virus. In fact, of the more than 6,200 specimens tested by U.S., WHO and NREVSS collaborating labs last week, only about 13% were positive for influenza.
And this among samples highly suspected of being influenza.
The truth is, it is very difficult to differentiate between influenza infections, RSV (respiratory syncytial virus), respiratory adenoviruses, parainfluenza viruses, rhinoviruses, coronaviruses, metapneumovirus, (and others) without using sophisticated lab tests.
While commonly viewed as `lesser’ respiratory viruses, the rogue’s gallery of non-influenza viruses above are all perfectly capable of causing severe illness, and even death. Particularly in the very young, very old, or those with comorbid medical conditions.
In a clinical setting - most influenza-like-illnesses go unidentified. Viral respiratory infections are generally self-limiting illnesses, treatment is pretty much the same regardless of etiology, and so there is little point in trying to identify the cause of every illness.
When doctors do test for influenza, they generally use an RIDT (Rapid Influenza Detection Tests), but as we’ve seen reported often over the years (see MMWR: Evaluating RIDTs) these tests often suffer from a lack of sensitivity (and to a lesser extent) specificity. Less often, samples will be sent for more detailed PCR testing.
Researchers – with better tools available today – are indentifying `new’ viruses all of the time. A few well distributed viruses that until recently, were unknown, include:
- The human metapneumovirus (HMPV) was identified in Dutch children with bronchiolitis about a dozen years ago. Since then, it has been found to be ubiquitous around the world, and responsible for a significant percentage of childhood respiratory infections . . . yet until 2001, no one knew it existed.
- Human Bocavirus-infection (HBoV) wasn’t identified until 2005, when it was detected in 48 (9.1%) of 527 children with gastroenteritis in Spain (cite). It has since been found around the globe using PCR testing.
And the list grows longer every year.
Adding to our misery, it is not all that uncommon to be infected by more than one virus (or a viral/bacterial combination) at the same time. In 2008 a study (see Frequent detection of viral coinfection in children hospitalized with acute respiratory tract infection using a real-time polymerase chain reaction) looked at clinical samples taken from 254 children treated in Germany over a 10 month period, finding:
Respiratory syncytial virus (RSV) was the most frequently detected pathogen in 112 samples (44.1%), followed by human bocavirus (hBoV) in 49 (19.3%), and rhinovirus in 17 samples (6.7%).
Viral coinfection was detected in 41 (16.1%) samples with RSV and hBoV being the most dominating combination (27 cases, 10.6%). Viral coinfection was found in 10 cases (17%) of children with bronchitis (n = 58) and in 7 cases (23%) of bronchiolitis (n = 30). In patients with pneumonia (n = 51), 17 cases (33%) were positive for 2 or more viral pathogens.
This plethora of pathogens helps to explain – in part - why so many people who get the flu shot every year complain they still caught `the flu’.
Often, they’ve caught one of these ubiquitous `flu-like illnesses’ that are not covered by the shot.
And it also helps to explain why, in Montgomery, Texas, public health officials have not yet been able to determine the cause of a handful of unusually severe respiratory infections (see Montgomery County, Tx Influenza-Like Illness Investigation Update – Dec 18th. There are literally dozens of possibilities to be explored and ruled out.
Dr. Ian Mackay weighs in this morning on the discovery process in:
In places like Hong Kong, China, Indonesia, Saudi Arabia, and Egypt, the elimination process becomes even more difficult. There they not only have the above complement of seasonal flu and non-flu viruses, they have multiple strains of novel influenza (H5N1, H7N9, H9N2) to contend with, and in the Middle East, an emerging coronavirus as well (MERS-CoV).
All of which can present with more or less the same symptoms . .. at least at the start of the infection.
Testing is both imperfect and time consuming, and getting accurate results depends on more than just the laboratory diagnostics employed. Patient samples have to be taken at the right time (when the patient is shedding virus), from the right place (with respiratory viruses, deeper into the respiratory system is usually better), and packaged and transported properly to give the best chance of detection.
All of which means that we are often left to deal with the `fog of flu’. Incomplete, or sometimes erroneous reports, from around the world.
After nearly 9 years of doing this blog, I can only recommend patience and a healthy dose of skepticism when we see news reports of something `unusual’ or perhaps, even `alarming’ somewhere in the world. Early media reports are often misleading, and are designed more to drive web traffic or sell newspapers than they are to inform the public.
None of which is to suggest that we shouldn’t watch these reports with interest. We should, since one of these days, one of them could certainly herald the arrival of a serious threat.
But, as I wrote last year in Novel Viruses & Chekhov’s Gun, most of these threats will turn out to be less dangerous than they first appear.
As for that rare virus that genuinely poses a serious public health threat?
Well, that’s why we urge people to follow good flu hygiene year-round, stay informed, and have a good pandemic (and all threat’s disaster) preparedness plan in place (see Pandemic Preparedness: Taking Our Cue From The Experts.)
Because - regardless of what may come down the pike tomorrow - the advantage always goes to those who prepared today.