That we happened to have this datum in hand is, of course, a side effect of rapid, broad-spectrum viral antigen testing, a fairly recent development. Actually, these panels also detect certain bacterial pathogens, such as B. pertussis and C. pneumoniae. This information would have been very challenging to obtain only a few years ago.
When the patient - who was eighteen months old - was described as having hMPV, I offered an opinion about why that could not possibly be the reason for admission, though I don't doubt for a moment that the toddler tested positive for this pathogen. The problem is that the statement isn't counter-factual. Rather, it is operating from the wrong frame of reference. This makes it deeply and complexly wrong, and it illustrates a fairly pervasive misunderstanding about how infectious diseases operate, how clinical diagnoses are made, what such entities mean and how an appropriate plan of care comes about.
There are a few organisms that are so reliably linked to a single clinical entity that the disease state and the pathogen that cause it are one and the same thing. I suppose I would put primary varicella or measles in this category, or maybe Shigella or pertussis. Even with these examples, caution is in order. For a much larger group of agents, however, infection is not "destiny." With adenoviruses, coronavirus, rhinovirus, respiratory syncytial virus (RSV), hMPV and others the clinical presentation is incredibly variable. Many patients experience only the common cold. Others develop pharyngitis, largyngotracheitis, acute otitis media, bronchopneumonia or wheezing. Quick - what's the most appropriate disposition for a patient coming to the ED with Streptococcus pyogenes infection? S. pyogenes (or Group A B-hemolytic streptococci) in children can cause pharyngitis, acute otitis media (rarely), cellulitis, erysipelas, necrotizing fasciitis, pneumonia, sepsis and more. It is also an asymptomatic colonizer of the upper respiratory tract. So the right answers could include home with no treatment, admit to ICU, and anything in between. It depends on the diagnosis much more than the bug.
This obsession with getting the name, rank and serial number of each respiratory tract pathogen has engulfed adult medicine as well. It's not that these viruses are necessarily more virulent than "plain vanilla" cold viruses. In fact, rhinoviruses - which are of the vanilla variety - show up a fair amount, including cases of COPD exacerbation in the elderly. No, these ARE the names of the vanilla viruses that cause sniffles every winter. It's just that they're capable of more. We've known this for a number of years. But now we're putting a name to the face.
However, these pathogens are related only tangentially to the real problem before us - what clinical diagnosis and what disease state are we dealing with? How is it managed? When is in-patient care necessary?
What are these respiratory panels good for? Isolation, probably. Research and epidemiology. Influenza detection, because there are antiviral medications. That's probably about it. Identifying the cause of a fever? Be careful here. In the winter, a large number of people will be positive, and co-infection with another pathogen is not rare. Making a better diagnosis? Not so fast. Virological identification does not a diagnosis, nor a diagnostician make.
That 18 month old patient with hMPV - what would the right care be? Home with reassurance? The floor with bronchodilators? The PICU? Since her active problem was gastroenteritis, fluid replacement was the ticket. The same fluids we would have administered before the viral panel came to town.
