What Distinguishes Influenza From Other Respiratory Viruses?

Pediatricians see waves of patients with influenza coming through their offices each year -- but many of the illnesses are so mild that they are indistinguishable from other febrile respiratory viruses. For every child who has a raging fever, marked myalgias, dry cough, and headache, there are a handful of children who have been exposed to the same virus and who have a simple runny nose or cold/cough. Such was frequently the case during the nh1n1 epidemic of 2009. Many family members or classroom contacts of a documented case did not "get sick" or simply had a mild cough/cold. 
 
What distinguishes influenza from other respiratory viruses?
 
High fevers. The fever from influenza is often high and sustained. During the novel nh1n1 epidemic in 2009, many older kids were having fevers of 103°F to 104°F. For many of today’s parents who never experienced the prolonged fevers of measles, this was very scary.

More fatigue. Muscle fatigue can make infants and toddlers act “limp”.... toddlers sit quietly on their parent’s laps in the waiting room. Older kids will flop on the couch, and then feel dizzy when they try to get up and walk. Many people will feel “achy” muscles -- hence the term “grippe.”

Rapid onset of illness. A distinguishing feature of influenza is its rapid onset. Older children will remember the exact hour they started to feel sick. Often a child will go to school feeling fine, only to develop a high fever in the afternoon. Younger children often vomit before the onset of fever. 
 
The potential for viral pneumonia. The cough of influenza usually gets wetter and more productive as the days go on and the viral-induced cell debris begins to be coughed up. At the onset of the illness, the cough tends to be dry and irritating in older kids and croupy in younger kids. In a child with the flu, rhonchi sound more moist.

Influenza is potentially more pathogenic than so many of the other respiratory viruses because it can attach itself to the surfactant producing alveolar type II cells. This can lead to alveolar collapse and respiratory distress syndromes. During the nh1n1 epidemic, pregnant women were more prone to this form of pneumonia.

Capillary leakage is another type of pulmonary injury from influenza. One of my colleagues remembers a bad influenza season when he was a resident in the 1970s in which an otherwise healthy 3-year-old boy with influenza ended up being admitted to the PICU after coughing up blood with his cough in the ED.  
 
The potential for secondary bacterial infections. Influenza increases bacterial adhesion, particularly pneumococcus, to the respiratory epithelium. Before l995 and the widespread use of the Prevnar vaccine, there were many more hospitalizations for pneumococcal bacteremia and pneumococcal pneumonia than there are now. The other big pediatric pathogen, Haemophilus influenza, induced more serious illnesses (ie, meningitis, preseptal cellulitis, epiglottis) during the winter months when influenza causes microscopic injury to children’s respiratory linings.
 
Bacterial secondary infections usually develop several days after what seems to be classic viral symptoms of a clear runny nose. Infants and toddlers can be up all night with ear pain. A a raging strep throat can develop in a child who had been exposed to strep at school. Typically, the child has been starting to look a little better when the fever returns. If a secondary bacterial pneumonia is starting to develop, a very high fever and tachypnea will develop.    

OTHER COMPLICATONS OF THE FLU

Myocarditis. Fortunately, this complication is rare, but most pediatricians know of a patient who has had subclinical myocarditis. Besides homing onto to the respiratory lining, influenza can also infect muscles cells. When teenagers or athletes have what looks like influenza with myalgias, we recommend that they not push themselves with drills in sports teams. We advise them to take it easy until they are back to full strength and completely symptom free. For a teenager who has been running fevers for days, the “recovery phase” (ie, when they're not back to normal) might be 2 weeks or so.
 
Febrile seizures. Sometimes older toddlers and preschoolers experience a febrile seizure with the initial wallop of the immune response. Also, during the nh1n1 epidemic there were a few reports of infants with symptoms of encephalitis.
 
Autoimmunity. Patients with juvenile onset diabetes often have a flu-like or enteroviral syndrome before the onset of diabetes.    

FLU IMMUNITY

Many people have an extensive library of memory antibodies to flu. This is why many teachers, pediatricians, dentists, shop owners don’t get sick often after the initial few years of training.   Everyone else in their family might get sick, but they have only mild cold symptoms, or one day of achiness/fatigue. People who grow up in large families usually experience lots of “sibling sprinkles” throughout early childhood. By the time they go to college, they're more “immune” than a classmate who grew up in the country, didn't go to day care, or was home schooled. A child who has attended day care for 2 or 3 years has already developed lots of immunity to influenza.

Also, children who have received the influenza vaccine during the first couple of years of life and during the preschool years have much more resistance to influenza. Young children have a developing immune system and more “photographic” memory for flu vaccines. A vaccine given in 2010 can cause memory antibodies to activate years later.
 
Many people think that flu vaccines are not important. Some think it’s better to get the “natural illness.” The antigens to 3 different strains in each year’s flu vaccine might not completely “match” and the person might still get sick. However, a vaccine given to a child isn't wasted, because 1 of the 3 strains present in the vaccine can later appear.
 
Other people expect a flu vaccine to do more than it is capable of. They think the flu vaccine will prevent them from getting sick during the winter. This, of course, is not so. They can still get strep throat,  the Norwalk 'vomiting' illnesses, respiratory adenovirus, etc. Younger children are still at risk for respiratory syncytial virus, and human metapneumovirus, and parainfluenza viruses. They can still be back and forth to the doctor's office with one cold after another and an ear infection or croup, even though they had a flu shot.