Influenza C


There's an influenza you almost never hear about. We don't vaccinate for it. But you can still get it. 

It does however infect us. It just usually doesn't cause the problems that influenza A and B. But the virus, Influenza C, is more interesting - and causes more disease - than many doctors think.

The good news is that it has never been seen to cause a pandemic, unlike Influenza A. Moreover, Influenza C is expected to never cause a pandemic.

It also usually causes at most mild illness in adults

The symptoms usually are mild and are:

  • feels like a common cold
  • fever for 2 days
  • cough
  • runny nose

It is usually associated with mild upper respiratory infections, not lower respiratory tract disease, like pneumonia. Some cases may have no symptoms.

The bad news is we may have underestimated Influenza C's effects in some.

  • Small kids can get pretty sick from Influenza C

Influenza C may cause more cases of severe illness in young children than was appreciated. Influenza C causes in small children more lower respiratory disease, like pneumonia, than is often appreciated. In small children (under age 2), Influenza C and A can cause similar symptoms.

  • Influenza C can cause outbreaks, larger than appreciated

Influenza C can cause outbreaks, especially in children, and is found around the world. It is generally considered not a disease that causes epidemics.

However, it can cause more cases than we recognize, even, as Japanese researchers said, an epidemic, which was seen in Japan in 2004. Usually, Influenza spreads from school age children to families, but Influenza C is generally found in children under age 6, so the disease may spread more through preschool and day care exposures.

  • Influenza C may evolve more than we thought

Influenza C is characterized as having little genetic variability and evolving at a slower rate than other influenza types. It doesn't have as many variants as Influenza A has so infections are usually acquired only early in life, unlike Influenza A that can spread among all ages of populations, causing disease and further transmission. However, this does not mean that Influenza C can't change. There is relatively frequent reassortment in influenza C. It can create recombinants from reassortment. This doesn't mean it will mutate and create a large epidemic, or even a pandemic, like Influenza A. However, it may still cause more disease and hospitalizations than we give the virus credit for.

Why hasn't Influenza C been recognized?

It's hard to know when a kid has Influenza C. Clinical diagnosis is difficult. It looks much like other respiratory viruses and standard tests often don't look for it.

Moreover, the disease has not been studied as much. In part, this is because Influenza A and B have been more dangerous.

But there's also another reason:

Influenza C was first identified in 1947. However, there wasn't a cell line established to grow Influenza C, like there was for Influenza A and B. So Influenza C wasn't well identified or studied. Most cases were noted retrospectively in studies of antibodies in healthy people and so the disease was seen as possibly milder than it was. What the studies of antibodies showed was that influenza C is found around the world and that infection first occurs early in the first few years of life. These studies though, historically, were retrospective. Studies of children with Influenza C saw the illness it created as not much different - in children - from Influenza A.

Is Influenza C in other animals?

Yes, but unlike Influenza A, humans seem to be primary "reservoir". Influenza C has been isolated from pigs and humans, though. There can be spread between human and animals. However, there is less spread of Influenza C in animals - and not in the farm and migratory birds that spread Influenza A around the world.

My dog can get Influenza C, right?

Yes, like a lot of infections, especially influenza, we can share these infections (some of them at least) with animals. Dogs can be infected with type C. However, the dog influenza infections we see are usually type A. The influenza types well known to infect dogs, especially in the last few years, have been Type A influenza H3N8 virus and H3N2 virus. These viruses are thought to be a low risk to people. It has not been shown to spread to humans.

Is there a vaccine?

The seasonal flu vaccine does not protect against influenza C. There is not a vaccine approved for Influenza C in the US. 

Is Influenza C genetically different from other Influenza types?

Yes, Influenza C has diverged from A and B. Influenza C does not have the HA and NA glycoproteins on its surface (which are what the H1N1 or H5N1 refer to in Influenza A names). Instead, influenza C has fewer RNA segments (7) rather than the 8 the other influenza types have. Instead of HA and NA, influenza C has just one glycorpotein HEF (hemagglutinin-esterase-fusion) which combines the work HA and NA.

Is there treatment?

Because C is milder than Influenza A and B, there are not usually recommended specific treatments. Influenza treatment recommendations are designed for types A and B. However, the standard flu drugs do not work on C.

Drugs like Oseltamivir (Tamiflu), Zanamivir (Relenza), Laninamivir (Inavir), and Peramivir, which are neuraminidase inhibitors, cannot treat Influenza C, which has a different type of enzyme. The other treatments for influenza, Amantadine and Rimantadine, which are Adamantanes, used to be recommended for Influenza A, but now are not because of resistance and lack of noted efficacy, but they were not effective for non-A influenza.

When does it spread?

Influenza C usually spreads in winter and spring to early summer

Is that it? Just Influenza A, B, and C?

Actually, there is an Influenza D. Fortunately, it has only been found in cattle and pigs. It was first identified in 2011 in Oklahoma. It hasn't yet become a problem for us. Influenza though mutates - bit by bit and jump by jump. We can always be surprised by a new variant of influenza, but for now Influenza D isn't something worrisome.

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