In the hierarchy of influenza viruses, Influenza A - due to its ability to produce severe illness and to rapidly mutate and reassort with other influenza A viruses - is viewed as the biggest human health threat. Influenza B, while less apt to make big evolutionary leaps, comes in second due to its ability to cause serious illness.
Influenza C - which is rarely tested for - generally gets short shrift. And recently discovered Influenza D, even more so.From the CDC's website (bolding mine):
Types of Influenza VirusesBut our knowledge of influenza is constantly changing. Until a few years ago, Influenza B was viewed as a far `less serious' infection than influenza A - affecting mainly children - and producing relatively mild illness in adults.
There are four types of influenza viruses: A, B, C and D. Human influenza A and B viruses cause seasonal epidemics of disease almost every winter in the United States. The emergence of a new and very different influenza A virus to infect people can cause an influenza pandemic. Influenza type C infections generally cause a mild respiratory illness and are not thought to cause epidemics. Influenza D viruses primarily affect cattle and are not known to infect or cause illness in people.
Just over a year ago, in Influenza B: A Virus Not To Be Underestimated,
we looked at a number of studies over the past decade that have revised that perception, including:
Comparing Clinical Characteristics Between Hospitalized Adults With Laboratory-Confirmed Influenza A and B Virus Infection
Su Su Sandra S. Chaves Alejandro Perez Tiffany D'Mello Pam D. Kirley Kimberly Yousey-Hindes Monica M. Farley Meghan Harris Ruta Sharangpani Ruth Lynfield ...
We challenge the notion that influenza B is milder than influenza A by finding similar clinical characteristics between hospitalized adult influenza-cases. Among patients treated with oseltamivir, length of stay and mortality did not differ by type of virus infection.
While the role of recently identified Influenza D in human infection remains unclear (see Serological Evidence Of Influenza D Among Persons With & Without Cattle Exposure), we have seen growing evidence to suggest that influenza C may not be quite as benign as previously advertised.
A study published in Clinical Infectious Diseases (2017) found:
Detection of Influenza C Viruses Among Outpatients and Patients Hospitalized for Severe Acute Respiratory Infection, Minnesota, 2013-2016.
Thielen BK1,2, Friedlander H3, Bistodeau S3, Shu B4, Lynch B4, Martin K3, Bye E3, Como-Sabetti K3, Boxrud D3, Strain AK3, Chaves SS4, Steffens A4, Fowlkes AL4, Lindstrom S4, Lynfield R3.
Conclusion: We found seasonal circulation of influenza C with year-to-year variability. Detection was most frequent among young children but occurred in all ages. Some cases that were positive for influenza C, particularly those with comorbid conditions, had severe disease, suggesting a need for further study of the role of influenza C virus in the pathogenesis of respiratory disease.To this short, bu growing list, we can add a recent Research Letter published in the CDC's EID Journal, which details 3 serious Influenza C infections detected in children in Cameroon. I've only posted some excerpts, so follow the link to read it in its entirety.
Volume 25, Number 3—March 2019
Richard Njouom , Gwladys Chavely Monamele, Burcu Ermetal, Serge Tchatchouang, Sylvie Moyo-Tetang, John W. McCauley, and Rodney S. Daniels
We report 3 cases of influenza C virus in children hospitalized with severe acute respiratory infection in Cameroon. Two of these case-patients had grave clinical manifestations, but all 3 recovered. The lack of specific antiviral drugs for influenza C virus highlights the need to identify and describe cases involving this virus.
Four types of influenza viruses are known: A, B, C, and D (1). Unlike influenza types A and B viruses, influenza C viruses generally cause a mild respiratory illness (2). However, some cases of lower respiratory infections have been described in children (1–3). In recent years, severe illness due to influenza C virus has been reported from different geographic regions, but little data have come from Africa (3–6).
Previous detection of influenza C virus in Cameroon reported 2 cases among 561 patients with influenza-like illness (7). We identified 3 cases of influenza C virus infection among hospitalized patients with severe acute respiratory infection (SARI) in Cameroon.
Studies have shown variability in the circulation of influenza C virus, with peaks in winter and spring seasons (3,4). These 3 cases all occurred in December 2017, possibly indicating seasonality of influenza C infection in Cameroon, but identification of more cases is required to confirm this hypothesis.
The recent identification of severe illnesses related to influenza C virus infection in Cameroon and the lack of influenza C–specific antiviral drugs highlight the importance of integrating molecular testing for this virus into existing inpatient and outpatient sentinel surveillance systems and for in-depth studies of the epidemiology of influenza C viruses. This process could lead to predominant circulating strains of influenza C virus being included in seasonal influenza vaccines to protect vulnerable populations.
Prof. Njouom is head of the Virology Department and the National Influenza Centre at the Centre Pasteur du Cameroon. His areas of interest include molecular epidemiology of hepatitis and respiratory viruses and its impact on diagnosis, treatment, and pathogenesis.
While Influenza C (and Influenza D) will likely remain minor players in the flu world, if we've learned anything over the past 30 years, it is that what is believed to be a minor threat today can easily become a bigger threat tomorrow.
Making anything we learn about them now well worth the effort.