Influenza A & B - due to their high burden of disease and (influenza A's, in particular) wide host range - remain the primary focus of influenza research and thus far, only influenza A has demonstrated the ability to spark a pandemic.
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.
Not quite two years 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 ... Show more
Clinical Infectious Diseases, Volume 59, Issue 2, 15 July 2014, Pages 252–255, https://doi.org/10.1093/cid/ciu269
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.
But increasingly, attention is being paid to two other, lesser known, flu types; Influenza C (discovered in 1947) & influenza D (discovered in 2011).
Recently we've seen reports (see here and here) suggesting that Influenza C may not be quite as benign as previously advertised, and evidence suggesting that the more recently discovered Influenza D might have some zoonotic capabilities.
While it still isn't known whether Influenza D can cause symptomatic illness in humans, in 2016 - in Serological Evidence Of Influenza D Among Persons With & Without Cattle Exposure - researchers reported finding a high prevalence of antibodies against Influenza D among people with cattle exposure. They wrote:
IDV poses a zoonotic risk to cattle-exposed workers, based on detection of high seroprevalence (94–97%). Whereas it is still unknown whether IDV causes disease in humans, our studies indicate that the virus may be an emerging pathogen among cattle-workers.
In last February's blog J. Clinical Med. : A Review Of The Emerging Influenza D Virus, the authors wrote:
The mixed reports for IDV infections in humans further make it important to study the zoonotic potential of IDV, especially in people with occupational exposure to susceptible livestock. Since IDV shows potential to infect a wide range of host after IAVs, its zoonotic potential is a global concern.Last May, in Two New Influenza D Studies To Ponder, we saw evidence that Influenza D can infect Dromedary Camels, and a study that explored human cellular tropism, replication, and the development of antibodies to Influenza D.
All of which brings us to a new study, published this week in the journal Viruses, which describes the detection of a new genetic cluster of the Influenza D virus circulating in Italy.
by Chiara Chiapponi 1,*, Silvia Faccini 1, Alice Fusaro 2, Ana Moreno 1, Alice Prosperi 1, Marianna Merenda 1, Laura Baioni 1, Valentina Gabbi 1, Carlo Rosignoli 1, Giovanni L. Alborali 1, Lara Cavicchio 2, Isabella Monne 2, Camilla Torreggiani 1, Andrea Luppi 1 and Emanuela Foni 1
Viruses 2019, 11(12), 1110; https://doi.org/10.3390/v11121110 (registering DOI)
Received: 21 October 2019 / Revised: 21 November 2019 / Accepted: 27 November 2019 / Published: 30 November 2019
Influenza D virus (IDV) has been increasingly reported all over the world. Cattle are considered the major viral reservoir. Based on the hemagglutinin-esterase (HEF) gene, three main genetic and antigenic clusters have been identified: D/OK distributed worldwide, D/660 detected only in the USA and D/Japan in Japan.
Up to 2017, all the Italian IDV isolates belonged to the D/OK genetic cluster. From January 2018 to May 2019, we performed virological surveillance for IDV from respiratory outbreaks in 725 bovine farms in Northern Italy by RT-PCR. Seventy-four farms were positive for IDV. A full or partial genome sequence was obtained from 29 samples.
Unexpectedly, a phylogenetic analysis of the HEF gene showed the presence of 12 strains belonging to the D/660 cluster, previously unreported in Europe. The earliest D/660 strain was collected in March 2018 from cattle imported from France. Moreover, we detected one viral strain with a reassortant genetic pattern (PB2, PB1, P42, HEF and NP segments in the D/660 cluster, whilst P3 and NS segments in the D/OK cluster).
These results confirm the circulation of IDV in the Italian cattle population and highlight the need to monitor the development of the spreading of this influenza virus in order to get more information about the epidemiology and the ecology of IDV viruses.(Continue . . . )
Although the zoonotic risk from Influenza D remains unquantified, this is obviously a virus that continues to evolve, and to spread globally.
For now, it doesn't appear to pose a significant human health threat, but it is important to learn what we can about it now - while we have the luxury of time - rather than waiting until it evolves into something more challenging.