The influenza C virus is spread worldwide. The virus has not yet reached pandemic potential, but it continues to circulate together with pandemic Influenza A Virus and Influenza B Virus during the winter months.
In humans Influenza C Virus causes diseases of the upper and (less frequently) lower respiratory tract in humans. It is assumed that an infection with that virus is generally mild.
It was once thought that Influenza C Virus was a strictly human pathogen, but in recent years new research emerged that proved that the virus is also endemic in pigs and dogs. It is also known to override the barriers of transmission between those three hosts. Which means it can easily jump from one species to another.
 Influenza C Virus in a number of mongrel dogs, suggesting that the virus can cause natural infection in dogs. In Germany more than 50% of all dogs tested showed antibodies against Influenza C Virus.
The immune system of mammals often fails to recognize new antigenic variants of influenza C virus, which invariably arise in nature, resulting in outbreaks of diseases, although the structure of antigens in Influenza C Virus in general is much more stable than those of Influenza A Virus and Influenza B Virus.
There is no proof of recombination processes in the influenza C virus genome. The genome of influenza C virus comprises seven segments. Based on the comparison of sequences in one of its genes, six genetic or antigenic lineages of this virus can be distinguished (Yamagata/26/81, Aichi/1/81, Mississippi/80, Taylor/1233/47, Sao Paulo/378/82, and Kanagawa/1/76). However, the available genetic data show that all the seven segments of the influenza C virus genome evolve independently.
All is well then? Influenza C Virus only causes mild illness in humans and it doesn’t mutate that easily? Don’t count on it, because in 2013 new appeared that researchers had discovered in the American state of Oklahoma a completely new strain of Influenza C in pigs and later in cattle. It was designated C/Oklahoma/1334/2011 (C/OK).
 Speranskaia et al: Genetic diversity and evolution of the influenza C virus in Genetika - 2012
 Ohwada et al: Distribution of the antibody to influenza C virus in dogs and pigs in Yamagata Prefecture, Japan in Microbiology and Immunology - 1987
 Youzbashi et al: Verbreitung der Influenza-C-Virus-Infektion bei Hunden und Schweinen in Bayern in Tierärtzlicher Praxis - 1996