Influenza Virus Fact Sheet Updated in SHIC Library

The Swine Health Information Center (SHIC) Fact Sheet on influenza viruses C and D has been updated.  New information in the fact sheet on epidemiology, including host range, geographic distribution, and prevalence, as well as pathogenicity, all increase its relevance and value. Fact sheets are part of SHIC’s mission to protect the health of the US swine herd, providing guidance and resources for producers, practitioners, and diagnosticians who are on the front lines of swine health concerns is an ongoing effort. Influenza C virus (ICV) and influenza D virus (IDV) are potential emerging pathogens of pigs, although swine are not the primary host for either virus species. Swine can be naturally and experimentally infected with both ICV and IDV, but clinical illness occurs rarely. Currently, influenza A virus (IAV) is the only species of routine clinical significance in swine.

Although ICV is predominantly a human pathogen, pigs, cattle, horses, camelids, and dogs can also be infected. IDV is primarily found in cattle, however, it has been detected in domestic and feral swine, as well as other species. Both ICV and IDV seem to be widely distributed. In swine, however, ICV has been described only in China, Japan, and Great Britain. IDV has been identified in swine in France, Ireland, Italy, Luxembourg, China, and the United States. Most studies have found that seroprevalence of ICV and IDV in swine is relatively low. One report from China found that up to 40% of tested pigs were seropositive for IDV.

ICV was first isolated from healthy pigs in China in 1981. IDV was initially detected in US swine with influenza-like illness in 2011. ICV and IDV have since been detected in other swine populations. Influenza viruses spread through direct or indirect contact with respiratory droplets or by inhalation of infectious aerosols. Influenza viruses infect epithelial cells lining the respiratory tract. In cattle, IDV has been detected in the upper respiratory tract vs. the middle and lower respiratory tract in swine.

Knowledge of ICV and IDV in swine lags far behind IAV-S. There is no long-term surveillance for these species, and standard laboratory tests for influenza do not detect emerging strains. Non-specific cross-reactivity occurs in human HI testing, complicating the interpretation of laboratory data. More information is needed on both ICV and IDV to develop diagnostic tests and vaccines. Many questions remain regarding pathogenicity, temporal distribution, and zoonotic potential of emerging influenza viruses.

SHIC, launched in 2015 with Pork Checkoff funding, continues to focus efforts on prevention, preparedness, and response to novel and emerging swine disease for the benefit of US swine health. As a conduit of information and research, SHIC encourages sharing of its publications and research. Forward, reprint, and quote SHIC material freely. SHIC is funded by America’s pork producers to fulfill its mission to protect and enhance the health of the US swine herd. For more information, visit http://www.swinehealth.org or contact Dr. Sundberg at psundberg@swinehealth.org.