23 September 2011
The United States of America Centers for Disease Control and Prevention (CDC) reported a cluster of swine influenza cases in a recent issue of the Morbidity and Mortality Weekly Reports. Two cases of febrile respiratory illness caused by a novel swine-origin influenza A(H3N2) virus were identified in the United States in two children <5 years (a boy and a girl) on August 19 and August 26, 2011 respectively. To date, no epidemiologic link between the two cases has been identified and no additional confirmed human infections have been detected, however, investigations are continuing. The authors of the publication noted that influenza A viruses are endemic in many animal species and that sporadic cases of transmission of influenza A viruses between humans and animals occur. In recent years there have been increased reports of swine-origin influenza viruses. This is potentially due to increased influenza testing capabilities resulting in increased recognition of human and swine-origin influenza viruses, as well as genetic changes in swine influenza viruses and other factors. This particular genetic combination of swine influenza virus segments is unique and has not been previously reported in either swine or humans; as a result, no information is available regarding the capacity of the viruses to transmit in swine, humans, or between swine and humans at this stage. The virus in the identified cases is resistant to amantadine and rimantadine, but susceptible to the neuraminidase inhibitor drugs oseltamivir and zanamivir. The lack of known direct exposure to pigs in one case indicates the possibility that limited human-to-human transmission of the influenza virus occurred.
Since the publication date, two additional cases have been identified in the state of Pennsylvania, and all four infected patients (all children < 9 years) have recovered; three out of the four cases had direct contact with swine, in more than one location, indicating the virus is likely circulating within the swine population. It is difficult to adequately assess the virulence or severity of this virus based on such a small sample. It is notable, however, that all four cases occurred in young children and that cross reactivity between this novel swine origin influenza A(H3N2) virus and the 2011-12 northern hemisphere seasonal vaccine is seen in adults and older children in serological studies performed by the CDC. The extent of circulation of this virus in the swine population is unknown, highlighting the need to improve monitoring of influenza viruses in domestic animal populations both in the US and globally.
Nalluswami, K., et al. Swine-Origin Influenza A (H3N2) Virus Infection in Two Children — Indiana and Pennsylvania, July–August 2011. Morbidity and Mortality Weekly Report. September 9, 2011; 60(35).