Influenza A(H1N1)pdm09 Virus in Pigs, Réunion Island - - Emerging Infectious Disease journal - CDC
Table of Contents
Volume 18, Number 9–October 2012
Influenza A(H1N1)pdm09 Virus in Pigs, Réunion Island
Influenza A(H1N1)pdm09 virus, which caused the last influenza pandemic among humans, is a unique reassortant derived from swine influenza viruses of the triple reassortant swine North American lineage and the avian-like swine Eurasian lineage (1). Réunion Island, a tropical French overseas department in the southwestern Indian Ocean, was struck by the influenza pandemic during July–August 2009. The epidemic had a high attack rate in humans (estimated clinically at 12.5% and serologically at 40.0%) (2,3). A(H1N1)pdm09 virus was reported to cause a reverse zoonosis in pigs (4); thus, a long-term (2009–2011) serologic and virologic survey was designed to check for transmission of the virus to pigs on Réunion Island, where the pork industry is a key economic activity and no live pigs have been imported since 1978. At 6-month intervals, a local veterinary surveillance system conducts serologic surveillance for pathogenic swine influenza viruses (H1N1, H1N2, and H3N2) among local herds, and during the last 20 years, none have been detected.
AbstractDuring 2009, pandemic influenza A(H1N1)pdm09 virus affected humans on Réunion Island. Since then, the virus has sustained circulation among local swine herds, raising concerns about the potential for genetic evolution of the virus and possible retransmission back to humans of variants with increased virulence. Continuous surveillance of A(H1N1)pdm09 infection in pigs is recommended.
During a first step (November 2009–February 2010), seroprevalence rates for A(H1N1)pdm09 virus were assessed in 120 breeding pigs (>4 years old) from 57 farms. Blood was obtained from randomly selected pigs at the only slaughterhouse on the island, where pigs are held for <3 hours. We screened the samples for antibodies to influenza A viruses by using the ID Screen Antibody Influenza A kit (ID.vet, Montpellier, France), and titers were determined by using hemagglutination-inhibition (HI) assays (5) against all classical swine influenza viruses and A(H1N1)pdm09 virus (Table 1). Ninety-eight (81.7%; 95% CI 74.7%–88.5%) of the 120 serum samples were positive for A(H1N1)pdm09 virus (HI titers >20); the range of positive titers was 40–640, and 54.2% of the samples expressed high HI titers (160–640). Of the 98 serum samples, 5 reacted at low titer and with only 1 European A (H1N1) swine virus (titer <20), i.e., >4 dilutions lower than for A(H1N1)pdm09 virus, indicating cross reactivity (6). Thus, pigs from 47 (82.4%) of 57 tested farms had been infected by A(H1N1)pdm09 virus; the seroprevalence rate was 81%–100% for pigs on 79.0% of the farms. Farms with affected pigs were located throughout the island (Figure).