Seroprevalence of swine influenza virus on swine farms in the Philippines

Swine influenza virus (SIV) infection in pigs is dependent upon the infecting virus strain, age and immune status of the animals and concomitant pathogens. In the Philippines, SIV infection can either be quick onset and rapid recovery or can be less obvious and demonstrate traditional clinical signs of infection. Not all farms are vaccinating against SIV because the specific strains are not identified and prevalence of the different SIV subtypes circulating in swine herds is unknown. Therefore, the purpose of this study was to determine the seroprevalence of swine influenza viruses in farm herds that do not vaccinate.

A cross-sectional collection of sera was conducted between mid-2014 and mid-2015 in 13 farms (no SIV vaccination) located in major swine-producing provinces with respiratory diseases. Samples from each farm range from 20 to 40 sera (average of 36) taken from breeder sows, weanling, growers and finishers. A total of 475 pig sera were tested for influenza A antibody using Influenza A Virus Antibody Test kit (IDEXX Influenza A Test Kit) to measure exposure of a herd to Influenza A. The sera were also tested using hemaglutination-inhibition (HI) technique with European Isolates which included: SWI type H1N1-Best; SWI type H1N2-Gent and SWI type H3N2-Flanders 1/98. An inhibition of haemagglutination at dilution > = 4 (log2) are regarded to be positive.

Numbers and percentages of seropositive animals according to age group, aggregated across the 13 farms, were evaluated. Seroprevalence of influenza A viruses is 65%. Seropositivity in each subpopulation is variable with the highest % in growers and sows (80%). The seroprevalence of SIV subtypes as determined with the HI test was as follows: H1N1 132 sera (28%), H1N2 9 sera (2%) and H3N2 52 sera (11%). Seropositivity in each subpopulation was variable among H1 and H3 subtypes. The sow herd had the highest seroprevalence (59%) for H1N1 subtype-positive sera. The level of antibody seropositivity between the Influenza A antibody test and HI test was numerically different across all sera and among age groups. This difference could be attributed to the European strains of SIV used in the HI test.

This study demonstrated that natural infection rate of SIV is highly prevalent in Philippine swine herds and that it occurs in all stages of production. The three active influenza subtypes (H1N1, H1N2 and H3N2) are present. Further studies on strain prevalence should be conducted to provide clearer understanding on the prevalence of European SIV subtypes and American SIV subtypes in Philippine pigs

Disclosure of Interest: None Declared.