Frequency analysis of H1N1 and H3N2 antibodies in samples from pig farms in Mexico (2012-2015)

The objective was to investigate the frequency of antibodies against H1N1 and H3N2 subtypes of Influenza A virus (IAV) in pig farms in Mexico from May 2012 to June 2015, using the hemagglutination inhibition test (HI).

A total of 3,444 serum samples were collected from 96 pig farms located in 13 states: Chiapas (61), Coahuila (35), Guanajuato (198), Hidalgo (169), Jalisco (933), State of Mexico (35), Michoacan (74), Morelos (35), Puebla (450), Queretaro (168), Sonora (975), Veracruz (256) and Yucatan (55). Three IH tests were developed from the following Influenza A virus isolates: H1N1 classic A/swine/NewJersey/11/76 (H1N1c), H3N2 reference A/swine/Minnesota/9088-2/98 (H3N2r) and the Mexican isolate H3N2 A/swine/Mexico/Mex51/2010 (H3N2 iso51), since previous studies have shown that this strain has an antigenic behavior different to the H3N2 reference. Samples were considered positive > 1:80 dilutions.

At country level, 58.3% of the samples were positive to H1N1c, 34.7% to H3N2r and 33.6% to H3N2 iso51. Percentage positive to more than one test were: 25.7% to H1N1c and H3N2r, 30.6% to H1N1c and H3N2 iso51, and 18.9% to H3N2r and H3N2 iso51. There were 18.0% of samples that tested positive to the three tests. Approximately half of the farms sampled for this study had been vaccinated against Influenza, and all of them tested positive. At state level there was at least one farm testing positive to each one with the exception of Morelos; all samples from that state where sourced from one single farm not vaccinating against IAV. At farm level, 88.5% had positives to H1N1c, 84.3% to H3N2r and 74.0% to H3N2 iso51. By production category the serum samples breakdown is as follows: 2,905 growers-finishers, 339 breeding sows and 200 pigs of unknown category. 57.6% of growers-finishers were positive to H1N1c, 35.8% to H3N2r and 32.8% to H3N2 iso51. 58.4%, 25.7% and 31.6% of the samples from breeding sows were positive to H1N1c, H3N2r and to H3N2 iso 51, respectively.

Influenza A viruses are widespread in Mexican pig populations. Both H1N1 and H3N2 subtypes are present in commercial farms not vaccinating against those, suggesting they are common. The study shows evidence of the presence of the reference isolates, including the Mexican specific H3N2 (iso 51). The diagnostic results of an HI test for influenza virus will depend on the test. It is important to conduct various studies for the prevention and control of this disease, considering the likelihood that new subtypes may be affecting pigs. Currently, there are new diagnostic tools that will help to know in detail the current situation of the disease in Mexico.

Disclosure of Interest: None Declared.