TABLE 1.

Serological reactivity to H5N1 influenza viruses of pig sera collected in Vietnamese slaughterhouses^a

Yr and mo	Location	No. of serum samples tested	No. of sera positive for H5 antibody (%) in neutralization tests	Neutralization titer(s) of positive sera
2003 September	Hanoi	109	0
2003 October	Hanoi	106	0
2003 November	Hanoi	91	0
2003 December	Hanoi	100	0
2004 January	Hanoi	105	1 (1)	160
2004 February	Hanoi	156	0
2004 March	Hanoi	343	0
2004 April	Hanoi	603	5 (0.8)	80, 160, 160, 40, 40
2004 April	Can Tho	1,039	0
2004 April	Ho Chi Minh City	307	0
2004 May	Hanoi	102	1 (1)	80
2004 June	Hanoi	114	1 (0.9)	80
Total		3,175	8 (0.25)

^aSera were collected from 3,175 pigs in Vietnam during September 2003 through June 2004. Pigs were sampled in a slaughterhouse at three different locations as indicated. All sera were heat inactivated at 56°C for 30 minutes and tested in a neutralization test. Briefly, sera were screened at a dilution of 1/10 by mixing 100 50% tissue culture infective doses of MDCK cell-adapted viruses (A/Vietnam/3078/04 [H5N1]) with the antibody dilution, incubating for 1 hour at 37°C, and adding to a preformed MDCK cell monolayer. Cytopathic effect was read at 3 days when the virus back-titration confirmed the challenge virus dose to be 100 50% tissue culture infective doses. Positive and negative control sera and a virus back-titration were included. Sera positive in the screening test were retested after pretreatment with receptor-destroying enzyme derived from Vibrio cholerae (Denka Seiken, Tokyo, Japan) to remove nonspecific inhibitors. The antibody titer was determined by testing serial twofold dilutions from 1/10 to 1/640 in quadruplicate. The highest antibody dilution providing complete protection of the cell monolayer in >2 of the quadruplicate wells was regarded as the antibody titer. The sera giving positive neutralization tests were confirmed by Western blotting using H5 antigen (Fig. 1).