In October of 1993, there was decreased egg production and increased mortality among Mexican chickens, in association with serologic evidence of an H5N2 influenza virus. First isolated from chickens in May of 1994, after spreading widely in the country, the virus caused only a mild respiratory syndrome in specific pathogen-free chickens. Because eradication of the virus by destruction of infected birds posed major obstacles to the poultry industry in Mexico, we were able to conduct a "field experiment" to determine the fate of an avirulent virus after repeated cycles of replication in millions of chickens. By the end of 1994, the virus had mutated to contain a highly cleavable hemagglutinin (HA), but remained only mildly pathogenic in chickens. Within months, however, it had become lethal in poultry. Nucleotide sequence analysis of the HA cleavage site of the original avirulent strain revealed R-E-T-R, typical of avirulent viruses and unlike the K-K-K-R sequence characterizing viruses responsible for the 1983 outbreak in poultry in the United States. Both mildly and highly pathogenic isolates contained insertions and a substitution of basic residues in the HA connecting peptide, R-K-R-K-T-R, which made the HA highly cleavable in trypsin-free chicken embryo fibroblasts. Phylogenetic analysis of the HA of H5 avian influenza viruses, including the Mexican isolates, indicated that the epidemic virus had originated from the introduction of a single virus of the North American lineage into Mexican chickens. This sequence of events demonstrates, apparently for the first time, the stepwise acquisition of virulence by an avian influenza virus in nature.

The presence of highly pathogenic H5N2 avian influenza in domestic poultry in Mexico that is not being eradicated by conventional depopulation methods constitutes an imminent problem for poultry producers and agricultural authorities in the United States. The present report considers the candidate vaccines available to H5N2 influenza virus and establishes that a fowl pox-H5 recombinant can provide protection from lethal Mexican H5N2, and prevent shedding in the feces and transmission to contact birds. Inactivated and recombinant vaccines may be useful adjuncts to eradication if the H5N2 influenza virus spreads to the United States or the countries in Central America.

To determine sensitivity and specificity of immunofluorescence assay (IFA) for detection of Helicobacter pylori, we studied 151 patients. Biopsies of gastric mucosae were obtained for culture, histological testing, and IFA. H. pylori serum antibodies were tested by enzyme-linked immunosorbent assay. IFA was done on Formalinpreserved, paraffin-embedded biopsies by using rabbit anti-H. pylori and goat anti-rabbit gamma globulinfluorescein isothiocyanate conjugate. The sensitivity and specificity of IFA compared with culture and Warthin-Starry stain were 93 and 95%, respectively. IFA is an accurate method for the diagnosis of H. pylori infection.