The Centers for Disease Control and Prevention (CDC), in its latest update on the nation´s seasonal influenza activity, reported on a person who was infected with a swine influenza virus following several exposures to pigs, including a sick one.

The CDC said the patient was infected with a swine influenza A/H1N1 virus. Although human infections with swine flu viruses are uncommon, many years bring reports of isolated cases, the report said.

The Texas Department of State Health Services, in a flu surveillance activity report for the week ending Nov 15, said the patient got sick in mid October. His or her specimen was collected and the virus identified during routine influenza surveillance. Texas officials, who gave no details about the patient´s illness, said their investigation found no illnesses in his or her household or close contacts.

According to the CDC´s background information on swine flu, the agency receives about one human influenza isolate each year that tests positive for a swine influenza virus. H1N1 and H3N2 swine flu viruses are endemic in US pig populations.

In September, researchers from the CDC and public health officials from Wisconsin published a case report in Emerging Infectious Diseases on a healthy 17-year-old boy who had mild respiratory symptoms in December 2005, 3 days after helping his brother-in-law butcher pigs.

At an outpatient clinic a few days later, healthcare workers collected nasal wash specimens, which tested positive for influenza A and were forwarded to the Wisconsin State Laboratory of Hygiene. Though further testing isolated influenza A, the virus didn´t match human H3 or H1 subtypes or the H5 avian subtype. CDC investigators sequenced the virus, identifying it as a swine influenza A (H1N1) triple reassortant virus, A/Wisconsin/87/2005 H1N1.

The report said that triple reassortant H1N1 subtypes are the predominant genotype in North American pigs and that human swine flu illnesses often mimic seasonal flu infections. The authors recommended that clinicians ask patients with unexplained influenza-like illnesses about exposure to animals, including pigs, and visits to petting zoos and county fairs.

Human infections with novel influenza A subtypes now are nationally notifiable diseases in the United States, the group reported. Though human-to-human swine flu transmission is rare, the CDC said human infections with swine H1N1 viruses should be investigated to ensure that they are not spreading among humans—as spread could represent a pandemic threat—and to monitor changes in circulating viruses.

In 1988, an H1N1 swine flu virus was found in a previously healthy 32-year-old pregnant woman who died 8 days after she was hospitalized for pneumonia, according to the CDC. Four days before she got sick she had visited a swine exhibit at a county fair where a flu-like illness was widespread among the pigs. Follow-up studies showed that 76% of swine exhibitors had antibodies to the swine flu virus, though no illnesses were reported. However, researchers found that one to three healthcare workers who had contact with the woman experienced mild flu symptoms with antibody evidence of swine flu exposure.

In December 2007, researchers reported that a new swine flu subtype found recently in Missouri pigs—H2N3—combined genes from avian and swine flu viruses, could cause experimentally induced infections in mice, and was transmissible in pigs and ferrets. The findings, which appeared in Proceedings of the National Academy of Sciences (PNAS), bolstered the theory that pigs can serve as a mixing vessel for flu viruses and a possible source for a human pandemic strain.

The CDC said swine flu outbreaks in pigs typically occur in late fall and winter months. The agency said seasonal influenza vaccines are likely to partially protect against swine H3N2 viruses, but not the H1N1 subtype.

See also:

CDC week 46 flu surveillance report

Texas Department of State Health Services week 46 flu surveillance report

CDC swine flu information
http://www.cdc.gov/flu/swine/

Sep 2008 EID case report

Nov 4, 1988, MMWR report

Dec 20, 2007, CIDRAP News story "New swine flu virus supports ´mixing vessel´ theory"