Published Date: 2012-09-23 17:05:24
Subject: PRO/AH/EDR> Influenza (90): USA H3N2v, human cases under-estimated
Archive Number: 20120923.1305983
INFLUENZA (90): USA H3N2V, HUMAN CASES UNDERESTIMATED
A ProMED-mail post
ProMED-mail is a program of the
International Society for Infectious Diseases
Date: Fri 21 Sep 2012
Source: CIDRAP News [abridged, edited]
An investigation of one of the 1st swine-origin H3N2 influenza cases detected in the United States in 2011 suggests that for each confirmed case, there may be many more that go undetected. The investigation, triggered by a case related to a Pennsylvania fair in August 2011, revealed 3 confirmed cases, 4 probable cases, and 82 suspected cases of variant H3N2 (H3N2v), all of them in people who had attended the fair, according to a report published yesterday [20 Sep 2012] in Emerging Infectious Diseases [ref. below]. The virus could not be confirmed in most cases because most of those with suspected infections had recovered before the investigation, the report says. But among 6 children under age 4, an age-group in which retrospective serologic testing for novel flu viruses is more useful than for older groups, 4 tested positive for antibodies to the virus. "This finding suggests that illness in at least some suspected case-patients can be attributed to A(H3N2v) virus infection," says the report by investigators from the Centers for Disease Control and Prevention (CDC), the Pennsylvania departments of health and agriculture, and the Allegheny County Health Department in Pittsburgh. The investigators also found signs that greater exposure to pigs increased the risk of having a suspected case, though this finding was not statistically significant. They found no clear evidence of efficient or sustained human-to-human transmission of the virus.
The 3 confirmed cases discussed in the report were among 12 that were identified in 2011. So far this year , more than 300 cases have been found, nearly all of them over the summer in people who had exposure to pigs at county and state fairs. Also, a few cases of variant H1N2 and one case of variant H1N1 have cropped up recently, also associated with fairs. All of these variant strains have picked up the M (matrix) gene from the 2009 H1N1 virus.
The 1st infection in the 2011 Pennsylvania cluster was confirmed by the CDC 6 days after the fair closed, and it prompted the CDC and state and local officials to launch a hunt for other cases and possible risk factors, according to the new report. The investigators cast a wide net to find possible cases. They interviewed swine exhibitors at the fair and members of a national children's agricultural club in the county where the fair occurred. They used the media to encourage people to contact the state health department if they were sick after attending the fair. Also, they encouraged clinicians to obtain samples from patients with flu-like illness after swine or fair exposure.
In addition, the researchers questioned sick contacts of case-patients, and they interviewed visitors at another, later fair in the same county to find out whether they had gotten sick after attending the earlier fair. Finally, they used regular flu surveillance to find cases.
The team defined a suspected case-patient as a person who, within 7 days after attending the fair, had one or more flu signs or symptoms from at least 2 categories, such as fever, respiratory, gastrointestinal, and "constitutional" (fatigue, joint pain). Confirmation of a case was based on reverse-transcriptase polymerase chain reaction (RT-PCR). Serologic (antibody) testing of suspected case-patients was used to define probable cases. Children aged 13 and younger were asked to undergo such testing, because it was assumed they would have little previous exposure to viruses like H3N2v and, therefore, fewer cross-reactive antibodies, which make results hard to interpret. Previous studies indicated that no children younger than 4 have antibodies to H3N2v, but some children between 4 and 13 have cross-reactive antibodies, the report says. Consequently, a probable case-patient was defined as a suspected case-patient younger than 4 who had an H3N2v antibody titer over 40. Because of possible cross-reactive antibodies, children between 4 and 13 were considered seronegative if they had an antibody titer under 10 and indeterminate if titers were higher than that. On this basis, the investigation yielded 3 confirmed, 4 probable, and 82 suspected cases. The 3 confirmed cases included the initial case and are included in the official total of 12 H3N2v cases the CDC has listed for 2011, said Michael Jhung, MD, a medical epidemiologist in the CDC's Influenza Division and senior author of the study.
Of the 89 total case-patients, 19 (21 percent) were from households of exhibitors at the original fair, 29 (33 percent) were from households of ag club members, 4 (4 percent) had attended both the earlier and the later fair, 34 (38 percent) had called the state health department to report an illness, 10 (11 percent) were identified by another case-patient, and 2 (2 percent) were found through state flu surveillance. Some were identified by more than one method.
Illnesses followed swine exposure: The patients ranged in age from 6 months to 60 years, with a median of 12 years. Most of them got sick within 4 days after the swine show or the swine auction at the fair. The patients spent a median of 6 days at the fair, and 80 of 87 patients with available information reported direct or indirect exposure to swine there. Of the 3 confirmed case-patients, one was a girl under age 4, and the others were girls between 4 and 13 years old. One of the latter had a preexisting medical condition and was hospitalized for respiratory distress but recovered, according to the CDC. All 7 of the confirmed and probable case-patients attended the swine exhibit at the fair.
One contact of each of 4 case-patients (3 suspected and one confirmed) had flu-like illness, although none had attended the fair within 7 days before their illness. For 3 of the contacts, respiratory samples were obtained within the 1st week of illness and tested, and all were negative for flu, although one had a rhinovirus.
An inspection by a veterinarian on the 3rd day of the fair found no visibly sick pigs, though one had been removed earlier because of a fever, the report says. But in a post-fair survey of swine exhibitors, 8 of 80 households said their swine had shown signs of respiratory illness during or shortly after the fair. All of them had recovered or been slaughtered by the time of the survey. No pigs were tested for flu.
[Byline: Robert Roos]
ProMED-mail Rapporteur Kunihiko Iizuka
[The Abstract of the paper "Outbreak of Influenza A (H3N2) Variant Virus Infection among Attendees of an Agricultural Fair, Pennsylvania, USA, 2011" by Karen K. Wong et al. in Emerg Infect Dis 2012 December (Early online publication) http://wwwnc.cdc.gov/eid/article/18/12/12-1097_article.htm, reads as follows:
"During August 2011, influenza A (H3N2) variant [A(H3N2)v] virus infection developed in a child who attended an agricultural fair in Pennsylvania, USA; the virus resulted from reassortment of a swine influenza virus with influenza A(H1N1)pdm09. We interviewed fair attendees and conducted a retrospective cohort study among members of an agricultural club who attended the fair. Probable and confirmed cases of A(H3N2)v virus infection were defined by serology and genomic sequencing results, respectively. We identified 82 suspected, 4 probable, and 3 confirmed case-patients who attended the fair.
Among 127 cohort study members, the risk for suspected case status increased as swine exposure increased from none (4 percent; referent) to visiting swine exhibits (8 percent; relative risk 2.1; 95 percent CI 0.2-53.4) to touching swine (16 percent; relative risk 4.4; 95 percent CI 0.8-116.3). Fairs may be venues for zoonotic transmission of viruses with epidemic potential; thus, health officials should investigate respiratory illness outbreaks associated with agricultural events."
Subsequent to this investigation, for the 1st time since the summer outbreak began in July 2012, the CDC reported no new swine-origin variant flu cases, and the number of confirmed H3N2v cases remained at 305. - Mod.CP
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