Figures on prevalence of asymptomatic flu cases are all over the map
A meta-analysis of the influenza literature indicates that there is no consistent or typical figure for the percentage of flu infections that are asymptomatic, which adds to the difficulty of responding to outbreaks, according to a new report from Australian and British researchers published in Emerging Infectious Diseases.
In an analysis of 55 studies that met their criteria, they found that the prevalence of asymptomatic infection (meaning a total absence of symptoms) ranged from 5.2% to 35.5%, while the proportion of subclinical cases—those that didn't meet the investigators' criteria for acute febrile or influenza-like illness (ILI)—varied from 25.4% to 61.8%.
A statistical analysis showed that the wide range of prevalence figures "could not be explained by the type of influenza, the laboratory tests used to detect the virus, the year of the study, or the location of the study," the researchers wrote.
"We found no evidence to support a fixed asymptomatic rate (or even an informative range) between or even within influenza virus subtypes," they said. "For example, the prevalence of asymptomatic influenza A (H1N1) virus ranged from 0% to 65%, resulting in an overall failure to explain the extreme heterogeneity in this reported rate."
Assessing the prevalence of asymptomatic infections is important because asymptomatic carriers can serve as "insidious spreaders" of flu viruses, the authors noted.
To project the spread of cases and frame control strategies, it’s important to measure the prevalence of asymptomatic cases quickly when new flu subtypes and strains emerge and to track how the prevalence evolves over time and space, they wrote.
May 11 Emerg Infect Dis article
Analysis shows frequent two-way flu spread between wild birds, poultry
Poultry production may play a larger role in spreading avian influenza viruses than previously thought, according to a new genetic analysis that factored in ecological information such as trade data and poultry production levels.
Researchers from several institutions, including St. Jude Children's Research Hospital and the Massachusetts Institute of Technology, reported their findings yesterday in PLoS Pathogens.
They focused on H9 avian flu isolates, including those collected from North American wild birds between 1974 and 2013. They integrated the ecological data to assess spatial relationships and spread patterns. For comparison, they looked at patterns for H3 and H6 subtypes to see if the H9 pattern could be generalized.
Analysis revealed high rates of transmission from domestic to wild birds, with the highest rates of viral flow among domestic populations, hinting that poultry trade may be driving much of the viral spread within regions. Wild birds, however, were responsible for intercontinental dispersal of the viruses.
For example, the risk of viral emergence was enhanced in eastern China, though western China and Southeast Asia had a greater risk of receiving H9 viruses. The analysis of H3 viruses showed a similar pattern, but the results were less clear for H6.
The researchers concluded that integrating the different data sources reveals new insights and shows that the assumption of a one-way flow of viruses from wild birds to poultry is an incomplete picture and may hamper control efforts. Production systems that promote two-way transmission between wild and domestic birds pose a threat for generating potential pandemic flu strains and outbreaks that are tough to contain, the team wrote.
May 11 PLoS Pathog report
Egypt reports two H5N1 outbreaks in poultry
Egypt reported two recent H5N1 avian influenza outbreaks in poultry, according to notifications yesterday from the United Nations Food and Agriculture Organization (FAO).
One was detected in Sharqia governorate on May 4, affecting chickens, ducks, and other unspecified birds. The other was observed in Beni Suef governorate on May 5, affecting domestic ducks. The reports were noted on the FAO's EMPRES (Global Animal Disease Information System) database, which lists cases reported by national authorities.
Egypt has had a few human H5N1 cases this year, but the number is down steeply from an unprecedented surge in the first half of 2015 that resulted in at least 134 illnesses.
May 11 FAO report on Sharquia outbreak
May 11 FAO report on Beni Suef outbreak
May 11 FluTrackers post
Texas dengue outbreak in 2013 was linked to Mexican epidemic
An outbreak of 53 dengue virus cases in southern Texas in 2013 was linked to a concurrent epidemic in northern Mexico, demonstrating the risk of border-hopping outbreaks, according to a report yesterday in Emerging Infectious Diseases.
The 53 Texas cases were detected through surveillance that was enhanced because of the Mexican epidemic, which involved more than 5,500 cases, says the report by investigators from the Centers for Disease Control and Prevention (CDC) and Texas state and county health officials.
Almost half (26) of the cases were acquired locally, which "represents the largest number of locally acquired dengue cases in a single outbreak since dengue first reemerged in Texas in 1980," the report states.
Twenty-nine (55%) of the 53 Texas patients were hospitalized, a higher rate than is typically seen in dengue-endemic areas, the authors said. The reasons for it probably include detection bias based on severity of disease and an older population than is usually found in dengue-endemic regions.
Molecular phylogenetic analysis of dengue isolates showed that the viruses circulating in northern Mexico and southern Texas in 2013 were closely related to viruses that had recently circulated in Mexico and Central America, the report says.
The investigators tested 51 household members of 22 patients in the outbreak and found that 6 had recently been infected with dengue virus though they had not recently traveled, which suggested household transmission.
Some of the Texas cases were missed by tests in commercial laboratories: of 83 specimens that were initially negative for IgM antibodies at those labs, 14 were positive on polymerase chain reaction tests run by the CDC. The false-negatives probably resulted from testing too early in the illness course, because dengue virus IgM is generally not detectable until 3 to 5 days after onset, the authors said.
Because Aedes aegypti mosquitoes are established in southern Texas, future dengue epidemics in northern Mexico are likely to spill over to the US side of the border, the report warns. Residents of the region therefore should eliminate mosquito breeding sites and take steps to prevent mosquito bites.
May 11 Emerg Infect Dis report