According to a ProMED posting on July 17th and press reports, the Colorado Department of Health and Environment has reported six confirmed positive cases of avian influenza-H5N1 and 16 presumptive cases in workers engaged in depopulating an infected farm in Weld County, CO. According to the report, all of the affected workers showed conjunctivitis but ominously also exhibited mild respiratory symptoms and malaise suggesting a higher level of pathogenicity associated with a presumed bovine-adapted virus.
The affected complex undergoing exposure for the third time in three years housed approximately 1.5 million hens in eight aviary houses in addition to 170,000 hens among four conventional cage units. To date two houses holding replacement pullets situated about a mile from the laying complex are unaffected.
The question that arises relates to the prevalence rate among workers. Previously only one case of mild conjunctivitis occurred with the depopulation of as many as 50 million laying hens in previous outbreaks of H5N1 avian influenza. In the present situation if all the workers are positive and discounting any additional cases that may be incubating H5N1 infection the prevalence rate among the cohort of 170 is 14 percent. Depopulation of the 1.6 million complex was completed on July 16th. Initiation of the operation was delayed due to the Independence weekend when carbon dioxide for euthanasia was not readily available.
The Centers for Disease Control and Prevention is currently conducting field and laboratory investigations relating to the outbreak in cooperation with public health agencies in Colorado. It will be determined whether the virus isolated from the affected workers is in any way different as denoted by whole genome sequencing from a previous isolates of H5N1 responsible for a single case in a worker in 2023 together with a comparison of the isolates from previous and recent egg production flocks. Since there is a probability that the infection in the layer complex comprising 1.6 million hens was derived from a dairy farm, it is presumed that a bovine-adapted strain may have enhanced the ability to bind to mammalian receptor sites including conjunctival tissue and in the respiratory tract of humans.
The degree of protection afforded workers using PPE including respirators and face shields or goggles under hot conditions requires elaboration. Although PPE was provided, the question arises as to correct use and supervision especially the Tyvec suits, N95 masks and face shields that were apparently in short supply at the beginning of the operation. Midway through the depopulation goggles were supplied but canister respirators were not used. Whether depopulation of aviary houses with more dust contributes to exposure of workers compared to cage-housed flocks will have to be investigated. The question of whether cooling fans may have increased the level of suspended virus in the air of the houses will be evaluated either as an independent factor or in combination with dust in aviary-equipped houses. The fact that H5N1 infection has until now not been a problem with depopulation suggests a change in the virus facilitating infection of the respiratory mucosa through affinity with human sialic acid receptors. This has implications for future protective modalities. Since depopulation of a second complex in Weld County is underway the incidence rate among workers will be monitored and protective measures will require modification.
Cases among contract workers emerged on July 10th approximately four days after commencing depopulation. There were no cases among workers employed on the farm. The USDA was able to depopulate this laying complex over seven days from commencement of the operation, half the time required when the complex was previously infected, although there was a delay after diagnosis.
Notwithstanding the cases among contract workers, avian influenza in egg production flocks does not represent a risk to consumers. The virus is not considered to be vertically transmitted to eggs. None of the eggs packed by the complex entered the supply chain and were disposed of together with carcasses and manure on site.
The response of agencies in the State of Colorado and cooperation with the CDC team and USDA-APHIS is commendable given the incidence rate of avian and dairy herd outbreaks in the State.