Although no substantial or significant enough specific pathway or pathways have been identified as the main culprit in the massive outbreak of avian influenza this spring, the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service continues to uncover new lessons learned in helping producers better prepare to defend against the virus should it strike again.

In an update to its epidemiological analysis first released on June 5, the July installment highlights a case series investigating 81 turkey farms across the Midwestern United States and a case-control study focusing on egg layer flocks in Iowa and Nebraska.

APHIS reported that turkey farms typically follow biosecurity protocols, however fomites – such as equipment – are probably playing a role in this outbreak. Common procedures include spraying vehicle tires with disinfectant at the farm entrance, requiring visitors and employees to wear coveralls and disposable boot covers (or dedicated footwear) before entering the barns, using disinfectant footbaths at barn entrances, using rodent control, and caring for younger birds before caring for older birds.

Fomites were a person, farm equipment, farm vehicles and a shared mortality bin. For several farms in the case series, 7 to 11 days passed between the potential exposure event and the onset of HPAI clinical signs. APHIS noted that as feed trucks and renderers were frequent visitors to the farms, “they should be further explored as potential fomites for HPAI spread in this outbreak.”

The report also identified that there was a potential age predilection for HPAI as half of infected tom farms had 13-16-week-old birds when the outbreak occurred, while half of hen farms had 9- to 12-week-old birds. “Extra vigilance may be indicated when birds are at these life stages,” APHIS stated.

APHIS reported that in the majority of cases in the study, only 43% of case farms reported that biosecurity audits or assessments were conducted on the farm by the company or a third part. “Farms can decrease their HPAI risk by verifying that biosecurity procedures are being followed properly,” the report noted.

When evaluating the layer flocks in Iowa and Nebraska, APHIS also identified a number of risk factors for HPAI introduction as well as factors associated with lowering the risk of introduction. Factors associated with an increased risk of becoming infected with HPAI included being located within one of the 10-kilometer control zones; using rendering of dead birds as a disposal method; sharing of company trucks, trailers, bird removal and egg removal vehicles; sharing of equipment between farms like egg rack, pallets and flats; and visits by company service personnel who entered barns.

Factors associated with a lowered risk of infection included being more than 100 yards from a public gravel or dirt road, having wash stations for vehicles on the farm, and being more than 100 miles from the egg processing facility used by the farm.

The latest report looked at 28 case surveys compared to 30 control surveys that didn’t have the disease strike. “Based on this survey, it can be said that many producers believed that the virus was being spread via the air and that in some cases it may have spread by aerosolization of virus present on nearby, recently irrigated land,” the report said.

It was noted that many producers had no definite knowledge of whether trucking routes were being managed, but, conversely, larger companies had the ability to manage trucks and the routes that were taken.

A high proportion of producers mentioned specifically that the first ill birds on the barn were near a fan, and in most cases this was an intake fan bringing air into the barn. “For nine respondents, the first sick birds being near a fan and the participant believing that the virus was airborne were compatible responses,” APHIS said.

Perhaps the most striking theme to the interviewers was the noteworthy connectedness within four of the companies. Companies with four or more operations represented 16 of the 28 case surveys and 7 of the 30 control surveys. This company model is a common production type in the Iowa layer system and those surveyed were representative of the greater layer-hen industry in Iowa.

APHIS concluded, “Sharing of feed and other company trucks that make several trips back and forth from the main company site, which houses hens and often feed mills, to serve smaller pullet sites is one potential route of spread within an organization. In addition, the sharing of other pieces of equipment and common personnel cannot be ignored as a risk factor.”

The updated report can be read here.