PHOTO: ANP
DISEASE ▶▶▶
Analyzing the 2022 US avian influenza outbreak
From February to June 2022 the US battled a massive outbreak of avian influenza. In total, 186 cases were reported, resulting in the loss of 40.1 million birds. Lessons learned during the 2015 outbreak were clearly not remembered during the seven year hiatus without major incident.
BY HANS-WILHELM WINDHORST
Workers discard dead chickens at a farm in Iowa. It is particularly noteworthy that only two farms in Iowa, which had a capacity of more than 5 mil- lion layer places, accounted for a third of the total losses.
I
n 2016 the Animal and Plant Health Inspection Service of the United States Department of Agriculture (APHIS) stated in its final report on the 2015 epidemic that these were the most serious outbreaks ever in the USA. A total
of 227 outbreaks of the highly pathogenic influenza virus were recorded, killing 50.4 million poultry, including 43 million laying hens and 7.4 million turkeys. According to an analysis by Böckmann (2021), the overall economic damage was over US$ 3 billion. In the five years that followed, only minor outbreaks occurred which could be brought quickly under control. Between February and June 2022 there were again massive outbreaks of the virus. In total, 186 cases were documented, resulting in the loss of 40.1 million birds in commercial herds. Although the two epidemics occurred in almost the same time periods in 2015 and 2022 they showed significant differences, as comparisons will show.
The time period of the outbreaks The first outbreak of the H5N1 virus strain was detected on 8 February 2022 in a turkey farm in Indiana. More cases emerged in the days that followed. Almost simultaneously, farms in Kentucky were infected, one turkey and one broiler grower. In late February and early March additional outbreaks occurred in layer and broiler flocks in Delaware and Maryland, and on a broiler farm in Missouri. From mid-March several large layer flocks in Iowa and Wisconsin were affected. In Minnesota, the centre of turkey growing, the first case was reported on 26 March. In the weeks that followed the virus spread rapidly through Minnesota and the Dakotas. As of April 15 a cluster developed in Pennsylvania. Laying hen and duck farms were particularly affected. Outbreaks in layer farms in Colorado continued in the last week of April. During the last two weeks of May, the number of infections decreased rapidly but the virus flared up again in two large layer flocks in Colora- do in early June. The last outbreak was reported by APHIS on 9 June. A comparison with the situation in 2015 shows that the epidemic started earlier and the maximum phase was reached two weeks before that of 2015 (Figure 1). The epidemic ended about three to four weeks earlier in 2022 than in 2015. A de- tailed analysis may show that different weather patterns prob- ably played an important role but that improved biosecurity measures also led to a reduction in outbreaks.
Distribution by size classes While the 2015 epidemic almost exclusively affected laying hen and turkey farms, outbreaks in broiler and duck flocks also occurred in 2022. There was a clear imbalance between the proportion of outbreaks and animal losses for the individual poultry species. This makes it necessary to take a closer look at the distribution of herd sizes. A comparison of the average size of the infected farms soon reveals the special situation of layer flocks. While an average of 1.3 million hens per case was re- corded for layer farms, the average for broiler farms was 168,300 and for turkey farms only 42,600 birds. These values reflect the different farm sizes for these poultry species. In lay- er farms, the four lower size classes (up to 1 million hens) ac- counted for 55.5% of the cases but only 15.9% of the losses, while the larger farms affected accounted for 84.1% of hens that died or were culled. It is particularly noteworthy that only two farms in Iowa, which had a capacity of more than 5 mil- lion layer places, accounted for a third of the total losses. A
12 ▶ POULTRY WORLD | No. 6, 2022
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