Turkeys in a shed

Avian influenza (AI) has returned to the USA, with the authorities reporting its first highly pathogenic case since last June in a flock of 43,500 turkeys in Indiana.

Mortality topped 2% and the US Animal and Plant Health Inspection Service revealed on 15 January that the strain had been identified as H7N8 – different to the strains that led to the death of almost 50 million chickens and turkeys in the first half of 2015.

But it added that a further eight cases have since been detected on other turkey farms in the area. Even though these additional cases have been confirmed as low-pathogenic H7N8, the farms have all been quarantined and the birds culled.

See also: Broiler breeders at heart of Scottish bird flu outbreak

What is bird flu?

Avian influenza (AI) viruses are classified by a combination of two groups of proteins: hemagglutinin or “H” proteins, of which there are 16 (H1–H16), and neuraminidase or “N” proteins, of which there are nine (N1–N9).

Many different combinations of “H” and “N” proteins are possible. Each combination is considered a different subtype, and can be further broken down into different strains.

AI viruses are further classified by their pathogenicity (low or high)— the ability of a particular virus strain to produce disease in domestic chickens.

“It appears that there was a low-pathogenic virus circulating in the poultry population in this area, and that virus likely mutated into a highly pathogenic virus in one flock,” said Dr John Clifford, US Department of Agriculture chief veterinarian.

“Through co-operative industry, state and federal efforts, we were able to quickly identify and isolate the highly pathogenic case, and depopulate that flock.

“Together, we are also working to stop further spread of the low path virus, and will continue aggressive testing on additional premises within the expanded control area to ensure any additional cases of either high path AI or low path AI are identified and controlled quickly.”

The pathogenicity of a virus refers to its ability to produce disease.

Birds with low-pathogenic AI often show no signs of infection or only have minor symptoms.

Last year the US experienced the worst outbreak of AI in its history.

Some 211 commercial flocks and 21 backyard flocks were affected, mostly with the H5N2 virus on layer farms in the upper mid-west.

Indiana – the most easterly of all affected states, had just one case last year, in a backyard flock of 30 birds.

US poultry companies saw their stock market valuations drop on news of the latest outbreaks, though analysts suggest the market fall-out will not be as bad as last year, when a number of key overseas buyers closed their doors to all US exports.