Post Holdings, the owner of the Michael Foods egg business, has warned two further outbreaks of avian influenza mean 20% of its egg supply is affected by the disease.

In a statement, Post said chicken flocks at company owned and third-party farms in Nebraska and Iowa have tested positive for bird flu.

Previously, Post had warned that about 15% of its egg supply was impacted by avian influenza (AI). The company had estimated that the cost of bird flu would hit the bottom line by around US$20m this year. This negative impact was included in Post's full-year adjusted EBITDA guidance of US$585-610m.

These costs now look likely to be higher than initially estimated. "Post will update the estimated impact of AI as soon as practicable," the company said.