BARNEVELD, the Netherlands — Diligent family farmers like Marjan van de Vis have made the Netherlands the global leader in egg exports. But now she, like many others, finds her livelihood threatened since using a local company this year to rid her chickens of pesky lice.

That company, the authorities believe, appears to have mixed a controlled poison, fipronil, into its cleaning solution. Since last week, the resulting contamination has been found in exports of eggs and egg products to 16 European countries and China.

“Our beautiful family farm is hanging by a silk thread,” said Ms. van de Vis, who will now have to destroy 100,000 eggs that cannot be sold.

The scandal is now one of the biggest food scares in Europe since 2013, when horse meat was discovered being passed off as beef. As much as 20 percent of Dutch egg-laying chickens may be affected, according to the authorities, costing farmers millions and damaging the reputation of the roughly 700-million-euro Dutch egg sector.