JOHANNESBURG (Reuters) - The death toll from an outbreak of listeria in South Africa has jumped beyond 60 in the past month, health authorities said on Monday, adding they had closed a poultry abattoir where the bug that causes the disease had been detected.

Since monitoring of the outbreak began last January, 720 laboratory-confirmed cases of food poisoning due to the disease, also known as listeriosis, have been reported, the National Institute of Communicable Diseases (NICD) said.

That was up from 557 in December, since when recorded deaths had risen to 61 from 36.

A food microbiologist said the “alarming” outbreak appeared to be the biggest ever recorded and could spread further if it was not tackled urgently.

“Of the documented outbreaks globally that we know of ... our numbers are way above any of those other cases,” said Dr Lucia Anelich, who runs her own food safety consultancy.

The Department of Health said it had closed a poultry abattoir operated by Sovereign Foods in the capital Pretoria after detecting listeria there, and had banned the facility from preparing food in December.

The department said it did not yet know whether this abattoir was the source of the outbreak, which the NICD said was still unknown.

Sovereign Foods, which delisted from the Johannesburg stock exchange in November, said the prohibition on the abattoir was lifted on Monday after the listeria bacterium was not found in the latest samples from the plant.

“Despite being declared clean and free of the Lysteria bacterium, we are further strengthening steps to render products safer than they already are,” said Sovereign Foods head of production Blaine van Rensburg.

Listeria food poisoning is a bacterial infection that can be treated with antibiotics if diagnosed in time. The bacteria can be found in animal products including cold cut meats, poultry and unpasteurised milk, as well as fresh fruits and vegetables.

The disease can cause flu-like symptoms and diarrhea, and in more severe cases spread from the intestine to the blood, causing bloodstream infections, or to the central nervous system, causing meningitis.

Anelich said a listeriosis strain known as ST6 had been identified in nine out of 10 of the South African cases. That should make tracing the source easier, “because now we know that it probably originates from one processing facility”.

A health department official said the strain was not drug-resistant and that the deaths were due to delays in diagnosis, meaning cases were not treated in time.