US health officials are investigating a food poisoning outbreak linked to alfalfa sprouts that has sickened at least 55 people across 15 states. The Centers for Disease Control and Prevention (CDC) said on Friday that four people had been hospitalised.

The outbreak was first reported by state officials earlier this week after more than three dozen cases were identified in Minnesota and Wisconsin.

Cases have since been reported in Florida, Indiana, Iowa, Kansas, Michigan, New Hampshire, New York, Pennsylvania, North Carolina, South Carolina, North Dakota, South Dakota and Washington.

The US Food and Drug Administration (FDA) said 26 of 34 people interviewed as part of the investigation reported eating alfalfa sprouts.

The outbreak is unusual because it involves more than one type of bacteria. Health officials have identified both salmonella and several strains of E. coli among those who have fallen ill.

Shiga toxin-producing E. coli, or STEC, can cause stomach cramps and bloody diarrhoea. In some cases, infections can lead to a serious complication involving acute kidney failure.

Salmonella infections can cause diarrhoea, fever, vomiting, dehydration and stomach cramps.

Sprout Grower Linked to Outbreak

Officials have linked the illnesses to alfalfa sprouts produced by Everything Sprouts, a Minnesota-based grower. The sprouts were sold under the Calco and Everything Sprouts brands to restaurants and grocery stores.

The company has not issued a recall, the FDA said.

Last year, the FDA sent a warning letter to Everything Sprouts after an inspection identified several concerns. Among them was a leaking roof that was allowing water to fall on bean sprouts and workers’ smocks.

Health officials have not said that the conditions identified during that inspection caused the current outbreak.

The investigation into the source of the illnesses is continuing.