Five people died in a recent outbreak in Italy linked to unpasteurized dairy products, according to a study.

Infections were caused by consuming unpasteurized fresh cheese produced from contaminated cow’s milk. The incident was considered to be the largest and most severe outbreak associated with raw milk cheese in Italy, said researchers in the journal Emerging Infectious Diseases.

From November 2021 to May 2022, 37 cases of Streptococcus zooepidemicus in central Italy were identified, of which 23 were hospitalized. Human infection generally occurs through direct contact with infected animals or by consuming contaminated unpasteurized milk or other dairy products.

From November 2021 to February 2022, infections were detected in 18 hospitalized patients who lived in the province of Pescara. A range of symptoms were observed including septicemia, pharyngitis, arthritis, uveitis, and endocarditis. Five people died from severe meningitis.

Affected people were aged 6 to 98 with a median of 79 years old. Sixteen males and 21 females were sick.

Researchers obtained 21 isolates from 19 hospitalized patients that were sent to Istituto Zooprofilattico Sperimentale in Teramo, Italy. All 21 clinical strains were closely related, pointing to the same source of infection for the patients. 

Finding the outbreak source
A task force including physicians, veterinarians, epidemiologists, scientists, microbiologists, and communication experts was created by the local authority to evaluate the outbreak.

Investigations and whole genome sequencing (WGS) found unpasteurized fresh dairy products were the outbreak source.

Epidemiological analysis showed that 31 patients consumed soft or semi-soft cheeses purchased from local producers or dealers.

Streptococcus zooepidemicus was detected in an unpasteurized bulk cow milk sample taken from one dairy producer within the outbreak area. An official inspection then found 18 Streptococcus zooepidemicus strains from two bulk milk tanks and two cured raw milk cheese samples.

Further analysis showed these strains clustered with the clinical ones, indicating a strong correlation between them and samples from patients.

A review of data from the Istituto Zooprofilattico Sperimentale found a strain that was isolated from the milk of a cow with mastitis in November 2021; the animal belonged to the same operator whose products tested positive. After sequencing, it also clustered with the other strains isolated from patients and raw milk products.

The farmer with the positive cow mentioned construction work in the barn from October to November 2021, which might have caused stress, predisposing the animal to mastitis.

Local authorities established measures to limit and prevent Streptococcus Zooepidemicus spread in February 2022. All dairy products were recalled from the market and local dealers, ripening cheeses were destroyed, and officials required the pasteurization of milk used for cheese production.