Researchers have highlighted how international coordination played a role in solving a multi-country Salmonella outbreak linked to melons from Honduras.

Scientists said information sharing and harmonized data collection helped make the best use of limited resources to solve the outbreak during the Coronavirus pandemic.

Danish authorities reported a cluster of Salmonella Braenderup to the European Centre for Disease Prevention and Control (ECDC) in May 2021.

On the same day, Belgium reported an increase in Salmonella Braenderup cases, including four isolates clustered genetically with the Danish outbreak strain. A few days later, the UK reported an outbreak through the EU Commission’s Early Warning and Response System (EWRS). By July 2021, 348 cases had been reported from 12 European and United Kingdom countries, including 68 hospitalizations. 

ECDC oversight

With support from affected countries and the European Food Safety Authority (EFSA), ECDC formed an international investigation team in May to identify the source and prevent the outbreak’s spread. Two other meetings were held in May and June. A fourth meeting in August summed up lessons learned from the outbreak.

There is no standard approach to outbreak investigations for foodborne diseases that are often resource-intensive and involve several countries, said researchers in the journal Eurosurveillance.

Multinational outbreak investigations require collaboration across countries. This typically involves a number of organizations, such as health agencies, food and veterinary authorities, central and regional laboratories, and clinicians.

Consumption information was shared with affected countries through a standard line list, revealing that 124 of 197 cases reported having eaten melons within seven days before disease onset. Galia was the most commonly named type of melon. 

Only half of the cases reported eating melon of any type. One explanation is that cross-contamination plays a role at various stages in the supply chain. For example, during transport, export, or supermarket re-stocking, evidence from the UK showed melon boxes were re-used for other fresh produce, including other melons.

A Danish case-control study found no association between being sick and having melons, but studies in the UK and Germany found cases more likely to consume melons.

Coronavirus impact

Salmonella was isolated from two of 200 Galia melons in the UK in June 2021. Genomic analysis of these isolates matched the Salmonella Braenderup outbreak strain.

The outbreak strain was identified in Austria from a pooled peel sample of Galia, cantaloupe, and honeydew melons and in environmental samples in Finland from boot swab samples from a henhouse where chickens had been fed melon rind. In Germany, the outbreak strain was also found in a pooled feces sample from bears in a zoo.

Salmonella Braenderup matching the outbreak strain was detected on the surface of a washing tank in one of the Honduran facilities where Galia melons were packed, and corrective measures were taken onsite to prevent future contamination. Scientists say that washing melons immediately post-harvest, a procedure used by the implicated supplier, may lead to bacterial contamination externally and internally as melons can absorb contaminated water.

By July 2021, the outbreak ended, likely due to a shift in the supply chain of melons from Latin America to Southern European countries during the European summer growing season.

In January 2022, the European Commission increased official controls on imported Galia melons from Honduras due to the outbreak. These checks were removed in early 2023.

International coordination spared resources since it removed the need for full investigations in all affected countries. This was important given the outbreak occurred during the COVID-19 pandemic when public health and laboratory resources were already under significant strain or diverted to support the COVID-19 response. With microbiological testing of melons, many samples are required to find a positive, and there were lab resource shortages during the pandemic.

“The international-level coordination between countries facilitated by ECDC was key in concluding the outbreak’s source quickly and effectively. This was particularly important given the absence of confirmatory microbiological evidence of Salmonella in melons and the approaching end of the seasonal supply of Galia melons from Honduras,” said scientists.

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