An investigation led to (uncooked) ham products as the most likely vehicle of transmission.
From October 2016 to March 2017, 54 cases of S. Bovismorbificans were seen in the Netherlands. They were five to 90 years of age and 29 were female.
Cases were mostly living in the less densely populated regions in the east and south of the country.
Individual cases of salmonellosis are not notifiable in the Netherlands except for (para)typhoid fever.
Low background incidence aid
From October 2016 until January 2017, 32 S. Bovismorbificans cases (one to five per week) were reported to the Center for Disease Control (CIb) of the Dutch National Institute for Public Health and the Environment (RIVM).
This exceeded the three to 14 cases of this serotype per year seen from 2005 to 2015 and was the first possible outbreak in the Netherlands since implementation of the laboratory surveillance network.
Such a low background incidence helped detection of the increase and start of an outbreak investigation, said researchers.
Increase in Belgium and France
Through an urgent inquiry in the European Centre for Disease Prevention and Control’s Epidemic Intelligence Information System (EPIS) it was found there had been a rise in Belgium to 32 cases in 2016.
An increase was also observed in France with 47 cases in 2016.
Seven isolates from Belgian and two from French patients matched the Dutch outbreak strain.
A traceback investigation by the NVWA led to a Belgian meat processor and researchers said it was ‘likely’ that contaminated products were available in Belgian stores and possibly in France.
No evidence was found for possible contamination during production at the Belgian processor.
There were no positive samples of incoming meat and production of raw ham was separated from other meat products to prevent cross contamination.
Outbreak strain potentially more pathogenic
All available Dutch isolates of S. Bovismorbificans from human cases in 2016 and those from other affected countries were sequenced in January 2017 using whole genome sequencing (WGS).
A total of 24 cases participated in the case–control study with 37 controls. Fifteen were admitted to hospital.
“This suggests the outbreak strain might be more pathogenic than other Salmonella strains, thus leading to a higher burden of disease for this specific strain,” said researchers.
“However, as the number of cases was small, we expect the burden of disease from this outbreak to be limited as the total burden of disease from salmonellosis in the Netherlands is already low compared with some other foodborne pathogens.
“A more pathogenic strain, which we anticipated was the case in this outbreak, would probably lead to lower rate of under-reporting, suggesting that we may have seen a relatively high number of true cases compared with other Salmonella outbreaks.”
Low infectious dose
Food items with the highest frequency of consumption among cases were ham and cheese products.
Multivariable analysis confirmed cases were more likely to have eaten ham products than controls and to have shopped at an unnamed supermarket chain.
Only 15 of 24 cases reported exposure to ham products in the incubation period.
Researchers said this could be due to recall bias as many cases were interviewed four to six weeks after onset of symptoms or another explanation could be cross-contamination of other products within individuals’ homes.
“We assume it unlikely that other food products were involved in the outbreak, as no new cases were reported with the outbreak strain after the withdrawal of the contaminated batch. The low number of positively tested food samples from the incriminated batch of ham suggests that the infectious dose of the contaminated batch was probably not very high.”
In April 2017, four weeks after the last case in the Netherlands, one of the collected retail ham products (smoked Coburg ham, sliced at supermarket) tested positive for Salmonella.
Jumbo issued a recall of Coburger ham in April due to possible presence of Salmonella, demonstrated during a quality control.
Researchers added the identification of S. Bovismorbificans in a ham product from the Dutch supermarket, originating from a meat processor, led to a recall of related products and prevented further cases.
“Outbreak of Salmonella Bovismorbificans associated with the consumption of uncooked ham products, the Netherlands, 2016 to 2017”
Authors: Diederik Brandwagt, Cees van den Wijngaard, Anna Dolores Tulen, Annemieke Christine Mulder, Agnetha Hofhuis, Rianne Jacobs, Max Heck, Anjo Verbruggen, Hans van den Kerkhof, Ife Slegers-Fitz-James, Lapo Mughini-Gras, Eelco Franz