08/03/19: APHA identifies unusual Salmonella strain; almost 300 sick, one dead

An agency in the United Kingdom has released more information about a potential new strain of Salmonella Typhimurium in sheep behind almost 300 human illnesses and one death.

The Animal and Plant Health Agency (APHA) has been investigating two incidents of Salmonella in sheep flocks with associated human cases. In both flocks, there were high levels of mortality in groups of cull ewes bought from various livestock markets for fattening prior to slaughter.

Cases of Salmonella Typhimurium have been linked to consumption of lamb and mutton. The outbreak started in July 2017 and 283 cases had been reported as of October 2018. Control measures such s restricting movement of animals and enhanced hygiene was followed by a significant decline in cases in the latter part of 2017.

Infection is thought to be associated with eating sheep meat, not cooking meat properly, not washing hands thoroughly after handling raw meat, or through cross-contamination of other foods, surfaces and utensils in kitchens. It is also possible for farm staff to be infected through direct contact with infected animals. One animal haulier has become ill via this route.

APHA officials said scanning surveillance activities in 2017 identified a case of an unusual strain of Salmonella Typhimurium following an investigation into a high level of mortality in a sheep flock. However, not all infected animals on two affected premises displayed clinical signs. Animals and flocks without any clinical signs can carry and shed the Salmonella, which can spread the bacteria.

“This new strain of Salmonella Typhimurium is different from previous outbreak strains in U.K. in that it does not usually conform to a recognised phage type or is untypable. It is also sensitive to all antibiotics in the APHA antimicrobial resistance surveillance panel,” added the agency.

The strain was linked by Whole Genome Sequencing (WGS) to a national outbreak causing widespread foodborne illness. In 2018 further incidents were identified by APHA with the same strain. Domestic species affected so far include sheep, cattle and poultry.

Public Health England (PHE) has classified the outbreak cluster as Salmonella Typhimurium t5. 3225 by WGS. Isolates within this cluster vary by a maximum of five single nucleotide polymorphism mutations (SNPs), which is considered a reliable threshold for identification of isolates epidemiologically related within a human disease outbreak.

Examination of the genetic relationship of this strain to others at PHE suggests it is unusual in the U.K. and the relatively low level of genetic diversity suggests a recently emerged source of contamination.

APHA officials said Salmonella can be introduced to a farm by movement of animals, which may or may not be showing clinical signs of infection; equipment and machinery e.g. handling/dosing equipment; vehicles – especially contaminated wheels and footwells; clothing and footwear; contaminated feed and wild birds and pests.

“Veterinarians have a vital role to play in surveillance and to look out for cases of high mortality in ewes or sheep flocks (or cattle herds) with a high level of diarrhoea. The human illness may also be associated, but the absence of human cases is not a reason to not report such cases,” added the agency.

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