Postal marks used to trace salmonella sources

In the US, salmonella infects 1.2 million people every year, killing on average 450 of them.

An estimated 11% of cases arise from direct contact with live chickens – very many of them in people with a penchant for backyard hen-keeping who buy their birds from agricultural supply stores.

Standard public health practice requires that the source of any salmonella case be identified, whether it be a live chicken or a plate of eggs Florentine, and its origin discovered.

This is done for a very good reason: salmonella never travels alone, and a single tainted source can catalyse a large number of potentially fatal cases.

According to a report prepared by the US Centres for Disease Control and Prevention (CDC), however, finding a hatchery that provided infected hens to any given store can be a fiendishly difficult business – especially when time is of the essence.

This, write researchers led by epidemiologist Jennifer Sidge of the Michigan Department of Health and Human Services (MDHHS), is because of a curious practice known in the chicken trade as “drop-shipping”.

When a hatchery has more orders than it can handle, it will subcontract one or more other chicken farms and get them to ship out the overflow. These secondary hatcheries, however, will brand and label their shipments using the name and address of the first one.

It might seem like a sane commercial arrangement, but it plays merry hell with any attempt to trace a salmonella-infected bird back to its source, because no-one – not the very ill chicken-owner, nor the people who run the store that sold it – can know for certain where it came from. 

“During human Salmonella outbreak investigations, real-time environmental sampling at mail-order hatcheries rarely occurs because of challenges involved in tracing poultry to its source,” the researchers write.

A pilot scheme in the state of Michigan, however, appears to have solved the problem – using postage stamps.

Postage marks, anyway. In a two-year program, the bedding or liners inside boxes full of hens arriving at supply stores were sampled by gloved-up MDHHS staff, placed in sterile plastic bags and shipped off to labs for testing. At the same time, the postage details on each box were photographed.

A total of 136 samples yielded 35 samples positive for six strains of salmonella, four of them tied to human illness.

Under normal circumstances, such results would have thrown up many problems, because the shipping labels on the boxes do not necessarily name the originating hatchery. The postal service, however, never lies.

“The mail-order hatchery industry practice of drop-shipping can complicate traceback investigations because hatchery records must be requested and reviewed to discover the actual source of poultry,” Sidge and colleagues report. 

“Using postal service labels to determine hatchery of origin is an efficient method for tracking the source to a specific hatchery and simplifies traceback during outbreak investigations.”

The pilot program, they report, allowed health authorities to notify hatcheries of infection much more rapidly than is usually the case, allowing the chicken farmers to take action, and, thus, by implication, saving human lives.