Epidemiology SLP 4

SCREENING FOR DISEASES

PART V – Other Investigations

Tracebacks of food are often necessary to identify sources of contamination and quickly limit a public health threat by removing these sources. One purpose of a traceback is to ascertain the distribution and production chain for a food product so that an effective recall can be undertaken. Tracebacks can also clarify the point or points at which the implicated food was likely to have become contaminated and help determine how to prevent similar outbreaks in the future. Epidemiologic tracebacks can accomplish each of these goals, but are different from the more detailed, regulatory tracebacks which follow rules of legal evidence.

An epidemiologic traceback usually begins with the information available at the time of purchase of the implicated food item and extends back to the very beginning of its production. All production steps, from harvest to consumption, are examined.

Full tracebacks leading to formal product recalls can be time-consuming and result in many dead-ends. Pertinent information and records are often missing or poorly maintained. Traceback efforts may require hundreds of hours of tedious work and may extend to other states and countries.

MDCH and CDC decided to do an epidemiologic traceback of the alfalfa sprouts implicated in the Michigan outbreak.

Of the 16 patients who ate sprouts for whom the source of the sprouts could be traced, 15 led to a single sprouting facility, facility A in Michigan; in the remaining traceback, the patient could have eaten sprouts from either facility A or facility B in Michigan. (Figure 4) Facility A and B were the only facilities that sprouted alfalfa seed in the state. Sprouts grown by facility A at the time of the outbreak came from two lots of seed: one from Idaho and one from Australia.

At this point, the investigators became aware of a concurrent outbreak of E.coli O157:H7 infection in Virginia. CDC subtyped the strains from Virginia and identified the same PFGE pattern as in the Michigan outbreak. A case-control study conducted by the Virginia Department of Health (VDH) linked the concurrent outbreak of O157:H7 infections to alfalfa sprouts.

In Virginia, the source of sprouts could be traced for 13 patients; all led to one sprouting company in Virginia. The Virginia sprouting company was using a single lot of seed harvested in Idaho –the same lot as the one used at facility A in Michigan. Traceback of the seed to the distributor identified it as part of a 17,000 pound lot of which 6,000 pounds still remained.

Traceback results of the E.coli O157:H7 investigation of alfalfa sprouts in Michigan (MI) and Virginia (VA), 1997.

The implicated seed lot was a blend of 5 lots from fields of four farmers and was harvested between 1984 and 1996. The seed processor and the farmers were located in Idaho.

Inspection of the alfalfa fields revealed three possible sources of contamination: cattle manure, irrigation water, and deer feces. Although manure is not normally applied to alfalfa fields in Idaho, cattle feed lots were common in this area and the alfalfa fields of one farmer were adjacent to a feed lot. Manure may have leaked or been illegally dumped onto the alfalfa fields or run-off water from neighboring fields, contaminated by manure, may have been used to irrigate the alfalfa fields. In addition, three of four farmers occasionally saw deer in their fields and one field was situated next to a wildlife refuge.

The seed from each of the farmers was harvested and mechanically cleaned at the same seed processing plant. The seeds were then placed in 50 lb. bags. No further processing occurred. Most of the seed was produced to plant alfalfa fields (e.g., to produce hay for livestock feed); the relatively small amount of seed used for sprouting was not handled any differently than the raw agricultural commodity seed.

Note:*A 1988 outbreak of Salmonella Saint Paul infections in Europe was linked to mung bean sprouts. A small 1990 cluster of Salmonella Anatum infections in the United States was suspected to be linked to one grower’s alfalfa sprouts, but the source of contamination was not determined. A 1994 Salmonella Bovismorbificans outbreak in Finland and Sweden was traced to Australian alfalfa seed. In 1995, it was concluded that sprouts caused an international outbreak of Salmonella Stanley, affecting persons in more than 17 states in the United States and Finland. In that same year, another multinational outbreak of salmonellosis (due to S. entericaserotype Newport) was linked to alfalfa seeds after an increase in infections was detected in Oregon and British Columbia. In 1996, almost 10,000 cases of E. coli O157:H7 occurred among school children in Japan. The outbreak was ultimately shown to be caused by radish sprouts grown from seed imported from the U.S.

Part VI Control

The implicated seed lot was not distributed to any other sprouting companies in or outside the United States. The remaining 6,000 lbs. of seed was immediately removed from the marketplace. A sample of 500 grams of seed was cultured directly, and the same amount was sprouted and then cultured; neither yielded E. coli O157:H7.

The Idaho Division of Food and Drugs held meetings at which public health officials explained to seed growers the need to protect alfalfa and other seeds used in sprouting from contamination during growing, harvesting, and packing. Both MDCH and the VDH made public television and radio announcements about the risk of contaminated sprouting seeds and recommended that persons at high risk for complications from E. coli O157:H7 infection not eat sprouts.

The Center for Food Safety and Quality Enhancement began working with the sprout industry to identify ways to make sprouts safer for human consumption. In tests with artificially inoculated seed, treating the seed by soaking it in a chlorine solution* (2000 ppm hypochlorite in 57-60EC water) at the time of sprouting reduced the level of contamination by a thousand-fold. Irradiation has also been tested and appears to work well in decontaminating sprout seeds. However, this treatment leads to diminished sprouting ability and has not been approved by the FDA.

*Chemical treatment with a hypochlorite solution is a U.S. Food and Drug Administration (FDA) approved treatment of foods.

Source:

This information above is a modification of the case by:

Mahon, B. E.et al. (1997). An international outbreak of Salmonella infections caused by alfalfa sprouts grown from contaminated seeds. Journal of Infectious Diseases, 175(4), 876-882.