Alfalfa Sprout Seeds Seen As Source of Salmonella Outbreaks

Alfalfa Sprout Seeds Seen as Source of Salmonella Outbreaks

Jan. 12 /99

JAMA. 1999;281:158-162

Chris A. Van Beneden, M.D., M.P.H.


CHICAGO – Scientists studying outbreaks of salmonella from alfalfa sprouts consumed inOregon and British Columbia conclude current sprouting methods are inadequate to protect against salmonella. They cite the seeds rather than the handling methods as the culprit, according to an article in the January 13 issue of The Journal of the American Medical Association (JAMA).


Chris A. Van Beneden, M.D., M.P.H., with the Oregon Health Division in Portland and the Centers for Disease Control in Atlanta, and colleagues sought to determine the source of a salmonella outbreak in the winter of 1995-1996 in the Pacific Northwest. A similar outbreak had occurred in six other central and eastern states in the fall of 1995. The authors traced the problem to a particular group of seeds that originated from a Dutch distributor destined for Pacific Northwest growers. They write that further evidence indicates that contaminated seeds from a distributor caused the previously unexplained salmonella outbreaks in other states during the fall of 1995. “The knowledge that early in 1995 Denmark had implicated alfalfa sprouts in a salmonella outbreak greatly facilitated the focus of our investigation,” the authors add.


“Collectively, these observations indicate that alfalfa seeds, most likely contaminated from a single source, were distributed through multiple channels first to growers in Europe and then to North America, causing numerous localized manifestations of one large outbreak that extended over at least nine months.” Fewer than 5 percent of salmonella infections are thought to be reported, but the authors estimate more than 20,000 people in North America alone contracted salmonella infections from eating these alfalfa sprouts in 1995.


The authors say sprout-associated outbreaks are being increasingly recognized, explaining alfalfa sprouts were initially implicated as a vehicle of unidentified foodborne illness in 1973. Although alfalfa sprouts were implicated repeatedly in unpublished investigations in Finland during the 1980s and early 1990s, according to the authors, most epidemiologists gave little thought to sprouts as a potential vehicle until 1995 when a brief account of a large Scandinavian epidemic was published.


“Alfalfa sprouts are a well-suited vehicle for salmonellosis,” the authors write.. “Seeds are often stored for months or years under cool, dry conditions in which salmonellae are stable. During the three-to five-day sprouting process, numbers of salmonellae may increase by three to four orders of magnitude, decreasing little if at all during subsequent refrigeration.


Alfalfa sprouts are rarely washed or cooked before consumption, and consumers are left with little protection other than chance. “From farm to table, many opportunities exist for contamination of alfalfa seeds or sprouts. Crops can be easily contaminated with dirty water, runoff from adjacent farms, animal fertilizers used in previous growing seasons, or droppings from rodents or ruminants.”


They speculate that salmonella organisms likely reside in seed crevices and between the cotyledon and testa, areas not reached by chemical treatments. According to the authors, “the fundamental problem is that the commercial sprouting process contains no ‘kill step’ that would eliminate pathogens without compromising a seed’s germination potential.” Irradiation is currently being evaluated as an adjunct seed decontamination method.


“Until barriers to a pathogen-free seed are resolved,” the authors warn, “we conclude that alfalfa sprouts are a high-risk food for salmonellosis. All consumers, particularly those at greatest risk for severe disease (immunosuppressed, elderly and very young people), should consider this danger when deciding whether to eat alfalfa sprouts.”


(JAMA. 1999;281:158-162)


Editor’s Note: This study was supported in part by the Emerging Infections Program of the Centers for Disease Control and Prevention, Atlanta.