May 22, 2002 (CIDRAP News) Although clear evidence links a recent widespread outbreak of Salmonella infection in the United States and Canada with eating cantaloupe, just how the cantaloupe became contaminated remains unclear, according to federal and state health officials.
"We've figured out how to identify these outbreaks fairly well, but we're not sure how to prevent them," said William E. Keene, PhD, MPH, an epidemiologist with the Oregon Department of Human Services who has been involved in investigating the outbreak.
The Food and Drug Administration (FDA) announced last week that the I. Kunik Co. of McAllen, Tex., was issuing a nationwide recall of its Mexican-grown Susie brand cantaloupe because of the outbreak. Keene said the outbreak has involved more than 40 laboratory-confirmed cases of infection with S enterica serotype Poona in seven US states and three Canadian provinces.
The FDA is continuing to hunt for the source of the contamination, said Sebastian Cianci, an FDA spokesman in Bethesda, Md. "How exactly it's occurring we don't know," he said. Salmonella contamination of cantaloupe "has been an issue for a number of years," so the FDA wants to figure out the cause, he added.
Keene said reports linking the outbreak with the presence of iguana lizards in the cantaloupe fields are "semi-informed speculation." Past S Poona outbreaks have been associated with a wide variety of sources, including contaminated well water used for irrigation, he said. In the last few years S Poona has been linked with pet iguanas, he said. "It's quite possible, but as far as I know, it's not certain" that iguanas are a factor in the current outbreak, he added.
Cianci said the FDA is looking at the possibility that iguanas or other reptiles have contributed to the outbreak. "There may be other animals out there," he said.
The FDA said it had taken steps to prevent further importation of Susie brand cantaloupes. The agency recommended that consumers take a number of precautions, including washing hands before and after handling fresh produce, washing all fresh fruits and vegetables with cool water before eating, and scrubbing melons with a produce brush.
A widespread outbreak of S Poona infection was associated with cantaloupe consumption in 1991, when more than 400 laboratory-confirmed cases occurred in 23 states (MMWR 1991;40:549-52). In that case, the Centers for Disease Control and Prevention (CDC) reported, "Grown on the ground, melons may be contaminated on their surface with dirt, chemicals, animal excreta, or bacteria, including Salmonella. Although large produce companies wash and dip melons in a chlorine solution, field-packed melons do not receive such treatment."
Keene said it does not appear that Salmonella contamination is generally present on cantaloupe, which leaves room for hope that the sources will eventually be better understood and the problem prevented.
The current outbreak was first noticed May 10, when Oregon health officials learned from their counterparts in Washington state that five S Poona isolates had been identified there, according to Keene. The same day, he said, two cases were found in Oregon. The subsequent alert led to the reporting of cases in California, Nevada, Minnesota, Texas, Wisconsin, and three Canadian provincesBritish Columbia, Saskatchewan, and Ontario.
"We get about six of these [S Poona isolates] a year," Keene said. Consequently, "When we get two in one day, it's obvious" that they may be connected. S Poona ranks in the low 20s on the list of the most common Salmonella serotypes in the United States, he said.
Keene estimated that at least seven patients were hospitalized in the course of the outbreak but said he was not aware of any deaths. Most of the patients were either young children or elderly (75 or older), he said.
A case-control study that matched 26 outbreak patients with healthy controls helped connect the outbreak to cantaloupe, according to Keene. The study showed that 73% of the patients had eaten cantaloupe in the 5 days before they got sick, compared with only 21% of controls, he said.