Study: Raw milk outbreaks only tip of iceberg

Illness outbreaks that are blamed on raw milk are only the tip of a big iceberg, because non-outbreak (sporadic) cases may outnumber them by 25 to 1, according to estimates based on 10 years' worth of foodborne illness surveillance data in Minnesota.

Researchers at the Minnesota Department of Health (MDH) found that 3.7% of patients who had sporadic intestinal illnesses between 2001 and 2010 reported drinking raw milk in the week or two before they got sick. There were 530 such cases, compared with 21 cases that were part of known raw milk–related outbreaks.

Given how many enteric illness cases go undetected for each one that's reported, more than 20,000 Minnesotans may have had raw milk–related illnesses in the decade in question, according to the study, published yesterday in Emerging Infectious Diseases.

The researchers also found that a disproportionate share of the illnesses occurred in children and that most of the children under age 6 who got sick had drunk raw milk produced on their own farm or that of a relative.

The authors say people who drink raw milk, and policy makers who might consider relaxing restrictions on selling it, should be informed about the risks involved. The study was led by MDH epidemiologist Trisha J. Robinson, MPH, as first author.

Raw milk sales allowed

Minnesota allows the sale of raw milk at farms where it is produced, making it one of 30 states that allow some sales of the commodity, the report notes.

Minnesota requires the reporting of illnesses caused by Campylobacter, Cryptosporidium, Shiga toxin–producing Escherichia coli (STEC, including O157 and non-O157 strains), and Salmonella. As part of surveillance, residents who have lab-confirmed cases are interviewed by MDH staff about symptoms and possible sources of infection. Each questionnaire contains questions about raw milk consumption.

The researchers counted 20,034 infections with the relevant pathogens that were reported from 2001 through 2010. Almost 6,700 of these were excluded from the analysis because patients reported international travel or could not be interviewed, cases were linked to a known outbreak, or the infection involved a species or strain not historically associated with raw milk or cattle exposures.

That left 14,339 cases, including 6,747 Campylobacter, 1,742 Cryptosporidium, 1,069 STEC O157, 354 non-O157 STEC, and 4,427 Salmonella. Of these, 530 patients (3.7%) reported drinking fluid raw milk during their exposure period, defined as 14 days before illness onset for Cryptosporidium and 7 days for other pathogens.

Campylobacter infections accounted for 407 (77%) of the 530 cases, and those with Campylobacter cases had the highest percentage of raw milk consumption, at 6.0%. Twelve patients were co-infected with Campylobacter and one of the other pathogens.

Patients who were exposed to raw milk had a median age of 17 (range, 9 months to 92 years). Twenty-five percent were 5 years or younger, 38% were 10 or younger, and 59% were younger than 21. A sizable majority—62.6%—were male.

The analysis also revealed that summer was the peak season for illness, with 35% of cases (186 of 530) occurring in June, July, and August.

13% of patients hospitalized

As for illness severity, 70 patients (13%) were hospitalized, and 4 of 19 patients with STEC O157 infection and 1 of 12 with non-O157 STEC suffered hemolytic uremic syndrome, a form of kidney failure. An 11-month-old baby with STEC O157 died.

Investigators gleaned information on the source of the raw milk in 377 cases (71%). Almost half of the patients—including 76% of those aged 5 years or younger—were reported to have obtained it from either their own dairy farm (24%) or that of a relative (24%).

The analysis also revealed that among 464 patients with available information, 232 (50%) also reported contact with cattle or their environment during the exposure period.

The researchers acknowledge that some of the cases could have stemmed from these cattle exposures rather than from raw milk, but they say the high-risk nature of raw milk consumption makes it the most likely source of infection for most of the patients who did report cattle exposures. This conclusion is also supported by previous findings on demographics in raw milk–related outbreaks.

The authors used "underdiagnosis multipliers"—estimates of the ratio of undetected cases to confirmed cases for foodborne pathogens—to estimate that a total of 20,502 infections (Campylobacter, Crytsporidium, STEC, and Salmonella) were associated with raw milk in Minnesota during the study period.

On the basis of survey data indicating that 2.3% of Minnesotans drink raw milk, their estimate suggests that 17.3% of raw milk consumers in the state may have gotten sick from it during the period, they write.

Robinson, the lead author, said she and her colleagues were not especially surprised by their overall findings. "As we're involved with the surveillance of these diseases, we see them on a daily basis, but it certainly is more striking when we put it all together," she told CIDRAP News.

Personal relationship may not be protective

"One of the more surprising findings was really that three fourths of the children [age] 5 and under were served from their own farm or a relative," she added. "It goes to show that even if you know your farmer or your farmer knows you, can get sick."

She predicted that if investigators in other states did similar studies, they would find similar results suggesting that raw milk outbreak cases are far outnumbered by sporadic cases.

The report says the lower burden of illness in adults, compared with children, "may be a result of acquired immunity among older adolescents and adults who were exposed to pathogens during farm exposures in childhood." Robinson commented further that adults may be less apt to drink raw milk or may drink less of it if they do, though that is speculation.

The age distribution "really goes to show that we don't want to be exposing young children, especially since the complications of these diseases can be so severe in young children," she said.

In their report, the authors express the hope that their findings will spur the education of raw milk consumers and policy makers about the risks. They note that previous surveys showed that 30% to 50% of dairy producers were unaware that their raw bulk milk tank could contain pathogenic microbes.

"I hope that the word will continue to get out, and that people contemplating drinking raw milk will hear about the study and will think long and hard about it before deciding to drink raw milk or giving it to their children," said Robinson.

Earlier this week the American Academy of Pediatrics (AAP) recommended that all infants, children, and pregnant women avoid raw milk and called for a nationwide ban on the sale of raw-milk products.

Robinson TJ, Scheftel JM, Smith KE. Raw milk consumption among patients with non–outbreak-related enteric infections, Minnesota, USA, 2001–2010. Emerg Infect Dis 2013 (published online Dec 17) [Full text]

See also:

Dec 16 CIDRAP News scan "AAP says no to raw milk for kids and pregnant women, calls for ban"

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