Salmonella dublin and Raw Milk Consumption -- California

In 1981, 46 cases of human Salmonella dublin infection were reported in California, and in 1982, 70 cases were reported. In both years, 24% of patients reported using certified raw milk (CRM). In 1983, 123 S. dublin cases were identified--the most ever reported in a single year--and of the 99 persons providing information on raw milk use, 44% reported using CRM.

The demographic characteristics and risk factors for S. dublin patients have consistently differed from those infected with other Salmonella serotypes. S. dublin patients are much older, and many have underlying, debilitating disorders. In 1983, 61% of S. dublin patients were 40 years of age or over, and 17% were less than 20 years of age. By contrast, salmonellosis caused by other serotypes is generally a pediatric disease, with 60% of infections occurring among individuals under age 20. At least 24 of the 1983 S. dublin patients had cancer, particularly lymphoma and leukemia; five had acquired immunodeficiency syndrome (AIDS); and others had diabetes mellitus, were immunodeficient, or were taking systemic corticosteroids and/or other immunosuppressants for a variety of disorders. Nearly 80% were hospitalized, and 26% died.

S. dublin gave continued evidence of being a particularly invasive serotype, as 79% of the 1983 isolates were recovered from extra-intestinal (nonfecal) sites, principally from blood, but also from cerebrospinal fluid, peritoneal fluid, and lung. By contrast, only 10%-15% of Salmonella isolates from all other serotypes in any year are from extra-intestinal sites. The invasiveness of S. dublin does not simply reflect the older age of the infected host: when controls infected by other Salmonella serotypes were matched by age and sex to S. dublin patients, the percentage of controls with positive extra-intestinal cultures remained below 15%.

To evaluate the possibility that (1) raw-to-rare beef or liver or (2) raw milk may be sources of S. dublin infection, the 1983 patients were asked about these food exposures: 16% reported consuming raw-to-rare beef or liver, whereas 44% reported using CRM. Raw milk use among persons with infections due to other Salmonella serotypes was 8% in a previous study. Raw milk represents less than 1% of all market milk distributed in California.

Since 1977, nearly 200 Salmonella isolates have been made from CRM by 14 local, state, and federal laboratories. Antibiogram tests of human S. dublin isolates have shown a statistically significant association between the patterns of isolates recovered from CRM users and the antibiogram patterns of isolates recovered from CRM itself.

The risk of contracting S. dublin from CRM in California has been estimated: assuming that 12,000 gallons of CRM are produced per day and that each user drinks 1 pint daily, the rate of S. dublin disease for CRM users in 1983 was estimated at 458.3 per million population. This contrasts with a rate of 2.9 per million for S. dublin patients from the California population that did not report drinking raw milk. The relative risk of illness from S. dublin for CRM users was, therefore, 158.0 (458.3/2.9). The association between CRM ingestion and S. dublin disease in 1983 was about 15 times stronger than the well-accepted association between cigarette smoking and lung cancer. Just as the majority of smokers do not develop lung cancer, the majority of raw milk users apparently do not develop S. dublin disease. The number of S. dublin cases in California is not larger than it is because the population that drinks CRM is very small, and the contamination of CRM appears intermittent. Salmonellosis from raw milk is a potential hazard that merits greater appreciation by consumers, producers, and health-care providers. Adapted from California Morbidity (March 30, 1984) by SB Werner, MD, FR Morrison, DrPH, GL Humphrey, DVM, RA Murray, DrPH, J Chin, MD, State Epidemiologist, California Dept of Health Svcs; Div of Bacterial Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Raw milk/milk products have been repeatedly demonstrated as the vehicles for a variety of human pathogens, including Campylobacter, Streptococcus zooepidemicus, and Salmonella of many different serotypes. Although the California data relate to illnesses associated with infection with a single Salmonella serotype, data collected in other states document that raw milk continues to be responsible for a substantial percentage of sporadic enteric illnesses, as well as for occasional outbreaks (1-3).

Currently available S. dublin data are most useful in determining a relative risk of infection to raw milk drinkers, since S. dublin is a strongly host-adapted serotype to cattle, and since there is good information concerning the quantity of raw milk produced in California. This calculation of a relative risk in CRM users is one of the first attempted in an endemic situation. The magnitude of the estimated relative risk is high, perhaps because many of the raw milk users are already very ill and may be more susceptible to infection and subsequent serious disease, even from a small inoculum.

Certification of raw milk is a copyrighted designation that requires adherence to certain quality-control procedures, but it does not imply the milk is free of pathogens, such as Salmonella. The report from California and a previous report of campylobacteriosis (1) both dealt with disease associated with the consumption of CRM.

In 1979-1980, Salmonella dublin infections in the remainder of the United States were similar to those in California in 1983 in many respects, including the strong association with consumption of raw milk (2), indicating that this problem is not localized to California. In both studies, patients with S. dublin infections tended to be older and have underlying illnesses more often than patients with isolates of other Salmonella serotypes and, in addition, experienced higher rates of hospitalization and mortality. Previous outbreaks of S. dublin infections in Oregon and Washington State were also traced to consumption of raw milk (2,4).

Data concerning only S. dublin isolates underestimate the extent of disease associated with raw milk consumption. Not only are there likely to be S. dublin infections that are not confirmed microbiologically, serotypes of Salmonella other than S. dublin can also cause milkborne disease. Outbreaks of illness among consumers of raw milk due to S. typhimurium in Arizona (5), Oregon (3), and Quebec (6), indicate that varying but occasionally large proportions of infections caused by this very common Salmonella serotype may be associated with consumption of raw dairy products. In recognition of this and the economic burden of milkborne salmonellosis, Scotland banned the sale and distribution of raw milk in 1983 (7,8).

References

1. Potter ME, Blaser MJ, Sikes RK, Kaufmann AF, Wells JG. Human Campylobacter infection associated with certified raw milk. Am J Epidemiol 1983;117:475-83.

2. Taylor DN, Bied JM, Munro JS, Feldman RA. Salmonella dublin infections in the United States, 1979-1980. J Infect Dis 1982;146:322-7.

3. Oregon Department of Human Resources. Raw milk associated Salmonella outbreak in northeastern Oregon. Communicable Dis Summary 1983;32(10).

4. CDC. Salmonella dublin associated with raw milk--Washington State. MMWR 1981;30:373-4.

5. CDC. Salmonella gastroenteritis associated with milk--Arizona. MMWR 1979;28:117, 119-20.

6. Bezanson GS, Khakhria R, Bollegraaf E. Nosocomial outbreak caused by antibiotic-resistant strain of Salmonella typhimurium acquired from dairy cattle. Can Med Assoc J 1983;128:426-7.

7. Reilly WJ, Sharp JCM, Forbes GI, Paterson GM. Milkborne salmonellosis in Scotland, 1980-1982. Vet Rec 1983;112:578-80.

8. Cohen DR, Porter IA, Reid TMS, Sharp JCM, Forbes GI, Paterson GM. A cost benefit study of milk-borne salmonellosis. J Hyg Camb 1983;91:17-23.

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