Veterinary Services 

Centers for Epidemiology and Animal Health December 2003 

_________________________________________________________________________________________________________________________ 

Salmonella and Listeria in 

Bulk Tank Milk on U.S. 

Dairies 

Salmonella 

Salmonella bacteria are found in humans and 

virtually all livestock species. The bacteria also are 

present in all dairy environments. Salmonellosis 

(infection caused by Salmonella) in cattle is not 

uncommon. Clinical signs can range from severe 

septicemia (blood infections resulting in diarrhea, 

pneumonia, and arthritis) in calves, to diarrhea in 

adult cattle, to no signs at all. 

 During milking, bulk tank milk can be 

contaminated by Salmonella organisms in the 

udder or through fecal matter. In a 2000 study of 

New York dairy herds, Salmonella were isolated 

from 1.5 percent of 404 milk filters;1

 and a study of 

dairies in California in which individual cow milk 

samples were taken indicated that 84 percent of 64 

dairies had at least one culture-positive cow.2

 

Salmonella also are a leading cause of 

foodborne disease in humans, and consumption of 

both meat and milk has been implicated in 

salmonellosis outbreaks in people. In addition, 

strains of Salmonella resistant to multiple antibiotics 

have been isolated from dairy cows during 

salmonellosis outbreaks on dairy operations. These 

same strains have been isolated from ill people. 

Listeria 

Listeria bacteria are commonly found in dairy 

environments, particularly in soil, silage, and 

manure. Listeriosis (infection caused by Listeria) in 

cattle is generally caused by feeding spoiled silage. 

Clinical signs include depression, limb weakness, 

difficulty in swallowing, droopy lower lip, and 

walking in circles. Abortions that occur in late 

gestation also may be associated with listeriosis 

infections. Listerial mastitis is rare but may 

contribute to Listeria found in bulk tank samples. 

 

 

 

Listeriosis can be partially prevented by feeding 

properly fermented silage. Poorly fermented silage, 

which has a pH greater than 5.0, is ideal for Listeria

growth. Listeria also occurs in pockets of aerobic 

deterioration in properly put-up silage. These 

pockets are often indicated by mold growth and 

should not be fed to cattle. Other moist, preserved 

feeds such as wet brewer’s grains and silage made 

from other commodities also may support Listeria

growth. 

Listeria also can affect human health. Listeria

infections in people have been generally associated 

with the consumption of unpasteurized milk or milk 

products, although cases of human listeriosis 

associated with pasteurized milk have been 

reported. However, there is no evidence that the 

bacteria survive when subjected to correct 

pasteurization procedures.3

 

Dairy 2002 Study 

 

 The National Animal Health Monitoring System 

(NAHMS) Dairy 2002 study assessed the 

prevalence of Salmonella and Listeria in bulk tank 

milk. Between February 25 and June 30, 2002, 

dairy operations with at least 30 milk cows from 21 

States* were visited by animal health officials. 

Timing of bulk tank sample collection from 860 

herds ensured that 70 percent of each herd’s 

lactating cows were represented. 

___________________________________ 

*Regions/States: 

West: California, Colorado, Idaho, New Mexico, Texas, Washington 

Midwest: Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, Ohio, 

Wisconsin 

Northeast: New York, Pennsylvania, Vermont 

Southeast: Florida, Kentucky, Tennessee, Virginia 
Dairy 2002 Study Results (Salmonella) 

 Dairy 2002 data showed that 2.7 percent of 

dairies tested positive for Salmonella when a single 

bulk tank milk sample was cultured. The most 

common serotypes of Salmonella isolated were 

Montevideo (7), Newport (4), Muenster (2), 

Meleagridis (2), and Cerro (2). 

 Recently, many Salmonella Newport isolates 

have exhibited resistance to multiple types of 

antimicrobials. Each of the four S. Newport isolates 

from this study also exhibited resistance to multiple 

antimicrobials. 

 Higher percentages of large (500 or more head) 

and medium (100 to 499 head) operations had bulk 

tank samples test positive for Salmonella (6.7 and 

5.0 percent, respectively) than small (less than 100 

head) operations (0.4 percent) (Figure 1). 

________________________________________________________ 

0

5

10

15

20

Figure 1. Percent of Operations that had a Single Bulk 

Tank Milk Sample Test Positive for Salmonella, by 

Herd Size

Small

(Less than 100)

Medium

(100 to 499)

Large

(500 or more)

6.7

5.0

0.4

2.7

All Operations

Herd Size (Number of Dairy Cows)

Percent

________________________________________________________ 

 In addition, the West region had a higher 

percentage of operations (3.8 percent) with bulk 

tank samples that tested positive for Salmonella

than the Midwest (1.9 percent of operations) and 

Northeast (1.0 percent of operations) regions. No 

operations in the Southeast region had bulk tank 

samples test positive for Salmonella (Figure 2). It is 

important to note that more large herds are found in 

the West region than in the other regions. 

________________________________________________________

0

5

10

15

20

West Midwest Northeast Southeast

3.8

1.9

1.0

0.0

Figure 2. Percent of Operations that had a Single Bulk 

Tank Milk Sample Test Positive for Salmonella, 

by Region

Percent

Region

________________________________________________________ 

Dairy 2002 Study Results (Listeria) 

 Overall, 10.4 percent of operations had a single 

bulk tank sample test positive for Listeria. However, 

only 6.5 percent were positive for Listeria 

monocytogenes, the only Listeria species known to 

be pathogenic to humans. A higher percentage of 

large operations (7.8 percent) had bulk tank 

samples test positive for L. monocytogenes than 

medium (5.4 percent) and small (3.0 percent) 

operations (Figure 3). 
0

5

10

15

20

Small

(Less than 100)

Medium

(100 to 499)

Large

(500 or more) All Ops.

7.8

5.4

3.0

6.6

Figure 3. Percent of Operations that had a Single Bulk 

Tank Milk Sample Test Positive for Listeria 

monocytogenes, by Herd Size

Percent

Herd Size (Number of Dairy Cows)

________________________________________________________ 

The Southeast region had a higher percentage of 

operations (8.3 percent) with bulk tank samples that 

tested positive for L. monocytogenes than the 

Northeast (5.4 percent of operations), West (2.9 

percent of operations), or Midwest (2.7 percent of 

operations) regions (Figure 4). 

________________________________________________________ 

0

5

10

15

20

West Midwest Northeast Southeast

2.9 2.7

5.4

8.3

Percent

Region

Figure 4. Percent of Operations that had a Single Bulk 

Tank Milk Sample Test Positive for Listeria 

monocytogenes, by Region

________________________________________________________ 

Conclusions 

 Due the abundance of Salmonella on dairies, 

elimination of the organism from the environment or 

from cows is not practical. Specific 

recommendations to control Salmonella on dairy 

operations are difficult to provide. Results of the 

NAHMS Dairy ’96 study, the predecessor of Dairy 

2002, indicate that use of flush water systems, 

feeding brewer’s products, large herd sizes, and 

region of the country were associated with fecal 

shedding of Salmonella. 

In other studies, factors associated with 

Salmonella on dairy operations include addition of 

replacement animals without testing them, failure to 

routinely test feed components for Salmonella, poor 

control of wild birds and rodents, and inadequate 

sanitation in calving and calf-rearing areas. 

Improvements in any of these areas will reduce the 

risk of Salmonella transmission and decrease 

transmission of many other fecal-borne pathogens. 

Adherence to strict hygienic milking practices will 

prevent most Salmonella from entering bulk tanks. 

 Similarly, it is not possible to remove all Listeria

organisms from dairy environments, but reducing 

exposure to cattle can be accomplished by feeding 

properly fermented silage and reducing contact with 

manure. Listeria have been shown to stick to 

stainless steel surfaces in milking systems, 

indicating that strict adherence to proper cleaning 

and sanitization procedures of milking systems is 

key to keeping Listeria out of bulk tanks.4

Eliminating Listeria from milking systems should be 

a goal of all producers. 

References 

1. Hassan L, Mohammed HO, McDonough PL. Farmmanagement and milking practices associated with 

the presence of Listeria monocytogenes in New 

York state dairy herds. Prev Vet Med 2001; 51(1-

2):63-73. 

2. Smith BP, Da Roden L, Thurmond MC, Dilling GW, 

Konrad H, Pelton JA, Picanso JP. Prevalence of 

salmonellae in cattle and in the environment on 

California dairies. J Am Vet Med Assoc 1994; 

205(3):467-471. 

3. Rowan NJ, Anderson JG. Effects of above-optimum 

growth temperature and cell morphology on 

thermotolerance of Listeria monocytogenes cells 

suspended in bovine milk. Appl Environ Microbiol 

1998; 64(6):2065-2071. 

4. Wong AC. Biofilms in food processing environments. 

J Dairy Sci 1998; 81(10):65-70. 
For more information, contact: 

USDA:APHIS:VS:CEAH 

NRRC Building B, M.S. 2E7 

2150 Centre Avenue 

Fort Collins, CO 80526-8117 

970.494.7000 

E-mail: NAHMSweb@aphis.usda.gov 

www.aphis.usda.gov/vs/ceah/cahm

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