The latest USDA APHIS National Animal Health Monitoring System Dairy 2007 report indicates that the percentage of Salmonella-positive dairy operations, based on individual culture of feces from healthy cows, has increased with each NAHMS Dairy study, from 21.1% in 1996 to 39.7% in 2007. The percentage of Salmonella-positive cows also has increased, from 5.4% in 1996 to 13.7% in 2007.

The NAHMS Dairy 2007 report says that Salmonella and Campylobacter organisms are recognized as the two most common bacterial causes of foodborne illness in humans in the United States. Each year in the United States, Salmonella spp. are estimated to cause 1.4 million cases of disease with 500 deaths and Campylobacter spp. are estimated to cause 2.5 million cases of disease with 100 deaths. Both of these pathogens can cause fever, abdominal cramping, and diarrhea in humans. Severe cases can result in systemic infections and death.

Salmonella spp. also can cause disease, occasionally leading to death, in cattle. Clinical signs of salmonellosis in cattle include fever, diarrhea, anorexia, abortion, and decreased milk production. Cattle can shed Salmonella in their feces during and after episodes of clinical disease or without showing any clinical signs.

“If one looks back at some early studies done many years ago, using very sensitive techniques such as serology to detect ELISA antibody against Salmonella in dairy cows, the percentage of dairies with evidence of Salmonella was very high, running about 75%,” says University of California-Davis Professor Emeritus Bradford Smith, DVM, Dipl. ACVIM. “So it appears we are doing a better job now of detecting the presence of Salmonella on dairies in these recent studies.”

Smith says it is also possible that the actual prevalence on dairies is increasing as well. “The survey studies detect all Salmonella, and many of the serotypes and strains detected in surveys do not tend to cause significant disease, so there is a difference in reporting isolates from ill cattle rather than survey isolates.”

NAHMS examined Salmonella occurrence in cows on dairy operations in Dairy 1996, Dairy 2002 and Dairy 2007. Table 1 presents results from these three NAHMS studies. In each study, fecal samples were collected from cows that were healthy, sick, or soon-to-be-culled at the time of sampling. The results in table 1 are limited to healthy cows because these populations were comparable across the three studies.

For Dairy 2007, the percentage of positive operations was almost double that of Dairy 1996, and the percentage of positive cows more than doubled over the same time period. Slight differences in sampling methodology, such as the number of operations sampled, might account for some of the differences among the three studies. Salmonella might be becoming more common on U.S. dairies, however.

“Every year we make genetic improvements in our dairy cows, so that over time the quantity of milk per lactation goes up,” Smith notes. “That places more dietary and metabolic demands on the cows, and perhaps makes them more susceptible to colonization of the gut with Salmonella (and Campylobacter). Add that to the fact that our U.S. dairies are evolving to have ever greater numbers of cows per dairy, and there is an opportunity for a given bacterial strain to rapidly colonize a large number of animals.”

Herd size differences

Dairy 2007 indicates that herd-size differences were more pronounced in the East region than in the West region. Overall, large operations were more likely to be Salmonella-positive, with 61% of operations with 500 or more cows being Salmonella-positive compared with 41.5% of operations with fewer than 500 cows.

It appears that stress on cows and opportunity to spread from cow-to-cow is greater on large dairies, Smith says. “One sees less illness in the cows on farms that are really managed at a high level to minimize dietary and metabolic stress, and healthier cows tend to be less often colonized by potential pathogens. It would be interesting to see if the data bore out this thesis.”

Three serotypes — Meleagridis, Montevideo, and Mbandaka — ranked in the top six for each of the three NAHMS dairy studies. Of these three serotypes, S. montevideo has been among the top 10 serotypes identified from humans in every year from 1996 through 2006, the last year for which results are available. S. meleagridis and S. mbandaka were not among the top 10 serotypes in humans for any year from 1996 through 2006.

Salmonella shows little resistance

Now for the good news. Salmonella isolates have shown relatively little resistance to antimicrobial agents in the three NAHMS Dairy studies from 1996 to 2007. Campylobacter jejuni isolates collected during NAHMS studies have also shown little resistance to antimicrobials, with the exception of tetracycline (see Campylobacter sidebar).

Of all Salmonella isolates tested for antimicrobial susceptibility, 88.9, 83.0, and 92.8% of isolates were susceptible to all antimicrobials in the three respective tests. In each study, about 5% of Salmonella isolates were resistant to two or more antimicrobials.

“That is very encouraging news and reflects well on the dairy industry and the veterinary profession, as well as government regulators who make and enforce the rules concerning antimicrobial use in food-producing animals,” says Smith of this recent data. “However, when it comes to those few isolates that are causing actual disease on dairy farms, one tends to find that the responsible strains tend to be multi-drug resistant. In other words, the very pathogenic strains of typhimurium and newport are often resistant to a number of antimicrobials, so susceptibility testing is a must when disease outbreaks occur.”

Resistance to tetracycline was most commonly observed in each of the three NAHMS Dairy studies. Resistance to amikacin, ciprofloxacin, and nalidixic acid was not observed in any of the studies. Dairy 2007 was the first study in which resistance to ceftriaxone was observed, but it was observed in only a single isolate. Resistance to ceftriaxone in cattle is of interest because it is commonly used to treat severe Salmonella infections in children.

In Dairy 1996 and Dairy 2002, more S. typhimurium isolates were resistant to multiple drugs than other serotypes. In Dairy 2007, however, only one multidrug-resistant S. typhimurium was observed. Dairy 2007 is the first NAHMS Dairy study in which multidrug resistance has been observed in S. montevideo, which has been one of the top three serotypes identified in each of the previous NAHMS studies.

What is the Salmonella source?

Whether it’s a large or small dairy, introduction of Salmonella can come from introduced cows, so a closed herd has some advantages. But, Salmonella are most frequently introduced in feeds, according to Smith. “The mill or trucks can be contaminated, or rodents/birds can introduce Salmonella to the feed,” he says. “A high percentage (over 40% in most reports) of Salmonella contamination in rendered protein sources makes these items potentially dangerous.”

If the moist feed is handled poorly (or sits in bunks for days without being removed) the small numbers of Salmonella originally in the feed can multiply rapidly (doubling every 20 minutes on a hot summer day). “Salmonella colonization and infection rates are dose-related, so the number of organisms to which the cow is exposed is a very important factor,” Smith adds.

Overall sanitation and feed handling as well as how well cows are managed to prevent dietary and metabolic stresses play major roles in whether or not Salmonella spreads on that farm and whether it causes a disease outbreak. Most likely the risks are similar for Campylobacter, Smith says.

It seems likely that as long as we have bacteria, cows and humans, we will remain in competition with the microorganisms, and mankind will have to remain vigilant in order to stay ahead of the microorganisms, says Smith. “We have a good modified live vaccine for use in calves against Salmonella dublin, and it seems likely that we will continue to see advances in vaccines as an aid in minimizing the effect of Salmonella and perhaps Campylobacter.”

Smith notes that the veterinarian needs to be part of the production team, and help design preventive programs that are science-based and futuristic. “As a profession, we cannot wait for problems and then simply react, even if we react effectively. Veterinarians need to sell good management practices to their dairy clients for that farm to thrive. We also need to actively participate with researchers in vaccine trials, feed trials and tests of management strategies so that we can objectively evaluate them.

“Adopt those that have merit and discard those ideas that do not bear up under objective scrutiny,” Smith continues. “Our clients need to understand the importance of this scientific endeavor to their future and make the involvement of their veterinarian part of his/her paid duties.” 

Campylobacter prevalence/resistance

Previous NAHMS studies have found Campylobacter to be present on most U.S. dairy operations. Campylobacter data presented in the NAHMS report are for C. jejuni andC. coli, which are most commonly associated with human disease. These Campylobacter species are not important as disease-causing organisms in cattle. In the past, foodborne transmission of Campylobacter to humans was attributed primarily to handling and consumption of contaminated poultry meat. Molecular subtyping suggests, however, the role of nonpoultry sources of human infection is underestimated.

In Dairy 1996, Campylobacter was detected in at least one healthy cow on all sampled operations In Dairy 2002, culture methods found that 97.9% of operations sampled had at least one healthy cow shedding Campylobacter in its feces. In Dairy 2007, culture methods found that 92.6% of 121 operations had at least 1 healthy cow shedding Campylobacter in its feces, and all positive operations had at least 1 healthy cow shedding C. jejuni. Of the 1,885 healthy cows tested in Dairy 2007, 635 (33.7%) were positive for Campylobacter.

Antimicrobial-resistance testing was conducted on Campylobacter isolates from Dairy 2002 and Dairy 2007. In Dairy 2002, one-half of the C. jejuni isolates were susceptible to all antimicrobials against which they were tested, while in Dairy 2007, 36.6% of the C. jejuni isolates were susceptible to all antimicrobials.

Ciprofloxacin and erythromycin are important antimicrobials because they are often used to treat humans infected with Campylobacter. Very few of the C. jejuni isolates were resistant to ciprofloxacin or erythromycin in the Dairy 2002 and Dairy 2007 studies. Tetracycline had the highest percentages of resistant isolates, with 47.4% and 62.9% of the C. jejuni isolates from Dairy 2002 and Dairy 2007, respectively, showing resistance.