It is likely that fresh cows and calves are easy targets for Salmonella because it is an opportunistic organism.

“If you look hard enough on any dairy, you’re going to find Salmonella,” says Bradford Smith, DVM, PhD, noted Salmonella researcher at the University of California-Davis.

That’s not to suggest, however, that veterinary practitioners or their clients should become complacent about the presence of Salmonella in dairy herds. “It is not acceptable to ‘write off’ Salmonella as an uncontrollable, necessary evil of dairying,” cautions Smith. “Left unmanaged, it may cause serious physical and financial losses to dairy herds, and it is a threat to human health, regarding both workers on the dairy and the human food supply.”

Human, dairy prevalence
In fact, Salmonella is one of 10 major foodborne pathogens continuously monitored by the Centers for Disease Control and Prevention (CDC) in Atlanta. In the most recent annual summary of this data (2002), Salmonella climbed the ranks to become the number one human foodborne pathogen in terms of laboratory-diagnosed human infections. CDC estimates that about 1.4 million cases of human Salmonella infections occur each year; more than 500 are fatal.

Unfortunately, the news is similar for dairy cattle. The National Animal Health Monitoring System’s (NAHMS) Dairy 2002 report indicates that randomly collected fecal samples showed that Salmonella was recovered from 7.3 percent of tested animals, compared to 5.4 percent in 1996. In the same time period, the overall percentage of dairy operations reporting at least one Salmonella-positive cow increased from 27.5 percent in 1996 to 31.3 percent in 2002.

With the on-farm prevalence on the rise, approximately parallel to that of human clinical cases, Salmonella clearly is an organism that deserves the attention of dairy practitioners and their clients.

Wreaking havoc on the dairy
If you haven’t already worked through a clinical outbreak of Salmonella on one of your client’s dairies, “it’s probably only a matter of time before you will,” says Bruce Vande Steeg, DVM, Sierra View Animal Health, Escalon, Calif. Vande Steeg has seen his share of outbreaks in his northern California practice. “The severity has ranged from mild diarrhea in cows or calves to death losses as high as 10 percent in the milking herd,” he reports. “When cows start lying down and dying, it becomes a very depressing situation. It’s a strain on labor, and the dairy’s assets evaporate quickly.”

Indeed, dairies lose money when Salmonella strikes. In addition to mortality in adult dairy cows, losses are incurred via treatment costs, increased cull rates, reduced feed efficiency, higher labor costs and lower milk production. Calf mortality and morbidity also add to the total.

“Fresh cows and baby calves are the most susceptible animals to Salmonella on the dairy,” says Smith. Vande Steeg’s clinical experience confirms the fresh-cow phenomenon. “On our dairies with clinical outbreaks in adult cows, it almost always has happened first in the fresh pen,” he notes. “It can start a day or two before calving and up to about 20 days after.”

The clinical signs Vande Steeg has observed match those listed in other Salmonella case studies – depression, dehydration, fetid diarrhea, vaginal discharge, fever (106°-108°F), anorexia and decreased milk production. The organism often gains a foothold when a cow suffers another setback in the fresh period, such as a displaced abomasum (DA) or severe metritis. Or, a very highly infective dose of Salmonella can initiate the problem, knocking fresh cows off feed and setting off a cascade of subsequent fresh-cow complications, including sub-acute rumen acidosis, ketosis, metritis, septicemia and DAs. “Another unfortunate side effect is that those DA cows are not strong enough to tolerate surgery, so they often die or have to be culled,” says Vande Steeg. 

Calves with clinical Salmonella infections can suffer from diarrhea, fever, lethargy and inability to rise. Vande Steeg says the scours calves have are characteristically watery, fetid, yellow in color and may be tinged with blood and contain a fibrin or mucous cast. Infected calves also can become septicemic. In fact, Smith notes that ill calves sometimes are misdiagnosed with pneumonia, because the advanced nature of the infection can cause panting and shallow breathing. This is especially true for Salmonella infections.

Etiology explained
It is likely that fresh cows and calves are easy targets for Salmonella because it is an opportunistic organism. “The infective dose is much lower in immunocompromised animals,” explains Vande Steeg. “Fresh cows are undergoing a number of changes, and their rumen fauna can get out of balance. Young calves have naïve immune systems, so both groups are much more susceptible than other animals on the dairy.” For the same reason, heat stress often triggers whole-herd clinical outbreaks.

     

Bradford Smith, DVM, PhD, cautions that just because Salmonella can be found on virtually every dairy, it is not part of the normal environmental fauna.

A Salmonella outbreak can be a challenging opportunity to broaden your skills as a clinician, says California practitioner Bruce Vande Steeg, DVM.

Chuck Fossler, DVM, PhD, says keeping Salmonella populations low on dairies is important to protect herd health and safeguard human health and food safety.

This point again draws attention to the ubiquitous nature of Salmonella on dairies. Simply put, when the window of opportunity opens, Salmonella usually is there. One of the most recent Salmonella surveillance projects in the United States was a study of 129 organic and conventional dairies in Minnesota, Wisconsin, Michigan and New York, which were sampled five times over a one-year period to check for the presence of Salmonella in:

  • Fecal samples from cows and calves
  • Bulk-tank milk
  • Milk filters
  • Water
  • Feed
  • Floors from calving and sick pens

Chuck Fossler, DVM, PhD, analyzed data from the study for Salmonella as part of his graduate studies at the University of Minnesota. Fossler reports that 93 percent of the farms had at least one Salmonella-positive isolate, but 25 percent of farms accounted for 75 percent of the positive samples.

“These findings verify that the prevalence of Salmonella organisms can vary greatly from one dairy to the next,” notes Fossler, who is now an epidemiologist with USDA’s National Surveillance Unit of the Animal and Plant Health Inspection Service (APHIS), Ft. Collins, Colo. “Although we did not evaluate clinical incidence of the disease, it is likely that dairies with the highest environmental population also had the most clinical challenges with the disease, simply because the potential for exposure for their animals was so much higher.”

That exposure can take many routes. Salmonella organisms can be transmitted via the fecal-oral route, nasal secretions and saliva, milk and in-utero. The incubation period is a short one to four days, and the duration of clinical disease is typically one to seven days. Smith says clinically infected animals shed the organism in exponentially higher numbers than asymptomatic animals, and the duration of shedding (which usually lasts six to 12 weeks) has been documented for up to 18 months, even from clinically unaffected animals. He adds that warm, moist conditions are ideal for environmental growth of the organism, and it can survive for several months in the environment. 

Another challenge is the fact that virtually all living organisms can harbor Salmonella, including humans, rodents, pets, birds, insects and reptiles. Based on numerous field investigations of Salmonella outbreaks on dairies, Smith identifies the following vectors that he has identified in the spread of the organism on dairies:

  • Direct fecal contamination of feed or water sources: cows or calves with diarrhea are excreting billions of Salmonella
  • Wildlife, including rodents, birds and feral cats, causing fecal contamination of feed
  • Heavy fly infestations, especially in calves
  • Feeding unpasteurized colostrum and/or waste milk from infected cows to calves
  • Recycled flush water
  • Livestock transport trucks
  • Rendering trucks
  • Using farm equipment, such as skid-steer loaders, to handle feed after contact with manure and/or dead animals
  • Feedstuffs, such as cottonseed and green chop, contaminated with animal manure slurry or human sewage effluent

“We know that we have to protect humans from animals, but we also have to protect animals from people,” notes Smith. “Human sewage is an often overlooked cause of Salmonella contamination on farms in the southwestern United States.”

Herd size, seasonality and geography also can play a role in Salmonella incidence rates. Fossler’s research revealed that herds of 100 cows or more were more likely to have Salmonella on the premises, and that cattle were more likely to be Salmonella-positive in late summer (July through October). Vande Steeg adds that he has witnessed clinical outbreaks during all seasons, but outbreaks tend to occur during climatic extremes, such as heat waves or long stretches of damp, foggy, cool weather.

 

Farm equipment, such as skid-steer loaders used to moved manure and/or dead animals, can transmit Salmonella and other pathogens when handling feed.

Little research has been performed to directly evaluate regional incidence of clinical Salmonella outbreaks, but dairies in warmer climates appear to have been more widely affected in the past, perhaps in part because they tend to have larger dairies, and the bacteria can thrive and multiply more readily in the absence of harsh winter cold. However, Salmonella appears to be a growing problem on dairies in the Midwest and Northeast. Vande Steeg theorizes that this may be due in part to a trend toward larger herds that handle their manure in mass storage systems that do not freeze, even in the winter.

Cleaning house
Treatment of clinical Salmonella cases is the critical first step in controlling an outbreak. Smith and Vande Steeg concur on the following treatment protocol for calves and adult dairy cows (at appropriate doses for age and body weight):

  • Oral and/or intravenous fluid therapy
  • Anti-inflammatory therapy
  • Antimicrobial treatment, often with ceftiofur, florfenicol or ampicillin 

Smith notes that ongoing antimicrobial susceptibility testing is important, because some Salmonella strains are resistant to many antibiotics, and resistance patterns also change over time.

A complete diagnostic work-up needs to be performed concurrently with treatment so the root cause of the outbreak can be addressed. Vande Steeg says that starts with thorough physical exams of ill animals, and a walk-through of the dairy to ascertain sanitation. He looks for any recent changes in feedstuffs or management, new animals on the farm, recent cattle movement, etc. He also routinely performs cultures of organ tissue from on-farm necropsies, plus fecal samples from affected animals. Additionally, he may submit feedstuffs and samples from the dairy’s environment to further pinpoint the source of the problem. He also shakes rations to determine if fiber is adequate, because rumen acidosis is a primary condition that can allow Salmonella to entrench itself. In some cases, Vande Steeg may request serovar identification to add to his arsenal of information (see Pathology sidebar).

The practitioner notes that complete recovery in a herd may take a year or longer. Follow-up fecal sample culturing from randomly identified animals (or animals in a specific category, such as calves or fresh cows) may be advisable at an interval of about every two months until the situation is resolved. The purpose of this monitoring is to monitor whether the same serovar of Salmonella is present or if the dairy is dealing with a new problem and/or a new source of infection.

Vaccines may or may not be effective in controlling Salmonella. Smith is reluctant to recommend vaccination as a first-line defense against the disease, noting that current commercial killed bacterins in calves have shown little efficacy, and that commercial and autogenous killed bacterins provide only short-term protection in animals older than 12 weeks of age. He adds that the multitude of Salmonella serovars makes it impossible for commercial vaccine manufacturers to provide complete protection, and little cross-protection is afforded between serovars. In his experiments and field experience, the modified-live Salmonella vaccine currently on the market is the most effective calf vaccine against Salmonella.

Vande Steeg notes that even autogenous vaccines developed specifically for a dairy require three to four weeks to manufacture, which is too long to wait to properly address an outbreak. “I’m much more in favor of going all-out to address the environmental factors contributing to the situation, then possibly adding an autogenous vaccine as another layer of protection,” he states.   

Smith and Vande Steeg advise the following management and biosecurity efforts to limit the growth and spread of Salmonella on dairies:

  • Do not house fresh cows and sick cows in the same hospital facility or pen and isolate cows with diarrhea.
  • Clean up obvious sources of contamination, such as contaminated feed and/or water. Do not feed or water animals off the ground.
  • Work to maintain dry bedding at all times.
  • Test feedstuffs to check for the organism and, if present, investigate the source of contamination.
  • Monitor dry-matter intake and clean feedbunks routinely, so cows are not forced to consume old feed.
  • Do not make management changes that will cause cows to go off feed.
  • Store feedstuffs in a cool, dry, shaded area.
  • Initiate control efforts to reduce population of insects and rodents.
  • Quarantine new animals introduced to the herd for at least four weeks.
  • Establish a work routine that dictates care of the youngest animals in the herd first.
  • Avoid commingling of animals from different age groups.
  • Pasteurize waste milk fed to calves (or feed milk replacer) and monitor the process to ensure consistent results.
  • Do not feed colostrum from infected cows to calves, and practice hygienic colostrum harvesting, including sanitary equipment and prompt chilling. Do not batch colostrum from multiple cows.
  • Regularly and thoroughly sanitize and replace calf-feeding equipment.
  • Use only clean irrigation water on crops used as non-ensiled feed or at least verify that proper fermentation has eliminated Salmonella from lagoon water used for irrigation. Ensilage of corn kills Salmonella.
  • Cull carrier cows identified with persistent titers for Salmonella dublin.
  • Limit visitors, and provide disposable boots to those who do enter the dairy (see sidebar).
  • Fossler also found that Salmonella was less likely to be present in herds that used medicated milk replacer and fed monensin in calf and heifer rations.

Advice to practitioners
It’s important to remember that the veterinarian’s role in helping manage Salmonella spans far beyond the farm level and into the human food supply.

USDA identified Salmonella as one of the target organisms for monitoring in large slaughter plants in 1998. “Still, preventing fecal contamination absolutely is difficult,” advises Fossler. “And, even with the additional protection of irradiation, we still have the responsibility to keep the organism in check at the farm level. Everything we can do to send fewer numbers of Salmonella down the road with animals to slaughter will help to ensure a safe final product.”

Vande Steeg also notes that helping a client work through a Salmonella outbreak can be a challenging but highly rewarding experience. “It will force you to use your resources, and work as a true clinician,” he says. “It takes time and communication, and sometimes learning new skills, to peel back the layers of the possible causes and solutions. The journey will likely last several months, but it can take you interesting places as a veterinarian. Best of all, when you have successfully resolved the problem, you have helped your client address a serious health challenge, while making improvements on the dairy that will yield a host of secondary benefits, as well.” 

Resource:

Salmonella serology can be ordered from the Livestock Salmonella Research Laboratory at the University of California-Davis by contacting Dr. Brad Smith at BPSmith@UCDavis.edu.

Wash your boots

Could Salmonella be transmitted on dairies via the boots of workers, visitors or – worst of all – the herd veterinarian?

A research team led by John Kirk, DVM, MPVM, and Bill Sischo, DVM, PhD, at the University of California-Davis, conducted a study in 2000-2001 to find out. Rubber boots were cultured for Salmonella after being worn in calving, hospital and fresh-cow pens on 27 selected dairies. The herds were selected based on either the occurrence of Salmonella in bulk-tank milk or recent evidence of clinical infections in adult cows.

New rubber boots were walked through the calving, hospital, fresh pen and barn alleys of participating dairies. Between six and eight culture samples were then swabbed from contaminated boots. In addition, boots from four dairies were recultured 48 hours after casually being hosed off on the farm. Bulk-tank milk cultures for Salmonella also were performed on all participating dairies.

Salmonella was isolated from boots of 12 of the 27 dairies. It also was identified on the boots from one of the four dairies where reculturing was performed after washing. On 92 percent of the dairies where Salmonella was cultured from boots, the same serotypes were found in bulk-tank milk and clinically ill cows. The researchers concluded that the organisms detected in the environment were biologically related to the Salmonella associated with the cows and not just environmentally adapted serotypes.

The lesson to veterinary practitioners is clear: boots should be thoroughly washed and properly disinfected prior to arrival at a dairy, and sanitized again before moving to another location on the dairy and before departing. Better still, purchase a pair of boots for each of your herds, sanitize accordingly while you’re working and leave them at the dairy.

Salmonella pathology pointers

There are thousands of different Salmonella serovars. Fortunately, not all of them cause disease in cattle, and even fewer cause illness in both cattle and humans.

John Adaska, DVM, MPVM, is the assistant branch chief at the California Animal Health and Food Safety Laboratory System–Tulare Branch. The lab is a reference center for field veterinarians and producers, providing bacterial cultures, histology, toxicology, antimicrobial sensitivity and serologic testing. They perform whole necropsies and evaluation of field necropsy samples.

Adaska outlines the following classification of the most common Salmonella isolates from dairies:

  • Group B. Salmonella typhimirium, including Salmonella typhimirium variant copenhagen and Salmonella agona. Both are very pathogenic in cattle, and Salmonella typhimirium can be a dangerous human pathogen as well.
  • Group C1. Most serovars in this group are of little consequence to cattle, except Salmonella montevideo.
  • Group C2. Salmonella newport falls into this group, and it can be an important cattle and human pathogen.
  • Group C3. Very few are ever isolated from cattle.
  • Group D1. Salmonella dublin is the most critical serovar in this group. Not only can it cause disease in both humans and cattle, but it also can be carried and shed by infected cattle for at least 18 months.
  • Group E. Many isolates are sporadically identified from this group, but they are not typically associated with clinical disease in cattle.

Other serogroups are less common and rarely cause clinical disease.

In the most recent Report of the Committee on Salmonella from the United States Animal Health Association (2003), the most frequently identified on-farm (including bovine, porcine and avian samples) serovars were Salmonella typhimirium, Salmonella heidelberg, Salmonella newport, Salmonella kentucky and Salmonella seftenberg. The 10 most common serovars accounted for 66 percent of the total isolates reported. In the same report, Salmonella typhimirium and Salmonella newport also made the list of top five most frequently isolated serovars in humans.

Adaska says the human-bovine connection between some serovars is one important reason to request serotyping of pathology samples. “In most cases, it’s not enough to simply know the problem is Salmonella,” he notes. “To properly manage the disease, you need serotyping data, as well.”

In addition to identifying human pathogens, he cites these benefits of serotyping:

  • Over time, serotyping allows for monitoring changes in the causative organisms on a dairy or calf ranch. A change in a herd’s pathogen profile could indicate a new source of infection that needs to be addressed.
  • Serotyping logically leads to the next step of antimicrobial susceptibility testing, which is important for effective therapy and prudent antibiotic use.
  • Diagnostic laboratories report their findings to national databases that can track trends in organism shifts for states, regions and the entire country. For example, “Salmonella newport was fairly common in the 1980s, virtually went away in the ’90s, then came roaring back a few years ago,” Adaska reports. “When it did, it happened all across the country at about the same time, and it had an entirely different antimicrobial susceptibility profile than before.”

Because all veterinary diagnostic labs do not automatically culture for Salmonella, Adaska suggests that practitioners request it specifically, either on the submission form or via direct communication with the pathologist. Serotyping also needs to be requested specifically and may take an additional week or longer to perform.

“Don’t be afraid to speak directly to the pathologist and discuss their perceptions, especially on tough cases,” Adaska advises. “Practitioners should not be intimidated by labs. They see a larger body of samples and may have useful insights that you can’t see at the practice level. Most importantly, they’re there to serve you.”

Keeping dairy workers safe

Salmonella is a potentially infectious pathogen for both animals and humans. Multi-drug-resistant strains in particular, such as Salmonella typhimirium DT104 and Salmonella newport are of particular concern. 

Bradford Smith, DVM, PhD, offers these tips for protecting dairy employees from Salmonella infections:

  • Encourage hand washing after working with animals, particularly calves. This is especially important before workers eat with their hands.
  • Provide coveralls and boots to workers and launder them regularly on the dairy. Alternatively, provide footbaths containing bleach or bactericidal disinfectant and encourage workers to thoroughly scrub their boots before going home.
  • Do not allow workers to consume unpasteurized milk from the dairy.