The prevalence of salmonella on dairy operations can be relatively high even though animals aren’t exhibiting any clinical signs of salmonellosis. Reducing the burden of salmonella in dairy cattle is an important first step in improving productivity and profitability.

 

According to recent studies, the prevalence of salmonella bacteria in dairy cows and calves across the nation is increasing. An Ohio report indicated that 38 percent of the herds being tested for Johne’s disease had cattle that were shedding salmonella bacteria. The percentage of cattle shedding within each herd varied from 1 percent to 97 percent with an average of 15 percent of the cows carrying the bacteria. Veterinarians in California estimate that 95 percent of the herds in the state would test positive for salmonella. And, the situation is similar in herds in the Upper Midwest.

 

The economic impact of a clinical salmonella infection is significant and can be devastating to a dairy producer. Clinical signs include fever and watery diarrhea which can lead to rapid dehydration. In severe cases, damage to the intestinal wall allows the bacteria to enter the bloodstream, leading to septicemia, acute diarrhea and death. Undetected subclinical infection can also be costly in terms of decreased milk production and poor production efficiency.

 

Salmonella outbreaks can occur within herds at any time, especially when stresses such as summer heat, periods of high milk production or shipping compromise the immunity of the animals and make them more susceptible to bacterial infection. Studies by the National Animal Health Monitoring System (NAHMS) indicated that salmonella shedding in dairy calves increased six-fold in the summer heat.   Often, salmonella outbreaks start when a late-term heifer or cow with subclinical infection is brought into the herd, and the stress of shipping or the new environment can cause her to break with the disease. An additional concern is the ease with which salmonella bacteria can be transmitted across species from cattle to humans.

 

Regular vaccination is an important step in eliminating both clinical salmonellosis and subclinical shedding of salmonella. However, with more than 2,400 serotypes of salmonella bacteria, it is virtually impossible for dairy producers to protect their herds with traditional, whole-cell vaccines containing antigens against just one or two serotypes. Additionally, effective treatment protocols for salmonellosis have been difficult for several reasons:

 

• Salmonella bacteria can remain viable in the environment for as long as a year if they are not exposed to direct sunlight. Temperatures above 80°F cause the salmonella that may be present in sources of contamination such as feed, water and pen floors to multiply rapidly.

 

• Cows may not exhibit clinical signs of salmonella disease but may be carriers and intermittently shed large numbers of the bacteria. These animals serve as a reservoir for future outbreaks of the disease within the herd.

 

• All serotypes of salmonella are capable of causing infection in susceptible animals if bacteria are present in sufficient numbers.

 

• And, most recently, infection with Salmonella Newport, one of the 2,400 serotypes of salmonella bacteria, has become much more common, in part, because of the resistance it has developed to many of the antibiotics that are available for use in dairy animals.

 

SRP Salmonella Newport Bacterial Extract vaccine was developed to aid in the control of disease and fecal shedding caused by infection with Salmonella Newport in healthy cattle. Since its introduction more than five years ago, a significant number of dairy producers are routinely using this vaccine to aid in the control of salmonellosis.

 

SRP technology has changed the way salmonella vaccine is produced and, as a result, has helped overcome the obstacles that previously confronted veterinarians and producers as they attempted to manage their salmonella prevention programs. SRP technology utilizes specialized proteins on the outer membrane of the bacteria, known as siderophore receptors and porins (SRPs), highly conserved proteins common to multiple strains of salmonella bacteria.

 

Vaccines produced with SRP technology utilize SRPs, specialized iron-acquisition proteins found on the outer membrane of the cell wall in nearly all Gram-negative bacteria. Salmonella bacteria, like other Gram-negative bacteria, must have an adequate supply of iron if they are to survive, grow and reproduce. To acquire the necessary iron from the host animal (or person), the bacteria send out siderophores to “steal” the iron that is bound to other proteins in the animal’s system. The siderophores carrying this iron re-enter the bacteria cell through the special siderophore receptors or porins on the outer membrane of the bacteria. Once inside the cell, the iron is released and is available for cellular metabolic functions. 

 

SRP vaccines are made by culturing salmonella in an iron-restricted environment, causing the bacteria to develop siderophore receptors and porins. The SRPs are then harvested and separated from the extraneous cellular material leaving a very pure extract of SRPs that is virtually free of lipopolysaccharides (endotoxins) found in traditional vaccines produced with whole bacteria cells. The SRPs are then put in solution with an appropriate adjuvant and are ready for use as a vaccine in animals.

 

When an animal is vaccinated with SRPs, it recognizes these proteins as antigens and produces antibodies as part of its normal immune response. These antibodies are then available in the host animal’s immune system, ready to defend the host animal from a challenge by salmonella bacteria.  The antibodies respond to a salmonella infection in the host animal’s digestive system by locking on to, or blocking, the siderophore receptors and porins on the cell wall of the ingested bacteria, rendering them useless for transporting iron. As a result, the salmonella bacteria in the host become iron-deficient and are literally starved to death.

 

Sideophore receptors and porins are what microbiologists call “highly conserved” attributes or properties of bacteria cells. Because their function is critical to the life of the bacteria, nature has caused them to remain nearly identical in structure and function even as bacteria have evolved and different serotypes or strains have developed. In other words, the SRPs on one strain of salmonella are very similar to the SRPs on other strains of salmonella. Consequently, the antibodies that the animal develops recognize the challenge from SRPs from multiple salmonella serotypes and then block their iron-transporting function. 

 

Once the host animal has been administered an SRP vaccine, its system is primed to recognize the challenges from SRPs that it may encounter in the future. Once primed, the constant exposure to salmonella found in dairy barns causes a gradual increase in SRP antibodies which provide broad protection between annual vaccinations.

Serology and fecal shedding studies have demonstrated that vaccines developed with SRP technology are highly efficacious. Case studies on commercial dairies have further proven the effectiveness against salmonellosis and fecal shedding of salmonella bacteria. Researchers believe that potential benefits in production parameters may also be derived from effective control of both clinical and subclinical salmonella infections.

 

Researchers at Kansas State University studied the effects of SRP Salmonella Newport Bacterial Extract vaccine on salmonella shedding, somatic cell count and milk production. The study included 180 Holstein cows and heifers in a 1,200-cow confinement dairy operation. The dairy did not have a history of salmonellosis in adult cows but they had experienced some salmonellosis in neonatal calves on several occasions. The producers had not done any diagnostic work to determine the serotype of salmonella bacteria that had caused the problem.

 

The animals in the study were randomly assigned in pairs to two groups – Group A which was vaccinated with SRP vaccine according to label directions, and Group B which received a placebo vaccine with adjuvant only. The first dose of the vaccine and the placebo was administered 45 to 60 days prior to calving. Both groups were revaccinated with a second dose of their treatment 14 to 21 days before calving.

Daily milk production was monitored for the first 90 days in milk (DIM). Daily milk production for each cow was monitored using an electronic recording system that weighs milk on a continuous basis for precise measurement. In addition, DHIA personnel recorded the weight of the milk produced per cow every 45 days.

 

Cows vaccinated with SRP vaccine produced 2.5 lbs more milk per day than cows receiving the placebo treatment, thus in a standard DHIA 305-day lactation, SRP-vaccinated cows would produce 762.5 more pounds of milk than placebo-treated animals. 762.5 more pounds of milk means an extra $100 or more on the bottom line.

 

Milk samples were taken for somatic cell counts on the first day in milk, at 30-60 DIM and at 60-90 DIM. Somatic cell counts were measured by DHIA personnel using flow cytometry. Somatic cell counts were significantly reduced in SRP vaccinates compared to controls at the 30-60 DIM measurement and numerical reductions other DIM measurements as well.

 

Fecal samples for salmonella isolation and blood samples to detect salmonella antibodies were collected at the beginning of the trial, 7 to 14 DIM and again at 28 to 35 DIM. Fecal samples were analyzed at the Kansas State University Veterinary Diagnostic Laboratory for isolation and to determine serotype of the bacteria found. Serum from the blood samples was sent to a private laboratory where ELISA testing was used to determine antibody response to the SRP vaccine. 

 

There were no Salmonella Newport bacteria recovered from any of the fecal samples; however, S. Angona was recovered from 20.3 percent of the animals. There were numerical reductions in the number of bacteria recovered from the vaccinated animals; however, the differences were not statistically significant. This could be attributed to the fact that the entire 1,200-cow herd on the commercial dairy (except the 78 in the control group) were vaccinated on day one of the trial.

 

Vaccinating healthy cows with SRP vaccine prior to calving increased milk production even in the absence of detectable S. Newport or clinical salmonellosis. SRP vaccine also decreased somatic cell counts and caused a positive antibody response. The significant increase in milk yield and the resulting improvement on the bottom line, make vaccinating with SRP vaccine and important and profitable part of a salmonella management program.