Salmonella spp. infection occurs when a susceptible animal ingests the bacteria. Dairy cattle ingest feed or water that has been contaminated with feces from animals shedding the organism.
Salmonellosis has a wide spectrum of manifestations in cattle. The manifestations vary with virulence of the strain, infectious dose and immunity of the host. On many dairies, salmonellosis is an opportunistic infection.
Recent large, multistate studies within the U.S. dairy industry suggest that approximately half of all herds will have at least one culture-positive cow on any one day and that approximately one in 20 apparently healthy dairy cows will be shedding Salmonella at any one time in the U.S. These numbers increase when researchers refine their sample populations to sick cows, cull cows and cows at slaughterhouses.
There appears to be geographic variation in the level of feedstuff contamination whereby temperate, higher rainfall dairy areas have higher levels of Salmonella contamination of forages and other ration components than do the harsher Northern climates in the mid-Western and Eastern U.S.
It is apparent from research that organic dairies do not differ significantly from larger freestall farms in the prevalence of environmental and individual cow and calf Salmonella infection. Environmental isolation of Salmonella organisms is frequently possible (about 40 to 50 percent of farms) even on farms with no known history of clinical salmonellosis.
Where does it come from?
On a dairy, the source of the infection is usually feces from infected cows. It may be difficult to tell which cows are shedding bacteria because asymptomatic and subclinically affected cows can shed as many organisms in their manure as the cows that are sick with salmonellosis. Other sources of infection may be rodents, birds (including waterfowl), flies, feral cats, dogs, raccoons and, rarely, people.
Why doesn’t it run its course and end?
Salmonella outbreaks commonly last several months. Protracted problems can be the result of a number of factors – persistence in the environment, persistence of risk factors, carrier state or prolonged shedding or reinfection of susceptible animals. On some dairies, particularly those with large numbers of cattle, the disease may become endemic.
Reservoirs include:
•feces
•feed
•water
•sick cows and asymptomatic shedders
The most recent studies show that approximately 5 percent of apparently healthy dairy cows may be shedding the organism in their feces and that approximately 20 percent of all sick cows on the cull list shed Salmonella spp. This makes the on-farm location of these cattle, as well as cattle with obvious diarrhea, very important because clinically affected animals may shed billions of organisms per day.
Survival outside animals
In the environment, it survives for four to five years in water, soil, dust, moist areas out of direct sunlight and on or within foods. S. dublin can survive in dry feces for over a year; however, freezing at -4ºF kills 85 percent of Salmonella spp. in two days.
Crops irrigated with Salmonella-contaminated wastewater will lead to contamination of forages and water sources. Rendering kills Salmonella; however, post-processing contamination accounts for 50 percent of contamination of rendered feed products.
Carrier animals
Infection with a host-adapted Salmonella strain (S. dublin in cattle) can result in a cyclic, endemic disease that is maintained on a farm by carrier animals shedding in the feces or milk. The carriers can shed constantly or intermittently.
Since Salmonella spp. can cross from one species to another, other potential animal sources include dogs, birds, cats, people and pigs. Flies also pose a risk for spread.
Feces and oropharyngeal secretions on the following can be significant sources of cross contamination between cattle:
•medication equipment: esophageal feeder, stomach tubes, buckets, stomach pumps
•calf nipples
•nose tongs
•exam gloves
•coveralls
•boots (walking through feed areas)
•calves suckling•equipment: scoops and loaders that handle manure, wheels track, manure from pen to pen, buckets
Risk factors
•more frequent in dairy herds than beef herds, mixed dairy and beef herds and calf herds
•outbreaks more common in calving season
•more common in summer months
•outbreaks more common in large herds
•purchasing cattle from dealers rather than source herds
•expansion
•confinement
•sick and calving cows comingled
•high density of feral cats
•wild birds having access to feed storage facilities
•antimicrobial use prior to or at the time of exposure
•use of flush water systems
•feeding brewers’ products, animal bypass protein sources, vegetable or other fat sources to lactating cows
•allowing commodity storage areas, particularly those that drain poorly or can retain moisture, to become wet
Clinical signs
•pyrexia
•lethargy, depression
•decreased milk production
•anorexia
•dehydration
•increased salivation
•diarrhea progressing to dysentery
Treatment (or fighting your way out of an outbreak)
Supportive care applied early is the most effective in limiting the course and severity of disease.
•fluids – oral or IV, with electrolyte supplementation
•NSAIDs
•probiotics
•prudent (maximize the therapeutic effect while minimizing the development of resistance) antimicrobial use
Will antibiotics make a difference? In adult cattle, antibiotics may improve recovery and lessen the severity of the disease but will likely extend the duration of shedding. Calves and young stock are especially prone to becoming bacteremic, but subtherapeutic use of antibiotics in the milk replacer may actually make the disease worse.
Opportunist or primary pathogen?
In dairy cattle, salmonellosis commonly occurs close to parturition. In this circumstance, it is frequently opportunistic, riding on the coat tails of concurrent disease, dietary stress and the natural depression of immunity at this time of the cow’s cycle.
Control strategies
The strategies implemented should be prioritized and focused on minimizing the source of infection and maximizing host immunity.
•Adopt an all-in, all-out system in calf and heifer raising facilities.
•Maintain a closed herd or make purchases from low-risk herds.
•Manage new additions to minimize stress and infection of residents.
•Minimize stress by feeding good rations, providing adequate time and space for transitions and maintain clean, uncrowded maternity pens.
•Use different facilities for calving cows and sick cows.
•Avoid adult-to-calf contact. Isolate heifers from the lactating herd.
•Disinfect waterers in high-risk areas (dilute bleach twice daily).
•Scrape manure, remove organic debris, disinfect clean, non-porous surfaces and expose to sun or UV light.
•Minimize fecal contamination of feedstuffs, feeding surfaces, water troughs and equipment.
•Drain and level areas that collect water.
•Allow no access to pond water or feeding areas cohabited by birds and waterfowl.
•Isolate the entire group in which affected cows commingle.
•No shared bunk spaces, water source, feeding or manure-handling equipment. Leftover TMR from the cows should not be fed to the heifers.
•Segregate Salmonella test-positive cows at calving.
•Do not use colostrum or milk from test-positive cattle.
•Manure-handling equipment is not used to handle feed, and it is kept out of feed lanes or food storage areas.
•Make certain that feed delivery vehicles do not travel through manure or across manure-scraping lanes.
•Control rodents, birds and feral cat populations.
•Be vigilant about waste management, control of effluent and the distribution of recycled flush water.
•Vaccination will not stop infection, but selected vaccines may reduce the severity of infection and curtail the mortality rate.
•Pasteurization of waste milk and colostrum, even refrigeration may limit growth of salmonellae in contaminated colostrum and waste milk.
From the AABP Food and Water Symposium, 11 action steps were suggested to tackle herd salmonellosis:
1. Break the fecal-oral transmission link by minimizing fecal contamination of feedstuffs, feeding surfaces, water troughs and equipment.
2. Maximize host resistance of susceptible animals (transition animals and newborns) and minimize exposure dose.
3. Control anything in the livestock environment that can perpetuate the organism – rodents, flies, nuisance birds, feral dogs and cats.
4. Because many of the infected animals are subclinical, in an outbreak, handle all animals as if they were shedding.
5. Implement a sound sanitation program based on cleaning all organic matter – feces, saliva, milk and blood – prior to use of disinfectants (orthophenylphenol on surfaces and boots and chlorhexidine for equipment).
6. Look for development of newer vaccines that target signaling pathways and other unique strategies rather than relying on conventional bacterins for prevention and control.
7. A healthy intestinal environment gives cattle a competitive resistance.
8. Maximize rumen function by consistent dry matter intake (DMI) in transition and parturient cows. VFA’s are toxic to salmonella.
9. Recognize extended survival time of salmonellae in the environment and deal with potential for spread four to five years after outbreak.
10. Minimize the chance for salmonellae to replicate by minimizing time in moist, warm environmental conditions. Mix feeds in smaller batches and feed soon after mixing. Don’t feed the waste TMR to young stock.
11. Warn farm families about zoonotic potential and assist them in implementing these steps to minimize the risk.
Avoiding zoonoses
•outer layer of protective clothing is left at farm or at the site of contamination
•gloves and protective wear
•hand washing
•no eating or drinking in work areas
•don’t drink raw milk PD
References omitted but are available upon request at editor@progressivedairy.com
—Excerpts from 2006 Vita Plus Dairy Summit Proceedings
Simon Peek
Veterinarian
Should dairy producers be watching for salmonellosis particularly in dry, transition and fresh cows?
The late dry period, calving and post-fresh periods are highly susceptible time points during which heifers and cows can acquire new Salmonella infections. The calving pen/maternity areas are also highly relevant from the perspective of transmission of Salmonella to neonatal calves and, therefore, become an important focus for management in the control of Salmonella on problem dairies.
Immunologically, cattle are more susceptible to the acquisition of many infectious diseases during the transition and immediate postparturient period, so limiting risk and exposure at this time is critical. Several studies, alongside clinical experience, would suggest that exposure to Salmonella is fast becoming a given for cattle on many dairies in the U.S., so for many herds it becomes an issue of maximizing overall health and minimizing exposure at these times rather than eradicating the disease or reducing the exposure risk to zero.
