- Purpose and Note on Changing Salmonella Nomenclature
- Tackling Herd Salmonellosis Problems (11 factors to keep in mind)
- On-Line Resources
Purpose and Note on Changing Salmonella Nomenclature
The purpose of this webpage is to provide students and practitioners a quick overview of Salmonella enteritica and some pointers on dealing with herd outbreaks of salmonellosis, and links to other resources. This webpage is not intended to provide herd-specific advice and does not reflect the current state of the veterinary literature.
Changing Salmonella Nomenclature
Molecular biology research is providing a better understanding of the organism, one result being a better understanding of how the various serotypes are genetically related. While the serogrouping is based on the phenotypic expression of particular O and H antigens, molecular methods are based on the organism's genotype. Findings from these molecular methods are being integrated into the understanding of the relationships between these organisms. In the meantime, the same organism may have two different names in the scientific literature. The common salmonella serotypes of concern to bovine practitioners (e.g. S. anatum, dublin, montevideo, newport, typhimurium) are now classified into a single species, Salmonella enterica, and are subclassified by their traditional serovar name. Confusion will rein for some time because other salmonellas, such as S. enteritidis, are also classified under S. enterica. In the research literature what was S. typhimurium before the renaming is now Salmonella enterica serovar Typhimurium or S. Typhimurium rather than S. typhimurium.
Tackling Herd Salmonellosis Problems
Much bovine salmonellosis information is in the practitioner literature (Merck Manual, Cornell Consultant Bovine Salmonellosis). With that in mind, the following is based on what we have observed in investigations of salmonella outbreaks on livestock farms.
To slow a clinical outbreak:
Several practitioners have found that in the midst of a clinical outbreak in adult cows, once or twice daily monitoring of rectal temperatures in those animals most at risk of clinical disease, such as periparturient cows, and initiating systemic antibiotic treatment upon temperature rise may help. They have observed that body temperatures rise to 105-107oF a day or so before the diarrhea starts. Producers should institute such a program only under the guidance of their veterinarian within a proper veterinary-client-patient relationship, only after a complete on-farm outbreak workup and only after complete identification of the organism including antibiotic sensitivities.
Eleven characteristics of Salmonella and salmonellosis to keep in mind
- Salmonella infection of a farm is maintained primarily by transmission
of the agent from the feces of infected animals to the mouths of susceptible animals.
The primary transmission route is fecal-oral; the epidemiology of salmonellosis is primarily the epidemiology of fecal pollution. Transmission by inhaled aerosols occurs as well as by other routes (e.g., ocular, via teat streak canal, rectal) occurs, but less frequently.
Action: Enhance within herd biosecurity. Break the links in this chain by minimizing the opportunity for fecal contamination of feedstuffs, feeding surfaces, water troughs and equipment.
- Salmonella infection and subsequent clinical disease (the two are not synonymous) is a result of:
- The innate resistance of the host animal.
- The infectious dose received by the animal.
- The infectivity and virulence of the particular strain of the organism.
The livestock producer has the most impact on the salmonella cycle through maximizing 1) and minimizing 2) (and probably in that order) but cannot change 3) once the strain has invaded the farm. The only option for 3) is minimizing the chance of invasion by improving herd biosecurity.
1) Maximize the resistance of susceptible animals. Pay careful attention to the transition of the most susceptible animals (periparturient cows, newborn calves). Concentrate on those things known to decrease host resistance and remember that above all else salmonella is an opportunist. For newborn calves this means calving them in a clean maternity pen, getting 4 quarts of clean, high antibody concentration colostrum into them within the first 2 hours of life before they are exposed to enteric pathogens and moving them into a clean calf hutch immediately. For closeup and fresh cows, this means such things as getting them to appropriate body condition before dry off, maintaining that body condition score through the dry period, avoiding overcrowding of the closeup and fresh cows, avoiding dominance problems between heifers and mature cows and maximizing dry matter intake during the transition. The occurrence of any other periparturient problems, such as dystocia, displaced abomasum, ketosis and hypocalcemia, are clear indicators that cows are being stressed and thus are more susceptible to this disease.
2) Minimize the exposure dose. Considering the other points in this document, eliminate or control all means by which this infectious agent can get from infected animals to the mouths of susceptible animals either directly or indirectly, particularly by the contamination of feedstuffs, feeding surfaces, water sources, or oral treatment equipment such as esophageal feeders, oral speculums, stomach tubes, balling guns and hands.
One basic way to minimize exposure dose is to isolate susceptible animals from healthy appearing but potentially infected animals. This is a major weakness of many farm systems. For example, one of the strongest risk factors for a clinical salmonellosis problem in adult dairy cows is the practice of placing fresh cows in the hospital pen yet according to the NAHMS data this is a common practice. Another dangerous practice is holding back poor doing youngstock, potentially exposing susceptible younger animals in the group.
For general considerations of infectious disease transmission, see Epidemiology Concepts for Disease in Animal Groups
- Salmonella infects anything in the livestock environment that has an
Besides cows and calves, salmonella infections occur in feral cats, dogs, rodents, birds, waterfowl, flies, humans, fish, and indigenous wild mammals (raccoons, porcupines, deer). Under the right conditions, any of these species (even flies) can serve as biological multipliers of this organism.
Action: Initiate control programs for rodents, flies, nuisance birds, and feral dogs and cats and regularly monitor for their effectiveness. Rodenticides and insecticides alone are seldom sufficient. Rodent-proof and bird-proof feed storage and cattle housing facilities by removing nesting and roosting opportunities and by removing protective cover. To reduce fly populations remove fly breeding materials, such as damp straw and manure, weekly and establish integrated pest management practices.
- The majority of salmonella infections in a herd over time are subclinical;
the clinical infections are only the tip of the iceberg, even during outbreaks of clinical
Misunderstanding this "iceberg" effect by both practitioners and producers leads to inappropriate management of individuals in infected herds, meaning that often attention is paid only to the animals that are or were clinically ill. Although clinically affected animals shed much higher numbers of organism in the feces than do subclinically infected animals, the latter clearly shed enough to provide an infectious dose for many normal animals and are usually far more numerous in a herd.
Action: In an outbreak, handle all animals as if they were shedding, not just the sick ones. Institute procedures to protect all animals from all other animals, such as reducing contamination of water sources by installing guards and reducing contamination of feed.
- Septicemic animals shed the agent in oral and nasal secretions and urine as well as feces. These animals don't necessarily have clinical signs associated with enteric salmonellosis at the time.
Such animals are very dangerous because they contaminate water troughs, water bowls, nipples, oral treatment equipment, and human hands. Often this equipment (e.g. balling guns, esophageal feeders) is used without proper sanitizing between animals and transmits the infection to other animals that are in a most susceptible state.
This sanitation failure is likely the biggest weakness of many farm treatment programs.
Action: Implement a sound sanitation program for potentially contaminated equipment, including training of personnel. All organic matter (e.g., saliva, manure, milk, milk fat film, blood) must be removed prior to the application of a disinfectant. The disinfectant must be in sufficient concentration and remain in contact for a sufficient period to kill the organism. Contact time is dependent on temperature. For equipment, surfaces, and boots use disinfectants with known effectiveness against the target agents, such as chlorine dioxide or Virkon S. After contact with infected or potentially infected animals and their discharges, wash hands well, scrubbing for 20 seconds or use an alcohol rub carried by personnel. Strategically locate hand sanitation stations.
See Sanitation in the control of livestock infectious disease for additional information on sanitation procedures.
Don't share sick pen water troughs with other groups! Unless the trough is cleaned and sanitized, orally-shedding cattle leave long term contamination for other cattle.
- Salmonella has a complex relationship with its animal host, which is only beginning to be understood.
Why many animals are subclinically infected while some others are clinically affected is unknown but research is advancing in this area. For example, researchers have found that S. Typhimurium sends a protein signal through the wall of a nearby intestinal cell of the host. In response to this signal, the host cell flattens its brush border and builds a large, projecting pedicle with an interior cytoskelton. Salmonella then attaches securely to this pedicle. Salmonella and other pathogens are able "trick" the intestinal mucosal M cells into ingesting them, which gives the bacteria direct access to the reticuloendothelial system but are protected from it. However, this protein signal is only produced when the salmonella is near an enteric cell and is not produced during laboratory fermentation.
Because of this complex relationship, bacterins simply targeted at producing antibodies against antigens produced during standard laboratory fermentation are not likely to be highly efficacious. Other than anecdotal experiences, little empirical evidence suggests that current commercial bacterins are or are not beneficial and good clinical trials are needed (House and Smith, 1997). Bacterins targeted at specific parts of this relationship, such as blocking the Type III signaling, are more likely to provide protection.
- Salmonella are a small part of an extremely competitive, complex, dynamic microbial environment in intestinal tracts and this competition is a very important part in resistance to infection.
An understanding the aspects of intestinal microbiology is helpful in developing strategies to prevent salmonellosis (For a primer on mammalian intestinal microbiology, see Drasar and Barrow (1985)). Calves are born with a sterile intestinal tract that is at a neutral pH and provides an excellent environment for bacterial growth. Succeeding waves of microflora are established that change as the calf develops and its diet changes. The lactobacilli are normally the first to populate the tract. The strains of lactobacilli change with time and only those with specific characteristics that enable them to bind to the mucosal surface persist. The presence of these lactobacilli increases the resistance of the calf to salmonella infection. Over time, other flora populate the gut until over 400 species of bacteria are present, the numbers of each that are present varying along the length of the gut. The gram negative fecal coliforms represent less than 1% of the mass of bacteria present in the normal animal's gut with most being strict anaerobes. Many of these other species are very important in resisting salmonella infection. Normal mice require 10,000-fold the infective dose to establish salmonellosis as gnotobiotic mice or mice treated with streptomycin, which have no or altered competing flora. Because most of these other species are more sensitive to antibiotics than are salmonella, the use of antibiotics precipitates clinical salmonellosis in sub-clinically infected humans and animals in part by allowing overgrowth of the salmonella. Poorly absorbed oral antibiotics and antibiotics that are secreted into the gut will particularly predispose the gut to an overgrowth of salmonella by killing the more sensitive competitive microflora. Some research shows that even those antibiotics to which the salmonella are sensitive will cause a cessation of salmonella shedding and that shedding resumes when the antibiotic is withdrawn. Less clear in the ruminant is the effect of antibiotic use on the prolongation of the carrier state.
This suggests that antibiotics should be used with considerable prudence in salmonella-infected herds and then only in those cases with systemic involvement.
- Salmonella are usually killed by exposure to the volatile fatty acids of fully functioning normal rumens.
The level of VFAs in rumens of most cattle on continual full feed are toxic to most salmonella (Brownlie and Grau, 1967, Chambers and Lysons, 1979, Mattila et al. 1988). However, if dry matter intake drops for any reason, the VFA levels decline rapidly. A drop in DMI may be precipitated when animals don't have regular access to feed, such as during transport through the marketing system, for physiologic reasons such as impending parturition, subclinical ketosis and hypocalcemia, and sudden ration changes or ration maladaptation, and for husbandry failures, such as inadequate bunk and pen space and mixing submissive heifers with dominate cows at parturition.
Ration fats may also encapsulate the bacteria, protecting them from the rumen VFAs. S. Typhimurium DT104 may be more acid resistant and appears to be able to survive in the rumen of some chronic carrier cows.
Action: Maximize rumen function by maximizing a consistent dry matter intake in periparturient and early fresh cows.
- Salmonella survives for long periods under environmental conditions
common on the livestock farm.
Salmonella have several different survival mechanisms (reviewed by Foster and Spector, 1995) that enable the organism to survive sudden environmental changes and to survive for long periods in different environments. Moreover, the pathogen can quickly turn these different systems on and off in response to changing environments. Once some of these systems are turned on in the dehydrated organism, the organism becomes much more resistant to environmental factors and other control measures, such as heat or disinfectants, that would otherwise kill it. As a result, it survives very well on surfaces, in dust, and in dried manure that are protected from sunlight.
These survival times are very long if the organism isn't exposed to sunlight. In an experiment that simulated a barn floor under defecating cows, salmonella survived for 5 years (Forshell and Ekesbo, 1996). These researchers found S. Typhimurium in an empty slurry pit that had not been used for 4 years. Once dehydrated, some strains have been shown to survive exposure to 100o C for one hour (Kirby and Davies, 1990).
Salmonella survives well in water, surviving in lagoons where it can be recycled back to the herd in the flush water (Gay and Hunsaker, 1993).
- Salmonella replicates in moist environments (< 85% dry matter)
even with scarce nutrients.
Salmonella replicates rapidly in mixed feeds and on surfaces (wood!) that have been washed but have not been adequately sanitized. Other workers found that the bacteria survived for at least 119 days in contaminated pond water.
Salmonella replicates very well in composted manure solids used for bedding once it becomes wet, is contaminated and is at a sufficient temperature, which it will often be when laid upon by resting cows.
Salmonella is often present at low levels in many purchased feedstuffs, such as vegetable protein sources and both vegetable and animal fat sources. Under warm environmental conditions, mixing these contaminated feed ingredients with wet feeds, such as silage or haylage, may allow the organism to replicate until infectious doses for normal animals are reached if sufficient time between mixing and consumption passes. Placing wet mixed feeds on contaminated surfaces such as feed alleys may allow the same consequence.
Action: Minimize replication time, such as by not mixing large batches of moist feed that are stored or not mixing well in advance of consumption.
- Salmonella, particularly Salmonella Typhimurium DT104, in livestock is a significant zoonotic disease risk
for in-contact people, particularly young children.
Human disease due to salmonella is a particular risk to farm families, particularly young children, and to employees in contact with infected cattle.
- Veterinarians dealing with farms infected with this strain are remiss if they do not warn people associated with the farm about the hazard presented by the exposure of the very young, the elderly, people who are immunocompromised and those taking antibiotics to animals shedding this organism.
- Likewise, consumption of raw milk from the farm should be strongly discouraged.
- Outer garments and footwear exposed to infected animals and their discharges should not be brought into the household.
- Hands should be cleaned well, using soap and warm water and scrubbing or an alcohol-based handrub for at least 15 seconds, before returning to the household. Several times more effective than traditional soap and water for unsoiled hands, the alcohol-based hand sanitizers can be used anywhere. If hands are soiled, soap and water should be used first. The best handwashing guidelines are the following:
- In the household, food preparers should be particularly cautious about proper food handling to minimize opportunities for salmonella contamination and for the replication of this contamination. This includes refrigerating cool enough (<39oF),
heating hot enough (>160oF) and minimizing the time that foods requiring cooking or refrigeration are exposed to temperatures between 40oF and 140oF. Prolonged exposures in this temperature range, even
when thawing or cooling, allow minimal salmonella contamination to replicate to above infectious doses.
- 4 Steps to Food Safety
sodium hypochlorite (bleach) to sanitize (3 tablespoons per gallon of water) or to disinfect (3/4 cup per gallon of water) surfaces and items,
allowing at least two minutes of contact time.
- Guidelines for the Use of Chlorine Bleach as a Sanitizer in Food Processing Operations (Oklahoma State U FAPC-116 pdf)
- Minimize the opportunities for replication in moist, contaminated items such as dish cloths and sponges by disinfecting or changing and washing these frequently rather than reusing these between meals.
- As we have isolated this agent from clinically normal domestic pets associated with infected herds, in the presence of young children these animals should either be restricted to the household or from the household.
Keeping the above factors in mind when working with a farm facing a salmonellosis problem will enable you to detect the weak points in the management and the facilities of that farm and to determine the best places to begin breaking the salmonella cycle. In herds that don't have a salmonella problem, proactively increasing herd biosecurity will reduce the likelihood of having an outbreak. Begin implementing bovine food and water safety. Otherwise, it may be only a matter of time. Far from being a simple organism with simple functions, salmonella interact with the environment and their animal hosts in very complex ways that are only beginning to be understood. Salmonella is a very worthy adversary for the veterinary practitioner.
Current bovine salmonellosis resources -
- The Center for Food Security & Public Health - Disease Fact Sheets (Iowa State U)
- Cornell Consultant
- current Cornell Consultant salmonella references - cattle
- National Library of Medicine PubMed - bovine salmonellosis
- Dairy Studies
- Salmonella and Campylobacter on U.S. Dairy Operations, 1996–2007, pdf
- Prevalence of Salmonella and Listeria in Bulk Tank Milk and Inline Filters on U.S. Dairies, 2007, pdf
- E. coli O157 and Salmonella - Status on U.S. Dairy Operations (NAHMS Dairy '96)
- Salmonella and Campylobacter on U.S. Dairy Operations (NAHMS Dairy '02)
- Beef Cow-Calf Studies
- Salmonella on U.S. Beef Cow-calf Operations, 2007-08, pdf
- Beef Feedlot Studies
- Salmonella in U.S. Cattle Feedlots, 2007 - pdf
Other General On-line Sources: