Salmonellosis

Salmonella organisms may be carried by animals without any signs of illness, and are shed in faeces. Shedding rates may increase if carrier animals are stressed or if clinical infection develops. Organisms may also be spread by flies, rodents and birds. Animals are mostly infected by ingestion of organisms, although inhalation or conjunctival contamination may also result in infection. 

The response of an animal to Salmonella challenge will depend on multiple factors including number of bacteria and virulence, specific immunity to Salmonella from vaccination or prior exposure to that strain, and a wide range of factors influencing general health (immune capacity, concurrent disease, nutritional state, and stress).

Within individually exposed animals, the outcome may range from resistance (no disease at all), to mild disease, to severe infection and possibly death. The export process is associated with a range of stressors (feed/water deprivation, change in environment and feed, transport, mixing of animals, exposure to other diseases, and climatic stress) that may increase the level of Salmonella shedding and make animals more susceptible to infection. 

Cattle are at risk of exposure in assembly points, particularly if assembly points are continually populated with animals over time. Outbreaks of clinical disease may occur during assembly or at sea under favourable conditions (high density of stressed or susceptible animals and heavy exposure). 

There is particular interest in salmonellosis in sheep in the export process because inanition and salmonellosis are the two most common causes of death in sheep exported from Australia to other countries. 

The epidemiology of salmonellosis in export sheep is complex. A causal web for salmonellosis and inanition is shown in the section on Inappetence/inanition. Inappetence is recognised as a potential precursor that may predispose sheep to salmonellosis, but sheep may also develop salmonellosis without having inappetence. Salmonella are ubiquitous and many healthy animals will carry and shed organisms without disease. Many factors interact to influence the occurrence of disease, including Salmonella challenge (number of bacteria and virulence of a strain), specific immunity to Salmonella infection, as well as general health and immune capacity (influenced by innate immune response capacity, concurrent disease, nutrition, and stress). 

The prevalence of sheep shedding Salmonella on entry to export assembly depots has been shown to be very low. Clinically infected animals and passive shedders are thought to be the major potential sources of Salmonella organisms for other susceptible animals, either directly, or indirectly through contamination of water, feed and the environment. Salmonella organisms can survive for months to years in the environment, and under favourable environmental conditions, Salmonella are capable of proliferation in the environment to increase the level of environmental contamination. Assembly feedlots that receive large numbers of sheep over many months for consecutive export voyages may become heavily contaminated with Salmonella organisms over time, leading to increased risk of exposure of new arrivals to infection.

Factors that are likely to be associated with increased risk of heavy shedding in sheep include mixing of animals in saleyards or dealer operations, frequent or prolonged transport movements, reduced feed access, inappetence, or other stressors. 

Healthy sheep may be reasonably resistant to Salmonella exposure. Stressors such as transport, yarding, feed and water curfews or deprivation, inappetence, concurrent disease, and inclement weather can all reduce host resistance and increase the risk of salmonellosis. The outcome in individual infected animals may range from recovery, to acute, fulminant bacteraemia, endotoxaemia and death. 

Compromised animals exposed to heavy levels of virulent organisms may develop acute disease and begin dying while still in the assembly feedlot. In other cases, animals may be exposed in the feedlot and develop disease during the early part of the voyage. In exposed and infected animals, the disease usually runs its course over 14-21 days. 

Enteritis (mainly associated with salmonellosis), inanition, and the combination of both enteritis and inanition together, form the most common causes of mortality during export voyages (LIVE.0123, 2009).

At necropsy, intestinal contents are watery and usually blood stained, intestinal walls are red and thickened, and the mucosal surface is coated with necrotic debris and fibrin. Mesenteric lymph nodes are large and wet, and sometimes red. Affected animals may or may not have concurrent evidence of inanition, such as very low or absent rumen contents, poor body condition and evidence of fat mobilisation (fatty liver, enlarged gall bladder and serous atrophy of fat stores). 

Laboratory confirmation requires faeces submitted chilled for bacteriology. Additional specimens desirable from dead animals are sections of abdominal visceral organs, which should be submitted chilled and separated for bacteriology, and fixed in buffered formalin for histology. Samples from intestines may be stored in viral transport medium for polymerase chain reaction (PCR) testing to detect Salmonella genetic material. 

Differential diagnoses include ruminal acidosis, clostridial enterotoxaemia, coccidiosis, and bovine viral diarrhoea virus (BVDV) infection in cattle.

Antibiotics are recommended by a number of publications and veterinarians, but the efficacy of antibiotic use in treating salmonellosis is unclear, and there are risks of adverse effects. 

Several antibiotics, including potentiated sulphonamides, ampicillin and amoxicillin, have been recommended as effective treatments for salmonellosis in ruminants.

Antimicrobial use has also been associated with emergence of antimicrobial resistant Salmonella and increased duration of Salmonella shedding, and can actually interfere with normal digestive microflora, predisposing sheep to further gastrointestinal disease. Mass medication as prophylaxis in an attempt to prevent salmonellosis is not recommended. 

Blanket administration of antibiotics via drinking water is not recommended. Sick sheep are unlikely to drink enough and the healthy sheep that do drink may become sick because of disruption to normal gut flora.

Host resistance can be maximised by maintaining adequate nutrition and minimising stress. Management guidelines for achieving these goals throughout the export process are given in the Australian Standards for the Export of Livestock (ASEL). 

The key to minimising exposure to Salmonella pathogens is to reduce environmental Salmonella contamination. Management procedures to reduce Salmonella contamination include paddock rotations, isolation of sick animals, feed and water management to prevent faecal contamination, and control of flies and rodents (LIVE.0112, 2002). Disease prevention and control strategies need to be tailored to each facility to account for differences in variables such as soil type, drainage, and environmental conditions. 

It is important to use strategies to minimise the risk of inappetence and inanition since these conditions increase the risk of salmonellosis occurring. See the section on Inappetence/Inanition for more information. Spelling or rotating feedlot or assembly pens may be useful to avoid progressive build-up of environmental levels of pathogenic organisms and may reduce exposure risk. Scraping manure contaminated top-soil from assembly pens may also reduce environmental exposure. Changing flow patterns in assembly feedlots to move towards a one-way flow (separate receival yards and load out yards), and use of elevated sheds to house sheep are two examples of relatively high cost methods that may minimise exposure of livestock to Salmonella challenge. 

Salmonella vaccines are available for use in cattle, mainly as an aid in preventing salmonellosis in calves. Efficacy of these vaccines in older cattle to protect against salmonellosis in the export process is unclear. Preliminary research trials on oral Salmonella vaccination in sheep appear promising, but commercial products are not yet available and further work needs to be done. 

All people working with stock should be regularly reminded that Salmonella can be passed between animals and humans, and of the importance of personal hygiene, especially washing hands thoroughly with warm soapy water before eating.