Understanding Salmonella in Reptiles and Humans

Salmonella Bacterial Infection

Bacteria belonging to the genus Salmonella are rod-shaped Gram-negative facultative anaerobes up to 3 μm long. They belong to the bacterial family Enterobacteriaceae, and along with other major pathogens in this group are often attributed to causing illness within the small intestine, from which the bacteria can migrate and result in progression to full systemic body disease. The severity of infection with Salmonella can range from sub-clinical infections, in which the organism is present but causes no disease in the carrier animal, to major enterocolitis and fatal acute septicaemia in animals harbouring an active infection. Populations of Salmonella bacteria are found as part of the naturally occurring flora in the gastrointestinal tracts of most reptile species, where they are present as a sub-clinical (non-illness causing) infection. Provided their populations do not grow out of control, they generally do not result in any clinical or symptomatic illness, and in small numbers do not present any significant health hazard to the reptile host.

However, Salmonella is an opportunistic pathogen, and if an animal is very young or very old, subjected to stress factors, or has a compromised immune system, it is not uncommon for the normal populations of Salmonella in the gut to increase and cause enteric disease. Stress factors that may initiate an active infection of Salmonella bacteria include transportation or overcrowding, sudden changes in diet, extreme temperatures and surgery. A reptile that is suffering from an acute active infection of Salmonella will show signs of anorexia, depression, weakness and lethargy, and will pass copious amounts of exceptionally foul and liquid diarrhoea. Mucous and blood may be passed with the faeces, the animal rapidly becomes dehydrated and will quickly begin to show signs of weight loss. Gravid females may abort partially or fully developed eggs, which may lead to secondary infection if some of this tissue is retained within the oviduct. If left untreated, the Salmonella infection will quickly become systemic, resulting in septicaemia and often death. Animals that manage to survive an attack of septicaemic salmonellosis may be prone to persistent diarrhoea, pneumonia and arthritis.

A study in 2003 estimated that 3%-5% of all Salmonella infections reported in humans are the result of contact with reptiles. Reptile Associated Salmonellosis (RAS) in humans is characterised by severe diarrhoea, abdominal pain and discomfort. Nausea, fever, lethargy and headache may also be experienced. The incubation period for human infection is typically 12 to 36 hours, and the illness itself generally does not last longer than one week. Humans that suspect that they are infected are advised not to go to work and to visit their general practitioner, who may commission laboratory tests to confirm Salmonella as the active pathogen.

In both human and reptile enteritis, it is important that an infection by Salmonella is confirmed so that the most appropriate course of treatment is undertaken. The clinical picture presented to the veterinary surgeon is often the first indicator that the active pathogen is Salmonella bacteria, but diagnosis cannot be confirmed without positive laboratory tests. Faecal and blood samples should be taken from the patient and cultured on to brilliant green (BG) and xylose-lysine-deoxycholate (XLD) agar plates. If Salmonella bacteria are present, the colonies and adjacent medium on BG agar will turn red, indicating an alkaline reaction. In a positive test on XLD agar, the colonies on are typically red with black centres as a result of hydrogen sulphide production.

Once diagnosis is confirmed, the main principles of treatment are largely the same as those used for any other companion animal with acute diarrhoea, with good supportive nursing playing a key role in patient recovery. Firstly the animal must be isolated in quarantine housing designed specifically to be easy to disinfect and prevent the spread of contagious organisms. All food is withheld and a fluids and electrolyte solution should be administered intravenously to combat dehydration and shock. Once the patients’ condition is stable, small amounts of liquid or soft food may be given. Tube-feeding may be required for animals that are recovering from very severe infections, as they may be too weak to eat for themselves or suffering from anorexia. Humans with enteric salmonellosis are advised to rest, eat small bland meals and drink plenty of fluids.

There is controversy amongst veterinarians and microbiologists regarding the use of microbial drugs in the treatment of enteric and septicaemic salmonellosis. Evidence from scientific investigations suggests that the use of antibiotics and similar drugs can further disrupt the normal gut flora, promote a prolonged period of carrying and excreting the disease as a sub-clinical infection once the active infection is eradicated, and may result in the development of resistant strains. Because of this, antimicrobial treatment should only be used in animals that are acutely ill or of a very high monetary or conservation value. If drugs are to be used in the treatment of enteric Salmonella, then members of the fluoroquinolone class of antibiotics, such as enrofloxacin or ciprofloxacin should be used.

Salmonellosis is a non-notifiable disease, meaning the government Department for Environment, Food and Rural Affairs (DEFRA) does not need to be informed when an outbreak amongst humans or livestock occurs. Instead it is listed as ‘reportable’, meaning that any samples that give a positive result in diagnostic laboratory tests for the Salmonella pathogen must be reported to the Veterinary Laboratory Agency (VLA) in England and Wales, and to the local Divisionary Veterinary Manager in Scotland. The data gathered from these sample reports are then analysed and published in annual reports. These reports allow government officials, as well as veterinary and health professionals, to evaluate the main sources of Salmonella contamination, and take action to reduce its occurrence within that industrial sector.