Apparently healthy dogs can harbour Salmonella and might thereby serve as a potential source of human infection with implications for public health. Almost a quarter of all UK households are home to at least one dog [9] and it is therefore important that we understand the risk of transmission of zoonotic infections. This study showed that the prevalence of Salmonella in dogs located in the Midlands region of the UK is low (0.23 %; 95 % confidence interval 0.006 %, 1.27).

Other studies have estimated the prevalence of subclinical carriage of Salmonella in clinically healthy dogs to range from 0 to 44 % [23, 24]. The results of the current study are in line with the lower estimates, but are inconsistent with the larger estimates. The prevalence of subclinical shedding of Salmonella in normal apparently healthy household dogs has been reported for a number of different countries. A study of 150 dogs from Hawke’s Bay, New Zealand showed an absence of subclinical carriage [29]. By contrast, a study of 251 dogs visiting parks in three cities in south-western Ontario, Canada reported Salmonella in 1.2 % of the dogs [30]. Other studies have reported slightly higher prevalence. Rectal swabs collected from 437 household in northern Taiwan revealed that 2.1 % (9 dogs) were positive for Salmonella spp. [31]. A study of 1391 dogs across Trinidad reported a prevalence of 3.6 % [32] and investigations undertaken in Tehran, Iran and Florida, USA reported even higher prevalences of 4.4 % (21 of 474 dogs) and 15 % (n = 1,626) respectively [33, 34]. This considerable geographical variation in the prevalence of Salmonella serotypes reported in dogs could be due to the sample size, year of sampling, sampling strategies, and isolation methods performed, but may also be due to cultural differences in feeding or hygiene practices or favorable climate conditions for bacterial growth and survival.

The prevalence of Salmonella in dogs as reported in the literature is also highly variable depending on the immediate environment in which the animals live. For example, Salmonella isolation rates from stray dogs have been reported to be significantly higher than those from household dogs [31]. For this reason, in the current study faecal samples were collected and analysed for the presence of salmonellae from dogs housed in a variety of environments: households, rescue centres, boarding kennels, retired greyhound kennels and a pet nutrition facility. It is noteworthy that in this study all kennelled dogs tested negative for the presence of Salmonella, although other studies have reported a higher prevalence of Salmonella shedding in kennelled dogs. Rectal swabs from kennel dogs in Tehran, Iran indicated that 28 out of 181 (15.5 %) were positive for Salmonella [33]. In a shelter in Bursa, Turkey 11 % of dogs tested positive for Salmonella [35], and in Japan, 5.9 % of stray or unwanted apparently healthy dogs were positive for the presence of salmonellae in their intestinal contents [36]. In northern Taiwan rectal swabs collected from 491 stray dogs in a municipal animal shelter found 6.3 % (31 dogs) dogs were positive for salmonellae [31]. However, other studies involving strays or kennelled dogs show much lower rates of carriage that are more in line with the findings presented here. Ojo [37] failed to detect salmonellae in the intestinal contents of stray dogs (n = 100) in Trinidad, West Indies, and an analysis of rectal swabs collected from kennelled dogs in Istanbul, Turkey found only one out of 100 (1 %) to be positive for Salmonella [38].

Shedding of Salmonella in faeces is also known to be common among the racing greyhound population. Racing greyhounds in the USA have been shown to have high rates of subclinical shedding of Salmonella at 44 % [39], and a more recent study reported Salmonella in faeces from 11 % of asymptomatic greyhounds [40]. The high prevalences of Salmonella that are typically reported in greyhounds may be traceable to the high-protein raw meat diet provided for racing. A high prevalence of Salmonella in these raw meat diets has been reported, and identical enterotypes have been found in the faeces of dogs consuming the food, confirming that the diet is the likely source of Salmonella [41]. In some instances, the dogs may not be colonised by Salmonella and may just be passive carriers in which food-borne Salmonella is transiently passing through the intestines. However, studies have shown that raw meat diets contaminated with Salmonella can lead to abortions and high levels of morbidity and mortality in greyhounds through Salmonella infection [42, 43]. In contrast to other studies, none of the retired greyhounds in this study tested positive for Salmonella and this may be due to the fact that they were all fed commercial diets.

In recent years there has been a trend towards the feeding of commercially manufactured pet food in many countries including the UK [9]. This is consistent with the results of the accompanying questionnaire from this study, from which it was clear that the vast majority of dogs (93 % of all dogs sampled) were fed a commercial diet. This may be one of the reasons the overall prevalence of Salmonella in faeces from dogs in the five housing environments studied was relatively low. Although there have been reported instances of Salmonella outbreaks in dogs and humans that were shown to originate from commercial food sources [44], commercial pet foods are typically manufactured in such a way as to minimise the risk of contamination from Salmonella [45].

In the context of racing greyhounds, dogs fed raw meat diets may be at an increased risk of Salmonella exposure, a possibility supported by previous research. For example, in one study Salmonella was isolated from 80 % of the raw meat diet samples and from 30 % of the stool samples from dogs fed the diet [46]. Other studies have also demonstrated high levels of Salmonella in faeces from animals fed raw meat and offal diets [47]. Unfortunately, due to the overwhelming popularity of commercial dog foods used to feed dogs in the current study it is not possible to address this issue directly. This study showed that only two dogs (0.4 %) were fed a raw meat diet and neither of these dogs was positive for Salmonella shedding.

The cross-sectional nature of this investigation meant that only a single faecal sample was analysed from each dog, which may be the reason that the prevalence of Salmonella was low. However, although the prevalence of Salmonella might have been higher if more than one faecal culture was performed on each dog this study suggests that the exposure risk is low; however, Salmonella was still identified. The limitations of single faecal cultures for the isolation of Salmonella, due to intermittent shedding, are well documented. Dogs with experimentally-induced latent infection shed the agent irregularly for the subsequent 3–4 weeks. In rare cases this shedding continues for up to 100 days [48, 49]. Since the agent is being shed at intervals, sampling times are very important when searching the carrier status of the dogs and in the present study we can only conclude that the dogs were positive or negative for the presence of Salmonella at the time of sampling. As an illustration of the intermittent shedding, the Salmonella positive dog in this study was repeat-sampled over three consecutive days but all subsequent samples were negative for the presence of Salmonella.

Further to the point of variability, the Salmonella isolated from the household dog that tested positive was Salmonella arizonae, which was only isolated following pre-enrichment of the faecal sample, suggesting the microorganism was present in low numbers. This subspecies is predominately found in reptiles, such as snakes, lizards and terrapins but has also been reported in dogs [50]. In a report by Public Health England it ranked eleventh of the most frequently reported serotypes in 2013 [1]. Salmonella arizonae has also been reported to cause salmonellosis in humans that have consumed contaminated snake meat or ingested medicines with traces of snake [51, 52]. Typically human cases are from vulnerable groups including immunocompromised, elderly or very young children [1]. However it is the serotypes Enteritidis and Typhimurium that are amongst the most frequently isolated serotypes of Salmonella from clinical cases, both human and animal, in the UK [1], and so the presence of Salmonella arizonae in the dog in this study may be considered atypical.