Methicillin-resistant Staphylococcus aureus (MRSA) has been an important cause of hospital acquired infections in people since the 1970s (Tong et al., 2015b, Laupland, 2013). In recent years the epidemiology of MRSA has changed with the emergence of community acquired infections, first reported in Australia in remote Aboriginal communities in people without the usual healthcare associated risk factors (Udo et al., 1993). People of Aboriginal background remain disproportionately affected by community acquired MRSA infections, presumably as a consequence of social disadvantage and poor health infrastructure (Tong et al., 2015a).
Staphylococcus aureus is the most frequently isolated coagulase positive Staphylococcus species from the anterior nares of people; asymptomatic carriage is commonly reported in around 30% of the population (Verhoeven et al., 2014). Asymptomatic carriage of MRSA is also possible and has been associated with an increased risk of subsequent infection (Wertheim et al., 2005). The prevalence of asymptomatic nasal carriage of MRSA is highly variable between study populations with very high prevalence reported in Aboriginal populations both in Australia (Vlack et al., 2006) and overseas (Daley et al., 2016).
To the best of our knowledge, no studies have been conducted to estimate the prevalence of asymptomatic carriage of MRSA by dogs in Australia. Low rates of carriage (0-2%) have been reported from community-based dog population studies overseas (Vengust et al., 2006, Mouney et al., 2015, Gronthal et al., 2015, Bagcigil et al., 2007, Daley et al., 2016, Hanselman et al., 2008, Boost et al., 2008, Hanselman et al., 2009, Loeffler et al., 2011). It has been proposed that dogs and other household pets may play a role in the epidemiology of community acquired MRSA infections in people however this association is yet to be thoroughly investigated. This current study aimed to estimate the prevalence of asymptomatic carriage of MRSA in dogs from predominantly Aboriginal communities in remote NSW.
A survey of dogs and cats living in the communities of Bourke, Enngonia, Brewarrina, Goodooga and Weilmoringle in remote north-west NSW was conducted in September and October 2016 to estimate the prevalence of MRSA carriage. Dogs and cats were recruited from those participating in a community dog health program. Three skin swabs were collected from each animal (anterior nares, oropharynx and perineum); in addition a fourth swab was collected from skin lesions or wounds if present. Swabs were cultured on enrichment media followed by selective media for Staphylococcus to characterise background staphylococcal normal flora in addition to culture on media selective for methicillin-resistant Staphylococcus species. Species identification was confirmed by standard phenotypic testing followed by MALDI-TOF mass spectrometry.
A total of 258 animals were sampled from the 5 communities, 219 dogs and 39 cats. MRSA was isolated from 7 swabs from 5/219 dogs (prevalence estimate for the population 2.3%, 95% CI: 0.75-5.25%). Four out of the 5 dogs were positive for MRSA from the anterior nares swab, two of these dogs were also positive from their oropharynx swab. The fifth dog had skin lesions consistent with bacterial pyoderma and was positive for MRSA from the swab from these lesions. No cats were positive for MRSA and no dogs or cats were positive for any other methicillin-resistant Staphylococcus species including methicillin-resistant Staphylcoccus pseudintermedius (MRSP). Prevalence of MRSA carriage was not sufficiently high (n=5) to evaluate risk factors.
Full characterisation of Staphylococcus species normal flora to date is available for 97 animals; 85 dogs and 12 cats (all dogs and cats from Bourke and Enngonia). Staphylococcus sp. were isolated from 63 of 97 animals (64.9%); 55 of 85 dogs (64.7%) and 8 of 12 cats (66.7%). Staphylococcus aureus was isolated from six sites from five of 85 dogs (5.9%). Five of the six (83.3%) S. aureus isolates were MRSA. Staphylococcus aureus was not isolated from any cat samples. Staphylococcus felis was isolated from seven of the 12 cats (58.3%) and five of the 85 dogs (5.9%). Staphylococcus pseudintermedius was carried by 33 of the 85 dogs (38.8%) and three of the 12 cats (25.0%).
This study confirms that dogs in remote, predominantly Aboriginal communities in New South Wales can be asymptomatic carriers of MRSA and that the prevalence of MRSA carriage is high relative to other populations of healthy companion animals. It can be assumed that the direction of transmission of S. aureus is from humans to pets, which combined with a relatively low level of methicillin-susceptible S. aureus carriage suggests the prevalence of MRSA in the human population of these communities is likely to be high. Further studies to investigate the prevalence of MRSA carriage in the human population and to investigate risk factors for carriage of MRSA in pets in these communities are warranted to further understand the epidemiology of this important pathogen.