Twenty-four breeds were represented, only three of which were observed in more than one individual dog. Four dogs were of mixed breed. Mean and median age at the time of the inclusion sample was six years (range 1-12 years). Sixteen of the 31 sampled dogs were intact females, nine were intact males, three were spayed females, and three were neutered males.
Thirteen dogs were diagnosed as having post-operative infections at the time of the inclusion sample. Eleven dogs were diagnosed with dermatitis at the time of the inclusion sample. The remaining seven dogs were treated for traumatic injuries or secondary infection.
Six of the eleven dogs with dermatitis at the time of the inclusion sample had recurrence of skin disease later during the study. Signs of dermatitis were present on ten sample occasions, three of which yielded negative results from all sample sites. Only one of the 31 dogs (dog No. 20, Additional file 1
: Table S1) showed other clinical signs of infection during the sampling period. That dog had an oronasal fistula and a yellow, thick nasal discharge was noted by the owner daily during the entire study period.
All dogs received systemic treatment with antimicrobial agents to which the cultured MRSP were resistant prior to the first sample occasion in the study. Median length of treatment was 24 days. No dogs received further treatment with such antimicrobials during the study, and in approximately two thirds (n = 20) of the dogs, the treatment was ended prior to, or at the time of, the inclusion sample. Five dogs received systemic treatment with tetracycline, an antimicrobial agent to which the cultured MRSP isolates were susceptible. The treatment was ended prior to the first sample occasion in four of these dogs. The fifth dog (dog No. 31, Additional file 1
: Table S1) was the only dog in the study receiving any antimicrobial treatment after the first sample occasion of the study. The median length of treatment with tetracycline was 30 days (range 20-60 days).
The SmaI restrictions profiles showed 85% or more similarity between the two isolates compared by PFGE from each dog (Figure ). With the exception of isolates found in dog number 17 and 20, the MRSP-isolates tested for antimicrobial susceptibility showed similar antibiograms, being susceptible only to fusidic acid and tetracycline. The isolates with diverging results were found to be resistant to tetracycline.
Dendogram of the 26 pairs of isolates from 26 dogs compared by pulsed-field gel electrophoresis.
Twenty-nine out of the 31 dogs were sampled according to plan regarding length of follow-up. Two dogs were only available for sampling for 5.5 and 6 months, respectively. All samples were collected according to the planned minimum interval and 60% within the planned 3.5 month maximum interval. The median study period was 10 months (range 4.5-19), and the median number of sample occasions per dog was 3 (range 2-6). The median time between sample occasions was 2.5 months (Additional file 1
: Table S1, Figure ).
Overview of sampled dogs over months.
Five dogs remained MRSP-positive for more than 14 months (61 weeks). Nine dogs were sampled for 5-12 months (22-52 weeks) and remained positive. One of these dogs had a negative result on one sample occasion followed by positive results. Fifteen dogs were found to be negative within 12 months (52 weeks) and they remained negative 3.5 to 7.5 months later (15-33 weeks). Two dogs left the study at five and ten months, respectively, with negative results from all sample sites at the final sample occasion (Additional file 1
: Table S1).
The overall median length of MRSP carriage was 11 months (48 weeks) (Figure ). When the last positive sample occasion was used as the end of MRSP carriage the median time of carriage was 45 weeks. No significant differences in length of MRSP carriage appeared either between the three diagnostic categories (p = 0.31) or in the dogs with dermatitis on the following sample occasions in the study compared to dogs with no visible signs of skin disorder (p = 0.78). Possible influence of breed could not be evaluated from the material due to the wide variety of breeds being represented in the relatively small number of dogs. There was no significant difference in the length of carriage between the two age groups: ≤ 6 years versus > 6 years (p = 0.43).
Figure 3 Proportion of dogs that were MRSP-negative in relation to time from inclusion sample. The proportion of dogs found to be MRSP-negative (y-axis) in relation to time in weeks from inclusion sample (x-axis). MRSP negativity was defined as the cultures from (more ...)
A statistically significant difference in length of MRSP carriage was found between dogs treated with antimicrobials to which the cultured MRSP bacteria were resistant for a period of three weeks (21 days) or longer (n = 19) and dogs treated for a shorter period (n = 12) (p = 0.01). Median length of carriage was 11.3 months (48.5 weeks) for the "long-term treatment" group and 5.7 months (24.5 weeks) for the group treated for a shorter period (Figure ).
Figure 4 Length of MRSP carriage from inclusion sample related to length of antimicrobial treatment. The proportion of dogs found to be MRSP-negative (y-axis) in relation to time in weeks from the inclusion sample (x-axis) by length of systemic treatment with (more ...)
Two of the five dogs receiving systemic treatment with an antimicrobial agent to which their cultured MRSP isolates were susceptible (tetracycline) were found to be MRSP-negative on the following sample occasion. The first dog (dog no 1) was treated for three weeks and was found to be negative on the very first sample occasion in the study, 8 months (34 weeks) after the inclusion sample. The second dog (dog No. 31) was treated for 4 weeks in between being sampled at 8.5 and 10 months (37 and 43 weeks) after the inclusion sample. The dog was unavailable for further sampling. The remaining three dogs were found to be MRSP-positive on the sample occasions following end of treatment. Length of proven MRSP carriage was nine months (39 weeks) following 8.5 weeks of treatment (dog No. 5) and approximately five to six months (23 and 26 weeks) following 4.3 weeks of treatment (dogs No. 30 and 7). Median length of carriage of MRSP for the five dogs that received treatment with tetracycline was 34 weeks, compared to 49 weeks in non-treated dogs (p = 0.06).
MRSP could be isolated from all sample sites. On 9 of the in total 73 positive sample occasions, positive cultures were found from all four sample sites. On 21 occasions MRSP was found from only one of the sampled sites. The nose was the only sample site that never yielded a positive culture when all other sites were negative. MRSP was isolated from the nostrils in only 38% of the 73 positive sample occasions. From the corner of the mouth, perineum and pharynx MRSP was isolated in 58, 63, and 67%, respectively, of the positive sample occasions.
Nonpurulent wounds were present in 11 dogs on a total of 20 sampling occasions, excluding the inclusion samples. Ten dogs had one or several wounds when first sampled, and five of these still had wounds on the following sampling. Two of these dogs were found to be negative on both of these sample occasions. MRSP was isolated from wounds in 13 of the 16 positive samplings of dogs with wounds present (81%). All dogs with a positive MRSP wound culture also had simultaneous positive cultures from one or several other sampling sites.
The presence of nonpurulent wounds did not significantly increase the length of MRSP carriage (p = 0.94). It did however significantly increase the number of positive sample sites (p = 0.003). The median number of positive sample sites per positive sample occasion, excluding wounds, in each dog was 2.66 when wounds were present (9 dogs) and 2.0 when there were no wounds (18 dogs).
No significant association could be found between the median number of positive sample sites per sample occasion and presence vs. absence of dermatitis during the study (2.00 and 2.33 respectively, p > 0.9).