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Using the appropriate antibiotics for septic peritonitis in dogs | VETgirl Veterinary Continuing Education Podcasts

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In today’s VETgirl online veterinary continuing education podcast, we discuss the appropriate use of antibiotics for dogs with septic peritonitis. In patients with sepsis, early antimicrobial therapy is one of the cornerstones of treatment, along with resuscitation and source control. In human medicine, a landmark study of patients with septic shock revealed that for every one hour delay in antibiotic administration for the first six hours after presentation, mortality increased by 7.6%!(1) In critically ill, septic patients, antibiotics should be started before culture and sensitivity results are generally available, right? Well, the questions remain in both human and veterinary medicine about how we should choose those antibiotics, and whether or not our choices matter.

So, Dickinson et al (2) wanted to study two big questions: 1) in dogs with septic peritonitis, does appropriate empirical antibiotic therapy affect survival to discharge (and they hypothesized that it would); and 2) is antimicrobial therapy in the 30 days prior to hospital admission for septic peritonitis associated with inappropriate antimicrobial selection (in other words, are there more likely to be resistant bugs if the patient has just been on antibiotics, and they hypothesized that there would be)?

The authors retrospectively evaluated 86 dogs from the Royal Veterinary College between 2003-2011 that were diagnosed with septic peritonitis (using septic peritonitis as a common source of sepsis in our veterinary patients). Septic peritonitis was based on cytological confirmation or positive bacterial culture; all dogs went to surgery for surgical correction. Patients were separated into three groups based on illness severity. Dogs with “abdominal infection” had septic peritonitis with or without systemic inflammatory response syndrome (SIRS); those with “severe sepsis” had septic peritonitis with organ dysfunction; and those with “septic shock” had septic peritonitis with hypotension requiring vasopressor therapy post-operatively despite adequate fluid resuscitation. Antibiotic therapy was considered appropriate if at least one of the drugs given empirically before surgery and within 24 hours of diagnosis was effective against pathogens isolated based on susceptibility testing, and dosing and route were consistent with current standards.

Overall, 86 dogs were included in the study: Labrador retrievers were most common (n=10), followed by mixed breeds (8), with all other breeds represented by less than 6 dogs each. Ages ranged from 6 months to 12 years (mean 5.2 years), and mean weight was 24.7 kg. Based on the illness severity groups mentioned earlier, 37/86 (43%) dogs had “abdominal infection,” 31/86 (36%) had severe sepsis, and 18/86 (21%) had septic shock. 49/86 (57%) dogs survived to discharge; out of the 37 non-survivors, 28 were euthanized and 9 died while in the hospital. In the illness severity groups, mortality was 8% for dogs with abdominal infection, 55% for those with severe sepsis, and 94% for those with septic shock (p<0.001). The majority of dogs (64/86, 76%) had gastrointestinal leakage as their site of infection; other sources included the urogenital tract (n=8), intra-abdominal abscess (6), hepatic/biliary system (5) and primary peritonitis (1).

55% (47/86) of the dogs had a previous abdominal surgery performed within 30 days. This didn’t influence survival (p=0.065), but it was associated with inappropriate antibiotic choices (p=0.021). Antimicrobials were given to 73% (63/86) of dogs in the 30 days prior to hospitalization, with amoxicillin/clavulanic acid most commonly prescribed (65%). Previous antibiotics weren’t associated with outcome (p=0.3), but they were associated with inappropriate antibiotic selection (p=0.031).

94% (81/86) of cases had positive bacterial cultures with a total of 134 isolates identified. E. coli was the most common organism identified in 51% of samples (69/134), followed by Enterococcus (19/134), Clostridium (9/134) and Streptococcus (7/134). For empirical antibiotic choices, 44/86 (51%) dogs received a single agent and the remaining dogs received more than one antibiotic. The most common choices were amoxicillin/clavulanic acid (41/86), cefuroxime (34/86), fluoroquinolone (22/86), and metronidazole (22/86). The use of a single agent versus multiple antibiotics did not affect outcome (p=0.533) or the appropriateness of therapy (p=0.354). The appropriateness of the empirical antibiotics chosen could only be evaluated for 78 dogs because 1 dog had a culture that was not submitted, 1 culture was lost, 3 had no growth, and 3 had susceptibility panels that didn’t include the chosen antibiotics.

Overall, antibiotic choices were appropriate in 41/78 (52.6%) dogs; in the other 37 cases, the antibiotic choice was considered inappropriate based on culture and sensitivity results (not dose or route of administration). Of the 41 dogs that received appropriate antibiotics, 24 (58.5%) survived, whereas 20/38 (52.6%) of the dogs receiving inappropriate antibiotics survived (p=0.512). Appropriateness of antibiotic choice wasn’t associated with outcome when the illness severity groups were separated out, either.

So, what can we take away from this VETgirl podcast?

This is the first veterinary study examining empirical antibiotic choices in patients with sepsis, and the results are interesting since it suggests that in cases of septic peritonitis, choosing an appropriate antibiotic may not be associated with outcome. This is very different from studies of human patients with sepsis, where the evidence suggests that inappropriate antimicrobial choices increase the risk of mortality.

One of the areas where antibiotic choice may not be quite as critical in people is in cases of surgical sepsis, and this may relate to the results of this study. Because surgical source control is being performed in dogs with septic peritonitis in addition to antimicrobial therapy (compared to other cases of sepsis where only antibiotics may be used for treatment, for example in cases of pneumonia), antibiotic choices may not affect outcome as strongly. In combination with a small study population and a large percentage of patients that were euthanized or died, the authors hypothesize that this may be why they didn’t see an obvious benefit to choosing appropriate antimicrobials.

A limitation of this study was that timing of antimicrobial therapy wasn’t evaluated in this study, and even though all patients received antibiotics within 24 hours of hospitalization, the critical 6-hour time window mentioned earlier (in human medicine) couldn’t be examined based on the study records. This might be important in relation to outcome, and the authors note that it should be evaluated in future prospective studies.

One of the major take-away points from this study is that patients treated with antimicrobials in the previous 30 days were more likely to receive inappropriate antibiotics for their episode of septic peritonitis. While the authors couldn’t determine if this was due to changes in the patients’ enteric microflora from the previous antibiotics and subsequent resistant bacteria, or from a pre-existing resistant infection, it is important to keep in mind when choosing antibiotics for these patients.

This is an interesting study examining the appropriateness of antibiotics in dogs with septic peritonitis, and suggests that the choice of antibiotic may not significantly affect outcome. However, because this study is a small, single center retrospective, more data is needed before we can make definitive conclusions about this question. In conclusion, this study suggests that critically ill dogs with septic peritonitis may be more likely to receive inappropriate antibiotics if they received antibiotics or had abdominal surgery within the previous 30 days, and also suggests that the empirical choice of antibiotics for septic peritonitis may not affect outcome. VETgirl’s advice? Try to use antibiotics appropriately.

References:
1. Kumar A, Roberts D, Wood KE. Duration of hypotension before initiation of effective antimicrobial therapy is the critical determinant of survival in human septic shock. Crit Care Med 2006;34(6):1589-96.
2. Dickinson AE, Summers JF, Wignal J, et al. Impact of appropriate empirical antimicrobial therapy on outcome of dogs with septic peritonitis. J Vet Emerg Crit Care 2015;25(1):152-159.

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