Acute gastrointestinal (GI) disease is the most common reason for the empirical prescription of antibacterials in small animal practice, with a suspicion of Campylobacter infection being the most cited reason (German et al., 2010). Yet it can’t have escaped anyone’s notice that antibacterial resistance is a growing problem, and there is increasing pressure to reduce unnecessary usage.
While the tonnage of antibacterials prescribed in small animal practice is dwarfed by that in both medicine and the farm animal industry, the potential for zoonotic transmission of resistant organisms from companion animals is perhaps greater: owners are more likely to have close contact with their pet than any farm animal. Therefore, before the regulators restrict a vet’s right to prescribe, we need to ensure we are using antibacterials in a rational and safe manner.
Antibacterials are most frequently used in dogs with acute gastroenteritis, and since it can take several days for a stool culture to rule out a bacterial cause, it is likely that they are often used unnecessarily. While acute gastroenteritis in vaccinated dogs is often attributed to Campylobacter jejuni infection, a study in Germany using electron microscopic examination of faeces showed that over half of dogs with haemorrhagic diarrhoea had a viral infection. As well as parvovirus and distemper, there is an emerging range of enteric viruses infecting dogs, from astrovirus, circovirus, coronavirus and rotavirus, to bocavirus, kobuvirus and sakobuvirus, and even norovirus.
When Campylobacter spp. are identified on routine stool culture, the likelihood is that it is C. upsaliensis; indeed, this can be isolated from about 30 percent of young dogs in the UK. Although potentially a zoonotic pathogen, C. upsaliensis may actually be a commensal in dogs and not need any treatment.
C. jejuni can only be reliably identified by PCR after culture, and so antibacterials are usually started before a confirmatory result of infection is obtained. However, it can also be isolated from the stool of healthy dogs, and so isolation does not prove it is the cause of any signs. Furthermore, most C. jejuni infections appear to be self-limiting and antibacterials are often unnecessary, although pet owners often expect antibiotics to be prescribed when their dog has diarrhoea. Appropriate fluid therapy is the most important treatment, but on the principle of “first do no harm”, when there is an expectation to treat in likely self-limiting diarrhoea, it is safer to use adsorbents (eg kaolin, pectin, etc) and/or probiotics.
With the increasing popularity of feeding raw foods, there appears to be increasing numbers of dogs excreting Salmonella in their faeces
With the increasing popularity of feeding raw foods, there appears to be increasing numbers of dogs excreting Salmonella in their faeces. Freeze-thawing raw food before feeding does not kill all potential pathogens, and unless each batch is microbiologically tested, a risk of infection exists. However, use of antibacterials is not generally recommended as Salmonella infection may be asymptomatic or self-limiting, whereas treatment increases the chances of antibacterial resistance and induction of a carrier state. Only when there is evidence of sepsis is treatment warranted, and this is seen more frequently in cats than dogs.
Renamed “acute haemorrhagic diarrhoea syndrome” (AHDS), because there is no gastritis, the general consensus is still that dogs with haemorrhagic gastroenteritis (HGE)/AHDS should be given antibacterials because of the risk of sepsis. The presence of blood in the stool (Figure 1) indicates the mucosal barrier has been breached, and therefore bacteria can potentially enter the bloodstream. However, Unterer et al. (2011) in a randomised trial showed that administration of potentiated amoxicillin in cases of HGE did not affect morbidity or mortality, suggesting that the common use of this antibiotic is unnecessary.
Closer review of this study reveals that dogs with evidence of sepsis (pyrexia, raised WBC count) were excluded. Therefore, prescription of antibacterials is still justifiable in sick dogs with AHDS, and potentiated amoxicillin with or without metronidazole are reasonable choices as they cover the typical spectrum of enteric bacteria.
Although antibacterials have no efficacy against viral infections, use is justified in dogs with canine parvovirus (CPV-2) infection. Not only may the patient have bloody diarrhoea, they are also likely to be immunosuppressed; they are typically young with an immature immune system, and parvovirus can cause concurrent neutropenia and lymphopenia. Thus, parvo puppies are at real risk of bacterial sepsis and death, and should be given prophylactic antibiotics.
Although Giardia is sensitive to metronidazole, a high dose (25mg/kg q12h for 5 days) is required, and this is close to the neurotoxic dose. It is more rational to use fenbendazole (50mg/kg/day for a minimum 3 doses), as it is licensed for this use, is safe and is not an antibacterial.
Canine chronic enteropathies (CCE) are currently subdivided into food-responsive enteropathy, antibiotic-responsive diarrhoea (ARD) and steroid-responsive enteropathy (equivalent to idiopathic inflammatory bowel disease), based on their response to empirical treatment trials. Dogs where there is no suspicion of underlying neoplasia, where no bacterial pathogen has been isolated and where an empirical fenbendazole trial has failed are typically given a food trial, usually with a hydrolysed diet. Dogs that do not respond could be trialled with steroids next, but it is less likely to harm the patient if an antibiotic trial is commenced first. This approach has been criticised as it risks induction of resistance, and so critically important antibacterials should not be used, but trials with oxytetracycline or tylosin are justifiable.
ARD is seen most often in young German Shepherds and is managed with long courses of oxytetracycline (Figure 2) or tylosin. Metronidazole can also be used, but long courses can lead to toxicity. Treatment should be stopped periodically to see if relapse occurs as many dogs grow out of the problem by about two years of age.
Chronic bacterial enteritis
Most cases of bacterial enteritis are acute and selflimiting. However, persistent infection is a potential cause of diarrhoea if organisms can attach to the mucosal surface (eg attaching and effacing enteropathogenic E. coli (EPEC)) or can invade (eg Salmonella). Diagnosis cannot be confirmed by stool culture, as these organisms can be found on the stool of clinically healthy dogs. Identification of organisms in biopsies using fluorescent in situ hybridisation (FISH) analysis is needed.
Most cases of bacterial enteritis are acute and self-limiting
Boxer colitis (formerly histiocytic ulcerative colitis) is a severe condition seen in young Boxers and French Bulldogs. It is now known to be caused by infection with an “attaching and invading E. coli”, and can be cured by a prolonged course of fluoroquinolones (Mansfield et al., 2009).
However, granulomatous colitis is rare in comparison to other forms of colitis in Boxers. Inappropriate, empirical use of enrofloxacin in any Boxer with colitis in the USA led to emergence of resistance, with amikacin often being the only effective antibiotic left. Thus, confirmation of infection by FISH is mandatory before prescribing fluoroquinolones for Boxer colitis.
Acute bacterial enteritis is likely to be self-limiting and probiotics should be prioritised over antibacterials initially. Antibacterial usage is generally justifiable in haemorrhagic diarrhoea if the dog appears septic or has parvovirus infection.