Reconsidering antibiotic duration for canine aspiration pneumonia

Photo courtesy Dr. Greg Bishop

Aspiration pneumonia can be a nightmare. Conventional wisdom says, after stabilization, dogs with aspiration pneumonia should be treated for four to six weeks with antibiotics, and an additional one to two weeks after clear chest radiographs.1 While this may be a widespread belief in the veterinary community, the evidence base for such recommendations is actually quite thin. Extended antibiotic therapy is not an unreasonable idea, in fact, new research is whittling away the recommended duration of antibiotics for canine aspiration pneumonia. Is there, in fact, good evidence for recommendations for long duration of antibiotic therapy for aspiration pneumonia?

What is the ideal treatment for aspiration pneumonia? What would shorter courses of antibiotics work? Are there consequences of prolonged treatment? What is the incidence of disease relapse? Does extended treatment promote microbial resistance?

Current evidence and fresh findings

PICO questions (Population, Intervention, Comparison, Outcome), are the foundation for any evidence-based clinical inquiry: Are dogs with aspiration pneumonia (P) treated with short courses of antibiotics (I) versus “standard” courses (C) have different results? (O)? While there is not an extensive literature research does at least begin to answer the PICO question, and even better, seems to point in a consistent direction.

In the relevant studies, there was only one randomized clinical trial. It showed dogs with aspiration pneumonia treated with either 10-day or 21-day courses of antibiotics all got better without incident.2 The study was properly conducted, however, it was only a pilot study, with fewer than 10 cases.

Fortunately, the disease lends itself to observational studies, since the treatment variable in question is duration and easy to measure. One such study found “early” discontinuation of antibiotics at 21 days was just as effective as a 35-day course,3 while another found most dogs needed only one week of treatment to resolve.4 Both studies also utilized C-reactive protein (CRP), a clinical biomarker of inflammation that spikes during an aspiration pneumonia event but resolves faster than do thoracic radiographs.5

In another observational study, researchers found no difference in outcome between dogs treated with an average of 11 days versus an average of 21 days. About half of the dogs were clinically better by two weeks but were continued on antibiotics because they did not have normal chest X-rays.6

One final paper showed resolution by one week of treatment, and an average of nine days of antibiotic therapy with
no relapses.7

Uncontrolled studies have their limits, but with a broad range of cases and institutions, and the specific clinical question of duration, the picture is becoming clearer. It is quite obvious not all dogs need four to six weeks of antibiotics. The consistent finding is short courses of antibiotics are just as effective as longer ones for most dogs with aspiration pneumonia.

Knowledge gaps

The information gained from studies is clinically helpful, but it also generates more questions. For example, are antibiotics always necessary? In most cases, a bacterial infection is presumed when a dog aspirates, but it is not confirmed.8 In addition, do clinical signs of pneumonia, inflammatory leukogram, radiographic signs, or elevated CRP prove pathogenic bacteria are replicating in lung tissue? Does a positive culture from lung wash fluid even do that? Very few cases of aspiration pneumonia get cultured for antibiotic susceptibility testing,8 and when they do, guidelines may over-categorize the infection rate by three to four times.9 Under more judicious antimicrobial policies, most cases from a recent canine study would not have gotten antibiotic therapy, even with the documented presence of bacteria in the lungs.9 Further, another case series of aspiration pneumonia in dogs indicated all of the animals got better without antibiotics even though some of them were very sick.10

Other relevant questions include:

  • If supportive care is sufficient in some cases, do all dogs with aspiration pneumonia need antibiotics?
  • Are CRP levels (clinic cost about $30) a better clinical indicator than radiographs?
  • Are both CRP levels and radiographs misleading?
  • Could treatment decisions be based on clinical signs alone?

Current directions

While the picture is not yet crystal clear, and there are no systematic reviews and meta-analyses to guide the decision-making process, research findings might also be considered in light of the current International Society for Companion Animal Infectious Diseases (ISCAID) guidelines. These guidelines clearly state not all cases of canine aspiration pneumonia require antibiotics, and that dogs should be reevaluated after 10-14 days before continuing treatment.11

It is fairly clear radiographs lag behind clinical resolution, so they are probably not as helpful as traditionally thought.2,5,12 CRP might be a better clinical indicator,5 or perhaps a normal physical exam can guide as to the best time to discontinue antibiotics. Especially with a stable patient, withholding antibiotics pending worsening signs seems a perfectly justifiable strategy (i.e. a “delayed prescription”).11

Aspiration pneumonia is just one condition where new findings are shedding light on inappropriate antimicrobial practices. Here is a sobering excerpt from an American College of Veterinary Internal Medicine (ACVIM) consensus statement on antimicrobial resistance: “While recommendations are available in clinical guidelines, review articles or general (e.g. textbook) references, these have limited scientific foundation.”13

The fact cases of aspiration pneumonia resolved after prolonged antibiotic therapy does not also mean prolonged antibiotic therapy caused the resolution. Futher, given global concerns about antibiotic resistance, is prolonged antibiotic therapy a sustainable long-term practice?

The bottom line

Veterinarians still have much to learn about optimal treatment for canine aspiration pneumonia, but the pieces are coming together. So far, the evidence supports shorter durations of antibiotics much more than the old “four to six weeks” orthodoxy.

The science will keep evolving, and it is possible that in a decade or so we will not reach for antibiotics nearly so often in these cases.

Greg Bishop, DVM, is a small animal veterinarian and part-time veterinary technology instructor in Portland, OR. Dr. Bishop is a member of the Evidence-Based Veterinary Medical Association (EBVMA), with different members writing this column. While all articles are reviewed for content, the opinions and conclusions of the author(s) do not necessarily reflect the views of the EBVMA or Veterinary Practice News. For information about the association or to join, visit


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