When I was in vet school, I was under the impression that I would be performing ear cultures on a semi-regular basis. After all, how else would I know if and when I was treating a resistant infection?
Unfortunately, once you leave the ivory tower and enter “the real world,” bacterial cultures may become a lot less frequent.
When your clients have limited financial resources, spending a couple of hundred dollars on a culture and sensitivity can be a challenge to justify.
However, that doesn’t mean it’s any less important! It’s important for you to clearly understand the benefits, indications, and limitations to bacterial culture and sensitivity, so you will know when you should push for this test and when you can allow clients to decline it more readily.
When should I recommend a culture and sensitivity?
The following scenarios call for a culture and sensitivity:
- Chronic bacterial otitis with a poor response to medications
- Suspected or confirmed otitis media
- Bacterial otitis in a patient with a history of long-term topical or systemic antibiotics
- Any bacterial otitis involving rods (maybe... depending on who you ask!)
In most cases, it’s relatively easy to convince a client to go for a culture and sensitivity in a pet with recurrent or chronic otitis, or the possibility of otitis media. Convincing a client to authorize a culture on a first-time infection that happens to include rods, however, may be more of a challenge! Dermatologists differ on whether a culture and sensitivity is really necessary in this context. Some advocate for culture in all cases involving rods, while others recommend empirical treatment and say you should only move to culture if the pet does not respond. While you may have a preferred approach, based on what you learned from your professors, be aware that you may find yourself working in a practice that asks all doctors to follow a standardized approach, in order to provide consistency for clients and staff.
How should I collect a sample for culture and sensitivity?
Your patient should be off all antibiotics (topical and oral) and ear cleaners for several days before sample collection to prevent false negative results.
Ear samples for otitis externa can be collected from the external ear canal using a sterile swab. Samples for otitis media require myringotomy under anesthesia. Remember that a patient may have different organisms in the external ear canal and middle ear, so you may need to collect both samples.
What if my culture and sensitivity results don’t agree with my cytology?
Yes, this happens! In one study, only 68% of bacterial cultures agreed with the results seen on cytology.1 This same study also found that if two cultures were performed on the same ear swab, there was a 20% chance of obtaining differing results.1 Unfortunately, this suggests that cultures are not always accurate. Organisms differ in how well they will grow in culture, depending on a variety of factors.
When you receive results of a culture and sensitivity, evaluate them critically. You should always perform an ear cytology when submitting a culture and sensitivity, so you can use this information to determine whether your results are reasonable. If your culture results do not agree with what you have seen on cytology, consider submitting a fresh sample.
Should I always use the antibiotics suggested by culture and sensitivity?
Not necessarily. Culture and sensitivity testing is designed to examine expected serum levels of antibiotics. When you instill antibiotics topically into the ear, you are able to achieve concentrations that are dramatically higher than those tested in culture and sensitivity testing. In many cases, a given antibiotic may be effective even when the culture suggests possible resistance.
Are all bacteria listed on the culture and sensitivity report relevant?
Not always. Common commensals within the ear canal include Bacillus spp. and Micrococcus spp. Your sample may also be contaminated by your sampling technique. Review culture and sensitivity reports critically to determine which species warrant treatment. Staphylococcus spp. and Pseudomonas spp. should be the first species you look for and the species that you primarily target with treatment.
No Easy Answers on Bacterial Cultures
Given their many limitations, it’s not difficult to see why the need for otic cultures can be a controversial topic among veterinary dermatologists. However, these tests can offer significant benefits. Once you know which bacterial species are involved in your patient’s infection, you can make better treatment decisions. Additionally, an organism that is resistant to a large number of antibiotics at the concentrations tested suggests a need for more concern than an organism that is sensitive to all tested antibiotics. The key is to recognize that bacterial culture and sensitivity is just one of many tools to be used in the treatment of canine otitis. Don’t blindly make treatment recommendations based on your results; instead, evaluate results critically and in the context of the whole patient.
1. Graham-Mize, C. A., & Rosser, E. J., Jr (2004). Comparison of microbial isolates and susceptibility patterns from the external ear canal of dogs with otitis externa. Journal of the American Animal Hospital Association, 40(2), 102–108.