Antimicrobial stewardship differs by species, CDC epidemiologist says

The AVMA published the second edition of Antimicrobial Stewardship and Pathogens Affecting Animal Health in the United States on November 5, and its species-by-species structure makes one point clear: the right drug is not enough without the right timing and duration.

Dr. Alexandra M. Medley, veterinary epidemiologist and associate director for antimicrobial resistance at the CDC's Division of Foodborne, Waterborne, and Environmental Diseases, presented the report's highlights at AVMA Convention 2026 in Anaheim, California. Her session, "Safeguarding Animal Health Against Antimicrobial-Resistant Pathogens," walked through host-specific summaries of pathogens of concern, resistance consequences, and stewardship actions. The report was authored by the AVMA Committee on Antimicrobials.

Though pathogens can affect multiple species, she said, the host's environment, approved antimicrobials and vaccines, and zoonotic potential all shape how a veterinarian should practice stewardship.

What changes for companion animals?

The true prevalence of resistant pathogens in dogs and cats remains largely unknown, Dr. Medley said. One Health considerations differ from food animals: pets carry direct household transmission risk rather than occupational or food-safety risk.

The report strongly promotes cytology, cultures, and antimicrobial susceptibility testing where appropriate. "Investigate underlying causes of recurrent infections," Dr. Medley advised. "And remain vigilant for emerging resistance concerns that extend beyond bacteria."

That vigilance now includes fungi. Dermatophytes (Microsporum canis and Trichophyton spp) are the first fungal pathogens covered in the report. Most ringworm cases stem from M canis, and terbinafine resistance is emerging in severe Trichophyton indotineae infections that fail usual treatment. Veterinarians may struggle to find a laboratory offering antifungal susceptibility testing, and no species-specific breakpoints exist for these organisms.

Breakpoints, the report explains, are applied in susceptibility testing to specific species-pathogen-drug combinations to classify isolates as susceptible, intermediate, or resistant.

What does the cattle section say about BRD?

For cattle, stewardship centers on prevention, diagnostics, and management — tactics that cut antimicrobial use, preserve drug efficacy, and avoid the economic damage of disease. Bovine respiratory disease is the clearest example, because BRD drives substantial antimicrobial use.

Diagnostics remain imperfect. The report cites the lack of reliable, affordable, farm-level "cow-side" tests. BRD is driven largely by three pathogens — Pasteurella multocida, Histophilus somni, and Mannheimia haemolytica — but environmental factors, host factors, and viruses also contribute. Treatment decisions often hinge less on therapy duration and more on post-treatment meat or milk withdrawal intervals.

"Stewardship is 100% not simply about choosing the right drug, but the right drug at the right time and for the right amount of time," Dr. Medley said.

How do poultry and aquaculture differ?

These sectors have limited drug and vaccine options, so preserving existing treatments matters more. For food fish, only three antimicrobials carry FDA approval.

Stewardship starts before treatment, through biosecurity, age-appropriate vaccination, and good management. Ornithobacterium rhinotracheale, a major respiratory pathogen in turkeys, shows why: few treatment options exist, no widely available commercial vaccine exists, validated breakpoints are lacking, and susceptibility varies across isolates.

What is the equine concern?

For horses and other equids, infection prevention equals antimicrobial selection in importance, because resistant infections spread among horses, people, other animals, and the environment. MRSA is a pathogen of concern: S aureus is among the most commonly isolated gram-positive bacteria colonizing equid skin, and horses previously exposed to antimicrobials or to human-transmitted infections face much higher MRSA risk.

Most MRSA isolates are multidrug resistant, and few FDA-approved drugs are active against MRSA in horses. "This may be one of those cases where legally compliant extralabel drug use of human and animal drugs is often needed for treatment," Dr. Medley said.

What about pigs and small ruminants?

Swine stewardship decisions occur at the group level. The report recommends core principles of swine health management — "all in, all out" flow, biosecurity, minimized environmental stress — to prevent infections. Viral infections frequently predispose pigs to secondary bacterial infections, creating stewardship tradeoffs. Pathogens of concern include Salmonella spp, Streptococcus suis, Staphylococcus hyicus, Escherichia coli, Pasteurella multocida, Glaesserella parasuis, Mycoplasma spp, and agents of colitis or enteritis syndrome.

Salmonella illustrates the report's nuance: resistance patterns vary by serotype, and one herd or animal can host multiple serotypes at once. "Understanding the biology and, in particular, the epidemiology of the pathogen, is just as important as understanding your susceptibility report and your test results," Dr. Medley said.

For sheep and goats, the report delivers one of its most striking messages: for many pathogens of these species, validated clinical breakpoints simply do not exist, so resistance can be hard to recognize. Repeated treatment or retreatment with an alternative drug is often the first sign it is present.

"Every species-pathogen combination is different, but stewardship principles do remain remarkably consistent," Dr. Medley said. "It's prevention, diagnostics, thoughtful interpretation, and, ultimately, responsible antimicrobial use."

The AVMA supports practitioners with species-specific educational materials and policies on stewardship and judicious use, resources practitioners can use as the second-edition report informs day-to-day prescribing decisions.