Antimicrobial Resistance in Veterinary Medicine

by | Nov 18, 2021 | Veterinary Diagnostics

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Overuse of antimicrobial agents in veterinary medicine contributes to the spread of resistance genes which undermine our ability to prevent or treat infectious disease.

Overuse of antimicrobial agents in veterinary medicine is fueling antimicrobial resistance (AMR) in common human pathogens, threatening our ability to prevent and treat infectious disease. The WHO warns that, without urgent intervention, a post-antibiotic era will occur in which minor infections and injuries can be fatal.

Antimicrobial use policies in both companion animal and agricultural veterinary medicine must be updated in order to preserve their effectiveness in human medicine.

Veterinary Medicine, Antimicrobial Resistance, and Human Health

The veterinary and human sectors share antimicrobial classes used to treat infections. When resistance traits are developed in one context, they quickly spread and appear in other species and environments. This makes antimicrobial resistance a global One Health challenge.

The One Health approach recognizes that humans, animals, and the world we live in are linked. The Centers for Disease Control and Prevention (CDC) defines One Health as “a collaborative, multisectoral, and transdisciplinary approach—working at the local, regional, national, and global levels—with the goal of achieving optimal health outcomes recognizing the interconnection between people, animals, plants, and their shared environment.”

AMR transmission involves a dynamic and complex web of interactions (Figure 1). Resistance can spread between humans and animals through the food chain; environment (e.g., sewage, animal manure, soils); or direct contact. Poor hygiene, sanitation, and infection control are three key factors contributing to the transmission of AMR in the ecosystem. Increases in international travel, migration, and trade accelerate AMR propagation. These factors demonstrate the interconnectivity of humans, animals, and the environment and the need for all sectors to work together in combatting AMR.

Routes of transmission of antimicrobial resistance
Figure 1. Possible AMR transmission routes between humans, animals, and the environment.

In 2014, the US released a National Action Strategy for Combatting Antibiotic Resistance Bacteria (CARB). The resulting National Action Plan and USDA Action Plan both identify the need for a collaborative approach by human, veterinary, and environmental sectors to address this issue. The World Organization for Animal Health (OIE) is instrumental in the One Health initiative by collaborating with over 70 international organizations that play a role in the human-animal-ecosystems interface.

Companion Animal Veterinary Practice and Antimicrobial Resistance

Companion animals, especially cats and dogs, play an important role in AMR development, carriage, and transmission to humans. Pets can be reservoirs for resistance microorganisms, particularly when infections are not completely treated. Through physical contact, resistant microorganisms living on a pet’s skin or in saliva can be transferred to humans. This poses a zoonotic health risk.

American Veterinary Medical Association (AVMA) members reported that 82% of companion animal veterinarians were strongly or somewhat concerned about AMR infections in cats and dogs. Over the last decade, many small animal practitioners have been confronted with multi-drug resistant infections for which limited effective antimicrobials exist. Methicillin-resistant Staphylococcus (MRSA), methicillin-resistance Staphylococcus pseudintermedius (MRSP), vancomycin-resistant enterococci (VRE), extended spectrum beta-lactamase (ESBL) or carbapenemase-producing Enterobacteriaceae (CPE), and gram-negative bacteria are the most threatening AMR bacteria of pet origin to human health.

AMR Pathogens Affecting Companion Animals

The AVMA partnered with microbiologists, epidemiologists, and veterinary experts to identify a comprehensive list of antimicrobial resistant pathogens affecting animal health in the United States. The drug resistant bacterial pathogens that originated from cats and dogs include:

  • Staphylococcus species
    – S. aureus
    – S. pseudintermedius
    – S. schleiferi
  • Enterobacteriaceae
    – Escherichia coli
    – Proteus species
    – Klebsiella species
  • Acinetobacter species
  • Pseudomonas aeruginosa
  • Enterococcus species
    – E. faecalis
    – E. faecium
  • Campylobacter jejuni

Overuse of Antimicrobials Drives AMR in Companion Animals

Injudicious antimicrobial use is a key driver of the promotion and spread of AMR in companion animals. According to the CDC, up to 50% of antimicrobials are unnecessarily or inappropriately prescribed in human medicine. The percentage is likely similar in companion animal settings. Of veterinarians surveyed by the AMVA, 62% felt that antibiotic use in small animal practice impacts the AMR issue. Clinical veterinarians at a US veterinary teaching hospital reported that 88% of faculty members felt antimicrobials were overprescribed at the hospital, and 46% felt uncomfortable prescribing at least one class of antimicrobials (e.g., carbapenems) due to concerns about AMR and public health risks. Another study investigating the antimicrobial use practices of veterinary clinicians at a US veterinary teaching hospital reported that 33.9% of their clinicians prescribe antimicrobials with no documented evidence of infections and 61.3% sometimes prescribe antimicrobials for suspected, but not confirmed infections.

Veterinarians commonly use empirical therapy for treating infectious diseases in cats and dogs. In fact, they often prescribe antimicrobials on a “just in case” basis; a judgment veterinarians make from prior experience and clinical presentation. However, treating patients based on empiric antimicrobial selection may have several undesirable outcomes. Pets may adversely react to broad-spectrum antibiotics and develop diarrhea, inappetence, or vomiting. In addition, under-treatment can result in the antimicrobial therapy being ineffective. In both cases, the patient must schedule an additional visit to change the therapy, and this increases the overall treatment costs. The indiscriminate use of broad-spectrum antimicrobials also encourages the selection and spread of resistant bacteria, contributing to the global issue of AMR.

Figure 2 is an example of the typical clinical treatment approach for urinary tract infections (UTI). Veterinary professional organizations such as the International Society for Companion Animal Infectious Diseases recommend using culture and sensitivity test (C&ST, also known as Culture and Antibiotic Susceptibility Testing) as tools to support diagnosis and initial treatment of bacterial infections; however, only 4% of veterinary professionals use this testing method. One possible reason for its limited use is the long wait time for results, which is 3 to 5 days.

Clinical decision tree for urinary tract infection
Figure 2. A routine clinical workflow for diagnosing and treating patients presenting UTI symptoms in companion animals.

When a patient presents with suspected simple UTI symptoms, veterinarians often begin empirical therapy by prescribing a broad-spectrum antimicrobial to a pet. Without definitive C&ST test results to identify the source of the infection, over treatment or under treatment may be given. Over-treatment occurs when C&ST results are later obtained and show UTI absence. Under-treatment occurs when the antimicrobial therapy is ineffective. Over-treatment requires de-escalation of therapy and undertreatment requires escalation of therapy, both of which contribute to AMR emergence. Recurring UTIs under empirical therapy may require de-escalation or escalating therapies. Thus, both C&ST and empirical therapy methods for diagnosing and treating UTIs in companion animals contribute to AMR development.

Farm Animal Veterinary Practice and AMR

In 2019, over 11,468 tons of antimicrobials were sold for use on farm animals in the United States alone. On a global scale, the demand for animal protein to support human population growth is driving increased antimicrobial use in livestock animals. Researchers estimate antimicrobial consumption to rise by 67%, from 63,151 tons in 2010 to 105,596 tons in 2030.

Livestock farmers use antimicrobials in several different ways to maintain animal health and increase productivity.

  • Therapeutic use: treatment of sick animals infected by pathogens
  • Metaphylactic use (or disease control): treatment of both healthy and diseased animals belonging to the same group after the diagnosis of a contagious infectious disease to prevent an outbreak in the entire group
  • Prophylactic use (or disease prevention): treatment of a group of healthy animals to prevent potential infections from developing
  • Growth promotion use: adding antimicrobials into animal feed or water to promote faster or more efficient livestock growth

The widespread use of non-therapeutic antimicrobials in farm animals to encourage faster growth or to prevent illness is concerning. Antimicrobial long-term use in livestock creates ideal conditions for the development and spread of resistant strains.

The first case of antimicrobial resistance in livestock was reported in 1951 after streptomycin was fed to turkeys. Since then, reports of resistance to many classes of antimicrobials such as tetracyclines, sulfonamides, beta-lactams, and penicillin have emerged. Other examples include apramycin and ampicillin-resistant E. coli in newborn calves and CRE in livestock. In 2015, resistance to colistin was observed in pigs and is now found in up to 100% of farm animals in some places. Colistin resistance is easily transferred between organisms and can be acquired by humans.

From Agriculture to Human Disease

Animals raised on antimicrobials are a threat for harboring multidrug resistance superbugs and transmitting them to humans. These animals could act as reservoirs of resistant organisms that eventually find their way to humans through direct contact with animals, by exposure to manure, through consumption of undercooked meat, and through contact with uncooked meat.

Meat sold in supermarkets may contain bacteria resistant to antimicrobials. In fact, the Environmental Working Group (EWG) analyzed over 47,000 US federal government lab tests of bacteria on supermarket meat. EWG’s analysis shows resistant bacteria in 81% of raw turkey meat, 69% of raw pork, 62% of ground beef, and 36% of chicken.

Several other outbreaks of infectious disease caused by multidrug-resistant organisms, acquired through food sources, have brought the use of antibiotics in agriculture into public attention. In 2014, a multistate outbreak of multidrug-resistant Salmonella heidelberg in the US was linked to consumption of chicken meat. Incidences like this have prompted many public calls for “antibiotic-free meat.”

Global AMR Action Plans

Without prompt action, AMR will compromise our ability to treat infectious diseases and advance health and medicine. To prevent AMR emergence and spread, both the WHO and CDC developed action plans.

In 2015, the WHO developed a global action plan on AMR to ensure that infectious diseases can be treated and prevented with effective and safe medicines that are used responsibly.

As a part of the Antibiotic Resistance Solutions Initiative, the CDC fights AMR in food, farms, and animals by helping veterinarians have the tools, information, and training required to use antibiotics. This initiative supports the work of the FDA and USDA to improve antibiotic use in veterinary medicine and agriculture.

The US National Action Plan for Combating Antibiotic-Resistant Bacteria aims to slow the emergence of resistant bacteria and strengthen national One Health surveillance efforts to combat resistance.

The AVMA urges veterinarians to take action on AMR and implement the principles of antibiotic stewardship to improve disease prevention strategies and antimicrobial drug prescribing.

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