Antibiotic use in animals, humans can trigger AMR in each other: study establishes links

Reciprocity of antimicrobial consumption and resistance between animals and humans emphasises the need for integrated strategies to stop transmission across various One Health domains
AMR is the ability of bacteria and other microbes to resist the drugs used to inhibit or kill them. Representative photo: iStock.
AMR is the ability of bacteria and other microbes to resist the drugs used to inhibit or kill them. Representative photo: iStock.

recent study has found there are bidirectional associations between antibiotic consumption and antimicrobial resistance (AMR) rates in humans and animals. Antibiotic use by humans can lead to AMR in animals and antibiotics in animals can cause AMR in humans.

While antimicrobial consumption in animals was significantly associated with AMR in human pathogens (listed under critical priority by the World Health Organization), AMR in humans was associated considerably with animal AMR rates, noted the study published in The Lancet Planetary Health on April 4, 2023.

AMR is the ability of bacteria and other microbes to resist the drugs used to inhibit or kill them. Antibiotics are used in food-animals to promote growth and prevent diseases. This overuse and misuse can lead to the development of antibiotic-resistant bacteria, which can then be transmitted to humans.

Critical priority human pathogens include carbapenem-resistant A. baumannii, carbapenem-resistant E. coli, third-generation cephalosporin-resistant E. coli, carbapenem-resistant K. pneumoniae, cephalosporin resistant K. pneumoniae and carbapenem-resistant P. aeruginosa.

The study has been carried out by researchers from various universities as well as experts across the globe.

Carbapenem-resistant acinetobacter baumanii, third-generation cephalosporins-resistant escherichia coli, and oxacillin-resistant staphylococcus aureus (all antibiotic-bacterium specific pairs) were found to have significant associations with animal antibiotic consumption.

These bacteria are known to cause severe infections in humans such as pneumonia, bloodstream infections, meningitis, urinary tract infections, gastroenteritis, bacteremia, etc. This is one of the first studies that has identified the global two-way animal–human associations.

In addition, significant associations were also noted between animal antimicrobial consumption and AMR in food-producing animals and between human antimicrobial consumption and AMR, specifically in critical-priority and high-priority pathogens.

In view of antibiotic consumption in humans, higher consumption is found to be associated with greater resistance in carbapenem-resistant acinetobacter baumanii, carbapenem-resistant pseudomonas aeruginosa, third-generation cephalosporins-resistant escherichia coli, third generation cephalosporins-resistant klebsiella pneumoniae, oxacillin-resistant staphylococcus aureus and vancomycin-resistant enterococcus faecium.

Third generation cephalosporins and vancomycin belong to the highest priority critically important antimicrobials category; carbapenems belong to critically important antimicrobials category and oxacillin belongs to highly important antimicrobials category, as classified by WHO.

Antibiotic consumption in animals was positively associated with rates of third-generation cephalosporins-resistant E. coli, oxacillin-resistant S. aureus and carbapenem-resistant A. baumanii.

Overall, for the critical human pathogens, the study found that carbapenem and third-generation cephalosporin consumption in humans and antibiotic consumption in food-producing animals were linked to an increase in AMR. These were also positively associated with resistance in food-producing animals. 

The study adds to the growing body of evidence suggesting that antibiotic use in animals may be contributing to the rise of AMR in humans.

The reciprocity of antimicrobial consumption and resistance between animals and humans emphasises the need for integrated strategies to stop transmission across various One Health domains.

Adopting better preventative measures, such as biosecurity, vaccination and waste management, may fall into this category. A 2021 report by the Delhi-based non-profit Centre for Science and Environment showed how critically important antibiotics are misused and overused in the Indian food-animal production sector.

In the dairy sector, 21 critically important antimicrobials from six classes, 14 critically important antimicrobials from four classes in the poultry sector, and three critically important antimicrobials in the aquaculture sector were found to be in use.

In the dairy sector, third, fourth, and fifth-generation Cephalosporins and several other critically important antimicrobials were used. These findings can now be viewed in light of the associations established in the study to understand how the use of cephalosporins in animals poses a risk of resistance in human population.

Efforts to establish similar food-animal and human associations were also attempted in another report titled Antimicrobial consumption and resistance in bacteria from humans and animals.

It focuses on six antimicrobials classes: Third- and fourth-generation cephalosporins, fluoroquinolones, polymyxins, aminopenicillins, macrolides and tetracyclines, five out of which are critically important antimicrobials. 

In the report, associations between food-producing animals and humans were observed between the consumption of an antimicrobial class and bacterial resistance to the antimicrobials in this class in the same population.

For example, in the case of food-producing animals, a statistically significant positive association was observed for one or more years between the prevalence of extended-spectrum beta-lactamase-producing E. coli and AmpC beta-lactamase-producing E. coli and consumption of third- and fourth-generation cephalosporins.

In addition, resistance in bacteria in humans was associated with antimicrobial consumption in animals as well as resistance in bacteria from food-producing animals which, in turn, was related to antimicrobial consumption in animals.

For example, a statistically significant positive association was observed for one or more years between resistance to third-generation cephalosporins through a univariate analysis in invasive E. coli isolates from humans and the consumption of third- and fourth-generation cephalosporins, both in humans and in food-producing animals.

Globally, antibiotic sales are increasing with rising food demand, making antimicrobial resistance a serious concern in countries where antibiotics can be purchased without a prescription, such as low- and middle-income countries.

Some 93,309 tones of antibiotics were sold for use in food-producing animals worldwide in 2017, with that figure expected to rise to 104,079 tones by 2030, according to the study.

The associations presented in the study demonstrate a strong understanding of the importance of conserving antibiotics because, whether used in animals or humans, they will undoubtedly pose a risk to other counterparts and make disease treatment difficult for both.

In addition, the study has also identified determinants of AMR in humans and food-producing animals and established a positive association of antimicrobial resistance with indicators such as socioeconomic and demographic indicators, environmental indicators, health-related indicators and antibiotic policy and regulation in humans and animals.

Their specific indicators that establish positive association are presented in the table below. 

Socioeconomic and demographic indicators  
GINI index The extent to which the distribution of income between individuals or households within an economy deviates from a perfectly equal distribution
Mortality rate attributable to unsafe WASH Deaths attributable to unsafe WASH focusing on inadequate WASH services, expressed per 100,000 population
Population density Number of people divided by land area measured in km2
Net migration rate Annual difference in number of immigrants and emigrants
Median age of the population Median age of the population
Homeless people Annual average number of homeless people due to natural disasters per 1,000,000 people
Environmental indicators
PM2·5 Annual mean concentration of PM2·5 in urban areas
Average temperature (°C) Average 12 monthly temperature in Celsius
C. Health-related indicators
Cardiovascular death rate per 100,000 Annual number of deaths per 100,000 people due to cardiovascular disease
Obesity prevalence Crude prevalence of obesity in adults (BMI ≥30)
Antibiotic policy and regulation in humans and animals
Arable land (percentage of land area) Percentage of land area that is under temporary crops, temporary meadows for mowing or for pasture, land under market or kitchen gardens, and land temporarily fallow
Cattle density Global distribution of cattle expressed in total number of cattle per pixel

The study clearly shows AMR encompasses both human and animal aspects and a logical consideration of other dimensions is also required to address the issue on a global scale.

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