What do we know about the flow-on effects of antibiotic use in animals?
But there is concern that some of the drugs used in animals create opportunities for resistance to develop and cross into humans. The MPI data shows growing sales of the cephalosporin class of antibiotics, especially third- and fourth- generation cephalosporins. These include some of the limited therapies available for treating human illnesses, such as acute meningitis caused by Salmonella and multi-drug resistant E coli, and have been classed as “critically important” to human health by the World Health Organisation.
Sales figures also show a 25% rise in macrolides, one of a limited number of drugs able to treat Legionella, Campylobacter, MDR Salmonella and MDR Shigella infections. According to the ministry, macrolides in animals can give rise to macrolides-resistant Campylobacter spp in animals, especially Campylobacter jejuni in poultry.
“I find it mind boggling that you would use critical medicines out in the environment when you really should have very different types of drugs,” says microbiologist Siouxsie Wiles. “We need to make serious decisions about how we want our meat, and how expensive we want it to be, so animals can be raised without antibiotics. Yes, the food is cheap, but that’s because we are not seeing the real cost of it. If the real cost is we lose our ability to use modern medicine, to me that cost is not worth it.”
Although the NZ Veterinary Association has announced a vision that, by 2030, “New Zealand Inc will not need antibiotics for the maintenance of animal health and wellness”, livestock numbers continue to climb. Between 2011 and 2014, the national dairy herd grew from 4.6 million to 4.9 million. Sheep, beef cattle and pig numbers fell, but the number of chickens bred for meat jumped 11% and laying hens by 10%. As animal numbers rise, so, too, does the likelihood of outbreaks.
Antimicrobial use affects the environment. The antibiotics that animals – and we – ingest pass through the body and end up in soil, rivers, waterways and wastewater-treatment plants, encouraging the growth of antibiotic-resistant bacteria.
In crops, too, antibiotic-resistant bacteria in soil or leaf matter can transfer that resistance to more human-friendly bacteria. MPI figures also show a spike in antibiotic use in horticulture, largely the result of an outbreak of Psa (Pseudomonas syringae pv actinidiae). The antibiotic weapons of choice were kasugamycin and streptomycin; streptomycin, meanwhile, is used in human medicine to treat tuberculosis, urinary tract infections and pneumonia. Resistance to streptomycin has already been reported overseas.
“Whenever you use antibiotics, you create evolutionary pressure on natural selection, so giving an advantage for microbes that have greater resistance,” says Michael Baker, professor of public health at the University of Otago. “If you do that often enough and long enough, you will eventually see resistance to every known antimicrobial.”
Baker is a founding member of One Health Aotearoa, part of a global network promoting a unified approach to people, animals and the environment.
“We can’t just do what we’ve been doing and assume it will be okay. Saying dairying gives a great return so let’s just keep ramming a foot on the accelerator is very simplistic. Our environment is getting precarious and antibiotic resistance is one of the manifestations of that. There is a far more sophisticated approach to land use, sustainability and human and animal health. As a smart country, we need to be thinking that way.”