Overuse of antibiotics is accelerating the spread of drug resistant superbugs, experts warn.

Farmers are buying more of the antibiotics classified as critically important for human health, despite dire warnings about drug resistant superbugs.

Sales in four of the five antibiotic groups identified by the World Health Organisation (WHO) as being of critical importance increased in 2017, according to the Ministry for Primary Industries (MPI).

While total sales of antibiotics to the agricultural industry rose 3 per cent, sales of critically important drugs increased by 5.5 per cent overall and as much as 16 per cent in one group, the ministry's latest Antibiotic Sales Analysis shows.

It was unclear why antibiotic sales were increasing but possible causes included new or increased disease prevalence, stockpiling of the medicines or a reduction in "prudent use", the report says.

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Earlier this year, the WHO listed antimicrobial resistance – driven by antibiotic overuse in animals and people – as one of the top 10 threats to global health.

MPI agricultural compounds and veterinary medicines programmes and appraisals manager, Karen Booth, said any increase in the sale and use of critically important antibiotics was a concern.

At the end of each reporting period, industry and vets were asked to provide the ministry with comment on any factors that could contribute to changes in antibiotic sales volumes.

"Apart from an increase in poultry numbers over the last year, no other contributory factors were identified by industry," she said.

SUPPLIED Dr Heather Hendrickson says consumers need to understand the seriousness of the threat posed by antibiotic resistance because it could ultimately fall to them to force a change.

Nearly half of all antibiotics sold in New Zealand during 2017 were registered for use in pigs, poultry or both.

The majority contained the active ingredient zinc bacitracin, which is mixed into the daily feed of most meat (broiler) chickens to protect against necrotic enteritis.

It can also increase body mass gains in chickens, and is promoted by some manufacturers as a growth enhancement.

Booth said zinc bacitracin was used just like other antibiotics, for the management of disease, and was not an antibiotic of concern to human health.

STUFF Even if they're not sick, most of our chickens are medicated.

However, the medicine is mixed into chicken feed as a preventative measure, not a cure, and is classed as "important" for human health by the WHO.

Asked if farming chickens in conditions where they were less likely to develop and rapidly spread diseases would be preferable to doling out medication with their feed, Booth said animal health and welfare was a legitimate reason to use antibiotics.

"MPI supports changes in livestock production practices, including the preventative measures such as vaccination to reduce the need for antibiotics," she said.

Michael Brooks, executive director of the Poultry Industry Association, said there were discrepancies between the data supplied by the industry and the figures reported by MPI.

"Our industry data shows we don't use 20 per cent of what MPI claims was sold by the companies that supply the two industries," he said.

"Furthermore, the report found the overall use of zinc bacitracin was up 9 per cent, whereas meat chicken production rose only 3 per cent."

SUPPLIED The United Nations says drug resistant infections could kill 10 million people a year by 2050.

Both factors suggested the shortfall was made up by industries other than poultry, Brooks said.

A 28 per cent increase in sales of macrolides, classed as "highest priority critically important", and lincosamides registered for use in poultry and pigs was also not attributable to the poultry industry, he said.

Macrolides and lincosamides are monitored together because of similarities which mean cross-resistance can develop.

"Tylan, an animal antibiotic and member of the macrolide family of antibiotics, was the only macrolide used in the New Zealand poultry industry," he said.

"Tylan was used only in breeder meat chickens and only to treat illness that threatens their welfare."

Brooks said the poultry industry used 100 kilograms of macrolides in a national flock of 127 million birds, equal to 1.4 per cent of the more than seven tonnes used in New Zealand agriculture in 2017.

NZ Pork general manager David Baines said zinc bacitracin had been used in the commercial pork industry.

However, that use accounted for less than half a per cent of the total amount of the product sold in 2017, he said.

Baines said some farms had experienced an increase in pig diseases that would have been appropriately treated with the macrolide tylosin – the active ingredient in Tylan – if prescribed by a vet.

MIKE SCOTT/STUFF Federated Farmers dairy chairman Chris Lewis says antibiotics are already under tighter control.

"We cannot confirm the extent to which this may have contributed to the increase in sales during 2017 but have good information that indicates that just over half of the tylosin reported as sold in the period was used in the commercial pig industry," he said.

In April, the United Nations' Interagency Coordination Group on Antimicrobial Resistance warned deaths from infections resistant to common antibiotics, antivirals and anti-parasitic drugs could increase more than ten-fold to 10 million deaths annually by 2050.

At least 700,000 people die each year due to drug-resistant infections and more diseases, including respiratory tract and sexually transmitted infections, are failing to respond to drug treatment, according to the report.

Threats to food systems were just as alarming, due to the "rampant" use of anti-microbial agents on crops, livestock and fish.

"Misuse and overuse of existing antimicrobials in humans, animals and plants are accelerating the development and spread of antimicrobial resistance," the report warns.

Dr Heather Hendrickson, senior lecturer in molecular bioscience at Massey University, said consumers needed to understand the seriousness of the threat posed by antibiotic resistance.

"The way that we're [farming] poultry and pigs is in really high concentrations and we are putting antibiotics in their feed just to keep them alive and not getting sick."

Animals treated with antibiotics can't be slaughtered until the medicine has time to leave their system and any residue falls below levels set by MPI.

However, drug resistant bacteria carried in faeces or on the animal's body through the slaughter process could be brought into homes with the meat and spread to people, Hendrickson said.

"It's not the antibiotic in the animal per se, it's the bacteria that, despite all the washing and the steps that we take, is still hanging around.

"Bugs that are on the meat that, if they've been exposed to these antibiotics, are more likely to have resistance and then we get infections from those things that will be antibiotic resistant infections."

PHIL REID/STUFF The World Health Organisation has listed antimicrobial resistance as one of the top 10 threats to global health.

Sales of third generation cephalosporins and fluoroquinolones, both classed as "highest priority critically important", were also up in 2017, rising 15.6 per cent and 1 per cent respectively.

Critically important penicillin sales were up 9 per cent to nealry 14.5 tonnes, with just over quarter of that for use in cattle as injectables or dry cow therapy (DCT) for dairy cows.

DCT is treatment administered via the teat canal to cure existing mastitis and prevent new infections during the non-milking period.

Federated Farmers dairy chairman Chris Lewis said the industry had been looking into ways to reduce antibiotic use for a long time and the medicines were already under tighter control.

"It's probably easier to get antibiotics for my kids than my cows and the feedback from other farmers is that administering drugs on farms is getting tougher and tougher," he said.

"There is more education, discussion with vets and administration about what the best treatment options are."

Lewis said a farmer's decision on whether or not to use DCT was influenced by several factors, including finances, weather conditions and clinical requirements.

"When the payout is up, they'll spend on things like that. When it's low, they won't, but there will probably be a higher somatic cell count in those lower payout years."

Somatic cells are mainly white blood cells sent to fight infection in the udder and also include a small number of damaged udder cells.

PETER MEECHAM/STUFF Poultry Industry Association boss Michael Brooks says an antibiotic-free national flock is not an impossibility but would bring far higher rates of mortality.

They can always be found in milk but are present in higher numbers in cows with mastitis and farmers can be penalised for milk with a higher count.

Mark Bryan, chairman of the New Zealand Veterinary Association's antimicrobial resistance committee, said the increase in antibiotic use for DCT was likely due to a shift away from higher category antimicrobials where possible.

That meant more penicillin-based treatments had been authorised as the use of cephalosporin-based products decreased.

"It's important to note that the MPI data is weight based. Penicillin-based DCT is considerably heavier than cephalosporin DCT – anything from twice as heavy to almost four times as heavy – so this will be reflected in MPI reporting figures," he said.

Bryan said the Veterinary Association's view was that antibiotics should only be used when it was known or strongly suspected that susceptible bacteria were present and causing, or likely to cause, disease.

Every effort should be made to find the origin of the problem and identify the most effective treatment before antibiotics were used, he said.

In 2015, the association set the goal of wiping out prophylactic use of antibiotics for animal wellness by 2030.

While "ambitious", Bryan believed that goal was achievable.

"We are already the world's third lowest user of antimicrobials for animals.

"We do need to move carefully forward because animal welfare is always the top priority for veterinarians and whatever changes we make need to be made with this in mind," he said.

"This is why we expect this journey to take the full 15 years."

RNZ A Professor of Public Health at Otago University says Candida auris causes invasive infections and cannot be treated with normal anti-fungal medication.

However, Hendrickson disagreed.

"It's good that we're measuring sales and the vet association is saying, 'We're going to solve these problems at a very distant point in the future'.

"But I don't see how that 2030 goal is achievable if we're seeing increases," she said.

"I think we look good on paper because we have a lot of large animals and the numbers are worked out on milligrams of antibiotic per kilogram of animal weight.

"Any country with a high proportion of cattle and sheep is going to have a lower reported [milligrams of antibiotic per kilogram], even if every chicken is being pumped full of antibiotics."

Brooks said the poultry industry continued to look for ways to reduce the threat of disease in poultry sheds but there would always be a need for antibiotics.

"To have a completely antibiotic-free national flock is not an impossibility, but the ramifications on poultry welfare are significant, with far higher rates of mortality."

There would also be a less sustainable supply of chicken and the higher cost of farming the birds would push prices up for consumers.

Lewis said the dairy industry currently had no clear path to achieving the Veterinary Association's 2030 goal.

"At the moment, that goal is more aspirational than anything else because nobody's offering an alternative. Farmers need more tools in the toolbox."

Work was being done to breed cattle with lower cell counts and develop vaccines for mastitis but much of it was overseas and still at the trial stage, he said.

"In 10 years time, there could be a widely available solution and I hope there is."

KEVIN STENT/STUFF Drug resistant bacteria present on animals during slaughter could be brought into homes with the meat and spread to people, Massey University expert Dr Heather Hendrickson says.

​Hendrickson said it could ultimately fall to consumers to force a change.

"If there isn't consumer demand for change, I don't think politicians will want to get involved because it's a messy issue. Nobody wants to take on big agriculture," she said.

"But consumers can demand that they know more about how their meat is produced. If they were able to see how chickens were raised just by reading the label, that kind of pressure on industry will cause change."

Because nobody was demanding that information, it wasn't being given, she said.

"If people who eat meat don't care about the antibiotics and what's been done to the meat to get that low price, nothing is going to change.

"Consumers need to start thinking harder about their choices and we need to be growing animals in conditions where they're not dependent on mass antibiotics just to stay alive because the trade-off is human health down the line.

"We're using antibiotics to do things the way we want to and there is going to be a price and I don't think it's one that we want to pay."