Bacteria. AP Photo/Centers for Disease Control and Prevention The possibility that superbugs — bacteria that can't be killed by our best antimicrobials — might become widespread is a terrifying doomsday scenario. In such a future, driven by our extreme overuse of antibiotics, we'd effectively return to the dark ages when people routinely died of bacterial infections.

Over-prescription of antibiotics to human patients contributes to the development of resistant bacteria, but in the United States about 80% of total antimicrobial use goes toward raising animals for food. The Food and Drug Administration recognizes the threat this poses to human health and has encouraged farmers — who use antibiotics routinely, not just when animals are sick — to curb their use.

The problem isn't just in the US, though. It's global in scale — and according to a study published in Proceedings of the National Academy of Sciences, it's about to get much worse.

To get a handle on antimicrobial use in raising livestock worldwide, an international team of scientists mapped where in the world antimicrobials are used in livestock, and how much is used in each country.

They estimated that in 2010, the world used about 63,151 tons of antimicrobials in livestock. China's usage already exceeded that in the US, and the researchers project that — driven by a growing demand for meat around the world and a shift to large-scale farming practices — "antimicrobial consumption will rise by 67% by 2030, and nearly double in Brazil, Russia, India, China, and South Africa."

On the chart below, (A) shows the five countries that used the most antimicrobials in livestock in 2010, and (B) shows the researchers' projections for 2030:

(A) shows the five countries that use the most antimicrobials in livestock in 2010. (B) shows the researchers projections for 2030. Van Boeckel et. al., PNAS 2015

The researchers also created a heatmap of antimicrobial use in livestock around the world:

The darker the color, the more antibiotics used. Van Boeckel et. al., PNAS 2015

"Antimicrobial consumption hotspots" are in darker red on the map, including Sichuan province and the southeast coast of China, the south coast of India, the south of Brazil, and the midwestern and southern regions of the United States.

Armed with this map, we have a better idea of how much we use antimicrobials for raising livestock around the world, and which areas might be more likely to produce a drug-resistant bug.

Cows, chickens, and pigs

The researchers also made separate maps to show antimicrobial use for raising different species of animals.

In Europe, the antimicrobial use for raising cattle wasn't as intense as for raising chickens and pigs. This is partly because antimicrobial use is proportionately lower for cows, and partly because chicken and pigs are raised closer together than cows.

Map A shows antimicrobial use for cows, B for chickens, and C for pigs:

Map A shows antimicrobial use for cows, B for chickens, and C for pigs. Van Boeckel et. al., PNAS 2015

The researchers did a similar breakdown for chicken (map A) and pigs (B) in Asia, including predictions for where antimicrobial use will increase significantly (purple on the maps).

Limitations

The researchers acknowledged some assumptions in their model that could have affected the maps that came out. Because data about antimicrobial consumption were only available from 32 high-income countries, they had to use that data to estimate antimicrobial use for low and middle-income countries with similar farming practices. That could have made their estimate greater than the real amount of antimicrobial use.

But the researchers also think they overestimated antimicrobial use in the United States because they couldn't get data from individual states, so it might even out.

The study also did not differentiate between antimicrobial compounds used only in animals and those used in both humans and animals, which are are a greater concern for their role in stimulating drug-resistant bugs.

Still, this analysis is an important first step at assessing the scale of the problem. The next step is figuring out a solution.