Livestock need drugs to maintain a high food production. Their rearing at high densities create perfect conditions for the spread of disease-causing microbes and parasites that need to be combated using drugs. However there are many negative consequences that accompany the benefits brought by these drugs, especially when they are misused.
Antibiotics are used to cure livestock of bacterial infections and in some countries also as growth rate enhancers. The problem with the overuse of these antimicrobial livestock drugs is that it creates an ideal breeding ground for pathogenic bacteria to acquire resistance to multiple antibiotics.
Evolution is never idle. Also, the antibiotics can pass up the food chain and into us humans, which also doesn’t help with the antibiotic resistance problem. In the EU, the use of antibiotics specifically as growth enhancers has been banned since 2006. But it is still allowed in many other countries like the US, where almost 25 million pounds of antibiotics are given to healthy livestock per year. In the US, between 70-80% of the total antibiotic consumption is by cattle, pigs and poultry. With the increase in meat consumption in countries like China, India and Brazil the use of antibiotics in livestock may increase up to around 68% by 2030 (Boeckel et. al., 2015).
Antibiotic resistance in pathogenic bacteria is becoming a major worry for the modern human world. According to the CDC, in the USA 2 million people a year develop antibiotic-resistant bacterial infections, and 23,000 die of these infections a year. A woman was recently found in Pennsylvania with E. coli bacteria resistant to multiple antibiotics. The worrying part is that the bacteria were also resistant to the most potent antibiotic called colistin that isn’t even used widely these days due to its toxicity – it is used as a last resort. The plasmid gene called mcr-1 conferred the resistance, and it is possible that this gene can be shared with other pathogenic bacteria. It’s unclear where she picked up the bacteria as she hadn’t left the US in 5 years.
Maybe she acquired the bacteria through the consumption of undercooked meat or even just meat preparation. There is also the possibility that she acquired the bacteria from an agricultural worker. At the John Hopkins University in North Carolina, Christopher Heaney and his team carried out a study where they swabbed the noses of hog-farm workers in eastern North Carolina over a long period. 86% of the workers carried drug-resistant Staphylococcus auereus (over half of the samples were immune to three or more drugs) at some point during the study period, and one worker carried MRSA throughout the whole study (Nadimpalli, 2014). The bacteria could stay on the workers when not at the farm for days. These drug-resistant bacteria could potentially be spread into the rest of the population through these workers.
Or maybe the woman simply acquired the bacteria from the wind! McEachran et. al. (2015) carried out a study where they compared the particulate matter downwind and upwind of large-scale cattle farms. They found that particulate matter downwind of the farms contained antibiotics, ruminant-associated bacteria and a much greater amount of tetracycline-resistance genes. Also, the bacterial communities from downwind and upwind matter were distinct.
Not only can we ingest the antibiotics used in livestock through meat eating, but also through eating plants! The manure produced by livestock that are treated with antibiotics is frequently used as fertilizer for crops. In the US, manure fertilises around 9.2 million hectares of agricultural land. In two separate experiments in 2005 and 2007, researchers at the University of Minnesota grew crops in manure-treated soil within greenhouses. In the first study (Kumar et. al., 2005), they found that corn, lettuce and potato absorbed chlortetracycline and in the second study (Dolliver et. al., 2007) they found sulfamethazine in corn, lettuce and potato. The amount of antibiotics found in the crops increased with the amount in the manure.
As well as this being a problem with bacteria developing antibiotic resistance, this may have other side effects too. Antibiotics have been linked to the development of asthma and allergies, and the drugs may be responsible for their rise over the last 20 years. Run-off can cause antibiotics to enter into natural aquatic ecosystems. Grazing can be a pathway of antibiotics into terrestrial ecosystem. An ecological side effect that is particularly worrying is the effect that antibiotics have on methane emissions. Researchers at the University of Colorado studied the bacterial communities in the dung of cows treated with tetracycline and those who were not. The microbes in the dung treated with the antibiotic were less diverse and dominated by a genus known to have resistant strains.
Dung from cows treated with the antibiotic were found to have a methane emission nearly twice as high as those from untreated cows (Hammer et. al., 2016). If you don’t know, methane is an extremely potent greenhouse gas. The reason for the increase may be due to the antibiotics changing the structure of the bacterial communities inside the cows and within the dung, reducing the competition experienced by methane-producing microbes (archaea).
The team also found that antibiotics restructured the bacterial community within the guts of dung beetles. And there are other livestock drugs affecting dung beetles too. A team of researchers from Spain and France found that dung beetles that ingest the commonly used ivermectin (used against parasite of cattle) lose their ability to interact with their environment due to the drugs interfering with their senses and locomotor capability (Verdu et. al., 2015). In fact, dung beetles have been declining rapidly in the last 30 years and the presence of drugs in manure is a contributing factor. They provide very important ecosystem services of removing dung (which reduces methane emissions), recycling nutrients, aerating pastures through tunnelling and regulating pests and are an important food source for groups like birds.