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Does antibiotic use in farm animals contribute to the spread of resistant bacteria?

Even though human medicine is currently considered by the EU to be the primary source of resistant bacteria in humans (EMA, 2018), antibiotic usage in animals is a key factor contributing to the wider range of resistance. Its effect is particularly pronounced in intensive animal production. The spread of resistant bacterial populations can be transmitted from animals to humans via direct contact between them, or through the food chain and the environment. Additionally, transmission can also occur through the introduction of animals from other farms.

Antimicrobial-resistant infections in humans can cause prolonged illnesses, increased frequency of hospitalization, and treatment failures that can prove fatal. Some types of bacteria that cause serious infections in humans have already developed resistance to most or all of the available treatments and we are quickly running out of treatment options. WHO recommends an overall reduction in antibiotics usage within food-producing animals to help preserve their effectiveness for human medicine.

Sources: Farm antibiotics, EMA, WHO, OECD

What are the main reasons for antibiotics use in food-producing animals?

Antibiotics are used for the treatment (therapeutic use) of a sick animal or group of animals that have been diagnosed with an infection and/or clinical disease.  Additionally,  antibiotics may be given to animals that are not currently ill but are at a high risk of acquiring an infection. Control treatment or metaphylaxis aims to prevent the spread of infectious disease to animals in close contact, ones at considerable risk, and those who might already be infected or incubating the disease.

An animal may also be treated with antibiotics after undergoing surgery or injurious trauma – preventive use or prophylaxis. Preventive treatment should only be applied to animals diagnosed at high risk of bacterial disease and not as a routine practice or to compensate for poor hygiene and inadequate husbandry condition.

Besides being used for treatment, control, and prevention, antibiotics have been added to animal feed to promote faster growth by accelerating the rate of weight gain and/or increasing feed conversion efficiency in animals. The use of antimicrobials as growth promoters is being prohibited in a growing number of countries, including in the EU; however, it is still a common practice in many parts of the world.

Sources: Q Chang, 2015, Farm antibiotics, FAO, MSU

Are there alternatives to antibiotics in husbandry?

The first steps towards reducing antibiotic use in husbandry are improving hygiene and making changes to animal housing and other husbandry practices (WHO, 2017). Otherwise, there are a number of alternatives to antibiotics; these include vaccines, prebiotics, pro-biotics, in-feed enzymes, antimicrobial peptides, bacteriophages, heavy metals, and more. All of these alternatives have different levels of efficacy and target growth promotion and/or disease prevention and/or disease treatment (PEW, 2017).

Vaccines are the most prominent and widely used alternative to antibiotics as they pre-emptively introduce either an attenuated or killed version of the bacteria to the animal. In turn, the animal develops resistance and will be able to prevent later infection by itself (Frontiers in Microbiology, 2014).

Bacteriophages are viruses that act as parasites to bacteria by infiltrating them and then replicating once inside; in time, this kills the bacteria. This method has a number of advantages; for instance, it is highly specific to particular bacteria and will not harm helpful local bacteria in animals. Additionally it replicates inside the target bacteria meaning the injected dose is low (Frontiers in Microbiology, 2014).

Another alternative includes improving the immune system of the animal to encourage a more effective response against bacterial infection. This can also be used in conjunction with antibiotics to increase the treatment’s efficacy. At the moment, only a few bacteriophages are allowed on the market and only as post-slaughter decontaminants. One example is a phage that kills Listeria in food post-slaughter (Frontiers in Microbiology, 2014).

How can improved sanitation in husbandry lead to a decline antibiotics usage?

It is important to realise that an unclean production space is a breeding ground for a wide array of microbes. Proper cleaning and sanitation will help remove the majority of microbes along with the visible dirt, in turn resulting in fewer microbial infections and therefore reducing the need for antibiotics.

What is the withdrawal time for antimicrobials?

The withdrawal time or withdrawal period for antimicrobials is the time between the last antimicrobial dose given to the animal and the time in which the level of residues in the tissues (muscle, liver, kidney, skin/fat) or products (milk, eggs and meat) is lower than or equal to the Maximum Residue Limit (MRL). This is done to avoid un-prescribed consumption of antimicrobials by humans via the food chain. In terms of regulation, this period “ensures that the food does not contain levels of the medicine that exceed the maximum residue limit” (EMA, 2018). In the EU, the MRL is first recommended by the EMA’s Committee for Medicinal Products for Veterinary Use and then adopted by the European Commission. It then becomes a legally binding food safety standard (EMA, 2018).

How does antibiotic use in animals affect humans?

A number of antibiotics used for veterinary purposes are also used to treat humans; For example, critically important antimicrobials for medicine (WHO, 2018) are also used in animals such as glycopeptides, cephalosporins, macrolides, and quinolones (AMRLS, 2011).  Scientific evidence has shown that overusing antibiotics in animal husbandry, especially in low doses for growth promotion, can lead to antimicrobial resistance. It is crucial that important antibiotics for human medicine do not lose their efficacy and these antibiotics of “highest priority critically important” should be avoided at all costs in animal production.

Bacterial pathogens that acquire antibiotic resistance in animals might then spread to humans through food and lead to infections that are harder, or impossible, to treat. However, this only occurs at a very small rate with bacteria such as salmonellae and campylobacters. The main issue is an overall increase in AMR and the indirect spread to humans as 60% of all human diseases originate from animals. This is especially concerning when it deals with bacteria resistant to antimicrobials that are crucial for human medicine such as the critically important antimicrobials listed above.

Sources: WHO, WHO, Journal of Antimicrobial Chemotherapy

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