1.2 The role of antimicrobials and different types of use

Antimicrobials play an essential role in promoting animal health and welfare, and therefore are crucial to food security. Antimicrobials can be administered to animals orally in feed or water, or non-orally, for example by injection or via the intra-mammary route.

Antimicrobials may be used in various ways for food-producing animals:

  • Therapeutic use: Only animals presenting signs of disease are treated. This is the most appropriate use of antimicrobials.
  • Metaphylactic use: Antimicrobials are administered to healthy animals belonging to the same group as animals with clinical signs. This pattern of treatment can occur in poultry and aquaculture production, when antimicrobials are administered in water or feed even though signs are observed only in a few animals. Although therapeutic use is the most appropriate approach, in these systems it is not practical to treat individual animals.
  • Prophylactic use: Antimicrobials are administered to a herd or flock of animals at risk of a disease outbreak but are not currently showing signs of disease.

Antimicrobials have also been used for growth promotion purposes, in order to reduce the time and amount of feed needed to grow the animals to market weight. Such antimicrobials are usually administered in feed and are applied at a sub-therapeutic dosage – that is, a lower dose than needed to treat disease. This type of use is banned by the European Union (EU) due to the risk of the development of AMR and is regarded as inappropriate by the WHO (World Health Organization), the FAO (Food and Agriculture Organization) and the OIE (Office International des Epizooties, or World Organisation for Animal Health).

  • Consider the following four examples of AMU and decide whether they are therapeutic use, prophylactic use, metaphylactic use or growth promotion.

    A farmer gives three unwell pigs an injection of an antimicrobial.

  • a. 

    Therapeutic use


    b. 

    Prophylactic use


    c. 

    Metaphylactic use


    d. 

    Growth promotion


    The correct answer is a.

  • A broiler unit uses low doses of an antimicrobial mixed into the feed to increase productivity.

  • a. 

    Therapeutic use


    b. 

    Prophylactic use


    c. 

    Metaphylactic use


    d. 

    Growth promotion


    The correct answer is d.

  • A fish farmer adds antimicrobials to the feed of their fish after a neighbouring fish farm has an outbreak of disease.

  • a. 

    Therapeutic use


    b. 

    Prophylactic use


    c. 

    Metaphylactic use


    d. 

    Growth promotion


    The correct answer is b.

  • A dairy farmer treats all their healthy lactating cows with an antimicrobial after four of the cows show signs of mastitis.

  • a. 

    Therapeutic use


    b. 

    Prophylactic use


    c. 

    Metaphylactic use


    d. 

    Growth promotion


    The correct answer is c.

There are risk factors associated with the use of antimicrobials that increase the development of resistance in animals and potential transmission to humans and the environment (RISKSUR, 2015).

Examples of factors that may change the risk of AMR development and transmission include:

  • unnecessary or inappropriate use, such as using the wrong antimicrobial or treating animals that do not need to be treated
  • non-therapeutic use; that is, prophylactic and metaphylactic use, or growth promotion
  • the route of administration and formulation, or oral administration versus non-oral administration
  • the bioavailability of the antimicrobials, which determines the amount absorbed versus the amount excreted – if a drug has low bioavailability, more of the antimicrobial will be excreted in the faeces.

Using antimicrobials appropriately, as discussed in the Antimicrobial stewardship in animal health module, can reduce the risk of AMR emergence.

1.1 Antimicrobials in food production animals

1.3. The importance of regulating AMU in animals