Analysis of trends in use per species

The use of different active ingredients measured in kilograms is not directly comparable due to their differences in antimicrobial potency, pharmacokinetics , and, consequently, the dosage prescription.

Daily doses (ADD)

To provide insight into the true exposure of animals to antibiotics, the use should be determined per animal species and expressed in the number of Animal Daily Dose (ADD) per animal year (Jensen et al., 2004; abbreviated: 'dd per ay'). The ADD is the defined average maintenance dose of a specified medicine per kg of a specified animal per day, applied for its main indication. This unit conforms to international developments in this field and developments in the human health sector. With the ADD approach the calculation and comparison of the total antibiotic use on different farms is possible, even when different active ingredients are involved. Expressing the use per animal year also enables comparisons of farms with different production and vacancy periods.

Furthermore, the ADD approach offers an opportunity to study the relationship between antibiotic use and the occurrence and trends in antimicrobial resistance. With an ADD approach also a comparison of countries could be possible, but only when based on reliable usage data per animal species. The now often reported difference in grams per kg of biomass, as calculated from national total sales figures, is reasonably suitable to adjust trends in the sales for changes in the size of the animal population within a country, but not for country comparisons. A comparison of countries based on overall averages is strongly influenced by animal demographics and therefore a very inaccurate indication of the true differences in antimicrobial exposure, per animal species. To get an appropriate certainty about the true differences between countries it is necessary to have reliable information about the use per animal species (Bondt et al., 2012).

Calculating number of daily dosages

The use of different active ingredients becomes comparable when the amount of active ingredients in each antibiotic preparation is measured as the number of daily dosages. The number of daily dosages per animal year was determined by calculating the total number of kilograms of animal that can be treated with each active ingredient, the so-called treatable weight. This was then divided by the total weight of the average present livestock on the farm, assuming that the average treatment is administered to animals with an average weight. The following daily dosages box gives an example of the calculation of the number of daily dosages per animal year.

Example: Calculation of the number of daily dosage

For example, a farm with 150 fattening pigs with an average weight of 70.2 kg used 2 litres of antibiotic preparation X during the course of one year (X contains 40% = 400 mg/ml of active ingredient a) and 20 kg of antibiotic preparation Y (Y contains 25% = 250 mg/g of active ingredient b). Antibiotic preparation X: the defined daily dosage of active ingredient a is 10 mg per kg of animal weight per day. Antibiotic preparation Y: the defined daily dosage of active ingredient b is 50 mg per kg of animal weight per day.

Antibiotic X can be used to treat (2,000 * 400)/10 = 80,000 kg of animal weight. Antibiotic Y can be used to treat (20,000 * 250)/50 = 100,000 kg of animal weight. Consequently, the farm has used antibiotics for treatment of a total of 180,000 kg of animal weight. The farm has an average of 150 fattening pigs per year, with a total weight of 10,530 kg. 180,000 kg were treated in that year, equivalent to 180,000/10,530 = 17.1 daily dosages. Consequently, an average fattening pig  on the farm in that year was administered a prescribed dosage of antibiotics on 17.1 days. In this example the farm uses 17.1 daily dosages per animal year of antibiotic preparation X plus Y.

Animal weights

The calculations in the sample survey are based on the average weight per animal during the animals' presence on the farm. The following average weights have been used: dairy cows 600 kg, veal calves 172 kg (i.e. the weighted average of white veal calf 164 kg and rosé veal calf 192 kg), broilers 1 kg, fattening pigs 70 kg, sows 220 kg, maiden gilts 107.5 kg, piglets (< 25 kg) 12.5 kg, breeding boars 350 kg (ASG, 2010). On dairy farms the number of daily dosages is based on the weight of the dairy cows only, because this category of animals gets almost all of the antibiotics. On sow farms the size of the 'population at risk' is based on the weight of all present animals (including piglets, gilts, breeding boars). For an illustrative calculation of the number of daily dosages for young calves on dairy farms (from birth to weaning at 56 days of age) the average weight of 56.5 kg has been used.