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Keeping chickens in the back garden just as pets or for their eggs is becoming more popular. This makes it more likely that veterinary professionals who normally deal with companion animals will be asked about the health of chickens. As chickens are technically a food-producing species, this means that there are strict rules about what medicines can be used to treat them. This module explains the main considerations relating to the use of medicines in chickens.

By doing this module you will:

  • understand how eggs are made 

  • know the factors that influence how drug residues can end up in eggs

  • be aware of medicines law relating to the treatment of food-producing animals

  • understand the significance of withdrawal periods for medicines

  • know where to get more information

Eggs have three main components: yolk, albumen and shell.

  • The yellow yolk of an egg mainly consists of fats, protein, cholesterol and vitamins A and D.

  • The egg whites (albumen) are mainly protein. 

  • The shell mainly consists of calcium carbonate.  

The yolk of the egg takes the longest time to develop. It forms in the hen’s ovary (where it is called a yolk follicle) over a period of about 2 weeks before ovulation. During this time some of the components of the yolk are synthesised in the liver and transported via the circulation to the yolk follicles in the ovary. A hen’s ovary usually contains up to 14 yolks at various stages of development

After release of the developed yolk follicle from the ovary (ovulation), the other components of the egg are assembled over the following 24 hours or so: the egg whites are laid down during the first 4 hours and the egg shell is formed over the next 20 or so hours. The egg is then laid. 

When a hen is treated with a medicine, the active ingredient (drug) is distributed around the body in the circulation. As well as entering the various body tissues, the drug can also end up in the egg yolk or albumen. What happens to a drug in the body, including whether it is likely to be deposited in eggs and how long it persists, depends on how soluble it is in fat or water and also on whether it binds to protein in the body. So drugs that are more soluble in fat than in water (so-called lipophilic drugs) may be more likely to deposit in the egg yolk than non fat-soluble drugs, because of the fatty nature of the yolk. Drugs that bind to proteins in the body are more likely to be deposited in eggs because of the high protein content. However, it can be difficult to predict whether or not a drug will be deposited in eggs and so specific studies are needed to find out. For example, although the antibiotics doxycycline and oxytetracycline are very similar drugs (they belong to the same class – the tetracyclines), there is a big difference in the time they persist in eggs at the end of treatment (9 days for oxytetracycline vs. 25 days for doxycycline). This difference is due to the higher fat solubility of doxycycline compared to oxytetracycline. 

There are also other factors that affect how much of a drug will be deposited in an egg and how long it persists in the body. These include the route of administration, the dose and how well the drug is absorbed.

Given that the yolks develop over the 2 weeks or so before ovulation and that a hen’s ovary contains up to 14 yolks at various stages of development, it is easy to understand that there can be a risk of drugs being present in eggs for some time after stopping a drug treatment. In other words, when a hen is being treated with a medicine, there is a potential for the drug to be present in eggs laid during the treatment and for at least 2 weeks after the treatment is stopped. 

Even tiny amounts of a veterinary drug in edible animal products (eggs, meat and milk) can potentially pose risks to human health, through direct toxic effects, allergic reactions or increased bacterial resistance to antibiotics. The risk to people consuming the animal product depends on various factors, including the particular drug used to treat the animal, the amount of drug residue, the quantity of tissue or food product consumed by a person, the duration of exposure, and also the health of the person consuming the food. Often the risk is minimised because the animal product is only consumed sporadically.

Of course, sometimes it is necessary to treat a food-producing animal with a medicine for welfare reasons. The laws affecting food production require that all foods, including eggs, obtained from animals treated with veterinary medicines must not contain any residue that might represent a hazard to the health of the consumer. Therefore before a veterinary medicine intended for food-producing animals can be authorised for use anywhere in the European Union, the safety of the active ingredients and their residues in food must first be evaluated. This includes determining the maximum concentration of residue considered safe in a food product obtained from an animal that has received a veterinary medicine. This figure (which is usually stated in micrograms per millilitre) is the Maximum Residue Limit (MRL). 

The withdrawal period is the number of days between stopping a drug treatment and the egg being judged fit for human consumption, that is when the concentration of drug or any metabolite does not exceed the Maximum Residue Limit. For example, if the chicken is treated with a medicine that requires a withdrawal period of 6 days for eggs, the eggs may be consumed from day 7 after the last dose, at the earliest. For some products (e.g. Flubenvet, which contains the anthelmintic fenbendazole), the withdrawal period is zero days, which means that it is safe to eat eggs during treatment with this medicine. For medicines that are authorised for use in laying hens, the withdrawal period for eggs is included in the summary of product characteristics (SPC).

There is a list of drugs that are approved for use in food-producing animals (it is Table 1 in European Commission Regulation 37 ⁄2010). You can see the Table by clicking this link and scrolling down until you get to Table 1 ‘Allowed substances’. The table gives the established Maximum Residue Limit for the specific species and tissues or products. For example the anthelmintic fenbendazole is listed with an Maximum Residue Limit for eggs. Pharmaceutical companies use this information to establish withdrawal periods for medicines. The information is also useful for vets when they are prescribing for food-producing animals under the Prescribing Cascade.

There is also a short list of drugs that are prohibited  (Table 2 in the regulation). These are banned from use in food-producing animals because they are considered hazardous to human health at any residue level. 

The Veterinary Medicines Regulations require that any drug used in a food-producing animal must be listed in the table of ‘allowed substances’. This means that it is not lawful to use either a prohibited drug or one that is not listed in in a food-producing animal. Some parasiticides that are widely available to buy over the counter contain unlisted drugs such as fipronil or imidacloprid. By law these must not be used in food-producing animals and it would be illegal for a veterinary professional to prescribe them for such animals. It may be that some owners choose to use unapproved (unlisted) medicines on their hens. However it is very important for people to understand that any animal treated with an unlisted drug (and any food, including eggs derived from it) must never enter the food chain. This includes through informal egg sales or even using the eggs to make cakes that are sold to others. 

There are many medicines authorised for use in laying hens. However, many of them are designed for mass medication on commercial farms, and so they may not be practical for use in backyard chickens because of the large pack sizes and high concentrations of active ingredient. For example one pack might contain enough to treat around 2,000 to 2,500 laying hens for one day! Dispensing the required quantity from the bulk pack is an option, but may not be economical. An exception is diluted forms of Flubenvet (which contain the anthelmintic flubendazole) , which is available in small quantities for use in domestic poultry: these are Flubenvet 1% medicated premixture and Flubenvet final medicated feed, which is in a ready-to-use form. Both these forms are in supply category POM-VPS and so can be prescribed by a vet, pharmacist or an SQP qualified to supply avian medicines.

When prescribing a medicine for a food-producing animal, the prescriber must inform the owner of the appropriate withdrawal period to ensure that drug residues are absent or do not exceed the Maximum Residue Limit. For a medicine that is authorised for use in laying hens (e.g. Flubenvet) this is a simple matter because the withdrawal period is stated in the product information (zero days for Flubenvet). However, if a vet prescribes a medicine that is not authorised in laying hens and for which a withdrawal period is not included in the product information, it is the prescribing vet’s responsibility to set one. To do this a vet must take into account known information about the use of the product in the species for which the product is authorised plus any other relevant information. Sources of information that help include product SPCs, the EU table of ‘allowed substances’, published drug residue studies and the relevant pharmaceutical company technical information department. Information sources are listed at the end of this module.

When a medicine is prescribed for a food-producing animal, the following information must be recorded and retained by the supplier for at least 5 years:

  • date of examination of the animal(s)

  • name and address of the owner

  • identification and number of animals treated

  • results of the clinical assessment

  • brand name and batch number

  • name and quantity of active ingredient

  • dose and duration of treatment

  • withdrawal period.

Remembering to record the batch number and withdrawal period is particularly important as this will not be normal practice for companion-animal practices.  

The product details, including the batch number and the withdrawal period must also be provided to the owner to enter into their records. The owner is legally responsible for retaining this information for 5 years. If the required quantity of medicine is removed from a bulk pack and dispensed in another container, a copy of the package leaflet, SPC or data sheet must be provided to the owner. Owners must be advised not to eat or sell eggs or use them to make products (e.g. cakes) during the withdrawal period. 

  • Although the main components of eggs are assembled over around 24 hours, the constituents of the yolk accumulate over the previous 2 weeks. This means that there can be a risk of drugs or metabolites accumulating for some time after stopping drug treatment. 

  • The Maximum Residue Limit is the maximum concentration of residue accepted in a food product obtained from an animal that has received a veterinary medicine.

  • The prescriber is responsible for informing the owner about the implications of treatment on egg consumption, including any appropriate withdrawal period before eggs are safe to eat.

  • The law requires that information about the treatment of a food-producing animal is recorded by the supplier and the owner.

  • Veterinary professionals are well placed to help educate pet owners about the appropriate use of medicines in backyard/pet chickens.

 Sources of Information and Advice

  • NOAH Compendium of SPCs for most authorised veterinary medicines and datasheets for some medicated feeds and Small Animal Exemption Scheme products. 

  • Pharmaceutical company technical information departments. For information on specific authorised products.

PODCAST

If you prefer, you can listen to the whole audio presentation of this module using the following podcast. Don't forget that you can also download the podcast to your iPod, music player, tablet or smartphone using the Download link on the right of the audio player. 

How we produced this module

Our modules start with a detailed outline and electronic literature search. We commission a collaborating author, who is a specialist in the module topic, to write a draft module. The collaborating author on this module was Grant Hayes MRCVS. The draft was circulated unsigned to other poultry specialists, practising first-opinion vets. We also consulted pharmaceutical companies and the Veterinary Medicines Directorate. There is a rigorous editing and checking process and the result is a module that is evidence-based, impartial and relevant to clinical practice. The final module is unsigned because it is the result of collaboration.

Cki

How we produced this module

Our modules start with a detailed outline and electronic literature search. We commission a collaborating author, who is a specialist in the module topic, to write a draft module. The collaborating author on this module was Grant Hayes MRCVS. The draft was circulated unsigned to other poultry specialists, practising first-opinion vets. We also consulted pharmaceutical companies and the Veterinary Medicines Directorate. There is a rigorous editing and checking process and the result is a module that is evidence-based, impartial and relevant to clinical practice. The final module is unsigned because it is the result of collaboration. 

References 

Baytril 10% oral solution. (June 2018) Summary of product characteristics. Bayer plc.

British Small Animal Veterinary Association. Backyard poultry [online].  https://www.bsava.com/Resources/Veterinary-resources/Medicines-Guide/Backyard-poultry [Accessed 7 June 2019]

Cornejo J et al. (2011) Study of enrofloxacin and flumequine residues depletion in eggs of laying hens after oral administration. J Vet Pharmacol Therap 35: 67–72.

European Medicines Agency (2019). Withdrawal period [online] https://www.ema.europa.eu/en/glossary/withdrawal-period [Accessed 7 June 2019]

European Commission (2010). Commission Regulation (EU) No 37/2010 of 22 December 2009 on pharmacologically active substances and their classification regarding maximum residue limits in foodstuffs of animal origin. Official Journal of the European Union [online]: https://ec.europa.eu/health//sites/health/files/files/eudralex/vol-5/reg_2010_37/reg_2010_37_en.pdf

European Commission (2009). Regulation (EC) No 767/2009 of the European Parliament and of the Council of 13 July 2009 on the placing on the market and use of feed. Official Journal of the European Union [online]: https://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2009:229:0001:0028:EN:PDF 

Exzolt 10mg/mL solution for use in drinking water in chickens. (August 2017) Summary of product characteristics. Intervet International BV.

Goetting V et al. (2011) Pharmacokinetics of veterinary drugs in laying hens and residues in egg: a review of the literature. J Vet Pharmacol Therap 34: 521-6.

Panacu AquaSol 200mg/mL suspension for use in drinking water for pigs and chickens. (August 2016) Summary of product characterstics. Intervet International BV.

Solomon SE. (1997) Egg & Egg Shell Quality. (Manson Publishing) ISBN 0-8138-2827-9

The Veterinary Medicines Regulations. 2013 [online] http://www.legislation.gov.uk/uksi/2013/2033/contents/made. [Accessed 5 June 2019] 

Veterinary Medicines Directorate. (2019) Exemption from authorisation for medicines for small pet animals [online]. https://www.gov.uk/guidance/exemption-from-authorisation-for-medicines-for-small-pet-animals [Accessed 12 June 2019]

Whithead M, Roberts V. (2014) Backyard poultry: legislation, zoonoses and disease prevention. J Small Anim Pract 2014; 55: 487–96.

Yoshimura H et al. (1990) Residues of doxycycline and oxytetracycline in eggs after medication via drinking water to laying hens. Food Addit Contam 8: 65–9.