Prescribing for backyard or pet hens
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Companion-animal vets might sometimes be consulted about the health of backyard or pet hens, which might be kept for egg-laying. As chickens are technically a food-producing species, it means that there are specific legal obligations related to medical treatment. There are also practical difficulties because of the few licensed (authorised) products in pack sizes suitable for treating small numbers of birds, as well as the need to interpret various sources of information to determine an egg withdrawal period.
By doing this module you will:
understand how the formation and composition of eggs relate to drug deposition
know the legal obligations concerning the use of medicines in chickens
be aware of practical problems relating to prescribing for backyard hens
understand how to interpret information for determining egg withdrawal times
know where to get further information
A hen’s ovary contains all of the ova for the lifetime of lay of the bird. The active ovary displays a phenomenon described as follicular hierarchy, in which there is concurrent, sequential development of maturing follicles ranging in size and colour, reaching up to approximately 40mm in diameter and a vivid yellow appearance just before ovulation. A typical commercial layer hybrid (brown feathers) breed bird will have approximately 12–14 yellow follicles of various sizes visible on the ovary at any one time. (Solomon 1997)
The yellow yolk of an egg mainly consists of a lipoprotein complex with high cholesterol and vitamins A and D content. The egg whites (albumen) are predominantly multi-layered complex proteins in a water matrix laid on a proteo-carbohydrate sac that envelops the yolk. The yolk is suspended in the albumen by polar cords of twisted albumen. The shell consists of a calcium carbonate deposit on an organic shell membrane. (Solomon 1997)
Eggs develop from ovulation to egg laying (oviposition) in just over 24 hours: the egg whites are laid down during the first 4 hours after ovulation and the egg shell is formed over the next 20 or so hours. However it is important to note that the yolk takes the longest to develop: precursors of yolk lipoproteins are synthesised in the liver and transported via the circulation to the yolk follicles in the ovary where they accumulate over about 12–14 days before ovulation. (Goetting et al. 2011)
In essence eggs have three main components: yolk, albumen and shell, made up predominantly of lipoproteins, protein and calcium carbonate, respectively. These components are the main factors affecting drug residue levels in eggs. (Goetting et al. 2011)
Following administration of a medicine, the drug or its active metabolites or inactive ingredients (excipients) in the formulation might preferentially deposit in the yolk or albumen depending on their relative solubility in fat and water and their protein binding affinity. (Goetting et al 2011). The amount deposited can also be affected by the route of administration (e.g. oral, topical, parenteral), on how well the substance is absorbed, and on the dosage and duration of administration. The amount of drug deposited in an egg and how long it persists can vary between class of drug (e.g. penicillins, tetracyclines) and between the individual drugs in a class (e.g. doxycycline, oxytetracycline). In a study of doxycycline and oxytetracycline administered orally to hens (at a dose of 0.5g/L for 7 consecutive days) the concentrations of antibiotic in the yolk increased daily to a peak at 2 days after cessation of medication and then declined gradually (Yoshimura et al. 1990). Oxytetracycline was detected in the yolk for 9 days after withdrawal whereas doxycycline was detected in the yolk for 25 days after drug withdrawal (although not necessarily at unsafe levels). Despite the similarity in structure of the two drugs, the difference in persistence is thought to be related to the higher lipophilicity of doxycycline, resulting in greater oral absorption and tissue penetration.
Given that the yolk lipoproteins accumulate in the ovarian follicles over the 2 weeks or so before ovulation and that a hen’s ovary contains up to 14 yolks at various stages of development, there can be a risk of drugs or their metabolites accumulating to above the safe limits defined for safe egg consumption for some time after stopping drug treatments.
Edible tissues containing veterinary drug residues can pose potential risks to human health, including direct toxic effects, allergic reactions and increased bacterial resistance to antibiotics (Goetting et al. 2011). The risk varies depending on various factors, including the drug, the residue level, the quantity of tissue or food product consumed, the duration of exposure, and the health of the person consuming the food. Often the risk is minimised by sporadic consumption.
EU law requires that foodstuffs, 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 (EC 2010). Therefore before a veterinary medicine intended for food-producing animals can be authorised in the EU, the safety of the active ingredients and their residues must first be evaluated. The MRL is the maximum concentration of residue accepted by the EU in a food product obtained from an animal that has received a veterinary medicine (EC 2010).
The EU approval status and MRLs for drugs used in food-producing animals are listed in European Commission Regulation 37 ⁄2010. The regulation contains a list of ‘allowed substances’ (in Table 1 in the regulation), together with the established MRLs for the specific species and tissues or products. The regulation also contains a list of ‘prohibited substances’ (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. New substances and MRLs are added to the lists from time to time. You can view the ‘live’ up-to-date lists of ‘allowed’ and ‘prohibited’ substances by following this link and scrolling down to reach the tables.
If a medicine ingredient (excipient) does not have a pharmacological action, no MRL is required. The European Medicines Agency (EMA) has published a list of excipients that do not fall within the MRL regulation. The so-called ‘out-of-scope’ list covers excipients and other substances (e.g. naturally-occurring substances) for which an MRL is not required or not appropriate. The list only includes substances on which the EMA has received enquiries, and is not exhaustive. You can view the list by following this link.
The Veterinary Medicines Regulations do not define ’food-producing animals’ but it is generally accepted that it means any animal that is fed, bred or kept for the production of food for human consumption. A chicken could therefore be considered non food-producing if it falls outside of this definition. However, EU law (specifically Article 3(c) of Regulation (EC) No 767/2009 on the placing on the market and use of feed) defines a ‘food-producing animal’ as any animal that is fed, bred or kept for the production of food for human consumption, including animals that are not used for human consumption, but that belong to a species that is normally used for human consumption in the Community (EC 2009). According to this definition all chickens are considered to be food-producing animals. Legally, the definition of a food-producing animal can be interpreted either way.
The Veterinary Medicines Regulations require that any drug used in a food-producing animal must be listed in the table of ‘allowed substances’ (Table 1 of Commission Regulation (EC) 37/2010). (EU Table 1).
When presented with a clinical case of backyard poultry, the primary concern is likely to be the health of the pet rather than the requirement to be able to continue to eat the eggs. In the event of significant or chronic health challenges egg production will obviously be sacrificed. Unfortunately, poultry are a predated species and often appear asymptomatic until they become seriously ill, at which stage mortality rates tend to be high.
Unlike for horses, there is no provision for chickens to be classed as non-food producing. However, from a pragmatic stance, when chickens are kept purely as pets, and providing the vet can justify the prescribing decision, the use of a drug not listed in EU Table 1 on welfare grounds would be considered acceptable. (personal communication, VMD). In such a case, the vet must make it very clear to the animal owner that neither the animal nor food products derived from it (such as eggs) can ever enter the food chain. In reality, it might be difficult to be certain of this (e.g. if chickens are sold or passed on to someone else). Ultimately, the prescribing vet is responsible for medicine selection under the Prescribing Cascade; therefore if any produce that ended up in food for human consumption was found to have non-compliant residues (i.e. unlisted or ‘prohibited substances’), the prescribing vet could be considered liable. Unlisted substances include: fipronil, imidacloprid, cefuroxime, fusidic acid, methadone, propodol, lufenuron (Whitehead and Roberts 2014).
When prescribing a medicine for a food-producing animal, the vet must inform the owner of the appropriate withdrawal period to ensure that drug residues are absent or do not exceed the MRL (VMR 2013). The withdrawal period is the number of days between cessation of drug treatment and the egg being judged fit for human consumption (i.e. when the concentration of drug or metabolite does not exceed the MRL) (EMA 2019). Eggs should not be eaten during the treatment period, unless there is a zero-day withdrawal for the drug.
When choosing a treatment, the rational first choice (and in keeping with the Prescribing Cascade) is a product authorised for use in laying hens that has an established egg withdrawal period. There are many such products – some are listed in the table below. All of the drugs in these products are ‘allowed substances’ for food-producing animals and have established MRLs for eggs (or, as for amprolium, no MRL is required). However, these products are designed for mass medication on commercial farms, and so are impracticable for use in backyard chickens because of the large pack sizes and high concentrations of active ingredient. For example a 100g pack of Phenocillin is 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. Diluted forms of Flubenvet (which contain the anthelmintic flubendazole) are 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.
Products authorised for use in laying hens
There are also products authorised for use in poultry that are not authorised for use in laying hens (see the table below). They contain ‘allowed substances’, but the SPCs state that they are “not for use in animals for which eggs are produced for human consumption”, because an MRL is set for poultry meat but not for eggs.
Products authorised for use in chickens, but not authorised for use in laying hens
There are also a few products that contain ‘allowed substances’ for which an MRL has been set for eggs, but the SPC for the product does not support its use in birds producing eggs for human consumption (see the below for examples). For some products, the in-water formulation is authorised for use in animals producing eggs for human consumption, whereas the in-feed formulation is not. These anomalies are likely to be due to regulatory or commercial considerations.
Products containing drugs with established egg withdrawal periods that are not authorised for use in laying hens
If there is no suitable authorised product to treat a chicken, the prescribing vet might consider it appropriate to prescribe a medicine under the Cascade.
Given that most medicines authorised for chickens are impractical for use for individual animals due to pack size or concentration of the active ingredient, general practice vets may need to resort to prescribing for chickens under the Cascade. A general practice vet is more likely to have a dispensary suited to small animals or more traditional farm animals. Any drug listed in the table of ‘allowed substances’ may be considered for the treatment of a food-producing animal under the Cascade.
Certain medicines containing parasiticides (e.g. ivermectin – an ‘allowed substance’) are marketed for use in small pet animals (e.g. cage birds, rabbits) and are exempt from needing to be authorised as veterinary medicines (VMD 2019). If such products are used for species other than those for which they are recommended, under the Prescribing Cascade they are at the same level as ‘specials’ or extemporaneous preparations. See the related module on Specials for more on prescribing specials.
Vets need take into account the risks when prescribing medicines for use in backyard chickens, and set a suitable withdrawal period to ensure that no illegal residue levels enter the food chain.
For products authorised for in use chickens, any established withdrawal period for eggs will be stated in the summary of product characteristics (SPC) (e.g. zero days for fenbendazole [Panacur Aquasol SPC]). However if no withdrawal period for eggs is published, it is the prescribing vet’s responsibility to set one (VMR 2013). When medicines are used that have no defined egg withdrawal period, including authorised medicines used in an unauthorised way (for example at a different dose or for a different duration from that authorised), the Veterinary Medicines Regulations define a minimum statutory withdrawal period of 7 days for eggs, unless a longer withdrawal period is stated on the product’s SPC (VMR 2013). However, given that eggs take around 14 days for the yolks to mature, some may decide to err on the side of caution and advise a minimum egg withdrawal of 14 days unless the information gathered to make the decision indicates a longer period.
For an authorised product that has a defined egg withdrawal period the advice to give to clients is a simple matter. However when setting the withdrawal period for a product for which no egg withdrawal period is published, 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 will help include product SPCs, the EU table of ‘allowed substances’, and published drug residue studies (links to sources of information are included at the end of this module). The relevant pharmaceutical company technical information department is likely to be able to help by sharing available information.
In practice it is difficult for any vet, especially for those unfamiliar with poultry practice, to ensure that they are in possession of all the known information about drugs used under the Cascade in order to define an appropriate egg withdrawal because the available information is often unrelated to chickens or eggs. The only way to ensure an appropriate egg withdrawal is to test eggs, which is impractical. The decision about the egg withdrawal is the responsibility of the prescribing vet and should be made before prescribing or administering the treatment.
When prescribing for a food-producing animal under the Cascade, the withdrawal period for eggs must not be less than 7 days (VMR 2013). However, given that egg yolks are formed over 14 days, some may prefer to err on the side of caution and consider a minimum withdrawal of 14 days.
If the medicine is authorised for another food-producing animal, there may be a withdrawal period specified in the SPC for meat or milk, which could be used if longer than the statutory minimum.
Further information on which to base a withdrawal period may be available in the published literature. The relevant pharmaceutical company may be able to help identify relevant information.
Take into consideration the formulation. For example, a drug administered in a long-acting (depot) formulation is likely to persist in tissues longer than if administered in an immediate-acting formulation.
Bear in mind that drug metabolic processes in chickens may be different from mammals.
The following are examples of how available information can be used to make a decision about egg withdrawal.
Enrofloxacin, a broad spectrum fluoroquinolone antibiotic, is authorised for oral administration in chickens as Baytril 10% oral solution, and other brands, for adding to drinking water). Baytril is also available as a 2.5% solution authorised for use in calves and exotic animals (so use in chickens is ‘off-label’). Enrofloxacin is an ‘allowed substance’ for food-producing animals. However the SPC for Baytril 10% solution states “Do not use in laying hens producing eggs for human consumption” (Baytril 10% SPC) because no MRL has been set for eggs and so use of the product in commercial egg production is limited. However, this does not necessarily preclude the use of enrofloxacin in backyard chickens. Some vets will take the SPC statement to mean that if enrofloxacin is used in egg-laying birds, the eggs can never be used for human consumption. However, the SPC also includes the statement “Do not administer to layer replacement birds within 14 days of coming into lay”. This is intended to guide the ‘on-label’ use of the 10% product in juvenile replacement birds before point of lay. Some might interpret this as the basis for applying a 14-day egg withdrawal (for either the 10% or 2.5% formulations). This withdrawal duration is supported by the results of studies of enrofloxacin residues in hens’ eggs (Cornejo et al 2011; Goetting et al 2011). However, those who prefer a more cautious approach might decide on a withdrawal period of 4 weeks. Another important consideration is whether enrofloxacin is a suitable choice of antibiotic and is in keeping with antimicrobial stewardship principles.
Fluralaner, a relatively new isooxazoline ectoparasiticide, is widely used in companion-animal practice and now in poultry practice. Fluralaner is an ‘allowed substance’ for food-producing animals and the brand Exzolt solution for adding to water is authorised for the treatment of red mites in chickens and has a zero-day egg withdrawal (Exzolt SPC). If this product, which is intended for large-scale use, is not considered suitable for individual chickens, it may seem reasonable to consider using Bravecto spot-on (a brand of fluralaner licensed for cats and dogs) at an equivalent dose while recommending the minimum 7 days’ statutory egg withdrawal. However, although administration of the drug fluralaner is appropriate, it is also important to consider the suitability of the spot-on formulation, including its efficacy in chickens and the potential risk of adverse effects (e.g. from the solvent in the product). A search of the published literature and contact with the pharmaceutical company that markets Bravecto revealed a lack of published data on the efficacy and safety of Bravecto spot-on formulation in chickens. The lack of available information at present suggests that fluralaner spot-on may not be a suitable choice of treatment for a backyard chicken.
Irrespective of the recommended withdrawal period, the considerations and due diligence work that led to the decision should be documented and retained, and written consent obtained from the owner before drug administration.
When administering a product for a food-producing animal, the following information must be recorded and retained by the practice for at least 5 years (VMR 2013):
date of examination of the animal(s)
name and address of the owner
identification and number of animals treated
results of the vet’s clinical assessment
brand name and batch number
name and quantity of active ingredient
dose and duration of treatment
Remembering to record the batch number and withdrawal period is particularly important as this will not be normal practice for companion-animal vets. A VMD or RCVS Practice Standards Scheme inspection will include checking record-keeping in respect of food-producing animals, including chickens.
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 (VMR 2013). 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 in products that will be sold (e.g. cakes) during the withdrawal period.
If consulted about the health of a backyard/pet chicken, companion-animal vets need to take into account the legislation on use of medicines in the treatment of food-producing animals
Although the main components of eggs are assembled in the 24 hours or so before lay, the components 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 prescribing vet 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.
There are few authorised products for chickens available in quantities and concentrations suitable for the treatment of small numbers of chickens and so treatment under the Cascade may be necessary.
When prescribing under the Cascade, the considerations and due diligence work that lead to the treatment decision should be documented and retained, and written consent obtained from the owner before drug administration.
Sources of Information and Advice
‘Live’ list of ‘allowed substances’ for food-producing animals maximum residue limit in tissues and produce: Table 1 of EC COMMISSION REGULATION (EU) No 37/2010
VMD Product Information Database of authorised veterinary medicines. Contains all authorised products and links to SPCs
NOAH Compendium Withdrawal Periods table Withdrawal periods published in SPCs are collated in this table by product name. Not all authorised products included.
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.
BSAVA Backyard Poultry General guidance in the BSAVA Medicines Guide
VMD The Cascade: Guide to prescribing unauthorised medicines
Veterinary Medicines Directorate. email@example.com. Include Cascade use in the subject line.
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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.
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.