the dietary exposure of consumers to injection site residues, and therefore any risks
associated with ingestion, they frequently result in very long withdrawal periods
being assigned to the products. This discourages the development of long-acting
injectable products by the veterinary pharmaceutical industry as well as their use by
farmers. Possible approaches to decreasing the withdrawal period of long-acting
injectable products were discussed in a recent publication (EMEA-CVMP2008b).
At the international level, the food safety risk assessment of residues of veteri-
nary drugs at injection sites is not consistent amongst jurisdictions (Reeves 2007 ).
A major impediment to developing a harmonised risk assessment procedure is the
scant objective information available on the probability of dietary exposure to
injection site residues (EMEA-CVMP, 2005 ). In general terms, data are required
in relation to the prevalence of injection site tissues in animals at slaughter, the fate
of injection site tissues, and the incidence of residues present in injection site
tissues. The prevalence of injection site tissues in animals at slaughter is con-
founded by differences in regional animal husbandry practices, which may prevent
the findings of a survey for one category of livestock production, or for one region,
being extrapolated to another. Similarly, little objective information on the fate of
injection site tissues is presently available. Injection site tissue, when identified, is
trimmed and discarded by meat-processing personnel to prevent it entering the
human food supply; however, the proportion of the total number of injection site
residues that are identified and trimmed is not known. Not all injection site tissues
contain drug residues. Injection site tissue resulting from vaccination or consisting
solely of scar tissue does not represent a chemical food-borne hazard for consumers.
The paper published by the Committee for Medicinal Products for Veterinary
Use (CVMP) of the European Medicines Agency (EMEA) notes eleven possible
approaches to the risk assessment of injection site residues. One proposal involved
increasing the muscle MRL to facilitate a decrease in the withdrawal period of
injectable products. The paper recognised that this would disrupt the tissue distri-
butional relationships of residues and penalise those products administered by non-
injectable routes. A different approach involved using the ARfD instead of the ADI
as the permissible exposure standard. Acute toxicity considerations of the injection
site residues are a current practice in Australia, Canada and the USA. The validity
of this approach depends on the ingestion of injection site residues being a rare
event. Acute toxicity considerations may shorten the withdrawal period, but this is
only when the ARfD exceeds the ADI for the veterinary drug. From a residue
surveillance perspective, this approach requires a sampling protocol that is able to
distinguish between injection site muscle and non-injection site muscle. A dual
sampling protocol for achieving this goal was proposed at the Codex Committee for
Residues of Veterinary Drugs in Foods during the 1990s and early 2000s, but it was
not adopted.
Sanquer et al ( 2006 ) demonstrated that the ADI-MRL concept for assessing
chronic dietary intake of residues is not appropriate for use with injection site
residues. These workers performed a qualitative exposure assessment to estimate
the number of days on which an injection site, or part of an injection site, was
ingested by EU consumers during 1 year. The risk assessment was modelled on all
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