Did Foot-and-Mouth Disease-Control Operations Affect Rodenticide Exposure in Raptors?

Abstract

The source outbreak of the 2001 foot-and-mouth (FMD) epidemic in Britain was on a farm in northeast England (Northumberland) in February. The disease then spread rapidly throughout England, Wales, and the Scottish borders. This was due to transportation of infected animals and associated airborne transmission of the disease before the outbreak was recognized and control measures implemented (Scudamore 2002). There were just over 2,000 confirmed FMD cases, most of which occurred between March and October (DEFRA 2002). Part of the strategy for containing the disease was the slaughter of all livestock on infected premises, on neighboring holdings, and on other holdings considered to have close contact with infected animals. Thus, the total number of premises on which animals were slaughtered exceeded the number of confirmed FMD cases. Animals were killed on the farm premises, and the scale of the cull (peak slaughter rates of half a million animals per week) meant that there could be a delay of some days between slaughter and carcass disposal. Scavengers, particularly the brown rat (Rattus norvegicus), which might feed on the abundant carcasses, were considered a potential route of FMD transmission and a general hygiene problem. Rat control was carried out on all affected premises by teams, which included private contractors, led by the Ministry of Agriculture, Fisheries and Food (now, the Department for Environment, Food and Rural Affairs [DEFRA]). Ministry teams baited solely with the second-generation anticoagulant (SGAR), difenacoum, whereas contractors used difenacoum and another SGAR, bromadiolone (P. Butt, DEFRA, personal communication). We are not aware of any published records for the amount of bait used during the epidemic, but it was estimated that typically no more than 20 kg bait was used on each premises, although more than 100 kg, and occasionally more than 300 kg, was required on a small proportion (,10%) of farms with very high rat populations (P. Butt, DEFRA, personal communication). The average amount of difenacoum used per farm growing grassland and fodder crops in Britain is estimated to be approximately 14 kg per year (Garthwaite et al. 1999). Thus, the amount of difenacoum used over a few days on each treated premises during FMD clean-up operations could typically be up to 40% more than the amount normally used over a whole year, and in some cases, it exceeded normal annual usage more than 20-fold. Routine use of difenacoum and other SGARs for pest control in Britain is known to be associated with unintentional widespread exposure of a range of nontarget species, particularly avian and mammalian predators and scavengers (McDonald et al. 1998; Newton et al. 1999b; Shore et al. 2001; Burn et al. 2002; Shore et al. 2003a,b). Most of this exposure is thought to be secondary, caused by predators feeding on contaminated rodents, and exposure can result in mortality (Joermann 1998, Luttik et al. 1999, Carter and Burn 2000), although relatively few deaths have been diagnosed either in exposure studies (Newton et al. 1999b) or during investigations of mortality incidents (Barnett et al. 2000a,b; Barnett et al. 2002a,b, 2003). However, it has been argued that the levels of exposure and mortality in predators could increase if there was greater SGAR use (and associated increased numbers of rodents with SGAR residues), or if there were dietary shifts by predators toward eating more poisoned rodents (Shore et al. 2003a). The concern during the FMD outbreak was that the largescale use of SGARs during the epidemic could pose an increased risk to nontarget species by increasing the number of contaminated rodents available to predators on and around treated premises. Our aim was to determine whether the high use of SGARs, especially difenacoum, on FMD-infected premises was associated with increased SGAR exposure in 2 bird of prey species that are relatively common in Britain, the barn owl (Tyto alba) and the buzzard (Buteo buteo). We examined barn owls because their exposure to SGARs was already well characterized (Newton et al. 1990, Newton et al. 1999b, Shore et al. 2005), and we believed this would aid us in the interpretation of our data. There were no such previous studies of rodenticide residues in buzzards from England and Wales, but it was anticipated that this species might be at high risk of exposure during the FMD epidemic. This is because buzzards largely feed by scavenging (Newton 1979), and 1 E-mail: rfs@ceh.ac.uk