Routes of Uptake and Their Relative Contribution to the Toxicologic Response of Northern Bobwhite (Colinus Virginianus) to an Organophosphate Pesticide

Abstract

Ingestion of contaminated food is considered the primary route of exposure in birds to agricultural chemicals. Routes of exposure other than ingestion are not often considered in risk assessments of agricultural chemicals to avian wildlife. However, recent studies demonstrated anorexic or avoidance behaviors in birds exposed to organophosphate (OP) insecticides. These behaviors would tend to limit exposure if ingestion alone were considered. The contribution, if any, of dermal, preening, and respiratory pathways to the exposure of birds to pesticides under field conditions is unknown. In addition, oral exposures are currently assessed in artificial environments that do not reflect real‐life exposure scenarios. To determine the relative contribution of these pathways and to assess exposures under ecological conditions, 270 northern bobwhite (Colinus virginianus) were exposed to simulated aerial crop applications of methyl parathion in an environmentally controlled wind tunnel. The wind tunnel environment consisted of a 25‐cm cotton plant canopy, a 5‐cm‐thick floor of silt‐loam, a temperature of 25°C, 50% RH, UV intensity similar to summer sunlight, and a wind speed of 3.2 km/h. Inhalation, preening, and dermal routes were isolated in groups of birds exposed to each application. Five birds from each group were collected at 1, 4, 8, 24, and 48 h post‐spray to determine cholinesterase (ChE) response to the exposures. Contaminated and uncontaminated darkling beetle (Tenebrio molitar) larvae were presented to free‐ranging sprayed birds in the wind tunnel to assess oral uptake. ChE response was determined at 4, 8, 24, and 48 h postspray. All exposures were replicated. All four routes contributed to the inhibition of brain ChE at different post‐spray periods. Dermal uptake and preening were major contributors to the overall dose and toxic response of birds to methyl parathion. Inhalation was the major route of exposure at 1 h post‐spray. At 4 h post‐spray, uptake through preening caused the greatest inhibition of brain ChE activity. Oral ingestion resulted in less than 20% inhibition of brain ChE during the test. Routes of uptake in order of contribution to toxicologic response from 8 to 48 h post‐spray were dermal > preening ≥ oral > inhalation.

Risk Assessment for Birds and Mammals ‐ Revision of Guidance Document under Council Directive 91/414/EEC (SANCO/4145/2000 – final of 25 September 2002) ‐ Scientific Opinion of the Panel on Plant protection products and their Residues (PPR) on the Science behind the Guidance Document on Risk Assessment for birds and mammals

“…However, birds in that experiment were able to preen so that some of their exposure was also oral (though not dietary). Driver et al, (1991) working with methyl parathion in a wind tunnel, were able to separate the various routes of exposure more clearly and showed that the dermal route was more important than others in their experimental setup. In an analysis that will be repeated and expanded below, Mineau (2002) showed that an understanding of the relative dermal toxicity of insecticides was very important in understanding acute impacts in a large sample of field studies.…”

“…Within each growth stage a conservative (early) value is used. Non-dietary exposure + /++ This parameter is ignored, however, the true contribution uncertain, but could, in short term, increase risk by up to two times or more although this is very uncertain as based on bird studies (Driver et al, 1991). Variation of toxicity between species and/or stages within species ---There is very little data on toxicity in mammalian species other than the standard species used for human toxicology.…”

Decision‐making Scheme for the Environmental Risk Assessment of Plant Protection Products

“…This decision requires expert judgement, using knowledge of the behaviour and ecology of the species concerned, and information from previous experiences with similar products. Recent research is beginning to address the relative importance of different exposure routes (Driver et al, 1991), and in the future may provide a stronger basis for identifying the critical mechanisms of exposure.…”

“…Very little is known of exposure by inhalation, or of its toxic implications for wildlife, although recent experimental studies have helped to extend knowledge in this field (Driver et al, 1991). At present, it is appropriate to follow an approach similar to that for dermal toxicity (see Note 19), making use of acute oral toxicity values (LD50 x 5), rather than requiring direct measures of inhalation toxicity.…”

“…L'exposition par inhalation et ses constquences toxiques pour la faune sauvage sont tres ma1 connues, malgre quelques etudes expkrimentales recentes (Driver et al, 1991). A I'heure actuelle, la demarche a suivre est la meme que dans le cas de la toxicite cutante (voir note 19): utiliser 5 fois la toxicite aigue par administration orale (DL50 x 5 ) plut6t que des mesures directes de toxicite par inhalation.…”