Organophosphate insecticides, such as parathion, malathion, diazanon, and chlorpyrifos, are widely used in agriculture; these neurotoxins act by inhibiting acetylcholinesterase at nerve endings, thus increasing cholinergic signaling. In addition to this acute toxicity, organophosphates can contribute to neurodevelopmental abnormalities in rodents through a variety of other mechanisms, including changes in transcription factors and cell signaling pathways required for neural cell differentiation, increased oxidative stress and DNA damage, and altered apoptosis and cell migration (Slotkin and Seidler, 2007). The crawling behavior, short stature, hand-to-mouth activity, tendency to mouth or ingest non-food items, and higher basal metabolic rate of infants and small children contribute to pesticide exposures and body burdens that differ from those of adults. Moreover, because both glial cells and neurons in the human central nervous system continue to develop and mature after birth, the potential for significant neurobehavioral and cognitive consequences of childhood pesticide exposure warrants attention from both clinicians and toxicologists (Weiss et al., 2004).
Although much of the data on the effects of organophosphate (OP) pesticide exposure has been obtained using rodent models of nervous system development and function, there are a few studies of developmental, reproductive, and behavioral consequences in wild birds. In the Lower Rio Grande Valley of Texas, an intensely agricultural area, cotton fields are regularly sprayed with OP insecticides; a number of wild bird species, including the White-winged Dove, are exposed to these compounds through drinking contaminated irrigation water. To examine potential mechanisms for the decline in White-winged Dove populations in South Texas, Burkepile and colleagues (2002) used captive breeding pairs of these birds to examine the effects of the OP methyl parathion on reproductive behavior.
The sublethal effects of OP pesticides on wild bird populations is a somewhat controversial topic, and reports include anorexia, lethargy, incorrect migratory orientation, decreased coordination, decreased egg production and fertility, teratogenesis in embryos, territory abandonment, reduced time at the nest, and reduced predator avoidance among the outcomes. Burkepile et al. (2002) used 3 different chronic and periodic exposure paradigms for parathion (delivered in drinking water), at several different doses, with captive breeding pairs of doves, and then measured eggs laid and hatched, young fledged, and nest attentiveness. Acetylcholinesterase inhibition in brain was also measured in euthanized birds, at the end of each experiment. The authors found that chronic exposure to parathion at levels greater than or equal to 4.5 ppm had negative effects on egg laying and incubation, and significantly reduced fledgling success. Doves exposed to these levels of parathion, which correlated with a 25% or greater inhibition of acetylcholinesterase, were also less attentive to their nests than were control birds, a behavioral change that could have major embryo growth consequences in a “constant incubator” species.
Burkepile et al. (2002) emphasized that their experiments were designed to approximate OP pesticide exposure in agricultural settings, and suggested that their results indicate that declines in White-Winged Dove populations in the Lower Rio Grande Valley may be due to ingestion of contaminated irrigation water. However, the authors also found that the captive birds avoided parathion-containing water when given a choice, and recommend that changes in the timing of pesticide spraying and crop irrigation might offer wild birds an opportunity to avoid contaminated water.
^’^ Second post in the Wednesday Wings series
White-winged Dove, Zenaida asiatica
Image Source: J. Fromer (via Wikipedia)
*Burkepile, N.A., Hewitt, D.G., Waggerman, G.L., Small, M.F., Hellgren, E.C. (2002). Effects of methyl parathion on white-winged dove productivity and reproductive behavior. J. Wildlife Management 66, 202-211.
Slotkin, T.A., Seidler, F.J. (2007). Comparative developmental neurotoxicity of organophosphates in vivo: Transcriptional responses of pathways for brain cell development, cell signaling, cytotoxicity, and neurotransmitter systems. Brain Res. Bull. 72, 232-274.
Weiss, B., Amler, S, Amler, R.W. (2004). Pesticides. Pediatrics 113, 1030-1036.
*main peer-reviewed article
Burkepile, N.A., Hewitt, D.G., Waggerman, G.L., Small, M.F., Hellgren, E.C. (2002). Effects of methyl parathion on white-winged dove productivity and reproductive behavior.. Journal of Wildlife Management, 66(1), 202-211.