Fenthion poisoning in ravens

Felipe Reggeti, Neil Pople, Richard Davis, Nick Schrier

Animal Health Laboratory, University of Guelph, Guelph, ON (Reggeti, Schrier); Veterinary Diagnostic Services, Manitoba Agriculture (Pople); Fish and Wildlife Branch, Manitoba Natural Resources and Northern Development (Davis), Winnipeg, MB.

AHL Newsletter 2022;26(1):17.

Sick and dead ravens were found in proximity to a pile of fish offal, beef bones and coyote carcasses in Manitoba.  The pile was suspected to be poisoned bait; therefore, local conservation officers proceeded to submit beef bones and five dead ravens to Veterinary Diagnostic Services, Manitoba Agriculture for diagnostic testing.  On postmortem (PM) examination, the birds were all adults in good body condition.  One raven had been shot and another had been scavenged and looked more autolyzed than the others.  Only one bird had identifiable stomach contents, consisting of fish bones and scales.  There were no other significant gross findings. 

Considering multiple deaths and the lack of significant PM findings, toxicosis was suspected.  To investigate exposure to pesticides, brains from three of the ravens were submitted to Prairie Diagnostic Services in Saskatoon to be evaluated for acetylcholinesterase (AChE) activity, and the results were as follows: - 2.07 μmol/min/g - 1.50 μmol/min/g

Since the reported AChE activity in brain from healthy chickens is 15.99 +/- 3.18 μmol/min/g (1), these results indicated marked enzyme inhibition (less than 25% of normal activity), consistent with organophosphorus (OP) or carbamate toxicosis.  To further investigate the presumptive pesticide exposure, small pieces of soft tissues from the beef bones and liver from one raven were submitted to the Animal Health Laboratory  for pesticide screening.  Fenthion was identified in the liver sample by gas chromatography-mass spectrometry (GC-MS), while no toxins were found in the beef carcass.  Since fish bones and scales were present in the gastric contents of one bird, it is possible that only the fish offal was poisoned, or perhaps scavengers consumed most of the meat on the beef bones.  Regardless, it was speculated that the pile of animal remains was the most likely source of the pesticide because affected animals were in close proximity to it, and fenthion is highly toxic to birds.

Fenthion is an OP compound of intermediate oral acute toxicity in mammals (LD50 in rats = 255 mg/kg bw), but it is highly toxic to birds (LD50 = 2.5 mg/kg bw) and freshwater invertebrates (LC50 = 0.024 µg/L), creating potential risks for non-target species toxicosis.  Organophosphorus compounds act by phosphorylation of a serine residue in the active site of AChE, causing “irreversible” enzyme inhibition.  This leads to accumulation of acetylcholine at the synapse, overstimulating muscarinic and nicotinic cholinergic receptors and potentially resulting in a “cholinergic syndrome”.  Intoxicated animals and humans may present with increased salivation, lacrimation, urination, defecation, increased gastrointestinal motility, emesis and miosis (SLUDGEM); and in severe cases, convulsions, respiratory depression and death (2).

Fenthion-containing products were first registered in Canada in 1961 to control mosquitos, flies, birds and insects for companion animals and agricultural, industrial and residential buildings.  Fenthion was not considered to pose an unacceptable risk to humans and the environment when used following the specifications, as concluded by the Canada Pest Management Regulatory Agency (PMRA) in a re-evaluation of fenthion (PACR2003-05) in 2003 (3).  However, the manufacturer decided to discontinue these products in the USA and Canada.  As a result, Health Canada provided a phase-out timeline for fenthion-containing products, allowing their use until December 31, 2006, as recommended by the PMRA in a subsequent re-evaluation of fenthion (RRD2004-10) in 2004 (3).  Although any remnants of fenthion-containing products in Canada should have been disposed of thereafter, cases of toxicosis are still identified.   AHL


1. Tor E and Galey F. Determination of cholinesterase activity in brain and blood samples using a plate reader. J AOAC Int 1994;77(5):1308-1313.  

2. Meerdink GL. Anticholinesterase insecticides. In Clinical Veterinary Toxicology. Plumlee KH, ed. Mosby, 2004;178-180.

3. https://www.canada.ca/en/health-canada/services/consumer-product-safety/reports-publications/pesticides-pest-management/decisions-updates.html