Suspected ethanol toxicity in juvenile blackbirds and redwings
- 1AHVLA Diseases of Wildlife Scheme, AHVLA – Penrith, Merrythought, Cumbria CA11 9HS
- 2AHVLA Diseases of Wildlife Scheme, AHVLA – Shrewsbury, Kendal Road, Shrewsbury, Shropshire SY1 4HD
- 3Clinical and Forensic Toxicology, Medical Toxicology Laboratory, St Thomas' Hospital, London SE14 5ER
DURING August 2011 the Cumbrian police were called to a primary school where the bodies of 12 juvenile blackbirds (Turdus merula) in varying stages of decomposition had been found. One additional bird was alive but unwell. Foul-play was considered as a possible cause of the deaths.
The carcases were radiographed, revealing no obvious abnormalities, and then taken to Animal Health and Veterinary Laboratories Agency (AHVLA) – Penrith and examined under the Diseases of Wildlife Scheme. All birds were in good body condition but some had feathers missing from the breast and neck areas. Postmortem examination and histopathology revealed no obvious abnormalities apart from signs of trauma in some birds. However, all 12 birds had berries present in their gastrointestinal tracts. Salmonella species, avian influenza virus, West Nile virus and Usutu virus were not detected by AHVLA testing. The school was visited with the police; a further two skeletal birds were found, both with berries in body cavities. A few blackbirds were seen on rowan trees (Sorbus species) at the school. Under the trees, the grass was covered by berries, similar to those found inside the birds. Many of the berries showed bruising, suggesting they had been partially eaten.
Rowan berries are consumed by wild birds and are not normally considered to be poisonous; however, a faint odour of fermentation was noted from the berries found in the birds. Samples of two livers and a pooled gizzard content were sent to the St Thomas' Hospital medical toxicological laboratory in London, where 430 ppm ethanol was found in one of the liver samples, but no ethanol was found in the remaining two samples.
Suspected ethanol toxicity has been reported before in wild birds (eg, Fitzgerald and others 1990) associated with a liver ethanol concentration of 238 ppm. Although endogenous ethanol production in birds is unlikely, ethanol metabolism in wild birds is not well understood and it is, therefore, difficult to interpret liver concentrations. Previously published reports suggest that frost-damaged berries were involved and, although frost was not likely in this incident, the berries on the ground were damaged and presumably vulnerable to yeast infestation with subsequent fermentation and alcohol production.
Staff from the wildlife rescue centre were interviewed, without mention of suspicions, and they reported that on admission the live bird was unsteady on its feet and placed both wings on the ground to support itself and lent against the walls of the enclosure to maintain posture ‘as though it was drunk’. The bird made a full recovery and was released two days later. In summary, the 12 birds were in good condition without obvious disease processes and all had eaten berries tentatively identified as rowan. We cannot explain why only one of the three samples that was tested was ethanol positive. The findings indicated a diagnosis of suspected ethanol toxicity caused by ingestion of fermenting berries. Some of the birds had died from traumatic injury, possibly related to in-flight collisions secondary to intoxication.
A similar diagnosis was reached in 1999 from a submission of redwings (Turdus iliacus) to the Diseases of Wildlife Scheme. Many redwings were seen feeding on berries in a holly tree (Ilex species) in January; several were seen to fall out of the tree and die after landing on the concrete. Carcases were examined at AHVLA (formerly VLA) – Shrewsbury. Laboratory analysis for a range of agricultural chemicals did not detect any residues. Holly berries were present in the birds' intestinal tracts and 1360 ppm of ethanol was detected in crop and gizzard contents, 465 ppm in small intestinal contents and 95 ppm in the liver of one bird.
- British Veterinary Association