Comparative Study of Anesthesia-related Mortality in Raptors

 

Bradly Fountain, Michelle Willette, Dana Franzen-Klein, Sandra Allweiler, Mark Ponder, Julia Ponder

College of Veterinary Medicine, University of Minnesota, Minneapolis, Minnesota

 

Abstract

The risk of anesthetizing birds includes many confounding variables. Foremost may be the multitude of different species, but also the traditional factors of anesthetic protocols such as age, history, and overall health status. The aim of this study is to investigate the relationship between some of these factors in a sample of wild raptors and the risk of anesthetic-related mortality to pursue greater understanding of how these factors can affect future anesthetic studies and protocols. A retrospective approach was used to record the age category (nestling, fledgling, hatch year, after hatch year, two year, three year, four year and adult), body condition score (BCS, on a 1-5 scale), and health status (healthy, trauma-related disease, metabolic-related disease, and stable to recovering while in-clinic) in five different species of raptor (Bald Eagles, Red-Tailed Hawks, Great-Horned Owls, Cooper’s Hawks and Merlins). Cases involving these species were studied to record the status on intake, the number of anesthetic events the animal underwent, the status of the bird at the end of each anesthetic event and the final outcome of the case (euthanized or died due to anesthetic-related causes or due to other reasons or released back to the wild). Preliminary results show a significant correlation between species of the patient and anesthetic-related mortality, with both Cooper’s Hawks and Merlins exceeding the normal distribution. Results also show a significant correlation between age category of the patient and anesthetic-related mortality, with fledglings exceeding the normal distribution. The BCS showed no correlation. As of this writing, the health status of the patients relationship with mortality risk has not been measured.

 

Introduction

Anesthesia is a commonly used method for both surgical operations and general restraint of birds in the clinical setting. However there are many confounding variables in avian medicine that are not present in other small mammal practices, including the wide variety of species differences and a lack of literature for the traditional aspects of anesthetic protocols, such as history, weight, and general health. It is largely unknown what risk factors in small mammal anesthesia translate to avian medicine, especially in wildlife. We hypothesize that species, age category or body condition score on presentation will affect risk of anesthetic-related mortality. To test this, a selection of records from the Gabbert Raptor Center, a wildlife rehabilitation hospital associated with the University of Minnesota in Saint Paul, were reviewed to gather data from a selection of species over two years. Preliminary findings suggest a relationship between species or age category with the risk of anesthetic mortality, but did not find a correlation with body condition score. This information can be used to further explore the relationships between these common anesthetic risk factors and the variety of avian patients seen in both wildlife and pet clinics.

Methods

For the purposes of this study, anesthetic-related death was defined as: due to any progressive systemic, neurological, respiratory or other symptoms known to be related to anesthesia; due to any unknown causes within 48 hours post-anesthesia; due to any reason during anesthesia other than the choice to euthanize due to prognosis.

The records for five species (Bald Eagles, Red-Tailed Hawks, Cooper’s Hawks, Great Horned Owls and Merlins) were selected from all cases between 2017 and 2018 at the Gabbert’s Raptor Center.

Species, age category if known, weight, body condition score on a 1-5 scale, reason for intake, and the final outcome for the case were recorded. Those euthanized on intake due to severity of injury or poor prognosis for rehabilitation to release were excluded. Outcomes within 48h from each anesthetic event were recorded. Cases with outcomes other than those released back to the wild were subjected to further review by clinicians at the Raptor Center to confirm if anesthetic-related.

Table 1: Raw data of all included records from 2017-2018

Data was subjected to chi-square testing, wherein anesthetic-related death was found to not be independent of species or age category (p=0.026 and p=0.00026 respectively) but independent of BCS.

Table 2: Chi-square data. Risk of mortality is dependent on species (p<0.05) and age category (p<0.0005)

Table 3: p-value distributions for data in Table 2

Linear probability implies that Cooper’s Hawks have 3.5%
increase in probability of dying from an anesthetic event (significant at the 5% level). The other bird species are statistically insignificant. Having an abnormal weight increases the chance of dying from anesthesia by 2.8% (significant at the 1% level). Finally, being a Fledgling also increases the probability of dying by 3.8% (significant at the 10% level).

Table 4: Linear probability data and graph. Cooper’s Hawks have a significant (p<0.05) increase in risk of mortality, as do fledglings (p<0.01) and animals outside of normal weight ranges (p<0.1)

Logistic regression was run for just species with BAEA as the excluded category. COHA still significant at the 1% level. The estimates suggest that the odds of a Cooper’s Hawks dying from anesthesia are about exp(1.2) = 3.9 times larger than that of a Bald Eagle.

Table 4: Logistic Regression Data and graph.

By odds ratio, Cooper’s Hawks are statistically different from all other birds, besides Merlins. The other species have overlapping confidence intervals, suggesting no significant difference at a 5% level.

Table 5: Odds of Anesthetic Death Ratio by Species

Conclusions

The null hypothesis of independence between species or age category and anesthetic-related mortality can be rejected. The data suggests that Cooper’s Hawks are at significantly higher risk of anesthetic-related mortality than Bald Eagles, Red-tailed Hawks, Great Horned Owls and Merlins, with the four other species relatively similar.

The null hypothesis of independence between BCS on intake and anesthetic-related mortality cannot be rejected, however abnormal weight may be.

More research and statistical analysis must be performed to discover any correlations between age and species, number of anesthetic events involved in a case, and overall health of the animal undergoing anesthesia.

Broadening or supplementing the scope by studying different groups of species, such as common household psittacines, may further our understanding.

 

Acknowledgements

This project was supported by the Boehringer Ingelheim Veterinary Scholars Program and the College of Veterinary Medicine, University of Minnesota

Thank you to the Gabbert’s Raptor Center for your time, photos, and expertise

 

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