Determinants of mortality of juvenile harbour seals (Phoca vitulina) infected with lungworm submitted to a Dutch seal rehabilitation centre

Since the seal populations in the North Sea are again thriving, the rationale behind seal rehabilitation is currently under discussion. Seals frequently require rehabilitation as a result of a lungworm infection, with these infections most commonly seen in young seals. The need for triage support is addressed by the organisations involved in seal rehabilitation to ensure adequate decision making on whether or not a seal should be taken into rehabilitation. It is still unclear which parameters influence seal mortality in rehabilitation, these parameters are essential to enable triaging of stranded seals. Therefore, the aims of this study were: to estimate the proportion of lungworm infected juvenile harbour seals in a rehabilitation centre; to determine the survival rate among lungworm infected juvenile harbour seals; and to study determinants of mortality in the lungworm infected juvenile harbour seals. Data was collected retrospectively from all harbour seals admitted to a Dutch rehabilitation centre between September 2017 and August 2019 (n = 208). Eleven parameters were evaluated using univariable logistic regression with a 95% confidence interval (p < 0.05) to study the association between the determinants and the outcome - survival or death. All associated parameters with a p-value <0.2 were used in multivariable logistic regression. The multivariable model demonstrated that high body temperature at intake (high vs normal body temperature OR = 0.32; p = 0.01); intake from August to December (Augustus-December vs January-May OR = 0.40; p = 0.02); and whether the seal was previously admitted to a rehabilitation centre (yes vs no OR = 0.12, p < 0.01) were good determinants of mortality. The results of this study could be used to further develop triage-support that aids in the decision to leave the seal on the beach; admitting the seal to a rehabilitation centre; and/or to euthanise the seal, in order to prevent further suffering.