Lungworm infection in Dutch wild-living seals: Evaluating quantitative measurements

Background
Seals in Dutch waters frequently have lungworms, specifically Parafilaroides gymnurus (PG) and Otostrongylus circumlitus (OC). Lungworm infections can induce severe respiratory disease, jeopardizing health and survival of affected seals. We evaluated quantitative lungworm measurements to determine prevalence and severity of lungworm infestations in stranded seals.
Materials & Methods
Postmortem examinations were conducted on 22 harbour seals (Phoca vitulina) and two grey seals (Halichoerus grypus) found dead on Dutch beaches (n=17) or euthanased (n=7), between October 2021 and July 2023. Macroscopic lungworm infestation was assessed semi-quantitatively as none, mild, moderate, or severe depending on the visible lungworms within tissue. From a subset of harbour seals (n=20), one lung was weighed and flushed with digestive solution as per modified Inderbitzen method (MIM) to determine lungworm type and quantity. Lung samples were collected from all specimens for histology.
Results
Macroscopic examination revealed lungworm infestations in 13/24 seals, with five mild, four moderate, and four severe infestations. MIM identified lungworms in 16/20 seals, thirteen showing PG and fifteen showing OC infestations. Histological examination detected lungworms in 17/24 seals, all PG, three combined with OC. PG was observed in cranial (15/24), middle (16/24), and caudal (14/24) lung lobes. Consolidated measurements identified lungworms in 17/24 seals. Pearson’s correlation analysis suggested histology as the most sensitive method for identifying PG (correlation coefficient 1.00; p<0.01), whereas OC is better detected by MIM (correlation coefficient 0.87; p<0.01).
Conclusion
We found lungworms in 71% of examined seals. Future examinations are needed to better understand the implications of lungworm infections.