New Neurological Exam Built for Harbor Seal Pups Could Reveal Effects of Prenatal Mercury Exposure

At The Marine Mammal Center in Sausalito, California, a stranded harbor seal pup can be tested with a fish head dangled on a string above it. The stimulus is part of a new neurological exam built specifically for Pacific harbor seal pups (Phoca vitulina richardii), described in a study in Frontiers in Veterinary Science. Cara Field, the center's director of conservation medicine, said the fish head turned out to be useful and, apparently, quite tasty.

The exam exists because the standard neurological workup was designed for land-based quadrupeds. Seals have flippers, reduced external ears, thick blubber, and bodies built for swimming and diving, so the conventional approach fits poorly. Stranded pups arriving at rehabilitation centers can have a range of problems, neurological disorders among them, and clinicians needed a better way to tell what they were dealing with.

Over two consecutive years, the team examined 45 stranded pups at the center. After the first year they revised their draft into a 33-test protocol. Many tests come from the small animal neurological exam, and others were created around how a pup's body works. New components include a sloped ramp to evaluate proprioception and motor function, a "handstand" to check neck reflexes, and the dangled fish head, which tests balance, strength, coordination, and cranial nerve function, including the field of vision.

To check reliability, the investigators repeated the exam on 17 pups. Of the 33 tests, 26 gave a dependable result in at least 80% of those repeat exams. Some pups' temperaments made certain results hard to interpret. The authors suggested the protocol could be adapted for other phocid seals.

The researchers also applied it to three pups with suspected neurological deficits. In all three, the exam correctly identified where in the nervous system the lesion lay, and imaging, necropsy, or both later confirmed it. The case reports show the exam's value in separating neurological disease from look-alike problems. One pup with poor balance and coordination had MRI findings of middle ear fluid consistent with otitis media and a spinal cord lesion at C1-C2; either could have affected its balance. It improved with antimicrobial treatment and was released. Imaging in two other pups found porencephaly, fluid-filled cavities in the brain, in one and severe hydrocephalus in the other. Three pups that seemed weak on repeat exams were diagnosed with primary malnutrition, their deficits considered secondary to systemic illness, and all three recovered enough for release. Christine Thomson, the study's first author and a veterinary neurologist at Animal Referral Hospital in Brisbane, said a diagnosis allows appropriate treatment and rehabilitation.

The exam belongs to a larger program led by Todd O'Hara, a veterinary toxicologist at the Texas A&M College of Veterinary Medicine and Biomedical Sciences, who studies contaminants in marine mammals and their prey along the Pacific Coast of North America. Mercury biomagnifies in marine food webs, concentrating at each step from prey to predator, so fish-eating mammals can be heavily exposed. Pups add another angle: some contaminants pass from mother to offspring during pregnancy and through milk, so young animals can reveal very early exposure. Daniela Murillo Cisneros of Centro de Investigaciones Biológicas del Noroeste noted that regional differences in currents and food webs can shape maternal exposure and, in turn, what pups receive.

An earlier study at the center, published in Science of the Total Environment in 2021, measured total mercury in blood and hair from 267 pups admitted between 2015 and 2019. Some of the hair was lanugo, the coat grown in utero. Higher mercury was negatively correlated with responses to touch and with movement, during both intensive care and pool stages, and was linked to longer rehabilitation stays. The authors called the associations modest but statistically significant, with wide variation between pups. Mercury showed no relationship with survival. First author Marianne Lian said the findings support the idea that fetal mercury exposure puts wild animals at risk of neurodevelopmental abnormalities.

O'Hara's group has also studied Steller sea lion pups in Alaska, where thorough neurological exams were hard to do at remote rookeries. O'Hara said a pup may look healthy yet carry subtle abnormalities that could hurt its survival after release. Next steps include studies of California sea lions in Mexico and continued community-based work on mercury and other trace elements in Alaska's harbor seals.