The exam is part of a research program looking at how mercury exposure before birth may affect young marine mammals.
Stranded harbor seal pups admitted to rehabilitation centers can have a range of health problems, including neurological disorders.1 Yet the standard neurological examination was designed for land-based quadrupeds and is a poor fit for seals.2 In a study published in Frontiers in Veterinary Science, veterinarians and researchers developed and tested an exam protocol made for Pacific harbor seal (Phoca vitulina richardii) pups. They worked with stranded animals at The Marine Mammal Center in Sausalito, California.2
Flippers, reduced external ears, thick blubber, and a body built for swimming and diving all make a seal a poor fit for a conventional neurological workup.1
Designing an exam around those differences is one piece of a broader research program involving Todd O'Hara, DVM, PhD, DABVT, a veterinary toxicologist and professor at the Texas A&M University College of Veterinary Medicine and Biomedical Sciences. Working with collaborators along the Pacific Coast of North America, O'Hara studies which contaminants marine mammals and their prey carry and how those exposures may affect the animals' health.1
“As the harbor seals are wild animals and primarily aquatic, we had to be creative and develop new ways to assess their neurological function,” Cara Field, DVM, PhD, DACZM, director of conservation medicine at The Marine Mammal Center, said in a news release from Texas A&M.1 “Fish-head-on-a-string was a useful stimulus and, apparently, quite tasty.”
Over 2 consecutive years, the team examined 45 stranded pups in care at the center.2 After the first year, the researchers revised their draft protocol into a 33-test exam. The final version borrows many tests from the small animal neurological exam. It also adds new tests created around how the pups' bodies function.2 New components include:
The dangling object started out as a metal nut. The team switched to a fish head after some pups tried to bite the nut.2
To check reliability, the investigators repeated the exam on 17 pups. Of the 33 tests, 26 gave a dependable result in 80% or more of those exams. Some pups' temperaments made certain results difficult to interpret.2 The authors suggested the protocol could be adapted for use in other phocid seals.2
The researchers also used the protocol on 3 pups with suspected neurological deficits. For all 3, the exam correctly identified where in the nervous system the lesion was located. Imaging, necropsy, or both later confirmed each location.2
According to Texas A&M, part of the exam's value lies in separating neurological disease from other problems that can cause similar signs.1 The study's case reports show how that can play out:
“By developing a bespoke neurological examination protocol, we can identify pups with neurological dysfunction and pursue further diagnostic tests to identify the cause, such as intoxication, meningitis, or developmental problems,” said Christine E. Thomson, BVSc, PhD, DACVIM (Neurology), DECVN, in the release.1 “With a diagnosis, appropriate treatment and rehabilitation can be undertaken.” Thomson is the study's first author and a veterinary neurologist at Animal Referral Hospital in Brisbane, Australia.2
Mercury biomagnifies in marine food webs, building to higher concentrations at each step from prey to predator. As a result, fish-eating marine mammals can end up with greater exposure.1 Pups add another dimension. Some contaminants can reach offspring from the mother during pregnancy and through milk. Studying young animals can therefore reveal exposures that started very early in life.1
“Differences in ocean currents and environmental conditions can shape local food webs and prey composition,” Daniela A. Murillo Cisneros, PhD, a postdoctoral fellow at Centro de Investigaciones Biológicas del Noroeste in Mexico, said in the release.1 “These regional food-web differences can influence maternal exposure and, in turn, the exposure their pups receive.”
An earlier study at The Marine Mammal Center examined whether mercury levels at admission were related to pups' behavior during rehabilitation. The study was published in Science of the Total Environment in 2021. Researchers measured total mercury in blood and hair samples from 267 harbor seal pups admitted between 2015 and 2019.3 The hair was partly lanugo, the coat pups grow in utero. The pups' behavior was assessed weekly throughout rehabilitation.3
Mercury concentrations in both blood and hair were negatively correlated with the pups' responses to touch and with their movement. The pattern appeared in both the intensive care and pool stages of rehabilitation. Higher mercury concentrations were also linked to longer stays in rehabilitation.3 The investigators described the associations as modest yet statistically significant. They also noted wide variation from one pup to the next. Mercury concentrations showed no relationship with survival.3
“Measuring these adverse effects can be especially difficult in a wild animal,” Marianne Lian, DVM, PhD, the study's first author and a former doctoral student of O'Hara's, said in the release.1 “These findings supported that fetal exposure to mercury can put wild animals at risk for neurodevelopmental abnormalities.”
O'Hara's group has also studied Steller sea lion pups in Alaska. However, thorough neurological exams were hard to carry out at the region's remote rookeries. According to Texas A&M, harbor seal pups in rehabilitation gave the researchers a more workable way to study contaminants' neurological effects. The university also suggests the findings could eventually inform research in other pinnipeds.1
“An animal may appear healthy while still having subtle neurological abnormalities that could affect its ability to survive after release,” O'Hara said in the release.1 “Being able to identify those effects gives rehabilitation organizations, such as the Marine Mammal Center, a more complete picture of the animal's health and whether it is ready to successfully return to the wild.”
O'Hara approaches the research from a One Health perspective. People and marine mammals can draw on many of the same marine food sources. Studying contaminants in those shared food webs may therefore shed light on risks to both.1 Next steps for the group include studies of California sea lions in Mexico. The group also plans continued community-based work with harbor seals in Alaska, centered on mercury and other trace elements.1
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