Increases in resource consumption and waste production associated with booming human development in the past hundred years have profoundly altered our environment. However, the question remains: how can we measure these changes adequately? One tool at our disposal is the study of animals in the wild. Wildlife, especially top predators, provides a good marker of environmental health and an abundance of these predators often reflects a healthy ecosystem.
I recently had the opportunity to visit The Marine Mammal Center (TMMC) in Sausalito, CA , where I met with fellow Western University 2009 graduate, Dr. Vanessa Fravel. The center offers state of the art facilities for the study and rehabilitation of stranded marine mammals such as California sea lions, Northern elephant seals or harbor seals and thus provides us with a better understanding of the interactions between wildlife, environment and public health. Just as with other animals, marine mammals can share a wide variety of diseases with humans and diagnosis of such pathogens in these species can help identify an outbreak in the environment and thus lead to preventive actions to aprotect the human population.
Such zoonoses include bacterial diseases (leptospirosis, brucellosis) but also fungal (coccidiomycosis), viral (influenza A & B, rabies) and parasitic (Anisakis). A more comprehensive list of zoonotic diseases shared between people and marine mammals can be found on this UC Davis webpage. Many of these diseases are present in both marine and terrestrial environments and studying their prevalence in wild marine mammals can help identify if these pathogens are naturally present in these animals’ environments or if they represent runoff from a land-based outbreak.
Centers like TMMC are also very well placed to assess maritime ecosystem health, through evaluation of predatory species numbers or prevalence of environmental toxins. One such toxin has gained much attention in the marine mammal field in the past. Domoic acid has been linked to several sea birds deaths as well as marine mammals strandings, including a recent event involving sea lions in Santa Monica, CA in the USA. The toxin is produced by algal diatom Pseudonitzschia sp. The algae are naturally present in the environment and can be found at every level of the oceanic food chain beginning by absorption of the toxin by filter feeders such as clams and small fishes. While non-toxic at low levels, predators such as sea lions eat a large amount of these fish, leading to very high domoic acid tissue levels.
The toxin principally affects the hippocampus and the most recognized clinical appearance of affected animals is neurologic in nature, represented by bouts of seizures as well as altered behavior. The signs are irreversible and animals are gradually not able to hunt and feed properly leading to death. However, the toxin can affect several other organs and has been detected in cardiac tissues of subclinical animals. Antemortem diagnosis of domoic acid poisoning can be difficult but MRI scan of the brain can reveal lesions in the hippocampus, however most diagnoses are made during necropsy once the animal has died.
The amount of fish people eat compared to marine mammals is minimal and as such, domoic acid poisoning in people does not lead to such extreme symptoms as seizures. However, the toxin is recognized to be the etiology of Amnesic Shellfish Poisoning (ASP) in humans through ingestion of contaminated shellfish, especially in periods of diatom blooms. Symptoms in humans also reflect involvement of the hippocampus and include short-term temporary memory loss, which can be permanent in severe cases.
While naturally present in the environment, scientists agree that blooms of domoic acid-producing algae are increasing. Reasons for this trend are most likely multifactorial and it remains to be seen whether causes include natural algal life cycle, global weather changes or increased nitrogenous agricultural waste runoff into the ocean (a phenomenon often associated with blooming of various algal species).
Once again, a One Health perspective allows us to recognize the inter-dependence between human, animal and ecosystem health and it is only through careful analysis of all of these factors that we can improve public health through an efficient and long lasting approach.