A Systematic Review of Changes in Marine Mammal Health in North America, 1972-2012: The Need for a Novel Integrated Approach

Marine mammals are often cited as “sentinels of ocean health” yet accessible, synthesized data on their health changes that could effectively warn of ocean health changes are rare. The objectives of this study were to 1) perform a systematic review of published cases of marine mammal disease to determine spatial and temporal trends in disease from 1972–2012, including changes in regions and taxa affected and specific causes; and 2) compare numbers of published cases of neoplasia with known, hospital-based neoplasia records to explore the causes of discrepancy between numbers of published cases and true disease trends. Peer-reviewed literature was compiled, and data were collected from The Marine Mammal Center database in Sausalito, California for comparison of numbers of neoplasia cases. Toxicoses from harmful algal blooms appear to be increasing. Viral epidemics are most common along the Atlantic U.S. coastline, while bacterial epidemics, especially leptospirosis, are most common along the Pacific coast. Certain protozoal and fungal zoonoses appear to be emerging, such as Toxoplasma gondii in southern sea otters in California, and Cryptococcus gattii in cetaceans in the Pacific Northwest. Disease reports were most common from California where pinniped populations are large, but increased effort also occurs. Anthropogenic trauma remains a large threat to marine mammal health, through direct mortality and indirect chronic disease. Neoplasia cases were under-reported from 2003–2012 when compared to true number of cases, and over-reported in several years due to case duplication. Peer-reviewed literature greatly underestimates the true magnitude of disease in marine mammals as it focuses on novel findings, fails to reflect etiology of multifactorial diseases, rarely reports prevalence rather than simple numbers of cases, and is typically presented years after a disease first occurs. Thus literature cannot guide management actions adequately, nor inform indices of ocean health. A real-time, nationally centralized system for reporting marine mammal disease data is needed to be able to understand how marine mammal diseases are changing with ecosystem changes, and before these animals can truly be considered ‘sentinels of ocean health’.海洋哺乳动物常被称为‘海洋健康哨兵（sentinels of ocean health）’，但可获取且经系统整合的、能有效预警海洋健康变化的海洋哺乳动物健康变化数据却极为稀缺。本研究的目标为：1）对已发表的海洋哺乳动物疾病病例开展系统综述，以明确1972年至2012年间疾病的时空分布趋势，涵盖受影响区域、类群及具体病因的变化；2）对比已发表的肿瘤（neoplasia）病例数与已知的医院存档肿瘤病例记录，探究已发表病例数与真实疾病趋势之间存在差异的原因。研究人员汇编了同行评议文献，并从加利福尼亚州索萨利托的海洋哺乳动物中心（The Marine Mammal Center）数据库中提取数据，用于肿瘤病例数的对比分析。有害藻华引发的中毒事件呈上升趋势。病毒疫情在美国大西洋沿岸最为频发，而细菌疫情（尤其是钩端螺旋体病（leptospirosis））则多见于太平洋沿岸。部分原生动物和真菌人畜共患病正呈蔓延态势，例如加利福尼亚南部海獭感染的弓形虫（Toxoplasma gondii），以及太平洋西北部鲸类（cetaceans）感染的盖特隐球菌（Cryptococcus gattii）。疾病报告多来自鳍足类（pinniped）种群规模庞大的加利福尼亚州，不过相关研究投入的增加也是原因之一。人为创伤仍是威胁海洋哺乳动物健康的重大因素，可直接造成动物死亡，也可引发间接慢性疾病。与真实病例数相比，2003年至2012年的肿瘤病例存在报告不足的情况，而部分年份则因病例重复上报导致报告数虚高。同行评议文献严重低估了海洋哺乳动物的真实疾病负担：这类文献多聚焦于新发现，无法反映多因素疾病的病因，极少报告患病率（prevalence）而仅报道简单的病例数，且通常在疾病首次出现数年后才发表。因此，现有文献无法充分为管理决策提供指导，也无法用于构建海洋健康指标。要了解海洋哺乳动物疾病如何随生态系统变化而演变，让这些动物真正成为‘海洋健康哨兵’，亟需建立一套实时的全国集中式海洋哺乳动物疾病数据报告系统。