Sources and Types of Disease Agents in Cook Inlet
A number of potential sources of disease-causing agents exist in and around Cook Inlet. Disease agents may include pathogens (such as bacteria, viruses, and fungi), parasites, and harmful algal blooms (HABs). Belugas may be exposed to disease agents through: interactions with, or proximity to, other infected belugas or other species; ingestion of contaminated material or organism; open wounds; or inhalation. Natural sources of disease include other belugas, other wild animals, and environmental and water-borne pathogens of natural origin. Anthropogenic sources of disease include untreated sewage outfalls; malfunctioning septic systems; pet waste; runoff from agricultural operations; and discharge from vessels (URS Corp. 2011). No comprehensive survey of disease sources or their characteristics are available. Transfer of disease and parasites between belugas and other wild or domestic species are poorly understood, and endemic disease and parasite loads of CI belugas in comparison to other populations are unknown. For an in-depth review of available information on this topic, see Appendix IX.H – Cause of Death Analysis.
Diseases have the potential to compromise health, reduce reproductive potential, and increase the chance of mortality. Diseases can have population-level effects throughout a species range. Although disease outbreaks among CI belugas are currently expected to be intermittent, climate change and increased pollution could cause an increase in disease frequency. In 2011, 62% of CI belugas photographically identified in Eagle Bay had signs of some level of current or previous infection (McGuire et al. 2014c).
The necropsy record of stranded CI beluga carcasses shows only low levels of parasitism, and parasites that were present did not appear to have a significant negative impact (i.e., were not attributed to be the cause of death). Additionally, parasites most likely would only have a detrimental effect to the individual whale, and not result in population-wide effects. Thus, based on the available data, the threat of parasites to CI belugas currently appears to be of low relative concern.
Although HABs have the potential to detrimentally impact a large portion of the population, the reported incidence of HABs in Cook Inlet, and Alaska in general, has been very low (RaLonde 2001; Alaska Sea Grant 2012). However, LeFefebvre et al (2016) reported evidence that HAB toxins (e.g., domoic acid and saxitoxin) are present throughout Alaska waters at levels high enough to be detected in marine mammals. The authors concluded that current climate trends may result in conditions favorable to the growth of HABs, increasing the health risks to northern marine mammals. Burek-Huntington et al. (2015) found that domoic acid was present in very low levels in two of 17 CI belugas tested, and saxitoxin was present at just above detection level in one out of 15 CI belugas tested. In the one case where HABs were detected in a fetus, they were not detected in the mother, (Burek-Huntington 2015), suggesting that there may be a greater threat to CI beluga calves than adults. In addition to potential increases in the prevalence of HABs in Alaska, climate change is rapidly altering the global movement of pathogens, bringing diseases to new areas. Guimarães et al. (2007) modeled the dynamics of an infectious disease spreading through a reproductively isolated group of killer whales in the Pacific Northwest. That study’s results indicated that small populations, such as the CI beluga population, are susceptible to population-wide disease outbreaks.
Currently, the incidence of disease as a factor in the deaths of CI belugas appears to be low, and there is little evidence to suggest diseases of concern are present in other mammals in the area. We assume some unknown level of disease is present in CI belugas, with a medium to high probability that disease will occur in the future. Moreover, a population-wide outbreak of a novel (new) disease could be catastrophic to the CI beluga population. As such, despite a low relative concern from parasites and a low incidence of disease currently, the threat to CI beluga recovery due to increases in HABs or a disease outbreak associated with novel pathogens in the future is of medium relative concern, and the overall relative concern for the impact of disease agents is medium.