Human health also may be threatened
The deaths of 107 bottlenose dolphins and thousands of fish off the Florida Panhandle earlier this year brought attention to the growing number of emerging diseases in marine mammals. Brevetoxin, a neurotoxin produced by a dinoflagellate associated with red tides, was implicated in the deaths, but a host of other diseases have emerged in marine mammals during the past several years. These include distemper, brucellosis, toxoplasmosis, sarcocystosis, and papillomavirus and West Nile virus infections.
Dr. Tom Reidarson, the director of veterinary services at Sea World in San Diego, gave a presentation on these emerging diseases at the AVMA Annual Convention. He said that the common thread linking their emergence was runoff from urban areas. Runoff introduces or contributes to the growth of disease-causing organisms, and it may inhibit the ability of marine mammals to fight off disease. Human health is also threatened by runoff, because people gather food and participate in recreational activities in the same coastal waters in which many affected species live.
"Is living in an urban center hazardous to them and us?—I would say yes," Dr. Reidarson said.
Dr. Reidarson also said that, although some diseases are not novel for a specific species, improved diagnostic techniques have increased the likelihood that a definitive diagnosis will be made.
"Most of these organisms have been around for a long time, but we're getting better at diagnosing them," he said.
Papillomavirus infections in wild and captive manatees are a good example. In the past several months, researchers have sequenced the genome of the papillomavirus that infects manatees. They discovered it is a primitive virus that has probably latently infected manatees for a long time. However, a number of environmental factors are contributing to greater susceptibility to developing clinical disease.
In other instances, human activities have been directly implicated in the emergence of disease in marine mammals. Runoff containing fertilizer chemicals that reaches the ocean is believed to contribute to algal blooms known as red tides. These algal blooms produce high concentrations of neurotoxins that kill otherwise healthy marine mammals and other animals. These toxins bioaccumulate in fish and shellfish and, as a result, have a greater impact on species such as dolphins, otters, and sea lions, which are higher in the food chain. The toxins can also contaminate aquatic plants, thus making manatees susceptible to toxicosis.
Dr. Reidarson said that outbreaks of distemper have also caused the deaths of many marine mammals in recent years.
A record number of southern sea otter strandings in California in 2004 have been attributed to Sarcocystis neurona and Toxoplasma gondii infections. Dr. Reidarson said many of the otters that have been necropsied have also had signs of exposure to domoic acid, a neurotoxin produced by algal blooms common off the California coast.
Researchers believe that feces of opossums and domestic cats in fresh water runoff contaminate coastal waters and bioaccumulate in the bivalves that otters eat. This could have serious implications for human health, because it may be risky for humans to eat raw shellfish collected from those areas. Recently, drinking water in Vancouver, Canada, contaminated with T gondii oocysts caused several infections in humans.
Exposure to chemicals and other contaminants in runoff may compromise the immune systems of marine mammals and make them more susceptible to disease. Dr. Reidarson said that, although runoff is diluted once it enters the ocean, it can still harm marine mammals, other animals, and humans. The public needs to be more proactive about reducing runoff.
"The solution to pollution is not dilution," he said. "I think we have to realize we have a responsibility."
Dr. Reidarson also emphasized the importance of veterinarians who work in the marine mammal field and study these diseases.
"I think our role as veterinarians working with marine mammals is important," Dr. Reidarson said.