It sounds almost impossible. How could a tiny mosquito possibly kill one of the top predators in the ocean? According to a new peer-reviewed case report published in the Journal of Marine Mammals and Their Ecology, it’s because they lived in captivity.
Orca Deaths Linked to Captivity in New Study
“Orca (Orcinus Orca) Captivity and Vulnerability to Mosquito-transmitted Viruses,” co-authored by John Jett and Jeffrey Ventre, queries the role of captivity and husbandry procedures in lowering the immune system of captive orcas. The duo, who are former SeaWorld trainers, directly correlated the death of two SeaWorld killer whales to their environment and disease-carrying mosquitoes.
“Although unreported in wild orca populations,” Jett and Ventre noted, “mosquito-transmitted diseases have killed at least two captive orcas in U.S. theme parks.”
The two male orcas in question were Kanduke, held at SeaWorld of Florida, and Taku – housed at SeaWorld Texas. Kanduke died in 1990 with St. Louis Encephalitis Virus (SLEV) implicated in his death. Male orca Taku, succumbed to West Nile Virus (WNV) in 2007. Mosquitoes are the main vectors for both of these diseases.
UV Lowers Orca’s Ability to Fight Infection
“Unlike their wild counterparts who are rarely stationary,” Jett and Ventre said, “captive orcas typically spend hours each day (mostly at night) floating motionless (logging) during which time biting mosquitoes access their exposed dorsal fins.”
Furthermore they added, “high ultraviolet radiation … poor dentition [and persistent] antibiotic use,” all contributed to lowering an orca’s immune system, leaving him susceptible to mosquito-transmitted diseases.
In wild orca populations, this was not the case. No reports of disease transmission exist in wild killer whales, according to the study, although blood samples taken from bottlenose dolphins (Tursiops truncatus) residing in shallow inland marshes and mangroves in Florida, Charleston and South Carolina, suggested possible exposure to WNV – but no manifestation of the disease.
“To our knowledge,” wrote Jett and Ventre, “no other literature exists describing wild cetacean exposure to WNV or SLEV.” Based on their collective experiences they added, “we conclude that the captive environment represents unique exposure risks to orcas living within WNV and SLEV-prone geographic regions.”
What About Other Cetaceans Held in Captivity at SeaWorld?
Dr. John Jett has B.S. and M.S. degrees in Environmental Science and a Ph.D. in Health and Human Performance with an emphasis on waterway management and marine mammal conservation issues. He is also a visiting Research Professor in Environmental Science and Laboratory Manager for the Biology Department at Stetson University. Dr. Jett told Decoded Science:
It is certainly reasonable to guess that dolphins at SeaWorld have been exposed, though I know of no studies that have confirmed this. Besides Taku, other orcas held in Texas SeaWorld possessed positive titers, while other orcas tested in Florida SeaWorld did not.
According to the Centers for Disease Control, in a limited number of human cases, WNV has “been spread through blood transfusions, organ transplants, breastfeeding and even during pregnancy from mother to baby.” So we asked about the potential for an infected pregnant female orca to transmit the disease to her unborn calf.
Jett told Decoded Science:
Based on Chapa JB et al., Obstet Gynecol. 2003;102:229-31, WNV can be transmitted to a fetus if the mother is in later stages of pregnancy. No literature (that I know of) looks at this phenomenon in cetaceans, much less orcas. That said, as both are mammals, with many similarities in fetal development (e.g., placenta, etc.), I’m guessing the virus could be transferred in utero. This is just a guess.