This paper demonstrates the broad applicability of using pan viral microarray-based diagnostics to determine potential pathogens that were not initially considered in the deaths of marine mammals this paper demonstrates this by the discovery of West Nile virus (WNV) as the cause of death in a killer whale.
In 2007 a 14-year-old male killer whale at the Marine Park in San Antonio died without notable premonitory signs. After conventional diagnostic assays were performed the final diagnosis was septicaemia secondary to a primary viral infection that caused swelling of the brain. To determine the organism responsible, DNA micro-arrays with highly conserved sequences from > 1000 viruses were selected to screen for known and novel viruses. The results from these assays demonstrated a significant homology with viruses of the family Flaviviridae; in particular West Nile virus. Reverse transcription PCR primers targeting WNV were then used to confirm the microarray results which yielded a sequence with a 99% identity and hundred percent acid identity to the WNV strain OK 03. This diagnosis of WNV was further supported by performing an immunohistochemical staining on brain tissue which demonstrated abundant WNV antigen. These findings broaden the known host tropism of WNV to include cetaceans in addition to the previously known pinnipeds.
The authors also evaluated WNV exposure within the same cohort as well as a geographically distant cohort of whales by using serologic testing. They found the serum from the affected whale and its five cohort killer whales from the San Antonio Park which have regular contact with each other gave positive results. Five other whales were tested in Orlando which had no contact to the whales in San Antonio and were found to be negative for West Nile.
The authors of this paper suggest that health evaluations of free ranging and captive cetaceans should include WNV serology to assess exposure rates. Although this report focuses on killer whales' "loafing" behaviour (which would allow a mosquito bite) it is also seen in many coastal dolphins thus exposing them to possible WNV infection. Potential viral shedding can occur in many ways and, until we know the implications of this infection in Marine animals, WNV should be considered as a cause of death in marine mammals.
Review of : St. Leger J, Wu G, Anderson M, Dalton L, Nilson E, Wang D.; 2011; West Nile virus infection in killer whale, Texas, USA, 2007; Infect Dis; Accessed DEC/2011; http://dx.doi.org/10.3201/eid1708.101979
4 comments:
Interesting paper .... I think West Nile Virus is exclusively mosquito borne. It causes infection mainly in birds, but can cause human infection. I'm perplexed to see how a mosquito could bite a whale! How would it penetrate the blubber to reach a blood vessel? Do the authors say anything about this?
Hi Colin,
Yes, I believe that West Nile Virus is exclusively mosquito borne. The paper describes how because the whale spent periods of time stationary at the surface of the water (loafing) this will give the mosquitoes opportunity to fed upon the whales.It doesn't describe penetration into the blubber but I guess that it must have a blood supply for the skin surrounding the blubber. As whales are so big and spend a long periods of time loafing this will allow mosquitoes to feed for longer and more frequently increasing chances of virus transfer.
Previously it wasn't thought that cetacea could be infected with West Nile Virus, The authors speculate that this may also be applicable to coastal bottle nose dolphins, however as little is known about transfer in these animals, screening of wild populations would need to be carried out to understand the level of threat.
Hi Theo, a bit of Googling reveals that blubber is highly vascularized; there must be capillaries near the surface, but the skin of a whale must surely be much thicker than other mammals. I found another example of a captive Orca succumbing to a different mosquito-borne virus .... but I'm even more pussled by the fact that West Nile virus has been decribed in frogs, turtles, alligators and crocodiles. How would a mosquito penetrate the skin of a croc? The only place I can think of is the eyelid or nostrils. Maybe the blowhole is is a more likely spot for whales to be bitten - the epidermis must be much thinner there. This could be a QI question!
Also due to the natural geographic distribution of killer whale populations(in cold oceans far away from land) they would not usually be exposed to mosquito transmission to flaviviruses, therefore killer whales are a naive population and so may be more susceptible to WMV infection
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