Abstract

The acute death of three harbor seals, Phoca vitulina, and one Hawaiian monk seal, Monachus schauinslandi, between August 20 and September 9, 2003, prompted a serosurvey for titers to West Nile Virus (WNV) in the mammal and avian population at SeaWorld San Antonio(SWSA), San Antonio, Texas. During this time period, one other harbor seal showed similar clinical signs as the seals that died but recovery was uneventful. Ninety-six mammals and twenty-three birds were tested. Samples were sent to the Texas Veterinary Medical Diagnostic Laboratory (TVMDL) and/or to Cornell University for titers.

WNV is a member of the family Flaviviridae and genus Flavivirus Japanese Encephalitis Antigentic Complex. This complex includes the St. Louis encephalitis virus.¹ WNV was first reported in the United States in 1999 when 62 human cases and seven deaths were reported in the New York City area.² It has now spread across the U.S. and into Canada and Latin America. As of October 29, 2003, Texas had 470 human cases reported to the Center for Disease Control (CDC) with 17 deaths.³ Between July and September 2003, Bexar county in Texas had 13 blue jays, 2 Western scrub jays, 1 short-eared owl, and 2 horses die of WNV.⁴ Only one other case was reported earlier in 2003 and that was a blue jay reported in May.

On August 20, 2003, a harbor seal was found dead in the exhibit at SWSA. Two hours later, another harbor seal displayed clinical signs of weakness, head tremors, and was unresponsive to external stimuli. He died 15 minutes later. Another harbor seal was discovered on the morning of August 22 with similar signs of weakness, head tremors, and shivering. He was immediately given emergency care to include steroids, fluids, antibiotics, and atropine. The next day, August 23, a Hawaiian monk seal presented with clinical signs of depression, anorexia, and had a brown, syrupy diarrhea. The animal died during the physical exam. A fourth harbor seal was presented for examination on September 4, 2003 with anorexia and tremors. This animal died on September 9 after treatment with antibiotics and steroids. Gross pathology showed all animals to be in good body condition. The lungs were similar in that they were dark reddish-brown in color with a meaty consistency. The two harbor seals that died in August had a mild pneumonia caused by Pseudomonas aeruginosa. Histopathology was consistent in that all four animals had a multifocal, nonsuppurative encephalitis indicative of a viral infection such as WNV.

The close proximity of the deaths of the harbor seals and monk seal prompted a serosurvey of most of the mammals and a random sampling of avian species residing at SWSA for WNV exposure. Ninety-six mammals and twenty-three birds were bled and serum submitted to the TVMDL and/or to Cornell University for titers and/or polymerase chain reaction (PCR) for WNV. TVMDL tests by Hemagglutination Inhibition (HI). The basic principle of an HI test is the inhibition of hemagglutination induced by viruses which are capable of binding to and agglutinating red blood cells (RBCs) of a particular host, usually different from that of the natural host. If antibodies are present, RBCs are not agglutinated because the antibodies inhibited binding to the RBCs. If antibodies are absent, RBCs are agglutinated by the virus. Cornell tests by Serum Neutralization (SN), where the basic principle is the inhibition (i.e., neutralization) of viral replication or of toxin action by specific antibodies. If antibodies are present, virus does not replicate and damage cells in the culture, or toxin does not damage the cells. If antibodies are absent, virus replicates and produces cell damage, or toxin causes cell damage.

The species tested and the results are listed below:

a. Includes 1 African grey, 2 red front macaws, 2 galahs, 1 North American ruddy duck, 4 Attwater Prairie chickens, 5 flamingos, 2 bar-headed geese, 2 golden conures, and 1 spoonbill.
b. Includes 1 red front macaw, 3 Attwater prairie chickens, 1 flamingo, 2 bar-headed geese, and 3 lorikeets.
c. Includes 3 lorikeets, 2 bar-headed geese, and 1 spoonbill.

Serum from 25 animals that had a positive titer for WNV through TVMDL was also sent to Cornell to correlate the results. Those are as follows:

 TVMDL with a titer of 1:40 or greater--12 positive and Cornell 9 (75%) positive.

 TVMDL with a titer of 1:20--13 positive and Cornell 2 (15%) positive.

There were no cetaceans with titers to WNV identified by the Cornell laboratory although there were 12 with titers identified by TVMDL.

It would appear that there was a high risk of exposure to WNV in late July through August at SWSA as evidenced by titers to the virus during that time period. There is a strong correlation between TVMDL and Cornell if the titers are 1:40 or greater through TVMDL. Of the mammals, only phocids showed any adverse reactions to exposure to WNV restricting vaccinations with WNV Vaccine (Fort Dodge Animal Health, Fort Dodge, Iowa 50501) to that group of animals.

All the harbor seals and Hawaiian monk seals were vaccinated with one ml I.M. twice at a 4 week interval regardless of titer results. No reaction to the vaccine was observed.

Acknowledgements

The authors are grateful to the Animal Care, Aviculture, and Animal Training staffs for all the time and effort put forth to collect blood from our animal collection for the survey. We also want to thank the Laboratory staff for preparation, receipt, and documentation of the samples. Lastly, we want to thank CDC and Cornell Diagnostic Laboratory for support of the survey.

References

1.  www.cdc.gov/ncidod/dvbid/westnile/virus.htm

2.  www.cdc.gov/ncidod/dvbid/westnile/surv&controlCaseCount03.htm