Infection by Leptospira spp., Brucella spp., phocine distemper virus (PDV), caliciviruses, and influenza A virus have all been documented in free-ranging pinniped populations, with some implicated in significant morbidity or mortality.1-5,7-9,11,13,15 This study investigated the infectious disease exposure status of adult free-ranging Pacific walrus (Odobenus rosmarus divergens) in Alaska. Serology was performed for the five main serovars of Leptospira interrogans (Pomona, Hardjo, Icterohaemorrhagiae/Copenhageni, Grippotyphosa, Canicola), Brucella spp., PDV, influenza A virus, and 26 strains of calicivirus (San Miguel sea lion virus [SMSV]) and vesicular exanthema of swine virus [VES]). Preliminary data have previously been reported.12
Blood samples were collected from 20 male and 20 female walrus at St. Lawrence and Round Islands, Alaska, during May 1994, 1995, and 1996 and October 1995 and 1996. Walrus ages ranged from 6–31 years, based on visual counts of lower canine cementum growth layer groups (J. Garlich-Miller, personal communication).6
Leptospira serology was performed by microscopic agglutination and Brucella serology was performed by tube or card agglutination. Serology for PDV and calicivirus (SMSV strains 1, 2, 4–13 and VES strains A48, B51, C52, D53, E54, F55, G55, H54, I55, J56, K54, 1934B, Tillamook, and Walrus) 10 were performed by virus neutralization in tissue culture. Sera from 38 walrus were tested for antibodies to influenza A virus by agar gel immunodiffusion (AGID). Subtype specificity of positive sera were determined by hemagglutination-inhibition (HI) tests (hemagglutinin subtypes H1–H14) and neuraminidase-inhibition (NI) tests (neuraminidase subtypes N1–N9).
All walrus were negative to four Leptospira interrogans serovars (Pomona, Hardjo, Icterohaemorrhagiae/Copenhageni, Canicola) but three males sampled on Round Island had low titers (1:100, 1:100, 1:200) to L. interrogans serovar Grippotyphosa. All walrus were negative for Brucella. Two walrus (females sampled at St. Lawrence Island) had low PDV titers (1:22, 1:45) which were interpreted as nonspecific, and 25 were negative at a dilution of 1:20. Samples from the remaining 13 walrus sera were toxic at low dilutions but were negative at the lowest dilution (1:20 to 1:80) at which the test could be performed.
Eight walrus (four females and two males from St. Lawrence Island and two males from Round Island) were AGID positive for Influenza A. Antibodies were specific for hemagglutinin subtype H10, with titers ranging from 1:8 to ≥1:32. In addition, two sera had antibodies to neuraminidase subtypes N7 (H10N7); two were positive for subtypes N2, N3, and N7; one was positive for subtypes N2 and N7; one was positive for subtypes N2, N3, N5, N6 and N7; and two were negative for specific neuraminidase antibodies. These results are consistent with exposure of this population to multiple serotypes of influenza A virus.
Seven walrus were positive for one or more strains of calicivirus. Two females sampled at St. Lawrence Island had a titer to VES G55 (1:60, ≥1:230); one female sampled at St. Lawrence Island had a titer to VES F55 (1:140); one female sampled at St. Lawrence Island had a titer to VES E54 (1:140); one female sampled at St. Lawrence Island had a titer of 1:57 to both SMSV 12 and VES 1934B; and two males (one each from St. Lawrence and Round Islands) had titers to SMSV 12 (1:180, ≥1:230). These results are consistent with a population which has a low level of exposure to several marine caliciviruses.
This study extends the assessment of the Pacific walrus population’s infectious disease exposure status. Although L. interrogans serovar Pomona infection is common in Northern fur seals (Callorhinus ursinus)13 and California sea lions (Zalophus californianus),15 the walrus were infrequently seropositive to Leptospira spp. and were seronegative to the known pinniped pathogen L. interrogans serovar Pomona. Three walrus were seropositive to L. interrogans serovar Grippotyphosa, a serovar which has also recently been documented associated with renal disease in Pacific harbor seals and in an asymptomatic California sea lion and elephant seals in a rehabilitation center.14 Brucella spp. can cause abortion and infertility in other species, pinniped PDV epizootics have occurred,4,8,11 and both are emerging diseases in pinniped populations. Atlantic walrus (Odobenus rosmarus rosmarus) seropositive to both Brucella spp.9 and PDV5 have been documented but few, if any, Pacific walrus in this study were seropositive for these pathogens. A number of walrus demonstrated serologic evidence of influenza A infection, a disease which has caused mass mortalities in Atlantic harbor seals (Phoca vitulina).7 Serologic evidence of influenza A infection has also recently been documented in one ringed seal (Phoca hispida), but not numerous other pinniped species including walrus, from Alaska.3 This study demonstrated exposure to caliciviruses in the population, consistent with previous studies in the Pacific walrus, California sea lion, Steller sea lion (Eumetopias jubatus), Northern fur seal, and various phocids from the eastern Pacific and Bering Sea.1,2
Populations without previous exposure to infectious agents are more susceptible to significant health consequences when first exposed. The results of our study continue to suggest that the Pacific walrus population has had limited exposure to many of these potentially pathogenic bacteria and viruses. As a naive population they could be adversely affected if these agents were introduced.
A number of Fish and Wildlife Service employees, their designates, and native Alaskan hunters are gratefully acknowledged for assistance with sample collection. The authors thank John Sykes for assistance with data entry.
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