Abstract

Since the 1970's sporadic epizootics of leptospirosis due to Leptospiral interrogans var pomona have been recognized in California sea lions (Zalophus californianus). Between 1991 and 2004, there have been 7 episodes whereby the number of sea lions with leptospirosis accounted for more than 25% of the annual submissions to the Marine Mammal Center, Sausalito, CA. In those years with more intense outbreaks in California, there are historic and anecdotal reports of adult and subadult male sea lions succumbing to leptospirosis during northern migration from breeding and bachelor rockeries in California, along the western seaboard of the United States and British Columbia.

Between January and June, 2004, there were 8 sea lion mortalities attributed to leptospirosis in California, with a dramatic increase to 197 confirmed cases between July and August, followed by a progressive decline from October through to December (Table 1). In California, 23 females were presented for rehabilitation and the remaining 283 animals were subadult and adult males. From 5 August to 30 September, 2004, there were 29 documented strandings along the Oregon coast consisting of 3 stellar sea lions (Eumetopias jubatus), 3 unknown species, and 21 California sea lions (Z californianus). Between 10 August and 22 December, 2004, 58 California sea lions were identified along the Washington State coast (Table 2). In British Columbia, the first California sea lion mortality was detected 5 September, 2004 along the mid coastal region of western Vancouver Island (Ucluelet and Tofino) and a total of 1 unspecified and 13 moribund and dead California sea lions were observed through to 15 November, 2004. The time lag between initial identification of the epizootic in California and detection of animals in Washington State and British Columbia is attributed to migration and eventual decompensation of infected animals. It is important to note that the number of detected or reported animals is likely an under representation of the overall incidence of disease due to sparsely human populated regions, animals lost to sea, inaccessibility of stranding sites and other factors.

Sea lions stranded in Washington State and British Columbia were typically found near fresh water estuaries, rivulets or creeks. This phenomenon is attributed to dehydration, secondary to renal failure. In addition, animals were often emaciated and occasionally icteric. For logistical reasons and financial constraints, not all animals that stranded were necropsied and presumptive diagnoses were based on gross findings or select confirmatory diagnostic tests. In Washington State and British Columbia, 50% of dead animals (n=31) were positive for leptospirosis by serology, histopathology or polymerase chain reaction (PCR)(Table 2). PCR with L interrogans var pomona specific primers were positive and consistent for representative sea lion samples recovered from California, Washington State and British Columbia.

Review of in-house case records and laboratory results confirm previous detection of Leptospira interrogans var pomona in a small number of unspecified California sea lions in British Columbia 1984 and possibly, in 1992. Based on PCR results, exposure most likely occurred in California, with clinical disease and death associated with migration. This condition is not only of concern to California sea lions, but also to other susceptible marine mammal species, terrestrial wildlife, companion animals, livestock and humans. At present, there is little known about the epidemiology and pathogenesis of leptospirosis in sea lions and the risk to other susceptible species posed by northern migrations. In addition to leptospirosis, 3 animals were also culture positive for Salmonella spp and multiple individuals had significantly elevated liver mercury (280-300 ppm) and selenium values as well as elevated blood urea nitrogen (BUN) in ocular fluid (180-250 ppm). Enhanced surveillance for moribund or dead sea lions and continued communication of emerging disease concerns between respective stranding networks, public health officials, veterinarians and biologists should improve recovery of stranded animals and further resolve the epidemiology of this condition.

Table 1. Sea lion strandings for 2004.

Those animals listed in California were confirmed Leptospira positive; whereas, in Oregon, Washington State and British Columbia, diagnostic confirmation occurred only in select cases and involved other otariid species. In the Pacific Northwest, the leptospirosis status of indigenous sea lions between January, 2004 and December, 2004 is unknown.

Table 2. Total number of sea lions in Washington and British Columbia (Ucluelet, Vancouver Island) reported and numbers sampled and tested for Leptospirosis from August to December 22, 2004 (E.j.= Eumetopias jubatus, Zc= Zalophus californianus). Fifteen of 31 tested animals were positive for Leptospira interrogans var pomona by polymerase chain reaction, histopathology or serology.