Diseases in New Zealand Sea Mammals

Diseases in New Zealand This article provides a comprehensive sea mammals review of diseases in New Zealand sea Introduction mammals. New Zealand’s marine fauna has many unique features associated with the geographical location and isolation of the country. The Padraig J. Duignan wide latitudinal range of New Zealand territories, ranging from the sub-tropical Kermadec Islands to the Ross Dependency in Antarctica, presents a wide range of habitats suitable for marine species, both main New Zealand islands but which now has breeding resident and those that migrate seasonally between polar waters concentrations only at the Auckland Islands and Campbell Island and the tropics. Little is known about the off-shore movements of during the winter months. In the North, the Hauraki Gulf and Bay New Zealand’s sea mammals, but they are potentially at greater risk of Plenty are home to Bryde’s whales (Balaenoptera edeni) which from transmission of infectious agents than are our terrestrial fauna. occasionally become stranded in the Firth of Thames. Of most Thus, it is prudent to regard our species as part of the greater Pacific significance among the New Zealand cetacean fauna is Hector’s Basin marine province. dolphin (Cephalorhynchus hectori). It is one of the smallest and Based on stranding and sighting records since the later nineteenth rarest oceanic dolphins in the world and is an endemic species with century, New Zealand has a diverse complement of sea mammals. populations off the east and west coasts of the South Island and a These include 38 members of the order cetacea (whales, dolphins relict population between Taranaki and the Manukau Harbour in and porpoises) and seven members of the class pinnipedia (otariidae the North Island(1). Because of its coastal distribution it is vulnerable or eared seals and phocidae or true seals). to human activities such as set-net fishing, and the Banks Peninsula marine mammal sanctuary was established to protect Hector’s The cetaceans are broadly divided into those with baleen dolphins from this activity. (mystacoceti) and those with teeth (odontoceti). Among the former are the great baleen whales some of which are of considerable Three otariid species frequent our coasts. Most common is the New importance in New Zealand waters. These include the southern right Zealand fur seal (Arctocephalus forsteri). Because there are rookeries whale (Eubalaena mysticetus), which used to be common around the on or close to the mainland, weaners frequently come ashore in page Surveillance 27(3) 2000 9 inappropriate locations and suffer the misguided attention of people only on viral and bacterial infections. Compared with North or their dogs. The New Zealand (or Hooker’s) sea lion (Phocarctos America and Europe, relatively little research has been conducted on hookeri), which is our only endemic species, is encountered less health and disease among New Zealand’s sea mammals. A frequently on the main islands. It is classified as threatened because consequent serious lack of baseline data on native species became of its small population size and limited breeding sites on the apparent during an investigation of sea lion mass mortality in 1998. subantarctic islands(2). Occasional pups are born on the Otago coast The prevalence of potentially pathogenic bacteria, viruses, protozoa, and, out of their breeding season, sea lions are sighted quite and metazoan parasites in these species is unknown, and laboratory commonly around the southern parts of the South Island. The third findings were therefore hard to interpret. Complicating this lack of otariid to frequent our coasts is the sub-antarctic fur seal published data is a paucity of tissue and serum archives which could (A tropicalis). It does not breed in New Zealand, but strandings and be used to determine the historical prevalence of putative pathogens sightings are not uncommon. in sea mammals. Among the true seals (phocidae) stranded or sighted around our Viruses coasts are the leopard seal (Hydrurga leptonyx), the southern Avian influenza: The potential of viral infections to regulate marine elephant seal (Mirounga leonina) and, rarely, Antarctic species such mammal populations was first recognised with the discovery that as the crabeater seal (Lobodon carcinophagus) and Weddell seal fatal avian influenza infections could be transmitted from birds to (Leptonychotes weddelli). both seals and cetaceans in the eastern North Atlantic(9),(10). Although Natural mortality no illness has been attributed to influenza A among marine mammals of the Pacific, a strain of this virus, designated A/whale/ Some understanding of the natural history of a species of interest is PO/19/76 (H1N3), was isolated from a minke whale (Balaenoptera needed when investigating causes of disease or deaths in marine acutorostrata) caught in the South Pacific during the 1975/76 mammals, and deciding what is an acceptable level of mortality is whaling season(11). A close antigenic, genetic and biological the first step. For example, as with most mammals with a high relationship was demonstrated between isolates of influenza A from reproductive rate, mortality of fur seals can exceed 50% in the first a tern and the whale(12). Close associations have also been year of life. That contrasts with the southern right whale in which a demonstrated between isolates from ducks and seals in the Atlantic, calf is produced only every third or fourth year. Thus, very few dead leading to the conclusion that introduction of avian viruses into sea calves could constitute an epidemic. mammals has occurred on several independent occasions and that For most marine mammals, we have too little understanding of these introductions were recent(12). Thus, although no investigation normal population dynamics, so it is hard to decide when unusual of influenza infection has occurred among New Zealand’s marine numbers of deaths have occurred. For some, in which population mammals, the potential for transmission exists. studies have been conducted, elevated mortality rates can be more Morbillivirus: Perhaps of greater significance for marine mammal easily signalled and decisions as to whether or not to investigate population dynamics are the distemper viruses (family made. Thus, higher than normal rates of stranding, emaciation, and Paramyxoviridae, genus Morbillivirus). Recent reviews cover the death were recognised in New Zealand fur seals during the recent El extensive literature on this topic(6),(7),(13). In 1987, a field-strain of Niño/Southern Oscillation (ENSO) excursions. These events usually canine distemper virus (CDV) killed thousands of Baikal seals cause decreased availability of prey, but occasionally they result in (Phoca sibirica) in Siberia and may also have killed thousands of blooms of toxin-producing algae that can cause paralysis. Deaths in Caspian seals (Phoca caspica) in 1997(14),(15). Since 1988, three new fur seals on the Kaikoura coast during the summer of 1998 coincided members of the genus morbillivirus have been described: phocine with blooms of Gymnodinium mikimotoi (3). However, these deaths distemper virus (PDV) first isolated from harbour seals; porpoise were not investigated. Recently, an algal toxin, domoic acid, was morbillivirus from harbour porpoises (Phocoena phocoena); and implicated in the deaths of California sea lions, Zalophus dolphin morbillivirus from striped dolphins (Stenella coeruleoalba). californianus (4), suggesting that similar mortalities may occur in Each of these isolates were from animals that died during epidemics this country. in Europe that killed thousands of animals. The porpoise and Mass deaths in marine mammals appear to be occurring with dolphin viruses are genetically similar and may represent strains of a increased frequency worldwide. Various aetiologies have been host-adapted cetacean morbillivirus (CeMV). Reverse transcriptase implicated, including bacteria(5), viruses (6),(7), and biotoxins (4),(8). The PCR for morbillivirus P and N genes also suggest that the first such event in New Zealand occurred among New Zealand sea morbillivirus of long-finned pilot whales (Globicephala melas) is lions on the Auckland Islands in 1998, and some of the findings from another variant of CeMV(16). Based on the high sero-prevalence of the ensuing investigation are included in this review. specific antibody in both long-finned and short-finned (G macrorhynchus) pilot whales in the western North Atlantic, it was Infectious diseases hypothesised that these species may be the reservoir and vectors for This review and synthesis of unpublished observations will focus CeMV(17) (18). Similar data from the eastern North Atlantic support page 10 Surveillance 27(3) 2000 this hypothesis(19). A clear parallel exists between this situation and Calicivirus: Since 1972, 20 calicivirus serotypes (family that of PDV among phocid seals of the North Atlantic. For that Caliciviridae) have been isolated from at least five pinniped and one virus, serological evidence suggests that harp seals (Phoca cetacean species(34). All of these isolates were made from species groenlandica) are the most probable reservoir host(20),(21),(22). found on the Pacific coast of North America and in the western Arctic(35). Based on serological surveys, several other whale species Serological and pathological surveys of marine mammals in the in the North Pacific appear to have been infected by these Canadian Arctic and along the Atlantic seaboard of North America serotypes(35),(36). This pattern of infection

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