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CANINE DISTEMPER VIRUS
IDENTIFIED AS CAUSE OF CASPIAN SEAL EPIZOOTIC
Thousands of Caspian seals (Phoca caspica)
have died in the Caspian Sea since April 2000. An international team of
scientists, working as part of the Caspian Environment Programme's Ecotoxicology
Project (ECOTOX), has now concluded that canine distemper virus (CDV) infection
was the primary cause of the epizootic. The results of these investigations have
been subjected to peer-review and details will be published in a forthcoming
issue of the scientific journal, Emerging Infectious Diseases.
Numerous possible contributors to seal mortality
have been investigated in the past, including pollution from land-based sources,
climatic effects such as absence of ice in the North Caspian Sea, parasitic
worms, and other disease vectors. In 1997, a single Caspian seal was determined
to have canine distemper virus infection, but seal disease could not be
attributed directly to this virus at that time. The present study was designed
to examine recent massive mortality in Caspian seals with these various causes
in mind.
Scientists from the Institute of Zoology, Regents
Park, London; the Seal Rehabilitation and Research Center, Pieterburen, The
Netherlands; the Sea Mammal Research Unit, University of St. Andrews, Scotland;
and the Tara Seal Research Centre, Portaferry Northern Ireland, visited areas of
seal mortality in Kazakhstan and Azerbaijan during May and June 2000 to carry
out post-mortem examinations and collect tissue and blood samples from dead
seals. They worked in collaboration with staff from the Geological Institute of
the Azerbaijan Republic Academy of Sciences, Baku, Azerbaijan, the Laboratory of
Virus Ecology, Institute of Microbiology and Virology, Almaty, Kazakhstan and
Akademgorodok, Institute of Zoology, Almaty, Kazakhstan. Blood and tissue
samples, collected from dead seals found on the Caspian coasts of Kazakhstan,
Azerbaijan and Turkmenistan, were then analysed in the Veterinary Sciences
Division of the Department of Agriculture and Rural Development, Belfast,
Northern Ireland, the Institute of Animal Health, Pirbright, England and the
Institute of Virology, Erasmus Medical Center, Rotterdam, The Netherlands. Algal
toxin analysis was performed at Dundee University, Scotland; bacteriology was
performed at the Scottish Agricultural College Veterinary Laboratory, Inverness,
Scotland;
Microscopic lesions, characteristic of canine
distemper, were found in the seal tissues and infection with CDV was confirmed
by serological and molecular methods. These findings in seals from several
geographically dispersed regions of the Caspian Sea provide strong evidence that
CDV infection was the primary cause of the epizootic. Other environmental
factors may have contributed to the deaths, including pollution from land-based
sources, warm winter temperatures which inhibited ice formation, and so on.
However, the CDV appears to be the primary factor causing the majority of deaths
in the epizootic.
Canine distemper virus is a member of the
morbillivirus group of viruses. In 1987-1988 it caused high mortality in Baikal
seals (Phoca siberica) in Lake Baikal in Siberia and is suspected of
having caused a die-off of crabeater seals in Antarctica in 1955. Other
morbilliviruses have caused several major epizootics among aquatic mammal
populations in various regions of the world in recent years.
Further studies are underway to determine the
potential role of pollution in the recent epizootic, which is continuing in some
areas of the Caspian Sea. Our work has shown that one hypothesized cause for the
deaths, harmful (toxic) algal blooms, definitely did not play a role in the
recent deaths.
This investigation was supported by the World Bank
through a donation by the Japanese Consultant Trust Fund, and by the Offshore
Kazakhstan International Operating Company. Details of the results may be
obtained from the Emerging Infectious Diseases website at www.cdc.gov/ncidod.
For further information, please contact:
Dr. Susan Wilson, Tara Seal Research Centre, 7
Millin Bay Road, Portaferry, Northern Ireland BT22 1QD. E-mail: suewilson@marinelife.demon.co.uk.
FAX: +44 (0)28 42728600
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