Evolution and ecology of influenza A viruses.

If stretched end to end, the estimated 1030viruses in the oceans would span farther than the nearest 60 galaxies. This reservoir of genetic and biological diversity continues to yield exciting discoveries and, in this Review, Curtis A. Suttle highlights the areas that are likely to be of greatest interest in the next few years. Viruses are by far the most abundant 'lifeforms' in the oceans and are the reservoir of most of the genetic diversity in the sea. The estimated 1030 viruses in the ocean, if stretched end to end, would span farther than the nearest 60 galaxies. Every second, approximately 1023 viral infections occur in the ocean. These infections are a major source of mortality, and cause disease in a range of organisms, from shrimp to whales. As a result, viruses influence the composition of marine communities and are a major force behind biogeochemical cycles. Each infection has the potential to introduce new genetic information into an organism or progeny virus, thereby driving the evolution of both host and viral assemblages. Probing this vast reservoir of genetic and biological diversity continues to yield exciting discoveries.

Marine viruses — major players in the global ecosystem

Mass mortalities due to disease outbreaks have recently affected major taxa in the oceans. For closely monitored groups like corals and marine mammals, reports of the frequency of epidemics and the number of new diseases have increased recently. A dramatic global increase in the severity of coral bleaching in 1997—98 is coincident with high El Nino temperatures. Such climate-mediated, physiological stresses may compromise host resistance and increase frequency of opportunistic diseases. Where documented, new diseases typically have emerged through host or range shifts of known pathogens. Both climate and human activities may have also accelerated global transport of species, bringing together pathogens and previously unexposed host populations. T he oceans harbor enormous biodiver- sity by terrestrial terms (1), much of which is still poorly described taxo- nomically. Even less well known are the dy- namics of intermittent, ephemeral, threshold phenomena such as disease outbreaks. De- spite decades of intense study of the biolog- ical agents structuring natural communities, the ecological and evolutionary impact of diseases in the ocean remains unknown, even when these diseases affect economically and ecologically important species. The paucity of baseline and epidemiological information on normal disease levels in the ocean chal- lenges our ability to assess the novelty of a recent spate of disease outbreaks and to de- termine the relative importance of increased pathogen transmission versus decreased host resistance in facilitating the outbreaks. Our objectives here are to review the prevalence of diseases of marine taxa to evaluate wheth- er it can be concluded that there has been a recent increase. We also assess the contribut- ing roles of human activity and global cli- mate, and evaluate the role of the oceans as incubators and conveyors of human disease agents. Is There an Increase in Diseases in the Ocean?

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Emerging Marine Diseases--Climate Links and Anthropogenic Factors

Seasonal changes in plasma glucocorticoid concentrations in free-living vertebrates.

Pandemics are the most dramatic presentation of influenza. Three have occurred in the twentieth century: the 1918 H1N1 pandemic, the 1957 H2N2 pandemic, and the 1968 H3N2 pandemic. The tools of molecular epidemiology have been applied in an attempt to determine the origin of pandemic viruses and to understand what made them such successful pathogens. An excellent example of this avenue of research is the recent phylogenetic analysis of genes of the virus that caused the devastating 1918 pandemic. This analysis has been used to identify evolutionarily related influenza virus genes as a clue to the source of the pandemic of 1918. Molecular methods have been used to investigate the avian H5N1 and H9N2 influenza viruses that recently infected humans in Hong Kong. Antigenic, genetic, and epidemiologic analyses have also furthered our understanding of interpandemic influenza. Although many questions remain, advances of the past two decades have demonstrated that several widely held concepts concerning the global epidemiology of influenza were false.

Global Epidemiology of Influenza: Past and Present

More than 400 harbor seals, most of them immature, died along the New England coast between December 1979 and October 1980 of acute pneumonia associated with influenza virus, A/Seal/Mass/1/180 (H7N7). The virus has avian characteristics, replicates principally in mammals, and causes mild respiratory disease in experimentally infected seals. Concurrent infection with a previously undescribed mycoplasma or adverse environmental conditions may have triggered the epizootic. The similarities between this epizootic and other seal mortalities in the past suggest that these events may be linked by common biological and environmental factors.

http://science.sciencemag.org/content/sci/215/4536/1129.full.pdf

Mass mortality of harbor seals: pneumonia associated with influenza A virus

Biological and environmental influences on circulating adrenal and thyroid hormones were investigated in 36 wild and 36 semidomesticated Atlantic bottlenose dolphins, Tursiops truncatus, matched by age, sex, and time of year when the samples were collected. Serum concentrations of thyroxine (free [fT41 and total [tT4]), triiodothyronine (free [fT3], total [tT3], and total reverse [rT37), cortisol, and aldosterone were determined by radio-immunoassay, Wild female dolphins had significantly higher levels of tT4, fT4 and fT3, an effect that was possibly related to reproduction and lactation. Semidomesticated females had higher tT3 than their wild counterparts. fT4 declined with age in wild dolphins, whereas rT3 was greatest in the older animals. Cortisol and aldosterone were both higher in wild animals sampled after a variable interval of up to four hours after encirclement by capture net. The pattern of adrenal hormone release suggested a mild stress response. Levels of both adrenal hormones were low in semidomesticated dolphins conditioned to present voluntarily their tails for blood sampling, an approach that appears to yield specimens representative of resting values for these constituents.

Dolphin thyroid and adrenal hormones: circulating levels in wild and semidomesticated tursiops truncatus, and influence of sex, age, and season

Bovine thyroid stimulating hormone administered to three beluga whales, Delphinapterus leucas, was effective in producing an increase in circulating levels of triiodothyronine and thyroxine. A single dose of 10 I.U. of thyroid stimulating hormone resulted in a 145% increase in triiodothyronine and a 35% increase in thyroxine after nine hours in a whale tested within two hours after capture. The response was less pronounced in an animal tested with the same does on two occasions after four and eight weeks in captivity. In the third whale, 10 I.U. of thyroid stimulating hormone given on each of three consecutive days produced a marked increase in triiodothyronine and thyroxine. The elevation of thyroxine concentration persisted for at least two days after the last injection of thyroid stimulating hormone. A subsequent decrease in thyroxine to levels below baseline signalled the suppression of endogenous thyroid stimulating hormone. This preliminary study helps to establish a protocol for testing thyroid function in cetaceans.

D. J. St. Aubin

Papers

Mass mortality of harbor seals: pneumonia associated with influenza A virus

Dolphin thyroid and adrenal hormones: circulating levels in wild and semidomesticated tursiops truncatus, and influence of sex, age, and season

Stimulation of thyroid hormone secretion by thyrotropin in beluga whales, Delphinapterus leucas.

Tissue levels of ascorbic acid in marine mammals