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Morbilliviruses and morbillivirus diseases of marine mammals.

In recent years, serious disease outbreaks among seals and dolphins were attributed to infection with established or newly recognized morbilliviruses. The first identification of a morbillivirus as causative agent of mass mortality among marine mammals was in 1988, when the previously unrecognized phocine distemper virus (PDV) caused the death of 20,000 harbor seals (Phoca vitulina) in northwestern Europe. A similar epizootic among Baikal seals (Phoca sibirica) in Siberia in 1987 was later attributed to infection with canine distemper virus (CDV). A morbillivirus isolated from stranded harbor porpoises (Phocoena phocoena) between 1988 and 1990 proved to be yet another new member of the genus Morbillivirus, distinct from PDV and CDV and more closely related to rinderpest virus and peste-des-petits-ruminants virus: porpoise morbillivirus. A similar virus, dolphin morbillivirus, was the primary cause of mass mortality among striped dolphins (Stenella coeruleoalba) in the Mediterranean from 1990 to 1992. In this review, current knowledge of the genetic and antigenic relationships of these viruses is presented, and the origin and epizootiological aspects of the newly discovered morbilliviruses are discussed. In addition, the possible contributory role of environmental contaminant-related immunosuppression in the severity and extent of the different disease outbreaks is discussed.

Animals↗

The evolution of morbilliviruses: a comparison of nucleocapsid gene sequences including a porpoise morbillivirus.

Sequence data for the nucleocapsid protein (N) gene of the porpoise morbillivirus including the very conserved middle section of the protein and the hypervariable C terminus are reported. Analysis of dissimilarity indices based on an alignment of the N proteins of various morbilliviruses identifies a variable region of the N protein from amino acids residues 121 to 145 and a hypervariable part from amino acids 400 to 517. This type of analysis can be usefully applied when protein sequences of five or more morbillivirus species are available. Regions of variability between species identified by this index also represent regions of variation within one species e.g. measles virus (MV). Hence, comparative analysis of different morbilliviruses provides an insight into the potentially variable parts of viral proteins. From the great and unexplained nucleotide sequence conservation observed within MV, it would appear that the various morbilliviruses have diverged from each other a very long time ago. However, the data do not yet allow us to estimate the time span of these divergences. The relatedness and the number of different morbillivirus species provides a unique database for study of the evolution of RNA viruses.

Amino Acid Sequence↗

Completion of the sequence of a cetacean morbillivirus and comparative analysis of the complete genome sequences of four morbilliviruses.

The gene encoding the large (L) protein and the genome termini of the dolphin strain of cetacean morbillivirus (CeMV) were sequenced. The CeMV genome is 15702 nucleotides long and has been compared with other available morbillivirus genome sequences in regards to the "rule of six" and the "phase" of any particular nucleotide, defined as its position within a given hexamer, which here is defined as a group of six nucleotides starting from the 3' end of the genomic RNA. With exception of the position of the start of the F gene, the phase of the transcription start sites of each gene is strictly conserved between the morbilliviruses, but each gene is in a different phase. The lengths of gene transcripts differ between viruses by multiples of six nucleotides with exception of the M and F transcripts. The differences between the various morbilliviruses result from deletions or insertions of multiples of six nucleotides in the 3' and 5' UTRs of the different viral genes. The four bases were distributed non-randomly over the six positions in the hexamer boxes. However, the distribution patterns of each of the four bases indicated that multiples of three were more prevalent than those of six nucleotides. This reflected the positions of nucleotides in codons and codon usage in the reading frames. The L protein of CeMV was found to be 2183 amino acids in length and similar to that of MV and RPV. The CeMV L protein sequence was found to be equidistant between those of the CDV/PDV and MV/RPV subgroups of the morbilliviruses. This concurs with the analyses carried out on the other structural proteins.

3' Untranslated Regions↗

Morbillivirus infections, with special emphasis on morbilliviruses of carnivores.

Morbilliviruses infections cause significant mortality in human beings and animals. Measles virus is responsible for up to two million childhood deaths annually in the developing world, while rinderpest and peste des petits ruminants cause severe epizootics in domestic and wild ruminants in areas of the world where they remain endemic. Canine distemper virus (CDV) is a cause of fatal disease in many species of carnivores. Distemper is controlled by vaccination in domestic dogs and farmed mink, but it may be impossible to eradicate the virus because of its global distribution and wide variety of susceptible host species, which includes both freshwater and marine seals. Research is currently under way to develop new recombinant vaccines, since the currently available live attenuated vaccines for CDV are not safe for use for all species and many valuable zoo animals need to be protected from CDV. New morbilliviruses with potentially disastrous ecological consequences for marine mammals have been discovered in the past decade; phocid distemper virus (PDV) in seals and the cetacean morbillivirus (CMV) has been found in dolphins, whales and porpoises. Reverse transcription, coupled with the polymerase chain reaction (RT/PCR) and nucleic acid sequencing, has been used to characterise the morbilliviruses and has given insights into the evolution of this virus genus.

Animals↗

Comparative analysis of the gene encoding the nucleocapsid protein of dolphin morbillivirus reveals its distant evolutionary relationship to measles virus and ruminant morbilliviruses.

A morbillivirus of uncertain origin recently killed hundreds of Mediterranean dolphins. This is the first report of the nucleotide and deduced amino acid sequence of a dolphin morbillivirus (DMV) gene. The sequence of the nucleocapsid (N) gene including boundaries was determined. When the DMV N gene coding region was compared with the corresponding sequences of other morbilliviruses a distant evolutionary relationship between these viruses and DMV was apparent. Phylogenetic analysis of the sequence data provided further evidence that DMV is not closely related to any known morbillivirus, whereas phocine distemper virus exhibits a relatively close relationship to canine distemper virus.

Amino Acid Sequence↗

Nucleotide and deduced amino acid sequences of the matrix (M) and fusion (F) protein genes of cetacean morbilliviruses isolated from a porpoise and a dolphin.

Morbilliviruses have been isolated from stranded dolphins and porpoises. The present paper describes the cloning and sequencing of the porpoise morbillivirus (PMV) F gene and of the dolphin morbillivirus (DMV) M and F genes and their flanking regions. The gene order of the DMV genome appeared to be identical to that of other morbilliviruses. A genomic untranslated region of 837 nucleotides was found between the translated DMV M and F gene regions. The predicted DMV M protein were highly conserved with those of other morbilliviruses. Both the deduced PMV and DMV F0 proteins exhibited three major hydrophobic regions as well as a cysteine rich region, a leucine zipper motif and a cleavage motif allowing cleavage of the F0 protein into F1 and F2 subunits. Apparently the DMV F0 cleavage motif was not modified by adaptation of DMV to Vero cells. The predicted PMV and DMV F proteins were 94% identical. Comparisons with the corresponding sequences of other morbilliviruses demonstrated that the cetacean morbillivirus does not derive from any known morbillivirus but represents an independent morbillivirus lineage.

Amino Acid Sequence↗

Immunohistological and serological investigation of morbillivirus infection in harbour porpoises (Phocoena phocoena) from the German Baltic and North Sea.

The role of morbillivirus infection as a cause of disease or death in harbour porpoises (Phocoena phocoena) from the German North and Baltic Sea was investigated by serology, histology and immunohistochemistry. Blood and tissue samples of lung, brain and lymph nodes from 74 stranded or by-caught harbour porpoises from German waters were collected between 1991 and 1997. According to dentinal growth layers and body length, animals were grouped into four age classes (neonates, 0-1, 1-4, 4-16 years of age). Formalin-fixed, paraffin-embedded sections were stained by hematoxylin and eosin (HE). Immunohistology was done in all lung tissues using the avidin-biotin-peroxidase technique and a polyclonal canine distemper virus (CDV) nucleoprotein-specific antibody, which cross-reacts with porpoise morbillivirus (PMV) antigen. A virus neutralization assay for detection of (Onderstepoort-strain) CDV- and PMV-specific antibodies was performed. Due to the cytotoxicity of some sera, only titres of 1:20 or greater were considered positive. PMV or CDV-specific neutralizing antibody titres were found in 88 and 50% of the animals, respectively. Titres were always highest against PMV indicating infection with a homologous porpoise virus strain. There were no significant differences in neutralizing antibody titres between animals of the different age groups. No histological lesions specific for morbillivirus infection were detected and by immunohistology all cases were negative for morbillivirus antigen. The absence of morbillivirus antigen and the lack of characteristic morbillivirus-specific lesions showed that morbillivirus infection was not a cause of death or illness in the investigated population. However, the high incidence of PMV-specific antibodies in all age groups indicated a continuous spread of infection with a morbillivirus among harbour porpoises from the German Baltic and North Sea.

Age Factors↗

Morbillivirus infections in aquatic mammals: a brief overview.

Since 1987, at least eight morbillivirus infection (MI) epidemics have caused mass mortality of several free-living pinniped and cetacean populations around the world. The responsible agents, all belonging to the genus Morbillivirus (family Paramyxoviridae), have been characterized as either "canine distemper virus" strains, infecting pinnipeds, or as three new morbilliviruses, namely "phocid (phocine) distemper virus" , "porpoise morbillivirus" and "dolphin morbillivirus" . The last two agents are currently gathered under the common denomination of "cetacean morbillivirus". At post-mortem examination, a commonly occurring macroscopic lesion is represented by more or less severe bilateral pneumonia, with consolidation, congestion and oedema of both lungs, which fail to collapse. Histologically, a non-suppurative broncho-interstitial pneumonia, characterized by type II pneumocyte hyperplasia and by formation of endobronchial, endobronchiolar and endoalveolar "Warthin-Finkeldey type" syncytia, as well as a multifocal, non-suppurative encephalitis, associated with a severe and generalized lymphoid tissue depletion, are common pathological findings. Furthermore, eosinophilic viral inclusions are often detected, at both the intracytoplasmic and intranuclear level, within bronchial and bronchiolar epithelial, pulmonary syncytial, neuronal and other cell types. These inclusions, along with lymphoid and other cellular elements, are often found to be immunohistochemically positive for morbillivirus antigen. Among the still debated, or even controversial issues regarding MI in sea mammals, the one related to the origin of their causative agents is of particular concern. Another intriguing issue regards the synergistic effects, if any, associated with chronic exposure to a number of environmental pollutants, such as organochlorines and heavy metals. In fact, it is also unknown whether and how these chemicals contribute towards modulating the pathogenic and pathogenetic activity primarily displayed by sea mammal morbilliviruses.

Animals↗

Prevalence of morbilliviruses among pinniped and cetacean species.

Since 1987, morbillivirus infections have caused serious disease outbreaks with high mortality among aquatic mammals. Baikal seals (Phoca sibirica) in Siberia were involved in an outbreak caused by a virus closely related to canine distemper virus (CDV) in 1987. Harbour seals (Phoca vitulina) and grey seals (Halichoerus grypus) in north-western Europe were struck by a newly recognised morbillivirus of seals (PDV1). A serological survey has indicated that these morbillivirus infections frequently occur among several pinniped species. Besides pinnipeds, the presence of morbillivirus infections among cetaceans, including whales, dolphins and porpoises, has been demonstrated since the outbreak of PDV among seals in north-western Europe. Morbillivirus was isolated from several stranded harbour porpoises (Phocoena phocoena). This virus proved to be very similar to a virus which was isolated during a disease outbreak with high mortality among striped dolphins (Stenella coeruleoalba) in the Mediterranean area. The viruses isolated from these cetacean species were quite different from the viruses isolated from the seals. They proved more related to the ruminant morbilliviruses, peste des petits ruminants virus and rinderpest virus. The potential transmission of the dolphin morbillivirus to the endangered population of the Mediterranean monk seal (Monachus monachus) has been considered. Studies are presently being conducted into the possibility of inducing protection against morbillivirus infection in this species by vaccination with an immune stimulating complex (ISCOM) preparation based on CDV.

Animals↗

Morbillivirus infections in aquatic mammals.

Infections with morbilliviruses have caused heavy losses among different populations of aquatic mammals during the last 5 years. Two different morbilliviruses were isolated from disease outbreaks among seals in Europe and Siberia: phocid distemper virus-1 (PDV-1) and phocid distemper virus-2 (PDV-2) respectively. PDV-1 was characterized as a newly identified morbillivirus, most related to canine distemper virus (CDV), whereas PDV-2 most probably is a strain of CDV. Morbilliviruses were also isolated from porpoises--porpoise morbillivirus (PMV)--and dolphins--dolphin morbillivirus (DMV)--which had stranded on the coasts of Europe. PMV and DMV proved to be closely related to, but distinct from 2 ruminant morbilliviruses, rinderpest virus (RPV) and peste-des-petits-ruminants virus (PPRV). Serological surveys carried out among pinniped and cetacean species in the seas of Europe and North America indicated that infections with these newly discovered morbilliviruses or closely related viruses commonly occur among aquatic mammal species.

Animals↗

Emerging morbillivirus infections of marine mammals: development of two diagnostic approaches.

In the last 13 years, four viruses belonging in the Morbillivirus genus of the Paramyxoviridae family have emerged as significant causes of disease and mortality in marine mammals. The viruses involved are canine distemper virus (CDV) in seals and polar bears, dolphin morbillivirus (DMV) and porpoise morbillivirus (PMV) in cetaceans, and phocine distemper virus (PDV) in pinnipeds. The two cetacean morbilliviruses (DMV and PMV) are now considered to be the same viral species, named cetacean morbillivirus (CMV). All three morbillivirus species (CDV, CMV, and PDV) are genetically and antigenically related and cross-react in various serological tests. The diagnosis of morbilliviral infections in marine mammal specimens poses two challenges. First, various marine mammal species can be infected by more than one closely related but distinct morbilliviruses, making definitive virus identification unattainable by classical virology methods. Second, standard immunological reagents such as anti-species conjugates are unavailable for most marine mammal species, rendering definitive serological diagnosis difficult by classical serological techniques. The objectives of this study were to develop two diagnostic approaches that alleviate these difficulties, providing simple, rapid, and cost-effective diagnostic methods. For nucleic acid detection, reverse transcription-polymerase chain reaction (RT-PCR) and restriction endonuclease digestions were used to differentiate the three viruses. For antibody detection, a monoclonal antibody-based competitive enzyme-linked immunosorbent assay (c-ELISA) was used on sera from several species, thus avoiding the need for multiple anti-species enzyme conjugates.

Animals↗

Comparative analysis of the attachment protein gene (H) of dolphin morbillivirus.

DMV, dolphin morbillivirus, a paramyxovirus of uncertain origin recently emerged in Mediterranean dolphins. This study presents the complete nucleotide sequence of the hemagglutinin (H) gene including the gene boundaries. The single open reading frame of the DMV H gene encodes a protein of 604 residues which exhibits overall sequence characteristics similar to the H genes of other morbilliviruses. When compared to its closest homologues, measles virus (MV) and rinderpest virus (RPV), DMV has, respectively, 44 and 46% of amino acid residues in identical positions. The primary sequence of the DMV H protein is markedly less conserved than that of the fusion protein. The comparative data at the genomic level correspond with cross-neutralization studies with different morbilliviruses. Retrospective serogical studies dating back to 1983 indicate DMV-like infections in whales of the eastern Atlantic. The presented data support and extend previous studies suggesting that this novel morbillivirus is one of the phylogenetically oldest morbilliviruses known to circulate today. The relationship of DMV and established morbilliviruses to the newly emerged candidate morbillivirus infecting horse and man is discussed.

Amino Acid Sequence↗

Round table on morbilliviruses in marine mammals.

Since 1988 morbilliviruses have been increasingly recognized and held responsible for mass mortality amongst harbour seals (Phoca vitulina) and other seal species. Virus isolations and characterization proved that morbilliviruses from seals in Northwest Europe were genetically distinct from other known members of this group including canine distemper virus (CDV), rinderpest virus, peste des petits ruminants virus and measles virus. An epidemic in Baikal seals in 1987 was apparently caused by a morbillivirus closely related to CDV so that two morbilliviruses have now been identified in two geographically distant seal populations, with only the group of isolates from Northwest Europe forming a new member of the genus morbillivirus: phocid distemper virus (PDV). Because of distemper-like disease, the Baikal seal morbillivirus was tentatively named PDV-2 in spite of its possible identity with CDV. The appearance of morbilliviruses in the Mediterranean Sea causing high mortality amongst dolphins should further increase the research activities on protection strategies for endangered species of marine mammals.

Animals↗

A review of the 1988 European seal morbillivirus epizootic.

An epizootic of morbillivirus infection killed thousands of common seals (Phoca vitulina) in European seas in 1988. Most of the affected seals had respiratory signs and the main post mortem finding was acute pneumonia. The histopathological changes were similar to those of canine distemper. Six common porpoises (Phocoena phocoena) found stranded on the coast of Northern Ireland in late 1988 had similar lesions. Morbillivirus infection also killed several thousand Siberian seals (Phoca siberica) in Lake Baikal in 1987 and 1988. A morbillivirus (phocine distemper virus) has been isolated from affected seals in several European countries and studies of the antigenicity of the virus indicate that it has several unique epitopes that distinguish it from the other known morbilliviruses. Biochemical studies of the viral proteins, RNA and nucleotide sequence confirm that it is a new morbillivirus. There is seroepizootiological evidence of morbillivirus infection in Greenland harp seals (Pagophilus groenlandicus), ringed seals (Phoca hispida) and Dutch common seals several years before the 1988 epizootic. Antibodies to a morbillivirus have also been found in bottlenose dolphins (Tursiops truncatus) from the eastern coast of the USA. Further studies are required to determine whether these sea mammal populations have been infected with phocine distemper virus.

Animals↗

A morbillivirus antibody survey of Atlantic walrus, narwhal and beluga in Canada.

A longitudinal serologic survey was conducted for morbillivirus antibodies in Atlantic walruses (Odobenus rosmarus rosmarus), narwhal (Monodon monoceros), and beluga (Delphinapterus leucas) from the Northwest Territories, Nunavut and the St. Lawrence estuary (Canada). Sixty-five of 131 (50%) walruses sampled between 1984 and 1993 had detectable morbillivirus neutralizing antibodies. Positive walrus were identified from four of five Arctic sampling sites, to as far back as 1984. Prevalence of morbillivirus neutralizing antibodies in walruses from Foxe Basin ranged from a high of 76% (n = 21) in 1993 to a low of 22% (n = 28) in 1984. Limitations in sample acquisition may have produced underestimates for the 1984 data. There are no reports of clinical morbillivirus infection in walruses. Our results are consistent with the hypothesis that a morbillivirus similar or identical to phocine distemper virus (PDV) has circulated among walrus populations of the eastern Canadian Arctic, at least since the early 1980s. No narwhal (n = 79) or beluga (n = 445) from Arctic waters were identified as having antibodies to dolphin morbilivirus (DMV) above the threshold serum dilution of log2 4. Also, none of the beach-cast cetacean carcasses (n = 28) from the Gulf of St. Lawrence and the St. Lawrence estuary were positive for antibodies to DMV. This indicates that Gulf of St. Lawrence, St. Lawrence estuary, and Arctic cetaceans either have not been exposed to DMV or an antigenically related morbillivirus, or are not susceptible to infection.

Animals↗

Comparison of the deduced matrix and fusion protein sequences of equine morbillivirus with cognate genes of the Paramyxoviridae.

The nucleotide sequence of the matrix protein of equine morbillivirus (EMV) was determined to be 1062 nucleotides and coded for a deduced protein of M(r) 40148 having a net charge of + 19 at neutral pH. The matrix protein gene was separated from the P and F genes by intercistronic regions of 546 and 469 nucleotides, respectively. The nucleotide sequence which coded for the F protein was 1641 nucleotides and coded for a deduced protein of 546 amino acids having an M(r) of 60,447 and a charge + 4 at neutral pH. Partial sequence information was also determined for the P/V proteins. M, P and F protein sequence comparisons revealed that a greater homology existed between EMV and known members of the morbillivirus genus than with other members of the Paramyxoviridae and that this homology resided within the central half of the protein for the fusion protein, the C-terminal half of the matrix protein and at certain sites with the P protein. Far greater homology was seen between the morbilliviruses and EMV than for the other paramyxoviridae. It was inferred from phylogenetic analyses that EMV was a distantly related member of the morbillivirus genus. A conserved sequence of 18 nucleotides (assumed to be the transcriptional editing site) was present in the P gene of EMV. Insertion of a single nucleotide residue within this site generated the C-terminus of a V-like, cysteine rich protein. Likewise, a conserved 'CTT' intergenic region presumed to be the transcription termination and polyadenylation signal was present in EMV between the P-M-F genes. The close sequence homology of these sites with that of morbilliviruses also inferred that EMV was a member of the morbillivirus genus.

Amino Acid Sequence↗

Nucleic acid hybridization analyses confirm the presence of a hitherto unknown morbillivirus in Mediterranean dolphins.

In 1990 an epidemic caused by a morbillivirus was noticed among Mediterranean dolphins. RNA was extracted from the tissues of dolphins and from cell cultures infected with a corresponding dolphin morbillivirus isolate. By nucleic acid hybridization this RNA was compared to RNA extracted from animal tissue or cell cultures infected with canine distemper virus (CDV), phocine distemper virus (PDV) or measles virus (MV). The presence of morbillivirus RNA in the dolphin tissue was demonstrated. Morbillivirus N, P, M and F gene mRNAs were detected in the RNA from dolphin morbillivirus infected cells. These mRNA species seemed to be of approximately the same size as the corresponding mRNA species of CDV, PDV and MV. The results of the comparison demonstrated that the dolphin morbillivirus is genetically different from CDV, PDV and MV. No indication of a close relationship between the dolphin isolate and either CDV, PDV or MV was found.

Animals↗

Full genome sequence of peste des petits ruminants virus, a member of the Morbillivirus genus.

Peste des petits ruminants virus (PPRV) causes an acute febrile illness in small ruminant species, mostly sheep and goats. PPRV is a member of the Morbillivirus genus which includes measles, rinderpest (cattle plague), canine distemper, phocine distemper and the morbilliviruses found in whales, porpoises and dolphins. Full length genome sequences for these morbilliviruses are available and reverse genetic rescue systems have been developed for the viruses of terrestrial mammals, with the exception of PPRV. This paper presents the first published full length genome sequence for PPRV. The genome was found to be consistent with the rule-of-six and open reading frames (ORFs) were identified that encoded the eight proteins characteristic of morbilliviruses. At the nucleotide (nt) level, the full length genome of PPRV was most similar to that of rinderpest, the other ruminant morbillivirus. However, at the protein level five of the six structural proteins and the V protein showed a greater similarity to the dolphin morbillivirus (DMV) while only the C and L proteins showed a high relationship to rinderpest.

Base Sequence↗