Unusual striped dolphin mass mortality episode related to cetacean morbillivirus in the Spanish Mediterranean sea
- Consuelo Rubio-Guerri†1Email author,
- Mar Melero†1,
- Fernando Esperón2,
- Edwige Nina Bellière2,
- Manuel Arbelo3,
- Jose Luis Crespo4,
- Eva Sierra3,
- Daniel García-Párraga4 and
- Jose Manuel Sánchez-Vizcaíno1
© Rubio-Guerri et al.; licensee BioMed Central Ltd. 2013
Received: 20 February 2013
Accepted: 17 May 2013
Published: 23 May 2013
In the last 20 years, Cetacean Morbillivirus (CeMV) has been responsible for many die-offs in marine mammals worldwide, as clearly exemplified by the two dolphin morbillivirus (DMV) epizootics of 1990–1992 and 2006–2008, which affected Mediterranean striped dolphins (Stenella coeruleoalba). Between March and April 2011, the number of strandings on the Valencian Community coast (E Spain) increased.
Necropsy and sample collection were performed in all stranded animals, with good state of conservation. Subsequently, histopathology, immunohistochemistry, conventional reverse transcription polymerase chain reaction (RT-PCR) and Universal Probe Library (UPL) RT-PCR assays were performed to identify Morbillivirus. Gross and microscopic findings compatible with CeMV were found in the majority of analyzed animals. Immunopositivity in the brain and UPL RT-PCR positivity in seven of the nine analyzed animals in at least two tissues confirmed CeMV systemic infection. Phylogenetic analysis, based on sequencing part of the phosphoprotein gene, showed that this isolate is a closely related dolphin morbillivirus (DMV) to that responsible for the 2006–2008 epizootics.
The combination of gross and histopathologic findings compatible with DMV with immunopositivity and molecular detection of DMV suggests that this DMV strain could cause this die-off event.
KeywordsCetacean morbillivirus Dolphin morbillivirus Mediterranean sea Mass mortality Striped dolphin
Over the last 20 years, epizootics caused by Cetacean morbillivirus (CeMV) (genus Morbillivirus, family Paramyxoviridae) have occurred among cetacean populations worldwide and have caused mass mortality . Three main CeMV groups have been described: dolphin morbillivirus (DMV) , porpoise morbillivirus (PMV)  and pilot whale morbillivirus (PWMV) [4, 5].
The spread of DMV infection in striped dolphins (Stenella coeruleoalba) in the Mediterranean Sea caused around 1000 deaths in 1990–1992 . This outbreak started in 1990 in the Gulf of Valencia, in the Spanish Mediterranean Sea , and propagated along European Mediterranean coasts over the following months [2, 8]. Then in 2007, a new DMV outbreak occurred off the Spanish Mediterranean coast. It affected approximately 100 striped dolphins  and up to 60 long-finned pilot whales (Globicephala melas) , and subsequently spread to the French Mediterranean coast .
In the last two decades, the annual mean mortality rate of dolphins stranded off the Mediterranean coast of Valencia has been 28.4 animals per year. However this rate lowers to 18.3 animals per year if the 1990 and 2007 outbreak years are excluded .
In the present study, 37 dolphins are reported as stranded dolphins in 2 months, which represents more than the annual mean in that region. An evaluation of Morbillivirus infection revealed the overwhelming positivity of the stranded animals, suggesting that DMV could be responsible for this increase in strandings, which might be the third DMV epizootic in the Mediterranean Sea.
Thirty-seven dolphins stranded along the Valencian Mediterranean coast between March and April 2011: 26 striped dolphins (S. coeruleoalba), three bottlenose dolphins (Tursiops truncatus) and eight dolphins of undetermined species (poor level of conservation hampered species identification).
Necropsies were performed according to standard protocols of the European Cetacean Society . Stranded dolphins were recovered from the Valencia Mediterranean coast of Spain (39°N, 0°W) by the Marine Mammal Stranding Network of the Conselleria de Infraestructuras, Territorio y Medio Ambiente of Valencia. A detailed post-mortem examination could be carried out on 11 animals (nine striped dolphins and two bottlenose dolphins) since other animals were poorly preserved.
Fresh tissue samples (brain, lung, kidney, liver, lymph node, tonsil, thymus, spleen and skin) were fixed in 10% neutral buffered formalin for histopathology, refrigerated for microbiology, and tissue samples were frozen for molecular diagnosis. Immunohistochemical staining with a Canine Distemper Virus monoclonal antibody specific for nucleoprotein, IgG2B isotype (CDV-NP. VMRD®, Inc.), was carried out on selected samples of brain, lung, kidney, urinary bladder, stomach and intestine. Frozen tissues were homogenized using a Bullet BlenderTM (Next Advance, Inc., Averill Park, NY), and total nucleic acid was extracted using the NucleoSpin RNA II Kit (Macherey-Nagel) for RNA extraction and the High Pure PCR Template Preparation Kit for DNA extraction, following the manufacturers’ instructions in both kits. For the molecular CeMV diagnosis, real-time RT-PCR assays, based on the Universal Probe Library (UPL) platform, that target, a sequence within the fusion protein gene, was carried out .
CeMV infection was confirmed by sequencing the real-time RT-PCR products. For the phylogenetic analysis, in addition to the fusion protein gene, the DMV phosphoprotein (P) and nucleoprotein (N1 and N2) genes were amplified by conventional RT-PCRs assays according to published protocols [4, 15] in some positive sample. A BLAST analysis was used to compare the obtained phosphoprotein and nucleoprotein sequences with all the CeMV sequences available in GenBank. A phylogenetic analysis was performed using the MEGA 4.0 software . P-distance matrices were calculated, and tree topology was inferred by the neighbor-joining maximum composite likelihood method to test the reliability of the topology by bootstrapping 1000 replicates generated with a random seed.
Brucella spp. and Toxoplasma gondii (T. gondii) diagnoses were carried out in the animals whose non suppurative encephalitis was observed in the histopathological analysis. The molecular identification of Brucella spp. was performed in the brains of suspected animals by TaqMan Real time PCR, targeting the insertion sequence IS711 of Brucella spp. . Additionally, T. gondii DNA was detected by nested PCR, in which the target formed part of the sequence of repetitive gene B1 (194 bp, 97 bp) using the method described by Montoya et al. .
Necropsy, histopathology and morbillivirus detection results in the Stenella coeruleoalba individuals stranded in the 2011 epizootic
Stranding date (2011)
Age (length, cm)
Main gross lesions
Main histological lesions
Tissues tested for CeMV (positive samples underlined by UPL RT-PCR)
New born (108.7)
Poor body condition, severe pneumonia
Focal broncointersticial pneumonia with few giant cells, necrotic areas in reticular and perivascular cells of lymph nodes
Brain, Lung, Pulmonary lymph node, Kidney
New born (112.3)
Numerous injuries, severe enteritis in large intestine
Multifocal lymphocytic bronchointerstitial pneumonia, bronchoalveolar hemorrhages, eosinophilic lymphadenitis, nephritis
Brain, Thymus, Lung, Pulmonary lymph node, Kidney
Sub adult (170)
Suboptimal body condition, gas bubbles in meninges vessels, severe pneumonia, mild gastritis
Severe non-suppurative meningoencephalitis, intranuclear inclusion bodies mainly in astrocytes
Brain, Lung, Prescapular and Pulmonary lymph nodes, Kidney
Poor body condition, severe pneumonia, enteritis in large intestine with numerous parasitic granulomas
Verminous bronchopneumonia, chronic nephritis
Brain*, Lung, Pulmonary lymph node, Kidney, Liver
Cachexia, injuries, severe brain congestion, internal hemorrhage, atelectasis, large pulmonary bullae
Verminous bronchopneumonia, pleuritiswith bacterial colonies, systemic intravascular bacterial embolism
Brain, Lung, Pulmonary lymph node, Kidney
Pneumonia in cranial left lung, enlarged and congestive prescapular and pulmonary lymph nodes
Severe non-suppurative meningoencephalitis
Brain, Lung, Pulmonary lymph node, Kidney, Tongue ulcer
Poor body condition, maxilla and mandible fracture, brain congestion, numerous injuries, pneumonia, atelectasis, enlarged and hemorrhagic pulmonary and abdominal lymph nodes, hepatic congestion
Mild non-suppurative meningoencephalitis
Brain, Lung, Pulmonary lymph node, Kidney
New born (123)
Suboptimal body condition, temporal fracture, tonsillitis, tongue ulcers, pulmonary fibrosis, severe pneumonia in right lung, enlarged and congestive pulmonary lymph nodes, severe enteritis in large intestine
Severe brain hemorrhages
Brain, Lung, Pulmonary lymph node
New born (109)
Poor body condition, pneumonia, atelectasis, enlarged and congestive pulmonary lymph nodes, tonsillitis, enteritis in both small and large intestines
Brain, Lung, Pulmonary lymph node
According to the molecular diagnosis performed by UPL RT-PCR assays, seven of the eleven analyzed dolphins were positive in DMV/PMV UPL PCR, which represents 63.6% of positivity to CeMV. They were all striped dolphins. After considering that the most affected specie in the last Mediterranean DMV epizootics was striped dolphin, the percentage was calculated in relation to all the analyzed striped dolphins, which changed to 78% of positivity (seven positive striped dolphins as compared to nine analyzed striped dolphins) (Table 1). Furthermore, the systemic form of the disease was found in five of these seven animals, which contained at least two positive tissues. However, only one of the seven positive animals analyzed by UPL RT-PCR was positive for phosphoprotein by conventional PCR, and this positivity was restricted to brain.
The microbiological investigations found no pathologically significant microorganisms. As regards Brucella spp. and T. gondii PCRs, negative results were identified in the three animals with non suppurative encephalitis.
The 37 strandings in just 2 months might represent an unusual mortality event if we consider the annual number of stranded dolphins has not been exceeded since 1990, except in 1990 and 2007, on Valencian Mediterranean coasts . DMV was considered the causative agent of this increase in strandings in 1990 and 2007 [7, 9]. In addition, this is not the first description of DMV in the Mediterranean Sea in 2011. In Italy, DMV has been reported in striped dolphins, bottlenose dolphins and fin whales on almost the same dates as this report [21–24]. Accordingly, Morbillivirus infection finding might be related with the rise in strandings in 2011.
DMV detection by the UPL RT-PCR assay in 78% of the analyzed striped dolphins highlights the important role that DMV plays in this unusual episode of mass strandings. At the same time, the recognition of DMV in at least two tissues in five animals may indicate the general spread of this virus, most likely by the circulatory system. Failure to detect CeMV in the central nervous system (CNS) in all the positive animals can be explained by the fact that distribution of CeMV brain infection is not homogeneous  and that the CNS samples in this study were not collected uniformly; thus it was impossible to determine from which brain region each sample had been taken. In addition, Toxoplasma gondii and Brucella spp. have been related with encephalitis in stranded dolphins [26, 27], and even together with CeMV . However in our study, any of the three cases with non suppurative encephalitis can be related with these pathogens.
A comparison made of the striped dolphin outbreaks in 1990, 2007, and 2011 suggests a change in DMV epidemiology in the Western Mediterranean Sea. In 2011, mortality was even lower, lesions were less severe, and mostly younger animals were affected. Since enzootic infections in wildlife are characterized by milder lesions and lighter pathogen loads than epizootics , it is possible that DMV epidemiology in striped dolphins in the Western Mediterranean is changing from epizootic to enzootic infection, as suggested by others [20, 25]. Systematic serological surveys are urgently required to address this question.
In conclusion, the presence of DMV-compatible lesions, the antigen detection in one of the animals and the molecular detection of DMV genomic sequences all suggest that DMV is associated with this unusual mass mortality episode in striped dolphins in the Western Mediterranean Sea. Further research to define how the virus circulates and causes epidemics in the Mediterranean Sea, and why only striped dolphins were affected in 2011, is warranted.
Reverse transcription polymerase chain reaction
Universal probe library
Pilot whale morbillivirus
- S. coeruleoalba:
Central nervous system.
We thank Prof. Miró Corrales and Dr. Montoya from Complutense University of Madrid and Dr. Fuentes from Parasitology Service of the National Centre of Microbiology in the Carlos III Institute of Health for the Toxoplasma gondii diagnosis. This work has been supported by a collaborative agreement between the Conselleria de Infraestructuras, Territorio y Medio Ambiente of Valencia of Valencia; the Oceanographic Aquarium of the Ciudad de las Artes y las Ciencias of Valencia; the VISAVET Center of Complutense University of Madrid; and the Pfizer Foundation. This work was cofinanced by Project CGL2009-08125 of the Spanish National Research Plan. The authors thank the Institut Cavanilles de Biodiversitat i Biologia Evolutiva, University of Valencia, for collaboration on necropsies, and Elena Neves, Belén Rivera, Verónica Nogal and Rocío Sánchez for technical assistance. Mar Melero is the recipient of a PhD student grant from the University Complutense of Madrid and Consuelo Rubio is the recipient of a FPU grant from the Spanish Ministry of Education.
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