Members of the genus Morbillivirus (family Paramyxoviridae), collectively known as morbilliviruses, are RNA-containing viruses. They cause a number of important diseases, including measles in humans, canine distemper in dogs, and rinderpest in bovines. Since July 2013, a serious morbillivirus outbreak has occurred among bottlenose dolphins (Tursiops truncatus) predominantly along the eastern coast of the United States (from New York to Florida), resulting in the strandings and deaths of almost 1500 of these marine creatures. This recent cetacean morbillivirus epidemic is larger and more significant than a similar morbillivirus outbreak that occurred in 1987–1988, during which 742 bottlenose dolphins died. See also: Animal virus; Canine distemper; Cetacea; Disease; Emerging diseases in marine mammals; Measles; Paramyxovirus; Ribonucleic acid (RNA); Rinderpest; Virus; Virus classification

Bottlenose dolphins can contract the morbillivirus disease through the inhalation of aerosolized particles containing the virus. They also can be infected via direct contact with other infected dolphins. The virus can enter a dolphin through its eyes, mouth, urogenital tract, or skin wounds, in addition to the respiratory tract. The lungs and brain of infected dolphins are the major organs of the body that are affected. Infected dolphins appear thin and display numerous skin lesions. They are stressed by breathing difficulties, which are often related to pneumonia, and act abnormally. Moreover, these dolphins become prone to other illnesses because the morbillivirus infection weakens their immune systems.

One theory about the current epidemic relates to the span of time that has occurred since the outbreak in 1987–1988. Dolphins that were infected (but not killed) during the previous morbillivirus outbreak thereafter carried antibodies to the virus, protecting them against further infection. These resistant dolphins in turn offered protection to other dolphins (especially young dolphins) lacking natural immunity because these unprotected individuals had less chance of coming into contact with the pathogen—the epidemiological phenomenon called herd immunity. However, many of the virus-resistant dolphins have died of other causes since 1987–1988, leaving later generations susceptible again to illness from reintroduction of the morbillivirus. See also: Antibody; Immunity; Immunology

So far, the cetacean morbillivirus outbreak has been limited to bottlenose dolphins in the northern Atlantic Ocean. It has not spread to other marine mammals. In addition, the dolphin-killing pathogen is not a threat to humans. Unfortunately, there is no vaccine or effective treatment for dolphins suffering from the infection. Investigations now in progress are comparing the genetic sequence of the current morbillivirus strain to that of the viral strain responsible for the 1987–1988 outbreak. It is possible that researchers might find mutations that explain the current outbreak's potency. Another possibility is that the current outbreak may have been triggered by environmental stresses or anthropogenic toxins that have been reported to reduce the disease immunity and fitness of bottlenose dolphins. See also: Ecosystem; Environmental toxicology; Epidemiology; Marine ecology