Abstract
Influenza viruses induce highly contagious respiratory diseases in human and other animal populations worldwide. Annual human influenza epidemics result in at least 200,000 hospitalizations and 36,000 deaths in the United States, and global mortality has been estimated between 300,000 to 500,000. The three influenza pandemics of the 20th century are believed to have arisen from interspecies transmission. The most devastating of these, the Spanish flu pandemic of 1918–1919, killed 20 million and affected over 200 million people. Interspecies transmission of animal influenza viruses (e.g., avian, swine, etc.) occurs on a global scale, heightening concerns that the next human pandemic may emerge from the ever-expanding animal-human interface.
Influenza viruses are classified into three genera (influenza virus A, B, C) within the family Orthomyxoviridae. Orthomyxoviruses are segmented RNA viruses, and segment reassortment aids rapid evolution and novel host adaptation. Influenza A viruses infect a wide variety of species, including humans, domesticated animals (pigs, horses, and poultry), aquatic birds, and marine mammals. Influenza B viruses infect humans, ferrets, and marine mammals. Although Influenza C viruses infect humans, dogs, and pigs, they are less common than the other two genera and have been infrequently documented in widespread human epizootics.
Since the late 1970s, influenza A and B viruses have been detected by viral isolation, serologic methods, and reverse transcriptase PCR in wild populations of cetaceans and pinnipeds. In 1979–1980, approximately 20% of the northeastern US harbor seal (Phoca vitulina) population died from an H7N7 outbreak that likely arose following transmission from an avian host. This influenza A virus replicated to high titers in mammalian (e.g., ferrets, cats, pigs) but not avian hosts. Experimental transmission studies with this virus produced a lethal systemic disease in squirrel monkeys, and several investigators exposed to the affected harbor seals developed severe conjunctivitis.
Despite growing evidence of influenza A and B viruses within wild marine mammal populations, the level of surveillance for respiratory viruses resulting in marine mammal morbidity and mortality at institutions harboring captive populations of pinnipeds, cetaceans, and sirenians is lacking. Furthermore, a survey of participants at research and rehabilitation facilities, zoos, aquaria, and swim-with-the-dolphin programs reported a 4% incidence of respiratory illness associated with contact with marine mammals. The mixing of exotic species in proximity, and the intimate contact between livestock, trainers, husbandry staff, and the general public provides an environment for the interspecies transmission of contagious diseases such as influenza viruses. Public access to partial information, a poor understanding of infectious disease processes through web based search engines, and the tendency to overreact without baseline information may lead to an increased but potentially unfounded fear of infectious disease exposure from any animal contact. Basic research into respiratory viruses, while initially concerning for those managing these species, will in the long run provide a more accurate picture of pathogen transfer potential and likely result in less over reaction, better disease management and prevention in zoo and aquarium populations.
Here we outline the first concerted surveillance effort to detect and characterize influenza A and B viruses in marine mammals in zoos, aquariums, and stranded US marine mammal populations using a combination of virologic (cell culture), serologic (microneutralization), and molecular (qRT-PCR for both virus detection and transcription of host anti-viral and immune response genes) based techniques.