Ranaviruses have been implicated as one of the causes of worldwide amphibian declines. These viruses can be deadly to amphibians and have caused mass mortality events in North America. Clinical signs of ranavirus infection may be vague but include hemorrhaging of the skin, lethargy, and swelling (edema). Multiple internal organs may be affected; however, the kidney and liver often are targeted, resulting in hemorrhage and necrosis. In some species, mortality can occur as early as within a few days of exposure. Subclinical infections also occur in some (less susceptible) species. Although this pathogen can infect all life stages of amphibians, larvae (tadpoles) are most often affected. Transmission is most commonly through casual contact, cannibalism, or water exposure. Ranaviruses can infect other lower vertebrates as well, including reptiles and fishes, and have been known to cause extensive morbidity and mortality in these groups. Treatment is currently not available; however, Japanese researchers have developed a vaccine for fish that may eventually prove useful in captivity. Additionally, we found that Nolvasan® can be used to inactivate the virus, which is especially important for equipment (e.g., nets, boots) used in the field or in captive facilities.
The University of Georgia has been collaborating with other researchers, veterinarians and biologists to investigate this pathogen. One example is a large scale collaborative study between the UGA Veterinary Diagnostic and Investigational Laboratory and the University of Tennessee Center for Wildlife Health. This study is designed to investigate ranaviruses in free-ranging amphibians. The preliminary findings suggest that anthropogenic stressors (i.e., those that affect water quality) may cause some species (e.g., green frogs) to be more susceptible to the virus. When coupled with metamorphosis, these stressors can prove especially devastating because during metamorphosis, cortisol levels increase and the larval immune system is dismantled so that an adult immune system can be built. During this critical time period, these creatures logically have an increased susceptibility to pathogens. Thus, additional stressors may prove detrimental to wild populations as well as captive ones (e.g., zoological and ranaculture facilities).
Amphibians are used as fish bait, pets, exhibit specimens (e.g., in zoos), food (e.g., frog legs), and research specimens, all of which may result in shipment of live animals. In many cases, animals are either collected from the wild (generally as egg masses or larvae) or obtained from captive facilities. Recent studies suggest that ranavirus strains found in captive amphibian facilities (e.g., bait shops, ranaculture facilities) may be more virulent than wild strains. Because people frequently buy amphibians from captive facilities and accidentally or intentionally release them into the environment, novel and highly virulent strains can be released into natïve amphibian populations. Moreover, humans can inadvertently transport virus particles among watersheds on footwear, clothing, or recreation equipment. Thus, humans potentially contribute to the spread of this pathogen.
Of course ranavirus is not the only pathogen that negatively impacts amphibians. During the past few years, we have investigated die-offs of various amphibian species in captivity and in the wild. In some cases, other pathogens such as Batrachochytrium dendrobatidis or Aeromonas hydrophila were involved. Batrachochytrium dendrobatidis is a fungus that causes chytridiomycosis in amphibians, and has contributed to worldwide amphibian declines and even extinction of some species. Aeromonas hydrophila is a bacterium that is commonly associated with infections (some severe) in lower vertebrates, but generally is a secondary invader. It was previously thought to be the primary agent in ‘red leg’ disease; however, recent evidence suggests that ranaviruses are the primary pathogen of this disease and A. hydrophila is secondary (opportunistic).
The good news is that, in May 2008, the World Organization for Animal Health (http://www.oie.int/) listed both ranavirus and chytridiomycosis as reportable diseases. Currently, the OIE is in the process of establishing guidelines for pre-shipment testing and disease surveillance. These listings will serve as valuable tools to unite veterinarians and biologists in attempting to control the spread of these deadly pathogens, and hopefully halt the worldwide amphibian decline.