Kathryn A. Converse

Epizootiology of sixty-four amphibian morbidity and mortality events in the USA, 1996-2001.

Abstract: A total of 44 amphibian mortality events and 20 morbidity events were reviewed retrospectively. The most common cause of amphibian mortality events was infection by ranaviruses (Family: Iridoviridae). Ranavirus epizootics have abrupt onset and affect late‐stage larvae and recent metamorphs. Mortality events due to ranavirus infections affected only widespread and abundant amphibian species, and there was a clear association with high population densities. Chytrid fungal infections accounted for seven mortality events in postmetamorphic anurans only. Chytrid epizootics are insidious and easily overlooked in the field. While both ranavirus and chytrid fungal epizootics were associated with > 90% mortality rates at affected sites, only the chytrid fungal infections were linked to multiple amphibian population declines. Three primitive fungal organisms in the newly erected clade, Mesomycetozoa, caused morbidities and mortalities in anurans and salamanders.

Hind Limb Malformations in Free-Living Northern Leopard Frogs (Rana pipiens) From Maine, Minnesota, and Vermont Suggest Multiple Etiologies

BACKGROUND Reports of malformed frogs have increased throughout the North American continent in recent years. Most of the observed malformations have involved the hind limbs. The goal of this study was to accurately characterize the hind limb malformations in wild frogs as an important step toward understanding the possible etiologies. METHODS During 1997 and 1998, 182 recently metamorphosed northern leopard frogs (Rana pipiens) were collected from Minnesota, Vermont, and Maine. Malformed hind limbs were present in 157 (86%) of these frogs, which underwent necropsy and radiographic evaluation at the National Wildlife Health Center. These malformations are described in detail and classified into four major categories: (1) no limb (amelia); (2) multiple limbs or limb elements (polymelia, polydactyly, polyphalangy); (3) reduced limb segments or elements (phocomelia, ectromelia, ectrodactyly, and brachydactyly; and (4) distally complete but malformed limb (bone rotations, bridging, skin webbing, and micromelia). RESULTS Amelia and reduced segments and/or elements were the most common finding. Frogs with bilateral hind limb malformations were not common, and in only eight of these 22 frogs were the malformations symmetrical. Malformations of a given type tended to occur in frogs collected from the same site, but the types of malformations varied widely among all three states, and between study sites within Minnesota. CONCLUSIONS Clustering of malformation type suggests that developmental events may produce a variety of phenotypes depending on the timing, sequence, and severity of the environmental insult. Hind limb malformations in free-living frogs transcend current mechanistic explanations of tetrapod limb development.

Raptor mortality due to West Nile virus in the United States, 2002

West Nile virus (WNV) has affected many thousands of birds since it was first detected in North America in 1999, but the overall impact on wild bird populations is unknown. In mid-August 2002, wildlife rehabilitators and local wildlife officials from multiple states began reporting increasing numbers of sick and dying raptors, mostly red-tailed hawks (Buteo jamaicensis) and great horned owls (Bubo virginianus). Commonly reported clinical signs were nonspecific and included emaciation, lethargy, weakness, inability to perch, fly or stand, and nonresponse to danger. Raptor carcasses from 12 states were received, and diagnostic evaluation of 56 raptors implicated WNV infection in 40 (71%) of these cases. Histologically, nonsuppurative encephalitis and myocarditis were the salient lesions (79% and 61%, respectively). Other causes of death included lead poisoning, trauma, aspergillosis, and Salmonella spp. and Clostridium spp. infections. The reason(s) for the reported increase in raptor mortality due to WNV in 2002 compared with the previous WNV seasons is unclear, and a better understanding of the epizootiology and pathogenesis of the virus in raptor populations is needed.

Duck plague epizootics in the United States, 1967-1995

In 1967, the first confirmed diagnosis of duck plague (DP) in the USA was made from pekin ducks (Anas platyrhynchos domesticus) on commercial duck farms on Long Island, New York. Within 10 mo, DP was confirmed as the cause of death in migratory waterfowl on a Long Island bay. This paper reviews 120 DP epizootics reported from 1967 to 1995 that involved waterfowl species native to North America or were reported in areas with free-flying waterfowl at risk. Duck plague epizootics occurred in 21 states with the greatest number reported in Maryland (29), New York (18), California (16), and Pennsylvania (13). The greatest frequency of epizootics (86%) was detected during the months of March to June. At least 40 waterfowl species were affected with the highest frequency of epizootics reported in captive or captive-reared ducks including muscovy ducks (Cairina moschata) (68%), mallard ducks (A. platyrhynchos) (18%) and black ducks (A. rubripes) (14%). The greatest number of waterfowl died in three epizootics that involved primarily migratory birds in 1967 and 1994 in New York (USA) and 1973 in South Dakota (USA). The greatest number of DP epizootics reported since 1967 appear to have involved flocks of non-migratory rather than migratory waterfowl; therefore, in our opinion it remains unknown if DP is enzootic in either non-migratory or migratory waterfowl.

The impact of disease in the American white pelican in North America

Abstract Records of reported die-offs of the American White Pelican (Pelicanus erythrorhynchos) held by the U.S. Geological Survey National Wildlife Health Center from 1978 through 2003 indicate that type C botulism (caused by Clostridium botulinum) was the major cause of mortality. In 1996, over 15,000 birds, including 8,500 American White Pelicans, were estimated to have died from type C botulism at the Salton Sea in California. This was the largest documented die-off of any pelican species and was estimated to represent 15-20% of the western metapopulation. This event was also notable in that it was the first time that fish, specifically Tilapia (Oreochromis mossambicus), were implicated as the source of type C botulinum toxin for birds. Type C botulism has recurred in both North American species of pelicans at the Salton Sea every year since, although the magnitude of mortality is much lower. West Nile virus caused mortality in both adult and immature American White Pelicans, but may have a more significant impact on nestlings. Emaciation and mortality is common in pelican nesting colonies. Further clarification of the cause of nestling mortality and the ability to differentiate background mortality from mortality due to infectious disease agents such as West Nile virus and Newcastle disease virus would be important in determining the impact of disease in pelicans. Although definitive conclusions cannot be drawn, the number of die-offs and the estimated losses of American White Pelicans appears to have increased along with declines in number in western U.S.

Mortality of Passerines Adjacent to a North Carolina Corn Field Treated with Granular Carbofuran

Red-winged blackbirds (Agelaius phoeniceus) were collected during an epizootic in southeastern North Carolina (USA). Activity of brain cholinesterase (ChE) was inhibited by 14 to 48% in three of five specimens, and returned to normal levels after incubation. Gastrointestinal tracts were analyzed for 30 anti-ChE agents. Carbofuran, the only compound detected, was present in all specimens at levels from 5.44 to 72.7 μg/g wet weight. Application of granular carbofuran in an adjacent corn field, results of necropsy examinations, and chemical analyses are consistent with a diagnosis of carbofuran poisoning in these specimens.