The development and refinement of amphibian medicine comprise an ongoing science that reflects the unique life history of these animals and our growing knowledge of amphibian diseases. Amphibians are notoriously fastidious in terms of captive care requirements, and the majority of diseases of amphibians maintained in captivity will relate directly or indirectly to husbandry and management. Investigators have described many infectious and noninfectious diseases that occur among various species of captive and wild amphibians, and there is considerable overlap in the diseases of captive versus free-ranging populations. In this article, some of the more commonly reported infectious and noninfectious diseases as well as their etiological agents and causative factors are reviewed. Some of the more common amphibian diseases with bacterial etiologies include bacterial dermatosepticemia or "red leg syndrome," flavobacteriosis, mycobacteriosis, and chlamydiosis. The most common viral diseases of amphibians are caused by the ranaviruses, which have an impact on many species of anurans and caudates. Mycotic and mycotic-like organisms cause a number of diseases among amphibians, including chytridiomycosis, zygomycoses, chromomycoses, saprolegniasis, and ichthyophoniasis. Protozoan parasites of amphibians include a variety of amoeba, ciliates, flagellates, and sporozoans Common metazoan parasites include various myxozoans, helminths (particularly trematodes and nematodes), and arthropods. Commonly encountered noninfectious disease etiologies for amphibians include neoplasia, absolute or specific nutritional deficiencies or overloads, chemical toxicities, and inadequate husbandry or environmental management.

During the summer and fall of 1997, an unusually high prevalence of skin lesions in fishes from Chesapeake Bay tributaries as well as two fish kills in the Pocomoke River stimulated significant public concern. Atlantic menhaden Brevoortia tyrannus were the most frequent target of the acute fish kills and displayed skin lesions that were attributed to the presence of the toxic dinoflagellate Pfiesteria piscicida. Hence, the penetrating skin ulcers so commonly found in this species are now widely viewed by the general public and some scientists as Pfiesteria‐related and to be caused by exposure to Pfiesteria toxin. We examined, histologically, 121 menhaden with these ulcers collected from both Maryland and Virginia waters of the Chesapeake Bay in 1997 and 31 from the Pocomoke and Wicomico rivers in 1998. All of the deeply penetrating ulcers, as well as raised lesions (with or without eroded epithelium), were characterized by deeply penetrating fungal hyphae surrounded by chronic, granulomatous inflammation. These lesions had an appearance identical or similar to epizootic ulcerative syndrome (EUS), an ulcerative mycotic syndrome of fishes in other parts of the world caused by the fungal pathogen Aphanomyces invadans. They were also identical to ulcerative mycosis of menhaden previously reported along the Atlantic coast of the USA as associated with Aphanomyces spp. In 1998, using methods for isolation of A. invadans, we were able to culture from affected menhaden an Aphanomyces sp. that by preliminary tests is similar or identical to A. invadans. We believe these findings suggest that factors other than Pfiesteria toxin need to be considered as the cause or initiator of these lesions.

The cause of deeply penetrating ulcers of Atlantic menhaden Brevoortia tyrannus has been the subject of significant research efforts in recent years. These lesions and the associated syndrome termed ulcerative mycosis have been observed along the East Coast of the United States since at least the early 1980s. Although Aphanomyces spp. were isolated from these lesions in the mid to late 1980s, similar lesions could not be reproduced by experimental infections of Atlantic menhaden with these isolates. The identical characteristic histologic appearance of granulomatous inflammation surrounding the penetrating fungal hyphae occurs in fish with epizootic ulcerative syndrome (EUS), as reported throughout South Asia, Japan, and Australia. Aphanomyces invadans has been found to be the causative agent of EUS in all of these countries. Using methods developed for the study of EUS, we successfully isolated an organism for which the DNA sequence, morphology, temperature and salinity growth characteristics, and infectivity of chevron snakehead Channa striata are identical to A. invadans. Using the polymerase chain reaction assay for A. invadans, we were able to demonstrate the presence of the organism from Atlantic menhaden lesions collected in U.S. estuarine waters from Delaware to South Carolina. In addition, the organism was present in lesions on a bluegill Lepomis macrochirus from a farm pond in Georgia and channel catfish Ictalurus punctatus from a farm pond in Louisiana.

Eastern Tubifex tubifex worms were exposed to Myxobolus cerebralis spores at 9, 13, 17, and 20 C in 1-L jars that contained sand, mud, or leaf litter as substrata. Beginning 60 days after exposure, water from each jar was filtered daily and examined for the presence of waterborne triactinomyxon spores (TAMs). On discovering a single TAM from an experimental jar, 48 T. tubifex worms from that jar were placed individually into 24-well plates. Spores released from individual infected T. tubifex worms were quantified to determine the first day of TAM release from infected worms, the infection rate, the total number of TAMs released per worm, and the duration of release. No TAMs were found in any of the jars incubated at 20 C or in uninfected, control worms at any temperature. The total number of TAMs released by infected worms in mud and sand was highest at 13 C compared with other temperatures. Infection rates among individual worms increased with temperature between 9 and 17 C. Higher temperatures (up to 17 C) induced earlier TAM releases among infected worms, and substratum did not influence this production parameter. The average duration of TAM release decreased as the temperature increased from 9 to 17 C, and there was a significant effect of substratum in the groups maintained at 13 and 17 C. In all temperature treatments between 9 and 17 C, the duration of release was least in the worms maintained in leaf litter, as was the total number of TAMs released during the experimental period and the median number of TAMs per production day.

Cytochemical and immunochemical analysis of cells may be helpful in identification and characterization of amphibian blood cells and splenic cells for evaluation of the health of these animals.

Three strains of rainbow trout and steelhead Oncorhynchus mykiss were evaluated for the presence of whirling disease in field and laboratory trials. In the field exposures, fingerling Salmon River steelhead and Cayuga Lake and Randolph strains of rainbow trout were placed in wire cages in an earthen, stream-fed pond in New York State that was known to harbor Myxobolus cerebralis. Control fish were held at another hatchery that was free of whirling disease. In the controlled trials at the National Fish Health Research Laboratory, fingerling steelhead and Cayuga Lake and Mount Lassen rainbow trout were exposed to triactinomyxons at low (200 triactinomyxons/fish) or high (2,000 triactinomyxons/fish) levels for 2 h. Controls of each group were sham-exposed. Following an incubation period of 154 d for laboratory trials and 180 d for field trials, cranial tissue samples were taken for spore enumeration (field and laboratory trials) and histological analyses (laboratory only). Clinical signs of disease, including whirling behavior, blacktail, and skeletal deformities, were recorded for each fish in the laboratory trial at the terminal sampling. No clinical evidence of disease was noted among fish in the field trials. Clinical signs were noted among all strains in the laboratory trials at both exposure levels, and these signs were consistently greatest for the Mount Lassen strain. Whirling and skeletal deformities were more evident in the steelhead than in the Cayuga Lake rainbow trout; blacktail was more common in the Cayuga Lake fish. In both field and laboratory trials, spore counts were significantly higher for Cayuga Lake rainbow trout than in steelhead. In laboratory trials, moderate to marked cranial tissue lesions predominated in all three strains.

Avian influenza viruses (AIV) affect many species of birds including waterfowl and may persist in sediment in aquatic habitats. Sediment samples were collected from two areas representative of prime migration and overwintering waterfowl habitat in Dorchester County, Maryland in the fall and winter of 2013-2014. Samples were screened for the presence of AIV via reverse transcriptase-quantitative PCR targeting the matrix gene. Although 13.6% of sediment samples were positive for the AIV matrix gene across all collection dates and locations, differences in detection were noted with location and collection season. Percentage of AIV-positive sediment samples recovered corresponded to trends in waterfowl abundance at collection sites both temporally and spatially. These findings provide further support for the assertion that the presence of AIV in the aquatic environment is likely affected by the total number, site-specific density, and array of waterfowl species.

Myxobolus cerebralis, the myxosporean parasite that causes whirling disease, has a number of deleterious effects on its salmonid host. Although it is well established that juvenile salmonids in the active stages of whirling disease mount an immune response to the pathogen, the occurrence and longevity of any related immunomodulatory effects are unknown. In this study, postyearling rainbow trout Oncorhynchus mykiss infected with M. cerebralis were examined for leukocyte functions and for resistance to Yersinia ruckeri, a bacterial pathogen of salmonids. Compared with uninfected controls, M. cerebralis‐infected fish showed lower proliferative lymphocyte responses to four mitogens (concanavalin A, pokeweed mitogen, phytohemagglutinin, and lipopolysaccharide). Conversely, M. cerebralis‐infected fish displayed greater bactericidal activity of anterior kidney macrophages than did uninfected fish. After bath challenges with Y. ruckeri, M. cerebralis‐infected fish had slightly lower survival and a more rapid onset of mortality than did the control fish. Renal tissue and fecal samples from M. cerebralis‐infected and uninfected survivors were cultured for the presence of Y. ruckeri, and no difference in prevalence was noted between the two groups. Because immunomodulatory changes in the M. cerebralis‐infected fish involved functional enhancement and suppression of different leukocyte populations, disease resistance among M. cerebralis‐infected fish in the later stages of whirling disease will probably vary with the secondary pathogen and the nature of immune response the pathogen evokes.