During July through September 1995. about 1.000 adult largemouth bass Micropterus salmoides died in Santee-Cooper Reservoir, South Carolina. Necropsy of two moribund largemouth bass (each weighing about 2 kg and measuring about 50 cm) revealed no parasites or bacterial infection. Fathead minnow (FHM) cell cultures inoculated with filtered homogenate from the swim bladder of one fish and the swim bladder and liver of the second tish developed cytopathic effect 48 h after inoculation. Media from infected cell cultures contained up to I0 8 -9 TCID50/mL of a 0.45-jjim filterable agent (TCID50 is the tissue culture infective dose that affects 50% of innoculated cultures). Electron microscopy of infected FHM cells revealed a cytoplasmic, icosahcdral virion that averaged 132 nm from facet to facet and 145 nm from corner to corner. Viruses acquired an envelope upon passing through the plasma membrane. Studies of virus replication inhibition by pyrimidine analogs were inconclusive in classifying the nucleic acid. The virus was isolated from all organs of experimentally infected adult largemoulh bass 8 and 26 d postinjection. No infected fish died, but an inflamed, necrotic lesion developed at the injection site. Although virulence of the virus is uncertain, this is the first published report of a vircmia associated with a mass mortality of wild centrarchids. Tentatively classified in the Iridoviridae, we propose "largemouth bass virus" (LMBV) as its name.
Diseases of fishes caused by Aeromonas spp. are common, have broad host ranges and may cause high mortality. Treatments of captive-reared populations using antimicrobials are limited with concerns for bacterial resistance development and environmental dissemination. This study was done to determine whether selected plant-derived essential oils were bactericidal to Aeromonas spp. Initially, twelve essential oils were evaluated using a disk diffusion assay to an isolate of A. salmonicida subsp. salmonicida, cause of fish furunculosis. The greatest zones of inhibition were obtained with oils of cinnamon Cinnamomum cassia, oregano Origanum vulgare, lemongrass Cymbopogon citratus and thyme Thymus vulgaris. Minimum bactericidal concentrations (MBC’s) were determined for these four oils, Allimed® (garlic extract, Allium sativum) and colloidal silver to sixty-nine isolates representing nine Aeromonas spp. The lowest mean MBCs (0.02–0.04%) were obtained with three different sources of cinnamon oil. MBCs for three sources of oregano and lemongrass oils ranged from 0.14% to 0.30% and 0.10% to 0.65%, respectively, and for two thyme oils were 2.11% and 2.22%. The highest concentration (5%) of Allimed® tested resulted in MBCs to twelve isolates. A concentration of silver greater than 15 mg/L would be required to determine MBCs for all but one isolate.
Largemouth bass virus (LMBV) is a recently discovered iridovirus that causes a fatal disease of largemouth bass, Micropterus salmoides (Lacepède). Fish can become infected by waterborne LMBV, but oral transmission of this virus has not been demonstrated previously. Largemouth bass were gavaged with guppies, Poecilia reticulata Peters, which had been injected with LMBV, and then sampled periodically during a 7-week observation period. The dose of LMBV averaged 10 5.6 tissue culture infectious doses -50% cytopathic endpoint (TCID 50 ) per largemouth bass. Five of 24 largemouth bass exposed to LMBV became infected with the virus, but none of the fish had clinical signs typical of LMBV disease. Virus titres in largemouth bass were highest in swim bladder (10 5.5-9.5 TCID 50 g )1 ) and were 10 5.2 TCID 50 g )1 or lower in cutaneous mucus, head kidney, trunk kidney, spleen, gonad and intestine. These results indicate that LMBV can be transmitted orally to largemouth bass, but further study is needed to determine the factors affecting pathogenicity of the virus.
Largemouth bass virus (LMBV) is an iridovirus widely distributed in southeastern U.S. populations of wild largemouth bass Micropterus salmoides. However, this virus has not been previously reported from fish hatcheries. During 1999, 15 state hatcheries in 10 states in the southeastern USA were surveyed for LMBV. The virus was isolated from largemouth bass broodfish at five hatcheries, but juvenile largemouth bass were infected in only one of these facilities. There were no indications of the disease caused by LMBV in either adults or juveniles at the time samples were collected. The virus was not isolated in concurrent samples of forage fish from seven of the hatcheries, including three where LMBV was found. Although transmission of LMBV from infected populations to their offspring in hatcheries does not always occur, this virus could be spread by transport of infected broodfish or by stocking infected juvenile largemouth bass.
A polymerase chain reaction (PCR) method was developed for largemouth bass virus (LMBV). This iridovirus can cause a lethal disease of largemouth bass Micropterus salmoides, but also subclinically infects largemouth bass and other species of fishes. Oligonucleotide primers were designed to specifically amplify the major capsid protein gene of LMBV. The protocol for sample processing and PCR provided a method that was more sensitive than cell culture for detection of LMBV in fish. The specific amplification of LMBV also provided an improved method for confirming the identity of cell-culture isolates presumptively identified as LMBV.
Nine severely anemic (hematocrit between 1 and 10%) channel catfish Ictalurus punctatus from commercial aquaculture ponds were examined. The total number of circulating erythrocytes ranged from 17,300 to 995,000 cells/^L and leukocytes ranged from 9,700 to 133,500 cells/jiL. The erythrocytes often had clumped hemoglobin and were occasionally bilobed. Hematopoietic tissue in the spleen, head kidney, and trunk kidney was necrotic. Steatosis (fatty change) of the liver was severe, and intestinal lesions included intussusception, epithelial desquamation, submucosal edema, and necrosis of the mucosa and muscularis.
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