Document Category: Disease and Parasites

Epizootics of a malignant blood cancer (lymphosarcoma) affects feral populations of northern pike and muskellunge. Overall frequencies of occurrence of the disease in northern pike and muskellunge as high as 20.9% and 16.0%, respectively, were found. The disease in feral muskellunge caused high mortalities while in northern pike spontaneous regressions are common. The disease is transmitted precutaneously during the act of spawning. The disease does not seem to be transmitted to progency via the egg. These species should be stocked as eggs or fry, not as adults, if the spread of lymphosarcoma is to be restricted.

Feral muskellunge and northern pike in widely separated geographical regions in North American and northern pike in Europe suffer epizootics of lymphosarcoma. The disease is highly contagious and virus associated. The tumor is transmitted to fish via physical contact during spawning activities. To prevent spread of the disease, it is recommended that stocking be limited to the progeny of disinfected eggs propagated under quarantine conditions.

A DNA vaccine containing the glycoprotein (G) gene of the North American viral hemorrhagic septicemia virus (VHSV) genotype IVb was developed to evaluate the immune response of fish following vaccination and evaluate its efficacy in protecting a susceptible species, the muskellunge (Esox masquinongy) , against VHSV‐IVb challenge. Seven weeks (539 degree‐days) following vaccination with 10 μg of either pVHSivb‐G or a control plasmid, Muskellunge were challenged by immersion with 105 plaque‐forming units (pfu)/mL of VHSV‐IVb. Fish vaccinated with pVHSivb‐G had a relative percent survival (RPS) of 45%. Vaccinated fish also had significantly lower mean viral titers in tissues (4.2 × 102 pfu/g) and viral prevalence (4%) than fish receiving the plasmid control vaccine (3.3 × 105 pfu/g; 82%). Neutralizing antibodies were detected 28 d (308 degree‐days) postchallenge (11 weeks postvaccination) in 100% of muskellunge vaccinated with pVHSivb‐G compared with only 12% of plasmid‐control‐vaccinated Muskellunge, suggesting robust induction of a secondary, adaptive immune response. In addition, pVHSivb‐G–vaccinated rainbow trout Oncorhynchus mykiss challenged 7 d (100 degree‐days) postvaccination with the heterologous novirhabdovirus, infectious hematopoietic necrosis virus (IHNV), experienced an RPS of 61%, compared to control fish, suggesting induction of an early and transient nonspecific antiviral immune response. This study provides an important starting point for VHSV‐IVb vaccine development and useful information about the antiviral immune response elicited by DNA vaccination in a nondomesticated fish species.

Blue spot disease (esocid herpesvirus‐1) was observed in populations of northern pike (Esox lucius) and muskellunge (Esox masquinongy) in northern Wisconsin during the 1984–1992 spring spawning periods. Prevalence of blue spot disease was as high as 34% for northern pike and 29% for muskellunge. Prevalence differed among lakes and between sexes in some lakes. In general, larger and older northern pike were less likely to have observable lesions. The pathogenesis of blue spot disease is unknown. Clinical signs are present for only a short period when water temperatures are between 2 and 13°C, and they are not visible shortly after fish spawn, when water temperatures reach 14°C.

Viral hemorrhagic septicemia virus (VHSV) has been found in fish populations throughout the Great Lakes basin since 2003. It is a single-stranded RNA virus that affects a number of fish species, including muskellunge (Esox masquinongy) a major predator in these waters. The purpose of this experiment was to compare the virulence of four strains of VHSV IVb (MI03, vcG002, FPL2013-002 and FLP20140991). Age-0 muskellunge were randomly assigned to one of the strains and exposed to either a high (5 x 105 plaque forming units/mL) or low (5 x 104 plaque forming units/mL) dose for one hour by immersion. Fish were then monitored for clinical signs of infection, such as petechial hemorrhages, lethargy and death, whereupon brain and pooled organ samples were harvested using aseptic technique. Quantitative reverse transcription polymerase chain reaction assays in muskellunge were performed along with viral isolation in order to confirm the presence of HVSV. Results of the Cox proportional hazard regression models did demonstrate a difference when comparing the time to death of the high dose versus the low dose, but no difference was observed when comparing the time to death of the four isolates over the course of the experiment. When comparing viral load in muskellunge pooled spleen, hear, liver and anterior and posterior kidneys or separate brain, there were no differences between the strains or the doses detected. Future studies with lower doses closer to the LD50 may differentiate changes in virulence properties of VHSV IVb.

The Great Lakes strain of viral haemorrhagic septicaemia virus (VHSV) isolated from adult subclinical muskellunge, Esox masquinongy days post‐infection (p.i.). The median lethal intraperitoneal injection dose (IP‐LD days after exposure in waterborne challenged fish, whereas fish infected by the i.p. route experienced the first mortality by 5 (Mitchill), in Lake St. Clair, MI, USA was shown to be highly pathogenic in juvenile muskellunge through intraperitoneal (i.p.) injection and waterborne challenge. Mortality began as early as 350) was approximately 2.21 plaque forming units (PFU) as opposed to the median lethal immersion challenge dose (IM‐LD5010 × ) of 1.74mL PFU −1. A high, medium and low dose of infection caused acute, subacute and chronic progression of the disease, respectively, as was evident by the cumulative mortality data. Clinical signs of disease observed in dead and moribund fish were very pale gills, dermal petechial haemorrhages along the flanks, severe nuchal haemorrhages, intramuscular haemorrhages at the fin–muscle junction and focal haemorrhaging on the caudal peduncle. Internal lesions included livers that were pale, discoloured and friable, and kidneys that were either congested or degenerative in appearance, and petechial to ecchymotic haemorrhages on the swim bladder wall. Histopathologic examination demonstrated massive haemorrhages in the swimbladder wall and muscle, severe vacuolation and multifocal necrosis of the liver, multifocal necrosis of the gills and depletion of lymphoid tissues within the spleen. Kidney tissues also exhibited a mixed pattern of degeneration that included tubular necrosis, interstitial oedema and congestion. Virus was recovered from kidney and spleen tissues through tissue culture and reverse transcriptase‐polymerase chain reaction (RT‐PCR).

In 2011, the Fathead Minnow nidovirus (FHMNV; Genus Bafinivirus , Family Coronaviridae , Order Nidovirales ) was isolated from pond‐raised juvenile Muskellunge Esox masquinongy suffering from lingering mortality at the Wild Rose Hatchery in Wild Rose, Wisconsin. Moribund Muskellunge exhibited tubular necrosis in the kidneys as well as multifocal coalescing necrotizing hepatitis. The FHMNV was also isolated from apparently healthy juvenile Muskellunge at the Wolf Lake State Fish Hatchery in Mattawan, Michigan. The identity of the two syncytia‐forming viruses (designated MUS‐WR and MUS‐WL from Wild Rose Hatchery and Wolf Lake State Fish Hatchery, respectively) as strains of FHMNV was determined based on multiple‐gene sequencing and phylogenetic analyses. The pathogenicity of the MUS‐WL FHMNV strain was determined by experimentally infecting naive juvenile Muskellunge through intraperitoneal injection with two viral concentrations (63 and 6.3 × 103 TCID50/fish). Both doses resulted in 100% mortality in experimentally infected fish, which exhibited severely pale gills and petechial hemorrhaging in eyes, fins, and skin. Histopathological alterations in experimentally infected fish were observed mainly in the hematopoietic tissues in the form of focal areas of necrosis. Phylogenetic analysis of concatenated partial spike glycoprotein and helicase gene sequences revealed differences between the MUS‐WL FHMNV, MUS‐WR FHMNV, and two other FHMNV originally isolated from moribund Fathead Minnows Pimephales promelas including the index FHMNV strain (GU002364). Based on a partial helicase gene sequence, a reverse transcriptase PCR assay was developed that is specific to FHMNV. These results give evidence that the risks posed to Muskellunge by FHMNV should be taken seriously.

The incidence of the following species of helminths recovered from the digestive tract of 218 muskallunge (Esox m. masquinongy) from various localities in the St. Lawrence watershed is recorded: Azygia augusticauda, A. longa, Triaenophorus nodulosus, Proteocephalus pinguis, Neoechinorhynchus cylindratus, Leptorhynchoides thecatus, Metabronema salvelini, and Rhaphidascaris canadensis. One hundred and ninety-two, or 88% of the fish examined were found to harbor one or more species. The most commonly found species were T. nodulosus and A. longa. In all cases the number of worms recovered per host was small.

Yearling tiger muskellunge (northern pike Esox lucius × muskellunge E. masquinongy) being cultured within the Colorado Division of Wildlife’s fish hatchery system were found to have external lesions that were grossly and microscopically consistent with descriptions of esocid lymphosarcoma. This neoplasia has been described as a tumor of adult northern pike and muskellunge; a retroviral etiology has been proposed for it. However, esocid lymphosarcoma has not previously been reported in tiger muskellunge. Owing to concerns about the potentially infectious nature of the condition in a hatchery environment, an experiment was conducted to determine whether the lesion could be transmitted to naive young-of-the-year tiger muskellunge in the laboratory by means of cell-free filtrates. At 32 weeks postchallenge, grossly visible lesions were observed on the challenged fish. Histological evaluation of these lesions confirmed that they were esocid lymphosarcoma. We believe that this is the first report of the natural occurrence of this disease in tiger muskellunge as well as in any esocid that was not an adult.

A new host, Esox masquinongy, the Muskellunge, for the Silver Lamprey, Ichthyomyzon unicuspis, is reported. Fifteen Silver Lamprey/Muskellunge interactions were documented in a 90-km section of the lower Ottawa River, on the Ontario as well as the Quebec side, from Ottawa to Hawkesbury between 1992 and 2001. Sites of attachment were predominantly on the back. Number of marks per host varied between 1 and 31 with a mean of 10.6. There was evidence of cytolytic activity of the buccal gland secretions. Shallowness of the fresh wounds indicated blood feeding rather than flesh feeding. Survival of the host was indicated by the presence of healed wounds in 26.7% of the cases. Muskellunge over 122 cm in total length were preferred over smaller individuals. Lampreys appeared to be more highly concentrated in the 50-km stretch of the lower Ottawa River, between Thurso and Hawkesbury, than they were in the 40-km stretch upstream, between Ottawa and Thurso.