Introduced Pathogens and Native Freshwater Biodiversity: A Case Study of Sphaerothecum destruens

A recent threat to European fish diversity was attributed to the association between an intracellular parasite, Sphaerothecum destruens, and a healthy freshwater fish carrier, the invasive Pseudorasbora parva originating from China. The pathogen was found to be responsible for the decline and local extinction of the European endangered cyprinid Leucaspius delineatus and high mortalities in stocks of Chinook and Atlantic salmon in the USA. Here, we show that the emerging S. destruens is also a threat to a wider range of freshwater fish than originally suspected such as bream, common carp, and roach. This is a true generalist as an analysis of susceptible hosts shows that S. destruens is not limited to a phylogenetically narrow host spectrum. This disease agent is a threat to fish biodiversity as it can amplify within multiple hosts and cause high mortalities

Control of Rickettsial Infections in White Sea Bass (Atractoscion nobilis)

This project was designed to address an emerging disease problem encountered in restoration efforts for the white seabass (Atractoscion nobilis) in California. Once an abundant species, declines have encouraged hatchery-based enhancement of this popular sport fish. Infections with a rickettsial-like intracellular pathogen caused serious losses in the hatchery in 1998. Losses in the hatchery exposed serious knowledge gaps with respect to the type of intracellular parasite causing the disease, the potential sources of the parasite, what diagnostic methods could be developed to detect it, what impacts could the parasite have on other marine or anadromous species (e.g., Pacific salmon) and lastly what means might be developed to control the disease caused by the parasite

Initial Steps Towards Evaluating the Potential Disease Impacts of Propagated Marine Fish on Wild Stock: Examination of a New Herpes-like Virus

The project hypothesis is that with the development and application of the appropriate tools for pathogen detection, we can gain major initial insights into the potential impacts of diseases among propagated fish upon wild fish populations

Initial Steps Towards Evaluating the Potential Disease Impacts of Propagated Marine Fish on Wild Stock: Examination of a New Herpes-like Virus

The project hypothesis is that with the development and application of the appropriate tools for pathogen detection, we can gain major initial insights into the potential impacts of diseases among propagated fish upon wild fish populations

Infectious Hematopoietic Necrosis Virus Transmission and Disease among Juvenile Chinook Salmon Exposed in Culture Compared to Environmentally Relevant Conditions

The dynamics of IHNV infection and disease were followed in a juvenile Chinook salmon population both during hatchery rearing and for two weeks post-release. Cumulative weekly mortality increased from 0.03%–3.5% as the prevalence of viral infection increased from 2%–22% over the same four-week period. The majority of the infected salmon was asymptomatic. Salmon demonstrating clinical signs of infection shed 1000 pfu mL-1 of virus into the water during a 1 min observation period and had a mean concentration of 106 pfu mL-1 in their mucus. The high virus concentration detected in mucus suggests that it could act as an avenue of transmission in high density situations where dominance behavior results in nipping. Infected smolts that had migrated 295 km down river were collected at least two weeks after their release. The majority of the virus positive smolts was asymptomatic. A series of transmission experiments was conducted using oral application of the virus to simulate nipping, brief low dose waterborne challenges, and cohabitation with different ratios of infected to naïve fish. These studies showed that asymptomatic infections will occur when a salmon is exposed for as little as 1 min to &gt;102 pfu mL-1, yet progression to clinical disease is infrequent unless the challenge dose is &gt;104 pfu mL-1. Asymptomatic infections were detected up to 39 d post-challenge. No virus was detected by tissue culture in natural Chinook juveniles cohabitated with experimentally IHNV-infected hatchery Chinook at ratios of 1:1, 1:10, and 1:20 for either 5 min or 24 h. Horizontal transmission of the Sacramento River strain of IHNV from infected juvenile hatchery fish to wild cohorts would appear to be a low ecological risk. The study results demonstrate key differences between IHNV infections as present in a hatchery and the natural environment. These differences should be considered during risk assessments of the impact of IHNV infections on wild salmon and trout populations.</p

Infectious Hematopoietic necrosis Virus Transmission and Disease Among Juvenile Chinook Salmon Exposed in culture Compared to Environmentally Relevant Conditions

The dynamics of IHNV infection and disease were followed in a juvenile Chinook salmon population both during hatchery rearing and for two weeks post-release. Cumulative weekly mortality increased from 0.03%–3.5% as the prevalence of viral infection increased from 2%–22% over the same four-week period. The majority of the infected salmon was asymptomatic. Salmon demonstrating clinical signs of infection shed 1000 pfu mL-1 of virus into the water during a 1 min observation period and had a mean concentration of 106 pfu mL-1 in their mucus. The high virus concentration detected in mucus suggests that it could act as an avenue of transmission in high density situations where dominance behavior results in nipping. Infected smolts that had migrated 295 km down river were collected at least two weeks after their release. The majority of the virus positive smolts was asymptomatic. A series of transmission experiments was conducted using oral application of the virus to simulate nipping, brief low dose waterborne challenges, and cohabitation with different ratios of infected to naïve fish. These studies showed that asymptomatic infections will occur when a salmon is exposed for as little as 1 min to &gt;102 pfu mL-1, yet progression to clinical disease is infrequent unless the challenge dose is &gt;104 pfu mL-1. Asymptomatic infections were detected up to 39 d post-challenge. No virus was detected by tissue culture in natural Chinook juveniles cohabitated with experimentally IHNV-infected hatchery Chinook at ratios of 1:1, 1:10, and 1:20 for either 5 min or 24 h. Horizontal transmission of the Sacramento River strain of IHNV from infected juvenile hatchery fish to wild cohorts would appear to be a low ecological risk. The study results demonstrate key differences between IHNV infections as present in a hatchery and the natural environment. These differences should be considered during risk assessments of the impact of IHNV infections on wild salmon and trout populations.</p

Sampling strategy for the treatment groups <i>Abramis brama</i>, <i>Cyprinus carpio</i> and <i>Rutilus rutilus</i>.

<p>List of organs and organ numbers which have been tested for the presence of <i>Sphaerothecum destruens</i> DNA. Organs tested included the kidney (K), liver (L), intestine (I), gill (Gi) and gonad (Go). n: number of fish sampled.</p>*<p>: at 28 d.p.e. the liver, kidney and intestine of 10 <i>C. carpio</i> were tested for <i>S. destruens</i>.</p>**<p>: gill and gonad tissues were analyzed in only 13 of the 22 <i>R. rutilus</i> mortalities.</p

Kaplan-Meier survival curves for <i>Abramis brama</i>, <i>Rutilus rutilus</i> and <i>Cyprinus carpio</i> following infection with <i>Sphaerothecum destruens</i>.

<p>Cumulative proportion of (A) Bream <i>Abramis brama</i>, (B) Roach <i>Rutilus rutilus</i> and (C) Carp <i>Cyprinus carpio</i> surviving following exposure to <i>S. destruens</i>. Treatment fish (solid line) were exposed to an average concentration of 8.6×10⁴<i>S. destruens</i> spores ml⁻¹ whilst control fish (dotted line) were sham exposed. Time: days post exposure.</p

Mortality pattern in <i>Abramis brama</i> as a result of infection with <i>Sphaerothecum destruens</i>.

<p>The cumulative percentage mortality in the treatment groups (n = 60 individuals in total) and daily mortalities are presented for 26 days post exposure with <i>S. destruens</i>.</p