E. Tellervo Valtonen: Research

Main Research Interests 2008à

 

The evolution of a more virulent pathogen as a consequence of intensive fish farming?

The emergence of infectious diseases may be triggered by changes in the ecology, epidemiology or evolution of a pathogen. In aquaculture, pathogen transmission among fish is radically changed due to high densities of homogenous subsets of fish and treatments against pathogens. Persistence of several co-occurring pathogen strains enhance the possibility for repeated outbreaks further. We have studied an increase in the occurrence and mortality caused by a bacterial fish disease Flavobacterium columnare in salmon fingerlings at a fish farm in northern Finland during the last 22 years, as well as a trend towards more serious symptoms and an increase in the use of antibiotic treatments against the disease. Our earlier experiments with strains isolated from disease outbreaks in Finland have showed that virulent strains produced worse symptoms in fish, had higher infectivity, higher tissue degrading capacity and higher growth rate than non-virulent strains. Combined with our recent findings that the bacterium can transmit more efficiently from dead fish than from living fish, and maintain its infectivity in sterilized water for months, we study if fish farms provide an environment, which promote the existence for more virulent strains during the warm water periods that have increased in length during the past decades. Our data are also consistent with evolutionary increases in columnaris disease virulence over the last two decades in Finland. We hypothesize that intensive fish farming will lead to the evolution of a number of more virulent pathogens.

 

 

Increasing Water Temperature and Disease Risks in Aquatic Systems: Climate Change increases the risk of some, but not all, diseases.

 

Global warming may impose severe risks for aquatic animal health if increasing water temperature leads to increase in the incidence of parasitic diseases. Essentially, this could take place through a temperature-driven effect on the epidemiology of the disease. For example, higher temperature may boost the rate of disease spread through positive effect on parasite fitness in weakened host. Increased temperature may also lengthen the infective season leading to higher total prevalence of infection and more widespread epidemics. However, to date, general understanding of these relationships is limited because of scantiness of long-term empirical data. We are working on one of the first long‑term multi-pathogen data sets on the occurrence of natural pathogenic bacterial and parasite infections in relation to increasing temperatures in aquatic systems. We analyse a time-series of disease dynamics in two fish farms in northern Finland in 1986-2006. We first demonstrate that the yearly mean water temperature increased significantly over the study period and that the increase was most pronounced in the late summer (July-September). Second, we show that the prevalence (i.e. proportion of fish tanks infected each year) increased with temperature. Interestingly, this pattern was observed in some of the diseases (Ichthyophthirius multifiliis, Flavobacterium columnare), whereas in the other diseases, the pattern was the opposite (Ichthyobodo necator), or absent (Chilodonella spp.). These results demonstrate the effect of increasing water temperature on aquatic disease dynamics, but also emphasise the importance of biology of each disease, as well as role of local conditions, in determining the direction and magnitude of these effects.

 

 

Vertebrate diets derived from trophically-transmitted  fish parasites

 

Parasites that are transmitted through predator-prey interactions may be used as indicators of trophic relationships between organisms. Yet, they are rarely used as such in the construction of topological (predator-prey) food webs. We are constructing food webs of vertebrate trophic interactions using observed diet alone, trophically-transmitted parasites alone, and the combination of the two based on data from 31 species of fish from the Bothnian Bay, Finland. Standard food web metrics were compared among the three webs. Food web metrics fell within the range of those calculated for other webs elsewhere. The sub-web constructed from parasite data alone had higher mean number of links and connectance than that using observed diet. Connectance and the mean number of links per species increased by 38% using parasites compared versus observed diet as indicators of trophic relationships.  Both measurements doubled when parasites and observed diet were considered together compared to observed diet alone, illustrating the complementarity of the two methods. A positive correlation was found between the mean number of parasites and the number of prey taxa in the diet among the fishes. Omnivorous fish had the highest diversity of both parasite species and prey items, while benthophagous fish had among the lowest. Mean total abundance and mean total prevalence of parasites correlated positively with fish size, with piscivores being the largest with the highest abundance and prevalence, while planktivores and benthivores had the lowest. Trophically-transmitted parasites may be used to help construct vertebrate sub-webs and derive information about food web processes. Parasites alone provided equivalent if not more information than observed diet. However, resolution is improved by using parasites and observed diet together.

 

Transmission patterns of trematodes in the world of narrow opportunities, Ichthyocotylurus pileatus and Australapatemon sp. as models.

 

Parasites with complex life cycles face two major challenges for transmission in northern latitudes. First, they have to cope with the general unpredictability associated with the series of transmission events required for completion of the cycle, and second, transmission has to be completed within a narrow temporal window because of strong seasonality. Despite this, parasites show high transmission success, suggesting operation of effective transmission mechanisms. We explored the transmission of Ichthyocotylurus pileatus (Trematoda) from its snail (Valvata macrostoma) to fish (Perca fluviatilis) hosts by including some key characteristics in dynamics of the cercarial release from snails. The transmission took place within few weeks mainly in July thus verifying the narrow temporal window for transmission. The output of the short-lived cercariae from the snails was low and variable in magnitude, nevertheless resulted in rapid and high rate of infection in newly hatched fish. The cercarial release showed a strong diurnal rhythm with most of the cercariae released in early evening and night, which might represent the most likely mechanism underlying the high rate of transmission in this species. We emphasise the importance of holistic approaches which combine aspects of multiple host species in studies on transmission of complex life cycle parasites.

 

Co-infections of multiple parasite species in hosts may lead to interspecific and intraspecific interactions between the parasites and subsequently shape the structure of the parasite community. Such interactions are particularly likely in species that have very high abundances and infect the same location with a host. We also investigate seasonality of establishment, interspecific interactions and site segregation in two species of trematodes, Ichthyocotylurus variegatus and I. pileatus, co-infecting their second intermediate host, perch (Perca fluviatilis).

Australapatamon sp. (Trematoda) infection are studied both in the first intermediate host Valvata macrostoma and in two second intermediate leech species, Helodella stagnalis and Erpobdella octoculata. Seasonality in production of cercariaae from Valvata as also metacercariae in leeches have been studied. In laboratory experiments both the susceptibility of the two leech spp as also the differences in activity the these hosts have been studied.