Ecological changes affect pathogen epidemiology and evolution and may trigger the emergence of novel diseases. Aquaculture radically alters the ecology of fish and their pathogens. Here we show an ...increase in the occurrence of the bacterial fish disease Flavobacterium columnare in salmon fingerlings at a fish farm in northern Finland over 23 years. We hypothesize that this emergence was owing to evolutionary changes in bacterial virulence. We base this argument on several observations. First, the emergence was associated with increased severity of symptoms. Second, F. columnare strains vary in virulence, with more lethal strains inducing more severe symptoms prior to death. Third, more virulent strains have greater infectivity, higher tissue-degrading capacity and higher growth rates. Fourth, pathogen strains co-occur, so that strains compete. Fifth, F. columnare can transmit efficiently from dead fish, and maintain infectivity in sterilized water for months, strongly reducing the fitness cost of host death likely experienced by the pathogen in nature. Moreover, this saprophytic infectiousness means that chemotherapy strongly select for strains that rapidly kill their hosts: dead fish remain infectious; treated fish do not. Finally, high stocking densities of homogeneous subsets of fish greatly enhance transmission opportunities. We suggest that fish farms provide an environment that promotes the circulation of more virulent strains of F. columnare. This effect is intensified by the recent increases in summer water temperature. More generally, we predict that intensive fish farming will lead to the evolution of more virulent pathogens.
Wild-caught European perch
(Perca fluviatilis)
were exposed in the laboratory to untreated bleached pulp and paper mill effluent in two separate experiments. The first experiment was conducted at ...7–8°C using effluent concentrations of 5 and 10%, and the second experiment was conducted at ambient river temperature of 4–20°C using an effluent concentration of 1%. Trichodinid ciliates were identified and enumerated at the end of the exposure using a mucus subsampling technique from gill and skin as well as a formalin immersion technique, which provided total counts on each fish. Four different trichodinid species were identified on the fish. Prevalence of infection, mean number, and mean density of
Trichodina
spp. decreased on fish exposed to effluents compared with controls. Prevalence of infection, mean number, and mean density of
Trichodinella epizootica
decreased on fish exposed to 5% and 10% effluents but increased on fish exposed to 1% effluents compared with controls. These results demonstrate that trichodinid ciliates vary in their susceptibility to at least certain types of contaminants and cautions against using trichodinids as environmental indicators without delineating species.
We used nestedness analysis to seek non-random patterns in the structure of component communities of metazoan parasites collected from 31 sympatric fish species from the northeastern Bothnian Bay, ...the most oligohaline area of the Baltic Sea. Only 8 marine parasite species were found among the 63 species recorded, although some marine fish species reproduce in the bay and others occasionally visit the area. Marine parasite species can utilize both freshwater and marine fish species as intermediate or final hosts, and marine fish can harbour freshwater parasite species. This exchange of parasite species between marine and freshwater fish has probably resulted from ecological factors acting over short time scales rather than from evolutionary processes acting over longer time; the key factor probably being the immediate presence of suitable intermediate and definitive hosts. Marine fish were expected to harbour species-poor parasite communities consisting mainly of generalists acquired from the sympatric freshwater fish species, which would result in a nested pattern among the different component communities. However, an anti-nested pattern was found in the component communities of metazoan parasites of fishes from the Bothnian Bay. A likely explanation for the observed pattern is that there are specialist parasite species, the majority of which are cestodes, in some of the freshwater fish species which otherwise have depauperate parasite communities.
In benthic habitats, predators can generally not be detected visually, so olfaction may be particularly important for inducing anti-predation behaviors in prey organisms. Manipulative parasites ...infecting benthic hosts could suppress these responses so as to increase the probability of predation and thus trophic transmission. We studied how infection with the acanthocephalan Echinorhynchus borealis affects the response of the benthic amphipod Pallasea quadrispinosa to water conditioned by burbot (Lota lota), the parasite's definitive host. In normal lake water, refuge use by infected and uninfected amphipods was similar, but when exposed to burbot-conditioned water, uninfected amphipods spent much more time hiding than infected amphipods. Thus, rather than affecting ambient hiding behavior, E. borealis infection seems to alter host response to a predator. A group of amphipods sampled from a postglacial spring that is devoid of fish predators exhibited only a weak response to burbot-conditioned water, perhaps suggesting these anti-predator behaviors are costly to maintain. The hiding behavior of spring and infected amphipods was very similar. If the reduced refuge use by the spring amphipods reflects adaptation to a predator-free environment, this indicates that E. borealis severely weakens its host's anti-predator behavior. Presumably this increases the likelihood of parasite transmission.
Chemokines and their receptors participate in the development of multiple sclerosis (MS) by guiding immune cells into the brain tissue. A CCR5 Δ32 deletion mutation abolishes functional CCR5 on the ...cell surface and may reduce cell entry into the lesion sites. To analyse the significance of this mutation in MS, we compared the frequencies of CCR5 genotype in peripheral blood mononuclear cells from 89 MS patients and 119 healthy controls. The CCR5 genotype was further compared with the CCR5 RNA and surface protein expression in 48 MS patients and their controls. In all MS patients, the Δ32/32 genotype was found with 6.7% frequency, whereas it was present only in 0.8% of the controls (6/89 vs 1/119, P = 0.01). Specifically, the Δ32/Δ32 genotype was increased (11.5%, P = 0.05) among primary progressive MS patients, whereas it was present only in 4.8% in other MS subtypes and only in 0.8% of the controls. The amount of CCR5 protein on CD4+ cells analysed in 48 MS patients (nine primary progressive MS, 18 secondary progressive MS, 21 relapsing–remitting MS) and 13 controls decreased with genotype, being 8.9% in wt/wt, 7.7% in wt/Δ32 and 4.3% in Δ32/Δ32. CCR5 surface expression analysed on these 48 MS patients and 13 controls was significantly decreased in Δ32/Δ32 MS patients as compared with that in wt/wt genotype individuals (P = 0.004). The significantly increased number of Δ32/Δ32 individuals among our MS patients suggests that this genotype could contribute as a general risk factor for MS. However, neither the levels of RNA or surface protein correlated with MS subtype, neurological disability as expressed by expanded disability status scale, or disease progression index. Our results suggest that the lack of CCR5 does not protect from MS, but rather it may predispose to the chronic course of the disease. This would further imply that in view of the redundancy in the chemokine system, CCR5 ligands must be assumed to function through other closely related chemokine receptors.
In inflamed colonic mucosa, the equilibrium between absorptive and secretory functions for electrolyte and salt transport is disturbed. We compared the expression of three major mediators of the ...intestinal salt transport between healthy and inflamed colonic mucosa to understand the pathophysiology of diarrhea in inflammatory bowel disease. Expression levels of the cystic fibrosis transmembrane regulator (CFTR) (Cl- channel), SLC26A3 (Cl-/HCO exchanger) and SLC9A3 (Na+/H+ exchanger) mRNAs were measured by real-time quantitative RT-PCR in peroperative colonic samples from controls (n = 4) and patients with ulcerative colitis (n = 10). Several samples were obtained from each individual. Tissue samples were divided into three subgroups according to their histological degree of inflammation. Expression of CFTR and SLC26A3 proteins were determined by immunohistochemistry and Western blotting from the same samples, respectively. Increased expression of CFTR mRNA was observed in all three groups of affected tissue samples, most pronounced in mildly inflamed colonic mucosa (5-fold increase in expression; P < 0.001). The expression of the CFTR protein was detected from health and inflamed colon tissue. Although the expression of the SLC26A3 mRNA was significantly decreased in severe ulcerative colitis (P < 0.05), the SLC26A3 protein levels remained unchanged in all groups. The expression of SLC9A3 mRNA was significantly changed between the mild and severe groups. Intestinal inflammation modulates the expression of three major mediators of intestinal salt transport and may contribute to diarrhea in ulcerative colitis both by increasing transepithelial Cl- secretion and by inhibiting the epithelial NaCl absorption.
While host stress in vertebrate populations has often been linked to outbreaks of epidemics, which are attributed to the immuno-compromise of the stressed hosts, no predictions have been made about ...the response of invertebrate host populations to stressful conditions. Experiments conducted on individual invertebrate hosts, however, suggest that starved hosts may be a poor resource for parasites and that heavily infected old hosts may be more susceptible to stress, causing parasite populations to decline when their host population faces food shortages. In this epidemiological experiment, we exposed infected and uninfected Daphnia magna populations, which had been kept for many generations under a constant high food supply, to reduced food resources. Using the microsporidian gut parasite Glugoides intestinalis, which is exclusively horizontally transmitted, we tracked changes in parasite and host population size as well as host body length to elucidate how food shortages for the hosts influence host and parasite population dynamics. In both infected and uninfected populations, food shortage led to an approximately equal reduction in host density and changes in host body length distribution. Large hosts suffered a higher mortality than smaller hosts, which significantly reduced the mean body length in the host populations; however, this change was stronger in the infected populations and went hand-in-hand with a reduction in parasite spore load (a measure of intensity of infection) and prevalence. This effect disappeared after six weeks of food shortage, when the populations reached a new equilibrium. Our results indicate that in this system food stress impairs parasite spread and that host mortality is an important factor in regulating parasite abundance at the population level.
Some parasites have been shown to manipulate host behavior so that parasite transmission to the next host is enhanced. Infection with Triaenophorus crassus Forel (Cestoda) caused alterations in the ...activity and microhabitat selection of the first intermediate host Cyclops strenuus Fischer (Copepoda) in the laboratory. Infected copepods made more starts to swim but spent less time swimming than uninfected copepods. These changes were independent of the intensity of infection. In a water column illuminated from above, infected copepods approached the surface, whereas uninfected ones remained close to the bottom. In the dark both infected and uninfected copepods stayed near the bottom. Finally, infection with T. crassus increased the probability of C. strenuus being eaten by the second intermediate host, whitefish (Coregonus lavaretus L. s.l.), in the laboratory. In experimental infections, 10-day-old procercoids had significantly lower infectivity for whitefish than older (12-, 14-, and 21-day-old) procercoids. Behavioral changes were detected in infected copepods containing procercoids 12 days old or older but not in experiments with 10-day-old procercoids. These results may indicate that T. crassus changes the behavior of the copepod host only after it has become infective to the next host, which is consistent with the active manipulation hypothesis.
Patterns of accumulation of Triaenophorus crassus in its second intermediate host whitefish Coregonus lavaretus s.l. were studied between 1991 and 1996 from two host populations in two separate areas ...of Lake Saimaa, Finland. Whitefish were infected commonly with several T. crassus plerocercoids and the parasites were aggregated into the oldest hosts. In one host population the annual parasite accumulation was 0·9 parasites in all host age groups between 3 and 8 years. In the other host population the annual accumulation was 1·6 parasites in 3–5‐year‐old fish, but increased up to 3 to 4 parasites per year in fish over 5 years old. The increase did not coincide with the period of maturation or any increase in whitefish growth, both of which could alter the food intake of the fish. The sharp increase in the annual accumulation suggests a threshold intensity above which the probability of acquiring further parasites increases. In spite of a heavy aggregation of parasites there was no evidence of parasite‐induced host mortality. The annual increase in mean abundance was not correlated with the mean annual weight increase in 2–4‐year‐old fish within cohorts. However, evidence of a negative effect of parasites on whitefish growth was revealed by back‐calculation of lengths of uninfected and infected whitefish and correlations between length or weight of fish and intensity of infection with fish age. Both analyses suggested that larger young fish harboured more parasites than the smaller ones while in older fish the reverse was true, a pattern that has not been shown earlier for parasitized fish.
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