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Infective Stages (infective + stage)
Selected AbstractsA long-term study of non-native-heartworm transmission among coyotes in a Mediterranean ecosystemOIKOS, Issue 3 2003Benjamin N. Sacks In Mediterranean ecosystems, abiotic factors are known to affect vertebrate population dynamics, but little is known about how these factors affect population dynamics of parasites. We conducted a 9-year investigation of the roles of temperature, precipitation, and vector abundance as determinants of transmission of the non-native canine heartworm (Dirofilaria immitis), a dangerous parasite of pets, among coyotes (Canis latrans), an important reservoir, in north-coastal California. Dates of heartworm transmission and total annual transmission were determined, respectively, from lengths and numbers of heartworms found in known-age coyotes. Vector host-seeking activity was assessed through weekly mosquito trapping. Within years, heartworm transmission occurred only when cumulative temperatures were sufficient to allow larval heartworms to develop to the infective stage (as predicted by an existing degree-day model), and when suitable vectors were available. Most (95%) heartworms infected their hosts between 1 July and 14 September. The onset of transmission periods always occurred after the peak in vector host-seeking activity and varied annually. Transmission periods ended before temperatures became limiting due to absence of vectors. The timing of host-seeking activity of the primary vector species, Ochlerotatus sierrensis, also was correlated with the onset of warming temperatures such that parasite and vector phenology were synchronized. For this reason (partly), the variation in timing of seasonal warming had no detectable effect on total annual transmission. Abundance of host-seeking Oc. sierrensis was positively correlated with annual precipitation, and annual heartworm transmission was positively correlated with abundance of host-seeking Oc. sierrensis. Annual transmission also was positively correlated with abundance of a less numerous vector species, Anopheles punctipennis, and was directly correlated with precipitation. This study demonstrates that multiannual variability in temperature, which affects seasonality of transmission, has little effect on annual transmission, but that precipitation is a driving force determining annual transmission. These findings imply that in California, and possibly other Mediterranean climate zones, it is especially important to preventively treat pets in summers following high-rainfall winters. [source] Transmission dynamics of an iridescent virus in an experimental mosquito population: the role of host densityECOLOGICAL ENTOMOLOGY, Issue 4 2005Carlos F. Marina Abstract., 1.,The transmission of insect pathogens cannot be adequately described by direct linear functions of host and pathogen density due to heterogeneity generated from behavioural or physiological traits, or from the spatial distribution of pathogen particles. Invertebrate iridescent viruses (IIVs) can cause patent and lethal infection or a covert sub-lethal infection in insects. Aedes aegypti larvae were exposed to suspensions of IIV type 6 at two densities. High larval density increased the prevalence of aggression resulting in potentially fatal wounding. 2.,The overall prevalence of infection (patent + covert) was positively influenced by host density and increased with exposure time in both densities. The survival time of patently infected insects was extended by , 5 days compared with non-infected insects. 3.,Maximum likelihood models based on the binomial distribution were fitted to empirical results. A model incorporating heterogeneity in host susceptibility by inclusion of a pathogen-free refuge was a significantly better fit to data than an all-susceptible model, indicating that transmission is non-linear. The transmission coefficient (,) did not differ with host density whereas the faction of the population that occupied the pathogen-free refuge (,R) was significantly reduced at high host density compared with the low density treatment. 4.,The transmission of free-living infective stages of an IIV in Ae. aegypti larvae is non-linear, probably because of density-related changes in the frequency of aggressive encounters between hosts. This alters host susceptibility to infection and effectively reduces the proportion of hosts that occupy the pathogen-free refuge. [source] Seasonality and the dynamics of infectious diseasesECOLOGY LETTERS, Issue 4 2006Sonia Altizer Abstract Seasonal variations in temperature, rainfall and resource availability are ubiquitous and can exert strong pressures on population dynamics. Infectious diseases provide some of the best-studied examples of the role of seasonality in shaping population fluctuations. In this paper, we review examples from human and wildlife disease systems to illustrate the challenges inherent in understanding the mechanisms and impacts of seasonal environmental drivers. Empirical evidence points to several biologically distinct mechanisms by which seasonality can impact host,pathogen interactions, including seasonal changes in host social behaviour and contact rates, variation in encounters with infective stages in the environment, annual pulses of host births and deaths and changes in host immune defences. Mathematical models and field observations show that the strength and mechanisms of seasonality can alter the spread and persistence of infectious diseases, and that population-level responses can range from simple annual cycles to more complex multiyear fluctuations. From an applied perspective, understanding the timing and causes of seasonality offers important insights into how parasite,host systems operate, how and when parasite control measures should be applied, and how disease risks will respond to anthropogenic climate change and altered patterns of seasonality. Finally, by focusing on well-studied examples of infectious diseases, we hope to highlight general insights that are relevant to other ecological interactions. [source] Development of proliferative kidney disease in rainbow trout, Oncorhynchus mykiss (Walbaum), following short-term exposure to Tetracapsula bryosalmonae infected bryozoansJOURNAL OF FISH DISEASES, Issue 8 2002M Longshaw The initial site of infection in the fish host for Tetracapsula bryosalmonae, causative agent of proliferative kidney disease (PKD) is poorly understood. Following the recent recognition that freshwater bryozoans harbour the infective stages to salmonid fish, experimental transmission studies were undertaken to investigate (1) the route of entry of the parasite into the fish host and (2) the minimum exposure time required to induce clinical signs of PKD. In-situ hybridization (ISH) studies were carried out on naïve rainbow trout exposed to the naturally infected bryozoan Fredericella sultana for up to 90 min. The sporoplasm of T. bryosalmonae was detected entering the fish via mucous cells in the skin epithelium within the first minute of exposure. In addition, T. bryosalmonae cells were infrequently detected in the skeletal musculature of exposed experimental fish up to 72 h post-exposure. The route of migration through the fish to the kidney and spleen was not determined. All fish exposed to infected, disrupted bryozoans for 10, 30 and 90 min and maintained for up to 8 weeks developed clinical PKD. [source] Introgressive hybridization of human and rodent schistosome parasites in western KenyaMOLECULAR ECOLOGY, Issue 23 2008MICHELLE L. STEINAUER Abstract Hybridization and introgression can have important consequences for the evolution, ecology and epidemiology of pathogenic organisms. We examined the dynamics of hybridization between a trematode parasite of humans, Schistosoma mansoni, and its sister species, S. rodhaini, a rodent parasite, in a natural hybrid zone in western Kenya. Using microsatellite markers, rDNA and mtDNA, we showed that hybrids between the two species occur in nature, are fertile and produce viable offspring through backcrosses with S. mansoni. Averaged across collection sites, individuals of hybrid ancestry comprised 7.2% of all schistosomes collected, which is a large proportion given that one of the parental species, S. rodhaini, comprised only 9.1% of the specimens. No F1 individuals were collected and all hybrids represented backcrosses with S. mansoni that were of the first or successive generations. The direction of introgression appears highly asymmetric, causing unidirectional gene flow from the rodent parasite, S. rodhaini, to the human parasite, S. mansoni. Hybrid occurrence was seasonal and most hybrids were collected during the month of September over a 2-year period, a time when S. rodhaini was also abundant. We also examined the sex ratios and phenotypic differences between the hybrids and parental species, including the number of infective stages produced in the snail host and the time of day the infective stages emerge. No statistical differences were found in any of these characteristics, and most of the hybrids showed an emergence pattern similar to that of S. mansoni. One individual, however, showed a bimodal emergence pattern that was characteristic of both parental species. In conclusion, these species maintain their identity despite hybridization, although introgression may cause important alterations of the biology and epidemiology of schistosomiasis in this region. [source] | ||||||||||