AbstractsBiology & Animal Science

Environmental opportunist pathogens are a class of organisms that are able to both infect multicellular hosts and grow in the outside-host environment as free-living organisms. Environmental opportunism differs from obligate parasitism in that direct host-to-host contact is not necessary for disease transmission and that there are environmental pathogen reservoirs which in suitable conditions act as sources of infection. Because of this, environmental opportunist pathogens form a persistent threat to human health, livestock, and wildlife, and cannot be eradicated by treating hosts. Three well-known examples of pathogens of this class are Vibrio cholerae, Flavobacterium columnare, and Bacillus anthracis, all of which cause sporadic outbreaks. Between infections, these pathogens are subject to multiple biotic and abiotic environmental pressures in the outside-host environment. While environmental opportunist pathogens are not dependent on live hosts for transmission and thus benefit from increased virulence, balancing between the two environments, within-host and outside-host, might incur trade-offs and thus limitations to their spread. In this thesis I have developed mathematical models of environmental opportunist pathogen dynamics and studied the effects of environmental variation and outsidehost interactions on patterns of pathogen outbreaks. The studies included in the thesis address (i) the origin of a sigmoidal dose-dependent infectivity response, (ii) the effect of competition in the outside-host environment on opportunist pathogen outbreaks, (iii) the effect of environmental variation on environmental opportunist dynamics, and (iiii) how environmental variation enables invasions of emerging opportunist pathogen strains. The modelling approach has enabled identification of factors such as alleviation of competitive pressure and certain kinds of environmental variation as outside-host environmental factors that promote outbreaks. Additionally, modelling results can be used to suggest control strategies to reduce the probability of environmental opportunist pathogen outbreaks. Ympäristössämme elää opportunistisia taudinaiheuttajia, jotka voivat tietyissä oloissa aiheuttaa vakavia epidemioita. Tällaisia taudinaiheuttajia ovat esimerkiksi kolera (Vibrio cholerae), pernarutto (Bacillus anthracis) ja flavobakterioosi (Flavobacterium columnare). Ympäristön opportunistiset taudinaiheuttajat aiheuttavat jatkuvan uhan ihmisten ja muiden eläinten terveydelle, sekä ruoan tuotannolle. Koska nämä taudinaiheuttajat pystyvät elämään luonnossa vapaana, niitä ei voida hävittää hoitamalla tartunnan saaneita isäntiä. Toisaalta vapaana eläessään ne ovat alttiita ympäristöolojen, kuten esimerkiksi lämpötilan ja sademäärän vaihtelulle, sekä vuorovaikutuksille toisten lajien kanssa. Tässä väitöskirjatyössä esitetään uusia matemaattisia malleja, joilla voidaan kuvata ympäristössä elävien opportunististen taudinaiheuttajien vuorovaikutusta ympäristönsä sekä taudin isäntälajin kanssa. Näiden mallien pohjalta voidaan paremmin ymmärtää…