Host Diversity and Multi-Host Pathogens PDF

Host community and ecology of multi-host pathogens  Host community and multi-host pathogens Multi-host pathogens infect multiple hosts Hosts differ in their reservoir competence for multi-host pathogens Structure of host community will influence epidemiology of m...

Host community and ecology of multi-host pathogens  Host community and multi-host pathogens Multi-host pathogens infect multiple hosts Hosts differ in their reservoir competence for multi-host pathogens Structure of host community will influence epidemiology of multi-host pathogens. Example of host community structure and the risk of Lyme disease Dilution hypothesis states that host biodiversity dilutes risk of infectious disease. Amplification hypothesis states that host biodiversity increases risk of infectious disease Feeding preferences of arthropod vectors can influence epidemiology of vector-borne diseases. Example of Culex mosquitoes their avian hosts and the epidemiology of West Nile virus (WNV) Control strategies of multi-host pathogens. Example of culling or vaccinating badgers to control incidence of bovine tuberculosis in the UK  Reservoir competence of hosts Reservoir competence – ability of animal species to be a host for the pathogen. From the perspective of the host rather than pathogen. Reservoir competence refers to all the host traits that are important for the life cycle of the pathogen (susceptibility, pathogen abundance in tissues, clearance, transmission) For any multi-host pathogen, hosts differ in their reservoir competence Host reservoir competence and host abundance/density are the key host features that will determine the host’s contribution to the R 0 of the pathogen The composition of the host community will therefore affect the prevalence and incidence of multi-host pathogens Example of host community and risk of Lyme disease  Blacklegged ticks and Borrelia burgdorferi are generalists Borrelia burgdorferi sensu stricto (Bbss) is a tick-borne bacterium that causes Lyme disease in dogs and humans Ixodes scapularis Peromyscus leucopus Blacklegged tick (I. scapularis) is vector. Blood-seeking stages include larva, nymph, and adult female ticks Bbss is transmitted between vertebrate hosts by ticks during blood meal Bbss is a multi-host pathogen; found in many vertebrate hosts I. scapularis is a generalist tick; feeds on Borrelia burgdorferi Turdus migratorius dozens of vertebrate hosts  Nymphal infection prevalence and Lyme disease risk Risk of Lyme disease depends on density of infected nymphal ticks (DIN) DIN is density of nymphs (DON) * nymphal infection prevalence (NIP) NIP is % of nymphs infected with Bbss Uninfected nymphs (orange) versus infected nymphs (green) NIP is 50% (5/10) in left panel and 80% (8/10) in right panel Risk of Lyme disease higher in right panel (NIP = 80%) versus left panel (NIP = 50%) Assumption: density of nymphs (DON) is same in two panels  Variation in reservoir competence for Bbss among hosts Vertebrate species vary in the factors that contribute to R0 of Bbss Factors: body burden, moulting percentage, reservoir competence, and host density Host tick burden: Number of ticks fed per host is higher for larger than smaller hosts. Host tick burden (body burden) = number of ticks per host Deer and white-footed mice carry 239 versus Moulting percentage = % of engorged larvae that moulted into nymphs 27.8 ticks Reservoir competence = susceptibility, duration of infection, and Host reservoir competence: Percentage of transmission of Bbss from infected hosts to feeding ticks Bbss-infected ticks produced by host is higher Host density = number of individuals per hectare for white-footed mouse (92.1%) than deer (4.6%) LoGiudice. 2003. PNAS 100:567-571. Host density: Density of deer (0.25/ha) is lower than white-footed mouse (100/ha)  Host community and risk High biodiversity Low biodiversity of Lyme disease Community with high biodiversity (left): white-footed mice, possums, and deer Community with low biodiversity (right): only contains white-footed mice White-footed mice are good reservoir hosts for Bbss. Possums and deer are poor reservoir hosts for Bbss Assumption: Density of infected nymphs (DIN) is the same in two communities Question: What is the relationship between NIP and biodiversity in host Time = 8:40 AM community?  NIP versus diversity of host community Calculate NIP for different host communities (panels) and density of mice (X-axis) Add other host species and re-calculate NIP Panel A: simplest community; only mice Panel B: add chipmunks and deer Panel C: add possums and carnivores Panel F: NIP observed in I. scapularis nymphs at study site in New York state NIP is higher in communities with low LoGiudice. 2003. PNAS 100:567-571. biodiversity that are dominated by white- footed mice  The dilution effect hypothesis suggests that there is a negative relationship between (human) disease risk and host diversity. That is, high host diversity “dilutes” disease risk.  Forest fragmentation, biodiversity, and Lyme disease risk Forest fragmentation is common in areas where Lyme disease is endemic White-footed mouse is generalist species that thrives in fragmented landscapes Density of white-footed mouse was highest in small forest fragments Question: Why is density of white-footed mice higher in intact forest habitat? Question: What are the relationships between forest fragmentation, density of white-footed mice, biodiversity, and risk of Lyme disease?  NIP and forest habitat fragmentation Fragmented forest habitats are good habitats for white-footed mice White-footed mice are good hosts for I. scapularis ticks and B. burgdorferi Predict that nymphal infection prevalence (NIP) will decrease with area of forest patch Highest NIP in highly fragmented habitats with a small patch area Density of infected nymphs (DIN) also had a negative relationship with patch area Area (ha)  Important assumptions of the dilution hypothesis Vector (tick) and pathogen (Bbss) are both generalists (multi-host parasite and multi-host pathogen) Vertebrate hosts vary in their reservoir competence for the pathogen Competent hosts achieves higher relative abundance in species-poor habitat Conversely, increasing biodiversity favours noncompetent hosts No increase in tick density with the addition of noncompetent hosts  Conclusions on biodiversity and Lyme disease risk Vertebrate host species differ in reservoir competence for B. burgdorferi White-footed mice are highly competent reservoir hosts for B. burgdorferi Highly fragmented forests with low biodiversity and high density of white- footed mice have higher NIP than intact forest habitats with high biodiversity Conservation of habitat and biodiversity can reduce risk of Lyme disease Dilution hypothesis: preserving biodiversity will reduce zoonotic risk!  Amplification hypothesis Time = 8:50 AM Amplification hypothesis states that biodiversity increases risk of infectious disease Leishmaniasis is a VBD caused by protozoan parasite (Leishmania) and transmitted by sand flies In Central America, cutaneous leishmaniasis is called, “chiclero’s ulcer.” Cichleros collect chicle, a latex produced by sapodilla trees Humans who live and work in intact rain forests are more likely to suffer from leishmaniasis Positive association between biodiversity and risk of leishmaniasis supports the amplification hypothesis Wood. 2014. Ecology 95:817-832.  Effects of biodiversity on 70 common human parasites Effects of biodiversity on 70 human parasites Wedges indicate frequency of positive, negative, neutral, and unknown responses of infectious disease to biodiversity “Amplification effect” = biodiversity increases disease risk for humans (blue wedge; e.g., chiclero’s ulcer) Biodiversity has no effect on disease risk for humans (yellow wedge) “Dilution effect” = biodiversity will reduce the disease risk (red wedge; e.g., Lyme disease) Wood. 2014. Ecology 95:817-832.  Host heterogeneity influences disease prevalence Host heterogeneity matters for the epidemiology of multi-host pathogens Host heterogeneity factors: host abundance, reservoir competence, body burden of arthropod vectors Abundance: Some host species are more abundant than others Reservoir competence: Some host species have higher viremia and/or longer duration of infectious period Body burden: Larger hosts feed more vectors than smaller hosts Vector preferences: arthropod vectors prefer some hosts over others  West Nile virus and reservoir hosts West Nile virus (WNV) cycles between Culex mosquitoes and birds Birds are reservoir hosts because high viremia facilitates virus transmission to feeding mosquitoes Mammals are incompetent or dead-end hosts because they have low viremia and no transmission of virus to mosquitoes feeding mosquitoes Bird species differ in viremia, transmission to mosquitoes and hence reservoir competence  Viremia profiles for 10 orders of birds Study compared viremia profiles of birds from 10 different orders Birds experimentally infected with WNV and blood sampled for viremia Songbirds (Passeriformes) and shore birds (Charadriiformes) had highest viremia and of longest duration Parrots (Psittaciformes) and fowl (Galliformes) had lowest viremias and of shortest duration Birds with high viremias of long duration are most competent reservoir hosts for WNV  Variation in WNV competence among bird species Reservoir competence requires acquisition of WNV by host and transmission to mosquitoes RC ~ susceptibility (s), infectiousness to mosquitoes (i), and duration of viremia (d) Compare reservoir competence for WNV among different bird species Reservoir competence of bird species i (Ci) calculated as Ci = s*i*d Most important reservoir hosts for WNV are blue jay, common grackle, house finch, American crow, and house sparrow  Do mosquitoes prefer some bird species? Lab estimates of intensity and duration of WNV viremia in different bird species are not the whole story WNV is transmitted among bird species by mosquitoes belonging to genus Culex Blue jays are highly competent reservoir hosts, but what if mosquitoes don’t like to bite blue jays? Culex mosquitoes prefer some bird species, and this affects WNV epidemiology Do feeding preferences of vector exist and do they matter?  Question: How could you test whether mosquitoes have preferences for different bird species and whether those preferences influence the risk of infection with West Nile virus? Time = 9:00 AM  Null hypothesis of mosquito feeding preferences for birds Avian reservoir hosts for WNV differ in abundance in any ecosystem Null hypothesis: If mosquitoes have no feeding preferences, they should feed on birds in proportion to their abundance Example: Bird community consists of two species with 80% species A and 20% species B For mosquito blood meals, 80% and 20% should come from bird species A and B, respectively  Mosquito feeding preferences in birds Study on mosquito feeding preferences with respect to birds in Maryland and Washington DC Null hypothesis requires knowledge of relative abundance of bird species Study sampled birds at 5 different sites (5 columns in graph) Columns show relative abundance of different bird species at 5 sites Most common bird species were house sparrow (blue) > rock dove (white) > European starling (purple) Kilpatrick (2006). P Roy Soc B-Biol Sci 273: 2327-2333.  Blood meals in Culex mosquitoes Sampled Culex mosquitoes from 5 sites Determine bird species on which these mosquitoes had fed (PCR) Mosquitoes do not feed on birds in proportion to their abundance Mosquito blood meal analysis: most common bird species are robins; house sparrows much less important Mosquitoes had preference for robins (16.7x) and aversion for house sparrows (7.9x) Kilpatrick (2006). P Roy Soc B-Biol Sci 273: 2327-2333.  Contribution of bird species to risk of WNV Estimate the fraction of WNV-infected mosquitoes produced by each bird species Multiply relative abundance of each bird species, mosquito feeding preference, and reservoir competence of bird host Robins are