Modeling susceptible infective recovered dynamics and plague persistence in California rodent-flea communities

CRITICAL CONTACT RATE FOR VECTOR–HOST–PATHOGEN OSCILLATION INVOLVING CO-FEEDING AND DIAPAUSE

We consider the dynamic vector–host–pathogen interaction motivated by tick-borne diseases such as tick-borne encephalitis and Lyme disease. We stratify the vector population in terms of the stage before and after the vector’s contact with hosts when co-feeding transmission may take place, and we also consider the case where vector development may involve two time lags due to normal development and diapause. We derive threshold conditions for disease persistence and for nonlinear oscillations in the vector population and in the diseased vector and host populations. Our objective here is to use a simple mechanistic dynamic model to show that diapause and co-feeding transmission may generate periodic and irregular oscillations even when seasonal variations of the environmental conditions are ignored. These oscillations are not necessary in synchrony with the seasonality of vector development, and hence complicated oscillatory patterns of vector-borne disease dynamics in the field and surveillance observations should be expected.

Ectoparasites of Microtus californicus and Possible Emergence of an Exotic Ixodes Species Tick in California

California voles (Microtus californicus Peale) harbor fleas and ticks, may be infected with vector-borne pathogens, and could themselves suffer from disease and serve as a source of infection for people and other animals. Here we summarize publications, museum archives, and recent records of ticks and fleas from California voles. There have been 18 flea species reported on California voles with geographic locations reported for 13. During recent statewide surveys, we found six flea species, with the highest species richness in Humboldt County. We found three of five previously reported tick species as well as a tick resembling the eastern North American tick Ixodes minor Neumann (which we here designate Ixodes "Mojave morphotype") on isolated Amargosa voles and Owens Valley voles (Microtus californicus vallicola Bailey) in Inyo County in 2012 and 2014. Additional incidental observations of this Mojave morphotype tick were on a western harvest mouse (Reithrodontomys megalotis Baird) at the Mojave site and a montane vole (Microtus montanus Peale) in the Owens Valley, both in March, 2014. We cannot rule out that this tick species has been present in remote areas of California but gone unrecognized, but these data are consistent with recent introduction of this tick, possibly from migrating birds. Changes in the ectoparasite fauna suggest changing ecologies of vectors and vector-borne pathogens that could influence animals and people as well.

We are connected: flea–host association networks in the plague outbreak focus in the Rift Valley, northern Tanzania

Context Plague is a serious health problem in northern Tanzania, with outbreaks since 2008 in two districts located in Rift Valley. There is dearth of knowledge on diversity of small mammal and flea fauna occurring in this plague focus. Knowledge on interactions between fleas and rodent species that harbour the plague bacterium, Yersinia pestis, is important for developing strategies for control and prevention of plague. Aims This study aims to show how rodents and fleas are associated with each other in the plague focus. Methods Animals were trapped bimonthly from 2009 to 2012 in different habitats. The fur of animals was brushed to collect fleas, which were identified and quantified. Network analysis methods, randomisation and rarefaction curves were used to show how hosts and fleas are associated. Key results Thirteen species of rodents were associated with 26 species of fleas of which Dinopsyllus lypusus, Xenopsylla brasiliensis and X. cheopis are confirmed efficient vectors of Y. pestis. Randomisation and rarefaction curves established that Lophuromys flavopunctatus had significantly higher flea species richness (n = 9) than did all other hosts, whereas Xenopsylla cheopis and Dinopsyllus spp. showed greater host species richness than did other species of fleas. There was no significant correlation between host sex and flea abundance (χ2 = 0.8, d.f. = 6, P = 0.371), but significant differences between reproductive states (adults had more fleas than did subadults) were observed, which probably reflected typical positive correlation between size and flea abundance (χ2 = 4.1955, d.f. = 1, P = 0.040). Conclusions The plague outbreak focus in northern Tanzania has a diverse fauna of rodents and fleas with multiple patterns of association and connectivity. Implications Existence of diverse populations of rodents associated with a large number of flea species, some of which are efficient plague vectors, increases the potential for persistence and transmission of plague to humans in northern Tanzania.

Flea (Siphonaptera) species richness in the Great Basin Desert and island biogeography theory

Numbers of flea (Siphonaptera) species (flea species richness) on individual mammals should be higher on large mammals, mammals with dense populations, and mammals with large geographic ranges, if mammals are islands for fleas. I tested the first two predictions with regressions of H. J. Egoscue's trapping data on flea species richness collected from individual mammals against mammal size and population density from the literature. Mammal size and population density did not correlate with flea species richness. Mammal geographic range did, in earlier studies. The intermediate-sized (31 g), moderately dense (0.004 individuals/m(2)) Peromyscus truei (Shufeldt) had the highest richness with eight flea species on one individual. Overall, island biogeography theory does not describe the distribution of flea species on mammals in the Great Basin Desert, based on H. J. Egoscue's collections. Alternatively, epidemiological or metapopulation theories may explain flea species richness.

Host resistance, population structure and the long-term persistence of bubonic plague: contributions of a modelling approach in the Malagasy focus

Although bubonic plague is an endemic zoonosis in many countries around the world, the factors responsible for the persistence of this highly virulent disease remain poorly known. Classically, the endemic persistence of plague is suspected to be due to the coexistence of plague resistant and plague susceptible rodents in natural foci, and/or to a metapopulation structure of reservoirs. Here, we test separately the effect of each of these factors on the long-term persistence of plague. We analyse the dynamics and equilibria of a model of plague propagation, consistent with plague ecology in Madagascar, a major focus where this disease is endemic since the 1920s in central highlands. By combining deterministic and stochastic analyses of this model, and including sensitivity analyses, we show that (i) endemicity is favoured by intermediate host population sizes, (ii) in large host populations, the presence of resistant rats is sufficient to explain long-term persistence of plague, and (iii) the metapopulation structure of susceptible host populations alone can also account for plague endemicity, thanks to both subdivision and the subsequent reduction in the size of subpopulations, and extinction-recolonization dynamics of the disease. In the light of these results, we suggest scenarios to explain the localized presence of plague in Madagascar.

Bubonic plague, known to have marked human history by three deadly pandemics, is an infectious disease which mainly circulates in wild rodent populations and is transmitted by fleas. Although this disease can be quickly lethal to its host, it has persisted on long-term in many rodent populations around the world. The reasons for this persistence remain poorly known. Two mechanisms have been invoked, but not yet explicitly and independently tested: first, the spatial structure of rodent populations (subdivision into several subpopulations) and secondly, the presence of, not only plague-susceptible rodents, but also plague-resistant ones. To gain insight into the role of the above two factors in plague persistence, we analysed a mathematical model of plague propagation. We applied our analyses to the case of Madagascar, where plague has persisted on central highlands since the 1920s and is responsible for about 30% of the human cases worldwide. We found that the long-term persistence of plague can be explained by the presence of any of the above two factors. These results allowed us to propose scenarios to explain the localized presence of plague in the Malagasy highlands, and help understand the persistence of plague in many wild foci.

Emergence, spread, persistence and fade-out of sylvatic plague in Kazakhstan

Predicting the dynamics of zoonoses in wildlife is important not only for prevention of transmission to humans, but also for improving the general understanding of epidemiological processes. A large dataset on sylvatic plague in the Pre-Balkhash area of Kazakhstan (collected for surveillance purposes) provides a rare opportunity for detailed statistical modelling of an infectious disease. Previous work using these data has revealed a host abundance threshold for epizootics, and climatic influences on plague prevalence. Here, we present a model describing the local space-time dynamics of the disease at a spatial scale of 20 × 20 km(2) and a biannual temporal scale, distinguishing between invasion and persistence events. We used a Bayesian imputation method to account for uncertainties resulting from poor data in explanatory variables and response variables. Spatial autocorrelation in the data was accounted for in imputations and analyses through random effects. The results show (i) a clear effect of spatial transmission, (ii) a high probability of persistence compared with invasion, and (iii) a stronger influence of rodent abundance on invasion than on persistence. In particular, there was a substantial probability of persistence also at low host abundance.

Arthropod vectors and vector-borne bacterial pathogens in Yosemite National Park

Ticks, fleas, and vector-borne pathogens were surveyed in diverse small mammals in Yosemite National Park, California, from 2005 to 2007. A total of 450 unique captures of small mammals was collected during a 3-yr period and yielded 16 species of fleas and 10 species of ticks, including known vectors of Anaplasma phagocytophilum and Borrelia burgdorferi and plague. Serology was performed for A. phagocytophilum, spotted fever group Rickettsia spp., B. burgdorferi, and Yersinia pestis. A. phagocytophilum exposure was identified in 12.1% of all wild small mammals tested, with seropositive animals in 10 species, notably Belding's ground squirrels (Spermophilus beldingi), jumping mice (Zapus princeps), and voles (Microtus sp.). Spotted fever group Rickettsia spp. exposure was detected in 13.9% of all small mammals tested, with seropositive animals in eight species. Additionally, 37.0% of rodents in five species tested were seropositive for B. burgdorferi. No individuals were seropositive for Y. pestis. No animals were polymerase chain reaction positive for any pathogen tested. These results provide baseline data for future research and prediction of emerging vector-borne disease in Yosemite National Park, as well as adding to the known ranges and host species for tick and fleas in California.