Disease spread patterns of Banana streak virus in farmers, fields in Uganda

Three surveys were conducted to establish the disease spread patterns of Banana streak virus (BSV) in farmers, fields in Uganda. Transects were traced both across the fields and from infection foci within a field. BSV incidence in adjacent quadrats was also determined to quantify statistically the spatial relationships of infected plants in the fields. Severity assessment along transects across fields revealed clusters of plants with moderate to high severity and clusters of plants with no BSV or low severity. Symptom severity decreased away from foci of infection (b=?0.014; P=0.0081). Observed frequency of infected quadrat counts differed from corresponding expected frequency of infected quadrat counts (Poisson, s distribution, x²; P<0.01). BSV– infected plants, therefore, were aggregated in well‐established fields. Aggregation of infected plants in farmers, fields and the decrease of severity away from infection foci suggest the likely involvement of a slow moving vector in BSV transmission.     

Partitioning the aggregation of parasites on hosts into intrinsic and extrinsic components via an extended Poisson-gamma mixture model

It is well known that parasites are often highly aggregated on their hosts such that relatively few individuals host the large majority of parasites. When the parasites are vectors of infectious disease, a key consequence of this aggregation can be increased disease transmission rates. The cause of this aggregation, however, is much less clear, especially for parasites such as arthropod vectors, which generally spend only a short time on their hosts. Regression-based analyses of ticks on various hosts have focused almost exclusively on identifying the intrinsic host characteristics associated with large burdens, but these efforts have had mixed results; most host traits examined have some small influence, but none are key. An alternative approach, the Poisson-gamma mixture distribution, has often been used to describe aggregated parasite distributions in a range of host/macroparasite systems, but lacks a clear mechanistic basis. Here, we extend this framework by linking it to a general model of parasite accumulation. Then, focusing on blacklegged ticks (Ixodes scapularis) on mice (Peromyscus leucopus), we fit the extended model to the best currently available larval tick burden datasets via hierarchical Bayesian methods, and use it to explore the relative contributions of intrinsic and extrinsic factors on observed tick burdens. Our results suggest that simple bad luck-inhabiting a home range with high vector density-may play a much larger role in determining parasite burdens than is currently appreciated.     

Predator modulation of plant pathogen spread through induced changes in vector development rates

1. Predation on vectors of pathogens can indirectly influence infection spread. In addition to the consumptive aspect of predation, non-consumptive, predator-induced changes in various vector traits can lead to trait-mediated indirect effects on pathogen spread, potentially operating in various directions and magnitudes. 2. A widespread non-consumptive effect of predation is the alteration of individual prey development rates. Yet, the implications of this phenomenon for the spread of vector-borne plant pathogens have not been studied. It is hypothesized that the epidemiological effects of predator-induced changes in vector development rate depend on the pattern in which the transmission biology of the vector changes along its ontogeny. 3. A general epidemiological model was developed that considers the role of predation in the infection dynamics of a plant pathogen, while incorporating vector stage structure to allow for variation in its development rate. 4. By contrasting scenarios that represent typical plant disease systems, this study confirms that the magnitude of the effect of altered development rate on infection prevalence depends on the disparity between juvenile and adult vectors in their pathogen transmission potential. 5. The model also reveals that the effect of predator-induced change in development rate can impact pathogen spread counterintuitively. Specifically, slowing down vector development can result in increased pathogen prevalence due to apparent competition between infected and uninfected vector populations. 6. More detailed, stage-specific studies of non-consumptive predator effects on vectors are likely to advance our understanding of plant disease ecology, and the development of more effective biological control practices in agriculture.     

A model for Chagas disease with controlled spraying

Chagas disease is a vector-borne parasitic disease that infects mammals, including humans, through much of Latin America. This work presents a mathematical model for the dynamics of domestic transmission in the form of four coupled nonlinear differential equations. The four equations model the number of domiciliary vectors, the number of infected domiciliary vectors, the number of infected humans, and the number of infected domestic animals. The main interest of this work lies in its study of the effects of insecticide spraying and of the recovery of vector populations with cessation of spraying. A novel aspect in the model is that yearly spraying, which is currently used to prevent transmission, is taken into account. The model's predictions for a representative village are discussed. In particular, the model predicts that if pesticide use is discontinued, the vector population and the disease can return to their pre-spraying levels in approximately 5–8 years.     

Modelling interference between vectors of non-persistently transmitted plant viruses to identify effective control strategies

Aphids are the primary vector of plant viruses. Transient aphids, which probe several plants per day, are considered to be the principal vectors of non-persistently transmitted (NPT) viruses. However, resident aphids, which can complete their life cycle on a single host and are affected by agronomic practices, can transmit NPT viruses as well. Moreover, they can interfere both directly and indirectly with transient aphids, eventually shaping plant disease dynamics. By means of an epidemiological model, originally accounting for ecological principles and agronomic practices, we explore the consequences of fertilization and irrigation, pesticide deployment and roguing of infected plants on the spread of viral diseases in crops. Our results indicate that the spread of NPT viruses can be i) both reduced or increased by fertilization and irrigation, depending on whether the interference is direct or indirect; ii) counter-intuitively increased by pesticide application and iii) reduced by roguing infected plants. We show that a better understanding of vectors’ interactions would enhance our understanding of disease transmission, supporting the development of disease management strategies.     

Dynamics of plant mosaic disease propagation and the usefulness of roguing as an alternative biological control

In this research article, an epidemiological model is formulated for mosaic disease considering plant and vector populations. Plant host population has been divided into three compartments namely healthy, latently infected and infected ones, and vector population is divided into two compartments: non-infective and infective vectors. The system possesses three equilibria: plant-only, disease-free and endemic equilibrium. Plant-only equilibrium is always unstable; disease-free equilibrium is stable when the basic reproduction number, R₀, is less than unity and unstable for when it crosses unity, and ensure existence of an endemic equilibrium which may be stable or can undergo a Hopf bifurcation. Finally, impulse periodic roguing with varied rate and time interval is adopted for cost effective and eco-friendly disease control and future direction of agriculture management. The dynamics of the impulsive system has also been analysed. Detailed numerical simulations are employed to support the analytical results. We found that roguing is most cost effective and useful management for mosaic disease eradication of plants if applied at proper rate and interval.     

Volatiles from potato plants infected with potato leafroll virus attract and arrest the virus vector, Myzus persicae (Homoptera: Aphididae)

The influence of viral disease symptoms on the behaviour of virus vectors has implications for disease epidemiology. Here we show that previously reported preferential colonization of potatoes infected by potato leafroll virus (genus Polerovirus) (luteovirus) (PLRV) by alatae of Myzus persicae, the principal aphid vector of PLRV, is influenced by volatile emissions from PLRV-infected plants. First, in our bioassays both differential immigration and emigration were involved in preferential colonization by aphids of PLRV-infected plants. Second, M. persicae apterae aggregated preferentially, on screening above leaflets of PLRV-infected potatoes as compared with leaflets from uninfected plants, or from plants infected with potato virus X (PVX) or potato virus Y (PVY). Third, the aphids aggregated preferentially on screening over leaflet models treated with volatiles collected from PLRV-infected plants as compared with those collected from uninfected plants. The specific cues eliciting the aphid responses were not determined, but differences between headspace volatiles of infected and uninfected plants suggest possible ones.     

Control measures for Chagas disease

Chagas disease, also known as American trypanosomiasis, is a potentially life-threatening illness caused by the protozoan parasite, Trypanosoma cruzi. The main mode of transmission of this disease in endemic areas is through an insect vector called triatomine bug. Triatomines become infected with T. cruzi by feeding blood of an infected person or animal. Chagas disease is considered the most important vector borne infection in Latin America. It is estimated that between 16 and 18 millions of persons are infected with T. cruzi, and at least 20,000 deaths each year. In this work we formulate a model for the transmission of this infection among humans, vectors and domestic mammals. Our main objective is to assess the effectiveness of Chagas disease control measures. For this, we do sensitivity analysis of the basic reproductive number R? and the endemic proportions with respect to epidemiological and demographic parameters.     

Temporal and Spatial Development of Papaya Ringspot in Veracruz, Mexico

The temporal progress and spatial distribution of papaya ringspot virus (PRV) and populations of aphid vectors were monitored in two papaya (Carica papaya) plantations in the state of Veracruz, México. The incidence of PRV had a typical sigmoidal curve and the logistic model was more acceptable for describing the disease progress than the Gompertz model. The rate (r^L of increase in disease incidence (0.034 and 0.023/unit/day in Plot A and B, respectively) differed (P = 0.05) between the plots; differences in number of aphids trapped could account for the difference. The initial determination of an apparent regular spatial pattern early in the epidemic, obtained with a quadrat size determined by Greig-Smith's method (n = 8 plants/quadrat) at 50 % disease incidence, was inconsistent with our biological observations in the field. An arbitrarily selected, square quadrat size (n = 9 plants/quadrat), was more consistent with visual observation for describing the spatial pattern in the field. With 9 plants/quadrat a random pattern was found. Aphid populations had a bimodal distribution at both sites with the highest population peak in December–February and a secondary peak in August–September and change in disease incidence was generally related to the aphid population level in the previous month. None of the five potential aphid vectors (Myzus persicae, Aphis gossypii A. nerii, A. citricola and Macrosiphon euphorbiae) of PRV in Mexico colonized the papaya plants, however, which may explain the absence of clustering of disease.     

A computer simulation of the epidemiology of the rice dwarf virus

An epidemiological system model was developed to evaluate the role of factors which were responsible for the prevalence of rice dwarf virus (RDV) transmitted by the green rice leafhopper, Nephotettix cincticeps. Simulation tests were conducted by varying values of the following three parameters: the vector density, the coefficient of the efficiency of feeding acquisition of RDV of the vector, and that of efficiency of RDV transmission by the vector. The effect of each parameter was assessed in terms of changes in percentages of infected insects and of infected rice hills. Both the percentages of infected insects and of infected rice hills increased rapidly with increasing vector density within a range of low vector density. The former increased linearly when the acquisitive coefficient was increased. But the percentage of infected rice hills was affected to a lesser extent. The percentage of infected insects and that of infected rice hills increased exponentially with increasing values of the transmission coefficient. The results obtained from the simulation tests were discussed in relation to the ecological factors which caused the recent prevalence of RDV.     

Clonal diversity of a malaria parasite, Plasmodium mexicanum, and its transmission success from its vertebrate-to-insect host

Infections of the lizard malaria parasite Plasmodium mexicanum are often genetically complex within their fence lizard host (Sceloporus occidentalis) harbouring two or more clones of parasite. The role of clonal diversity in transmission success was studied for P. mexicanum by feeding its sandfly vectors (Lutzomyia vexator and Lutzomyia stewarti) on experimentally infected lizards. Experimental infections consisted of one, two, three or more clones, assessed using three microsatellite markers. After 5 days, vectors were dissected to assess infection status, oocyst burden and genetic composition of the oocysts. A high proportion (92%) of sandflies became infected and carried high oocyst burdens (mean of 56 oocysts) with no influence of clonal diversity on these two measures of transmission success. Gametocytemia was positively correlated with transmission success and the more common vector (L. vexator) developed more oocysts on midguts. A high proportion (74%) of all alleles detected in the lizard blood was found in infected vectors. The relative proportion of clones within mixed infections, determined by peak heights on pherograms produced by the genetic analyser instrument, was very similar for the lizard’s blood and infections in the vectors. These results demonstrate that P. mexicanum achieves high transmission success, with most clones making the transition from vertebrate-to-insect host, and thus explains in part the high genetic diversity of the parasite among all hosts at the study site.     

The influence of plant spacing on density-dependent versus frequency-dependent spore transmission of the anther smut Microbotryum violaceum

The anther smut fungus Microbotryum violaceum is a pollinator-transmitted plant disease. As for other vector-borne diseases, frequency-dependent transmission patterns are predicted, in contrast to the density-dependent transmission of passively spread diseases. Frequency dependence will, however, only arise if vectors compensate for varying plant spacings. To test this assumption, we set up experimental populations of the host plant, Silene latifolia, with varying disease density (number of diseased plants per plot) and frequency (proportion of plants diseased), and three different plant spacings. We measured spore deposition on healthy flowers in these plots on two dates. Spore deposition decreased considerably from the first to the second census, perhaps related to the concomitant decrease in inflorescence sizes of diseased plants. At our first census, spore deposition rates varied with disease frequency, and the effect of frequency depended on plant spacing. While spore deposition was positively frequency dependent at the 1.5-m inter-plant spacing, no effect of disease frequency was found at a spacings of 0.5 m or 3 m. Nor was there an effect of disease density on spore deposition at the first census. At the later census, on the other hand, spore deposition increased almost significantly with increasing disease density (P = 0.08). This difference in deposition pattern together with a significant decrease in spore receipt indicates changes in pollinator spectrum and/or activity. The correlation of spore numbers among flowers within plants, an indication for intra-plant moves by vectors, was significant at 0.5 m and 1.5 m but not at 3 m. Floral traits and sex of individual plants influenced the number of spores they received. On the first census date, spore deposition increased with increasing inflorescence size in female but not in male plants. On the second census date, neither sex nor number of open flowers had an effect on spore receipt. None of the experimental plants became infected, however, probably because of the unusually hot and dry weather. Received: 14 May 1998 / Accepted: 19 November 1998     

Impact of disease frequency and host density on pollination and transmission of an African anther-smut fungus

The vast majority of flowering plants rely solely on insects for pollination. A number of pathogens have evolved mechanisms to exploit these close associations and use pollinators as vectors of infective propagules. Factors that affect pollinator movements and successful pollination may in turn also influence successful transmission of fungal spores. Here we investigate the effect of host density and the frequency of diseased Oxalis lanata individuals infected by the anther-smut fungus, Thecaphora capensis, on the likelihood of receiving pollen and fungal spores. Specifically, we determined the numbers of spores and pollen grains deposited on stigmatic surfaces of selected flowers under natural and standardized conditions where host density and disease frequency varied. The effect of host flower density and diseased flower frequency on pollen and spore transfer was variable under natural conditions and these factors interacted significantly. However, an increase in host density and disease frequency significantly influenced pollen and spore deposits under standardized conditions. The effect of host density was, however, not linear and an optimal flower density for pollen and fungal spore transmission was found. Similar to other systems of vector-borne disease, the transmission of anther-smut of Oxalis lanata is more frequency-dependent than density-dependent. This study represents a first step towards understanding the disease transmission process of T. capensis on Oxalis and lays the foundation for future comparative studies between this and other systems to develop and test general hypotheses of disease dynamics in vector-borne disease transmission systems.     

Pathogen effects on vegetative and floral odours mediate vector attraction and host exposure in a complex pathosystem

Pathogens can alter host phenotypes in ways that influence interactions between hosts and other organisms, including insect disease vectors. Such effects have implications for pathogen transmission, as well as host exposure to secondary pathogens, but are not well studied in natural systems, particularly for plant pathogens. Here, we report that the beetle‐transmitted bacterial pathogen Erwinia tracheiphila – which causes a fatal wilt disease – alters the foliar and floral volatile emissions of its host (wild gourd, Cucurbita pepo ssp. texana) in ways that enhance both vector recruitment to infected plants and subsequent dispersal to healthy plants. Moreover, infection by Zucchini yellow mosaic virus (ZYMV), which also occurs at our study sites, reduces floral volatile emissions in a manner that discourages beetle recruitment and therefore likely reduces the exposure of virus‐infected plants to the lethal bacterial pathogen – a finding consistent with our previous observation of dramatically reduced wilt disease incidence in ZYMV‐infected plants.     

Transmission of NPV in uniform- and mixed-age populations ofHeliothis zea [Lep: Noctuidae] on caged soybean

Transmission of a nuclear polyhedrosis virus (NPV) was examined in uniformaged and uniform- vs mixed-age populations ofHeliothis zea (Boddie) on caged soybean. Larval collections revealed viral disease outbreaks occurred in all treatments following release of infected larvae (Primary infected larvae). Transmission of NPV in uniform-aged populations was related to the density of primary infected larvae released in the population but not to the size at death of primary infected larvae (P<0.05). In mixed-age populations horizontal transmission in the oldest larvae in the population was equal to that in uniform-aged populations, providing that primary infected larvae in the mixed-aged population were all the age of the oldest noninfected cohorts. As the mixed-age population aged, transmission increased and was generally higher than that in the uniform-aged populations. Transmission was also higher when primary infected larvae were medium sized at death than when small or large at death. The concentration of virus deposited on foliage and in soil after all larvae had died on plants was related to density of primary infected larvae released. In soil, but not on foliage, the virus concentration was related to the size at death of primary infected larvae released. This material is based upon work supported in part by the U.S. Department of Agriculture under Agreement No. CRSR-2-1000.     

Herbivore arthropods benefit from vectoring plant viruses

Plants infected with pathogens often attract the pathogens’ vectors, but it is not clear if this is advantageous to the vectors. We therefore quantified the direct and indirect (through the host plant) effects of a pathogen on its vector. A positive direct effect of the plant‐pathogenic Tomato spotted wilt virus on its thrips vector (Frankliniella occidentalis) was found, but the main effect was indirect; juvenile survival and developmental rate of thrips was lower on pepper plants that were damaged by virus‐free thrips than on unattacked plants, but such negative effects were absent on plants that were damaged and inoculated by infected thrips or were mechanically inoculated with the virus. Hence, potential vectors benefit from attacking plants with virus because virus‐infected plants are of higher quality for the vector's offspring. We propose that plant pathogens in general have evolved mechanisms to overcome plant defences against their vectors, thus promoting pathogen spread.     

Non-independence of individuals in a population of Drosophila melanogaster: effects on spatial distribution and dispersal

We used a simple quadrat analysis to globally categorise the spatial distribution of Drosophila melanogaster individuals. Individuals were spatially aggregated and this was not only due to sexual attraction. This aggregation seemed to be maintained during dispersal, hence explaining the great variability of this behaviour observed in our work and by other authors. Thus, individuals are expected to arrive as small groups into a new patch. This may induce costs through competition and benefits by ensuring the presence of reproductive mates and/or if there is a phenomenon of local resource enhancement by the presence of conspecifics, hence influencing the evolution of dispersal.     

Empirical evidence for key hosts in persistence of a tick-borne disease

An important epidemiological consequence of aggregated host-parasite associations occurs when parasites are vectors of pathogens. Those hosts that attract many vectors will tend to be the focus of transmission. But to what extent, and can we identify characteristics of these key hosts? We investigated these questions with respect to the host-tick relationship of the yellow-necked mouse, Apodemus flavicollis, a critical host in the maintenance of the zoonotic disease, tick-borne encephalitis. Transmission of the virus occurs when ticks feed in a 'co-feeding' aggregation. Thus, the number and frequency of co-feeding groups provides an estimate of the potential rate of virus transmission. We recorded the spatio-temporal variations in co-feeding on a population of rodents in conjunction with recording individual host characteristics. Using Lorenz curves, we revealed conformation of tick-borne encephalitis transmission potential to the 20/80 Rule, where the 20% of hosts most infested with ticks were accountable for 80% of transmission potential. Hosts in the transmission cohort were identified as the sexually mature males of high body mass. Therefore control efforts targeted at this group would substantially reduce transmission potential compared to non-targeted control of the population, which resulted in a linear reduction in transmission potential. Focusing on the 'wrong' functional group would have little impact upon transmission potential until a considerable proportion of the population had been subject to control. However, individuals can change their functional status over time making it difficult to predict the contribution of these individuals to future transmission.