The effect of transmission route on plant virus epidemic development and disease control

Report of natural Mayaro virus infection in Mansonia humeralis (Dyar & Knab, Diptera: Culicidae)

BACKGROUND

Arboviruses are a group of viruses transmitted to vertebrate hosts by certain blood-feeding arthropods. Among urban vectors of arboviruses, mosquitoes of the genus Aedes are the most common. However, other mosquitoes may be susceptible to infection and involved in the transmission, such as Mansonia spp. Therefore, this study aimed to investigate whether Mansonia humeralis can be infected with the Mayaro virus (MAYV).

METHODS

These insects were collected from 2018 to 2020 in chicken coops of rural communities in Jaci Paraná in Porto Velho, Rondônia, Brazil, while performing blood-feeding on roosters. The mosquitoes were randomly grouped in pools from which the head and thorax were macerated and checked for the presence of MAYV by quantitative reverse transcription polymerase chain reaction (RT-qPCR). The positive pools were used to infect the C6/36 cell line, and on different days post-infection, the supernatant of the infected cells was subjected to viral detection by RT-qPCR.

RESULTS

A total of 183 pools of female mosquitoes were tested, of which 18% were positive for MAYV; some samples from insect pools inoculated into C6/36 cells showed in vitro multiplication capacity between 3 and 7 days post-infection.

CONCLUSIONS

This is the first report of Ma. humeralis mosquitoes that are naturally infected by MAYV, indicating that these vectors may be potential transmitting agents of this arbovirus.

Dispersion pattern of Mansonia in the surroundings of the Amazon Jirau Hydroelectric Power Plant

Mansonia spp. are voracious hematophagous mosquitoes whose mature stages usually breed in freshwater bodies containing aquatic vegetation. The reduction in water flow leads to a proliferation in aquatic plants, increasing their populations. Besides, some species are potential vectors of pathogens such as arboviruses and microfilariae. We evaluated the degree of active dispersion of females of Mansonia spp. in the surrounding area of the Jirau hydroelectric power plant in the Amazon, Rondônia, Brazil, using mark-release-recapture techniques. The flight behavior of the recaptured specimens was summarized with a set of average and maximum distances traveled. We show that the dispersal movement of Mansonia spp. is predominantly performed by random, low, and short flights, with a tendency to remain near the breeding sites in certain vegetation fragments. However, the maximum distances traveled were 2000 m from the release point for Mansonia amazonensis during 2018 and Mansonia humeralis during 2019.

The Epidemiology of Plant Virus Disease: Towards a New Synthesis

Epidemiology is the science of how disease develops in populations, with applications in human, animal and plant diseases. For plant diseases, epidemiology has developed as a quantitative science with the aims of describing, understanding and predicting epidemics, and intervening to mitigate their consequences in plant populations. Although the central focus of epidemiology is at the population level, it is often necessary to recognise the system hierarchies present by scaling down to the individual plant/cellular level and scaling up to the community/landscape level. This is particularly important for diseases caused by plant viruses, which in most cases are transmitted by arthropod vectors. This leads to range of virus-plant, virus-vector and vector-plant interactions giving a distinctive character to plant virus epidemiology (whilst recognising that some fungal, oomycete and bacterial pathogens are also vector-borne). These interactions have epidemiological, ecological and evolutionary consequences with implications for agronomic practices, pest and disease management, host resistance deployment, and the health of wild plant communities. Over the last two decades, there have been attempts to bring together these differing standpoints into a new synthesis, although this is more apparent for evolutionary and ecological approaches, perhaps reflecting the greater emphasis on shorter often annual time scales in epidemiological studies. It is argued here that incorporating an epidemiological perspective, specifically quantitative, into this developing synthesis will lead to new directions in plant virus research and disease management. This synthesis can serve to further consolidate and transform epidemiology as a key element in plant virus research.

Larvicidal toxicity of Metarhizium anisopliae metabolites against three mosquito species and non-targeting organisms

Background

The fungal toxin acts as effective, low-cost chemical substances for pest control worldwide and also an alternative to synthetic insecticides. This study assessed the larvicidal potential of Metarhizium anisopliae fungi derived metabolites against Aedes aegypti, Anopheles stephensi, Culex quinquefasciatus and non-targeted organisms at 24hr post treatment.

Method

Isolation of entomopathogenic fungi M. anisopliae from natural traps confirmed by using 18s rDNA biotechnological tools. Crude extracts from M. anisopliae solvent extraction and their secondary metabolites were bio-assayed following WHO standard procedures against Ae. aegypti, An. stephensi and Cx. quinquefasciatus, Artemia nauplii, Eudrilus eugeniae, and Solanum lycopersicum after 24 hr exposure. Histopathological analysis of E. eugeniae treated with fungi metabolites toxicity compared to those treated with Monocrotophos after 24hrpost-treatment. M. anisopliae metabolites were characterized using GC-MS and FT-IR analysis.

Results

The larvicidal activity was recorded in highest concentration of 75μg/ml, with 85%, 97% and 89% mortality in Ae. aegypti, An. stephensi and Cx. quinquefasciatus respectively. M. anisopliae metabolites produced LC₅₀ values in Ae. aegypti, 59.83μg/ml, in An. stephensi, 50.16μg/ml and in Cx. quinquefasciatus, 51.15μg/ml respectively. M. anisopliae metabolites produced lower toxic effects on A. nauplii, LC₅₀ values were, 54.96μg/ml respectively. Bio-indicator toxicity results show 18% and 58% mortality was recorded in E. eugeniae and A. nauplii and also there is no phytotoxicity that was observed on S. lycopersicum L. under semi-field condition. E. eugeniae histopathological studies shows fungal metabolites showed lower sub-lethal effects compared to synthetic chemical pesticide at 24hrs of the treatment. The GC-MS and FT-IR analysis identified five major components of active ingredients.

Conclusion

Findings of this study indicate that, M. anisopliae ethyl acetate derived secondary metabolites are effective against larvae of Ae. aegypti, An. stephensi and Cx. quinquefasciatus mosquito species, lower toxicity effects were observed on non-target organisms such as, Artemia nauplii, Eudrilus eugeniae as well as, no toxicity effect were observed on Solanum lycopersicum. Further research should be conducted in laboratory for separation of single pure molecule and be tested semifield conditions.

Plant Virus Epidemiology: Applications and Prospects for Mathematical Modeling and Analysis to Improve Understanding and Disease Control

In recent years, mathematical modeling has increasingly been used to complement experimental and observational studies of biological phenomena across different levels of organization. In this article, we consider the contribution of mathematical models developed using a wide range of techniques and uses to the study of plant virus disease epidemics. Our emphasis is on the extent to which models have contributed to answering biological questions and indeed raised questions related to the epidemiology and ecology of plant viruses and the diseases caused. In some cases, models have led to direct applications in disease control, but arguably their impact is better judged through their influence in guiding research direction and improving understanding across the characteristic spatiotemporal scales of plant virus epidemics. We restrict this article to plant virus diseases for reasons of length and to maintain focus even though we recognize that modeling has played a major and perhaps greater part in the epidemiology of other plant pathogen taxa, including vector-borne bacteria and phytoplasmas.

Modeling Virus Coinfection to Inform Management of Maize Lethal Necrosis in Kenya

Maize lethal necrosis (MLN) has emerged as a serious threat to food security in sub-Saharan Africa. MLN is caused by coinfection with two viruses, Maize chlorotic mottle virus and a potyvirus, often Sugarcane mosaic virus. To better understand the dynamics of MLN and to provide insight into disease management, we modeled the spread of the viruses causing MLN within and between growing seasons. The model allows for transmission via vectors, soil, and seed, as well as exogenous sources of infection. Following model parameterization, we predict how management affects disease prevalence and crop performance over multiple seasons. Resource-rich farmers with large holdings can achieve good control by combining clean seed and insect control. However, crop rotation is often required to effect full control. Resource-poor farmers with smaller holdings must rely on rotation and roguing, and achieve more limited control. For both types of farmer, unless management is synchronized over large areas, exogenous sources of infection can thwart control. As well as providing practical guidance, our modeling framework is potentially informative for other cropping systems in which coinfection has devastating effects. Our work also emphasizes how mathematical modeling can inform management of an emerging disease even when epidemiological information remains scanty. [Formula: see text] Copyright © 2017 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Modeling the effects of cross-protection control in plant disease with seasonality

Cross-protection in plants has been widely used to control losses caused by virus diseases in the world. Here, a non-autonomous plant-virus disease model was developed including cross-protection. Global dynamics of the model was discussed. Under the quite weak assumptions, integral form conditions were resolved for permanence of the system and extinction of diseases. Furthermore, we looked into the sufficient conditions that plants could be protected against the detrimental effects of infection by an infection with the mild virus isolates. Last, we performed numerical simulations. Our investigations suggested that cross-protection played an important role in controlling the spread of the challenging virus in plants.

Predictive Models for Tomato Spotted Wilt Virus Spread Dynamics, Considering Frankliniella occidentalis Specific Life Processes as Influenced by the Virus

Several models have been studied on predictive epidemics of arthropod vectored plant viruses in an attempt to bring understanding to the complex but specific relationship between the three cornered pathosystem (virus, vector and host plant), as well as their interactions with the environment. A large body of studies mainly focuses on weather based models as management tool for monitoring pests and diseases, with very few incorporating the contribution of vector's life processes in the disease dynamics, which is an essential aspect when mitigating virus incidences in a crop stand. In this study, we hypothesized that the multiplication and spread of tomato spotted wilt virus (TSWV) in a crop stand is strongly related to its influences on Frankliniella occidentalis preferential behavior and life expectancy. Model dynamics of important aspects in disease development within TSWV-F. occidentalis-host plant interactions were developed, focusing on F. occidentalis' life processes as influenced by TSWV. The results show that the influence of TSWV on F. occidentalis preferential behaviour leads to an estimated increase in relative acquisition rate of the virus, and up to 33% increase in transmission rate to healthy plants. Also, increased life expectancy; which relates to improved fitness, is dependent on the virus induced preferential behaviour, consequently promoting multiplication and spread of the virus in a crop stand. The development of vector-based models could further help in elucidating the role of tri-trophic interactions in agricultural disease systems. Use of the model to examine the components of the disease process could also boost our understanding on how specific epidemiological characteristics interact to cause diseases in crops. With this level of understanding we can efficiently develop more precise control strategies for the virus and the vector.

Disentangling Effects of Vector Birth Rate, Mortality Rate, and Abundance on Spread of Plant Pathogens

Models on the spread of insect-transmitted plant pathogens often fix vector population size by assuming that deaths are offset by births. Although such mathematical simplifications are often justified, deemphasizing parameters that govern vector population size is problematic, as reproductive biology and mortality schedules of vectors of plant pathogens receive little empirical attention. Here, the importance of explicitly including parameters for vector birth and death rates was evaluated by comparing results from models with fixed vector population size with models with logistic vector population growth. In fixed vector population size models, increasing vector mortality decreased percentage of inoculative vectors, but had no effect on vector population size, as deaths were offset by births. In models with logistic vector population growth, increasing vector mortality decreased percentage of inoculative vectors and decreased vector population size. Consequently, vector mortality had a greater effect on pathogen spread in models with logistic vector population growth than in models with fixed vector population size. Further, in models with logistic vector population growth, magnitude of vector birth rate determined time required for vector populations to reach large size, thereby determining when pathogen spread occurred quickly. Assumptions regarding timing of vector mortality within a time step also affected model outcome. A greater emphasis of vector entomologists on studying reproductive biology and mortality schedules of insect species that transmit plant pathogens will facilitate identification of conditions associated with rapid growth of vector populations and could lead to development of novel control strategies.

Modeling huanglongbing transmission within a citrus tree

The citrus disease huanglongbing (HLB), associated with an uncultured bacterial pathogen, is threatening the citrus industry worldwide. A mathematical model of the transmission of HLB between its psyllid vector and citrus host has been developed to characterize the dynamics of the vector and disease development, focusing on the spread of the pathogen from flush to flush (a newly developing cluster of very young leaves on the expanding terminal end of a shoot) within a tree. This approach differs from that of prior models for vector-transmitted plant diseases where the entire plant is the unit of analysis. Dynamics of vector and host populations are simulated realistically as the flush population approaches complete infection. Model analysis indicates that vector activity is essential for initial infection but is not necessary for continued infection because infection can occur from flush to flush through internal movement in the tree. Flush production, within-tree spread, and latent period are the most important parameters influencing HLB development. The model shows that the effect of spraying of psyllids depends on time of initial spraying, frequency, and efficacy of the insecticides. Similarly, effects of removal of symptomatic flush depend on the frequency of removal and the time of initiation of this practice since the start of the epidemic. Within-tree resistance to spread, possibly affected by inherent or induced resistance, is a major factor affecting epidemic development, supporting the notion that alternate routes of transmission besides that by the vector can be important for epidemic development.