Increased Biting Rate and Decreased Wolbachia Density in Irradiated Aedes Mosquitoes

Background: Releasing considerable numbers of radiation-sterilized males is a promising strategy to suppress mosquito vectors. However, releases may also include small percentages of biting females which translate to large numbers when releases are large.Currently, the effects of irradiation on the host-seeking and host-biting behaviors have not been exhaustively investigated. Information is also lacking regarding the effects of sterilizing treatment on the endosymbiotic bacterium Wolbachia, which is known to affect the vector competence of infected mosquitos.Methods: To ascertain the effects of irradiation on females, the pupae of two Aedes albopictus strains, differing in their natural or artificial Wolbachia infection type, and Ae. aegypti—which is not infected by Wolbachia—were treated with various doses of X-rays and monitored for key fitness parameters and biting behavior over a period of two weeks. The effect of radiation on Wolbachia was investigated by qPCR and FISH analysis.Results: Partial Ae. albopictus female sterility was achieved at 28 Gy but the number of weekly bites more than doubled compared to that of the controls. Radiation doses of 35 and 45 Gy completely inhibited progeny production but did not significantly affect the survival or flight ability of Ae. albopictus females and caused a tripling of the number of bites per female per week (compared to untreated controls). These results were also confirmed in Ae. aegypti after treatment at 50 Gy. Wolbachia density decreased significantly in 45-Gray-irradiated females, with the greatest decreases in the early irradiation group (26±2-hour-old pupae). Wolbachia density also decreased as adults aged. This trend was confirmed in ovaries but not in extra-ovarian tissues. FISH analysis showed a strongly reduced Wolbachia-specific fluorescence in the ovaries of 13±1-day-old females.Conclusions: These results suggest that, under SIT programs, the vector capacity of a target population could increase with the frequency of the irradiated females co-released with the sterile males due to an increased biting rate. In the context of a successful suppression, the related safety issues could be generally negligible, but they should be conservatively evaluated when large scale programs relying on imperfect sexing and high overflooding release ratios are run for long time in areas endemic for arboviral diseases. Also, the effects of irradiation on the vector competence deserve further investigation.

Anopheles salivary antigens as serological biomarkers of vector exposure and malaria transmission: A systematic review with multilevel modelling

BackgroundEntomological surveillance for malaria is inherently resource-intensive and produces crude population-level measures of vector exposure which are insensitive in low-transmission settings. Antibodies against Anopheles salivary proteins measured at the individual-level may serve as proxy biomarkers for vector exposure and malaria transmission, but their relationship is yet to be quantified.MethodsA systematic review of studies measuring antibodies against Anopheles salivary antigens (PROSPERO: CRD42020185449). Multilevel modelling estimated associations between seroprevalence with Anopheles human biting rate (HBR) and malaria transmission measures.ResultsFrom 3981 studies identified in literature searches, 42 studies across 16 countries were included contributing 393 meta-observations of anti-Anopheles salivary antibodies determined in 42,764 samples. A positive non-linear association between HBR and seroprevalence was found; overall a 50% increase in HBR was associated with a 13% increase in odds of seropositivity (OR: 1.13, 95%CI: 1.06-1.20, p<0.001). The association between HBR and Anopheles salivary antibodies was strongest with concordant, rather than discordant Anopheles species. Seroprevalence was also significantly positively associated with established epidemiological measures of malaria transmission: entomological inoculation rate, Plasmodium spp. prevalence, and malarial endemicity class.ConclusionsAnopheles salivary antibody biomarkers can serve as a proxy measure for HBR and malaria transmission, and could monitor vectorial capacity and malaria receptivity of a population to sustain malaria transmission. Validation of Anopheles species-specific biomarkers are important given the global heterogeneity in the distribution of Anopheles species. Salivary biomarkers have the potential to transform surveillance by replacing impractical, inaccurate entomological investigations, especially in areas progressing towards malaria elimination.FundingAustralian National Health and Medical Research Council, Wellcome Trust.

How do I bite thee? Let me count the ways: Heterogeneity and temperature dependence in Ae. aegypti biting habits drive individual mosquito arbovirus transmission potential

In models of mosquito-borne transmission, biting rate is the most influential parameter, and understanding the role of temperature on this process is important for assessing the transmission potential of arboviruses like dengue (DENV). Further, the process of biting is assumed to be relatively homogeneous across individuals, with time-between-bites described by an exponentially distributed process. However, these assumptions have not been addressed through laboratory experimentation. We experimentally determined daily biting habits of Ae. aegypti at three temperatures (24°C, 28°C, and 32°C) and determined that there was a high degree of individual heterogeneity in biting habits (number of bites, timing of bites, etc.). There was a significantly higher proportion of females that bit at 28°C compared to both 24°C and 32°C. We further explored the consequences of biting heterogeneity using an individual-based model designed to examine whether a particular biting profile determines whether a mosquito is more or less likely to 1) become exposed given a single index case of DENV and 2) transmit to a susceptible individual. Our results indicate that biting is heterogeneous among individuals and this heterogeneity affects transmission potential of DENV. Understanding individual-level heterogeneity in biting is important and may suggest a role for high-frequency biters as significant contributors to the transmission of DENV.

An Age-Structured Model for Transmission Dynamics of Malaria with Infected Immigrants and Asymptomatic Carriers

An age-structured (children and adults) model for the transmission dynamics of malaria with asymptomatic carriers and infected immigrants has been analyzed. We first analyze a model without infected immigrants. It shows that the disease-free equilibrium exists and is stable when and unstable for . Also, we compute the sensitivity indices of the basic reproduction number. The basic reproduction number is most sensitive to the mosquito biting rate. Besides, the sensitivity of the basic reproduction number shows that the children's class parameters are more sensitive than those of adults. In the presence of infected immigrants, the model does not admit a disease-free equilibrium. The sensitivity of endemic equilibrium shows that the asymptomatic carrier parameters are more critical than that of infected immigrants. Also, the inflow of infectious immigrants is sensitive than that of infected immigrants. The results obtained indicate that strategies that target asymptomatic carriers and infected immigrants can help control malaria. Keywords: Age-structure; malaria; immigrants; asymptomatic carrier; nonlinear ODE model

SIR-SI Mathematical Model for Zika Virus Progression Dynamics in India: A Case Study

Viral diseases are very hazardous for humanity because in the case of most viral diseases, drugs are not effective. At present, the whole world is living with the fear of COVID-19. From time to time, several viral diseases have been troubling human life. In this article, we have tried to capture the progression dynamics of Zika Virus (ZIKV) infection in the Indian scenario. A constructed model is based on compartment modelling. In the model, Susceptible-Infected-Recovered (SIR) structure is used for the human population and Susceptible-Infected (SI) structure is used for mosquito population. The value of the basic reproduction number (R0) is computed 0.36 at baseline values of parameters involved in the model. The lower value of R0 suggests that infection was unable to spread in the human population. Sensitive analysis for R0 revealed that the most accountable parameter in the spread of infection was mosquito biting rate. The modelling technique might be useful for other diseases also.

Biting behaviour and infectivity of Simulium damnosum complex with Onchocerca parasite in Alabameta, Osun State, Southwestern, Nigeria

A longitudinal study was carried out to investigate species composition, seasonal abundance, parity and transmission potential of Simulium damnosum complex in Alabameta community in Osun State, Southwestern, Nigeria. Adult Simulium damnosum complex were collected along Owena River, Alabameta, by two dark complexioned vector collectors from 07:00hr to 18:00hr weekly using collecting tubes from November 2014 to April 2015. The flies were morphologically identified and dissected for the purpose of detecting Onchocerca parasite using dissecting microscope. The Monthly Biting Rate (MBR) of flies was determined using World Health Organization standard formula. A total of four hundred and forty flies were collected during the study period with all of them identified as forest species of Simulium damnosum complex. There was significant variation in monthly collection of the flies with the month of November having the highest number of flies (194) (44%) while the month of April recorded the lowest number of flies (31) (7%) (p<0.05). The morning biting peak (09hr - 11hr) (137) was higher than the evening biting peak (15hr -17hr) (64) (p<0.05) while nulliparous flies (294) (67%) were more abundant than the parous flies (146) (33%) (p<0.05). There was absence of infection (zero infectivity) of the flies (p<0.05). The zero infectivity in the flies may plausibly indicate the possibility of zero transmission of Onchocerca parasite in the community which if sustained over a period of time may signify the possibility of onchocerciasis elimination. Also, the presence of forest species of the flies reduces the risk of resident’s intense exposure to blinding savannah strain of onchocerciasis.

The microbiome and mosquito vectorial capacity: rich potential for discovery and translation

Microbiome research has gained considerable interest due to the emerging evidence of its impact on human and animal health. As in other animals, the gut-associated microbiota of mosquitoes affect host fitness and other phenotypes. It is now well established that microbes can alter pathogen transmission in mosquitoes, either positively or negatively, and avenues are being explored to exploit microbes for vector control. However, less attention has been paid to how microbiota affect phenotypes that impact vectorial capacity. Several mosquito and pathogen components, such as vector density, biting rate, survival, vector competence, and the pathogen extrinsic incubation period all influence pathogen transmission. Recent studies also indicate that mosquito gut-associated microbes can impact each of these components, and therefore ultimately modulate vectorial capacity. Promisingly, this expands the options available to exploit microbes for vector control by also targeting parameters that affect vectorial capacity. However, there are still many knowledge gaps regarding mosquito–microbe interactions that need to be addressed in order to exploit them efficiently. Here, we review current evidence of impacts of the microbiome on aspects of vectorial capacity, and we highlight likely opportunities for novel vector control strategies and areas where further studies are required.

An entomological study of onchocerciasis vectors, Simulium damnosum s.l., in Kinshasa, Democratic Republic of Congo

Background Onchocerciasis, a parasitic disease caused by the filarial worm Onchocerca volvulus transmitted through the bite of Simulium (black flies), is a cause of global concern, with the African population being majorly affected. This study focused on the bite rates, bite cycle, and transmission potential of Simulium damnosum s.l. in two sites with river blindness outbreaks in Kinshasa, DRC: Gombe (S1) and Mont-Ngafula at Kimwenza (S2). Methods From August 1, 2019, to July 31, 2020, we captured adult female black flies near breeding sites along the Congo River at S1 and Lukaya Valley at S2. Collections using human baits at the two sites were conducted for five days/month. Results A total of 6082 black flies of species Simulium squamosum (classified based on other entomological surveys) were captured during the study period. The daily cycle of aggression revealed two peaks: one between 8 and 9 a.m. and the other between 4 and 5 p.m. Low bite rates were observed between 11 a.m. and 1 p.m. The annual biting rate/person reached 13,463 in S1 and 23,638 in S2, with a total of 37,101 bites/person. The average daily biting rate, 37 ± 10 and 69 ± 23 bites/day/person in S1 and S2, respectively, did not differ significantly (P = 0.8901). The high density of the host population can disrupt the transmission of vector-borne diseases by diluting the transmission indices. There was no evidence of onchocerciasis transmission at the study sites because of inadequate laboratory facilities in the DRC. Various larval supports have been identified: at Gombe: aquatic plants, plastic bags, dead leaves, and rocks; at Kimwenza: Ledermaniella ledermanii (the most abundant species at the site), plastic bags, artificial waste, and aquatic plants. Conclusions The study provides further evidence for the need for alternative strategies to eliminate the parasite in the formerly hyper-endemic foci.

Influence of tropical upright pasture structural and chemical characteristics on lamb grazing time

Tropical pasture canopy characteristics can alter lamb ingestive behavior. Our study evaluated the ingestive behavior of young lambs in different tropical pastures to identify which variables interfere in their grazing activity. Two years of study were carried out with 54 weaned lambs distributed in three different pasture canopies: 1) monoculture of an upright grass, guinea grass (Panicum maximum; GG); 2) monoculture of a shrubby legume pigeon pea (Cajanus cajan; PP) and 3) contiguous paddock with half GG and half PP (GP). The experiment was set out in a randomized complete block design (3 blocks). Lamb ingestive behavior was observed from sunrise to sunset with records every 5 minutes. To identify the main variables that affected lamb grazing activity, a multivariate analysis of the Decision Tree was performed. Our results showed that there was no difference in the ingestive behavior parameters of young lambs in different canopies (P > 0.05). There was interaction among the canopies and the experimental periods for the variables idleness time and biting rate (P ≤ 0.05). Lambs in all canopies showed more idleness time in the first evaluation period. Lambs in canopies containing grass (GG and GP) exhibited greater bites per minute throughout the experimental period. Lamb grazing time increased 40% as experimental period progressed and plants matured. The Decision Tree identified leaf:stem ratio as the variable that most influenced lamb grazing time in GG and GP canopies while in the PP, grazing time was directly related to canopy height. The behavior of young lambs on tropical pasture is variable as there is a change in the behavioral response to canopy characteristics over time. In addition, the grazing time of these animals can be estimated by means of variables related to canopy structural characteristics (leaf:stem ratio and height) together with chemical variables.

Investigation of Malaria Vectors (Diptera: Culicidae) in Agricultural Settlements in the Amazon Region of Brazil

Background: Environmental changes resulting from the urbanization process represent a challenge for malaria control. The majority of malaria cases in South America occur in rural areas, areas of recent occupation, mining and indigenous areas of the Amazon region. Although these areas have a significant impact on malaria cases, few entomological studies have been carried out in areas of recent occupation. This study aimed to describe the density, natural infection rate and hematophagic behavior of anopheline species in two settlements in the state of Rondonia, Brazil in order to understand how malaria transmission occurs in areas that have been settled at different times.Methods: An area of recent occupation, denominated Acampamento Fortaleza (AF), and an old settlement, denominated Projeto de Assentamento Florestal Jequitibá (PAFJ), were studied. Peridomicile collections of anopheles were carried out using the Protected Human Attraction Technique (PHAT). The risk and potential for malaria transmission were assessed using the human biting rate (HBR), sporozoite rate (SR) and the entomological inoculation rate (EIR).Results: The results confirmed that Nyssorhynchus darlingi, the main vector responsible for the transmission of malaria in the state of Rondônia, is the predominant species in the two studied locations. Although settlement of the two study sites has occurred at different times, the species richness found was low, showing that the environmental changes caused by anthropological actions probably favor the adaptation of the Ny. darlingi species. Of the 615 anopheline mosquitoes assessed, 7 (1.1%) were positive for Plasmodium infections. The EIR revealed that Ny. darlingi contributes to the transmission of malaria in both locations, since it was responsible for 0.41 infectious bites in humans at night in PAFJ and 0.16 in AF. In the two study sites, the biting occurred more frequently at dusk.Conclusions: Ny. darlingi is the principal vector found in the studied locations. Its prevalence occurs in areas of recent colonization but, even when present in a low density, this species could maintain transmission of malaria in an older settlement. The entomological information obtained in this study is important and may aid the selection of vector control actions in these locations that are considered as having a high risk of malaria transmission.