Ivermectin as an endectocide may boost control of malaria vectors in India and contribute to elimination

In vivo evaluation of three isoxazolines against Cx. tritaeniorhynchus (Diptera: Culicidae): A novel approach to control Japanese encephalitis vector

As Culex tritaeniorhynchus the primary vector of JE is a highly zoophagic and exophilic nature, a novel vector control approach use of isoxazolines drugs (Sarolaner, Fluralaner and Afoxolane) to reduce the survival of blood-fed mosquito was studied by in vivo model using Rattus norvegicus as an experimental animal. All the drugs are orally administered to the animal with therapeutic dosage. The drugs Fluralaner, Sarolaner and Afoxolaner induced 100 % mortality of Cx. tritaeniorhynchus up to 32, 7 and 20 days respectively after administered to the R. norvegicus. When Cx. tritaeniorhynchus was fed on animals treated with Fluralaner, the median survival duration was 12 h from days 1 to 7 of post-treatment and 24 h from days 8 to 28.

Model development to assess the impact of a preventive treatment with sarolaner and moxidectin on Dirofilaria immitis infection dynamics in dogs

BACKGROUND

Dirofilaria immitis is a mosquito-transmitted filarial parasite causing heartworm disease in dogs. The parasite may cause a significant disease burden to the dog population in high prevalence areas and is mainly managed through prophylactic treatments.

METHODS

In this modelling study, the effect of a prophylactic treatment with moxidectin and sarolaner on heartworm disease dynamics was investigated in dogs. A compartmental model was developed to investigate different epidemiological settings considering different values for prevalence and host preference.

RESULTS

When the mosquito host preference to dogs is low, a treatment compliance of only 40% decreases the proportion of infectious dogs. When the host preference of the mosquitoes however increases, an exponential increase in infectious dogs was observed, and a higher treatment compliance is required. In high transmission environments, with a high prevalence and a high mosquito host preference, a high treatment compliance (up to 100%) is required to have an impact on the number of infected animals. Notably, in scenarios with higher host preference towards dogs, more mosquitoes are exposed to sarolaner through the blood meal, leading to higher mortality of these mosquitoes and resulting in fewer infected and infectious dogs.

CONCLUSIONS

The preventive efficacy, as measured by the number of non-infected dogs, increases with increasing treatment compliance, but the extent of the treatment effect differs with the epidemiological setting. Adding sarolaner to a heartworm prevention has a complimentary impact on mosquito survival and heartworm disease transmission, although this effect depends on the epidemiological settings, emphasizing the true complexity of disease dynamics of a vector-borne disease.

Addressing challenges in vector control: a review of current strategies and the imperative for novel tools in India's combat against vector-borne diseases

Vector-borne diseases (VBDs) exert a substantial burden across the world, especially in tropical countries. Malaria, chikungunya, dengue, visceral leishmaniasis, lymphatic filariasis and Japanese encephalitis are among the public health concerns for India. One of the major pillars for the containment of VBDs is vector control and different tools have been employed for several decades. These range from chemical insecticides used in indoor residual sprays, space sprays, fogging, treated bednets and larvicides to biological control methods such as larvivorus fishes and environmental control and modification measures such as source reduction. However, these methods are increasingly becoming less effective due to several reasons such as insecticide resistance, outdoor biting, behavioural changes in vectors for biting and resting, climate change, movement of population, vector incursion to newer areas and others. It is essential to develop and test new tools for vector control to surmount these challenges. Though focusing on India's public health concerns, the new tools enumerated here can be tested by any country with similar epidemiological and environmental conditions. The promising new vector control tools are insecticide-treated nets with synergist and/or pyrrole chlorfenapyr, alternatives/additions to synthetic pyrethroids like neonicotinoids, clothianidin for indoor residual spray, newer formulations such as Bacillus sphaericus for use in larvicides, attractive toxic sugar baits, especially to curtail outdoor transmission, endectocides like ivermectin for use in animals/humans, insecticidal paints, spatial repellents, insecticide-treated wearables and others. Genetic modification technologies (Sterile Insect Technique/Incompatible Insect Technique/Wolbachia transfection) are also upcoming strategies. Among the six VBDs, India is committed to the elimination of three (malaria, visceral leishmaniasis and lymphatic filariasis) and it will require additional and/or novel tools to overcome the roadblocks in our current journey to the goal of control/elimination of these VBDs.

The synergistic effect of climatic factors on malaria transmission: a predictive approach for northeastern states of India

The northeast region of India is highlighted as the most vulnerable region for malaria. This study attempts to explore the epidemiological profile and quantify the climate-induced influence on malaria cases in the context of tropical states, taking Meghalaya and Tripura as study areas. Monthly malaria cases and meteorological data from 2011 to 2018 and 2013 to 2019 were collected from the states of Meghalaya and Tripura, respectively. The nonlinear associations between individual and synergistic effect of meteorological factors and malaria cases were assessed, and climate-based malaria prediction models were developed using the generalized additive model (GAM) with Gaussian distribution. During the study period, a total of 216,943 and 125,926 cases were recorded in Meghalaya and Tripura, respectively, and majority of the cases occurred due to the infection of Plasmodium falciparum in both the states. The temperature and relative humidity in Meghalaya and temperature, rainfall, relative humidity, and soil moisture in Tripura showed a significant nonlinear effect on malaria; moreover, the synergistic effects of temperature and relative humidity (SI=2.37, RERI=0.58, AP=0.29) and temperature and rainfall (SI=6.09, RERI=2.25, AP=0.61) were found to be the key determinants of malaria transmission in Meghalaya and Tripura, respectively. The developed climate-based malaria prediction models are able to predict the malaria cases accurately in both Meghalaya (RMSE: 0.0889; R²: 0.944) and Tripura (RMSE: 0.0451; R²: 0.884). The study found that not only the individual climatic factors can significantly increase the risk of malaria transmission but also the synergistic effects of climatic factors can drive the malaria transmission multifold. This reminds the policymakers to pay attention to the control of malaria in situations with high temperature and relative humidity and high temperature and rainfall in Meghalaya and Tripura, respectively.

Insecticidal activity of Simparica and Simparica Trio against Aedes aegypti in dogs

BACKGROUND

Aedes aegypti is one of the main species responsible for the transmission of mosquito-borne pathogens worldwide. The isoxazoline Sarolaner has excellent efficacy as an acaricide against ticks and mites and as an insecticide against fleas, and potential efficacy against other insects.

METHODS

In each of two laboratory studies, 24 dogs were randomly allocated (n = 8/group) to an untreated control group, a Simparica-treated group (at the minimum dose of 2.0 mg/kg sarolaner), or a Simparica Trio-treated group (at the minimum dose of 1.2 mg/kg sarolaner, 24 µg/kg moxidectin and 5 mg/kg pyrantel), based on pre-treatment mosquito counts. Treatments were administered orally once on day 0. Each dog was exposed to 50 unfed female adult A. aegypti mosquitoes for 1 h on days 1, 7, 14, 21, 28 and 35. After each exposure, mosquitoes were counted for each dog and characterized as live, moribund or dead, and as fed or unfed. Dead mosquitoes were counted and removed at 12, 24 and 48 h post-exposure in study 1 and at 24, 48, 72, 96 and 120 h post-exposure in study 2. In study 2, mosquito eggs were collected from 72 h post-exposure until 120 h post-exposure. Insecticidal efficacy was calculated based on the reduction of the arithmetic mean live fed-mosquito counts in each of the treated groups versus the untreated control group for every timepoint post-exposure.

RESULTS

Adequate challenge was demonstrated in both studies, with arithmetic mean live fed-mosquito counts ranging from 35.5 to 45.0 for the untreated group. Mean mosquito counts for dogs treated with Simparica and Simparica Trio were significantly (P < 0.0001) reduced within 48 h after exposure on all study days. In study 1, Simparica treatment provided ≥ 96.8% reduction in the arithmetic mean live fed-mosquito counts for 28 days, and Simparica Trio treatment provided ≥ 90.3% reduction for 21 days. In study 2, Simparica treatment provided ≥ 99.4% reduction for 35 days (from 48 h onwards), and Simparica Trio treatment provided ≥ 97.8% reduction for 28 days (from 72 h onwards).

CONCLUSIONS

Both studies demonstrated that a single oral dose of Simparica or Simparica Trio provides high efficacy against mosquitoes in dogs within 24-72 h after exposure for an entire month.

Surveillance and Control of Malaria Vectors in Hainan Province, China from 1950 to 2021: A Retrospective Review

Malaria is a serious mosquito-borne tropical disease impacting populations in tropical regions across the world. Malaria was previously hyperendemic in Hainan Province. Due to large-scale anti-malarial intervention, malaria elimination in the province was achieved in 2019. This paper reviews the literature on the ecology, bionomics, and control of malaria vectors in Hainan from 1951 to 2021. We searched PubMed, and the China national knowledge infrastructure (CNKI) database for relevant articles published and included three other important books published in Chinese or English in order to summarize research on species, distribution, vectorial capacity, ecology, the resistance of malaria vectors to insecticides, and malaria vector control in Hainan Province. A total of 239 references were identified, 79 of which met the criteria for inclusion in our review. A total of six references dealt with the salivary gland infection of Anophelines, six with vectorial capacity, 41 with mosquito species and distribution, seven with seasonality, three with blood preference, four with nocturnal activity, two with flight distance, 13 with resistance to insecticides, and 14 with vector control. Only 16 published papers met the criteria of addressing malaria vectors in Hainan over the last 10 years (2012–2021). Anopheles dirus and Anopheles minimus are primary malaria vectors, mainly distributed in the southern and central areas of Hainan. Indoor residual spraying with DDT and the use of ITNs with pyrethroid insecticides were the main interventions taken for malaria control. Previous studies on ecology, bionomics, and resistance of vectors provided scientific evidence for optimizing malaria vector control and contributed to malaria elimination in Hainan Province. We hope our study will contribute to preventing malaria reestablishment caused by imported malaria in Hainan. Research on malaria vectors should be updated to provide scientific evidence for malaria vector control strategies post-elimination as the ecology, bionomics, and resistance of vectors to insecticides may change with changes in the environment.

Sugar Bait Composition Containing Ivermectin Affect Engorgement and Mortality of the Mosquito Aedes aegypti (Diptera: Culicidae)

Toxins and attractants have been studied and used in sugar baits for mosquitoes. However, little importance has been given to the combination of sugar concentration, bait composition, and their relationship with mosquito engorgement and mortality. Therefore, the present study evaluated the effects of three concentrations of sucrose on baits with and without an attractant (concentrated guava juice), on engorgement and mortality rates of adult Aedes aegypti (Linnaeus, Diptera: Culicidae). Toxic sugar baits (TSB) and attractant toxic sugar baits (ATSB) containing 10, 50, and 70% sucrose and 100 ppm ivermectin (IVM) were prepared to assess engorgement and mortality rates. Subsequently, different concentrations of IVM (0.312-100 ppm) in TSB and ATSB were prepared with sucrose concentrations of 10 and 70% to determine the lethal concentrations (LC50 and LC90) values. Engorgement on the baits was observed under a stereomicroscope, and mortality was followed up to 48 h after feeding. In general, more mosquitoes engorged on TSB regardless of the sugar concentration, while higher concentrations of sugar in ATSBs resulted in higher numbers of mosquitoes engorging. A large increase in the LC90 of IVM was observed for females feeding on ATSBs and TSBs with 70% sucrose relative to those feeding on baits with lower sugar concentrations. No such effect was observed for males.

Potential of ivermectin as an active ingredient of the attractive toxic sugar baits against the Indian malaria vectors Anopheles culicifacies and Anopheles stephensi

BACKGROUND

Attractive toxic sugar bait (ATSB) is a novel vector control tool that exploits the sugar feeding behavior of mosquitoes. The current study aims to evaluate the efficacy of ivermectin-based ATSB against insecticide susceptible and resistant strains of major Indian malaria vectors - Anopheles culicifacies and Anopheles stephensi. ATSB with different concentrations of ivermectin were tested against mosquito vectors under standard laboratory conditions.

RESULTS

Dose-response analysis of ivermectin-ATSB showed 7.8 and 19.8 ppm as 50% and 90% lethal concentration (LC₅₀ and LC₉₀ ) values for insecticide susceptible An. culicifacies. In the case of insecticide susceptible An. stephensi, the LC₉₀ value was 35 ppm which was significantly higher in comparison to the LC₉₀ for An. culicifacies. The LC₅₀ of insecticide-resistant An. culicifacies and An. stephensi were 10.6 and 15.9 ppm respectively whereas LC₉₀ values were 36.9 and 61.0. Ivermectin-ATSB resulted in 99 ± 0.8% mortality of An. culicifacies and 93 ± 3.8% mortality of An. stephensi at an ivermectin concentration of 25 ppm. In another set of experiments, the ATSB solution containing standardized dose of ivermectin was sprayed on Allysum plant and mortality of both Anopheline vectors was recorded. Here, we observed > 90% mortality in both An. stephensi and An. culicifacies.

CONCLUSION

Our study demonstrates the potential of ivermectin-based ATSB in killing Indian malaria vectors irrespective of the method of application. Further field trials with ivermectin containing ATSB may pave the way for its usage in the national vector control program. © 2022 Society of Chemical Industry.

Challenges in Understanding the Bionomics of Indian Malaria Vectors

Many factors influence the success or failure of malaria vector control program such as political will, leadership, sustained funding, robustness of healthcare system and others. In addition, updated knowledge and information about the triad of host, parasite, and vector is of paramount importance. Vector bionomics studies that determine mosquito behavior in terms of feeding, resting, biting, mating, breeding, longevity, vectorial capacity, and response to different insecticides are a step towards enhancing our understanding. In the present work, we have compiled studies conducted in India over the past two decades (2000-2020) to identify gaps in our knowledge of malaria vector bionomics and the research that needs to be done in the future. We retrieved district-level data of India's six primary malaria vector species. According to our findings, vector bionomics studies have been undertaken in ∼50% and ∼15% of the country's high (annual parasite index > 1) and low (annual parasite index < 1) malaria-endemic districts respectively. Most of the research studies focused on mosquito density, insecticide susceptibility status, and parasite detection, whereas other vital bionomics parameters were neglected. Surveys conducted were incomplete, and vector bionomics data were not captured sufficiently. The absence of vector bionomics data can be a blind spot and the lack or inadequate understanding of vector bionomics can lead to use of inappropriate vector control tools. Thus, there is an urgent need to initiate comprehensive bionomics studies on India's primary and secondary malaria vectors.

Malaria control initiatives that have the potential to be gamechangers in India's quest for malaria elimination

Malaria continues to have devastating effect on people's lives especially in developing countries. India is slated for malaria elimination by 2030. Though India has sustained a decline in malaria burden at the national level the epidemiological picture remains heterogenous. India's road to malaria elimination plan is riddled with many roadblocks. Major challenges include insufficient surveillance, slow and aggregated data reporting especially in exigent situations like cross-border areas and vulnerable high-risk groups. More than half of total malaria cases were due to Plasmodium vivax (P. vivax) in India as reported by national malaria control programme in 2019. This translates into substantial burden of P. vivax malaria in absolute numbers. P. vivax malaria, which is difficult to resolve as compared to other species, poses a threat to India's elimination plans by virtue of its tendency to develop hypnozoites, due to poor compliance to primaquine (PQ), due to host factors like G 6 PD deficiency and other genes that affect PQ metabolism. Also, India's malaria endemic areas largely coincide geographically with tribal regions which are poor in healthcare infrastructure. The tribal population disproportionately bears a huge burden of malaria. They also harbour more G6PD deficient individuals than non-tribal regions. Therefore, in addition to inadequate diagnostic facilities (for both malaria and G6PD testing) these remote rural and tribal communities suffer from lack of timely treatment, incomplete radical treatment due to poor compliance and thus repeated episodes of P. vivax due to relapses and/or reinfections. Another challenge is that the the current diagnostic tools in the national programme in India and other countries are mostly available only via the programme and are able to detect patent infections on the whole. These therefore miss low-density infections which are another major limitation for their use in malaria endemic countries. Drug and insecticide resistance need to be constantly monitored as they have direct impact on the efficacy of the current tools. Need for better vector control products for the diverse entomological requirements is also felt. India is the second most populous country in the world with majority of its population at risk of malaria. Despite many agencies (government and non-government) working in the field of malaria, there needs to be more synergy at the local or central level for malaria control. Here, we have proposed solutions for specific facets of the malaria programme. Surveillance, data visualization and analysis can all be supported through over the counter availability of rapid diagnostics, adoption of molecular tools like PCR (requiring additional infrastructure and expertise), mobile applications for data capture and use of malaria data dashboard. Management could be augmented by inclusion of tafenoquine for treatment of P. vivax malaria with a companion point-of care diagnostic which has been developed to assess G6PD enzyme activity. A switchover to artemether-lumefantrine for the entire country can also be considered. Vector control can be strengthened by commercial availability of insecticidal bednets and exploration of novel vector control tools like ivermectin. Lastly, enhancing synergy amongst various stakeholders would also catalyze the malaria elimination plans.

Funding

The authors have received no funding for this paper.