A cytoplasmic polyhedrosis virus in midgut cells of Anopheles stephensi and in the sporogonic stages of Plasmodium berghei yoelii

Although it has been known for some time that nuclear and cytoplasmic polyhedrosis viruses may infect the larval stages of mosquitos capable of transmitting mammalian malaria this paper reports for the first time the presence of a cytoplasmic polyhedrosis virus in adult Anopheles stephensi. The virus was shown to be present in sporogonic stages of Plasmodium berghei yoelii with which the mosquitos were infected. It is possible that other viruses may affect both vector and malaria parasites. Further studies are required of viral infections in mosquitos as a possible method of biological control.     

Partial suppression of malaria parasites, and of the transmission of malaria, in Aedes aegypti (L.) doubly-infected with Semliki Forest virus and Plasmodium gallinaceum Brumpt

Laboratory experiments were undertaken with Aedes aegypti infected with both Semliki Forest virus (SFV) from infant mice and Plasmodium gallinaceum from fowls to determine if such double infection of mosquitos suppressed their ability to transmit the malaria parasite, a possibility suggested to explain reduction in malaria transmission in Uganda in 1960 when Anopheles funestus and Anopheles gambiae were transmitting both malaria and o''nyongnyong virus to the African population. In general, transmission of fowl malaria was not prevented by SFV infection in Aedes, although some malariometric indices, and consistently the mean oocyst count, were slightly lower in doubly-infected mosquitos than in controls. In one experiment, however, 60% of the Aedes infected 8 days previously with SFV died within 48 hours of ingesting a malarious meal. Mortality was selectively in favour of the survival of SFV-infected mosquitos negative for, or least heavily infected with, malaria; depression in the presence of the virus of the intensity of malaria infection in the individual Aedes also occurred. Some physiological factor—a stress in adult life or, possibly more important, suboptimal larval nutrition—appears to have been crucial to eliciting the adverse effect on the mosquitos themselves and on their malaria infections. Suppression of the development of a malaria parasite in a mosquito, and of malaria transmission, by concurrent infection of the vector with an arbovirus can happen, but is by no means inevitable.     

Review of data on susceptibility of mosquitos in the USSR to imported strains of malaria parasites

Studies on the susceptibility of mosquitos in the USSR to imported species and strains of human malaria parasites have revealed that Anopheles atroparvus, A. messeae, and A. sacharovi are highly susceptible to strains of Plasmodium vivax from Africa, Asia, and South America. There was no significant variation in the level of adaptation to the various vector species.     

Field experiments on the behaviour of malaria vectors in an unsprayed hut and in a hut sprayed with DDT in Northern Nigeria

Investigations on the behaviour of malaria vectors in DDT-sprayed houses are of special interest in malaria control campaigns. In the Northern Region of Nigeria the behaviour of the local vectors was studied by means of experimental huts fitted with exit window-traps. The results obtained show a marked reduction in the number of mosquitos entering the sprayed hut and a depression of the biting rate. The most striking observation is the large number of A. gambiae and of A. funestus that left the sprayed hut and the considerable proportion that escaped unharmed. Both figures show a very marked increase in subsequent months after spraying, indicating a rapid loss of the lethal action of the DDT residue along with a long-lasting irritant effect. It is concluded that under the particular local conditions the application of 1.9 g/m² DDT twice a year may not be sufficient to interrupt malaria transmission in the Northern Region of Nigeria.     

The efficiency of sporozoite transmission in the human malarias, Plasmodium falciparum and P. vivax

Reported are malaria sporozoite and inoculation rates over a 1-year period in eight epidemiologically defined villages of different endemicity in Madang Province, Papua New Guinea. In the study, more than 41 000 wild-caught mosquitos were analysed for Plasmodium falciparum and P. vivax sporozoites by ELISA. In a given village the entomological inoculation rates correlated strongly with the prevalences of both these malarial parasites in children. However, the prevalence of P. falciparum infections in children was much higher than that of P. vivax, despite similar inoculation rates for the two species. These data suggest that in Papua New Guinea P. falciparum is more efficiently transmitted than P. vivax from mosquito to man. The increased efficiency of transmission of P. falciparum may be due to the heavier sporozoite densities in wild-caught mosquitos naturally infected with P. falciparum sporozoites that were tenfold greater than the sporozoite densities in mosquitos infected with P. vivax.     

The vectors of human infections by Wuchereria species in endemic areas and their biology

In this paper the author has compiled an up-to-date list of the principal natural vectors of human Wuchereria infections, arranged by zoogeographical regions and countries, and gives data on natural and experimental filarial infection rates in vectors in different parts of the world. As an aid to workers in areas where Wuchereria and Plasmodium infections co-exist, he also provides a list of anophelines as filarial vectors, including in this list the results of natural and experimental studies of Wuchereria infection both alone and in combination with malaria.     

Description of Anopheles gabonensis,a new species potentially involved in rodent malaria transmission in Gabon,Central Africa

The genus Anopheles includes mosquito vectors of human malaria and arboviruses. In sub-Saharan Africa, the anopheline fauna is rich of nearly 150 species, few of which are anthropophilic and capable of transmitting pathogens to humans. Some of the remaining species are found in forests far from human environments and are vectors of wildlife pathogens. The diversity and the biology of these species have yet to be fully described. As a contribution to furthering knowledge of sylvan Anophelinae, using morphological and molecular tools we describe a new Anopheles species collected in Gabon (Central Africa), which we have named Anopheles gabonensis n. sp. We also molecularly screened this species to detect infections by Plasmodium parasites. The results showed the species to have been infected by Plasmodium vinckei, a rodent parasite. We discuss the role of An. gabonensis n. sp. in the transmission of P. vinckei in the rainforest areas of Central Africa and its potential to transfer pathogens to humans.     

Progress with viral vectored malaria vaccines: A multi-stage approach involving “unnatural immunity”

Viral vectors used in heterologous prime-boost regimens are one of very few vaccination approaches that have yielded significant protection against controlled human malaria infections. Recently, protection induced by chimpanzee adenovirus priming and modified vaccinia Ankara boosting using the ME-TRAP insert has been correlated with the induction of potent CD8⁺ T cell responses. This regimen has progressed to field studies where efficacy against infection has now been reported. The same vectors have been used pre-clinically to identify preferred protective antigens for use in vaccines against the pre-erythrocytic, blood-stage and mosquito stages of malaria and this work is reviewed here for the first time. Such antigen screening has led to the prioritization of the PfRH5 blood-stage antigen, which showed efficacy against heterologous strain challenge in non-human primates, and vectors encoding this antigen are in clinical trials. This, along with the high transmission-blocking activity of some sexual-stage antigens, illustrates well the capacity of such vectors to induce high titre protective antibodies in addition to potent T cell responses. All of the protective responses induced by these vectors exceed the levels of the same immune responses induced by natural exposure supporting the view that, for subunit vaccines to achieve even partial efficacy in humans, “unnatural immunity” comprising immune responses of very high magnitude will need to be induced.     

Prevalence and molecular characterization of Anaplasma phagocytophilum in roe deer (Capreolus capreolus) from Spain

Anaplasma phagocytophilum can infect a wide range of vertebrates; nevertheless, some genetic variants are associated with particular species of tick vectors and animal hosts. It has been suggested that roe deer (Capreolus capreolus) mainly acts as a reservoir of several A. phagocytophilum non-pathogenic variants for other animal species. The aim of this study was to determine the prevalence and identify the genetic variants of A. phagocytophilum in roe deer from Spain in order to assess host-pathogen associations and their pathogenic potential.The spleens of 212 roe deer hunted in Spain were individually collected and analysed by a commercial qPCR kit in order to detect the presence of A. phagocytophilum DNA. Positive samples were further characterized at groESL, 16S rRNA and msp2 partial genes. The possible influence of several intrinsic (age and sex) and extrinsic factors (ecological area) on A. phagocytophilum prevalence was analysed using a logistic regression.Overall, 41.5 % of the samples resulted positive to A. phagocytophilum. The percentage of infected roe deer was significantly higher in the Mediterranean and Oceanic areas than in the Continental and Mountain regions; nevertheless, prevalence was not related to age or sex. Sequence analysis at groESL and 16S rRNA genes allowed the identification of three ecotypes (I to III) and four variants (“Y”, “X”, “W”, “I”), respectively. A high percentage of roe deer from Spain is infected with different variants of A. phagocytophilum; these results have implications for public and animal health since some of these ecotypes and variants have been previously identified in both human and animal clinical cases.     

Natural mortality in two filarial vectors

Little has been written about the survival of filarial mosquitos in nature, although methods are available for determining natural mortality in the field. Every filarial infection in a mosquito provides some information about the number of days it has survived after infection. This can be used to determine the probability of survival of an infected mosquito population in the field. The parous rate of the vector provides another method for estimating natural mortality. Several estimates of natural mortality in two mosquitos, Culex fatigans and Anopheles peditaeniatus, in South India have shown a daily mortality of from 14% to 24% during a season favourable for survival. Information on natural mortality can be obtained during routine dissections of mosquitos for filarial larvae, but a more positive approach to the problem of the identification of filarial infections in mosquitos is needed.     

Seasonal infection of Culex mosquitos and swine with Japanese encephalitis virus

A year-round study of the infection rates of JE virus in Culex mosquitos was made during 1970-71 in 2 hamlets in Taoyuan County, China (Province of Taiwan). JE virus was recovered from 5 of 314 pools of C. annulus, and from 1 of 22 pools of C. tritaeniorhynchus; these recoveries occurred during a 14-day period in July 1971. None of the 288 pools of C. p. fatigans, which had been collected between October and April, was positive. In addition, sentinel swine were assessed for antibody and virus. All the pigs became highly immune by 23 July; in each hamlet all the pigs had become infected within 1 week, virus being detected in them for only 1-2 weeks. Virus-positive mosquitos appeared to have obtained their infections at about the time that viraemia was occurring in the sentinel swine. These observations illustrate once again the lower infection rate and shorter duration of virus-positive mosquitos in China (Province of Taiwan) as compared with Japan.     

Laboratory infection of Anopheles pharoensis with Wuchereria bancrofti.

A. pharoensis was infected from a donor with a high count of W. bancrofti microfilariae. Of the freshly dissected mosquitos, 81.6% were found to have ingested microfilariae, with an average of 12 ± 2.2 microfilariae per mosquito. An infectivity rate of 41.9% was observed in mosquitos dissected between the eleventh and fifteenth days after feeding. A mean of 7.6 ± 1.2 third-stage larvae was found in infective mosquitos. Although A. pharoensis has not yet been found naturally infected with third-stage larvae of W. bancrofti, these studies suggest that it is a potential vector of Bancroftian filariasis.     

Knowledge of mosquitos in relation to public and domestic control activities in the cities of Dar es Salaam and Tanga.

A study of community awareness of mosquitos and related subjects in the residential areas of two Tanzanian cities (Dar es Salaam and Tanga) showed that residents were well aware of mosquitos. Almost all claimed to use some form of domestic mosquito control product for their personal protection, and many spend a significant portion of the household income on this. The problems of nuisance-biting and malaria transmission are usually not separated and are considered to be the result of poor environmental hygiene, for which both residents and local authorities are responsible. Although Culex mosquitos are not a primary target of the Urban Malaria Control Project (UMCP), the persistence of nuisance-biting has made residents sceptical and dissatisfied with insecticide spraying. The residents'' priorities are evidently not the same as those of the health authorities, yet mutual cooperation is essential. In order to maintain community support, campaigns aimed at malaria vectors should consider the need for additional measures to control Culex mosquitos, such as those now being tried by the UMCP. Mosquito breeding sites are non-specifically associated with rubbish and standing water of all kinds, and so the actions that the community considers necessary for mosquito source reduction tend to be poorly targeted. Residents do not recognize that some sources produce malaria mosquitos while others produce nuisance mosquitos. The environmental anti-mosquito measures currently promoted by health education and other forms of propaganda are also poorly targeted. While some of them are directed at important Culex breeding sites, others are aimed at sites of little importance for mosquitos of any kind.(ABSTRACT TRUNCATED AT 250 WORDS)     

The development and trend of insecticide-resistance in Anopheles aconitus D?nitz and Anopheles sundaicus Rodenwaldt

The spread of insecticide-resistance in the two main vectors of malaria in Java, Anopheles aconitus Dönitz and A. sundaicus Rodenwaldt, has been of concern to workers in the Indonesian malaria eradication programme since DDT-resistance was first detected, in A. sundaicus, in 1954. The history of development of resistance in these two vectors has been summarized in this paper by assembling the available records in chronological order, thus facilitating reference.     

Tests for the existence of genetic variability in the tendency of Anopheles culicifacies species B to rest in houses and to bite man

Experiments were carried out in Sri Lanka on the malaria vector Anopheles culicifacies using the mark—release—recapture technique. Collections were made in cattle-baited huts fitted with exit traps, and in nightbiting catches on human subjects. The same individual mosquitos were caught biting cattle and man on different occasions and the numbers caught demonstrated an overall preference for the cattle. The mean interval between successive blood meals in the field was estimated to be 2.3 days.     

Anopheles minimus: its bionomics and role in the transmission of malaria in Assam, India.

Indoor, day-resting collections of Anopheles minimus mosquitos from human dwellings in the study area in Assam, India, indicated that these insects were prevalent throughout the year and that their maximum abundance occurred from March to August. A. minimus was identified as a vector of malaria, and sporozoite infections were recorded every month of the year, with the highest rating occurring in October. The mosquito was highly anthropophilic and fed on human hosts (indoor) all through the night, but feeding was more pronounced between 01:00 and 04:00, the person-biting rate was 13.7 per night. Breeding occurred throughout the year in slow-flowing streams with grassy banks. A. fluviatilis was also identified as a vector of malaria in the study area but occurred in low density, and sporozoite infections were only seasonal.     

Dates of onset of relapses and the duration of infection in induced tertian malaria with short and long incubation periods

The course of induced tertian malaria has been studied in patients in psychoneurological hospitals in Moscow over a long period. Various strains of P. vivax were used to infect patients; the sporozoites were transmitted by Anopheles maculipennis atroparvus bred under laboratory conditions. There are marked differences between the dates of onset of relapses, reckoned from the primary manifestations of the disease, in patients developing malaria after long and short incubation periods. P. vivax strains which have a short incubation period are characterized by producing in patients a series of relapses following a long latent period; strains with a long incubation period are characterized by the occurrence of relapses during the first 3 months after the end of the primary series of attacks. However, some P. vivax strains give rise to both types of relapse, depending on the incubation period. In any event, the duration of the disease does not, as a rule, exceed 2 years, including the incubation period. The number of mosquitos (and hence the number of sporozoites) used to infect a patient does not have any noticeable effect on the number and frequency of relapses. In short-incubation tertian malaria, the use of quinocide during the period of treatment of the primary manifestations greatly reduced the number of relapses; in long-incubation tertian malaria similarly treated with quinocide, no relapses occurred.     

A unique methodology for detecting the spread of chloroquine-resistant strains of Plasmodium falciparum,in previously unreported areas,by analyzing anophelines of malaria endemic zones of Orissa,India

Generally, clinical data is referred to study drug-resistance patterns of Plasmodium falciparum in an area. This is only possible after a clear manifestation of drug-resistance parasites inside the human host, and thereafter detection by healthcare persons. The detection of spread of drug-resistant P. falciparum in a population, before any pathological symptoms detected in humans is possible by analyzing the anopheline vectors, transmitting malaria. In the present study we implemented a new strategy to detect the spread of chloroquine-resistant (CQR) strains of P. falciparum by the major malaria vectors prevalent in selected endemic regions of Orissa, India. We screened P. falciparum positive vectors by using polymerase chain reaction (PCR)-based assay and thereafter detected K76T mutation in the Pfcrt gene, the chloroquine-resistance marker, of parasites present within the vectors. This study showed higher transmission rate of chloroquine-resistant P. falciparum parasites by Anopheles culicifacies and Anopheles fluviatilis. This study will help in assigning chloroquine-resistant P. falciparum sporozoite transmission potential of malaria vectors and suggest that by adopting the mentioned methodologies, we can detect the spreading of the drug-resistant P. falciparum in its transmission. This approach of studying the anophelines during regular vector collection and epidemiological analysis will give the knowledge of chloroquine-resistance pattern of P. falciparum of an area and help in devising effective malaria control strategy.     

Subtle periodicity of pupation in rapidly developing mosquitos: With particular reference to Aedes vittats and Aedes aegypti

Aedes vittatus and Ae. aegypti are important vectors of yellow fever and other arbovirus diseases in Africa and they complete many generations, through all developmental stages, in a single year. Recent studies on Drosophila have offered precise techniques for analysing periodicities in populations of single developmental events in individuals. If the variability of pupation times is low, periodicities may not be recognizable by an obviously polymodal pattern in a single synchronized population. Analysis of 8 synchronized populations of Ae. vittatus and Ae. aegypti, evenly spaced through 24 hours, provided a sensitive method of recognizing even subtle periodicities in rapidly developing mosquitos. By this means Ae. vittatus has been shown to exhibit a weak diurnal periodicity of pupation while a strain of Ae. aegypti showed not the slightest periodicity of pupation under the same light—dark cycle. The authors stress that periodicity of emergence or pupation in mosquitos can have important consequences for epidemiology and vector control and should be verified for each vector species for which control measures are envisaged.     

Past and current biological factors affecting malaria in the low transmission setting of Botswana: A review

Malaria continues to be one of the top infectious agents contributing to morbidity and mortality in sub-Saharan Africa. Annually, Botswana accounts only for a small proportion of cases (<<1%). Despite significantly reduced incidence rate, the country still experiences sporadic outbreaks that hamper the goal of malaria elimination. This review evaluated previous and current biological factors that impact malaria in Botswana, specifically focussing on the vectors, the parasite and the host. This was accomplished via a literature review evaluating these variables in Botswana. Current literature suggests that Anopheles arabiensis is the main malaria vector in the country. Several other potential vectors have been found widely distributed throughout Botswana in high numbers, yet remain largely unstudied with regards to their contribution to the country's malaria burden. We also report the most up to date list of all Anopheles species that have been found in Botswana. Plasmodium falciparum is responsible for the vast majority of symptomatic malaria in the country and some drug resistance markers have been documented for this species. Plasmodium vivax has been reported in asymptomatic subjects, even though a large proportion of the Botswana population appears to be Duffy antigen negative. Very little is known about the true distribution of P. vivax and no point of care testing infrastructure for this species exists in Botswana, making it difficult to tailor treatment to address possible recrudescence or relapse. Due to a genetically diverse population with a substantial Khoisan contribution into the Bantu genetic background, several phenotypes that potentially impact prevalence and severity of malaria exist within the country. These include sickle cell trait, Glucose-6-Phosphate Dehydrogenase deficiency, and Duffy negativity. This review highlights the information that currently exists on malaria in Botswana. It also postulates that a comprehensive understanding of these aforementioned biological factors may help to explain malaria persistence in Botswana.