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Review of vector control prior to the OCP.

It is a little over 60 years since Blacklock, in an elegant study, demonstrated that in Sierra Leone human onchocerciasis was transmitted by a Simulium fly. Amazingly within 6 years of Blacklock's discovery an attempt was made, mainly using environmental management, to control Simulium in the Chiapas focus of Mexico, though this was not successful. Later Buckley was successful in eradicating Simulium neavei from the small Riana focus in Kenya by clearing riverine forest. However, it was not until DDT became widely available that Simulium control entered its heroic phase with campaigns in Kenya, Uganda and Zaire, some of which were highly successful. This led to an interest in controlling the vector in West Africa. The very different pattern of disease with very large foci virtually contiguous across tens of thousands of square kilometers suggested a difficult proposition and early attempts were not very successful. There is much of interest in the East and Central African schemes and in the West African forerunners of the OCP. In this talk an attempt will be made to capture something of the flavour of these pioneering efforts, to give credit to the men involved, and to highlight the successes against both the S. neavei and S. damnosum complex, and the advances which contributed to the successful planning of the OCP.

Africa↗

[Use of traps in tsetse control: effects of treated and untreated supports].

Tsetse control trials were carried out in forest zone of Côte d'Ivoire with biconical traps. Some of the traps were impregnated with deltamethrin (1, 2 g per trap) and others were not. Results show a rapid and effective action of the treated supports. In nine days, the tsetse population had been reduced by 85% on the treated supports and by 60% on the non treated supports. After one month, a rapid decrease of the efficacy of the treated traps was observed. The fall in the effectiveness (more masked in the treated traps) was certainly due to heavy rains (875 mm) which caused an important increase in growth of vegetation.

Animals↗

[Studies on the bionomics of Phlebotomus chinensis in mountain cave and its relation to control in northern Sichuan].

After a survey of the gonotrophic cycle of 5147 female Phlebotomus chinensis collected from the caves in the mountain area 1500 m above sea level in Nanping County, Sichuan Province where this sandfly was proved to be the typical predominant wild species living mostly in the wild cave at a very high density, a large cave was sprayed with alphamethrin at a dosage of 50 mg/m2 in July, 1993. As a result, the sandfly density dropped to zero spanning over a period of two peak seasons. The authors consider that the residual effect of alphamethrin as long enough to suppress sandfly density in the cave for at least two years, and stress that the spray is a favourable measure in all the year round in reducing the sandfly population density and preventing visceral leishmaniasis.

Animals↗

Lessons from a national survey of Chagas disease transmission risk in Colombia.

In recent years, there has been a revitalization of large-scale programmes to control parasitic disease in developing countries. In 1997, the Governments of Colombia, Venezuela, Ecuador and Peru committed themselves to replicate the cost-effective elimination of Trypanosoma cruzi transmission achieved in the Southern Cone by using insecticides against the domestic triatomine vectors (in combination with blood-bank screening). Central American Governments launched a complementary initiative. All plan to interrupt vectorial transmission throughout the region by 2010 but specific targets are decided nationally. In this article, we highlight the novel approach taken by the Colombian Government for determining the geographic distribution of Chagas disease risk to select where to intervene first.

Animals↗

An entomological field evaluation of larval biology of sandfly in Kala-azar endemic focus of Bihar--exploration of larval control tool.

Knowing the exact breeding places inside the habitat is very important to plan the larval control strategy. Information regarding larval biology in relation to different seasons will be more useful to organize insecticide spray schedule at a particular month of maximum immature density to bring down the adult sandfly density. In the present study, maximum number of soil samples were found positive in the month of January and minimum in the month of September. Maximum positive soil samples were collected from cattle sheds, minimum in mixed dwellings and in case of human dwellings all soil samples were negative. Comparison of two methods for the isolation of immature stages showed that direct microscopic observation is superior to sugar flotation technique.

Animals↗

Prospects for the control of onchocerciasis in Africa with special reference to the Volta River basin.

Onchocerciasis is found in association with all the main river systems of northern tropical Africa, and there are endemic foci south of the Equator. Heavy and prolonged infection may cause blindness and intense pruritus. The vectors, Simulium damnosum and S. neavei, are also intolerable pests when they swarm. The disease and its vector together cause serious economic loss and are a main cause of the depopulation of river valleys in the savanna lands.The basin of the River Volta, in which the worst endemic area in the world is situated, is considered to be the most favourable area for a study of the problems involved in the large-scale control of onchocerciasis carried by S. damnosum. Mass treatment or prophylaxis are not practicable at present. The clinical condition progresses for many years in the absence of fresh infection, and drugs capable of mass application are needed. However, the first aim is to attack the larval stages of the vector with insecticides. DDT is ideal for this purpose in large, steadily flowing rivers, but a more suitable insecticide and formulation are needed for small, irregularly flowing streams.Research is needed into many aspects of the adult life of S. damnosum, including feeding and resting habits, dry season survival and flight range. One of the main practical problems is prevention of reinfestation of a treated river system.

Africa↗

Reinfestation sources for Chagas disease vector, Triatoma infestans, Argentina.

Reinfestation by Triatoma infestans after insecticide spraying has caused elimination efforts in the dry Chaco region to fail repeatedly. The sources and spatial extent that need to be considered to understand the reinfestation pattern and to plan a comprehensive control program were studied in 2 adjacent rural communities in northwestern Argentina from 1993 to 1997. The effects of external, residual, and primary sources on the reinfestation pattern were evaluated by using geographic information systems, satellite imagery, spatial statistics, and 5-year retrospective data for 1,881 sites. The reinfestation process depended on primary internal sources and on surrounding infested communities. In the dry Chaco, successfully reducing the risk for reinfestation in a community depends on treating all communities and isolated sites within 1,500 m of the target community. In addition, during the surveillance phase, spraying all sites within 500 m of new foci will delay reinfestation.

Animals↗

Onchocerciasis in West Africa after 2002: a challenge to take up.

Initially planned for a 20 year life time, the Onchocerciasis Control Programme in West Africa (OCP) will have finally continued its activities for nearly three decades (vector control alone from 1975 to 1989, then vector control and/or therapeutic treatment until 2002). Although onchocerciasis is no longer a problem of public health importance nor an obstacle to socio-economic development in the OCP area, the control of this filariasis is not over because OCP never aimed at eradication, neither of the parasite (Onchocerca volvulus), nor of its vector (Simulium damnosum s.l.). In 2003, the eleven Participating countries of OCP will take over the responsibility of carrying out the residual activities of monitoring and the control of this disease. This mission is of great importance because any recrudescence of the transmission could lead in the long run to the reappearance of the clinical signs of onchocerciasis, if not its most serious manifestations. For epidemiological and operational reasons, and given the disparity in national health policies and infrastructures, the capacities of the countries to take over the residual activities of monitoring and control of onchocerciasis are very unequal. Indeed, the interventions to be carried out are very different from one country to another and the process of integrating the residual activities into the national health systems is not taking place at the same pace. This inequality among the countries vis-a-vis the challenges to be met does not, however, prejudge the epidemiological situation after 2002 whose evolution will also depend on the effectiveness of the provisions made before that date by OCP, then after 2002, by the Regional Office for Africa of the World Health Organization which is currently setting up a sub-regional multidisease surveillance centre.

Africa, Western↗

Differences of susceptibility of five triatomine species to pyrethroid insecticides - implications for Chagas disease vector control.

As pyrethroids are presently the favored group of insecticides to control triatomines, we performed a series of bioassays to determine the intrinsic activity of some of the main compounds used in the control campaigns, against five of the main species of triatomines to be controlled. Comparing the insecticides it can be seen that lambdacyhalothrin is more effective than the other three pyrethroids, both considering the LD50 and 99 for all the three species with comparable results. On Triatoma infestans the LD50 of lambdacyhalothrin was followed by that of alfacypermethrin, cyfluthrin and deltamethrin. On Rhodnius prolixus the sequence, in decreasing order of activity, was lambdacyhalothrin, alfacypermethrin, deltamethrin and cyfluthrin. Some modifications can be seen when we compare the LD99, that has more to see to what happens in the field. T. brasiliensis showed to be as sensible to lambdacyhalothrin as T. infestans, the most susceptible for this product. By the other side T. sordida is the least susceptible considering the LD99 of this insecticide.

Animals↗

Population dynamics and control of Triatoma infestans.

Analysis of field populations of Triatoma infestans (Hemiptera: Reduviidae), after a 3-year study, shows that population growth rate is affected by both density-dependent and density-independent mortality. Although an equilibrium exists, apparently as a consequence of a density dependent-mechanism, population density fluctuates throughout the year because of the effect of monthly mean minimum temperature as a density-independent source of mortality. Simulation studies based on Moran curves shows that high population densities have an approximately constant extinction probability (around 0.20), independently of the season the population starts growing. However, at very low population densities, the extinction probability depends strongly on the season when the population begins to grow. Very low density populations beginning in winter or autumn have the highest extinction probability. The outcome of the simulation studies coincides with results observed in field populations affected by insecticide application at different seasons.

Animals↗

Barrier crops as a cultural control measure of non-persistently transmitted aphid-borne viruses.

Barrier crops have been used since the early 1950s as a cultural control strategy for reducing the spread of non-persistently transmitted aphid-borne viruses. Since then, this strategy has been investigated by several authors, resulting in a wide range of divergent conclusions on its effectiveness. In a series of field and laboratory experiments carried out in central Spain during 1995-1998, we investigated the efficacy and mode of action of various barrier crops for reducing the spread of Potato virus Y and Cucumber mosaic virus in pepper. The barriers acted as natural 'sinks' for non-persistent viruses and did not reduce the number of aphids landing in the protected crop, as suggested by some authors. There was a significant reduction in virus spread and an increase in yield in two of the four years of trials. Such differences in the effectiveness of the barrier crop strategy show that its efficacy depends on a series of factors such as the kind of virus spread pattern (monocyclic or polycyclic), the height of the barrier crop at the time of maximum risk of infection and the extent of competition between the barrier and the protected crop. Moreover, the barrier crop should not host any potential insect pest or pathogen able to damage the protected crop. It is concluded that use of barrier crops can be an effective crop management strategy to protect against virus infection, but only under specific circumstances.

Animals↗

[Epidemiological surveillance of Chagas disease].

Chagas disease still constitutes an important medical problem in affected countries. In some, the extent of the disease is still unknown and control programs have not been implemented. In others the disease has been reduced due to regular control programs and other economic and social factors. Epidemiological surveillance with community participation to guard against disease transmission is now the basic challenge. Applied research and in-depth reformulation of health systems are required to establish efficient and sustainable Chagas disease surveillance programs, considering low density of peridomiciliary vectors as the most relevant factor. In addition, a large population of already infected, poor individuals require specific medical attention and social security. As a consequence of health care decentralization, Federal institutions such as the Brazilian National Health Foundation (FNS) are being progressively decommissioned, and new participants must be engaged in the process. Communities themselves, together with regional and local institutions, must take charge of surveillance in order to guarantee its efficiency and sustainability.

Animals↗

Oral toxicity and repellency of borates to German cockroaches (Dictyoptera: Blattellidae).

The oral toxicities of boric acid and disodium octaborate tetrahydrate (DSOBTH) in dry-mixed, wet-mixed, and water-based solution baits were determined for German cockroaches, Blattella germanica L., in choice and nonchoice experiments. In dry-mixed, nonchoice bait tests, all cockroaches died within 1 wk. Time to mortality decreased as the concentration of boric acid increased. The LT50s for DSOBTH were not significantly different for any concentrations tested. In choice tests, the order of LT50s of boric acid and DSOBTH were 6.25% = 12.5% > 25% = 50% and 25% > 12.5% > 50% > 25%, respectively. Consumption of treated and control baits and the differences between the LT50s of the nonchoice and choice tests indicate repellency of the toxicants. Most German cockroaches died in 3-6 d in wet-mixed nonchoice tests, but all cockroaches survived the wet-mixed choice tests except at the lowest concentration of boric acid. This result indicated repellency. In nonchoice tests with water-based solutions, all cockroaches died in 5 d. In choice tests with boric acid solutions, mortality did not increase at concentrations > 1%. Choice/nonchoice ratios indicate no repellency of water-based solutions. In addition, consumption of treated and blank solutions was not significantly different for any concentration except 4% DSOBTH.

Administration, Oral↗

Systemic effects of neem on western flower thrips, Frankliniella occidentalis (Thysanoptera: Thripidae).

The systemic effects of neem on the western flower thrips, Frankliniella occidentalis (Pergande), were investigated in laboratory trials using green bean, Phaseolus vulgaris L., in arena and microcosm experiments. In arena experiments, systemic effects of neem against western flower thrips larvae on primary bean leaves were observed with maximum corrected mortality of 50.6%. In microcosm experiments using bean seedlings, higher efficacy in the control of western flower thrips were observed with soil applications of neem on a substrate mixture (i.e., Fruhstorfer Erde, Type P, and sand) in a 1:1 ratio (93% corrected mortality) compared with application on the commercial substrate only (76% corrected mortality). However, longer persistence of neem was observed with soil application on the commercial substrate, which showed effects against thrips for up to 6 d after application. In addition to systemic effects observed on all foliage-feeding stages of western flower thrips, mortality on contact and repellent effects were observed on soil-inhabiting stages after soil applications of neem. Finally, bean seedlings grown from seeds pregerminated for 3 d in neem emulsion were also toxic to western flower thrips.

Animals↗

Criteria for the selection of larvicides by the Onchocerciasis Control Programme in west Africa.

One of the weekly decisions the Onchocerciasis Control Programme has to make, in its operations in 11 west African countries, is the selection of one insecticide out of the six used that is most appropriate to the river stretches to be treated. This decision depends on several criteria, linked not only to the compounds themselves but also the hydrological conditions and blackfly populations involved. Given the great number of breeding sites (gites) to be treated, in 23,000 km of rivers at the height of the rainy season, this paper identifies the parameters needed to facilitate the choice of insecticide and to optimize the larviciding in terms of cost effectiveness, management of resistance and minimizing the environmental impact.

Africa, Western↗

Elimination of vector-borne transmission of Chagas disease.

The control of the vector-borne transmission of Chagas disease in Brazil was organized as a national program in 1975, when two large entomological and sero-epidemiological surveys were conducted in the country in order to identify areas at highest risk of transmission and to guide interventions regarding the chemical treatment of domestic vectors of the disease. The authors present the baseline data gathered through these studies and compare them with more recent data. The evaluation performed shows that the transmission by Triatoma infestans is virtually interrupted and that the transmission by other native species of triatominae from different regions of the country is possibly very low. It is emphasized the need to maintain permanent actions of entomological surveillance in order to prevent recurrent transmission.

Adolescent↗

Effects of a juvenoid, DPE-28, on biology and behaviour of Culex quinquefasciatus, the human filariasis vector.

Studies on the effect of a juvenoid, DPE-28 (2,4-dinitrophenyl-2',6'-di-tertiarybutyl phenyl ether) on biology and behaviour of Cx. quinquefasciatus showed that the developmental duration, sex ratio, mating success and blood feeding were considerably affected by the exposure of larvae and pupae to the compound. Exposure of fourth instar larvae to 0.007 (EI90) and 0.0019 (EI50) ppm of DPE-28 prolonged the duration of pupation by 58.6 and 52.4 hr and delayed the adult emergence by 35.4 and 17.7 hr in males and 36.8 and 21.1 hr in females respectively. Exposure of freshly ecdysed pupae to 10 and 5 ppm delayed the adult emergence with respect to the control by 54.3 and 32.4 hr in males and 55.2 and 33.2 hr in females respectively. The sex ratio of the adults emerged from treated larvae and pupae was also affected. The female mosquitoes that survived from the exposed fourth instar larvae and pupae exhibited a low blood engorgement ratio. This depression in blood feeding was more pronounced in adults emerged from treated pupae than that of treated fourth instar larvae. A significant proportion of adults emerged from treated larvae and pupae were able to feed only partially. Mating success of the treated populations declined considerably when crosses were made between the males and females emerged from treated fourth instar larvae and pupae. The adults emerged from treated larvae and pupae showed a significant reduction in the oviposition.

Animals↗

Differentiation of members of the Anopheles fluviatilis species complex by an allele-specific polymerase chain reaction based on 28S ribosomal DNA sequences.

Anopheles fluviatilis, one of the major vectors of malaria in India, is a complex of at least three cryptic species provisionally designated as species S, T, and U. Identification of the cryptic species of An. fluviatilis complex is of paramount importance in disease control program due to contrasting differences in their vectorial efficiency, preference for feeding on humans, and resting behavior. Species S, T, and U are morphologically indistinguishable at any stage of their life cycle and can be identified only by the examination of species-specific fixed inversions in the polytene chromosomes. We report an allele-specific polymerase chain reaction assay for the differentiation of members of An. fluviatilis complex, which is based on differences in nucleotide sequences in D3 domain of 28S ribosomal DNA. The assay was evaluated against chromosomally examined individuals from different localities with different sympatric associations and was found to differentiate unambiguously all the members of the complex.

Alleles↗