Methods for the rapid appraisal of African animal trypanosomosis in the Gambia

A technique for the rapid field assessment of African animal trypanosomosis (AAT) was developed during studies in the Gambia. This involved gathering indigenous information from rapid-appraisal questionnaires addressed to local informants, the results of single tsetse surveys and evaluations of the prevalence of trypanosome infections in village cattle. Local informants included livestock owners and herdsmen and trained personnel such as livestock assistants. The answers to the questionnaires were weighted in order to translate them into semi-quantitative ranked estimates (zero, low, medium, high or very severe) of the severity of AAT problems. A similar ranking was also defined for tsetse and prevalence data in the Gambia. The three assessment methods generally gave complementary results leading to similar conclusions about the severity of tsetse-trypanosomosis problems in a survey area; inconsistencies usually suggested that additional information was needed. The rankings of AAT intensity were used to develop management guidelines for minimising the impact of AAT at different levels through control interventions or improved livestock management. The methodology was designed to provide reliable, up-to-date and cost-effective assessments of AAT problems. Emphasis was placed on the importance of the involvement, priorities and perceptions of village livestock owners and herdsmen in making these assessments.

African horse sickness in Portugal: a successful eradication programme

African horse sickness (AHS) was diagnosed for the first time in southern Portugal in autumn 1989, following outbreaks in Spain. AHS virus presence was confirmed by virus isolation and serotyping. An eradication campaign with four sanitary zones was set up by Central Veterinary Services in close collaboration with private organizations. Vaccination began on 6 October. In February 1990, vaccination was extended to all Portuguese equines (170000 animals). There were 137 outbreaks on 104 farms: 206 of the equidae present died (16%) or were slaughtered (14%); 81.5% were horses, 10.7% were donkeys and 7.8% were mules. Clinical AHS occurred more frequently in horses than donkeys and mules. In the vaccinated population, 82 animals (62.2% horses and 37.8% mules and donkeys), died or were slaughtered due to suspected or confirmed AHS. One year after ending vaccination, December 1991, Portugal was declared free of AHS. Cost of eradication was US$1955513 (US$11.5/Portuguese equine).

Modelling the distribution and abundance of Culicoides imicola in Morocco and Iberia using climatic data and satellite imagery

Relative abundances of Culicoides imicola at 22 sites in Morocco were compared with climatic variables, altitude and the NDVI (Normalised Difference Vegetation Index, a satellite derived measure of photosynthetic activity) of the same sites. Abundances were negatively correlated with wind speed and positively correlated with the average and minimum NDVI (NDVImin). There were no significant correlations with air or soil temperatures, relative humidity, saturation deficit, rainfall, altitude or mean annual maximum or range of NDVI. The best 2-variable model, which combined wind speed and NDVImin as predictors, explained over 50% of the variance in abundance. It is suggested that wind speed affects the abundance of C. imicola by causing adult mortality while NDVImin provides a measure of the availability of C. imicola breeding sites. Data from 27 sites in Iberia yielded broadly similar results to those found in Morocco although the great abundance of C. imicola at Milfontes (Portugal) could not be accounted for. Several northern sites where the NDVImin is sufficiently high to suggest the presence of C. imicola but where it appears to be absent indicate that there may be a northern limit to the distribution of C. imicola in Iberia that is independent of NDVImin. The potential use of NDVImin to predict the distribution of outbreaks of African horse sickness was investigated using data from the 1989-1990 epizootic in northern Morocco. Within the cluster of outbreaks in Larache province is a corridor of very low NDVImin in which few or no outbreaks were reported.

The seasonal and geographical distribution of Culicoides imicola, C. pulicaris group and C. obsoletus group biting midges in central and southern Spain

Pirbright-type light traps were used to collect Culicoides biting midges (Diptera: Ceratopogonidae) at fifteen sites in twelve provinces of central Spain and Andalusia. A total of 293,625 Culicoides were collected in 1,387 samples over a two year period. These comprised approximately 9.2% Culicoides imicola, 11.4% C. pulicaris group, 1.6% C. obsoletus group and 12.2% C. circumscriptus. Culicoides imicola was present at ten of the fifteen sites; the five sites from which it was absent were the most eastern of the fifteen. The greatest abundance of this species was at Navalmoral in Caceres Province. Culicoides pulicaris group were present at all sites; C. obsoletus group were present at twelve sites. The annual peaks in abundance were: C. imicola, August-October; C. pulicaris group, May-June; and C. obsoletus group, March-June. The geographical and seasonal distributions of C. imicola are consistent with those of the outbreaks of African horse sickness (AHS) and bluetongue (BT) during epizootics in Spain, and support the contention that C. imicola was the major vector of AHS and BT viruses.

Effect of temperature on African horse sickness virus infection in Culicoides

This paper shows that both the infection rate and the rate of virogenesis of African horse sickness virus (AHSV) within vector Culicoides are temperature dependent. As temperature is reduced from permissive levels the lifespan of the vector itself is extended but the rate of virogenesis decreases and infection rate falls dramatically so that at 10 degrees C virtually all midges are free from virus by 13 days post infection (dpi). When vectors that had been kept at this temperature for 35 days were moved to a permissive temperature for 3 days; however, the apparent zero infection rate increased to 15.5%. It therefore appears that at low temperature (< or = 15 degrees C) AHSV does not replicate but virus may persist in some vectors at a level below that detectable by traditional assay systems and when the temperature later rises to permissive levels virus replication is able to commence. On the basis of this information an overwintering mechanism for AHSV is suggested. The temperature at which the immature stages of Culicoides are reared may also influence infection with AHSV. A 5-10 degrees C rise in larval developmental temperature resulted in an increase in oral infection rate of a normally non-vector species of Culicoides, from < 1% to > 10%. A mechanism is suggested.

Culicoides in relation to transmission of African horse sickness virus in The Gambia

Twelve light trap collections made near overnight shelters of horses and donkeys in four villages in the Central River Division of The Gambia captured fourteen species of biting midge of the genus Culicoides. Five species new to The Gambia were identified. This brought the number of recognized species of Culicoides (after a revision of C. schultzei) to twenty-nine in The Gambia. Species known or suspected as vectors of African horse sickness virus (AHSV) and bluetongue virus (BTV) comprised 83% of female captures, 65% of captures being C. imicola or its sibling species, C. miombo. Captures of female Culicoides in the late dry season were almost as large as in the early dry season, despite the extreme heat and dryness at this time of the year. Tests on batches of formalin-preserved female midges, using AHSV or BTV antigen capture ELISAs, did not show the presence of any virus amongst 2286 females in 240 aliquots. Nearly all Gambian equines are reportedly seropositive to AHSV and these results suggest that virus challenge from Culicoides vectors may be a factor in the health of Gambian horses and donkeys.

The relationship between climate and the distribution of Culicoides imicola in Iberia

The biting midge Culicoides imicola was captured at 17 of 27 farms in Spain and Portugal during a survey of its distribution following outbreaks of African horse sickness in Iberia that occurred between 1987 and 1990. Farms were sampled approximately twice weekly from October 1992 to February 1995. Farms were widely spaced apart (maximum 850 km) and had considerable variation in climate. Across sites, summer temperatures ranged from 18.3 degrees C-27.2 degrees C; in winter the range was 4.4 degrees C-11.6 degrees C. Relative humidities in summer ranged from 37.2% to 90.1%. Proximity to southern Spain (Seville) was the most significant predictor of the presence/absence of C. imicola, but high summer temperatures and possibly dry summer conditions, were also important. Vila Nova de Milfontes in Portugal, where C. imicola was abundant and the climate is relatively cool, was an exception to the climatic trends at the other 26 sites. This exception points to a lack of knowledge of climatic requirements for immature development of C. imicola. The absence of C. imicola from the three most easterly sites, which have apparently favourable climates, suggests a relatively recent invasion by this species into Iberia.

Spatial and seasonal distribution of Culicoides imicola in Iberia in relation to the transmission of African horse sickness virus

Collections of biting midges were made over 24 months from sixty sites spread across Iberia. Information on the distribution of the vector of African horse sickness virus, Culicoides imicola, from these 3119 samples showed that this species was annually present across south-western Spain as far as 3 degrees 53'W and throughout most of Portugal, up to 41 degrees 5'N. C. imicola was found in all areas where African horse sickness epizootics had occurred in 1987-90 and also in areas outside the epizootic zones. Seasonal patterns of capture success of C. imicola, from seventeen frequently sampled sites where the vector was present, usually showed a late summer-early autumn peak. At the sites furthest south there was a discrete peak, mostly in September or October, before and after which the numbers captured increased or decreased steadily. At higher latitudes peak abundances occurred as early as May or as late as November, population build up was less uniform and numbers often declined rapidly after the peak was reached. Both the distribution and seasonal abundance patterns closely matched transmission patterns of African horse sickness virus, which rose during late summer and caused most cases during the autumn months.

Observations on the prevalence of trypanosomosis in small ruminants, equines and cattle, in relation to tsetse challenge, in The Gambia

The prevalence of trypanosome infections in Djallonké sheep and West African Dwarf goats at different sites in The Gambia showed a significant, positive correlation with contemporary assessments of tsetse challenge. A similar correlation was observed in village N'Dama cattle which showed comparable prevalence values in the same areas. Trypanosome prevalences also tended to be higher in horses and donkeys in areas with high tsetse challenge compared with sites with relatively few flies. A ranking of the numbers of tsetse blood-meals from cattle, small ruminants and equines (1:0.06: > 0.03) corresponded with the estimated biomass of these livestock groups (1:0.09:0.05). Observations on the grazing ranges of livestock showed that, while cattle foraged widely into tsetse-infested habitat, sheep, goats and donkeys remained closer to the villages. This difference indicated that, under the management system practised in The Gambia, small ruminants and equines were probably exposed to a lower level of tsetse attack than cattle.

Simulation studies of African horse sickness and Culicoides imicola (Diptera:Ceratopogonidae)

A simulation model of African horse sickness in Spain was developed to investigate what factors affect the likelihood of an epidemic after the introduction of the virus. The model included 2 host species (horses and donkeys) and 1 vector species (Culicoides imicola Kieffer). Latin hypercube sampling was used for sensitivity analysis of the model, to include uncertainty in parameter estimates. In general, if an epidemic occurred most hosts were infected. The peak prevalence in midges was low, and never exceeded 3%. Midge population size, the recovery rate in horses, and the time of year when the virus was introduced were the most significant factors in determining whether or not an epidemic occurred. The uncertainty in interbloodmeal interval, removal rate (mortality and recovery) of infectious horses, midge population size, and transmission rates were significant factors in the size of the epidemic. These factors should be priorities for empirical research, and should be considered in the design of control strategies in areas at risk of virus introduction.

Proteolytic cleavage of VP2, an outer capsid protein of African horse sickness virus, by species-specific serum proteases enhances infectivity in Culicoides

Purified African horse sickness virus (AHSV) was fed, as part of a blood meal, to adult females from a susceptible colony of Culicoides variipennis, established in the insectories at the Institute for Animal Health, Pirbright Laboratory, UK. The meal consisted of heparinized blood obtained from ovine, bovine, equine (horse and donkey) or canine sources spiked with AHSV serotype 9 (AHSV9). The infectivity levels observed for C. variipennis varied significantly, according to the source of the blood sample. Comparison of the protein profiles obtained from AHSV9 incubated with the individual serum of plasma samples indicated that some species-specific serum proteases were able to cleave the outer capsid protein, VP2. The blood samples containing serum proteases that were able to cleave VP2 also showed an increase in infectivity for the insect vector when spiked with purified AHSV.

African horse sickness and the overwintering of Culicoides spp. in the Iberian peninsula

The presence at different latitudes and the seasonal distribution of two known or potential vectors of African horse sickness (AHS) virus--Culicoides imicola and C. obsoletus--were investigated in the Iberian peninsula using light trap collections. Culicoides imicola was present as far north as 41 degrees N but not at 43 degrees N (Asturias, Spain), whereas C. obsoletus was found at all latitudes. In the northern part of the distribution of C. imicola, adults of this species were present for only a few months of the year, but adults were continually present further south. Culicoides obsoletus could be found in all months of the year in the peninsula (as in southern Britain), despite cold winter conditions. These results were compared to data from the AHS outbreak in 1987-1990 in the Iberian peninsula, to indicate the potential for the disease to persist from year to year and expand more fully in the palaearctic regions of Europe.

Tsetse-trypanosomiasis challenge to village N'Dama cattle in The Gambia: field assessments of spatial and temporal patterns of tsetse-cattle contact and the risk of trypanosomiasis infection

The severity of the trypanosomiasis problem in a particular location is traditionally assessed in terms of a challenge index-the product of some measure of tsetse abundance and infection-rate-which is assumed to be proportional to the force of infection. However, this index masks variation in the force of infection between herds and among individuals within herds. It is also not comparable between sites since the relative abundance of tsetse to hosts may vary. We have studied spatial distribution of herds of cattle in relation to tsetse in The Gambia and calculated an index of challenge based on the ratio of vectors to hosts over the livestock ranging area. This index is strongly correlated with estimates of the force of infection calculated from the incidence of infection in susceptible zebu; and it provides information on heterogeneity in exposure of different herds to tsetse.

Productivity of trypanotolerant cattle kept under traditional management conditions in The Gambia

The productivity of trypanotolerant N'Dama cattle, kept under traditional management conditions in The Gambia, West Africa, was assessed by the regular, monthly collection of health and production parameters in two study areas. The study areas were selected because of differences in tsetse challenge. Performance traits were used to build up an index to estimate the productivity of village N'Dama cattle. The productivity index per 100 kg cow maintained per year varied from 37.2 kg in the study area of Keneba village (with a low tstse challenge) to 21.4 kg for cattle kept near the villages of Tuba and Sambelkunda, an area which had a high tsetse challenge. Average age at first calving was 4.5 or 5.0 years depending on the study area, calving intervals were 623 or 703 days and there was an average 12% loss of body weight in adult females during the dry season. The productivity indices of village N'Dama cattle in The Gambia compare favourably with similar indices for trypanotolerant and trypanosusceptible breeds elsewhere in Africa, and show that even under harsh conditions and with high tsetse challenge, they are able to effectively produce milk and meat for the rural population.

Cattle-tsetse contact in relation to the daily activity patterns of Glossina morsitans submorsitans in The Gambia

The daily flight activity patterns of one of the main vectors of animal trypanosomiasis in West Africa, Glossina morsitans submorsitans, were assessed using four different methods. Results from all the methods showed that there was some flight activity nearly every hour in all seasons but they differed in the level of contact between grazing cattle herds and G.m.submorsitans. In the late dry season, trap data indicated that there was negligible activity from midday to late afternoon, whereas observations of tsetse contact with cattle herds or hand-net collections on herd followings showed no fall in attack rates, on the cattle by G.m.submorsitans. Differences between trap and animal-baited collection data may be attributable to the type of G.m.submorsitans sampled by each method. Male G.m.submorsitans captured by traps were more fat depleted than those caught on ox-baited flyrounds or by hand-net collections on herd followings. All methods showed that male G.m.submorsitans were most fat depleted in the late dry season and least in the early dry season. It was concluded that the traps were mainly sampling the spontaneous flights of G.m.submorsitans. Hunger and endogenous rhythms increase the likelihood of spontaneous flights towards dusk, particularly in conditions such as those at midday in the very hot, late dry season. However, the presence of cattle herds in infested habitats probably activated nearby G.m.submorsitans and the continual movement through the grazing areas ensured contact with tsetse throughout grazing.(ABSTRACT TRUNCATED AT 250 WORDS)

Cattle migration and stocking densities in relation to tsetse-trypanosomiasis challenge in The Gambia

The local migration of village N'Dama cattle between two study sites, Niamina East and Bansang, 40 km apart in the inland region of The Gambia, West Africa, is described. The consequences of seasonal variations in local stocking densities on the epidemiology of African animal trypanosomiasis are reported. Tsetse abundance at each study site was monitored throughout the study period from trap catches, and cattle censuses at each site were carried out on a monthly basis. Detailed ecological, productivity and health data, including dietary intake and trypanosomiasis prevalence, were collected from selected study herds resident at the two sites and from a third group of (four) herds that migrated annually between the two areas to spend the late dry season period in Niamina East. It was shown that the migration strategy allowed migrants to maintain a high level of green grazing in the diet throughout the year. Cattle were moved to the area of highest tsetse density in the region to obtain this grazing, but it appeared that individual risk of trypanosome infection was diminished by a dilution effect created by locally high livestock densities. Trypanosomiasis prevalences in resident cattle at Niamina East were best correlated with the tsetse catch/trap/day 2 months previously, once this index of tsetse abundance had been corrected to allow for changes in relative stocking density.

Trypanosomiasis in cattle in Gambia: incidence, prevalence and tsetse challenge

The incidence of trypanosome infections, measured by a Berenil Index in experimental herds of 10 Zebu and 10 N'Dama cattle, was compared with tsetse challenge and with the prevalence of parasitaemia in local N'Dama at three villages in Gambia. Tsetse challenge was more strongly correlated with the incidence of parasitaemia in the Zebu than in the N'Dama. There was a strong correlation between prevalence and incidence of infection in the N'Dama. There was no correlation, however, between prevalence of infection in cattle and tsetse challenge unless the data were offset by 3-5 months. The Berenil Index in the Zebu increased at about twice the rate as in the N'Dama under corresponding levels of challenge. It is concluded that whereas incidence of infection in susceptible animals is best measured independently, it can, under stable conditions, be inferred from an assessment of tsetse challenge.

Trypanosome infections in warthogs (Phacochoerus aethiopicus) in the Gambia

The prevalence of trypanosome infections in warthogs (Phacochoerus aethiopicus) in The Gambia was found to be 11% of a sample of 62 animals. All isolates were identified as Trypanosoma simiae. Serological evidence indicated a higher level of exposure to T. simiae, but results were inconclusive for the presence of Trypanosoma congolense. The course of T. simiae infection in warthog piglets showed a rapidly rising parasitaemia, with a concomitant fall in packed cell volume, and resulted in a prolonged period of low-level parasitaemia. The same infections killed domestic piglets.

An analysis of survey measurements of tsetse challenge to trypanotolerant cattle in relation to aspects of analytical models of trypanosomiasis

The recent development of analytical models of trypanosomiasis has increased the general applicability of models to the strategic control of the disease. An analysis of detailed data on tsetse abundance and infection rates and trypanosome prevalence in village-based trypanotolerant cattle over 4 years in The Gambia showed that seasonal patterns of abundance in Glossina morsitans-infested areas were consistent, and that the rates of trypanosome infection remained relatively unchanging. However, there were two distinct seasonal trypanosome prevalence patterns in cattle, with peaks occurring either in May/June/July or November/December. The peaks of trypanosome prevalence therefore occurred either 4 months before or after the times of peak challenge from G. morsitans, not 1 or 2 months after as predicted by analytical models. In G. palpalis-infested areas there was little seasonal variation in abundance or trypanosome infections, but peak trypanosome prevalence still occurred mostly in June/July. Despite the incongruity between the months of peak prevalence and challenge, the overall annual prevalence rates and tsetse challenge indices showed a significant linear relationship. It is concluded that existing analytical models need to be refined to take into account trypanotolerance and the various influences on the expression of this trait.

Experimental infection of N'Dama cattle with trypanosomes using Glossina palpalis gambiensis caught in the wild

The transmissibility of trypanosome infection to N'Dama cattle by tsetse flies caught in the field was examined. Wild-caught Glossina palpalis gambiensis were transferred singly into small numbered cages and allowed to feed on 14 uninfected N'Dama cattle. Following a completed feed the tsetse were dissected and infection in the proboscis, the salivary glands and the gut was recorded. Each animal was bitten by a number of tsetse ranging from five up to 64 flies. Following dissection of the tsetse flies, seven of the cattle were found to have been bitten by a single infected tsetse, five by two, while the remaining two were each fed upon by three infected tsetse. The tsetse were harbouring either Trypanosoma vivax or a trypanosome species belonging to the Nannomonas subgenus or both species. The experimental animals were monitored daily over a period of three months for the appearance of trypanosomes in the blood and for antibodies in their sera. Other parameters such as body temperature, local skin reactions, packed red cell volume and weight changes were also measured. Trypanosomes were first detected eight days after the infective bite. Only five of the 14 cattle became infected, of which three had been exposed to a single infected tsetse fly. Trypanosoma vivax was detected in one animal, Trypanosoma congolense in two cattle and mixed infections of both species in the remaining two animals. These findings show that N'Dama cattle can become infected with trypanosomes through the bite of a single infected tsetse fly under field conditions. However, only five (possibly eight) of the 23 infected tsetse were able to transmit the parasites successfully.

Isolation and cultivation in vitro to the infective, metacyclic stage of Trypanosoma (Nannomonas) simiae from Glossina morsitans submorsitans

Two separate trypanosome isolations were made from a single Nannomonas-infected Glossina morsitans submorsitans from The Gambia. Inoculation of a piglet with the infected hypopharynx produced an infection with Trypanosoma simiae. DNA was isolated from the bloodstream forms to prepare a probe specific for this species. Trypanosomes isolated from the fly midgut were frozen in liquid nitrogen and then cultivated in vitro. Amplification of this population and elimination of a yeast contaminant were achieved by two passages through laboratory G. m. morsitans. Further cultivation in vitro resulted in the production of epimastigotes and, later, metacyclic forms. Two pigs inoculated with cultivated metacyclic forms developed infections with atypical, relapsing parasitaemias and extended survival time. Neither the metacyclic forms, nor bloodstream forms derived from them, infected calves. The identity of various stages of the in vitro cultivated, procyclic-derived stock was confirmed morphologically and with the T. simiae-specific DNA probe.

Ineffectiveness of historical data in predicting measles susceptibility

The Immunization Practices Advisory Committee ( ACIP ) has devised noninvasive, historical criteria for determining individuals who are susceptible to measles. These criteria, which involve proof of vaccination, are incorporated into school entrance regulations and are used to indicate people who require vaccination during outbreaks. In a recent measles epidemic in El Paso , TX, 120,000 records were screened using these criteria, and as a result 13,000 students were vaccinated. During this outbreak, 91 adolescents, who were susceptible to measles by ACIP criteria, were serologically tested for measles antibody. Although none of these students had documentation of vaccination, only 11.0% of them lacked measles hemagglutination-inhibiting (HAI) antibody at a titer of 5. Assuming a minimum cost for vaccine of +2.60 per dose, a conservative estimate of the cost to the El Paso Health Department for 20,000 doses of measles vaccine would be +52,000. If these data can be extrapolated to the total student population, then upwards of 85% of vaccinated students were already immune. Thus, +44,200 was spent unnecessarily. In addition, as the ACIP criteria did not select for measles susceptibility, an estimated 12,000 students in El Paso were not protected against measles. Other methods to determine measles susceptibility should be developed for optimal control of future outbreaks.

Assessment of the consequences of the house-spraying of malathion on the interruption of malaria transmission

Marked female Anopheles culicifacies were released into an enclosed palm-leaf hut at dusk and survivors recovered the following drawn by hand on several occasions before and after spraying with the insecticide malathion. Pre-spray releases yielded only 25-50% recovery which was thought to be due to concealment of resting mosquitoes in the palm-leaf. A mortality of 100% was recorded for over 2 months after spraying the insecticide but after 71 days small numbers of live recoveries were found. The likely effect on malaria transmission of the observed survival rates was assessed theoretically. Although the percentage survival found in the experiment would not seem to prevent the control of malaria the data suggest that the spraying regime would immediately give a great selective advantage to any resistance gene arising in the population.

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.Following blood-feeding on a cow in a hut, A. culicifacies were found to rest in the hut for 1 or 2 days, whereas A. subpictus and A. varuna generally exited on the night of feeding or on the following night. The same individual A. culicifacies were recorded resting in a hut for 1 or 2 days on different occasions. The apparent absence of genetic variability in host choice or indoor resting behaviour is considered encouraging for the prospects of control of malaria transmitted by A. culicifacies.

Field measurement of the effective dominance of an insecticide resistance in anopheline mosquitos

Anopheles culicifacies that were susceptible, heterozygous, or homozygous resistant to HCH and dieldrin were differentially marked with fluorescent dusts and released twice weekly into village huts in Pakistan that had been sprayed with four different dosages of HCH to see which of the genotypes died and which survived. The three highest dosages killed all three genotypes in the first four weeks, and heterozygotes and susceptibles for at least 12 weeks. The lowest dosage killed all the susceptibles throughout the period, and all but 0.07% of the heterzygotes. Thus the resistance is effectively recessive at the higher dosages and unlikely to be selected rapidly, as long as the gene frequency is low to start with and the houses are sprayed regularly. Similar releases of partially and completely resistant A. stephensi, and completely resistant A. subpictus, showed greater survival rates on exposure to the high HCH dosages than the same genotypes of A. culicifacies.