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PubMed1985

Modification of sand fly biting behavior by Leishmania leads to increased parasite transmission.

To attempt rodent-sand fly-rodent transmission of Leishmania major, laboratory-reared Phlebotomus doboscqi were fed on L. major-infected mice and then refed on uninfected mice 21 days later. Flies which refed either probed 1-2 times and took a full blood meal in less than 10 min or probed 3 or more times and took little or no blood during a period of 15 min or more. When dissected, 7 of 8 flies which experienced difficulty in obtaining a blood meal had flagellates in their cibaria, an observation supporting the hypothesis that parasites in this part of the alimentary canal modify normal blood feeding behavior. None of the infected females which probed 1-2 times had similar anterior station infections. Infected sand flies transmitted L. major to uninfected mice and a single fly, transferred from 1 mouse to the next while repeatedly attempting to take blood, infected 5 mice. During a year-long survey in Baringo District, Kenya, we collected 9,182 female sand flies. Only 2 of the 278 P. duboscqi captured during this collection were infected with L. major; however, 18 of the 789 small rodents from this area were infected with L. major. Parasite interference with normal blood feeding may explain how a relatively small population of P. duboscqi, only a few of which are infected with L. major, can amplify parasite transmission thereby maintaining a disproportionately large reservoir in local rodents.

Animals↗

PubMed1986

Dispersal and other population parameters of Aedes aegypti in an African village and their possible significance in epidemiology of vector-borne diseases.

Dispersal of Aedes aegypti aegypti adults within Shauri Moyo, an African village in the Rabai area north of Mombasa, Kenya, was studied using the mark-release-recapture method. A total of 920 mosquitoes were captured and uniquely marked, of which 828 (90%) were released and 332 (40%) recaptured. A great majority of mosquitoes were recaptured once, but some individuals were recaptured up to 10 times. Most females visited 1 or 2 houses (40.8%, 44.9%, respectively), but there were females that visited as many as 5 houses. Mosquito movement among houses of the village is summarized in tables. Dispersal ability of Aedes aegypti females was d = 0.592 and males d = 0.433. The maximum distance females and males moved per 24 hr was 154 m and 113 m, respectively. The mean distance rate was 57.0 m/day for females and 44.2 m/day for males. The size of the mosquito population at Shauri Moyo village was estimated to be 331.1 +/- 146.5 using the Lincoln index method, 533.3 by the Fisher and Ford method, 270.3 +/- 179.5 by Bailey's triple catch method and 380.1 +/- 92.9 by the Jolly-Seber method. The mean female and male survival was 10.7 and 5.8 days, respectively. The maximum survival of females was 42 days and males 18 days. The daily survival of females and males estimated by the Fisher and Ford method was 0.825 and 0.693, respectively. The daily survival estimated using the Jolly-Seber method was 0.852 for females and 0.532 for males. Longevity is discussed in relation to extrinsic incubation periods of yellow fever and dengue viruses. The recapture rates, survivorship, and size of the village population indicate that the Ae. aegypti population in Shauri Moyo village is fairly stable. Presence or absence of blood in the stomach, and developmental stages of the ovaries were measured, showing that the majority of females fed twice and some fed as many as 3 times during one gonotrophic cycle.

Aedes↗

PubMed1986

Rhodnius neivai: a new experimental vector of Trypanosoma rangeli.

Rhodnius neivai was as efficient as Rhodnius prolixus and Rhodnius robustus in transmitting by bite a Colombian strain of Trypanosoma rangeli following its inoculation into the hemocoel. Under conditions of the study the strain of T. rangeli had a high and constant infectivity to the salivary glands of R. prolixus, its natural vector in Colombia. Six species of Triatoma and Dipetalogaster maximus likewise inoculated did not develop metatrypomastigotes in the salivary glands. Of the 12 known species of Rhodnius, R. neivai is the eight to be demonstrated as an anterior station vector of T. rangeli.

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PubMed1987

Quantitative studies of onchocerciasis transmission by Simulium yahense and Simulium sanctipauli in the Firestone Rubber Plantation at Harbel, Liberia.

Black fly vectors of onchocerciasis from three ecologically different Simuliid breeding habitats in the Firestone Rubber Plantation at Harbel, Liberia, were surveyed by human-biting collections conducted at weekly intervals over a 13-month period. Black flies were identified morphologically and the monthly and seasonal contribution of different vector species to the transmission of Onchocerca volvulus at each site was determined. Simulium yahense was identified as the predominant vector species at each site. Greatest populations of this species occurred during wet season (May-Oct), but its impact on transmission of onchocerciasis was most profound during dry season (Nov-Apr) when parity, infection, and infectivity were high. S. sanctipauli was the only other vector species captured, but biting populations of this species were small, and during wet season confined primarily to the vicinity of its breeding site in the Farmington River. Dry season populations of S. sanctipauli were also characterized by lower human-biting rates, and by higher rates of parity, infection, and infectivity. Monthly transmission potentials at each site were attributed primarily to S. yahense, with peak monthly transmission occurring during the dry season months of January-April. Against the WHO standard of 100 as a "tolerable" annual level of onchocerciasis transmission, annual transmission potentials for the three sampling sites were 94, 1,877, and 4,900, with highest values being calculated for S. yahense breeding sites.

Animals↗

PubMed1988

Human malaria transmission studies in the Anopheles punctulatus complex in Papua New Guinea: sporozoite rates, inoculation rates, and sporozoite densities.

Malaria sporozoite rates and inoculation rates were measured over periods up to 25 months in the different anopheline species biting humans in 13 villages in Madang Province, Papua New Guinea. Analysis of three members of the Anopheles punctulatus complex, 68,458 An. farauti, 36,779 An. koliensis, and 11,667 An. punctulatus caught in landing catches was made using monoclonal antibody based ELISAs to detect sporozoites of Plasmodium falciparum and P. vivax. Sporozoite rates ranged from 0%-5.5% in An. farauti, 0.2%-3.8% in An. koliensis, and 0%-3.3% in An. punctulatus. In addition, over 3,000 An. longirostris were analyzed and sporozoites were not detected in this species. No significant differences were observed between the three vector species in the densities of P. falciparum sporozoites (geometric mean 2,320). However, the geometric mean P. vivax sporozoite density was significantly higher in An. punctulatus (350) than in either An. koliensis (160) or An. farauti (150). An. koliensis was less susceptible to infections of P. falciparum or P. vivax than either An. farauti or An. punctulatus. Variations in average sporozoite and inoculation rates were found among different villages, despite their close geographic proximity. Sporozoite and inoculation rates varied greatly within a village over time, but malaria transmission was perennial with a higher transmission during the wet season by An. koliensis and An. punctulatus.

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PubMed1989

An analysis of some factors determining the sporozoite rates, human blood indexes, and biting rates of members of the Anopheles punctulatus complex in Papua New Guinea.

The degree to which Anopheles punctulatus complex members feed on humans in different Papua New Guinea villages has a significant effect on sporozoite rates. Among villages, the human blood index (HBI) of the members of the complex varied with the average number of persons sharing a bednet. Although dogs are the preferred hosts by the 3 malaria vector species, the number of dogs did not significantly affect the HBI. The HBI was dependent upon the human-biting rate, implying increased avoidance of anophelines by people relative to other hosts at times of greater mosquito numbers. Human-biting rates and HBIs were also influenced by the distribution of alternative hosts relative to people.

Animals↗

PubMed1989

Annual transmission potential of bancroftian filariasis in an urban and a rural area of West Bengal, India.

The annual transmission potential of Wuchereria bancrofti was calculated from the number of infective larvae found in the number of Culex quinquefasciatus expected to bite an individual exposed 24 hr a day for a year in Calcutta and in a rural (Memari) area. In Calcutta, an average of 151 mosquitoes/night/person were collected; it was calculated that 55,028 mosquitoes could bite a person per year, including an average of 177 infective mosquitoes containing 319 infective larvae of W. bancrofti. At Memari, an average of 284 mosquitoes/night/person were collected; here with the resulting calculation was that 103,718 mosquitoes could bite a person per year, including an average of 137 infective mosquitoes containing 223 infective larvae. The annual transmission potential was higher in the urban area than that in the rural area. Annual transmission potential is useful knowledge in the epidemiological assessment of transmission dynamics for the formulation of control strategies.

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PubMed1992

Vectorial transmission of Trypanosoma cruzi: an experimental field study with susceptible and immunized hosts.

The dynamics of vectorial transmission of Trypanosoma cruzi and the level of host (guinea pigs) protection after immunization with attenuated parasites (TCC strain) was studied under natural climatic conditions in an endemic region of northern Argentina. The experimental design included two guinea pig corrals isolated by mosquito netting. One (controls) had 17 healthy and susceptible adult guinea pigs. The other had 19 guinea pigs immunized with attenuated T. cruzi TCC strain. Each corral was colonized in April 1988 with equal-sized populations of Triatoma infestans naturally infected by T. cruzi. To evaluate relevant variables in the natural transmission of Chagas' disease, corrals were sampled in both winter and late spring to assess vector populations, and to carry out parasitologic studies on both vertebrate and invertebrate hosts. In both corrals, vector density decreased in winter and reached a maximum in the hot season. The vector infection rate was very high (greater than 50%) throughout the experiment. Vector infectivity increased with temperature and vector age, but did not differ between the experimental and control corrals. The vector-host contact rate showed a close relationship with temperature, although a very high vector density decreased this rate, even with high ambient temperatures. Initial infections by T. cruzi occurred among guinea pigs only during the hot season. Vectorial transmission risk was estimated from the total number of bug bites per day, the proportion of infected bugs, and the daily incidence in the guinea pig population. During the hot season, this risk was 6.84 x 10(-4) in the control group and 1.82 x 10(-4) in the immunized group.

Animals↗

PubMed1992

Heightened transmission of stable malaria in an isolated population in the highlands of Irian Jaya, Indonesia.

Malaria at an elevation of 1,500 meters is uncommon and is usually unstable when it occurs. To confirm reports of a recent increase in transmission of stable malaria in the Oksibil Valley, which is at an elevation of 1,250-1,500 meters in the Jayawijaya Mountains of Irian Jaya, Indonesia, five malariometric surveys were conducted in four villages between May 1990 and July 1991. A total of 3,380 blood smears, representative of 1,949 persons, was examined. Prevalence rates over the survey period were consistent in each of the four villages, averaging 10% for infants, 50% for children 1-4 years of age, 35% for those 5-9 years old, 28% for those 10-14 years old, and 16% for adults (greater than 15 years old). The spleen rate for the those less than five years old was 96%, with an average enlarged spleen score of 2.32. Plasmodium falciparum accounted for 55% of the infections in the valley, but P. vivax was the predominant species in those less than 10 years old. In the village of Kutdol at an elevation of 1,500 meters, P. malariae was identified in 43% of the positive smears. Four cases were diagnosed as P. ovale. Infection with P. falciparum without obvious clinical symptoms was common in both adults and children. Entomologic and epidemiologic data suggested that the recent upsurge in transmission coincided with the replacement of traditional village huts with the more modern social housing. This replacement required the extensive construction of drainage ditches, which inadvertently also served as additional vector breeding sites. We suspect that this manipulation of the environment, in an effort to improve the quality of life, created conditions conductive for heightened transmission of stable malaria.

Adolescent↗

PubMed1993

A study of onchocerciasis with severe skin and eye lesions in a hyperendemic zone in the forest of southwestern Cameroon: clinical, parasitologic, and entomologic findings.

Prior to the initiation of an onchocerciasis control program based on the mass administration of ivermectin in the rain forest of southwestern Cameroon, a preliminary baseline study of the area was conducted. The results of this study showed that onchocerciasis was hyperendemic in the area. Skin symptoms and signs were observed including pruritus (67.4% of the population examined), onchocerca nodules (51.6%), skin depigmentation (18.5%), and hanging groins (5.7%). Except for pruritus, the prevalence of these symptoms increased with age. Of the eyes examined, 44.9% had microfilariae in the anterior chamber, 33.5% had choroidoretinitis, 28.0% had punctate keratitis, 8.3% had papillary abnormalities, and 3.6% had sclerosing keratitis. Vision in 10.5% of the eyes examined was classified as blind or very poor (visual acuity = 0-0.10), in 15.7% as poor (visual acuity = 0.11-0.39), and in 73.8% as good (visual acuity = 0.4-1.00). Unlike previous reports that have linked serious ocular damage mainly to savanna onchocerciasis, the present study showed that forest onchocerciasis also caused significant ocular pathology, including blindness. Parasitologically, positive skin snips were recorded for 92.7% of the persons examined, with both sexes being equally infected. The parasite load, expressed as the geometric mean number of microfilariae per skin snip, was 53.6, and was much higher in males than in females. The flv vector, Simulium squamosum, had a high infection rate of 7.5% infective females in Bakumba and 6.8% infective females in Ngbandi, the two fly-catching points. The transmission potential was 266 infective larvae per person per month in Bakumba and 189 in Ngbandi.

Adolescent↗

PubMed1993

The blackfly Simulium buissoni and infection by hepatitis B virus on a holoendemic island of the Marquesas archipelago in French Polynesia.

The hematophagous blackfly Simulium buissoni causes skin lesions on an island in the Marquesas archipelago that is holoendemic for hepatitis B virus (HBV). To test the hypothesis of the possible role of this fly in the transmission of hepatitis B, 506 children (age range 2-11 years) were examined for the presence of skin lesions, and attempts were made to detect HBV DNA in and on blackflies using two polymerase chain reaction methods. The mean number of skin lesions showed a positive correlation with the age of these children (r = 0.12, P < 0.05). Furthermore, it was significantly higher in the rural zone than in the urban zone (mean +/- SD 41.02 +/- 31.71 versus 17.73 +/- 13.43; P < 0.05), and showed a correlation with a higher infection rate (73.9% versus 41.3%). Of the 45 pools of 10 insects tested, HBV DNA was not detectable on the inside of the insect, but was detectable on the flies (1-10 particles/insect in three positive pools). Infection by HBV conveyed by the flies is theoretically possible, but their indirect role via the numerous skin lesions caused on children is likely to explain such a high level of transmission.

Adolescent↗

PubMed1993

Low-level Plasmodium falciparum transmission and the incidence of severe malaria infections on the Kenyan coast.

The transmission of Plasmodium falciparum was studied in relation to the incidence of severe malaria infections at Sokoke and Kilifi town, Kilifi District, Kenya. Intensive mosquito sampling during a one-year period yielded Anopheles gambiae s.l., An. funestus, and An. coustani. Anopheles gambiae s.l. was the predominant vector, comprising 87.9% and 97.9% of the total anophelines collected in Sokoke and Kilifi town, respectively. The proportion of An. gambiae s.l. with P. falciparum sporozoite infections was 4.1% (20 of 491) in Sokoke and 2.2% (3 of 138) in Kilifi town; no infections were detected in An. funestus or in An. coustani. Entomologic inoculation rates indicated that residents were exposed to only 8.0 infective bites per year in Sokoke and 1.5 in Kilifi town. Transmission was detected during only six months in Sokoke and three months in Kilifi town despite low-level, year-round vector activity. The yearly incidence of severe P. falciparum infections in children, 1-4 years of age was 24.1 per 1,000 in Sokoke and 4.2 per 1,000 in Kilifi town. Monthly patterns of transmission corresponded closely with the incidence of severe infections. At these sites on the coast of Kenya, the spatial and temporal incidence of severe malaria infections is associated with low-level P. falciparum transmission by vector populations.

Animals↗

PubMed1994

Variation of the vectorial capacity of some anophelines in western Venezuela.

The vectorial capacities of Anopheles nuneztovari, An. albitarsis s.l., and An. triannulatus to transmit Plasmodium vivax in western Venezuela were estimated in different seasons. In general, there was a decrease in the vectorial capacities of the three species during the dry season, which seems to be due mainly to a reduction in the mosquito biting populations. Comparing the observations made during the two wet seasons, there was a decrease in the vectorial capacities of the three species due to different factors for each species. In An. nuneztovari, the reduction in vectorial capacity seems to be due to a reduction in the human blood index (HBI); in An. albitarsis, it is probably due to a reduction in the HBI and a slight reduction in parity, whereas in An. triannulatus, the cause is probably a reduction in the mosquito biting population and in parity. The overall reduction in vectorial capacities of the three anopheline species in 1989 compared with the vectorial capacities in 1988 could partly explain the marked reduction in prevalence from 46.2 per 1,000 population in 1988 to 7.5 per 1,000 population in 1989.

Animals↗

PubMed1994

Effect of the developmental stage at infection on the ability of adult Anopheles stephensi to transmit Rift Valley fever virus.

The ability of adult Anopheles stephensi to transmit Rift Valley fever virus was determined for mosquitoes inoculated at selected times during development. None of 109 female An. stephensi inoculated as adults transmitted virus to hamsters. In contrast, 83% (50 of 60) of those inoculated as larvae transmitted virus by bite to hamsters. Transmission rates decreased as the stage of the mosquito at the time of inoculation changed from larva to pupa to adult. Transmission rates for adult mosquitoes inoculated as larvae, as pupae < 4 hr after pupation, as pupae > 24 hr after pupation, or as adults were 83%, 25%, 11%, and 0%, respectively. Viral titers recovered from mosquitoes were similar for all groups tested, regardless of stage at infection (larva, pupa, or adult) or of transmission status (transmitter or nontransmitter). Thus, differences in transmission rates may have been due to site-specific (i.e., salivary gland) replication, rather than a generalized increase in viral replication in mosquitoes inoculated at an earlier age.

Animals↗

PubMed1994

Plasmodium falciparum incidence relative to entomologic inoculation rates at a site proposed for testing malaria vaccines in western Kenya.

Relationships between Plasmodium falciparum incidence and entomologic inoculation rates (EIRs) were determined for a 21-month period in Saradidi, western Kenya, in preparation for malaria vaccine field trials. Children, ranging in age from six months to six years and treated to clear malaria parasites, were monitored daily for up to 12 weeks to detect new malaria infections. Overall, new P. falciparum infections were detected in 77% of 809 children. The percentage of children that developed infections per two-week period averaged 34.7%, ranging from 7.3% to 90.9%. Transmission by vector populations was detected in 86.4% (38 of 44) of the two-week periods, with daily EIRs averaging 0.75 infective bites per person. Periods of intense transmission during April to August, and from November to January, coincided with seasonal rains. Relationships between daily malaria attack rates and EIRs indicated that an average of only 7.5% (1 in 13) of the sporozoite inoculations produced new infections in children. Regression analysis demonstrated that EIRs accounted for 74% of the variation in attack rates. One of the components of the EIR, the human-biting rate, alone accounted for 68% of the variation in attack rates. Thus, measurements of either the EIR or the human-biting rate can be used to predict corresponding attack rates in children. These baseline epidemiologic studies indicate that the intense transmission patterns of P. falciparum in Saradidi will provide excellent conditions for evaluating malaria vaccine efficacy.

Animals↗

PubMed1995

Laboratory and field evaluations of a repellent soap containing diethyl toluamide (DEET) and permethrin against phlebotomine sand flies (Diptera: Psychodidae) in Valle del Cauca, Colombia.

The repellency and insecticidal efficacy of Nopikex, a soap formulation containing 20% diethyl toluamide and 0.5% permethrin, was evaluated against a laboratory colony of phlebotomine sand flies (Lutzomyia longipalpis). The repellency of Nopikex soap was also compared with that of a placebo soap against another species (Lu. youngi) in a forest near Tulua, Valle del Cauca, Colombia. In laboratory trials of the soap, no reduction in repellency was seen 4 hr after application, but within 8 hr, repellency decreased significantly to 67.0% of the initial value (P < 0.05) based on calculations of the coefficient of protection (CP). Under field conditions, the soap gave up to 100% protection immediately after application, but within 4 hours its CP value had decreased to a median value of only 44.3%. The placebo soap was also found to be somewhat repellent when compared with no treatment, giving a median CP value of 67.7 immediately after application. No significant mortality was seen in sand flies within 24 hr of exposure to the soap in the laboratory, even in those that had fed on protected volunteers.

Animals↗

PubMed1995

Relationships between Plasmodium falciparum transmission by vector populations and the incidence of severe disease at nine sites on the Kenyan coast.

The transmission of Plasmodium falciparum was studied in relation to the incidence of severe malaria infections at nine sites in the Kilifi District in Kenya. Intensive mosquito sampling during a one-year period yielded Anopheles gambiae s. l., An. funestus, An. coustani, An. squamosus, An. nili, and An. pharoensis. Anopheles gambiae s.l. was the predominant vector, comprising 98.4% of the total anophelines collected. Overall, 3.5% of 2,868 An. gambiae s.l. collected indoors and 0.8% of 261 collected outdoors contained P. falciparum sporozoites. Transmission was detected during 10 months, with peak periods from June to August and December to January. In eight of the nine sites, entomologic inoculation rates (EIRs) averaged only four infective bites per year (range 0-18); an annual EIR of 60 was measured for the site with the highest intensity of transmission. The incidence of severe malaria infections, ranging from 8.6 to 38.1 per 1,000 children (0-4 years), was not associated with EIRs. At these sites on the coast of Kenya, a high incidence of severe disease occurs under conditions of very low levels of transmission by vector populations. With respect to conventional approaches for vector control in Africa, decreases in transmission, even to levels barely detectable by standard approaches, may not yield corresponding long-term reductions in the incidence of severe disease.

Animals↗

PubMed1995

The effectiveness of insecticide-impregnated bed nets in reducing cases of malaria infection: a meta-analysis of published results.

The use of insecticide-impregnated bed nets to minimize human-vector contact may reduce the incidence of malaria. Consequently, several field trials have evaluated their effectiveness as a malaria prevention strategy. A meta-analysis of published reports of field trials that measured the incidence of infections was performed to provide a measure of the effectiveness of insecticide-treated bed nets in preventing clinical malaria. Subsetted analyses were performed on the 10 field trials to calculate pooled incidence rate ratios of infection among the study groups. For the studies comparing insecticide-impregnated bed nets with untreated bed nets, the summary incidence rate ratio for acquiring malarial infections was 0.757 (95% confidence interval [CI] = 0.612-0.938), representing a reduction of 24%. For the studies comparing permethrin-impregnated bed nets with controls without bed nets, the summary incidence rate ratio was 0.497 (95% CI = 0.417-0.592) (Rothman-Boice heterogeneity statistics = 17.27 [P = 0.004] and 23.55 [P = 0.0003], respectively). These data suggest that insecticide-impregnated bed nets are effective in preventing malaria, decreasing the incidence rate ratio by approximately 50% in field trials performed to date.

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