Biting Density Research Articles

Studies on the transmission and prevalence of Bancroftian filariasis in four coastal villages of Tanzania.

In surveys of Bancroftian filariasis in four coastal villages in Tanzania, microfilaria rates varied between 28·3 and 15·6% and hydrocoele rates in males over 20 years between 78–5 and 61–3%.Quantitative studies on the intensity of transmission were conducted over a period of 17 months. The estimated potential infective bites per person per year varied from 24 to 189. The number of infective bites was related to filarial endemicity.The vectors in order of importance were Anopheles gambiae s.l., Culex pipiens quinquefasciatus and An. funestus. In all villages An. gambiae s.l. played a role in transmission and in two it was the main vector. Of the total estimated bites by members of the gambiae complex, 16 223 (91·1%) occurred during or following the ‘long rains’ (April to early June). In contrast, transmission by C. p. quinquefasciatus occurred throughout the year. In one village in which C. p. quinquefasciatus was the predominant vector, the biting density of this species was 22 times greater than the bit...

Seasonal trends and diurnal patterns of man-biting activity of four species of Guatemalan black flies (Simuliidae).

Collections and dissections of black flies from human subjects were carried out over a 13-month period at two onchocerciasis-endemic sites in Guatemala. Simulium ochraceum comprised 87% of all flies collected. Populations of this species peaked during the first part of the dry season in November, while populations of S. metallicum, S. callidum and S. downsi were highest during the rainy season from June-October. Diurnal patterns of host-seeking activity were most pronounced for S. ochraceum, with a sharp peak occurring between 0700 and 0900 hours each day. Parous ratio was lowest at this time (27%), and rose to a peak of 63% between 1200 and 1400 hours each day. Eighty-eight percent of all parous S. ochraceum collected during 1200-1400 hours had large dilatations in the tunica of the ovarioles resulting from recent oviposition, thus indicating that this species oviposits in the morning and immediately seeks the subsequent blood meal. Parous biting density (the product of the total number of flies biting at a given time and the corresponding parous ratio) showed two distinct diurnal peaks, one in the early morning characterized by low parous ratio and high total number of flies, and the other in the early afternoon characterized by high parous ratio and low total number of flies. Moreover, the diurnal pattern of biting density of filaria-infected S. ochraceum was similar to that of parous biting density.

Seasonal variation in the microfilarial skin density of Onchocerca volvulus and in the biting activity of Simulium spp. in Guatemala

The density of Onchocerca volvulus microfilariae in the skin of 11 carriers was examined by carrying out a monthly examination in a single endemic area of Guatemalan onchocerciasis. The results obtained suggested the possibility of a seasonal variation in the microfilarial skin density. Almost all of the volunteers participating had shown a peak concentration of microfilariae in September, followed by August or October. The highest biting densities of Simulium ochraceum, the principal vector of onchocerciasis in Guatemala, occurred from September to the following January. It thus appears that the seasonal variation in the microfilarial density may correspond, to some extent, with the biting activity of the vector, S. ochraceum, and/or with climatic changes in the area.

Biting prevalence and malaria transmission patterns in the Anopheles punctulatus complex (Diptera: Culicidae) in Papua New Guinea.

Seasonal prevalence and malaria transmission patterns of Anopheles farauti, An. punctulatus and An. koliensis were studied in two coastal and two inland villages in the Madang district of northern Papua New Guinea. The monthly manbiting rates (average biting density of vector mosquitoes per man per night) were observed over a two-year period to establish typical seasonal prevalence patterns. Observations on the frequency of feeding and the human blood index were combined to derive the manbiting habit. A daily survival rate was determined from the dissection of mosquito ovaries by Polovodova's age-grading method. The extrinsic cycle in the mosquito was not observed, and the value for this parameter was extracted from the literature. All parameters were combined to estimate the monthly variations in vectorial capacity of each of the vector species, and the vectorial capacities of the individual species were added together to give a total monthly vectorial capacity of the punctulatus complex in each locality. The results showed that malaria transmission can be perennial, with a period of more intense transmission between August and January in the southern area, and between September and February in the northern. A period of very low transmission can be expected between February/March and July/August.

The biology of the tree-hole breeding mosquito Aedes geniculatus (Olivier) (Diptera: Culicidae) in southern England

AbstractThe biology of Aedes geniculatus (O1.) was studied in 1967–73 in Monks Wood National Nature Reserve, an oak-ash woodland, in southern England. Adult emergence from tree-holes occurred from May to September, and the sex ratio was approximately 1:1. Catches on man indicated that the maximum biting densities occurred in July and August and that females spend about two minutes obtaining a blood-meal. Significant positive correlations were found between their unfed weight and the weight of the blood-meal and also between female wing length and fecundity. The mean number of eggs per batch was 70. Studies with bamboo pots used as artificial oviposition sites in the field showed that the seasonal peak in oviposition spanned July and August, daily ovipositional activity was greatest in mid-afternoon and evening and ovipositing females preferred oviposition sites positioned within 4 m of the ground. A small proportion of eggs hatched during the summer in which they were laid, but most hatched in the following spring. Continuous or intermittent soaking failed to induce hatching until this time. In 'the laboratory, partially em'bryonated eggs required a minimum relative humidity of 88% to develop fully, but some embryonated eggs survived at 58%. Some eggs survived for 18–24 months in the field. The sampling of preadult stages dn tree-holes showed that larvae in the first two instars occurred almost throughout the year while third- and fourth-instar larvae were present in spring and summer. Pupae occurred only in the summer. It was concluded that A. geniculatus is univoltine in Monks Wood.

Onchocerciasis Transmission Potentials of Four Species of Guatemalan Simuliidae *

Two complete days of collections and dissections of all black fly species landing on two persons infected with Onchocerca volvulus were made at monthly intervals for 13 months within a single endemic area of Guatemala. A comparison of the infective biting densities and transmission potentials showed that Simulium ochraceum was the only significant vector.

Studies on the flight range and dispersal ofSimulium damnosum(Diptera: Simuliidae) in the rain-forest of Cameroon

Five marking/recapture experiments were carried out on Simulium damnosum in the rain-forest of the United Republic of Cameroon, in order to determine the regression in biting density at increasing distances from the marking point. Flies were captured unfed, marked with fluorescent dust, and then released on the bank of a large river. Recaptures were made over the six-seven days after marking, at seven stations situated from 200 m to 79 km along a large river, from the marking site. Marked flies, which showed fluorescent pigment when examined under an ultraviolet lamp, were recovered at all seven sites. The ratios of the number of recaptures per man-hour at each distant site to the recaptures per man-hour at the marking point were calculated to obtain values for the proportional biting density (Z). The value of Z declined in a logarithmic manner with increasing distance from the marking site, such that it fell to 0-1 at distances of 34-1 km on a large river, 5-5 km on a small river, and 1-5 km on a road leading away from a large river. The peak number of flies recaptured at the farthest sites along a large river (56 km and 79 km) occurred one day later than at the station nearest the marking point. The dispersal of marked flies which had fed on volunteers carrying Onchocerca volvulus microfilariae, was studied along a large river. Of three flies recaptured 24 km upstream from the marking point, one contained 10 infective larvae of O. volvulus.

A quantitative approach to the study of Bancroftian filariasis.

Abstract Exposure to infection with Wuchereria bancrofti was studied for 1 year in a suburb of Calcutta, India, and correlated with parasitological and clinical observations on the human study population. Blood surveys at the beginning and end of the study showed microfilaremia rates of 12.4% and 14.8%, respectively. The median microfilarial density remained constant at 18 per 20 mm3 of blood. A clinical survey at the end of the study revealed little severe filarial disease. However, between 30% and 40% of the males in the age groups above 20 years had milder symptoms suggestive of filariasis, mostly small hydroceles and cord abnormalities. Exposure to infection with stage III larvae was very heavy and continuous throughout the year. It was estimated that a person sleeing indoors during the year of this study would have been bitten by 1,850 “infective” mosquitoes carrying a total of 5,904 infective larvae. A person sleeping outdoors during the same period would have been bitten by 1,352 “infective” mosquitoes carrying 4,056 larvae. This heavy exposure was shown to be primarily due to the extremely high mosquito biting density of over 115,000 bites per person per year in the study area. The reasons for the discrepancy between the relatively moderate microfilaremia rates and lack of severe filarial disease which suggest low exposure to infection and the epidemiological data which suggests very high exposure to infection are discussed.

The biology of Anopheles claviger (Mg.) (Dipt., Culicidae) in southern England

AbstractThe biology of Anopheles claviger (Mg.) was studied for a varying number of years in three localities in southern England. The fluctuations in the proportions of the larval instars and pupae were studied by weekly sampling throughout the year. The population overwintered from October to April as pre-adults, and in mid-winter larval development was arrested, but was resumed in March. The construction of a simple life-table showed the existence of a large mortality of immature stages during the overwintering period. Females were sampled at two localities by human bait catches, and at one also by insect suction traps. There was an interval of 1–2 weeks between adult emergences in April and blood-feeding, during which time females apparently fed on sugar solutions. Maximum biting densities of nullipars in May and September showed that A. claviger was bivoltine. The physiological age structure of the population showed that most parous adults at bait had oviposited only once or twice, but a few females had laid four batches. Live Hygrobatid-type mites proved reliable in indicating nulliparity. Mature larvae developed in the eggs 5–7 days after oviposition and while most eggs hatched within the next three days, a few remained unhatched until the 30th day. Females of the second generation (September) laid fewer eggs than those of the first generation (May). About six days were required for blood digestion, and females supplied with sugar were able to live 46 days. The marked delays between successive gonotrophic cycles in natural populations would make parous rates unreliable for the estimation of daily mortalities.