Factors affecting transmission of Wuchereria bancrofti by anopheline mosquitoes. 1. Uptake of microfilariae

Ingestion of Wuchereria bancrofti microfilariae (mf) from humans by 639 Anopheles gambiae, 557 An. arabiensis, 117 An. melas and 9 An. funestus was investigated. The mf densities in blood fell into 3 groups; 0-32mf/ml, 107-122 mf/ml and 421-1140 mf/ml. In An. gambiae and An. arabiensis percentage of mosquitoes ingesting mf was strongly associated with mf density in host blood; in An. melas the association was much weaker. Mean number of mf ingested per mosquito was also strongly correlated to mf density in An. gambiae and An. arabiensis but not in An. melas. At low mf densities both An. gambiae and An. arabiensis concentrated mf, with concentration decreasing as density increased. From regression analysis, observed and expected uptake of mf would be equal at 622.9 mf/ml in An. gambiae and 391.6 mf/ml in An. arabiensis.     

Factors affecting transmission of Wuchereria bancrofti by anopheline mosquitoes. 2. Damage to ingested microfilariae by mosquito foregut armatures and development of filarial larvae in mosquitoes

Microfilariae (mf) of Wuchereria bancrofti from the midgut of 639 Anopheles gambiae, 557 An. arabiensis, 117 An. melas and 9 An. funestus were examined immediately after the mosquitoes had fed on carriers with different densities of mf. The percentages of mf damaged during ingestion were 57.1-60.0 in An. gambiae, 33.3-50.6 in An. arabiensis and 38.7-55.7 in An. melas. In each species the percentage of mf damaged was independent of mf density in the human host. A further 3657 An. gambiae, 2875 An. arabiensis, 347 An. melas and 32 An. funestus were examined 7 d or more after feeding on mf carriers. In An. gambiae and An. arabiensis, mean numbers of larvae per mosquito were strongly correlated to mf blood density, with similar regression slopes to those obtained from the regression of mf blood density on mean uptake of mf/mosquito. The ratio of mean numbers of larvae per mosquito to mean numbers of intact mf ingested per mosquito increased as the density of mf in the human host increased in An. gambiae and An. arabiensis, but decreased in An. melas as host mf density increased.     

Factors affecting transmission of Wuchereria bancrofti by anopheline mosquitoes. 4. Facilitation, limitation, proportionality and their epidemiological significance.

Quantitative understanding of the transmission dynamics of lymphatic filarial parasites is essential for the rational planning of control strategies. One of the most important determinants of transmission dynamics is the relationship between parasite yield, the success rate of ingested microfilariae (mf) becoming infective larvae in a mosquito vector, and mf density in the source of the human blood meal. Three types of relationship have been recognized in human filaria/mosquito couples--limitation, facilitation and proportionality; facilitation has hitherto been observed only in the couple Wuchereria bancrofti/Anopheles gambiae in Burkina Faso, in experimental studies on a high density mf carrier. The present paper demonstrates facilitation in W. bancrofti/An. gambiae and W. bancrofti/An. arabiensis in lower mf density carriers in The Gambia and Tanzania, and in W. bancrofti/An. funestus in Tanzania. Facilitation was not found in An. melas in The Gambia nor in An. merus in Tanzania. Analysis of published data shows limitation at low level mf densities in W. bancrofti/Culex quinquefasciatus in Sri Lanka, and in the same couple in India. Limitation also occurs in Brugia malayi/Aedes togoi in experimental cats; proportionality occurs in B. malayi/Mansonia bonneae in Malaysia. The epidemiological significance of these host/parasite relationships is discussed, and supporting evidence for its validity is presented from the published results of large-scale control programmes.     

Ingestion and development of Wuchereria bancrofti in Culex quinquefasciatus, Anopheles gambiae and Aedes aegypti after feeding on humans with varying densities of microfilariae in Tanzania

Laboratory observations were made on the transmission of Wuchereria bancrofti by Culex quinquefasciatus and Anopheles gambiae, natural vectors in East Africa, and by Aedes aegypti, an experimental vector. Mosquitoes were fed on human carriers with one to 2,667 microfilariae (mff/ml) of peripheral blood and the numbers of mff ingested were determined. The mosquitoes demonstrated a ‘concentrating effect’ and ingested 8.6 to 12-fold more mff than the expected number based on the density of mff in the carrier and the size of the blood meal. Studies on Culex showed that only 2.7 to 13% of the ingested mff developed into infective larvae (L3s). Emphasis was given to the transmission of Wuchereria from low density carriers with less than 10 mff/ml of peripheral blood—a value that approximates the threshold level of microfilaraemia detectable with conventional diagnostic techniques. By combining our laboratory data with published field data on the biting rates and longevity of mosquitoes in East Africa, we estimate that each low density carrier could serve as a source for nil to 15 L3s/year in Cx quinquefasciatus and 0.3 to 8 L3s/year in An. gambiae depending on the particular community. These estimates are relevant when formulating strategy to control bancroftian filariasis by mass chemotherapy with diethylcarbamazine. Experience with this drug has shown that a proportion of the carriers in a community continue to circulate mff at low levels after treatment. The present data demonstrate that these carriers serve as a reservoir of infection for mosquitoes.     

Comparative susceptibility of two members of the Anopheles oswaldoi complex, An. oswaldoi and An. konderi, to infection by Plasmodium vivax

We compared the susceptibility of Anopheles oswaldoi and An. konderi to infection by Plasmodium vivax based on the proportion of mosquitoes presenting oocysts and sporozoites. Anophelines were captured in the State of Acre and Rond?nia, Brazilian Amazon, and used to obtain F1 progenies. After emergence of adults, male genitalia of mosquitoes of each family were dissected in order to identify them as either An. oswaldoi or An. konderi. F1 progenies of field-captured An. oswaldoi, An. konderi and An. darlingi (used as control) were fed simultaneously on P. vivax-infected blood. Mosquitoes were dissected on day 10-12 after feeding and examined for the presence of oocysts and sporozoites. Both An. oswaldoi and An. konderi developed oocysts in the midguts, however, the percentage of oocyst-positive mosquitoes for An. oswaldoi (13.8%) was higher than for An. konderi (3.3%), and only An. oswaldoi developed salivary infection with sporozoites (6.9% of positivity). Infection rates in An. darlingi ranged from 22.5% to 30.0% for both oocysts and sporozoites. These results indicate that An. oswaldoi can transmit P. vivax and suggest that it is more susceptible than An. konderi. Although An. oswaldoi is an exophilic and zoophilic species, it may be involved in malaria transmission as possibly occurred in the State of Acre.