On the inefficiency of transmission of Wuchereria bancrofti from mosquito to human host.

The high biting rate of Culex pipiens fatigans in Rangoon, combined with a low prevalence of microfilaraemia due to Wuchereria bancrofti, suggested a poor efficiency of transmission. Data obtained by the WHO Filariasis Research Unit in Rangoon were analysed, and the efficiency of the parasite from Stage III larva in the mosquito to the production of microfilariae was estimated as 6.04-6.71 x 10(-5), indicating that an average of around 15 500 bites by "infective" mosquitos is necessary to produce 1 case of microfilaraemia. This surprising result prompted a step-by-step analysis of the success of the parasite through departure from the mosquito, penetration of host tissues, survival to maturity, and encountering a mate in the human host. It was hoped that the second approach would identify some discrepancy in the original analysis, but when all sources of loss were combined, it was estimated that the expected efficiency was 4.78 x 10(-5). The two estimates are within observational error of each other. The degree to which the two approaches agree leads to the conclusion that survival of the parasite is reasonably well known at all stages, and the calculations indicate that a high proportion of the population of Rangoon must be carrying parasites that are either dead or immature. A complete quantitative statement of the epidemiology of W. bancrofti seems to be attainable.

Observations on the flight activities of Culex pipiens fatigans in Rangoon, Burma.

Knowledge of the behaviour of a vector is often of value in devising suitable methods for assessing control or eradication measures. Flight behaviour is particularly important, since it is during this activity of a mosquito that trapping is possible. Trapping data have proved useful in the assessment of the action of insecticides on the vectors of malaria, and there is no reason why this should not also be the case with the vectors of filariasis.In this paper the difficulties of trapping Culex pipiens fatigans by conventional means are stressed. Moreover, since flight is a dynamic process, the interpretation of flight activities from data collected by traps, which are static, is difficult. Nevetheless, the results presented, however inadequate, show that adult C. p. fatigans move about more, both by day and at night, than is usually suspected and that the subject would repay closer study.

Dispersion studies of Culex pipiens fatigans tagged with 32P in the Kemmendine area of Rangoon, Burma.

The flight range and the dispersion of a vector are important factors when control or eradication measures are being considered and when general biological information is desired. The present work on Culex pipiens fatigans was carried out under conditions where breeding is intensive and housing congested. The radioactive tagging method adopted seemed to be harmless to the mosquito and gave excellent results.Radioactive adults emerging under normal conditions from larvae collected in the centre of the Kemmendine Experimental Area did not appear to differ in flight behaviour from radioactive adults released at one time in the centre. Mosquitos of both sexes dispersed fairly evenly in all directions from the release point; this fact is likely to be of practical value in control and biological experiments. Some mosquitos even crossed a river over one-third of a mile (500 m) wide and specimens were collected by hand more than (1/2) mile (800 m) from the release point without the use of lures or traps.The method also yielded valuable data on the daily mortality of adults and on the total mosquito population in the area. There seems little doubt that the radioisotope tagging technique can be a most valuable weapon in the hands of the biologist or epidemiologist.

Time of arrival of gravid Culex pipiens fatigans at an oviposition site, the oviposition cycle and the relationship between time of feeding and time of oviposition.

One of the most important activities in a female mosquito's life is the flight to the breeding place and the subsequent deposition of eggs. During this phase, motivated by endogenous and exogenous stimuli, the female is particularly exposed and susceptible to attack. It is therefore important to investigate these episodes in the gravid female's life.The work reported in this paper shows that gravid Culex pipiens fatigans females are easily trapped over breeding-water; there are two peaks in the arrival at a breeding site, one just after sunset and the other at sunrise. The oviposition cycle is biphasic, the two peaks coinciding, in calm weather, with the two arrival peaks; wind and rain cause marked disturbances in the oviposition cycle.The mean duration of the gonotrophic cycle depends on the time of feeding; this finding is of practical importance since the length of the cycle is often used to calculate the daily survival rate of adult mosquitos. It appears that oviposition is stimulated by a change in light: from light to dark for mosquitos ovipositing in the evening and from dark to light for those ovipositing in the morning.Apart from revealing some hitherto unknown behaviour patterns, the techniques evolved could also be used in the assessment of mosquito populations and hence the effects of control measures.

Qualitative and quantitative characteristics adult Culex pipiens fatigans populations according to time, site and place of capture.

The composition of a vector population is largely influenced by the pressures and modifying effects of the environment. Changes in the latter, whether natural or induced (e.g., by insecticides) will be reflected in the former. Thus if the existing characteristics of a vector population are determined, the parameters can subsequently be used to assess the effect of control measures.The effect of environmental conditions on the density and parity of Culex pipiens fatigans in catches in Burma is illustrated by a comparison of results obtained in Kemmendine, which is served by a bucket system for the removal of night-soil, with those from South Okkalapa, which has pit latrines; in the latter area there is a large preponderance of young mosquitos in the monsoon period, in contrast to the situation in Kemmendine.In Kemmendine, the inside-biting catch is reduced by about one-half if an inside-resting catch is made simultaneously. This suggests that C. p. fatigans rests indoors before biting indoors and that an efficient insecticide for inside application would markedly reduce the mosquito population. When a correction is made for the inside-resting catch it is shown that there is little to choose between inside- and outside-biting catches as measures of the activity of C. p. fatigans. The chances of acquiring infection are at least as great out of doors as indoors and this may account for differences in infectivity among different ethnic groups, according to the extent that they sleep out of doors.Differences in parity and in the state of infection of mosquitos according to the type of catch and the time of catching are discussed. Finally, it is noted that, although most male mosquitos are obtained in inside-resting catches, a considerable number, surprisingly, are taken on bait; the reason for this is not yet known.

Outdoor resting of Culex pipiens fatigans in Rangoon, Burma.

The sites occupied by resting adult mosquitos are of importance, since they may be accessible to treatment with insecticide. It has usually been assumed that Culex pipiens fatigans feeds and rests indoors, thus making itself vulnerable to indoor spraying. However, work in the Kemmendine Experimental Area of Rangoon, Burma, has now shown that many C. p. fatigans rest out of doors in a variety of shelters. The lower parous rate in outside catches as compared with indoor catches tends to confirm the view that newly hatched C. p. fatigans rest out of doors for some time before setting off to feed. The percentage of infected mosquitos is remarkably constant from site to site and from month to month and does not differ significantly from that obtained in indoor resting catches or on bait out of doors. It appears that the possibility of acquiring infection out of doors will have to be seriously considered in any anti-filariasis campaign.

The duration of egg, larval and pupal stages of Culex pipiens fatigans in Rangoon, Burma.

Laboratory experiments to determine the duration of the immature stages of Culex pipiens fatigans were carried out because such information is important from the point of view of control by larvicides. At a temperature of 25.1 degrees C+/-0.7 degrees C the mean incubation period is 27.11+/-0.57 hours. Females spend a longer time in the pupal stage than males (34.16+/-0.74 hours and 32.95+/-0.75 hours, respectively, at 28.6 degrees C+/-0.8 degrees C; there is no 24-hour pupating or emerging rhythm. The duration of larval life is longer for the female (135.3+/-4.4 hours) than for the male (118.4+/-2.4 hours). Larvae that take a long time to pupate also take a long time to emerge. Withholding of food for a few hours from first-stage larvae increases the duration of larval life but does not affect that of pupal life. These observations on the differences between the sexes in the duration of larval and pupal life are in agreement with observations made on Aedes aegypti in Uganda.

Evaluation of Wuchereria bancrofti infection in Culex pipiens fatigans in Rangoon, Burma.

In the past, several indices have been proposed to evaluate the threat of vectors to the community. Such indices, if accurately determined, could serve to assess the effectiveness of vector control campaigns. In this paper, a "risk of infection" index is introduced for the case of urban filariasis transmitted by Culex pipiens fatigans in Rangoon, Burma. This index is directly proportional to the three principal parameters that determine the extent of transmission-namely, the biting density of the vector, the proportion parous to total biting, and the proportion infective to total parous; all these parameters are easily determined from biting catches. Examples of the use of the index are given and it is noted that, in areas where conditions are not favourable to the vector throughout the year, or where the speed of development of the parasite in the vector may not be constant, these variables must be taken into account in calculating the index.

Experiments on the mating of Culex pipiens fatigans in the laboratory.

Few studies have been conducted into the mating and insemination of C. p. fatigans, despite the importance of this behavioural aspect for the production of fertile eggs and their development after a blood meal. Moreover, detailed knowledge of the behaviour of male mosquitos is required before the potentialities of genetic manipulation and sterile male techniques for the control and eradication of mosquitos can be fully investigated.The laboratory studies described in this paper show that high percentages of successful matings and insemination can be obtained in relatively small cages in the laboratory and suggest that the classical swarming of males, which takes place in the open, is not essential to mating.

Cane-sugar feeding in Culex pipiens fatigans.

The relatively poor results that have been obtained in controlling or eradicating Culex pipiens fatigans, the urban vector of Wuchereria bancrofti, have made it necessary to obtain as much information as possible about its biology. In this paper the unexpected finding of the influence of cane sugar in delaying oviposition in the gravid female is reported. This is an important finding for those who are investigating the biology of this mosquito in the laboratory and who make use of the age-old practice of keeping adult mosquitos alive by feeding them cane sugar. It is also reported that under certain conditions extensive cane-sugar feeding takes place in nature and it seems possible that this habit may have unsuspected repercussions on behaviour and physiology beyond the confines of the laboratory. One possible development would be the isolation of the attractant present in unrefined sugars and its use in traps in order to assess mosquito populations in the field.

Infection and reinfection of Culex pipiens fatigans with Wuchereria bancrofti and the loss of mature larvae in blood-feeding.

Although previous workers had found no evidence of resistance to superinfection in vectors of filariasis, it was considered desirable to reinvestigate the subject because of the epidemiological implications, since a mosquito that can incubate to maturity successive broods of filarial larvae will obviously be a more efficient vector than one that cannot. The results obtained indicate that a Culex pipiens fatigans mosquito that picks up an infection early in its life can, by taking subsequent infecting feeds, remain infective for the rest of its life.The movement of mature larvae in the vector and losses of larvae during feeding are of interest since from this information one can estimate the probable number of larvae deposited on the host at each feed and the period of infectivity of the vector. In the present experiment there were, on average, 6.1 larvae per infective mosquito before a second blood-meal; after the meal the figure was 3.6. Thus, 41% of the original infective larvae were lost; not all would be inoculated into the host, as some would die on the surface of his skin. The release of infective larvae through blood-feeding results from a combination of the passive pressure of the engorged blood in the abdomen and the active movement of the larva itself.