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.

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

Engorged females of three temperate simuliid species, Simulium ornatum, S. lineatum and S. equinum, were collected immediately after their bloodfeeds from a cow host. Midguts were dissected out at ...

Peritrophic membrane formation in three temperate simuliids, Simulium ornatum, S. equinum and S. lineatum, with respect to the migration of onchocercal microfilariae.

We studied the effect of 10 annual distributions of ivermectin for control of human onchocerciasis on the reduction of Onchocerca volvulus transmission by Simulium vector flies. Prevalence and infection load in the human population decreased, whilst the annual biting rates (ABR) of the vector remained unchanged. The annual transmission potential of infective larvae of O. volvulus fell to 40% of the pre-intervention level, but was still sufficient to maintain endemicity. However, recent immigration of herds of nomadic cattle into the study area has diverted the flies from man to cattle, creating an additional zooprophylactic effect. The predominant transmission of the bovine filaria O. ochengi to man also provides some concomitant cross-protective immunity against further infestations with O. volvulus. The effects of ivermectin on O. volvulus, combined with the zooprophylactic effects of the increased cattle population, have produced a complex beneficial influence on the transmission of human onchocerciasis.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3156.2001.00771.x

Combined benefits of annual mass treatment with ivermectin and cattle zooprophylaxis on the severity of human onchocerciasis in northern Cameroon

The impact of annual single-dose community-wide treatment on the transmission of Wuchereria bancrofti was investigated in 5 villages in the East Sepik Province where pretreatment prevalence of microfilaraemia ranged from 34% to 73%. Anopheles punctulatus and An. koliensis were the only carriers of the parasite. 3 villages received diethylcarbamazine citrate (DEC) in combination with ivermectin (IVR) and 2 received DEC alone. The rate and intensity of microfilaraemia were both reduced in all 5 villages. Reduction in prevalence was between 43% and 67% in the DEC+IVR study villages and between 24% and 27% in the DEC alone villages. Density was reduced by between 81% and 95% in the DEC+IVR villages and between 69% and 74% in the DEC alone villages. Breaks in perennial transmission (failure to detect infective mosquitoes in four or more consecutive monthly collections) occurred in all 3 communities treated with DEC+IVR. Transmission was almost completely interrupted in 2 villages, where infective mosquitoes were not detected during 11 of the 12 months following treatment. We concluded that repeated annual single-dose community-wide treatment with DEC+IVR could lead to complete interruption of transmission and ultimately elimination of lymphatic filariasis.

Towards eliminating lymphatic filariasis in Papua New Guinea: impact of annual single-dose mass treatment on transmission of Wuchereria bancrofti in East Sepik Province.

A technique for infecting mosquitoes with known numbers of Brugia pahangi microfilariae by enema is described. Virtually all mosquitoes receiving three microfilariae or more by this route contained infective larvae 10 days later. Within a range of 1 to 40 microfilariae, numbers of infective larvae recovered (Y) were related to the numbers of microfilariae administered (X) by the equation log10Y = 0.04 + 0.84log10X. Mosquitoes feeding on sugar for up to 20 days did not lose a significant number of infective larvae. A blood meal 10 days after infection reduced the proportion of females still infected, as well as the number of worms remaining in those still infected. A second blood meal on day 17 after infection only reduced the proportion of mosquitoes still infected. Adults reared on a low larval diet were less susceptible to infection, and a refractory mosquito strain did not support the development of third-stage larvae. Filarial worms had no effect on mosquito mortality until more than 30 microfilariae were administered.

Infection of Aedes aegypti with Brugia pahangi administered by enema: results of quantitative infection and loss of infective larvae during blood feeding.

Bain O

Papers

[Transmission of filariasis. Limitation of passages of ingested microfilariae to the hemocoele of the vector; interpretation].