A simple deterministic model for host-parasite relationship in Wuchereria bancrofti infection & its relevance to parasite regulation in human host.

A deterministic immigration-death model, which reflects the population dynamics of W. bancrofti in human host has been applied to study the relationship between vector and human infections. Application of the model showed that the rate of acquisition and loss of human infection were approximately equal (L = 0.130 and M = 0.129). The relationship of infective resting density (IRD) in vector population with maximum intensity (Imax) of infections and microfilaria prevalence (MFP) in human population were examined by using the least squares polynomial regressions. The fifth order polynomial regressions were found to be adequate to describe the observed pattern (Imax vs IRD: R2 = 0.8464, P = 0.0015; MFP vs IRD: R2 = 0.7246, P = 0.019). The observed relationships indicated that at an infective resting density of 0.26 per man hour or above, the density-dependent factors start regulating the human infections, which showed a declining trend, following this level.

Development of Brugia malayi in Mongolian gerbils previously exposed to Wuchereria bancrofti.

Twelve Mongolian gerbils, Meriones unguiculatus, were infected with 100 third-stage larvae of Wuchereria bancrofti. One month later these animals, along with 4 control animals, were given 100 third-stage larvae of Brugia malayi. Eleven of the 12 experimental animals and the 4 controls survived, and 8 of the experimental animals and all of the controls demonstrated microfilaremia after 3 months. The animals were killed at 6-months post-infection and examined for parasites. One W. bancrofti larva was found in one of the experimental animals, and 15% of the B. malayi given were recovered as adults from the testes, viscera, and carcass. Thirty-eight percent of the worms given to the controls were recovered from the testes, viscera, and pelt. The worms from the experimental animals also appeared to be smaller. This study suggests that gerbils are able to develop partial resistance to Brugia malayi following a previous infection with Wuchereria bancrofti.

Attempt to culture Wuchereria bancrofti in vitro.

Third-stage larvae of Wuchereria bancrofti recovered from laboratory raised Aedes togoi and Anopheles maculatus fed on a human volunteer were recovered by mass dissection methods and introduced into in vitro culture. LLC-MK2 cells were used as feeder cells, and the culture medium consisted of RPMI-1640 buffered with Hepes and sodium bicarbonate and supplemented with human AB serum. The third-stage larvae molted as early as 12 days and those surviving had all molted by 16 days. The fourth-stage parasites averaged in length from 1.4 mm to a maximum of 1.8 mm. Some larvae remained alive in culture as long as 40 days and while the worms were distorted in fixation, possible primodial cells of a spicule could be visualized in the rectal region. The cuticle also appeared to be separating in the posterior end. Although complete development was not achieved, it seems that with a continuing effort, success could be obtained using this culture system with feeder cells.

Further studies on the development of Wuchereria bancrofti in laboratory animals.

In an attempt to obtain a small animal laboratory model for Wuchereria bancrofti Mongolian gerbils and hamsters were infected with third-stage larvae of strains of the parasite from Indonesia and China. Gerbils were necropsied at 7 to 591 days and larvae recovered up to 219 days. The third molt occurred on or before 10 days and worms found after 14 days were in the fourth stage. More worms were recovered early in the infection (7 to 30 days) than in older infections. The worms were usually recovered from the testes, pelt and carcass and heart and lungs. An adult male worm was recovered on one occasion from the lung of a gerbil at 92 days. The parasite was also found to develop in hamsters with results similar to those in gerbils. Gerbils and hamsters were given antithymocyte sera before and after infection and more worms were recovered and the worms larger in size in some treated animals. Continued studies with geographic strains of W. bancrofti in various animals should be encouraged in order to find a small laboratory animal model for the parasite.

The effect of diethylcarbamazine treatment on the development of bancroftian microfilariae in Culex p. fatigans.

Bancroftian microfilarie survived after one to three courses of diethylcarbamazine treatment in carriers and the larvae able to reach the infective stage in Culex p. fatigans. The infection rate and the development of infective larvae per infected mosquito from DEC-treated carriers was much lower than from the untreated carrier. The fact that surving MF can develop to infective stage in their vector indicates that such filarial carriers may be important sources for transmission of filariasis in the endemic areas after suspension of control measures.