4.3 Bird-mosquito interaction
The parameters
associated with the dashed arrows in 
FIGURE 3 required a little extra consideration. In general, it seemed
reasonable that West Nile virus
transmission should depend on the mosquito biting rate, the proportion of bites
that actually transmit the disease, and the relative numbers of mosquitoes and
birds.
An adult female mosquito lives in a continuous cycle of feeding, laying eggs, feeding, laying eggs, and so on. It takes approximately 2.3 days for a mosquito to convert a blood meal into a batch of eggs, so the maximum biting rate for an individual mosquito is approximately once every 2.3 days. This gives an average daily per capita biting rate of 1/2.3 ≈ 0.44 bites per mosquito per day, denoted r. A realistic range for the gonotrophic cycle is approximately 1.9 to 2.9 days, giving a range of r ≈ 0.34 to 0.53 bites per mosquito per day. By assuming that mosquitoes were biting at their maximum rate, our model used so-called frequency-dependent transmission dynamics.
The remaining two parameters were disease transmission probabilities. From laboratory work, we knew that not every bite by an infectious mosquito or on an infectious bird leads to virus transmission. From infectious mosquitoes to birds, the transmission probability is ~0.88 (denoted p). From infectious birds to mosquitoes, it is only ~0.16 (denoted q).
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