Sporogonic cycle

When the female vector takes a blood meal there is a certain finite probability that malaria transmission takes place, either from the host to vector or vice versa. The probability of transmission to the vector during a blood meal from an infective host is considered a constant $P_{hv}=0.2$ (following [#!ermert:11a!#]), and thus the overall transmission probability $P_{h{\rightarrow}v}$ is the product of $P_{hv}$ and the proportion of hosts that are infective, namely the ratio of the population density of infected hosts $H_{inf}$ divided by the population density $H$:

$$P_{h{\rightarrow}v}=\frac{H_{inf}}{H} P_{hv}.$$ (6)

It should be noted that this assumes bites are randomly taken and not influenced by the host's infective state. Heterogeneous biting can impact the basic reproductive number considerably [#!smith:05a!#,#!smith:07a!#]. Heterogeneity of feeding habits is related to a wide array of factors, including host attractiveness to the vector and their vicinity to breeding sites, and heterogeneity in interventions such as the use of bednets, in particular the increased use by host suffering from clinical malaria. Some of these effects could easily be included in VECTRI if relevant data were available.

Each timestep, a proportion of vectors $P_{h{\rightarrow}v}$ become infected, and the parasitic development in the midgut of the vector begins, once again governed by a degree day concept:

$$R_{sporo}=\frac{T_{2m} - T_{sporo,min}}{K_{sporo}}.$$ (7)

After a number of days the sporozoites invade the salivary glands of the mosquito which subsequently becomes infective to humans. The mosquito is assumed to remain in this infective state until its death.