Database Reference
In-Depth Information
Bird Infected
Mosq3 Adults
Bird Susceptible
Total Susceptible
HIRate coef
BIRate coef
Human Infection Rate
Bird Infection Rate
Fig. 5.6
This module shows the human population according to their susceptibility to
encephalitis, the number with the disease at any given time, and the number of
immunes, with each of these elements of the population inflows of people either
through birth (Susceptibles) or inflows from a previous category. We sized the
population for Illinois using U.S. Bureau of Census data for 2000.
Two infection rates are calculated (Figure 5.6). A bird infection rate is calculated
as the product of the population of infectious birds, mosquito population, suscepti-
ble bird population, and coefficient of infection (adjusted to achieve transmission).
Similarly, a human infection rate is calculated as the product of susceptible humans,
mosquito population, infectious birds, and a coefficient of infection (adjusted to
achieve human transmission). The variable Total Susceptible is the sum of the array
of Human Susceptibles.
Several complications arise when matching the virus transmission rates to data
on human infection and mortality. Monath 5 indicates that the ratio of asymptomatic
to symptomatic infections may be as high as 338:1, and that transmission rates of
the virus typically do not differ between the age cohorts of the human population
even though the disease generally affects the elderly most. Actual mortality among
symptomatic humans ranges from 0% (age 10-14) to 18% (age 75+). Mortality
attributed to viral infection is highly correlated to the incidence of hypertension.
Since cardiovascular disease increases with age, this may be the determining factor
for mortality following viral infection. We did not model the incidence of hyperten-
sion in the population to determine the incidence of death. Instead, we chose to use
the mortality rates for each human age cohort relative to symptomatic infection as
reported by the Centers for Disease Control in Monath. Modeling the actual causes
of mortality is far more complicated and will require considerably more research.
The annual known human infection rate for Illinois is also obtained from CDC
data 6
(Table 5.1):
5 See footnote 2.
6 http://www.cdc.gov/ncidod/dvbid/arbor/arbocase.htm
Arboviral Encephalitis Cases Reported in Humans, by Type, United States, 1964-2000, CDC,
Atlanta, GA.
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