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where R
3 is the relative feeding index of dogs and chickens as sources of feed-
ing contacts compared to humans and the temperature follows a sine wave with an
annual average temperature of Argentina of 63 degrees F and a temperature range
of 52 to 72 degrees.
The initial number of dogs and chickens are estimated from the total human
population. We assumed that each household is comprised of 5 people, 2 dogs, and 2
chickens that are accessible to each human house.
Transmission of
T. cruzi
among insect vectors was determined as follows:
=
TR B)
∧
(CONT INF HOST)
VECTOR INFECTION RATE = 1
−
(1
−
,
(6.3)
where
TR B
=((
TR HB
∗
INF HU
)+(
TR DB
∗
INF DOG
))
/
BLOOD SOURCE
(6.4)
TR HB is the probability that, in one feeding by an initially uninfected bug on an
infected (seropositive) human, the bug acquires infection. It is 0.03.
5
TR DB is
the probability that, in one feeding by an initially uninfected bug on an infected
(seropositive) human, the bug acquires infection. It is 0.49.
5
INF DOG is the number
of infected dogs, calculated from the TOTAL DOGS and PREVALENCE OF
T.
CRUZI
AMONG DOGS (0.84)
12
.
CONT INF HOST is the average number of times an adult bug has had feeding
contact with an infected blood source:
CONT INF HOST
=
N FEEDING
∗
(INF HU
+
R
∗
INF DOG
)
/
(6.5)
BLOOD SOURCE
However, chickens are not included in infected blood sources because they are not
infected with
T. cruzi
.
The fatalities from
T. cruzi
among humans and the vector population dynamics
are shown in Figures 6.3 and 6.4, respectively. When there is no control of
T. cruzi
,
vector prevalence and the number of people dying of Chagas disease tend to increase
through time.
When cracked and unplastered walls are plastered well, household bug density
is decreased from 38 bugs to 0.43 bugs per household
13
. Since housing improve-
ments are a one-time event, their introduction in the model changes the initial bug
density and decreases the carrying capacity (K) from an assumed 500 to 200. This
demonstrates the effects of housing improvements and is displayed in Figures 6.5
and 6.6. House improvement itself decreases the insect population and human
12
Gurtler, R.E., Cecere, M.C., Rubel, D.N., Petersen, R.M., Schweigmann, N.J., Lauricella, M.A.,
Bujas, M.A., Segura, E.L. and C. Wisnivesky-Colli. 1991. Chagas disease in northwest Argentina:
infected dogs as a risk factor for the domestic transmission of
Trypanosoma cruzi
. Transactions of
the Royal Society of Tropical Medicine and Hygiene, 85, 741-745.
13
Gurtler, R.E., Cecere, M.C., Rubel, D.N. and J. Schweigmann. 1992. Determinants of the domi-
ciliary density of Triatoma infestans, vector of Chagas disease. Medical and Veterinary Entomol-
ogy, 6, 75-83.
