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HTLV-I TRANSMISSION DYNAMICS AND GLOBALIZATION
The Effect of Increased Host Population Heterogeneity on HTLV-I Persistence
J. Cole, A. Crawford, S. Palace, J. Quill
Faculty Sponsor: T. Livdahl
ABSTRACT
HTLV-I, a human retrovirus and the causative agent of ATLL, displays
highly cell-associated transmission. Consequently, HTLV-I is endemic
in genetically homogenous populations in regions of Japan, Jamaica,
and several South American countries. Since a degree of genetic
relatedness seems to be important in effective transmission, it is possible
that the disruption of isolated populations through globalization may
result in lower HTLV-I prevalence rates in currently endemic areas.
Several simulations were constructed using disease-modeling software
to determine the effect of persistent immigration on viral prevalence in a
hypothetical population prone to HTLV-I. Although this study did not
find that the lower transmission rates counteracted the effects of
exposing more individuals to the virus, vigorous efforts to quantify
transmission rates as a function of host relatedness should be made in
order to accurately assess risk of newly-exposed populations and
individuals.
MATERIALS / METHODS
Stella 9.0 software was used to represent plausible disease dynamics in a
simulated HTLV-I endemic population (Fig. 1, 2) [5, 6, 7]. Birth, death, and
immigration rates were chosen to yield a stable population size. Population
dynamics were graphed using Stella software until a stable state was reached.
Fig. 4: HTLV-I dynamics in a population with constant
immigration.
DISCUSSION
鲜
INTRODUCTION
The Human T-cell lymphotropic virus I (HTLV-I), a retrovirus, is the
causative agent in adult T-cell leukemia/lymphoma (ATLL) and
tropical spastic paraparesis. HTLV-I is endemic in numerous areas
throughout the world including southern Japan, the Caribbean, certain
areas of Africa, and the southeastern United States [1, 2].
The primary target of HTLV-I is the T lymphocyte, and the virus is
highly T-cell associated. HTLV-I is not easily transmissible, since cellcell contact is generally required. Two major transmission routes have
been described: vertical transmission through breast milk, and
horizontal transmission through sexual contact [1, 2].
The cell-associated transmission of HTLV-I requires relative genetic
homogeneity in host populations. As globalization increasingly
disrupts the isolation of such communities, it is possible that
transmission rates of HTLV-I will fall below the minimum rate at
which the virus can persist. To investigate this phenomenon, Stella
disease modeling software was used to replicate the effects of constant
immigration on a population in an HTLV-I endemic area. Prevalence
rates were compared in this model and in a control (no immigration)
model to determine potential effects of globalization on HTLV-I
distribution.
Fig. 1: Schematic for disease
Fig. 2: Schematic for disease dynamics in
dynamics in the control population. a population with constant immigration.
RESULTS
When the simulation populations reached a steady state, 30.8% of the control
population and 34.0% of the population with immigration were infected with
HTLV-I (Fig. 3, 4). Total population sizes did not change significantly, as the
simulation was designed to produce populations of stable sizes.
The proportion of infected individuals in the immigration-affected
population was not lower than that in the control population; it
appears as though the reduced transmission rates observed between
members of different populations do not negatively affect HTLV-I
prevalence to an extent that counteracts the risk of exposing a
greater number of individuals to the disease. This conclusion is not
wholly without precedence [3]; however, more research must be
done to definitively quantify transmission differentials between
unrelated populations to accurately assess the risk of disease
spread. The complexity of the transmission dynamics of HTLV-I
creates significant uncertainty as to the future of this disease in the
face of globalization. Further research is therefore required to
determine whether the global incidence of HTLV-I is likely to
increase or decrease.
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Review of the Literature. Leukemia & Lymphoma, 43: 127-132 (2002).
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The Journal of infectious diseases, 1 67: 1, 57-65 (1993).
Fig. 3: HTLV-I dynamics in an isolated population.
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