  Multistrain diseases have multiple distinct coexisting serotypes (strains).
For some diseases, such as dengue fever, the serotypes interact by
antibody-dependent enhancement (ADE), in which infection with a single serotype
is asymptomatic, but contact with a second serotype leads to higher viral load
and greater infectivity. We present and analyze a dynamic compartmental model
for multiple serotypes exhibiting ADE. Using center manifold techniques, we
show how the dynamics rapidly collapses to a lower dimensional system. Using
the constructed reduced model, we can explain previously observed synchrony
between certain classes of primary and secondary infectives (Schwartz et al.,
Phys. Rev. E 72: 066201, 2005). Additionally, we show numerically that the
center manifold equations apply even to noisy systems. Both deterministic and
stochastic versions of the model enable prediction of asymptomatic individuals
that are difficult to track during an epidemic. We also show how this technique
may be applicable to other multistrain disease models, such as those with
cross-immunity.
