D. Greenhalgh et al., Subcritical endemic steady states in mathematical models for animal infections with incomplete immunity, MATH BIOSCI, 165(1), 2000, pp. 1-25
Many classical mathematical models for animal infections assume that all in
fected animals transmit the infection at the same rate, all are equally sus
ceptible, and the course of the infection is the same in all animals. Howev
er for some infections there is evidence that seropositives may still trans
mit the infection, albeit at a lower rate. Animals can also experience more
than one episode of the infection although those who have already experien
ced it have a partial immune resistance. Animals who experience a second or
subsequent period of infection may not necessarily exhibit clinical sympto
ms. The main example discussed is bovine respiratory syncytial virus (BRSV)
amongst cattle. We consider simple models with vaccination and homogeneous
and proportional mixing between seropositives and seronegatives. We derive
an expression for the basic reproduction number, R-o, and perform an equil
ibrium and stability analysis. We find that it may be possible for there to
be two endemic equilibria (one stable and one unstable) for R-o < 1 and in
this case at R-o = 1 there is a backwards bifurcation of an unstable endem
ic equilibrium from the infection-free equilibrium, Then the implications f
or control strategies are considered. Finally applications to Aujesky's dis
ease (pseudorabies virus) in pigs are discussed. (C) 2000 Elsevier Science
Inc. All rights reserved.