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Global dynamics in a class of discretetime epidemic models with disease courses
Advances in Difference Equations volume 2013, Article number: 57 (2013)
Abstract
In this paper, a class of discrete SIRS epidemic models with disease courses is studied. The basic reproduction number ${R}_{0}$ is computed. The main results on the permanence and extinction of the disease are established. That is, the diseasefree equilibrium is globally attractive if ${R}_{0}<1$, and there exists a unique endemic equilibrium and the disease is also permanent if ${R}_{0}>1$.
MSC:39A30, 92D30.
1 Introduction
In recent years, more and more attention has been paid to the discretetime epidemic models. There are several reasons for that. Firstly, since the statistic data about a disease is collected by day, week, month or year, it is more direct, more convenient and more accurate to describe the disease by using the discretetime models than the continuoustime models; secondly, the discretetime models have more wealthy dynamical behaviors; for example, the singlespecies discretetime models have bifurcations, chaos and other more complex dynamical behaviors.
For a discretetime epidemic model, we see that at the present time, the main research subjects are the computation of the basic reproduction number, the local and global stability of the diseasefree equilibrium and endemic equilibrium, the extinction, persistence and permanence of the disease, and the bifurcations, chaos and more complex dynamical behaviors of the model, etc. Many important and interesting results can be found in articles [1–24] and the references cited therein.
In [4], the next generation matrix approach for calculating the basic reproduction number is summarized for discretetime epidemic models. As applications, six disease models have been developed for the study of two emerging wildlife diseases: hantavirus in rodents and chytridiomycosis in amphibians. The comparison of deterministic and stochastic SIS and SIR type epidemic models in discrete time is discussed in [3]. In [8, 9], the discretetime SIS type epidemic models with periodic environment and with diseaseinduced mortality in densitydependence, respectively, are investigated. In [11], Izzo and Vecchio proposed an implicit nonlinear system of difference equations which represents the discrete counterpart of a large class of continuous models concerning the dynamics of an infection in an organism or in a host population. They also studied the limiting behavior of the discrete model and derived the basic reproduction number. Izzo, Muroya and Vecchio in [10] proved the globally asymptotic stability of the diseasefree equilibrium for a general discretetime model of population dynamics in the presence of an infection. For the discrete epidemic model with immigration of infectives, by adopting the means of the nonstandard discretization method from continuous epidemic, Jang and Eiaydi in [12] studied the globally asymptotic stability of the diseasefree equilibrium, the locally asymptotic stability of the endemic equilibrium and the strong persistence of the susceptible class. Li and Wang in [15] discussed a SIS type discrete epidemic model with stage structure, where BevertonHolt type and Richer type recruitment rates were considered, the global stability of the diseasefree equilibrium and the dynamical complexity were investigated. In [17], the sufficient and necessary conditions for the global stability of the endemic equilibrium were established for a discrete epidemic model for the disease with immunity and latency in a heterogeneous host population. In [19], the bifurcations and chaos were proved in a discrete epidemic model with nonlinear incidence rates. The permanence and extinction are investigated in [20–22] for a class of discrete SIRS and SEIRS type epidemic models with time delays. In [24], a discrete mathematical model is formulated to investigate the transmission and control of SARS in China, where the basic reproductive number is obtained as a threshold to determine the asymptotic behavior of the model. Particularly, in [18] the authors studied the following class of disease epidemic models with the spread of an infection in a host population:
The global stability of diseasefree equilibrium and endemic equilibrium and the permanence of the disease were obtained.
However, we know that many diseases have different disease courses, for example, tuberculosis, syphilis, AIDS, etc. Therefore, taking into account the epidemic models with disease courses is very important since disease pathogen bacteria with different course may have different reproduction and survival capacities, which indirectly influences the population growth. Under a different disease course, the transmission rate, the mortality and other vital parameters will be different [25–27].
Motivated by the above results, in this paper, we consider a class of discretetime epidemic models with disease courses. We divide the total population into $m+2$ subgroups according to m disease courses. Let $x(n)$ be the number of susceptible individuals at the n th generation, ${y}_{j}(n)$ ($j=1,2,\dots ,m$) denote the number of infectious individuals who are in the j th course of a disease at n th generation, and let $z(t)$ denote the number of recovered individuals at the n th generation. We introduce the following assumptions.

(1)
The susceptible x has a constant input rate Λ and a natural death rate d.

(2)
The susceptible individuals of the $(n+1)$th generation are only infected by the infectious individuals of the n th generation, and ${\beta}_{j}$ is the constant transmission coefficient of which the susceptible is infected by compartment ${y}_{j}$.

(3)
After a susceptible individual contacts infectives and is infected, he/she will firstly enter compartment ${y}_{1}$, and then turn into compartments ${y}_{2},{y}_{3},\dots $ , finally into compartment ${y}_{m}$.

(4)
The infectious ${y}_{j}$ in the j th disease course admits the constant natural death rate d, the constant death rate induced by disease ${\alpha}_{j}$, the constant recovery rate ${\gamma}_{j}$ and the constant transmit rate ${\epsilon}_{j}$ from compartment ${y}_{i}$ to ${y}_{i+1}$.

(5)
The recovered z admits the constant natural death rate d, does not have permanent immunity, hence there is a constant transfer rate δ from the recovered class back to the susceptible class.
Base on the above assumptions, a class of discretetime epidemic dynamical models with m disease courses can be established as follows:
where ${p}_{i}=d+{\epsilon}_{i}+{\alpha}_{i}+{\gamma}_{i}$, $i=1,2,\dots ,m1$, ${p}_{m}=d+{\alpha}_{m}+{\gamma}_{m}$. For model (1), we always assume that the following basic hypotheses hold.
(H_{1}) For each $i=1,2,\dots ,m$, $d>0$, ${\epsilon}_{i}>0$, ${\alpha}_{i}\ge 0$, ${\beta}_{i}\ge 0$, ${\beta}_{m}>0$, ${\gamma}_{i}>0$, $\delta \ge 0$, $0<{p}_{i}<1$ and $0<\delta +d<1$.
(H_{2}) Any solution of model (1) satisfies the following initial conditions:
Remark 1 For model (1), we can easily see that when ${\beta}_{1}>0$ then model (1) describes a discrete SIRS type epidemic model with disease courses, where ${y}_{i}(n)$ ($i=1,2,\dots ,m$) denotes the number of infectious individuals in the i th course of the disease; and when ${\beta}_{1}=0$ then model (1) describes a discrete SEIRS type epidemic model with disease courses, where ${y}_{1}(n)$ is exposed and ${y}_{i}(n)$ ($i=2,3,\dots ,m$) denotes the number of infectious individuals in the i th course of the disease.
Remark 2 In model (1), based on the above assumption (1), we know that the disease incidence term is denoted by ${\sum}_{i=1}^{m}{\beta}_{i}{y}_{i}(n)x(n+1)$. This makes $x(n+1)$, i.e., the susceptible number of the $(n+1)$th generation, appear on both sides of the first equation. The reason for the above arguments is based on two considerations. On the one hand, it is influenced by the works given in [11, 18]; on the other hand, for the sake of convenience for mathematical analysis, especially, the positivity of solutions in model (1).
In this paper, by developing the methods given in [10, 11, 18], we will give the explicit expression of the basic reproduction number ${R}_{0}$. The criteria on the permanence and extinction of the disease will be established. That is, the diseasefree equilibrium is globally attractive if ${R}_{0}<1$, and there exists a unique endemic equilibrium and the disease is also permanent if ${R}_{0}>1$.
This paper is organized as follows. In Section 2, as preliminaries we will give several lemmas which will be used in the proofs of the main results. In Section 3, the basic reproduction number is calculated, the existence on the diseasefree equilibrium and endemic equilibrium is given and the theorem on the globally asymptotic stability of the diseasefree equilibrium is stated and proved. In Section 4, we will obtain the permanence of the disease. Conclusions are presented in the last section.
2 Preliminaries
Let k be any positive integer, we denote ${R}_{+}^{k}=\{({x}_{1},{x}_{2},\dots ,{x}_{k}):{x}_{i}\ge 0,i=1,2,\dots ,k\}$. For any sequence $\{f(n)\}$, we define
Firstly, on the positivity of solutions of model (1), we have the following result.
Lemma 1 For any solution $(x(n),{y}_{1}(n),{y}_{2}(n),\dots ,{y}_{m}(n),z(n))$ of model (1), it holds that $x(n)>0$, ${y}_{j}(n)>0$, $z(n)>0$ ($j=1,2,\dots ,m$) for all $n>0$.
Proof From model (1), we can easily obtain
and
Assume that $x(n)>0$, ${y}_{i}(n)>0$ ($i=1,2,\dots ,m$) and $z(n)>0$, then we further have
and
Therefore, by using the induction, we get $x(n)>0$, ${y}_{j}(n)>0$, $z(n)>0$ for all $n>0$ and $j=1,2,\dots ,m$. This completes the proof. □
Lemma 2 For any solution $(x(n),{y}_{1}(n),{y}_{2}(n),\dots ,{y}_{m}(n),z(n))$ of model (1), it follows that
Proof
Let
then we have
By using the induction, we can obtain the following inequality:
from which we have
From this, we finally have
This completes the proof. □
On the weak permanence and permanence of the disease of model (1), we have the following definitions.
The disease in model (1) is said to be weak permanent (permanent) if there exists a constant $h>0$ such that, for any solution sequence $(x(n),{y}_{1}(n),{y}_{2}(n),\dots ,{y}_{m}(n),z(n))$ of model (1), one has
From Lemma 2, Theorem 1.1.3 and Theorem 1.10 given in [28], we can immediately obtain the following result.
Lemma 3 If the disease in model (1) is weak permanent, then it also is permanent.
Similar to Lemma 2.3 in [11] and Lemma 5 in [18], we have the following result.
Lemma 4 For any solution $(x(n),{y}_{1}(n),{y}_{2}(n),\dots ,{y}_{m}(n),z(n))$ of model (1), the following inequalities hold:
Meanwhile, we also have
where
Proof From model (1) and Lemmas 1 and 2, we easily have that
and then
From the third equation of model (1), we directly have
Hence, we immediately obtain that ${\overline{y}}_{j}\ge {\epsilon}_{j1}{\overline{y}}_{j1}$ for $j=2,3,\dots ,m$.
Considering the second equation of model (1), we can obtain the following inequality:
and
Then we have
Similarly, from model (1), we easily obtain
and
Hence, from (7) and (8), it can be easily proved that
Hence, inequality (5) holds.
From inequalities (7), (8) and (9), it follows that
and
Hence, inequality (6) holds. This completes the proof. □
3 Global attractivity of diseasefree equilibrium
Let the constant
Firstly, on the existence of diseasefree equilibrium and endemic equilibrium, we have the following result.
Theorem 1 (1) Model (1) always has a diseasefree equilibrium ${E}_{0}(\frac{\mathrm{\Lambda}}{d},0,\dots ,0)$.

(2)
When ${R}_{0}>1$, model (1) also has a unique endemic equilibrium ${E}^{\ast}({x}^{\ast},{y}_{1}^{\ast},{y}_{2}^{\ast},\dots ,{y}_{m}^{\ast},{z}^{\ast})$, where
Proof The equilibrium of model (1) satisfies the following equations:
From the third equation to $(m+1)$equation of (11), we easily obtain
Substituting (12) into the second equation of (11), we further have
Having solved this equality, we obtain that ${y}_{1}=0$ and $x=\frac{1}{D}$.
When ${y}_{1}=0$, then from (12) we have ${y}_{i}=0$ for $i=2,3,\dots ,m$. Further, from the first and the last equations of (11), we have $z=0$ and $x=\frac{\mathrm{\Lambda}}{d}$. This shows that model (1) has a diseasefree equilibrium ${E}_{0}(\frac{\mathrm{\Lambda}}{d},0,\dots ,0)$.
When $x=\frac{1}{D}$, then from the last equation of (11), we have
From the first equation of (11), we further obtain
Substituting (12) into this equality, we further have
Hence,
Then, we further have
Thus, we finally obtain
Since ${\epsilon}_{i}<{p}_{i}{\gamma}_{i}$ for each $i=1,2,\dots ,m1$ and ${\gamma}_{m}<{p}_{m}$, we have
Then we further obtain
Hence, we can infer that
Thus, from (14) we obtain that ${y}_{1}>0$ if and only if ${R}_{0}>1$. Further, from (12) we obtain ${y}_{i}>0$ for $i=2,3,\dots ,m$. Finally, from (13), we also have $z>0$. Therefore, we prove that model (1) has a unique endemic equilibrium ${E}^{\ast}$. This completes the proof. □
Remark 3
Obviously, we have
The first term $\frac{{\beta}_{1}\mathrm{\Lambda}}{{p}_{1}d}$ denotes the ultimate number of the susceptible at the end of the first disease course which is infected by an infectious individual of the first disease course. The second term $\frac{{\epsilon}_{1}{\beta}_{2}\mathrm{\Lambda}}{{p}_{1}{p}_{2}d}$ denotes the ultimate number of the susceptible at the end of the second disease course which is infected by an infectious individual of the second disease course. And lastly, the final term $\frac{{\epsilon}_{1}\cdots {\epsilon}_{m1}{\beta}_{m}\mathrm{\Lambda}}{{p}_{1}\cdots {p}_{m}d}$ denotes the ultimate number of the susceptible at the end of the m th disease course which is infected by an infectious individual of the m th disease course. We see that ${R}_{0}$ is the sum of these ultimate numbers. This shows that ${R}_{0}$ certainly is the basic reproduction number of model (1).
Theorem 2 If ${R}_{0}<1$, then the diseasefree equilibrium ${E}_{0}$ of model (1) is globally attractive. That is, for any solution $(x(n),{y}_{1}(n),{y}_{2}(n),\dots ,{y}_{m}(n),z(n))$ of model (1), we have
Proof Since ${R}_{0}=D\frac{\mathrm{\Lambda}}{d}<1$, then by inequality (6) in Lemma 4, we can obtain ${\underline{y}}_{j}={\overline{y}}_{j}=0$ ($j=1,2,\dots ,m$). In fact, if for some $j\in \{1,2,\dots ,m\}$ such that ${\overline{y}}_{j}>0$, then by inequality (4) in Lemma 4, we can obtain ${\overline{y}}_{m}>0$. Hence, ${\sum}_{j=1}^{m}{\beta}_{j}{\overline{y}}_{j}>0$. From inequality (6) in Lemma 4 and ${R}_{0}<1$, we have
which leads to a contradiction. Hence, ${lim}_{n\to \mathrm{\infty}}{y}_{j}(n)=0$ ($j=1,2,\dots ,m$). Finally, from the expression of $x(n)$ and $z(n)$ of model (1), we can infer that (15) holds. This completes the proof. □
4 Permanence of disease
In this section, we mainly prove the permanence of model (1) when ${R}_{0}>1$. Firstly, we introduce several lemmas which will be used to study the permanence of model (1). Consider the following auxiliary system:
where ${\{g(n)\}}_{n=1}^{\mathrm{\infty}}$ is a given nonnegative bounded real sequence, and parameters ${p}_{i}$, ${\beta}_{i}$, ${\gamma}_{i}$, ${\epsilon}_{j}$, δ and d ($i=1,2,\dots ,m$, $j=1,2,\dots ,m1$) are defined as in model (1). We have the following result.
Lemma 5 For any constants $\eta >0$ and $M>0$, there exist a constant $\xi =\xi (\eta )>0$ and an integer $T=T(M,\eta )>0$ such that for any initial time ${n}_{0}\in {N}_{+}$ and initial value $({u}_{10},{u}_{20},\dots ,{u}_{m+1,0})\in {R}_{+}^{m+1}$ with $0<{u}_{i0}\le M$ ($i=1,2,\dots ,m+1$), if ${u}_{1}(n)\le \xi $ for all $n\ge {n}_{0}$, then we have
where $({u}_{1}(n),{u}_{2}(n),\dots ,{u}_{m+1}(n))$ is the solution of system (16) with the initial condition $({u}_{1}({n}_{0}),{u}_{2}({n}_{0}),\dots ,{u}_{m+1}({n}_{0}))=({u}_{10},{u}_{20},\dots ,{u}_{m+1,0})$.
Proof Firstly, we consider the last equation of system (16)
We can obtain that for any constants ${\eta}_{1}>0$ and $M>0$, there exist ${\eta}_{m+1}>0$ and ${T}_{m+1}>0$, with
such that for any initial time ${n}_{m+1}>0$ and initial value $0<{u}_{m+1}\le M$, if ${u}_{i}(n)\le {\eta}_{m+1}$ for all $n\ge {n}_{m+1}$ and $i=1,2,\dots ,m$, then we have
Consider the m th equation of system (16)
For the above constants ${\eta}_{m+1}>0$ and $M>0$, there exist a constant ${\eta}_{m}>0$ and an integer ${T}_{m}>0$, with
such that for any initial time ${n}_{m}>0$ and initial value $0<{u}_{m}({n}_{m})\le M$, if ${u}_{m1}(n)\le {\eta}_{m}$ for all $n\ge {n}_{m}$, then we have
Further consider the $(m1)$th equation
For the above constants ${\eta}_{m}$ and $M>0$, there exist a constant ${\eta}_{m1}>0$ and an integer ${T}_{m1}>0$, with
such that for any initial time ${n}_{m1}>0$ and initial value $0<{u}_{m1}({n}_{m1})\le M$, if ${u}_{m2}(n)\le {\eta}_{m1}$ for all $n\ge {n}_{m1}$, then we have
Repeating the above process for ${u}_{m2}(n),\dots ,{u}_{2}(n)$, respectively, finally we can obtain that for each ${u}_{i}(n)$ ($i=2,3,\dots ,m2$) and for the above obtained constants ${\eta}_{i+1}>0$ and $M>0$, there exist a constant ${\eta}_{i}>0$ and an integer ${T}_{i}>0$, with
such that for any initial time ${n}_{i}>0$ and $0<{u}_{i}({n}_{i})\le M$, if ${u}_{i1}(n)\le {\eta}_{i}$ for all $n\ge {n}_{i}$, then we have
Let $T={\sum}_{i=2}^{m+1}{T}_{i}$. Then, from the above discussions, we obtain that for any initial time ${n}_{0}>0$ and initial value $0<{u}_{i}({n}_{0})\le M$ ($i=1,2,\dots ,m+1$), if ${u}_{1}(n)<{\eta}_{2}$ for all $n\ge {n}_{0}$, then from (18) we have
We further have
Lastly, from (17) we have
This shows
This completes the proof. □
We further consider the following equation:
where the parameters are assumed to be as in system (1) and $0<\eta <1$. By calculating, we obtain that equation (19) has a positive equilibrium ${v}^{\ast}(\eta )$ with
Obviously, we have
Hence, there exists an ${\eta}_{0}>0$ such that when $0<\eta \le {\eta}_{0}$, we have
Therefore, we have the following result.
Lemma 6 For any constants $\epsilon >0$ and $M>0$, there exists an integer ${N}_{0}={N}_{0}(\epsilon ,M)>0$ such that for any initial ${n}_{0}>0$ and initial value ${v}_{0}$ with $0<{v}_{0}\le M$, we have
where $v(n;{n}_{0},\eta )$ is the solution of equation (19) with the initial condition $v({n}_{0},\eta )={v}_{0}$.
Proof For any solution $v(n;{n}_{0},\eta )$ of equation (19) with the initial condition $v({n}_{0},\eta )={v}_{0}>0$, we define a function
then
From this, we easily see that for any constant ε, there exists an integer ${N}_{0}={N}_{0}(\epsilon ,M)>0$ such that for any initial time ${n}_{0}>0$ and initial value ${v}_{0}$ with $0<{v}_{0}\le M$, when $0<\eta \le {\eta}_{0}$, then for all $n\ge {n}_{0}+{N}_{0}$, we have
This shows that the conclusion of Lemma 6 holds. This completes the proof. □
For any $x,y\in {R}^{m}$ with $x=({x}_{1},{x}_{2},\dots ,{x}_{m})$ and $y=({y}_{1},{y}_{2},\dots ,{y}_{m})$, if ${x}_{i}\le {y}_{i}$ ($i=1,2,\dots ,m$), then we denote $x\le y$.
Let $D\subset {R}^{m}$ and $f(n,u)=({f}_{1}(n,u),{f}_{2}(n,u),\dots ,{f}_{m}(n,u))$ be a function defined on $n\ge 0$ and $u\in D$. If for any ${u}_{1},{u}_{2}\in D$ with ${u}_{1}\le {u}_{2}$ we have
then the function $f(n,u)$ is said to be nondecreasing for $u\in D$.
Lemma 7 (See [29])
Let the domain $D\subset {R}_{+}^{m}$ and the function $f(n,x)$ defined on $n\ge 0$ and $x\in D$ be nondecreasing for $x\in D$. If the sequence ${\{x(n)\}}_{n=1}^{\mathrm{\infty}}\subset D$ for all $n\ge {n}_{0}$ satisfies $x(n+1)\ge f(n,y(n))$ ($x(n+1)\le f(n,y(n))$), then we have
where $y(n)$ is the solution of the difference equation $y(n+1)=f(n,y(n))$ with initial value $x({n}_{0})\ge y({n}_{0})$ ($x({n}_{0})\le y({n}_{0})$).
Now, we consider the following linear autonomous difference system:
where A is an $m\times m$ nonnegative matrix and $x(n)\in {R}^{m}$. Then we have the following result.
Lemma 8 Let $r=r(A)$ be the spectral radius of matrix A, then the following conclusions hold.

(1)
There exists an mdimensional column vector $e={({e}_{1},{e}_{2},\dots ,{e}_{m})}^{T}$ with ${e}_{i}>0$ ($i=1,2,\dots ,m$) such that $x(n)={\lambda}^{n}e$ is a solution of system (22).

(2)
For any ${x}_{0}=({x}_{10},{x}_{20},\dots ,{x}_{m0})$ with ${x}_{i0}>0$ ($i=1,2,\dots ,m$), there exist constants ${a}_{i}>0$ ($i=1,2$) such that
$${\lambda}^{n{n}_{0}}{a}_{1}e\le x(n)\le {\lambda}^{n{n}_{0}}{a}_{2}e\phantom{\rule{1em}{0ex}}\mathit{\text{for all}}\phantom{\rule{0.5em}{0ex}}n\ge {n}_{0},$$
where $x(n)$ is the solution of system (22) satisfying the initial condition $x({n}_{0})={x}_{0}$.
Proof (1) Since λ is an eigenvalue of matrix A and matrix A is a nonnegative matrix, we can obtain that there is a vector $e>0$ corresponding to the eigenvalue λ such that
From ${A}^{n}=A{A}^{n1}$, we have
Suppose that $x(0)=e$, then
Therefore, $x(n)={\lambda}^{n}e$ is a solution of system (22).

(2)
Let $b={min}_{1\le i\le m}\{{x}_{i0}\}$, $B={max}_{1\le i\le m}\{{x}_{i0}\}$, $c={min}_{1\le i\le m}\{{e}_{i}\}$ and $C={max}_{1\le i\le m}\{{e}_{i}\}$. Further let ${a}_{1}=b/C$ and ${a}_{2}=B/c$. Then we have ${a}_{1}e\le {x}_{0}\le {a}_{2}e$. Let ${x}_{1}(n)={a}_{1}e{\lambda}^{n{n}_{0}}$ and ${x}_{2}(n)={a}_{2}e{\lambda}^{n{n}_{0}}$. Then ${x}_{1}(n)$ and ${x}_{2}(n)$ are the solutions of system (22) with initial value ${x}_{1}({n}_{0})={a}_{1}e$ and ${x}_{2}({n}_{0})={a}_{2}e$, respectively. Then from Lemma 7 it follows that
$${\lambda}^{n{n}_{0}}{a}_{1}e\le {x}_{1}(n)\le x(n)\le {x}_{2}(n)\le {\lambda}^{n{n}_{0}}{a}_{2}e$$
for all $n\ge {n}_{0}$. □
Lemma 9 If ${R}_{0}>1$, then there exists a constant ${h}_{1}>0$ such that for any solution $(x(n),{y}_{1}(n),{y}_{2}(n),\dots ,{y}_{m}(n),z(n))$ of model (1), we have
Proof
Since
then we can choose a constant ${\delta}_{0}$ ($0<{\delta}_{0}<1$) such that
Hence, we have
Then, from (20) and (21), there exists an ${\eta}_{0}$ with $0<{\eta}_{0}\le {\delta}_{0}$ such that
Now, from Lemma 6, for above ${\delta}_{0}>0$, there exists an integer ${N}_{0}>0$ such that for any initial time ${n}_{0}>0$ and initial value ${v}_{0}$ with $0<{v}_{0}\le {M}_{0}$, where ${M}_{0}=\frac{\mathrm{\Lambda}}{d}+1$, we have
where $v(n;{n}_{0},{\eta}_{0})$ is the solution of equation (19) with $\eta ={\eta}_{0}$ and the initial condition $v({n}_{0},{\eta}_{0})={v}_{0}$. Hence, from (25) and (26), we have
Assume that $(x(n),{y}_{1}(n),\dots ,{y}_{m}(n),z(n))$ is any positive solution of model (1) with the initial condition $(x({n}_{0}),{y}_{1}({n}_{0}),\dots ,{y}_{m}({n}_{0}),z({n}_{0}))=({x}_{0},{y}_{10},\dots ,{y}_{m0},{z}_{0})$. Then, from Lemma 2, for $\epsilon =1$ there exists an integer ${N}_{1}>0$ such that when $n>{N}_{1}$, we have
Consider the following difference system:
where parameters ${p}_{i}$, ${\beta}_{i}$, ${\epsilon}_{i}$, ${\gamma}_{i}$, δ and d in system (29) are given as in model (1). Since $(x(n),{y}_{1}(n),\dots ,{y}_{m}(n),z(n))$ is the solution of model (1), then $({y}_{1}(n),\dots ,{y}_{m}(n),z(n))$ is the solution of system (29). From (28) and Lemma 5, for above ${\eta}_{0}>0$ and $M=\frac{\mathrm{\Lambda}}{d}+1$, there exist a constant ${\delta}^{\ast}>0$ and an integer ${N}_{2}>0$ with ${\delta}^{\ast}\le {\eta}_{0}$ such that for any initial time ${n}_{0}\ge {N}_{1}$ and initial value $({u}_{10},{u}_{20},\dots ,{u}_{m+1,0})\in {R}_{+0}^{m}$, if ${u}_{1}(n)\le {\delta}^{\ast}$ for all $n\ge {n}_{0}$, then we have ${u}_{j}(n)\le {\eta}_{0}$ for all $n\ge {n}_{0}+{N}_{2}$ and $j=1,2,\dots ,m+1$. Hence, if ${y}_{1}(n)\le {\delta}^{\ast}$ for all $n\ge {n}_{0}$, then we have
Now, we prove that if ${R}_{0}>1$ then
for any positive solution $(x(n),{y}_{1}(n),\dots ,{y}_{m}(n),z(n))$ of model (1). Suppose that the conclusion is not true, then there exists a positive solution $(x(n),{y}_{1}(n),\dots ,{y}_{m}(n),z(n))$ of model (1) such that ${lim\hspace{0.17em}sup}_{n\to \mathrm{\infty}}{y}_{1}(n)<{\delta}^{\ast}$. Hence, there exists an integer ${N}_{1}>0$ such that ${y}_{1}(n)<{\delta}^{\ast}$ for all $n\ge {N}_{1}$. From (30) we know that for any ${n}_{0}\ge {N}_{1}>0$, there exists an integer ${N}_{2}>0$ such that
Then, from (28) and the first equation of model (1), we obtain
Since from Lemma 7 and (27), for any ${n}_{1}\ge {n}_{0}+{N}_{2}$, there exists an integer ${N}_{0}>0$ such that
then replacing (32) into the second and the third equations of model (1), we have
Next, consider the following auxiliary system:
Obviously, system (34) is a linear autonomous difference system and we can rewrite it as follows:
where $w(n)={({w}_{1}(n),{w}_{2}(n),\dots ,{w}_{m}(n))}^{T}$ and
We have
From (24), we have
and, for a constant $\beta >max\{{\beta}_{1},{\beta}_{2},\dots ,{\beta}_{m}\}(\frac{\mathrm{\Lambda}}{d}{\delta}_{0})$, we further have
Then, from the intermediate value theorem, there exists a constant ${\mu}^{\ast}\in (1,\beta +1)$ such that $f({\mu}^{\ast})=0$. Therefore, we can obtain $\overline{\mu}=r(A)>1$. Let $w(n)$ be the solution of system (33) with the initial condition $w({N}^{\ast})=({y}_{1}({N}^{\ast}),{y}_{2}({N}^{\ast}),\dots ,{y}_{m}({N}^{\ast}))$, where ${N}^{\ast}={n}_{0}+{N}_{0}+{N}_{2}$. Then from (33), (34) and Lemma 7, we have
Further, from the second part of Lemma 8, there exists a constant ${a}_{1}>0$ such that
Hence, we have
From this, we obtain
which leads to a contradiction. This completes the proof. □
Lastly, directly from Lemma 3, Lemma 4 and Lemma 9, we can obtain the following result on the permanence of model (1).
Theorem 3 If ${R}_{0}>1$, then the disease in model (1) is permanent.
Proof In fact, from Lemma 9, we obtain that for any positive solution $(x(n),{y}_{1}(n),\dots ,{y}_{m}(n),z(n))$ of model (1),
Then, from inequality (4) in Lemma 4, we further have
Finally, from the last equation of model (1), we have
This completes the proof. □
5 Conclusions
In this paper, we study a class of discrete epidemic models with disease courses, that is, model (1). The basic reproduction number ${R}_{0}$ is calculated. It is shown that the global dynamics of model (1) is determined by the basic reproduction number ${R}_{0}$. If ${R}_{0}<1$, then we obtain that the diseasefree equilibrium of model (1) is globally asymptotically stable. This shows that when ${R}_{0}<1$ the disease in model (1) is extinct. If ${R}_{0}>1$, then we obtain that the endemic equilibrium of model (1) exists and the disease is permanent. Clearly, our condition given in this paper is the threshold condition between the extinction and the permanence of the disease. Hence, our results obtained in this paper extend the results given in [7, 11, 18] for the discrete epidemic models.
However, it is a pity that the case of the basic reproduction number ${R}_{0}=1$ is not discussed in this paper. From the results on the case ${R}_{0}=1$ obtained in [17, 18], we can guess that when ${R}_{0}=1$ then the diseasefree equilibrium of model (1) is also globally asymptotically stable. This shows that when ${R}_{0}=1$ the disease in model (1) is also extinct. The other one which is not obtained in our this paper is the global stability of the endemic equilibrium of model (1). From the results on the global stability of the endemic equilibrium obtained in [7, 17, 18], we can guess that when ${R}_{0}>1$ the endemic equilibrium of model (1) is globally stable. We will discuss these problems in our future work.
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Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant Nos. 11271312, 11001235, 10901130), the China Postdoctoral Science Foundation (Grant Nos. 20110491750, 2012T50836) and the Natural Science Foundation of Xinjiang (Grant No. 2011211B08).
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Keywords
 discrete epidemic model
 disease course
 basic reproduction number
 permanence
 global attractivity
 extinction