Discrete Dynamics in Nature and Society

Volume 2011, Article ID 730783, 22 pages

http://dx.doi.org/10.1155/2011/730783

## Global Stability of an Eco-Epidemiological Model with Time Delay and Saturation Incidence

Institute of Applied Mathematics, Shijiazhuang Mechanical Engineering College, Shijiazhuang 050003, China

Received 30 June 2011; Accepted 23 August 2011

Academic Editor: Her-Terng Yau

Copyright © 2011 Shuxue Mao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

We investigate a delayed eco-epidemiological model with disease in predator and saturation incidence. First, by comparison arguments, the permanence of the model is discussed. Then, we study the local stability of each equilibrium of the model by analyzing the corresponding characteristic equations and find that Hopf bifurcation occurs when the delay passes through a sequence of critical values. Next, by means of an iteration technique, sufficient conditions are derived for the global stability of the disease-free planar equilibrium and the positive equilibrium. Numerical examples are carried out to illustrate the analytical results.

#### 1. Introduction

Recently, more attention has been paid to the eco-epidemiology model which considers both the ecological and epidemiological issues simultaneously due to the fact that most of the ecological populations suffer from various infectious diseases which have a significant role in regulating population sizes (see, e.g., [1–6]). Mukherjee [7] discussed a predator-prey model with disease in prey. The criteria were derived for both local stability and instability involving system parameters. In addition, considering the time required by the susceptible individuals to become infective after their interaction with the infectious individuals, Zhou et al. [8] formulated a delayed eco-epidemiology model and found that the Hopf bifurcation occurs when the delay passes through a sequence of critical values. They also gave an estimation of the length of the time delay to preserve stability. On the other hand, in the predator-prey system, the disease not only can spread in prey but also can spread in predator. Therefore, Zhang et al. [9] studied an eco-epidemiological model with disease in predator and showed that a Hopf bifurcation can occur as the delay increased. The above-mentioned works all used bilinear incidence to model disease transmission.

Note that ecologically the assumption of standard incidence instead of the former bilinear mass action incidence is meaningful for large populations and a low number of infected individuals, a very good justification behind this assumption being found in [10]. Han et al. [11] proposed four modifications of a predator-prey model with standard incidence to include an SIS or SIR parasitic infection. Thresholds were identified, and global stability results were proved. When the disease persists in the prey population and the predators have a sufficient feeding efficiency to survive, the disease also persists in the predator population. Hethcote et al. [12] considered a predator-prey model including an SIS parasitic infection in the prey with infected prey being more vulnerable to predation. Thresholds were identified which determine when the predator population survives and when the disease remains endemic.

However, there are a variety of factors that emphasize the need for a modification of the bilinear incidence and standard incidence. For example, the underlying assumption of homogeneous mixing may not always hold. Incidence rates that increase more gradually than linearly in and may arise from saturation effects. It has been strongly suggested by several authors that the disease transmission process may follow saturation incidence. After studying the cholera epidemic spread in Bari in 1973, Capasso and Serio [13] introduced a saturated incidence rate into epidemic models with . A general saturation incidence rate was proposed by Liu et al. [14] and used by a number of authors; see, for example, Ruan and Wang [15] , Bhattacharyya and Mukhopadhyay [16] , and so forth. measures the infection force of the disease, and measures the inhibition effect from the behavioral change of the susceptible individuals when their number increases or from the crowding effect of the infective individuals. This incidence rate seems more reasonable than the bilinear incidence rate , because it includes the behavioral change and crowding effect of the infective individuals and prevents the unboundedness of the contact rate by choosing suitable parameters.

Motivated by the works of Zhang et al. [9] and Capasso and Serio [13], in this paper, we are concerned with the effect of disease in predator and saturated incidence on the dynamics of eco-epidemiological model. To this end, we consider the following delay differential equations: with initial conditions where , ), the Banach space of continuous functions mapping the interval into , here .

We make the following assumptions for our model (1.1). (A1)The prey population grows logistically with intrinsic growth rate and environmental carrying capacity .(A2)There is a spread of disease in predators which are divided solely into susceptible and infectious population. is the capturing rate of susceptible predators, is the growth rate of susceptible predator due to predation of prey. (A3)Susceptible predators become infected when they come in contact with infected predator, and this contact process is assumed to follow the saturation incidence rate , with measuring the force of infection and the inhibition effect. (A4) models death rate due to overcrowding, and is the time required for the gestation of susceptible predator. is the death rate of infected predator. All the above-mentioned parameters are assumed to be positive.

The paper is organized as follows. In the next section, the positivity of solutions and the permanence of system are discussed. By analyzing the corresponding characteristic equations, we find conditions for local stability and bifurcation results in Section 3. In Section 4, sufficient conditions are derived for the global stability of the disease-free planar equilibrium and the positive equilibrium of the system. Numerical examples are carried out to illustrate the validity of the main results. The paper ends with a conclusion in the last section.

#### 2. Permanence

To prove the permanence of system (1.1), we need the following lemma, which is a direct application of Theorem 4.9.1 in the study by Kuang [17].

Lemma 2.1. *Consider the following equation:
**
where and for all .*(1)*If , then .*(2)*If , then .*

Theorem 2.2. *All the solutions of (1.1) with initial conditions (1.2) are all nonnegative.*

*Proof. *Let be the solution of system (1.1) satisfying conditions (1.2). From the first and last equations of system (1.1), we have
Hence, and are positive.

We now claim that for all . Otherwise, there exists a such that and for all . Then . From the second equation of (1.1), we have
which is a contradiction.

Theorem 2.3. *All the solutions of (1.1) with initial conditions (1.2) are ultimately bounded. *

*Proof. *From the first equation of (1.1), we have
Hence, we get

From the second equation of system (1.1), for sufficiently large, we have
Hence, by Lemma 2.1, one can get

It follows from the third equation of (1.1) and the above inequality, that for sufficiently large, we have
Hence, one can see .

Now, we show that system (1.1) is permanent.

Theorem 2.4. *Suppose that *()*where is defined in (2.13), then system (1.1) is permanent. *

*Proof. *From the first equation of system (1.1), we have
It then follows that

Using the second equation of system (1.1), for sufficiently large, we have
Hence, by Lemma 2.1 and , one can derive that

From the third equation of system (1.1) and, above inequality, we have
Since holds, then

Therefore, the above calculations and Theorem 2.2 imply that there exist such that

#### 3. Local Stability

System (1.1) possesses the following equilibria.(1)The trivial equilibrium .(2)The axial equilibrium .(3) The disease-free planar equilibrium , where

(4) The unique positive equilibrium exists if , where

In the following, we discuss the local stability of each equilibrium of system (1.1) by analyzing the corresponding characteristic equations, respectively.

##### 3.1. Stability of Equilibrium

The characteristic equation of system (1.1) at the trivial equilibrium is of the form It is easy to see that (3.3) always has a positive root . Hence, is always unstable.

##### 3.2. Stability of Equilibrium

The characteristic equation of system (1.1) at the axial equilibrium is of the form There are two characteristic roots , , and another characteristic root is given by the root of

It is clear that . Hence, is always unstable.

##### 3.3. Stability of Equilibrium

Theorem 3.1. *The disease-free planar equilibrium is locally asymptotically stable if , and the equilibrium is unstable if .*

*Proof. *The characteristic equation of system (1.1) at the disease-free planar equilibrium is of the form

Clearly, is a negative eigenvalue. The second eigenvalue is given by the root of
Suppose that , then . It is a contradiction, so . The last eigenvalue is . The equilibrium is locally asymptotically stable if , and the equilibrium is unstable if .

##### 3.4. Stability of Equilibrium

The characteristic equation of system (1.1) at the positive equilibrium is of the form where

For , the transcendental (3.8) reduces to the following equation: We can easily get Therefore, the Routh-Hurwitz criterion implies that all the roots of (3.8) have negative real parts and we can conclude that the positive equilibrium is asymptotically stable in the absence of delay.

Theorem 3.2. *For system (1.1), if the condition holds, the positive equilibrium is conditionally stable. *

*Proof. *Substituting into (3.8) and separating the real and imaginary parts, one can get

Squaring and adding (3.12) we get
where
We know that provided that the condition holds. There is at least a positive satisfying (3.13), that is, the characteristic equation (3.8) has a pair of purely imaginary roots of the form . From (3.12), we can get the corresponding such that the characteristic (3.8) has a pair of purely imaginary roots
Let be the roots of (3.8) such that satisfying and . Differentiating the two sides of (3.8) with respect to , we get
Therefore,
If the conditions and hold, one can see
Therefore, the transversality condition holds, hence, the Hopf bifurcation occurs at and .

Theorem 3.3. *Suppose that the conditions and are satisfied. *(1)*The positive equilibrium of system (1.1) is asymptotically stable for all and unstable for . *(2)*System (1.1) undergoes a Hopf Bifurcation at the positive equilibrium when .*

#### 4. Global Stability

In this section, we study the global stability of equilibriums and . The strategy of proofs is to use an iteration technique and comparison arguments, respectively.

Theorem 4.1. *If *()* holds, then the disease-free planar equilibrium is globally asymptotically stable. *

*Proof. *Let be any positive solution of system (1.1) with initial conditions (1.2). Let the following hold:
In the following we shall claim that .

It follows from the first equation of system (1.1) that
By comparison, we obtain that
Since this inequality holds true for arbitrary sufficiently small, we conclude that , where
Hence, for sufficiently small, there is a such that, if , .

We, therefore, derive from the second equation of system (1.1) that, for ,
Hence, by Lemma 2.1, one can get
Hence, for sufficiently small, there is a such that, if , .

It follows from the third equation of system (1.1) that, for ,
Since holds, one can see
According to Theorem 2.2, we can get .

We derive from the first equation of system (1.1) that, for ,
By comparison we derive that
Since this inequality holds true for arbitrary sufficiently small, we conclude that , where
Hence, for sufficiently small, there is a such that, if , .

We derive from the second equation of system (1.1) that, for ,
Hence, by Lemma 2.1, one can get
Since this is true for arbitrary sufficiently small, we conclude that , where
Hence, for sufficiently small, there is a such that, if , .

Again, it follows from the first equation of system (1.2) that, for ,
A comparison argument yields
Since this inequality holds true for arbitrary sufficiently small, we conclude that , where
Hence, for sufficiently small, there is a such that, if , .

It follows from the second equation of system (1.1) that, for ,
By Lemma 2.1, one can derive that
Since this is true for arbitrary sufficiently small, we conclude that , where
Hence, for sufficiently small, there is a such that, if , .

We derive from the first equation of system (1.1) that, for ,
By comparison it follows that
Since this inequality holds true for arbitrary sufficiently small, we conclude that , where
Hence, for sufficiently small, there is a such that, if , .

We derive from the second equation of system (1.1) that, for ,
Hence, by Lemma 2.1, one can get
Since this inequality holds true for arbitrary sufficiently small, we conclude that , where
Hence, for sufficiently small, there is a such that, if , .

Continuing this process, we get four sequences such that, for ,
Clearly, we have
It follows from (4.27) that
Noting that and , we derive from (4.29) that
Thus, the sequence is monotonically nonincreasing. Therefore, it follows that exists. Taking , we obtain from (4.29) that
Noting that
it follows from (4.31) that
We derive from (4.33) and the third equation of (4.27) that
Similarly, one can derive from (4.27) and (4.34) that
It follows from (4.28), (4.33), and (4.35) that
We, therefore, have
Hence, the disease-free planar equilibrium is globally asymptotically stable. The proof is complete.

Theorem 4.2. *If ** holds, then the positive equilibrium is globally asymptotically stable.*

*Proof. *Let be any positive solution of system (1.1) with initial conditions (1.2). Let the following hold:
In the following we claim that .

It follows from the first equation of system (1.1) that
By comparison we obtain
Since this inequality holds true for arbitrary sufficiently small, we conclude that , where
Hence, for sufficiently small, there is a such that, if , .We obtain from the second equation of system (1.1) that, for ,
Hence, by Lemma 2.1, we derive that
Since it is true for arbitrary sufficiently small, we conclude that , where
Hence, for sufficiently small, there is a such that, if , .

It follows from the third equation of system (1.1) that
Since holds, one can see
Since it is true for arbitrary sufficiently small, we conclude that , where

We derive from the first equation of system (1.1) that, for ,
By comparison we derive that
Since this inequality holds true for arbitrary sufficiently small, we conclude that , where
Hence, for sufficiently small, there is a such that, if , .

We derive from the second equation of system (1.1) that, for ,
Hence, by Lemma 2.1 and , one can get
Since this inequality holds true for arbitrary sufficiently small, we conclude that , where
Hence, for sufficiently small, we get .

It follows from the third equation of system (1.1) that
Provided that , one can see
Since this inequality holds true for arbitrary sufficiently small, we conclude that , where

It follows from the first equation of system (1.1) that
By comparison we derive that
Since this inequality holds true for arbitrary sufficiently small, we conclude that , where
Hence, for sufficiently small, there is a such that, if , .

We obtain from the second equation of system (1.1) that, for ,
Hence, by Lemma 2.1, one can get
Since this inequality holds true for arbitrary sufficiently small, we conclude that , where
Hence, for sufficiently small, there is a such that, if , .

It follows from the third equation of system (1.1) that
Hence, by , one can see
Since this inequality holds true for arbitrary sufficiently small, we conclude that , where

We derive from the first equation of system (1.1) that, for ,
By comparison we derive that
Since this inequality holds true for arbitrary sufficiently small, we conclude that , where
Hence, for sufficiently small, there is a such that, if , .

We derive from the second equation of system (1.1) that, for ,
By Lemma 2.1, one can get
Since this inequality holds true for arbitrary sufficiently small, we conclude that , where
Hence, for sufficiently small, we get .

It follows from the third equation of system (1.1) that
Since holds, one can see
Since this inequality holds true for arbitrary sufficiently small, we conclude that , where

Continuing this process, we obtain six sequences such that, for ,
A direct calculation shows that
By induction, we can show that , . Therefore, the sequence is decreasing, and the sequence is increasing. Clearly, we have
Hence, the limits of the sequences and exist. Denote
We derive from (4.75) that
Since holds, . It, therefore, follows from (4.79) that . Accordingly, we derive from (4.75) that
By a simple calculation, we obtain
It follows from (4.75) and (4.81) that , . Hence, the unique positive equilibrium is globally asymptotically stable. The proof is complete.

In the following we will present two examples to verify our results obtained earlier.

*Example 4.3. *In system (1.1), we let . It is easy to show that . By Theorem 4.1 we see that the equilibrium of system (1.1) is globally stable (see Figure 1).

*Example 4.4. *In system (1.1), we let . It is easy to show that . By Theorem 4.2 we see that the equilibrium of system (1.1) is globally stable, as depicted in Figure 2.

#### 5. Conclusion

In this paper, we have incorporated the disease for the predator and the time delay into an eco-epidemiology model. A saturation incidence function was used to model the behavioral change of the susceptible predator when their number increases or due to the crowding effect of the infected predator. First, by comparison arguments, the permanence of system (1.1) was studied. Then, by analyzing the corresponding characteristic equations, sufficient conditions were derived for the local stability of each equilibrium of system (1.1). From Theorem 3.3, we showed that system (1.1) undergoes a Hopf bifurcation when the delay passes through a sequence of critical values. Next, by using the iteration technique and comparison arguments, we derived sufficient conditions for the global stability of the disease-free planer equilibrium and positive equilibrium of system (1.1). By Theorems 4.1 and 4.2, we showed that (1) if holds, the infected predator population becomes extinct and the disease will be eliminated; that is, only sound predator and prey coexist; (2) if holds, the prey, the sound predator and the infected predator coexist. The disease will not be eliminated, and the system is permanent.

#### Acknowledgment

This work was supported by the National Natural Science Foundation of China (no. 11071254).

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