Mathematical modelling of HIV epidemic and stability analysis

*Correspondence: fpeker@erciyes.edu.tr 2Department of Mathematics, Faculty of Science, Erciyes University, Kayseri, Turkey Full list of author information is available at the end of the article Abstract A nonlinear mathematical model of differential equations with piecewise constant arguments is proposed. This model is analyzed by using the theory of both differential and difference equations to show the spread of HIV in a homogeneous population. Because of the solution of this differential equations being established in a certain subinterval, solutions will be analyzed as a system of difference equations. After that, results will be considered for differential equations as well. The population of the model is divided into three subclasses, which are the HIV negative class, the HIV positive class that do not know they are infected and the HIV positive class that know they are infected. As an application of the model we took the spread of HIV in India into consideration. MSC: 39A10; 39A11


Introduction
Sexually transmitted diseases such as HIV are overwhelmingly observed in some populations. Countries are dealing with the growing impact of the epidemics on the youngest and most productive population groups; increasing numbers in children and adolescents; worsening situation among the poor and marginalized populations; a continuous aggravation of the existent health problems (such as tuberculosis); and above all, the diversion of resources from other health, welfare, and educational priorities. It is very important to understand how the transmission process of the infectious diseases works in order to avoid ulterior spread of these epidemics []. So mathematical models for transmission dynamics in HIV play an important role in better understanding of epidemiological patterns for disease control as they provide short and long term prediction for HIV and AIDS incidence []. The first mathematical model used for the explicit study of a sexually transmitted disease was a one sex model that was constructed by Cooke and Yorke in  [, ]. A two sex model developed specifically for gonorrhea was formulated by Lajmanovich and Yorke in  []. One STD (sexually transmitted disease) that many people are worried about getting is HIV. Mathematical modeling of the HIV epidemic has been studied by various authors recently [-]. In , Velasco-Hernandez and Hsieh analyzed the HIV epidemic in a male population []. In , Bachar and Dorfmatr considered the mathematical model where the population was divided into three sub populations ©2014 Bozkurt and Peker; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. http://www.advancesindifferenceequations.com/content/2014/1/95 []. Expanding [], Naresh has studied the effect of contact tracing on reducing the spread of HIV/AIDS in a homogeneous population with constant immigration of susceptibles.
Changes in population involved in a HIV transmission model can take place by discrete steps and continuous processes. In discrete models, difference equations reflect the change over the whole time step, whereas in continuous models, differential equations are developed to explore the changes in one variable with a decreasingly small change in another variable []. Because HIV population dynamics involves both continuous and discrete time arguments, for this type of problems, a different point of view came up and Buseenberg and Cooke used piecewise constant arguments to construct the mathematical modeling of biological structures in . By taking into account the work of Naresh [] we have constructed a new mathematical model. We added new terms and took into consideration both discrete time and continuous time. This model is analyzed by using the theory of both differential and difference equations to show the spread of HIV/AIDS in a homogeneous population. Because of the solution of these differential equations established in a certain subinterval, the solutions will be analyzed as a difference equations system. Furthermore, these results will be considered for differential equations, as well. So our mathematical model that is named a differential equation systems with piecewise constant arguments is as follows: In the equations above, t is the time and [|t|] is the exact value of t for t ≥ . The model monitors three populations; susceptibles S(t), HIV positives that do not know they are infected I  (t), HIV positives that know they are infected I  (t). The susceptibles are composed of individuals that have not contracted the infection but may get infected through contacts (sexual, blood transfusion etc.) with infectives. r  is the population growth rate of the susceptible population. The deaths (of natural causes) are given at a rate α  . p  is the rate of susceptible population per year. The susceptibles are lost from their class following contacts with the infectives I  and I  at a rate β  and β  , respectively. I  are populations that are HIV positive but do not know it, because disease symptoms have not yet appeared in this class. This population is generated by the HIV infection of susceptibles. r  is the population growth rate of the population I  . The deaths (of natural causes) are given at a rate α  . The population of this class decreases and becomes aware after screening at a rate θ . γ is the rate of individuals in class I  that detect the infection by tracing contacts with class I  . Following the contacts of class S and I  , class S becomes aware of the infection and this class is detected further which has the rate of ε  . And also, following the contacts of class S and I  , class S becomes aware of the infection and is detected further with the rate of ε  . r  is the population growth rate of the population I  . The deaths (of natural causes) are given at a rate α  . http://www.advancesindifferenceequations.com/content/2014/1/95

Local and global asymptotic stability analysis
In this section, the local and global behavior of the nonlinear system (.) under specific conditions is investigated. To show the consistence of the population classes with the model constructed in Section , the data of [] is used. Examples show the spread of the population over the time.

Equilibrium point of the model (1.1)
For t ∈ [n, n+] every equation in system (.) is a Bernoulli differential equation as follows: Solving (.) for t ∈ [n, n + ) and letting t go to n +  where n = , , , , . . . , we obtain a system of difference equations as follows: It is obvious that (.) must hold: System (.) is a system of difference equations. Hence, to explain the global behavior of system (.) we must consider the solution of (.), that is, the difference equations system (.). Firstly, we need to obtain the equilibrium points of this system (.), which are also the critical points of system (.). We have (.) http://www.advancesindifferenceequations.com/content/2014/1/95 Simplifying this, we get Substituting in (.) the expression we obtain To find the positive equilibrium point of (.), we have some assumptions in view of the demographic data in [].
Thus, the equilibrium points of system (.) are X = (S, I  , I  ). Here The Jacobian matrix for the equilibrium points is (.). This gives the linearizing equations of system (.), The characteristic equation is

Local and global stability of the positive equilibrium
In this part, the local and global stability of system (.) will be analyzed. For the proof of Theorem ., Theorem ., and Theorem ., it is assumed that conditions (i)-(viii) hold.
Theorem . Let X = (S, I  , I  ) the positive equilibrium point of system (.) and assume that .
Furthermore, suppose that .
, then the positive equilibrium point of system (.) is locally asymptotically stable.
Considering the condition (iii), we investigate where c  = a  a  a  + a  a  a  + a  a  a a  a  a a  a  a a  a  + a  , (.) can be written as In this case, we must only show the condition for the inequality Furthermore, if the following conditions hold, then the inequality (.) is available. If a  a  < a  a  , (  .   ) then we obtain Considering (.), (.), and (.), we obtain and we get Using (.) and (.) in the following, we obtain This completes the proof.
Theorem . Let {S(n), I  (n), I  (n)} ∞ n= , be a positive solution of system (.). The following statements are true.
(i) If then the solution of system (.) increases monotonically.
Theorem . Let system (.) be written as follows: where the first order partial derivatives of the functions f , g, and h with regard to S(t), I  (t), Proof Let the initial values be as follows: In this case, we must have to find that S(t + ) = f (S(t), I  (t), I  (t)) has no -cycle in I. In view of Theorem ., the following results can be obtained.
() f (S(t), I  (t), I  (t)) has no cycle in I, since for α  > β  we obtain where () Similarly, I  (t + ) = g(S(t), I  (t), I  (t)) has no cycle in I, since for α  > θ , we get ) has no cycle in I, since (.) and (.) hold, and we have This completes the proof.
Some additional assumptions of (.) are needed to verify global asymptotic stability for the positive equilibrium point x = (S, I  , I  ).
Let f , g, h be continuous functions and  variable β  = .. And also in Figure , we have changed β  = ., β  = .. In this case, one can say that β  is an important parameter for the spread of the populations over the time t. Considering Figure , it is seen that after  years an important increase of the I  class and decrease of the S class will happen for the selected values in system (.). When Figures  and  are compared, one can see that in the same years the I  class increases more in Figure  and that the S class decreases more in Figure . In Figure , for β  = . and β  = . after  years an important increase of class I  and decrease of class S will happen. In that case it is said that as the value of β  (per capita contact rate for susceptibles with unaware infectives (I  )) increases, the I  class decreases in earlier years.
In  the number of unaware infectives. It is seen that as the rate of θ and k become zero (see Figure ) i.e. the infectives who do not know that they are infected will continue maintaining sexual relationships in the community which will ultimately increase the infective populations.
In Figure  due to immigration, the susceptible population increases continuously, therefore, infection becomes more endemic and always persists in the population (see also Figure ). In Figure  the distribution of population with time is shown in different classes without immigration. It is seen that in the absence of immigration into the community, the susceptible population decreases continuously as the population is closed, which results in an increase in the infective population first and then it decreases as all infectives will develop AIDS will die out by disease-induced deaths. Thus the total population will be eradicating after some time. http://www.advancesindifferenceequations.com/content/2014/1/95  Thus, on changing the behavior and increasing the awareness about the HIV infection in the population the infection can be slowed down and may be kept under control.

Discussion
In this paper, a mathematical model with piecewise constant arguments is proposed to investigate the impact of the parameters (e.g. contact tracing, screening) on the spread of HIV in a population with variable size structure. The model is analyzed using Schur-Cohn criteria on difference equations and numerical simulation. The equilibrium point is found to be locally asymptotically stable and globally asymptotically stable under certain conditions. It is observed that if the aware HIV infectives, detected by screening and contact tracing, do not take part in spreading the disease, HIV infection reduces significantly. It is also found that the disease becomes more endemic due to immigration and the endemicity of the disease decreases when the infectives become aware of their infection http://www.advancesindifferenceequations.com/content/2014/1/95  after screening and contact tracing and do not take part in sexual interaction, whereas it increases in the absence of contact tracing. In the absence of screening and contact tracing, the infected people continue to spread the disease without taking any precaution due to unawareness of their infection.
Finally from the analysis, it may be speculated that the most effective way to reduce the infection rate and prevalence level is to educate the people about the HIV and make them aware of the consequences of practicing non-safe sex or any other kind of risky behavior. If the population presents a positive attitude toward preventive procedures, the disease may tend to vanish, even for relatively small random screening rates.