Predicting and preventing the emergence of antiviral drug resistance in HSV-2

Stochastic investigation of HIV infection and the emergence of drug resistance

Drug-resistant HIV-1 has caused a growing concern in clinic and public health. Although combination antiretroviral therapy can contribute massively to the suppression of viral loads in patients with HIV-1, it cannot lead to viral eradication. Continuing viral replication during sub-optimal therapy (due to poor adherence or other reasons) may lead to the accumulation of drug resistance mutations, resulting in an increased risk of disease progression. Many studies also suggest that events occurring during the early stage of HIV-1 infection (i.e., the first few hours to days following HIV exposure) may determine whether the infection can be successfully established. However, the numbers of infected cells and viruses during the early stage are extremely low and stochasticity may play a critical role in dictating the fate of infection. In this paper, we use stochastic models to investigate viral infection and the emergence of drug resistance of HIV-1. The stochastic model is formulated by a continuous-time Markov chain (CTMC), which is derived based on an ordinary differential equation model proposed by Kitayimbwa et al. that includes both forward and backward mutations. An analytic estimate of the probability of the clearance of HIV infection of the CTMC model near the infection-free equilibrium is obtained by a multitype branching process approximation. The analytical predictions are validated by numerical simulations. Unlike the deterministic dynamics where the basic reproduction number R₀ serves as a sharp threshold parameter (i.e., the disease dies out if R₀<1 and persists if R₀>1), the stochastic models indicate that there is always a positive probability for HIV infection to be eradicated in patients. In the presence of antiretroviral therapy, our results show that the chance of clearance of the infection tends to increase although drug resistance is likely to emerge.

Computational and Theoretical Analysis of the Association Between Gender and HSV-2 Treatment Adherence

Herpes simplex virus type 2 (HSV-2) is the most prevalent sexually transmitted infection in the world, despite the availability of effective anti-viral treatments. A mathematical model to explore the association between gender and HSV-2 treatment adherence is developed. Threshold parameters are determined and stabilities analyzed. Sensitivity analysis of the reproduction number and the numerical simulations suggest that treatment adherence for both females and males are equally important in keeping the reproduction as low as possible. The basic model is then extended to incorporate time-dependent intervention strategies. The Pontryagin's Maximum Principle is used to characterize the optimal level of the controls, and the resulting optimality system is solved numerically.

Global dynamics of deterministic and stochastic epidemic systems with nonmonotone incidence rate

This paper is devoted to studying the dynamics of a susceptible-infective-latent-infective (SILI) epidemic model that is subject to the combined effects of environmental noise and intervention strategy. We extend the classical SILI epidemic model from a deterministic framework to a stochastic one. For the deterministic case, the global stability analysis of the solution is carried out in terms of the basic reproduction number. For the stochastic case, sufficient conditions for the extinction of diseases are obtained. Then, the existence of stationary distribution and asymptotic behavior of the solution are further studied to illustrate the cycling phenomena of recurrent diseases. Numerical simulations are conducted to verify these analytical results. It is shown that both stochastic noise and intervention strategy contribute to the control of diseases.

Global dynamics of an epidemiological model with age of infection and disease relapse

In this paper, an epidemiological model with age of infection and disease relapse is investigated. The basic reproduction number for the model is identified, and it is shown to be a sharp threshold to completely determine the global dynamics of the model. By analysing the corresponding characteristic equations, the local stability of a disease-free steady state and an endemic steady state of the model is established. By means of suitable Lyapunov functionals and LaSalle's invariance principle, it is verified that if the basic reproduction number is less than unity, the disease-free steady state is globally asymptotically stable, and hence the disease dies out; if the basic reproduction number is greater than unity, the endemic steady state is globally asymptotically stable and the disease becomes endemic.

Mathematical assessment of the role of Dengvaxia vaccine on the transmission dynamics of dengue serotypes

A new mathematical model is designed and used to assess the impact of the newly-released Dengvaxia vaccine on the transmission dynamics of two co-circulating dengue strains (where strain 1 consists of dengue serotypes 1, 3 and 4; and strain 2 consists of dengue serotype 2). It is shown that the model exhibits the phenomenon of backward bifurcation when the disease-induced mortality in the host population exceeds a certain threshold value or if the vaccine does not provide perfect protection against infection with the two strains. In the absence of backward bifurcation, the disease-free equilibrium of the model is shown to be globally-asymptotically stable whenever the associated reproduction number is less than unity. It is shown that the community-wide use of the vaccine could induce positive, negative or no population-level impact, depending on the sign of a certain epidemiological threshold quantity (known as the vaccine impact factor). Simulations of the model, using data from Oaxaca, Mexico, show that, although the community-wide use of the vaccine will significantly reduce dengue burden in the community, it is unable to lead to the elimination of the two dengue strains. It is further shown that the use of Dengvaxia vaccine in dengue-naive populations may induce increased risk of severe disease in these populations.

MATHEMATICAL STUDY OF IN-HOST DYNAMICS OF HERPES SIMPLEX VIRUS TYPE 2 TO ASSESS THE IMPACT OF IMMUNE RESPONSE

A new deterministic model for Herpes Simplex Virus-2 (HSV-2) in vivo, which incorporates the cell-mediated and humoral immune responses, is designed and analyzed. The analyses of the model reveal that it has a globally-asymptotically stable (GAS) virus-free equilibrium (VFE) whenever the associated reproduction threshold is less than unity. Also, it has at least one virus-present equilibrium (VPE) when the reproduction threshold exceeds unity (and virus will persist in vivo under this condition). Furthermore, it is shown that a Herpes Simplex Virus-2 (HSV-2) vaccine will be effective in reducing HSV-2 burden in vivo if it reduces the ability of the virus without glycoprotein C (gC) to bind to the host cell or if it reduces the re-activation rate of latent HSV-2. Additionally, the vaccine will also be very effective if it results in an increase in the fraction of the re-activated latent viruses without gC. Numerical simulations of the model show that cell-mediated immune response is more effective (in controlling HSV-2 burden in vivo) than humoral immune response (the latter only offers marginal impact in reducing HSV-2 burden in vivo, except if its effectiveness level is very high). Thus, a future HSV-2 vaccine that boosts cell-mediated immune response is expected to be quite effective in controlling HSV-2 in vivo.

A model for the coupled disease dynamics of HIV and HSV-2 with mixing among and between genders

Evidence indicates that those with genital herpes (HSV-2) infections have greater risks of infection by HIV; and, once co-infected, are more likely to transmit HIV. To better understand the interactions between HIV and HSV-2, we construct a mathematical model that describes the joint dynamics. A new feature of this model is the inclusion of both heterosexual and homosexual interactions. We derive and interpret the basic and invasion reproduction numbers for HIV and HSV-2 using the approach of next-generation matrices. We then perform scenario analyses and conduct a sensitivity analysis to investigate the impact of the model parameters on the reproduction numbers and disease prevalences. We conclude that homosexual transmission drastically changes the disease prevalences; hence, it is important to account for this interaction as models that ignore homosexuality may greatly underestimate the disease burden.

IMPACT OF AGE-DEPENDENT RELAPSE AND IMMUNITY ON MALARIA DYNAMICS

An age-structured mathematical model for malaria is presented. The model explicitly includes the human and mosquito populations, structured by chronological age of humans. The infected human population is divided into symptomatic infectious, asymptomatic infectious and asymptomatic chronic infected individuals. The original partial differential equation (PDE) model is reduced to an ordinary differential equation (ODE) model with multiple age groups coupled by aging. The basic reproduction number [Formula: see text] is derived for the PDE model and the age group model in the case of general n age groups. We assume that infectiousness of chronic infected individuals gets triggered by bites of even susceptible mosquitoes. Our analysis points out that this assumption contributes greatly to the [Formula: see text] expression and therefore needs to be further studied and understood. Numerical simulations for n = 2 age groups and a sensitivity/uncertainty analysis are presented. Results suggest that it is important not only to consider asymptomatic infectious individuals as a hidden cause for malaria transmission, but also asymptomatic chronic infections (>60%), which often get neglected due to undetectable parasite loads. These individuals represent an important reservoir for future human infectiousness. By considering age-dependent immunity types, the model helps generate insight into effective control measures, by targeting age groups in an optimal way.

Modelling diseases with relapse and nonlinear incidence of infection: a multi-group epidemic model

In this paper, we introduce a basic reproduction number for a multi-group SIR model with general relapse distribution and nonlinear incidence rate. We find that basic reproduction number plays the role of a key threshold in establishing the global dynamics of the model. By means of appropriate Lyapunov functionals, a subtle grouping technique in estimating the derivatives of Lyapunov functionals guided by graph-theoretical approach and LaSalle invariance principle, it is proven that if it is less than or equal to one, the disease-free equilibrium is globally stable and the disease dies out; whereas if it is larger than one, some sufficient condition is obtained in ensuring that there is a unique endemic equilibrium which is globally stable and thus the disease persists in the population. Furthermore, our results suggest that general relapse distribution are not the reason of sustained oscillations. Biologically, our model might be realistic for sexually transmitted diseases, such as Herpes, Condyloma acuminatum, etc.

Mathematical models of viral latency

While viral latency remains one of the biggest challenges for successful antiviral therapy, it has also inspired mathematical modelers to develop dynamical system approaches with the aim of predicting the impact of drug efficacy on disease progression and the persistence of latent viral reservoirs. In this review we present several differential equation models and assess their relative success in giving advice to the working clinician and their predictive power for inferring long term viral eradication from short term abatement. Many models predict that there is a considerable likelihood of viral rebound due to continuous reseeding of latent reservoirs. Most mathematical models of HIV latency suffer from being reductionist by ignoring the growing variety of different cell types harboring latent virus, the considerable intercellular delay involved in reactivation, and host-related epigenetic modifications which may alter considerably the dynamical system of immune cell populations.

Modeling the synergy between HSV-2 and HIV and potential impact of HSV-2 therapy

Mounting evidence indicates that genital HSV-2 infection may increase susceptibility to HIV infection and that co-infection may increase infectiousness. Accordingly, antiviral treatment of people with HSV-2 may mitigate the incidence of HIV in populations where both pathogens occur. To better understand the epidemiological synergy between HIV and HSV-2, we formulate a deterministic compartmental model that describes the transmission dynamics of these pathogens. Unlike earlier models, ours incorporates gender and heterogeneous mixing between activity groups. We derive explicit expressions for the reproduction numbers of HSV-2 and HIV, as well as the invasion reproduction numbers via next generation matrices. A qualitative analysis of the system includes the local and global behavior of the model. Simulations reinforce these analytical results and demonstrate epidemiological synergy between HSV-2 and HIV. In particular, numerical results show that HSV-2 favors the invasion of HIV, may dramatically increase the peak as well as reducing the time-to-peak of HIV prevalence, and almost certainly has exacerbated HIV epidemics. The potential population-level impact of HSV-2 on HIV is demonstrated by calculating the fraction of HIV infections attributable to HSV-2 and the difference between HIV prevalence in the presence and absence of HSV-2. The potential impact of treating people with HSV-2 on HIV control is demonstrated by comparing HIV prevalence with and without HSV-2 therapy. Most importantly, we illustrate that the aforementioned aspects of the population dynamics can be significantly influenced by the sexual structure of the population.

Modeling the Impact of Behavior Changes on the Spread of Pandemic Influenza

We use mathematical models to assess the impact of behavioral changes in response to an emerging epidemic. Evaluating the quantitative and qualitative impact of public health interventions on the spread of infectious diseases is a crucial public health objective. The recent avian influenza (H5N1) outbreaks and the 2009 H1N1 pandemic have raised significant global concerns about the emergence of a deadly influenza virus causing a pandemic of catastrophic proportions. Mitigation strategies based on behavior changes are some of the only options available in the early stages of an emerging epidemic when vaccines are unlikely to be available and there are only limited stockpiles of antiviral medications. Mathematical models that capture these behavior changes can quantify the relative impact of different mitigation strategies, such as closing schools, in slowing the spread of an infectious disease. Including behavior changes in mathematical models increases complexity and is often left out of the analysis. We present a simple differential equation model which allows for people changing their behavior to decrease their probability of infection. We also describe a large-scale agent-based model that can be used to analyze the impact of isolation scenarios such as school closures and fear-based home isolation during a pandemic. The agent-based model captures realistic individual-level mixing patterns and coordinated reactive changes in human behavior in order to better predict the transmission dynamics of an epidemic. Both models confirm that changes in behavior can be effective in reducing the spread of disease. For example, our model predicts that if school closures are implemented for the duration of the pandemic, the clinical attack rate could be reduced by more than 50%. We also verify that when interventions are stopped too soon, a second wave of infection can occur.

When does overuse of antibiotics become a tragedy of the commons?

BACKGROUND

Over-prescribing of antibiotics is considered to result in increased morbidity and mortality from drug-resistant organisms. A resulting common wisdom is that it would be better for society if physicians would restrain their prescription of antibiotics. In this view, self-interest and societal interest are at odds, making antibiotic use a classic "tragedy of the commons".

METHODS AND FINDINGS

We developed two mathematical models of transmission of antibiotic resistance, featuring de novo development of resistance and transmission of resistant organisms. We analyzed the decision to prescribe antibiotics as a mathematical game, by analyzing individual incentives and community outcomes.

CONCLUSIONS

A conflict of interest may indeed result, though not in all cases. Increased use of antibiotics by individuals benefits society under certain circumstances, despite the amplification of drug-resistant strains or organisms. In situations where increased use of antibiotics leads to less favorable outcomes for society, antibiotics may be harmful for the individual as well. For other scenarios, where a conflict between self-interest and society exists, restricting antibody use would benefit society. Thus, a case-by-case assessment of appropriate use of antibiotics may be warranted.

Epidemiological impact of a genital herpes type 2 vaccine for young females

Genital Herpes, which is caused by Herpes Simplex Virus-1 or -2 (HSV-1, -2, predominantly HSV-2) is a sexually transmitted infection (STI) that causes a chronic latent infection with outbreak episodes linked to transmission. Antiviral therapies are effective in reducing viral shedding during these episodes, but are ineffective as a whole since many outbreaks are asymptomatic or have mild symptoms. Thus, the development of a vaccine for genital herpes is needed to control this disease. The question of how to implement such a vaccine program is an important one, and may be similar to the vaccination program for Human Papilloma Virus (HPV) for young females. We have developed a mathematical model to describe the epidemiology of vaccination targeting young females against HSV-2. The model population is delineated with respect to age group, sexual activity and infection status including oral infection of HSV-1, which may affect vaccine efficacy. A threshold parameter R(C), which determines the level of vaccine uptake needed to eradicate HSV-2, is found. Computer simulation shows that an adolescent-only vaccination program may be effective in eliminating HSV-2 disease, however, the success of extinction greatly depends on the level of vaccine uptake, the vaccine efficacy, the age of sexual maturity and safe sex practices. However, the time course of eradication would take many years. We also investigate the prevalence of infection in the total population and in women between 16-30 years of age before and after vaccination has been introduced, and show that the adolescent-only vaccination program can be effective in reducing disease prevalence in these populations depending on the level of vaccine uptake and vaccine efficacy. This will also result in a decrease of maternal-fetal transmission of HSV-2 infection. Another important, if commonsense, conclusion is that vaccination of some females reduces infection in men, which then reduces infection in women.

GLOBAL DYNAMICS OF A MULTI-GROUP EPIDEMIC MODEL WITH GENERAL RELAPSE DISTRIBUTION AND NONLINEAR INCIDENCE RATE

In this paper, we investigate a class of multi-group epidemic models allowing heterogeneity of the host population and that has taken into consideration with general relapse distribution and nonlinear incidence rate. We establish that the global dynamics are completely determined by the basic reproduction number R0. The proofs of the main results utilize the persistence theory in dynamical systems, Lyapunov functionals and a subtle grouping technique in estimating the derivatives of Lyapunov functionals guided by graph-theoretical approach. Biologically, the disease (with any initial inoculation) will persist in all groups of the population and will eventually settle at a constant level in each group. Furthermore, our results demonstrate that heterogeneity and nonlinear incidence rate do not alter the dynamical behaviors of the basic SIR model. On the other hand, the global dynamics exclude the existence of Hopf bifurcation leading to sustained oscillatory solutions.

Projecting the benefits of antiretroviral therapy for HIV prevention: the impact of population mobility and linkage to care

BACKGROUND

Recent mathematical models suggested that frequent human immunodeficiency virus (HIV) testing with immediate initiation of antiretroviral therapy (ART) to individuals with a positive test result could profoundly curb transmission. The debate about ART as prevention has focused largely on parameter values. We aimed to evaluate structural assumptions regarding linkage to care and population mobility, which have received less attention.

METHODS

We modified the linkage structure of published models of ART as prevention, such that individuals who decline initial testing or treatment do not link to care until late-stage HIV infection. We then added population mobility to the models. We populated the models with demographic, clinical, immigration, emigration, and linkage data from a South African township.

RESULTS

In the refined linkage model, elimination of HIV transmission (defined as an incidence of <0.1%) did not occur by 30 years, even with optimistic assumptions about the linkage rate. Across a wide range of estimates, models were more sensitive to structural assumptions about linkage than to parameter values. Incorporating population mobility further attenuated the reduction in incidence conferred by ART as prevention.

CONCLUSIONS

Linkage to care and population mobility are critical features of ART-as-prevention models. Clinical trials should incorporate relevant data on linkage to care and migration to evaluate the impact of this strategy.

A perspective on antiviral resistance

More than 25 years after the licensure of aciclovir and then penciclovir, followed by their respective prodrugs valaciclovir and famciclovir, cases of clinically relevant resistance to these drugs in immunocompetent individuals remain very rare. The aim of this review is to focus on the mechanism of action of these anti HSV drugs and then briefly compare this favourable outcome with that of CMV, HIV, HBV and influenza. A central theme is that resistance is an epiphenomenon of failure to suppress virus replication, so that improved potency and selectivity should be prioritised when developing new drugs rather than activity against resistant strains per se.

Dynamic modeling of herpes simplex virus type-2 (HSV-2) transmission: issues in structural uncertainty

The sexually transmitted infection (STI) Herpes simplex virus type-2 (HSV-2) is of public health concern because it is a very common frequently unrecognized lifelong infection, which may facilitate HIV transmission. Within HIV/STI modeling, structural uncertainty has received less attention than parametric uncertainty. By merging the compartments of a "complex" model, a "simple" HSV-2 model is developed. Sexual interactions between female sex workers (FSWs) and clients are modeled using data from India. Latin Hypercube Sampling selects from parameter distributions and both models are run for each of the 10,000 parameter sets generated. Outputs are compared (except for 2,450 unrealistic simulations). The simple model is a good approximation to the complex model once the HSV-2 epidemic has reached 60% of the equilibrium prevalence (95% of the 7,550 runs produced <10% relative error). The simple model is a reduced version of the complex model that retains details implicitly. For late-stage epidemics, the simple model gives similar prevalence trends to the complex model. As HSV-2 epidemics in many populations are advanced, the simple model is accurate in most instances, although the complex model may be preferable for early epidemics. The analysis highlights the issue of structural uncertainty and the value of reducing complexity.

Antiviral prophylaxis during pandemic influenza may increase drug resistance

BACKGROUND

Neuraminidase inhibitors (NI) and social distancing play a major role in plans to mitigate future influenza pandemics.

METHODS

Using the freely available program InfluSim, the authors examine to what extent NI-treatment and prophylaxis promote the occurrence and transmission of a NI resistant strain.

RESULTS

Under a basic reproduction number of R0 = 2.5, a NI resistant strain can only spread if its transmissibility (fitness) is at least 40% of the fitness of the drug-sensitive strain. Although NI drug resistance may emerge in treated patients in such a late state of their disease that passing on the newly developed resistant viruses is unlikely, resistant strains quickly become highly prevalent in the population if their fitness is high. Antiviral prophylaxis further increases the pressure on the drug-sensitive strain and favors the spread of resistant infections. The authors show scenarios where pre-exposure antiviral prophylaxis even increases the number of influenza cases and deaths.

CONCLUSION

If the fitness of a NI resistant pandemic strain is high, any use of prophylaxis may increase the number of hospitalizations and deaths in the population. The use of neuraminidase inhibitors should be restricted to the treatment of cases whereas prophylaxis should be reduced to an absolute minimum in that case.

Stochastic model of an influenza epidemic with drug resistance

A continuous-time Markov chain (CTMC) model is formulated for an influenza epidemic with drug resistance. This stochastic model is based on an influenza epidemic model, expressed in terms of a system of ordinary differential equations (ODE), developed by Stilianakis, N.I., Perelson, A.S., Hayden, F.G., [1998. Emergence of drug resistance during an influenza epidemic: insights from a mathematical model. J. Inf. Dis. 177, 863-873]. Three different treatments-chemoprophylaxis, treatment after exposure but before symptoms, and treatment after symptoms appear, are considered. The basic reproduction number, R(0), is calculated for the deterministic-model under different treatment strategies. It is shown that chemoprophylaxis always reduces the basic reproduction number. In addition, numerical simulations illustrate that the basic reproduction number is generally reduced with realistic treatment rates. Comparisons are made among the different models and the different treatment strategies with respect to the number of infected individuals during an outbreak. The final size distribution is computed for the CTMC model and, in some cases, it is shown to have a bimodal distribution corresponding to two situations: when there is no outbreak and when an outbreak occurs. Given an outbreak occurs, the total number of cases for the CTMC model is in good agreement with the ODE model. The greatest number of drug resistant cases occurs if treatment is delayed or if only symptomatic individuals are treated.

Modeling relapse in infectious diseases

An integro-differential equation is proposed to model a general relapse phenomenon in infectious diseases including herpes. The basic reproduction number R(0) for the model is identified and the threshold property of R(0) established. For the case of a constant relapse period (giving a delay differential equation), this is achieved by conducting a linear stability analysis of the model, and employing the Lyapunov-Razumikhin technique and monotone dynamical systems theory for global results. Numerical simulations, with parameters relevant for herpes, are presented to complement the theoretical results, and no evidence of sustained oscillatory solutions is found.

Suppressive Valacyclovir Therapy: Impact on the Population Spread of HSV-2 Infection

OBJECTIVES

Recent trial results demonstrate that the transmission probability of HSV-2 in monogamous couples is nearly halved by the use of valacyclovir as suppressive therapy.

GOAL

The goal of this study is to understand the potential impact of suppressive valacyclovir therapy on the transmission of HSV-2 within a population.

STUDY DESIGN

A mathematical model of HSV-2 epidemiology was developed which included suppressive therapy with the efficacy observed in the clinical trial. The model represented HSV-2 spread in an age and sexual activity stratified population where rates of viral shedding declined based on time since infection. The model tested the impact of a range of suppression coverage levels.

RESULTS

Suppressive therapy reduces the population incidence of HSV-2. With coverage rates of 3.2%, the incidence of HSV-2 would be reduced by between 1.8% and 2.8%. Higher coverage rates were estimated to reduce the incidence of new cases up to 13%. Starting suppression closer to the time of infection also reduces the incidence of new cases.

CONCLUSION

The impact of suppressive therapy on the HSV-2 epidemic is modest at current coverage levels but could be substantially increased with higher rates of diagnosis and a focus on coverage soon after infection.

A dynamic model for induced reactivation of latent virus

We develop a deterministic mathematical model to describe reactivation of latent virus by chemical inducers. This model is applied to the reactivation of latent KSHV in BCBL-1 cell cultures with butyrate as the inducing agent. Parameters for the model are first estimated from known properties of the exponentially growing, uninduced cell cultures. Additional parameters that are necessary to describe induction are determined from fits to experimental data from the literature. Our initial model provides good agreement with two independent sets of experimental data, but also points to the need for a new class of experiments which are required for further understanding of the underlying mechanisms.

The role of compensatory mutations in the emergence of drug resistance

Pathogens that evolve resistance to drugs usually have reduced fitness. However, mutations that largely compensate for this reduction in fitness often arise. We investigate how these compensatory mutations affect population-wide resistance emergence as a function of drug treatment. Using a model of gonorrhea transmission dynamics, we obtain generally applicable, qualitative results that show how compensatory mutations lead to more likely and faster resistance emergence. We further show that resistance emergence depends on the level of drug use in a strongly nonlinear fashion. We also discuss what data need to be obtained to allow future quantitative predictions of resistance emergence.

Perspectives on the basic reproductive ratio

The basic reproductive ratio, R0, is defined as the expected number of secondary infections arising from a single individual during his or her entire infectious period, in a population of susceptibles. This concept is fundamental to the study of epidemiology and within-host pathogen dynamics. Most importantly, R0 often serves as a threshold parameter that predicts whether an infection will spread. Related parameters which share this threshold behaviour, however, may or may not give the true value of R0. In this paper we give a brief overview of common methods of formulating R0 and surrogate threshold parameters from deterministic, non-structured models. We also review common means of estimating R0 from epidemiological data. Finally, we survey the recent use of R0 in assessing emerging diseases, such as severe acute respiratory syndrome and avian influenza, a number of recent livestock diseases, and vector-borne diseases malaria, dengue and West Nile virus.

Modelling entomological-climatic interactions of Plasmodium falciparum malaria transmission in two Colombian endemic-regions: contributions to a National Malaria Early Warning System

BACKGROUND

Malaria has recently re-emerged as a public health burden in Colombia. Although the problem seems to be climate-driven, there remain significant gaps of knowledge in the understanding of the complexity of malaria transmission, which have motivated attempts to develop a comprehensive model.

METHODS

The mathematical tool was applied to represent Plasmodium falciparum malaria transmission in two endemic-areas. Entomological exogenous variables were estimated through field campaigns and laboratory experiments. Availability of breeding places was included towards representing fluctuations in vector densities. Diverse scenarios, sensitivity analyses and instabilities cases were considered during experimentation-validation process.

RESULTS

Correlation coefficients and mean square errors between observed and modelled incidences reached 0.897-0.668 (P > 0.95) and 0.0002-0.0005, respectively. Temperature became the most relevant climatic parameter driving the final incidence. Accordingly, malaria outbreaks are possible during the favourable epochs following the onset of El Niño warm events. Sporogonic and gonotrophic cycles showed to be the entomological key-variables controlling the transmission potential of mosquitoes' population. Simulation results also showed that seasonality of vector density becomes an important factor towards understanding disease transmission.

CONCLUSION

The model constitutes a promising tool to deepen the understanding of the multiple interactions related to malaria transmission conducive to outbreaks. In the foreseeable future it could be implemented as a tool to diagnose possible dynamical patterns of malaria incidence under several scenarios, as well as a decision-making tool for the early detection and control of outbreaks. The model will be also able to be merged with forecasts of El Niño events to provide a National Malaria Early Warning System.

Evaluating the Performance of Chemical Control in the Presence of Resistant Pathogens

Resistance to chemical control is a major impediment to combating many socially and economically important diseases. Theoretical and experimental studies have shown that reducing the intensity of treatment can slow, or even prevent, the invasion of resistance, yet reducing treatment levels also results in a net increase in disease severity. Clearly there is a need to identify control strategies that balance the conflicting aims of resistance management and disease suppression. Using a mathematical model for the dynamics of fungicide resistance in crop pathogens, we present a broadly applicable measure of the performance of chemical control in the presence of resistant pathogen strains. We illustrate how to optimise fungicide performance with respect to the intensity of treatment as a function of the duration of treatment and the fitness of the resistant strain. We find that in the short term, fungicide performance is optimised at high levels of treatment despite rapid selection for resistance, while the long-term optimum performance is achieved when treatment renders the fungicide-sensitive and fungicide-resistant pathogens equally fit. We further present evidence that under prescribed conditions, the ratio of dose size and frequency, and the fungicide mode of action, can have a significant effect on fungicide performance.

Pathogens and parasites: strategies and challenges

The threat of emerging infections grows with the swelling tide of the human population and the continued disregard for the health of the environment. One of our most urgent challenges in public health is to understand the evolution and natural history of pathogens and parasites and how a sudden shift in virulence or in targeted host population may occur without warning. Viruses call for especially close watching. They are mostly genes and have mastered the art of manipulating other genes. Some are planktonic in the world's oceans, numbering 10 billion per liter of seawater; some are planktonic in our blood; some lie low inside cells; some take over a cell's replication machinery and explode the cell with new copies of themselves; and some splice their genes seamlessly into our chromosomes. The twin themes of genetic diversity and natural selection are explored in this review, with their relevance to viruses, the vertebrate immune system, virulence, and communicable disease epidemiology.

Potential effect of HIV type 1 antiretroviral and herpes simplex virus type 2 antiviral therapy on transmission and acquisition of HIV type 1 infection

Biological strategies for interrupting transmission of human immunodeficiency virus (HIV) type 1 should be directed at reducing infectiousness of and susceptibility to HIV-1. Potential antiretroviral interventions include reducing the likelihood of transmission of HIV-1 by reducing HIV-1 load in the blood and genital tract of HIV-1--infected person, prophylaxis after high-risk exposure, and pre-exposure prophylaxis for very high risk populations. Antiviral treatment of herpes simplex virus (HSV) type 2, the most common cause of genital ulcers, should be evaluated as a strategy for HIV-1 infection prevention by reducing infectiousness of and susceptibility to HIV-1, on the basis of biological and epidemiological data indicating that HSV-2 facilitates transmission and acquisition of HIV-1. The rationale for antiretroviral and HSV-2-specific interventions and studies to test these strategies are described.

The antiretroviral rollout and drug-resistant HIV in Africa: insights from empirical data and theoretical models

The U.S. Government has pledged to spend $15 billion in Africa and the Caribbean on AIDS. A central focus of this plan is to provide antiretroviral treatment (ART) to millions. Here, we evaluate whether the plan to rollout ART in Africa is likely to generate an epidemic of drug-resistant strains of HIV. We review what has occurred as a result of high usage of ART in developed countries in terms of changes in risky behavior, and the emergence and transmission of drug-resistant HIV. We also review how mathematical models have been used to predict the evolution of drug-resistant HIV epidemics. We then show how models can be used to predict the likely impact of the ART rollout on the evolution of drug-resistant HIV in Africa. At currently planned levels of treatment coverage, we predict that (over the next decade) in Africa: (i) the impact of ART on reducing HIV transmission (and prevalence) is likely to be undetectable (unless accompanied by substantial changes in behavior), (ii) the transmission rate of drug-resistant HIV will be below the WHO surveillance threshold of 5%, and (ii) the majority of cases of drug-resistant HIV that will occur will be due to acquired (and not transmitted) resistance. For the next decade, large-scale surveillance for detecting transmitted resistance in Africa is unnecessary. Instead, we recommend that patients should be closely monitored for acquired resistance, and sentinel surveillance (in a few urban centers) should be used to monitor transmitted resistance.

Calculating the Contribution of Herpes Simplex Virus Type 2 Epidemics to Increasing HIV Incidence: Treatment Implications

Herpes simplex virus type 2 (HSV-2) is the most prevalent sexually transmitted pathogen worldwide. There is considerable biological and epidemiological evidence that HSV-2 infection increases the risk of acquiring HIV infection and may also increase the risk of transmitting HIV. Here, we use a mathematical model to predict the effect of a high-prevalence HSV-2 epidemic on HIV incidence. Our results show that HSV-2 epidemics can more than double the peak HIV incidence; that the biological heterogeneity in susceptibility and transmission induced by an HSV-2 epidemic causes HIV incidence to rise, fall, and then rise again; and that HSV-2 epidemics concentrate HIV epidemics, creating a "core group" of HIV transmitters. Our modeling results imply that findings from HSV-2 intervention trials aimed at reduction of HIV incidence will be variable and that positive findings will be obtained only from trials in communities in which HIV incidence is steeply rising.

Modeling the emergence of the 'hot zones': tuberculosis and the amplification dynamics of drug resistance

'Hot zones' are areas that have >5% prevalence (or incidence) of multidrug-resistant tuberculosis (MDRTB). We present a new mathematical model (the amplifier model) that tracks the emergence and evolution of multiple (pre-MDR, MDR and post-MDR) strains of drug-resistant Mycobacterium tuberculosis. We reconstruct possible evolutionary trajectories that generated hot zones over the past three decades, and identify the key causal factors. Results are consistent with recently reported World Health Organization (WHO) data. Our analyses yield three important insights. First, paradoxically we found that areas with programs that successfully reduced wild-type pansensitive strains often evolved into hot zones. Second, some hot zones emerged even when MDR strains were substantially less fit (and thus less transmissible) than wild-type pansensitive strains. Third, levels of MDR are driven by case-finding rates, cure rates and amplification probabilities. To effectively control MDRTB in the hot zones, it is essential that the WHO specify a goal for minimizing the amplification probability.

Frequency-dependent incidence in models of sexually transmitted diseases: portrayal of pair-based transmission and effects of illness on contact behaviour

We explore the transmission process for sexually transmitted diseases (STDs). We derive the classical frequency-dependent incidence mechanistically from a pair-formation model, using an approximation that applies to populations with rapid pairing dynamics (such as core groups or non-pair-bonding animals). This mechanistic derivation provides a framework to assess how accurately frequency-dependent incidence portrays the pair-based transmission known to underlie STD dynamics. This accuracy depends strongly on the disease being studied: frequency-dependent formulations are more suitable for chronic less-transmissible infections than for transient highly transmissible infections. Our results thus support earlier proposals to divide STDs into these two functional classes, and we suggest guidelines to help assess under what conditions each class can be appropriately modelled using frequency-dependent incidence. We then extend the derivation to include situations where infected individuals exhibit altered pairing behaviour. For four cases of increasing behavioural complexity, analytic expressions are presented for the generalized frequency-dependent incidence rate, basic reproductive number (R0) and steady-state prevalence (i infinity) of an epidemic. The expression for R0 is identical for all cases, giving refined insights into determinants of invasibility of STDs. Potentially significant effects of infection-induced changes in contact behaviour are illustrated by simulating epidemics of bacterial and viral STDs. We discuss the application of our results to STDs (in humans and animals) and other infectious diseases.

[An epidemiological model of genital herpes for assessment of potential impact of therapy and prophylaxis: application to France]

OBJECTIVES

This study was performed to quantify the development of the number of cases of genital herpes and to assess the impact of different treatment strategies in France.

METHOD

A model for the natural history of herpes simplex virus genital infection is presented and applied to the French population. The model encompasses infection by herpes simplex virus type 1 (HSV-1) or type 2 (HSV-2), first episodes, recurrences, viral shedding and the effect of treatment on infected individuals.

RESULTS

In the Year 2000, 270,000 individuals would have suffered from genital herpes in France. A total annual cumulated number of 1.5 million episodes of recurrence and 23 million days of viral shedding were estimated. Seventy percent of viral shedding occurred in sub-clinical seropositive individuals. The expected number of attributable neonatal deaths remained very low. Systematic treatment of clinical episodes might reduce the number of days with lesions (- 27 p. 100), and is also effective on viral shedding (symptomatic: - 50 p. 100). Continuous treatment of the most severe patients (>or=6 recurrences per Year) might reduce viral shedding slightly more (- 85 p. 100). Antiviral treatment might have a major impact on the quality of life of these patients but would only slightly curb the number of new infections.

DISCUSSION

This model tries to integrate the various data currently available at international level on the epidemiology of genital herpes. However, many aspects are still not well documented and remain uncertain. It is therefore necessary to define various assumptions in order to simulate the natural history of the disease in a population. The lack of French data especially on the HSV-1 and HSV-2 seroprevalence profiles reinforces these uncertainties. Our results should hence be considered as exploratory. However, this modeling approach is the only possible way to integrate the multiple parameters describing the pathology and predicting of the public health impact of different interventions. This model is an open tool which may be modified when new data become available.

Predicting the emergence of drug-resistant HSV-2: new predictions

BACKGROUND

Mathematical models can be used to predict the emergence and transmission of antiviral resistance. Previously it has been predicted that high usage of antivirals (in immunocompetent populations) to treat Herpes Simplex Virus type 2 (HSV-2) would only lead to fairly low levels of antiviral resistance. The HSV-2 predictions were based upon the assumption that drug-resistant strains of HSV-2 would be less infectious than drug-sensitive strains but that the drug-resistant strains would not be impaired in their ability to reactivate. Recent data suggest that some drug-resistant strains of HSV-2 are likely to be impaired in their ability to reactivate.

OBJECTIVES

(1) To predict the effect of a high usage of antivirals on the prevalence of drug-resistant HSV-2 under the assumption that drug-resistant strains will be less infectious than drug-sensitive strains of HSV-2 and also have an impaired ability to reactivate. (2) To compare predictions with previous published predictions.

METHODS

We generated theoretical drug-resistant HSV-2 strains that were attenuated (in comparison with drug-sensitive strains) in both infectivity and ability to reactivate. We then used a transmission model to predict the emergence and transmission of drug-resistant HSV-2 in the immunocompetent population assuming a high usage of antivirals.

RESULTS

Our predictions are an order of magnitude lower than previous predictions; we predict that even after 25 years of high antiviral usage only 5 out of 10,000 immunocompetent individuals will be shedding drug-resistant virus. Furthermore, after 25 years, 52 cases of HSV-2 would have been prevented for each prevalent case of drug-resistant HSV-2.

CONCLUSIONS

The predicted levels of drug-resistant HSV-2 for the immunocompetent population are so low that it seems unlikely that cases of drug-resistant HSV-2 will be detected.

Herpes simplex virus resistance to acyclovir and penciclovir after two decades of antiviral therapy

Acyclovir, penciclovir, and their prodrugs have been widely used during the past two decades for the treatment of herpesvirus infections. In spite of the distribution of over 2.3 x 10(6) kg of these nucleoside analogues, the prevalence of acyclovir resistance in herpes simplex virus isolates from immunocompetent hosts has remained stable at approximately 0.3%. In immuncompromised patients, in whom the risk for developing resistance is much greater, the prevalence of resistant virus has also remained stable but at a higher level, typically 4 to 7%. These observations are examined in the light of characteristics of the virus, the drugs, and host factors.

Could widespread use of combination antiretroviral therapy eradicate HIV epidemics?

Current combination antiretroviral therapies (ARV) are widely used to treat HIV. However drug-resistant strains of HIV have quickly evolved, and the level of risky behaviour has increased in certain communities. Hence, currently the overall impact that ARV will have on HIV epidemics remains unclear. We have used a mathematical model to predict whether the current therapies: are reducing the severity of HIV epidemics, and could even lead to eradication of a high-prevalence (30%) epidemic. We quantified the epidemic-level impact of ARV on reducing epidemic severity by deriving the basic reproduction number (R(0)(ARV)). R(0)(ARV) specifies the average number of new infections that one HIV case generates during his lifetime when ARV is available and ARV-resistant strains can evolve and be transmitted; if R(0)(ARV) is less than one epidemic eradication is possible. We estimated for the HIV epidemic in the San Francisco gay community (using uncertainty analysis), the present day value of R(0)(ARV), and the probability of epidemic eradication. We assumed a high usage of ARV and three behavioural assumptions: that risky sex would (1) decrease, (2) remain stable, or (3) increase. Our estimated values of R(0)(ARV) (median and interquartile range [IQR]) were: 0.90 (0.85-0.96) if risky sex decreases, 1.0 (0.94-1.05) if risky sex remains stable, and 1.16 (1.05-1.28) if risky sex increases. R(0)(ARV) decreased as the fraction of cases receiving treatment increased. The probability of epidemic eradication is high (p=0.85) if risky sex decreases, moderate (p=0.5) if levels of risky sex remain stable, and low (p=0.13) if risky sex increases. We conclude that ARV can function as an effective HIV-prevention tool, even with high levels of drug resistance and risky sex. Furthermore, even a high-prevalence HIV epidemic could be eradicated using current ARV.

Antimicrobial use and antimicrobial resistance: a population perspective

The need to stem the growing problem of antimicrobial resistance has prompted multiple, sometimes conflicting, calls for changes in the use of antimicrobial agents. One source of disagreement concerns the major mechanisms by which antibiotics select resistant strains. For infections like tuberculosis, in which resistance can emerge in treated hosts through mutation, prevention of antimicrobial resistance in individual hosts is a primary method of preventing the spread of resistant organisms in the community. By contrast, for many other important resistant pathogens, such as penicillin-resistant Streptococcus pneumoniae, methicillin-resistant Staphylococcus aureus, and vancomycin-resistant Enterococcus faecium resistance is mediated by the acquisition of genes or gene fragments by horizontal transfer; resistance in the treated host is a relatively rare event. For these organisms, indirect, population-level mechanisms of selection account for the increase in the prevalence of resistance. These mechanisms can operate even when treatment has a modest, or even negative, effect on an individual host's colonization with resistant organisms.

Can behavior change explain increases in the proportion of genital ulcers attributable to herpes in sub-Saharan Africa? A simulation modeling study

BACKGROUND

The proportion of cases of genital ulcer disease attributable to herpes simplex virus type 2 (HSV-2) appears to be increasing in sub-Saharan Africa.

GOAL

To assess the contributions of HIV disease and behavioral response to the HIV epidemic to the increasing proportion of genital ulcer disease (GUD) attributable to HSV-2 in sub-Saharan Africa.

STUDY DESIGN

Simulations of the transmission dynamics of ulcerative sexually transmitted diseases (STDs) and HIV with use of the model STDSIM.

RESULTS

In simulations, 28% of GUD was caused by HSV-2 before a severe HIV epidemic. If HIV disease was assumed to double the duration and frequency of HSV-2 recurrences, this proportion rose to 35% by year 2000. If stronger effects of HIV were assumed, this proportion rose further, but because of increased HSV-2 transmission this would shift the peak in HSV-2 seroprevalence to an unrealistically young age. A simulated 25% reduction in partner-change rates increased the proportion of GUD caused by HSV-2 to 56%, following relatively large decreases in chancroid and syphilis.

CONCLUSION

Behavioral change may make an important contribution to relative increases in genital herpes.

Dynamics of bacterial phenotype selection in a colonized host

The population dynamics of Helicobacter pylori during colonization in an infected animal host provide a quantifiable experimental model of in vivo microbial phenotype evolution. Phenotype variability in H. pylori populations can be typed as polymorphic expression of Lewis antigens on their cell surfaces. The high mutational frequency of H. pylori for Lewis expression provides substrate for differential selection by the host. Experimental challenge and successful colonization of mice and gerbils allows tracking of H. pylori phenotype variability from the initial inoculation to the ultimate establishment of a quasispecies. Colonization data provide a quantitative experimental model of phenotype evolution in a relatively large population (>10(4) individuals) over a relatively long evolutionary time scale (>10(3) generations). A mathematical model is developed to interpret the data in terms of the dynamic processes occurring during colonization. The mathematical model distinguishes the roles of selection and mutation; quantifies the effects of initial phenotype diversity, mutational frequency, and selective advantage; and applies generally to phenotype evolution in biological populations.

Resistance in viruses other than HIV

The aim of this brief paper is to summarize the ways in which viruses develop resistance to currently licensed anti-viral drugs and to comment on their clinical relevance. Specific examples will be chosen to emphasize basic principles of the development of resistance and readers are referred elsewhere to a summary of resistance in HIV that follows the same principles.

The coreceptor mutation CCR5Delta32 influences the dynamics of HIV epidemics and is selected for by HIV

We explore the impact of a host genetic factor on heterosexual HIV epidemics by using a deterministic mathematical model. A protective allele unequally distributed across populations is exemplified in our models by the 32-bp deletion in the host-cell chemokine receptor CCR5, CCR5Delta32. Individuals homozygous for CCR5Delta32 are protected against HIV infection whereas those heterozygous for CCR5Delta32 have lower pre-AIDS viral loads and delayed progression to AIDS. CCR5Delta32 may limit HIV spread by decreasing the probability of both risk of infection and infectiousness. In this work, we characterize epidemic HIV within three dynamic subpopulations: CCR5/CCR5 (homozygous, wild type), CCR5/CCR5Delta32 (heterozygous), and CCR5Delta32/CCR5Delta32 (homozygous, mutant). Our results indicate that prevalence of HIV/AIDS is greater in populations lacking the CCR5Delta32 alleles (homozygous wild types only) as compared with populations that include people heterozygous or homozygous for CCR5Delta32. Also, we show that HIV can provide selective pressure for CCR5Delta32, increasing the frequency of this allele.

Live attenuated HIV vaccines: predicting the tradeoff between efficacy and safety

The utility of live attenuated vaccines for controlling HIV epidemics is being debated. Live attenuated HIV vaccines (LAHVs) could be extremely effective in protecting against infection with wild-type strains, but may not be completely safe as the attenuated strain could cause AIDS in some vaccinated individuals. We present a theoretical framework for evaluating the consequences of the tradeoff between vaccine efficacy (in terms of preventing new infections with wild-type strains) and safety (in terms of vaccine-induced AIDS deaths). We use our framework to predict, for Zimbabwe and Thailand, the epidemiological impact of 1,000 different (specified by efficacy and safety characteristics) LAHVs. We predict that paradoxically: (i) in Zimbabwe (where transmission is high) LAHVs would significantly decrease the AIDS death rate, but (ii) in Thailand (where transmission is low) exactly the same vaccines (in terms of efficacy and safety characteristics) would increase the AIDS death rate. Our results imply that a threshold transmission rate exists that determines whether any given LAHV has a beneficial or a detrimental impact. We also determine the vaccine perversity point, which is defined in terms of the fraction of vaccinated individuals who progress to AIDS as a result of the vaccine strain. Vaccination with any LAHV that causes more than 5% of vaccinated individuals to progress to AIDS in 25 years would, even 50 years later, lead to perversity (i.e., increase the annual AIDS death rate) in Thailand; these same vaccines would lead to decreases in the annual AIDS death rate in Zimbabwe.

Use of a glycoprotein G-based type-specific assay to detect antibodies to herpes simplex virus type 2 among persons attending sexually transmitted disease clinics

BACKGROUND

Most genital herpes simplex virus type 2 (HSV-2) infections are unrecognized, thus, strategies to reduce the sexual transmission of HSV-2 are partly dependent on serologic screening.

GOAL

To define performance characteristics of the Gull/ Meridian glycoprotein G-based HSV-2 enzyme-linked immunosorbent assay among sexually transmitted disease clinic attendees and correlates of test acceptance.

STUDY DESIGN

The cross-sectional study was conducted during two periods. Serologic testing was offered at a US $15 charge during the first period and at no charge during the second period. Sera were tested by a type-specific glycoprotein G enzyme-linked immunosorbent assay and Western blot analysis, with the latter test used as the reference standard.

RESULTS

Acceptance of HSV-2 testing was associated with free testing (odds ratio, 7.5; 95% CI, 6.0-9.9), older age, and white race. Sensitivity of the HSV-2 assay was 80.5% and specificity was 98.5%. The HSV-2 positive and negative predictive values were 95.8% (95% CI, 91.6-98.0%) and 92.2% (95 % CI, 89.6 -94.2%), respectively. Antibodies to HSV-2 were detected in 25.9% of 606 persons with no history of genital herpes.

CONCLUSION

Acceptance of HSV-2 serologic testing was cost sensitive. In this high-prevalence population, the positive predictive value of the enzyme-linked immunosorbent assay was sufficient to warrant its use without a confirmatory test. This assay could be useful in the screening of sexually active adults to detect unrecognized HSV-2 infection.

Epidemiology in the pharmaceutical industry: a perspective for infectious diseases

As access to observational data and databases expands so does the need for epidemiologists in industry. As an example, in the U.K. arm of the Department of Epidemiology at GlaxoWellcome Research and Development, the number of PhD-trained epidemiologists increased from zero in 1996 to eight in 1999. Their remit is often broad, including designing and co-ordinating, pharmacoepidemiologic research, studies of risk factors for disease and disease progression and pregnancy registries. Where appropriate, existing data sources, such as research databases, are employed for studies. Otherwise, primary data collection is pursued using standard methodology, for example, cross-sectional, case-control and cohort studies. This paper describes a structure for epidemiologic research in the pharmaceutical industry, the role of the industry epidemiologist and the scope of epidemiological studies. Two examples of observational studies of HSV-2/HIV interactions are given, and implications for research agendas discussed. The pharmaceutical industry is becoming increasingly committed to epidemiological research, and research agendas often involve extensive collaboration with leading external academic and public health groups.

Current recommendations for the treatment of genital herpes

The incidence of genital herpes continues to increase in epidemic-like fashion. Aciclovir (acyclovir) has been the original gold standard of therapy. The recent addition of famciclovir and valaciclovir as antiherpes drugs has improved convenience as well as the efficacy of treatment. Although aciclovir remains a widely prescribed and reliable drug, its administration schedule falls short of the ease of usage that the newer nucleoside analogues offer, for both episodic and suppressive therapy. Suppression of symptomatic disease and asymptomatic shedding from the genitalia have both become popular approaches, if not the primary targets of antiviral therapy. Knowing that asymptomatic disease leads to most cases of transmission strongly suggests that suppression with antiviral agents could reduce transmission risk in discordant couples. Unfortunately, the role for antivirals in reducing transmission remains to be proven in clinical trials. Neonatal herpes is now successfully treated using aciclovir. Current randomised clinical trials are examining aciclovir and valaciclovir administration, as well as safety and efficacy for post-acute suppressive therapy. Prevention of recurrences in pregnancy is also a topic under investigation, with a view to reducing the medical need for Cesarean section, or alternatively (and far less likely to be accomplished) to protect the neonate. Although resistance is largely limited to the immunocompromised and a change in resistance patterns is not expected, several drugs are available for the treatment of aciclovir-resistant strains of herpes simplex. Foscarnet is the main alternative with proven efficacy in this setting. Unfortunately, administration of foscarnet requires intravenous therapy, although a single anecdote of topical foscarnet efficacy in this setting has been published. Alternatives include cidofovir gel, which is not commercially available but can be formulated locally from the intravenous preparation. Less effective alternatives include trifluridine and interferon. Future possibilities for treatment of genital herpes include a microparticle-based controlled-release formulation of aciclovir and resiquimod (VML-600; R-848). The search for an effective therapeutic vaccine for genital herpes has not been successful to date, although a live virus glycoprotein H-deficient (DISC) vaccine is currently in clinical trials. Recent data suggest that seronegative women are protected (albeit, not fully) by a glycoprotein D recombinant vaccine with adjuvant. Despite the established safety and convenience of current treatment options, better suppressive options and topical treatment options are much needed. Studies using existing agents as potential tools to avoid Cesarean section, or transmission to neonate or partner are ongoing. Both vaccines and antivirals may eventually play a role in prevention of infection.

Mathematical models of the transmission and control of sexually transmitted diseases

BACKGROUND

The development of mathematical models to describe and interpret the epidemiology of sexually transmitted infections has involved the incremental addition of various forms of biological and behavioral complexity to simple mathematical templates.

GOAL

To review simple and complex models used in study of observed epidemiologic pattern.

STUDY DESIGN

An overview of modeling in sexually transmitted disease epidemiology identifies the function of different types of models.

RESULTS

Simple models have the advantage of transparency and analytical tractability and can illustrate the relative merits of different intervention options. However, real life is replete with complexities that can have effects that are difficult to predict in the absence of a mathematical framework.

CONCLUSIONS

Research should increasingly be based on robust parameterization of model structures and try to capture individual behaviors. Progress will be most rapid by interdisciplinary work where the clinician, epidemiologist, and mathematician work collaboratively to help improve our knowledge of how to best control infection and disease.