Effective antiviral treatment of systemic orthopoxvirus disease: ST-246 treatment of prairie dogs infected with monkeypox virus

Smallpox preparedness research has led to development of antiviral therapies for treatment of serious orthopoxvirus infections. Monkeypox virus is an emerging, zoonotic orthopoxvirus which can cause severe and transmissible disease in humans, generating concerns for public health. Monkeypox virus infection results in a systemic, febrile-rash illness closely resembling smallpox. Currently, there are no small-molecule antiviral therapeutics approved to treat orthopoxvirus infections of humans. The prairie dog, using monkeypox virus as a challenge virus, has provided a valuable nonhuman animal model in which monkeypox virus infection closely resembles human systemic orthopoxvirus illness. Here, we assess the efficacy of the antiorthopoxvirus compound ST-246 in prairie dogs against a monkeypox virus challenge of 65 times the 50% lethal dose (LD(50)). Animals were infected intranasally and administered ST-246 for 14 days, beginning on days 0, 3, or after rash onset. Swab and blood samples were collected every 2 days and analyzed for presence of viral DNA by real-time PCR and for viable virus by tissue culture. Seventy-five percent of infected animals that received vehicle alone succumbed to infection. One hundred percent of animals that received ST-246 survived challenge, and animals that received treatment before symptom onset remained largely asymptomatic. Viable virus and viral DNA were undetected or at greatly reduced levels in animals that began treatment on 0 or 3 days postinfection, compared to control animals or animals treated post-rash onset. Animals treated after rash onset manifested illness, but all recovered. Our results indicate that ST-246 can be used therapeutically, following onset of rash illness, to treat systemic orthopoxvirus infections.     

Immunologic hypo- or non-responder in natural dengue virus infection

Serologically defined primary dengue virus infection and/or subsequent homologous serotype infection is known to be associated with less severe disease as compared with secondary subsequent heterologous serotype infection. In geographical locales of high dengue endemicity, almost all individuals in the population are infected at some point in time and should therefore are at high risk of secondary infection. Interestingly, dengue viremia in healthy blood donors whose sera apparently lack detectable levels of specific antibody to dengue viral antigens has been reported. The incidence rate of potential immunologic hypo- or non-responders following natural primary dengue virus infection in dengue endemic regions, who do become immune responders only after repeated exposure, has not been described. These are the patients who may be diagnosed as primary infection in the subsequent infection, but actually are secondary infection. This concept has important implications with regards to the hypothesis of immunological enhancement of dengue pathogenesis, which has largely been advanced based on empirical observations and/or from in vitro experimental assays. The fact that dengue naïve travelers can suffer from severe dengue upon primary exposure while visiting dengue endemic countries underscores one of the major problems in explaining the role of immune enhancement in the pathogenesis of severe dengue virus infection. This evidence suggests that the mechanism(s) leading to severe dengue may not be associated with pre-existing enhancing antibody. Consequently, we propose a new paradigm for dengue virus infection classification. These include a) patients with naïve primary infection, b) those that are serologically defined primary in dengue endemic zones and c) those who are serologically defined secondary dengue virus infection. We submit that clarity with regards to such definitions may help facilitate the delineation of the potential mechanisms of severe dengue virus infection.     

Differential susceptibility epidemic models

We formulate compartmental differential susceptibility (DS) susceptible-infective-removed (SIR) models by dividing the susceptible population into multiple subgroups according to the susceptibility of individuals in each group. We analyze the impact of disease-induced mortality in the situations where the number of contacts per individual is either constant or proportional to the total population. We derive an explicit formula for the reproductive number of infection for each model by investigating the local stability of the infection-free equilibrium. We further prove that the infection-free equilibrium of each model is globally asymptotically stable by qualitative analysis of the dynamics of the model system and by utilizing an appropriately chosen Liapunov function. We show that if the reproductive number is greater than one, then there exists a unique endemic equilibrium for all of the DS models studied in this paper. We prove that the endemic equilibrium is locally asymptotically stable for the models with no disease-induced mortality and the models with contact numbers proportional to the total population. We also provide sufficient conditions for the stability of the endemic equilibrium for other situations. We briefly discuss applications of the DS models to optimal vaccine strategies and the connections between the DS models and predator-prey models with multiple prey populations or host-parasitic interaction models with multiple hosts are also given.This research was partially supported by the Department of Energy under contracts W-7405-ENG-36 and the Applied Mathematical Sciences Program KC-07-01-01.     

The effect of buprenorphine on the course of disease in laboratory rabbits infected with myxoma virus

The only method of assessing the virulence of myxoma virus is to record survival times of rabbits inoculated with the virus. This raises ethical concerns about using animals in experiments where death is the end point. We investigated whether or not the opioid analgesic buprenorphine could be used in rabbits without compromising the myxoma virus virulence assay and on the presumption that animals may suffer pain during the course of the disease. Thirty, 5-month-old New Zealand White rabbits were divided into two groups stratified for weight and gender, and inoculated intradermally with 100 pfu of the Standard Laboratory Strain (SLS) of myxoma virus. At day 6 post infection (p.i.), when eyelid swelling was first seen, each animal in one group was treated with 0.03 mg/kg buprenorphine, subcutaneously, morning and evening until death. Animals in the other group were untreated. Animals were weighed daily and rectal temperatures taken morning and evening. Intake of food and water was assessed as was general demeanor including respiratory effort. There was no significant difference in mean survival time, weight change, or demeanor between the two groups. Increased respiratory effort was seen from day 10 p.i. in animals surviving up to and beyond that time but again there was no difference between groups. Animals treated with buprenorphine refused food and water a day earlier than untreated animals, and had lower temperatures immediately prior to death. It was concluded that the opiate analgesic buprenorphine can be used without compromising the current virulence assay for the SLS of myxoma virus in New Zealand White rabbits but that the clinical signs of myxomatosis that could be attributed to pain were not abrogated.     

Duration of protection from reinfection following exposure to sialodacryoadenitis virus in Wistar rats

Wistar rats [Cr1:(WI)BR] were inoculated intranasally with approximately 10(3) median mouse lethal infective doses of sialodacryoadenitis (SDA) virus. Animals were subsequently selected at random, removed to a separate isolation room, and reinfected with SDA virus at 3, 6, 9, 12 or 15 months. Pre- and postinoculation serum samples were collected from all animals during the course of the study and evaluated for antibody titers to SDA virus. All experimental, control and sentinel animals, following inoculation with SDA virus, were necropsied and examined for lesions consistent with SDA. Salivary gland lesions were minimal to absent in rats reinfected with SDA virus for up to 12 to 15 months after the initial exposure and minimal to moderate in the respiratory tract at 12 or 15 months. SDA-associated lesions were extensive in age matched control animals examined at each time period of reinfection with SDA virus. Thus, prior exposure to SDA virus did protect against the development of typical salivary gland lesions for up to 15 months. Recovered animals were evaluated for their ability to transmit the virus following reinfection. Rats reinfected at 6 or 9 months were infectious to their naive cage mates. The results indicate that reinfection with homologous rat coronavirus can occur as early as 6 months after the initial infection, and such rats can transmit the infection to contact controls.     

Mathematical analysis of the global dynamics of a model for HTLV-I infection and ATL progression

Mathematical analysis is carried out that completely determines the global dynamics of a mathematical model for the transmission of human T-cell lymphotropic virus I (HTLV-I) infection and the development of adult T-cell leukemia (ATL). HTLV-I infection of healthy CD4(+) T cells takes place through cell-to-cell contact with infected T cells. The infected T cells can remain latent and harbor virus for several years before virus production occurs. Actively infected T cells can infect other T cells and can convert to ATL cells, whose growth is assumed to follow a classical logistic growth function. Our analysis establishes that the global dynamics of T cells are completely determined by a basic reproduction number R(0). If R(0)< or =1, infected T cells always die out. If R(0)>1, HTLV-I infection becomes chronic, and a unique endemic equilibrium is globally stable in the interior of the feasible region. We also show that the equilibrium level of ATL-cell proliferation is higher when the HTLV-I infection of T cells is chronic than when it is acute.     

Comparison of deterministic and stochastic SIS and SIR models in discrete time

The dynamics of deterministic and stochastic discrete-time epidemic models are analyzed and compared. The discrete-time stochastic models are Markov chains, approximations to the continuous-time models. Models of SIS and SIR type with constant population size and general force of infection are analyzed, then a more general SIS model with variable population size is analyzed. In the deterministic models, the value of the basic reproductive number R0 determines persistence or extinction of the disease. If R0 < 1, the disease is eliminated, whereas if R0 > 1, the disease persists in the population. Since all stochastic models considered in this paper have finite state spaces with at least one absorbing state, ultimate disease extinction is certain regardless of the value of R0. However, in some cases, the time until disease extinction may be very long. In these cases, if the probability distribution is conditioned on non-extinction, then when R0 > 1, there exists a quasi-stationary probability distribution whose mean agrees with deterministic endemic equilibrium. The expected duration of the epidemic is investigated numerically.     

The effect of infection with bovine viral diarrhea virus on the fertility of Swiss dairy cattle

Bovine viral diarrhea virus is a major cattle pathogen with a worldwide distribution. Animals may be infected with BVD virus transiently or persistently. Transient infection leads to protective immunity. Persistent infection is unique because it is associated with an immunotolerance that is specific to the infecting strain of BVD virus. Persistent infection results from viral invasion of fetuses between the second and fourth month of development. Such animals are of prime importance in the epidemiology of BVD because they shed large amounts of virus, and thus serve as a constant source of infection for non-immune animals. Infection of pregnant animals during the first two months of gestation may result in an increased rate of return to estrus. Animals infected in the period of five months to birth may abort or give birth to calves with malformations. The effects of BVD virus on fertility and gestation are well documented from experimental infection. However, much less is known of the extent of losses under field conditions. The main reason for this may be the multitude of other causes of increased return rates and gestation failures. In addition, the incidence of infection with BVD virus may vary over time and depends on management practices. In this study, we investigated the impact of BVD virus on gestation failures under field conditions in a large cattle-breeding area of Switzerland, where no specific measures to control BVD are in effect. Our approach consisted of relating seroconversions to BVD virus with the rate of return to estrus, abortion, and birth of calves with apparent malformations. These parameters of fertility were compared to those of animals immune to BVDV infection due to previous exposure to the virus and animals without seroconversion. Our data show that infection with BVD virus during the first 45 days of gestation did not influence the rate of return to estrus. By contrast, we observed a statistically significant increase in the abortion rate in mid-term gestation (Days 46 to 210) while no such effect was observed in the later stages of gestation. No clinically manifest malformations were observed in the offspring of animals that had seroconverted to BVD virus. In our study population the prevalence of BVDV antibody positive cattle varied only slightly between 78% and 80% over the period of observation. Our data showed that 7% (CI: 2.4-14%) of fetal deaths may be attributable to infection with BVD virus.     

Pest control through viral disease: mathematical modeling and analysis

This paper deals with the mathematical modeling of pest management under viral infection (i.e. using viral pesticide) and analysis of its essential mathematical features. As the viral infection induces host lysis which releases more virus into the environment, on the average 'kappa' viruses per host, kappain(1,infinity), the 'virus replication parameter' is chosen as the main parameter on which the dynamics of the infection depends. We prove that there exists a threshold value kappa(0) beyond which the endemic equilibrium bifurcates from the free disease one. Still for increasing kappa values, the endemic equilibrium bifurcates towards a periodic solution. We further analyse the orbital stability of the periodic orbits arising from bifurcation by applying Poor's condition. A concluding discussion with numerical simulation of the model is then presented.     

Endemic threshold results in an age-duration-structured population model for HIV infection

In this paper we consider an age-duration-structured population model for HIV infection in a homosexual community. First we investigate the invasion problem to establish the basic reproduction ratio R(0) for the HIV/AIDS epidemic by which we can state the threshold criteria: The disease can invade into the completely susceptible population if R(0)>1, whereas it cannot if R(0)<1. Subsequently, we examine existence and uniqueness of endemic steady states. We will show sufficient conditions for a backward or a forward bifurcation to occur when the basic reproduction ratio crosses unity. That is, in contrast with classical epidemic models, for our HIV model there could exist multiple endemic steady states even if R(0) is less than one. Finally, we show sufficient conditions for the local stability of the endemic steady states.     

Stability analysis of within-host parasite models with delays

We provide a global analysis of systems of within-host parasitic infections. The systems studied have parallel classes of different length of latently infected target cells. These systems can also be thought as systems arising from within-host parasitic systems with distributed continuous delays. We compute the basic reproduction ratio R0 for the systems under consideration. If R0< or =1 the parasite is cleared, if R0>1 and if a sufficient condition is satisfied we conclude to the global asymptotic stability (GAS) of the endemic equilibrium. For some generic class of models this condition reduces to R0>1. These results make possible to revisit some parasitic models including intracellular delays and to study their global stability.     

Analysis and simulation of a stochastic, discrete-individual model of STD transmission with partnership concurrency

Deterministic differential equation models indicate that partnership concurrency and non-homogeneous mixing patterns play an important role in the spread of sexually transmitted infections. Stochastic discrete-individual simulation studies arrive at similar conclusions, but from a very different modeling perspective. This paper presents a stochastic discrete-individual infection model that helps to unify these two approaches to infection modeling. The model allows for both partnership concurrency, as well as the infection, recovery, and reinfection of an individual from repeated contact with a partner, as occurs with many mucosal infections. The simplest form of the model is a network-valued Markov chain, where the network's nodes are individuals and arcs represent partnerships. Connections between the differential equation and discrete-individual approaches are constructed with large-population limits that approximate endemic levels and equilibrium probability distributions that describe partnership concurrency. A more general form of the discrete-individual model that allows for semi-Markovian dynamics and heterogeneous contact patterns is implemented in simulation software. Analytical and simulation results indicate that the basic reproduction number R(0) increases when reinfection is possible, and the epidemic rate of rise and endemic levels are not related by 1-1/R(0), when partnerships are not point-time processes.     

A Bayesian approach for inferring the dynamics of partially observed endemic infectious diseases from space-time-genetic data

We describe a statistical framework for reconstructing the sequence of transmission events between observed cases of an endemic infectious disease using genetic, temporal and spatial information. Previous approaches to reconstructing transmission trees have assumed all infections in the study area originated from a single introduction and that a large fraction of cases were observed. There are as yet no approaches appropriate for endemic situations in which a disease is already well established in a host population and in which there may be multiple origins of infection, or that can enumerate unobserved infections missing from the sample. Our proposed framework addresses these shortcomings, enabling reconstruction of partially observed transmission trees and estimating the number of cases missing from the sample. Analyses of simulated datasets show the method to be accurate in identifying direct transmissions, while introductions and transmissions via one or more unsampled intermediate cases could be identified at high to moderate levels of case detection. When applied to partial genome sequences of rabies virus sampled from an endemic region of South Africa, our method reveals several distinct transmission cycles with little contact between them, and direct transmission over long distances suggesting significant anthropogenic influence in the movement of infected dogs.     

Genome-Wide Comparison of Cowpox Viruses Reveals a New Clade Related to Variola Virus

Zoonotic infections caused by several orthopoxviruses (OPV) like monkeypox virus or vaccinia virus have a significant impact on human health. In Europe, the number of diagnosed infections with cowpox viruses (CPXV) is increasing in animals as well as in humans. CPXV used to be enzootic in cattle; however, such infections were not being diagnosed over the last decades. Instead, individual cases of cowpox are being found in cats or exotic zoo animals that transmit the infection to humans. Both animals and humans reveal local exanthema on arms and legs or on the face. Although cowpox is generally regarded as a self-limiting disease, immunosuppressed patients can develop a lethal systemic disease resembling smallpox. To date, only limited information on the complex and, compared to other OPV, sparsely conserved CPXV genomes is available. Since CPXV displays the widest host range of all OPV known, it seems important to comprehend the genetic repertoire of CPXV which in turn may help elucidate specific mechanisms of CPXV pathogenesis and origin. Therefore, 22 genomes of independent CPXV strains from clinical cases, involving ten humans, four rats, two cats, two jaguarundis, one beaver, one elephant, one marah and one mongoose, were sequenced by using massive parallel pyrosequencing. The extensive phylogenetic analysis showed that the CPXV strains sequenced clearly cluster into several distinct clades, some of which are closely related to Vaccinia viruses while others represent different clades in a CPXV cluster. Particularly one CPXV clade is more closely related to Camelpox virus, Taterapox virus and Variola virus than to any other known OPV. These results support and extend recent data from other groups who postulate that CPXV does not form a monophyletic clade and should be divided into multiple lineages.     

Experimental and Natural Infections of Goats with Severe Fever with Thrombocytopenia Syndrome Virus: Evidence for Ticks as Viral Vector

Background

Severe fever with thrombocytopenia syndrome virus (SFTSV), the causative agent for the fatal life-threatening infectious disease, severe fever with thrombocytopenia syndrome (SFTS), was first identified in the central and eastern regions of China. Although the viral RNA was detected in free-living and parasitic ticks, the vector for SFTSV remains unsettled.

Methodology/Principal Findings

Firstly, an experimental infection study in goats was conducted in a bio-safety level-2 (BSL-2) facility to investigate virus transmission between animals. The results showed that infected animals did not shed virus to the outside through respiratory or digestive tract route, and the control animals did not get infected. Then, a natural infection study was carried out in the SFTSV endemic region. A cohort of naïve goats was used as sentinel animals in the study site. A variety of daily samples including goat sera, ticks and mosquitoes were collected for viral RNA and antibody (from serum only) detection, and virus isolation. We detected viral RNA from free-living and parasitic ticks rather than mosquitoes, and from goats after ticks’ infestation. We also observed sero-conversion in all members of the animal cohort subsequently. The S segment sequences of the two recovered viral isolates from one infected goat and its parasitic ticks showed a 100% homology at the nucleic acid level.

Conclusions/Significance

In our natural infection study, close contact between goats does not appear to transmit SFTSV, however, the naïve animals were infected after ticks’ infestation and two viral isolates derived from an infected goat and its parasitic ticks shared 100% of sequence identity. These data demonstrate that the etiologic agent for goat cohort’s natural infection comes from environmental factors. Of these, ticks, especially the predominant species Haemaphysalis longicornis, probably act as vector for this pathogen. The findings in this study may help local health authorities formulate and focus preventive measures to contain this infection.     

Modelling to explore the potential impact of asymptomatic human infections on transmission and dynamics of African sleeping sickness

Gambiense human African trypanosomiasis (gHAT, sleeping sickness) is one of several neglected tropical diseases (NTDs) where there is evidence of asymptomatic human infection but there is uncertainty of the role it plays in transmission and maintenance. To explore possible consequences of asymptomatic infections, particularly in the context of elimination of transmission—a goal set to be achieved by 2030—we propose a novel dynamic transmission model to account for the asymptomatic population. This extends an established framework, basing infection progression on a number of experimental and observation gHAT studies. Asymptomatic gHAT infections include those in people with blood-dwelling trypanosomes, but no discernible symptoms, or those with parasites only detectable in skin. Given current protocols, asymptomatic infection with blood parasites may be diagnosed and treated, based on observable parasitaemia, in contrast to many other diseases for which treatment (and/or diagnosis) may be based on symptomatic infection. We construct a model in which exposed people can either progress to either asymptomatic skin-only parasite infection, which would not be diagnosed through active screening algorithms, or blood-parasite infection, which is likely to be diagnosed if tested. We add extra parameters to the baseline model including different self-cure, recovery, transmission and detection rates for skin-only or blood infections. Performing sensitivity analysis suggests all the new parameters introduced in the asymptomatic model can impact the infection dynamics substantially. Among them, the proportion of exposures resulting in initial skin or blood infection appears the most influential parameter. For some plausible parameterisations, an initial fall in infection prevalence due to interventions could subsequently stagnate even under continued screening due to the formation of a new, lower endemic equilibrium. Excluding this scenario, our results still highlight the possibility for asymptomatic infection to slow down progress towards elimination of transmission. Location-specific model fitting will be needed to determine if and where this could pose a threat.     

Reproducibility and usability of chronic virus infection model using agent-based simulation; comparing with a mathematical model

We created agent-based models that visually simulate conditions of chronic viral infections using two software. The results from two models were consistent, when they have same parameters during the actual simulation. The simulation results comprise a transient phase and an equilibrium phase, and unlike the mathematical model, virus count transit smoothly to the equilibrium phase without overshooting which correlates with actual biology in vivo of certain viruses. We investigated the effects caused by varying all the parameters included in concept; increasing virus lifespan, uninfected cell lifespan, uninfected cell regeneration rate, virus production count from infected cells, and infection rate had positive effects to the virus count during the equilibrium period, whereas increasing the latent period, the lifespan-shortening ratio for infected cells, and the cell cycle speed had negative effects. Virus count at the start did not influence the equilibrium conditions, but it influenced the infection development rate. The space size had no intrinsic effect on the equilibrium period, but virus count maximized when the virus moving speed was twice the space size. These agent-based simulation models reproducibly provide a visual representation of the disease, and enable a simulation that encompasses parameters those are difficult to account for in a mathematical model.     

Within-Host Models of High and Low Pathogenic Influenza Virus Infections: The Role of Macrophages

The World Health Organization identifies influenza as a major public health problem. While the strains commonly circulating in humans usually do not cause severe pathogenicity in healthy adults, some strains that have infected humans, such as H5N1, can cause high morbidity and mortality. Based on the severity of the disease, influenza viruses are sometimes categorized as either being highly pathogenic (HP) or having low pathogenicity (LP). The reasons why some strains are LP and others HP are not fully understood. While there are likely multiple mechanisms of interaction between the virus and the immune response that determine LP versus HP outcomes, we focus here on one component, namely macrophages (MP). There is some evidence that MP may both help fight the infection and become productively infected with HP influenza viruses. We developed mathematical models for influenza infections which explicitly included the dynamics and action of MP. We fit these models to viral load and macrophage count data from experimental infections of mice with LP and HP strains. Our results suggest that MP may not only help fight an influenza infection but may contribute to virus production in infections with HP viruses. We also explored the impact of combination therapies with antivirals and anti-inflammatory drugs on HP infections. Our study suggests a possible mechanism of MP in determining HP versus LP outcomes, and how different interventions might affect infection dynamics.     

Infection-age structured epidemic models with behavior change or treatment

We formulate infection-age structured susceptible-infective-removed (SIR) models with behavior change or treatment of infections. Individuals change their behavior or have treatment after they are infected. Using infection age as a continuous variable, and dividing infectives into discrete groups with different infection stages, respectively, we formulate a partial differential equation model and an ordinary differential equation model with behavior change or treatment. We derive explicit formulas for the reproductive number by linear stability analysis of the infection-free equilibrium, and explicit formulas for the unique endemic equilibrium, when it exists, for both models. These formulas provide mathematical theoretical frameworks for analysis of impact of behavior change or treatment of infection to the transmission dynamics of infectious diseases. We study several special cases and provide sensitivity analysis for the reproductive numbers with respect to model parameters based on those formulas.     

Evolution of cyclophilin A and TRIMCyp retrotransposition in New World primates

Host cell factors modulate retroviral infections. Among those, cyclophilin A (CypA) promotes virus infectivity by facilitating virus uncoating or capsid unfolding or by preventing retroviral capsid interaction with cellular restriction factors. In Aotus species, a retrotransposed copy of CypA inserted into the tripartite motif 5 (TRIM5) gene encodes a fusion protein which may block human immunodeficiency virus type 1 by targeting the incoming virus to ubiquitin-ligated degradation or by interfering with normal uncoating of the incoming particle, rendering those monkeys resistant to infection. In this study, we have extensively analyzed representative specimens from all New World primate genera and shown that the retrotransposed CypA copy is only present in Aotus. We have shown that this inserted copy diverged from its original counterpart and that this occurred prior to Aotus radiation, although no positive selection was observed. Finally, our data underscores the need for a precise taxonomic identification of primate species used as models for retroviral infections and novel antiviral approaches.