Antivirals for human use against rabies and prospects for their future application

Human Rabies Treatment-From Palliation to Promise

Rabies encephalitis has plagued humankind for thousands of years. In developed countries, access to preventive care, both pre-exposure and post-exposure, has significantly reduced the burden of suffering and disease. However, around the world, rabies remains a neglected tropical disease, largely due to uncontrolled dog rabies, and tens of thousands perish each year. Currently, the standard of care for management of rabies encephalitis is palliation. Heroic attempts to treat human rabies patients over the last few decades have yielded glimpses into our understanding of pathophysiology, opening the door to the development of new antiviral therapies and modalities of treatment. Researchers continue to investigate new compounds and approaches to therapy, yet there remain real challenges given the complexity of the disease. We explore and review some of the promising therapies on the horizon in pursuit of a salvage treatment for rabies.

Establishment and clinical validation of an in-cell-ELISA-based assay for the rapid quantification of Rabies lyssavirus neutralizing antibodies

Neutralizing antibodies (nAbs) prevent the entry of viruses into permissive cells. Since nAbs represent correlates of protection against the Rabies lyssavirus, the presence of sufficient nAbs indicates effective vaccination. Accordingly, Rabies lyssavirus-specific nAb titers need to be determined in routine diagnostics to identify individuals being at risk of Rabies lyssavirus infections due to insufficient immunity. The current gold standard for the quantification of Rabies lyssavirus-specific nAbs is the rapid fluorescent focus inhibition test (RFFIT). However, RFFITs are expensive and labor-intensive since multiple microplate wells must be evaluated one-by-one by trained personnel through microscopic inspection, which limits the number of samples that can be processed. To overcome this disadvantage, we established a novel assay for Rabies lyssavirus-specific nAbs relying on an in-cell-ELISA (icELISA)-based neutralization test (icNT). The icNT differs from the RFFIT in the readout phase, and can be automatically quantified in minutes using broadly available microplate readers. During the establishment, icNT parameters such as antibody concentrations, permeabilization procedures, blocking reagents, infectious doses, and the duration of infection were optimized. Afterwards, a dose-dependent detection of Rabies lyssavirus neutralization was demonstrated using the WHO Standard Rabies Immunoglobulin reference. A panel of 200 sera with known RFFIT titers revealed very good sensitivity and specificity of the icNT. Furthermore, the icNT showed very good intra- and inter-assay precision. By recognizing Rabies lyssavirus-specific antigens, the assay can be applied immediately to automatically quantify the concentration of Rabies lyssavirus nAbs in routine diagnostics or for various basic research questions such as screening for antiviral compounds.

A One Medicine Mission for an Effective Rabies Therapy

Despite the disease's long history, little progress has been made toward a treatment for rabies. The prognosis for patient recovery remains dire. For any prospect of survival, patients require aggressive critical care, which physicians in rabies endemic areas may be reluctant or unable to provide given the cost, clinical expertise required, and uncertain outcome. Systematic clinical research into combination therapies is further hampered by sporadic occurrence of cases. In this Perspective, we examine the case for a One Medicine approach to accelerate development of an effective therapy for rabies through the veterinary care and investigational treatment of naturally infected dogs in appropriate circumstances. We review the pathogenesis of rabies virus in humans and dogs, including recent advances in our understanding of the molecular basis for the severe neurological dysfunction. We propose that four categories of disease process need to be managed in patients: viral propagation, neuronal degeneration, inflammation and systemic compromise. Compassionate critical care and investigational treatment of naturally infected dogs receiving supportive therapy that mimics the human clinical scenario could increase opportunities to study combination therapies that address these processes, and to identify biomarkers for prognosis and therapeutic response. We discuss the safety and ethics of this approach, and introduce the Canine Rabies Treatment Initiative, a non-profit organization with the mission to apply a One Medicine approach to the investigation of diagnostic, prognostic, and therapeutic options for rabies in naturally infected dogs, to accelerate transformation of rabies into a treatable disease for all patients.

[New aspects of rabies control]

Rabies still causes about 60,000 human deaths per year, mainly in poor populations in Africa and Asia. However, since Louis Pasteur developed the first vaccine 130 years ago, prophylactic measures have been considerably improved and simplified. They now consist of the vaccine combined with purified rabies immunoglobulins of equine or human origin. In general, however, post-exposure prophylaxis protocols are long and expensive. Furthermore, the immunoglobulins used for associated serotherapy are costly and not widely available in developing countries. Approaches have been developed to deal with these two issues that offer hope for a paradigm shift for the benefit of exposed populations. Finally, mass rabies vaccination in dogs, which are the most cost-effective measure for preventing rabies in humans, are difficult to implement and sometimes have moderate effectiveness. The identification and analysis of the epidemiological drivers conditioning the circulation of the virus in dog populations allow a better understanding of the key control points that need to be associated with these campaigns for a better efficacy.

Update on lyssaviruses and rabies: will past progress play as prologue in the near term towards future elimination?

Rabies is an ancient, much-feared, and neglected infectious disease. Caused by pathogens in the family Rhabdoviridae, genus Lyssavirus, and distributed globally, this viral zoonosis results in tens of thousands of human fatalities and millions of exposures annually. All mammals are believed susceptible, but only certain taxa act as reservoirs. Dependence upon direct routing to, replication within, and passage from the central nervous system serves as a basic viral strategy for perpetuation. By a combination of stealth and subversion, lyssaviruses are quintessential neurotropic agents and cause an acute, progressive encephalitis. No treatment exists, so prevention is the key. Although not a disease considered for eradication, something of a modern rebirth has been occurring within the field as of late with regard to detection, prevention, and management as well as applied research. For example, within the past decade, new lyssaviruses have been characterized; sensitive and specific diagnostics have been optimized; pure, potent, safe, and efficacious human biologics have improved human prophylaxis; regional efforts have controlled canine rabies by mass immunization; wildlife rabies has been controlled by oral rabies vaccination over large geographic areas in Europe and North America; and debate has resumed over the controversial topic of therapy. Based upon such progress to date, there are certain expectations for the next 10 years. These include pathogen discovery, to uncover additional lyssaviruses in the Old World; laboratory-based surveillance enhancement by simplified, rapid testing; anti-viral drug appearance, based upon an improved appreciation of viral pathobiology and host response; and improvements to canine rabies elimination regionally throughout Africa, Asia, and the Americas by application of the best technical, organizational, economic, and socio-political practices. Significantly, anticipated Gavi support will enable improved access of human rabies vaccines in lesser developed countries at a national level, with integrated bite management, dose-sparing regimens, and a 1 week vaccination schedule.

Trying to treat the untreatable: experimental approaches to clear rabies virus infection from the CNS

Rabies virus causes an invariably fatal encephalitis following the onset of clinical disease. Despite the availability of safe and effective vaccines, the clinical stages of rabies encephalitis remain untreatable, with few survivors being documented. A principal obstacle to the treatment of rabies is the neurotropic nature of the virus, with the blood-brain barrier size exclusion limit rendering the delivery of antiviral drugs and molecules to the central nervous system inherently problematic. This review focuses on efforts to try and overcome barriers to molecule delivery to treat clinical rabies and overviews current progress in the development of experimental live rabies virus vaccines that may have future applications in the treatment of clinical rabies, including the attenuation of rabies virus vectors through either the duplication or mutation of existing genes or the incorporation of non-viral elements within the genome. Rabies post-infection treatment (PIT) remains the holy grail of rabies research.