The Unique Challenges of Viral Conjunctivitis

Its similarities to bacterial infection belie key differences that impact clinical care

By Joseph Capriotti, MD, Kevin P. Stewart, MD, and Jesse S. Pelletier, MD

External ocular infections caused by adenoviruses are among the most common eye infections seen worldwide. They lead to highly infectious community epidemics, seasonal outbreaks, lost productivity, significant patient discomfort and in some cases permanent visual compromise from long-term immune-mediated sequelae.¹ Though several therapeutic agents have been evaluated for acute viral conjunctivitis, none to date have been approved for therapeutic use in humans.^2-4

Both bacterial and viral pathogens cause acute infectious diseases of the ocular surface with similar clinical presentations. A recent evidence-based review⁵ concluded that signs and symptoms of acute bacterial and acute viral conjunctivitis are essentially identical. Most cases involve conjunctival hyperemia with varying chemosis, some component of ocular discharge and a constellation of symptoms that can include foreign body sensation, pain and itching. For this reason, we suggest the use of similar clinical criteria for one of the primary efficacy endpoints — namely, resolution of signs and symptoms — in both viral and bacterial clinical trials. However, there are key differences in the mechanism, host response and epidemiology of each etiologic agent. Consideration of these factors in vital in the evaluation of therapeutic agents in clinical trials.

Differing Relationships to the Ocular Surface

Bacterial conjunctivitis is commonly caused by normal ocular surface flora. When the balance between host defense and microbial colonization on the ocular surface is somehow disrupted, the commensal relationship can proceed to frank infection. Key factors that affect this pathogenic conversion appear to be more related both to host defense compromise and specific bacterial species present.⁶

In contradistinction to bacteria, adenovirus species are not typically found among the normal ocular flora.^7-9 A definitive study employing tissue culture in ocular swab samples obtained from the conjunctival fornicies was unable to demonstrate the presence of adenovirus from even a single sample of over 200 collected in asymptomatic patients presenting for routine eye exam.¹⁰ The absence of adenoviral colonization was similarly demonstrated in conjunctival specimens studied by tissue culture obtained from a series of patients with symptoms of non-infectious keratoconjunctivitis sicca.

The presence of adenovirus on the ocular surface would seem to indicate active or recent convalescent infection. This is a critical difference between bacterial and adenoviral conjunctivitis and must be considered when selecting efficacy endpoints for the evaluation of therapeutic agents. Anti-bacterials can clearly demonstrate their clinical utility by simply showing the resolution of clinical signs and symptoms in a shorter time period than would be expected with nonintervention. There is no need to show elimination of bacterial colonization, as bacterial sterility is not a feature of the healthy ocular surface. Bacterial conjunctivitis is a much rarer cause of community outbreaks and is less likely to be associated with person-to-person transmission.

Viral Transmission and Shedding

The differences in the transmissibility of bacterial and viral conjunctivitis merit careful consideration. Ocular adenoviral infection represents a significant public health problem in the US and worldwide.^12,15 Although exact numbers are difficult to determine, estimates^4,9,13,14 suggest the number of cases may be as high as 15-20 million per year in the United States. Adenovirus conjunctivitis is a reportable infection in Germany¹⁵ and is classified as a Category IV infectious disease by Japan's National Epidemiological Surveillance of Infectious Diseases (NESID) with mandated collection, analysis and publication of reports on occurrences.¹⁶

Adenoviral transmission between infected and uninfected hosts is particularly efficient in areas of high population density, overcrowding or poor hygiene. Studies on the rate¹⁸ of horizontal transmission to asymptomatic family member and close contacts suggest transmissibility of up to 50 percent.^19-21

Adenovirus is spread through droplets from the respiratory tract, stool, saliva and tears. Through a process known as viral shedding, infectious particles are transferred from the extracellular environment of lytic infected cells through a variety of fomites.²² Adenoviral particles, presumably shed from infected patients, have been isolated from multi-dose ophthalmic medications and diagnostic solutions.^23,24 Recovery of infectious adenovirus has been reported from samples obtained from inanimate hard surfaces and objects for up to 49 days.^25,26

Actively infected persons readily transmit adenoviruses. Viral shedding persists for 12-14 days after onset of clinical signs and symptoms. Transmission can be prevented by personal hygiene measures, including frequent hand washing; cleaning of towels, pillowcases and handkerchiefs; and disposal of contaminated facial tissues. Individuals who work with the public, in schools or in healthcare facilities in particular, should consider a temporary leave of absence from work to prevent infection of others, especially those who are already ill.

The most effective measures for limiting the severity of adenoviral conjunctivitis outbreaks rely on reducing the contamination of objects, workspaces and surfaces by aggressive steps to remove shed virus particles.^27,28 It follows that reducing shedding at the source — the infected ocular surface — would be a highly effective strategy for epidemic prevention and control. The clear relationship between shedding virus and infectivity necessarily affects our therapeutic options and our therapeutic requirements in acute viral conjunctivitis. The additional burden is placed on the evaluation of anti-adenoviral agents, given the pathogen's devastating potential to cause outbreaks. Therapeutic agents should aim for a reduction in viral shedding in addition to resolution of clinical symptoms.

Similar patterns of epidemic spread, droplet transmission and shedding are not typical features of bacterial conjunctivitis, though outbreaks have been reported in humans²⁹ and vector-dependent spread confirmed in animal models.^30-32 Bacterial conjunctivitis is a much less likely cause of outbreaks and is not a significant public health challenge (we acknowledge the enormous importance of C. Trachoma and defer its discussion as it is more commonly a chronic, endemic, recurrent infection with a distinct clinical course³³).

Though vertical transmission remains an important aspect of neonatal bacterial conjunctivitis, these cases are rare in the industrialized world and do not share the features of epidemic infection. Furthermore, neonatal conjunctivitis passed intrapartum from mother to newborn is easily eliminated through ocular administration of povidone-iodine at birth.³⁴

Ocular Immune Response in Bacterial and Adenoviral Infections

Both the innate and adaptive acquired immune systems play important roles in ocular defense.³⁵ While the predominantly extracellular bacterial pathogens are more effectively controlled by the innate ocular defense mechanisms, viral infections often lead to a more prolonged course. Viral exposures frequently involve a more robust acquired immune cascade with significant inflammatory damage.^36-38 It is precisely this exuberant immune reaction that leads to the signs and symptoms of viral conjunctivitis and immune-mediated sequelae.

It is often clinically beneficial to temper the ocular immune response in both viral and bacterial infections, with topical steroids frequently the agents of choice. Steroids have well-characterized effects on both innate and adaptive immunity. The features of the immune responses to viral and bacterial pathogens need to be considered along with the relative effects of steroid on each system: steroids have a more dramatic inhibitory effect on the adaptive system, and this is precisely the system that is most important at eliminating viral infections. It is expected that steroids would have less of an effect on the eye's ability to counter bacterial pathogens than they would on the elimination of viral organisms. Co-administration of potent topical steroids along with antibiotics does not lead to higher bacterial counts (measured as CFUs)³⁹ in the normal bacterial conjunctival flora.

It has been repeatedly shown that use of topical steroids in ocular adenoviral infection can prolong the duration of viral shedding and therefore lengthen the period of transmissibility in these cases.⁴⁰ For this reason, topical steroid monotherapy in ocular adenovirus infections is ill advised. Although topical corticosteroids (and oral steroids in some severe cases) can limit patient discomfort and prevent some immune-related inflammatory complications, even a short course of relatively low-potency corticosteroids without the addition of a suitable anti-viral agent can increase the duration of viral shedding and prolong the infectivity of affected patients.⁴¹

The addition of the topical steroid cripples the eye's immune response to viral pathogens. The effect on the ability to effectively clear viral infections is so pronounced that the addition of topical steroids can even reverse the effect of the most potent anti-virals.⁴² This in turn can potentiate the occurrence of community outbreaks and epidemic transmission in schools, places of business and medical facilities.⁴³ As described above, this additionally requires that effects on infectivity be considered along with symptom resolution in the clinical evaluation of anti-adenoviral therapies.⁴⁴

Detection of Adenoviral Infectivity

There are several techniques to detect adenovirus in ocular specimens. Despite recent advances in nucleic acid-based detection and availability of rapid point-of-care im munochromatographic tests for specific viral components, cell culture remains the only reliable method for the demonstration of viable, infectious virus.

Cell culture with confirmatory immunofluorescence (CC-IFA), a highly sensitive and specific test, is considered the “gold standard” for recovery of infectious virus from ocular samples. CC-IFA requires the presence of infectious virus and demonstrates unequivocally the ability of the recovered virus to cause a cytopathic effect (CPE) in a living cell. When combined with immunofluorescent staining, it provides a means to determine the presence, infectivity and identity of a viral specimen. A sample from a conjunctival swab is inoculated in susceptible cell line and followed over time to measure CPE.

The shell vial culture method enables more rapid identification of CPE.⁴⁵ This test utilizes shell vials (glass culture tubes that contain a coverslip coated with an A549 cell monolayer), centrifugation and visualization of adenovirus proteins inside host cells through binding of fluorescent dye. The culture tube is inoculated with the clinical specimen, centrifuged at low speed and incubated. It is hypothesized that the centrifugation enhances the adenoviral entry into the susceptible cells. The visualization technique is indirect, where a secondary antibody labeled with fluorochrome is used to recognize a primary antibody directed towards a conserved adenoviral epitope.

This test significantly shortens the time requirement and enhances the sensitivity and specificity. Positive results can be obtained from the visualization of even a single brightly stained cell, confirming that the adenoviral particles were capable of entering a cell, uncoating, replicating and producing infectious prodigy virions. In this way, CC-IFA in general and the shell vial method specifically provide an unequivocal way to determine the infectivity of an ocular specimen. It is for this reason that we propose assessment of infectivity by CC-IFA as a second primary endpoint for clinical trials designed to evaluate the efficacy of anti-adenoviral therapeutic agents.

Proposed Study Design for Acute Therapy in Acute Adenoviral Conjunctivitis

The ideal viral conjunctivitis therapy would alleviate patient symptoms, resolve clinical signs, decrease inflammatory damage, shorten the clinical course of infection, reduce viral shedding duration and decrease the period of infectivity. Evaluation of therapies should include analysis of clinical and infectious parameters and consider effects on the individual patient and the community as a whole. Use of separate primary efficacy endpoints is proposed that can demonstrate the following:

1. Resolution of signs and symptoms associated with viral conjunctivitis.
2. Decrease in infectious viral shedding measured by CC-IFA at the test-of-cure visit.

Resolution of signs and symptoms of the disease is the most clinically meaningful assessment, and derives from the similar clinical features shared by acute bacterial and acute viral conjunctivitis. The required analysis of viral shedding, which derives from the differences in transmission between bacterial and viral conjunctivitis, is important to ensure that symptomatic relief in individuals doesn't lead to prolonged infectivity. The requirement for all proposed agents to satisfy both of these endpoints is the most effective way to ensure that proposed anti-adenoviral therapies address both the infectious and inflammatory consequences of the disease. OM

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