A quantitative rabbit model of vaccinia keratitis.

In 1971, the United States Public Health Service recommended that routine childhood vaccination against smallpox be halted, in part because of the high risk of morbidity and mortality associated with the vaccine relative to the likelihood of disease occurrence.1 Smallpox was officially declared to be eradicated in 1980, 3 years after the last naturally occurring case was diagnosed.2 Vaccination requirements were waived for health care workers in 1976, international travelers in 1982, and military personnel in 1990. However, vaccinations of health care workers and military personnel resumed in 2001 because of concerns about the potential use of smallpox for bioterrorism. Adverse reactions to smallpox vaccination are common, and range from mild fever and muscle pain to systemic infection, encephalitis, myocarditis, and death.3–9 A recent study found that greater than one third of the study participants missed work due to mild-to-moderate symptoms after vaccination.10 One common adverse reaction to smallpox vaccination is ocular vaccinia, resulting from accidental transfer of vaccinia virus (VACV) from the inoculation site to the eye. Ocular vaccinia occurred in approximately 1 to 4 recipients per 40,000 primary vaccinations during the smallpox eradication effort and can manifest as blepharitis, conjunctivitis, iritis, and keratitis.9–11 Of these, corneal involvement is the most severe and can result in vision loss. In humans, vaccinia keratitis (VACVK) generally occurs in conjunction with conjunctivitis and blepharitis and begins as a fine granular opacification of the cornea. The disease can progress through the development of opacities in the superficial stroma to ulceration, endothelial keratitis, and diffuse interstitial keratitis.12 Corneal vascularization and uveal involvement (aqueous flare) are common.12 The estimated rates of keratitis during eradication efforts ranged from 6% to 30% of ocular vaccinia cases, depending on reporting conditions.12,13 Rabbits have previously been used to model both the pathologic course14–17 and treatment of vaccinia keratitis,18–23 and the current clinical guidelines for treating vaccinia keratitis in humans are based on the findings in these studies.24 However, the extent to which vaccinia keratitis in rabbits resembles the disease as it presents in humans has not been determined, and the disease parameters that have been monitored are limited in scope. This poses challenges for evaluating current and future therapies for vaccinia keratitis in a comprehensive manner. Information available in the literature does not allow relative comparison of treatment efficacy due to considerable variation in concentration of initial viral inoculum between studies. The establishment of a quantitative scoring system would greatly improve studies of the pathogenesis of VACVK and the testing of therapeutic modalities. A standardized concentration of viral inoculum would also greatly improve therapeutic intervention and virulence studies. The optimal virus inoculum would satisfy four main criteria: (1) It would result in a mean keratitis score near the middle of the scoring range; (2) it would result in disease in 90% to 100% of the animals; (3) it would result in disease in 90% to 100% of the animals within 48 hours after infection, to reduce potential variability in the onset of therapy; and (4) it would minimize the number of animals with conjunctival chemosis severe enough to make scoring keratitis difficult. We present the most complete quantitative analysis of the course of vaccinia keratitis in rabbits reported to date and determine the optimal viral inoculum for inducing vaccinia keratitis for therapeutic testing.