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Prions are small proteinaceous infectious agents known to cause central nervous system infections in both animals and humans. Interest in the pathogenesis of these diseases has grown since the emergence of bovine spongiform encephalopathy (BSE or "mad cow disease") in the United Kingdom and several other countries in Europe. Ingestion of meat products from animals infected with BSE has resulted in transmission of the disease to humans as a variant form of Creutzfeldt-Jakob Disease (vCJD). CJD has a long asymptomatic incubation period, is untreatable, and universally fatal. Hence concern has arisen over other possible routes of disease transmission. Because it is known that prions are found in low levels in the corneas of animals with experimentally induced prion disease, and that a case of human CJD transmission by corneal transplantation has occurred, the question of possible prion transmission through the reuse of diagnostic fitting of contact lenses has surfaced. This article reviews prion diseases of animals and humans, and the data regarding the presence and location of prions in the eye. Issues inherent in the question of corneal and contact lens transmission are discussed. Although some key information has yet to be derived, it appears that the chance of obtaining prion disease through contact lens use is negligible.

Purpose: Recent new reports of possible iatrogenic transmission of Creutzfeldt-Jakob disease (CJD) in Europe have prompted renewed scrutiny of current Eye Bank Association of America criteria for evaluation of potential corneal donors in this country. A prior study evaluated the risk of CJD occurring in U.S. corneal donors by using data to 1994. This report updates these data, analyses the risk by using data to 1997, and predicts potential risk into the next decade., Methods: EBAA data inclusive through 1997 were reviewed and correlated with incidence figures for CJD in the United States as provided by the Communicable Disease Center in Atlanta., Results: The annual incidence of CJD has remained stable at 1 case per million population. Thus approximately 270 new cases of CJD would be expected to occur each year in the United States. From this, the calculated risk of a prion-infected corneal donor appearing in the donor pool is 0.045 cases per year. If the data are corrected for age (90% of CJD patients are older than 60 years) and for possible infected but asymptomatic CJD patients (prevalence, 70 cases per million), at worst, 2.12 cases per year would appear for potential corneal donation (0.005% of all donors). Whereas donors completely without any neurologic symptoms cannot be screened by using any currently available laboratory method, those with a characteristic quadrate clinical prodrome including cognitive changes, speech abnormalities, cerebellar findings, and myoclonus could all be potentially excluded by using tightened medical record and historical screening criteria. Although no cases of bovine spongiform encephalopathy (mad-cow disease) or new variant CJD have been reported in the United States, if such should occur, only 4.2 cases of CJD would be expected in potential donors each year (0.009% of all donors). Tightening of exclusionary queries would significantly reduce the risk of even this number of patients appearing for corneal donation., Conclusions: Historical queries of potential corneal donors should be tightened to assure exclusion of donors with early neurologic alterations. Any patient undergoing autopsy for evaluation of possible central nervous system (CNS) disease should be absolutely excluded. With this approach, the risk of inclusion of CJD-infected transplant tissues derived from ocular sources is very small, and all previously reported cases would have been prospectively excluded from surgical use. Clearly, the benefits of corneal transplantation in the overall population continue significantly to outweigh the risks of transmission of prion disease.

A great deal of controversy and concern exists over potential transmission of central nervous system diseases by corneal transplant. The purpose of this study was to evaluate the available data relative to this question, pertaining especially to transmission of infectious dementia. From these data, determination of conveyance risks are possible, and rational policies for donor inclusion criteria can be constructed. Retrospective analysis of available published data regarding transmission of infectious dementias was performed. Risk of disease transmission was calculated from population data. Of the various forms of dementia, only rabies, hepatitis B, and Creutzfeldt-Jakob disease (CJD) have been transmitted by corneal transplantation. Transmission of the first two viruses is preventable by serologic testing. Prevention of CJD transmission relies on clinical history. Despite the possibility of transmission and the lack of available testing, slow virus disease (CJD) has been transmitted only once. That this case represents an extremely rare event is supported by a lack of successful transmission via corneal transplant in monkeys; lower levels of infectious agent in cornea than in brain; lack of successful transmission of similar human dementias, including Alzheimer's disease to primates; the apparent requirement for homozygosity at codon 129 of chromosome 20 for transmission; lack of transmission in 5-10% of CJD cases even after brain inoculation; and low numerical risk of transmission based on population data. Only 0.5-4 CJD infected donors per year would be expected. Current Eye Bank Association of America criteria for donor exclusion based on suspicious history are adequate to protect against accidental conveyance of transmissible dementia.