In the last century, microbiology textbooks have expanded their scope to include chapters on viruses, and recently, prions. The word "prion" was coined by Stanley B. Prusiner, MD, of the University of California at San Francisco, and is derived from the first two syllables of "proteinaceous" and "infectious."1 Unlike a virus, which contains nucleic acid, prions are composed entirely of proteins. Abnormal prions cause several transmissible spongiform encephalopathies (TSEs), which are degenerative neurologic conditions that present clinically as dementia, ataxia and eventual death.1 One of these diseases is bovine spongiform encephalopathy (BSE), first recognized as affecting British cattle in 1987.2

In the last two decades, bovine outbreaks of "mad cow disease" have been prominently reported, particularly in the United Kingdom. First described by two German neurologists in the 1920s,3 Creutzfeldt-Jakob Disease presents with similar symptoms as BSE, and a variant form (vCJD) is acquired through consumption of meat contaminated with BSE-infected neurological tissue.4,5 The various forms of CJD can also be transmitted iatrogenically. Although the likelihood of contracting CJD via ophthalmic surgery is small, the disease has grave implications, and the ophthalmic community has instituted measures to prevent iatrogenic transfer.

 

Peculiar Prions

Although researchers aren't entirely certain what purpose prions serve, they believe that they're normally involved in useful pathways. The cellular protein prion (PrPc) occurs naturally, is likely synthesized in neurons,6 and attaches to the cell membrane through a glycosylphosphatidylinositol anchor. It is easily digested by proteinase K.7 PrPc is found in many tissues, with especially high levels in the brain;6 proposed functions are varied and include copper metabolism, signal transduction, memory formation, neuronal differentiation and lymphocyte activation.8 It's not until something triggers a translational malfunction, which causes the prion to fold differently at the secondary and tertiary levels, that the prion becomes highly resistant to denaturation.9 PrPSC—the "SC" refers to "scrapie"—aggregates in nervous tissue to form amyloid plaques,10 resulting in dementia, ataxia, convulsions and death. There is no treatment.

Several TSEs have been recognized in mammals: scrapie in sheep; bovine and feline spongiform encephalopathies; chronic wasting disease in deer and elk; transmissible mink encephalopathy; and exotic ungulate encephalopathy in nyala, oryx and greater kudu. Recognized human forms include Kuru, fatal familial insomnia, Gerstmann-Sträussler-Scheinker syndrome and various forms of CJD.11 All TSEs are fatal, and involve progressively debilitating neurological symptoms. While the abnormal protein structure most commonly results from a translation malfunction, it can also be introduced orally and iatrogenically.12,13 The abnormal protein is not easily destroyed, and usual cooking and sterilization techniques leave it unscathed.11

 

Taboo Practices

Kuru was epidemic in Papua, New Guinea, in the first half of the 20th century, and traced to the mourning practice of cannibalizing dead relatives. Tribal men remained relatively free of the disease, as they ate the "good meat," while the women and children were relegated to consuming the brain. Kuru is notorious for its long incubation period, as symptoms of the "shaking death" usually didn't appear for several decades.14,15

Similarly, bovine spongiform encephalopathy is spread through cattle herds by feed made with animal by-products.16 Only three cases of BSE have been reported in the United States. The first was in December of 2003 and was traced to a cow imported from Canada.17 Some 200,000 cases of BSE have occurred worldwide, with the majority traced to U.K.-bred cattle. The United States has banned the import of beef from several European countries since 1989, and in 1997 expanded the restriction to include other countries with sub-par import restrictions.18 Japan has taken similar measures.19 In addition, most countries, including the United States, Japan and the United Kingdom, have banned the feeding of mammalian protein to ruminants. The extreme caution is warranted: BSE crosses the species barrier into humans through the consumption of any type of cattle meat that's been tainted with BSE-infected neurological tissue and, like Kuru, has an incubation period that's several decades long.12,15

 

Various Presentations

Of course, not all cases of CJD can be traced to cows. There are four semi-distinct forms of the disease. Familial CJD is hereditary, has been traced to a mutations on the PRNP (prion protein) gene on chromosome 20, and accounts for 5 to 10 percent of CJD cases.20 The sporadic form (sCJD) appears without warning, generally in people over the age of 50, has a clinical duration of approximately four to five months, and accounts for approximately 85 percent of cases of CJD.20 These forms of CJD are not related to the consumption of BSE-contaminated meat. Iatrogenic CJD accounts for less than 5 percent of all CJD cases,20 and is the result of transmission of mutant prion (familial CJD, vCJD, or sCJD) via contaminated surgical equipment, transplantation of infected tissue or infusion of infected blood.

In 1990, following the BSE outbreak, the U.K. reinstituted epidemiological surveillance of CJD. In 1996, a new variant CJD (vCJD) was announced as having affected 10 individuals, who all displayed clinical onset between 1994 and 1995. vCJD is different from sCJD in that it more often affects younger people, and has an average clinical duration of 13 to 14 months. Its neurological profile is also distinct from sCJD, as it presents with drastic behavioral changes, painful dysesthesias, particularly dense accumulation of prion protein in the brain and presence of the protein in lymphoid tissue.21,22

As of January 2008, 166 definite or probable cases of vCJD have been reported in the U.K.23 Only three have been reported in the United States, and all have been linked to exposure during time spent in the U.K.24 Worldwide, the incidence of vCJD is approximately 0.5 to 1 per million, and all cases have been traced to the consumption of infected cattle meat from the U.K.25

There is concern, however, over the long incubation period and symptoms. All cases thus far have shown homozygosity at codon 129 of the PRNP gene—it may be that heterozygous individuals have a longer incubation period, and that the true size of the epidemic has yet to reveal itself.14,26

 

Iatrogenic Transfer

Although fewer incidences of vCJD have been reported since 2000, other forms of CJD, which can also be transferred iatrogenically, remain comparatively common.23 However, the risk of developing or contracting any form of CJD, especially outside of the United Kingdom, is almost negligible. Given the low incidence of CJD, and the fact that the disease cannot be definitively diagnosed until after death, screening procedures for CJD-infected blood and tissues have been difficult to implement. Concerns about lengthy incubation time along with understandable public fear of the disease warrant some screening methods and reasonable preventative measures.

In 2003, the first cases of probable vCJD transmission through blood were reported.27,28 vCJD has also been shown to accumulate in the lymph, spleen and tonsils; therefore, the United States does not allow people who have resided for more than three months in a country where BSE is prevalent to donate blood.29 There has been one confirmed incident in which a cornea donor was found to have sCJD (diagnosed post-mortem and after the tissue donation) and the recipient later developed CJD; it occurred in the United States in 1974.13 At least two other possible transmissions of sCJD via corneal grafts have also occurred.30

Autopsies performed on sCJD and vCJD victims have shown no prion protein in any ocular structures other than the optic nerve and the retina. The retina seems to be particularly susceptible to prion-induced degeneration, although the degeneration is only clinically evident once the prions have replicated to maximal levels.31 In general, prions seem to be restricted to neurologic, immune-related and blood-fed tissues.21 Although no evidence exists of the transmission of CJD via ophthalmic instruments,32 during corneal transplants the blood-aqueous barrier is broken, compromising the immunologic privilege of the anterior chamber. If surgical instruments were previously contaminated, it is conceivable that prions could then be introduced to the patient. Hypothetically, the normal aqueous flow would wash any infectious agents out of the eye, and the prions should never come in contact with the retina or the optic nerve.

Concern has also been voiced over the transfer of epithelial cells on tonometers; again, no one has acquired CJD this way, but manual cleaning of the instruments to remove residual cells,33 the use of disposable instruments when appropriate and the use of cling film can help alleviate fears.34

Since 2004, the U.K. has adopted strict donor and surgical guidelines for ophthalmic surgeries. Extensive medical and behavioral histories must be taken of all potential donors in an effort to identify potential CJD victims; currently, this is the only way to exclude donors with a suspected CJD history. Reusable equipment must be identifiable and traceable, and disposable equipment is preferable. Careful cleaning of reusable instruments by following manufacturer's guidelines is emphasized. Records of all ocular donor tissue must be kept by eye banks, and records of recipients must be kept for at least eight years after the patient has died (or been lost to follow-up).35 The United States Food and Drug Administration in 2007 released a Guidance for Industry report with similar provisions,29 and although it's not binding, donor banks usually perform some CJD screening.

As the number of confirmed vCJD cases has been decreasing, it's likely that the current agricultural regulations have halted its progression. Given the proposed extended incubation period, however, it's conceivable that more cases of vCJD will be revealed in the coming decades. Since iatrogenic transfer of CJD is not limited to vCJD, as the sporadic and familial forms can also be transmitted iatrogenically, reasonable caution is warranted to protect against this disease.

Current methods of tracing patients and medical instruments should be maintained, and the medical history of donors should be examined for symptoms indicative of vCJD. Recent research has shown soaking instruments in sodium hydroxide prior to autoclaving can significantly reduce prion levels in vitro;36 this should be considered when developing sterilization policies. Concern and precautions against vCJD ought to be particularly keen in the U.K. and in the European Union. In the United States, the minimal incidence of BSE and meat trade restrictions should alleviate many of the concerns.

In general, the use of disposable instruments for corneal surgery is preferable to prevent contamination of any kind, but the risk of vCJD transmission during corneal surgery appears to be minimal. Ultimately, the use of disposable instruments is an economic decision, left to the discretion of individual institutions. 

 

Dr. Abelson, an associate clinical professor of ophthalmology at Harvard Medical School and senior clinical scientist at Schepens Eye Research Institute, consults in ophthalmic pharmaceuticals. Ms. Lilyestrom is a managing editor at ORA Clinical Research in North Andover.

 

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