The US incidence of CJD during 1979 through 2006 remained relatively stable at approximately 1 case per 1,000,000 persons; this incidence is similar to that reported in many other countries 
. In the present study, the large number of cases identified in ongoing US surveillance provides insights into the incidence of CJD by sex. These data demonstrate a majority (52.6%) of cases in females largely because of the higher number of women compared to men in the older age populations that experienced the highest CJD incidence rates. Incidence rates of CJD by sex were almost the same among persons <60 years of age and were higher among males relative to females as the age of the population groups increased. These findings are consistent with other studies that indicate a relative excess of cases among females 
but a higher incidence of CJD among the male population 
. The CJD incidence rates varied regionally, with the highest rate in the Northeast region and the lowest rates in the South and West regions. The low rate in the West is of particular interest due to the longstanding presence of chronic wasting disease (CWD) among cervids in parts of the region, particularly in Colorado and Wyoming.
Ongoing US surveillance data continue to demonstrate marked differences in CJD incidence by race and age. Similar to previous US reports 
, the age-adjusted incidence remains more than 2.5 times higher for whites than for blacks. Although the reasons for this disparity are unknown, it is possible that genetic differences and/or under-diagnosis among non-white patients could contribute. These continuing differences in incidence by race in the United States raise the possibility that the CJD incidence in countries where black populations predominate might be significantly lower than in countries where white populations predominate, an issue that deserves future study.
The incidence of CJD by age continues to show a pattern that is strikingly different from that reported for vCJD. Almost all vCJD cases, including all three US resident cases, died before 55 years of age. The US surveillance analysis illustrates that only about 11 percent of the CJD cases occurred in this younger US population. This relatively low proportion, and incidence rate, of CJD cases in this younger age group occurred despite an increased focus in US surveillance efforts on suspected cases in this age group since 1996 
. Cases of CJD <30 years of age in the United States remain extremely rare, with most of these cases being attributable to iatrogenic exposure or a genetic mutation.
The incidence of CJD dramatically increased with increasing age until it peaked in the 70–79 year age group. An explanation for the subsequent drop in incidence of CJD among those 80 years of age and older, a phenomenon consistently observed in earlier surveillance studies of CJD 
, remains uncertain. The rarity of CJD, however, in postmortem studies of possibly clinically unrecognized cases of CJD in the elderly has led to the suggestion that the decline in incidence among the elderly is real and unlikely an artifact of a varying sensitivity, by age group, of surveillance 
Since vCJD was initially recognized in the United Kingdom in 1996, CDC received reports of three subsequently confirmed cases of vCJD among US residents. The epidemiological data indicated that each of these cases was most likely infected in the United Kingdom (2 cases) and in Saudi Arabia (1 case) 
Through mid-2009, twenty cases of BSE were identified among cattle in North America, including three that were identified in the United States 
. In comparison, the United Kingdom reported more than 184,000 BSE cases as of September 30, 2008 and 168 cases of vCJD as of March 2, 2009 
. These BSE/vCJD data suggest that the greatest risk of vCJD in US residents will continue to be among persons who as a child or young adult consumed UK beef products during 1980–1996, the years when such products were most subject to BSE contamination 
. Such US residents would include those who were born and raised either in the United Kingdom or in another country where potentially BSE contaminated UK beef products were available for consumption. These persons would also include those who consumed such UK beef products as a child or young adult during visits abroad. The United States historically has imported few or no live cattle, beef products, or livestock nutritional supplements from the United Kingdom, and throughout the 1990s had banned the import of live ruminants and most ruminant products from known BSE countries 
. Because indigenous BSE cases in North America were initially documented in 2003 and have continued to occur through 2008, albeit in relatively low numbers 
, the results of ongoing US vCJD surveillance receives considerable attention, particularly among those concerned about the emergence of indigenous vCJD cases in the United States.
Limitations of using the US national multiple cause-of-death data include possible coding and reporting discrepancies, and misdiagnosis of CJD as a cause of death. However, the use of national death certificate data has been found to be a reasonably sensitive (≥80%) method compared to more active methods of CJD case ascertainment 
. Furthermore, the US prion disease surveillance includes activities carried out by the NPDPSC. The NPDPSC, established by CDC in collaboration with the American Association of Neuropathologists, makes prion disease testing available free-of-charge to US physicians that can help to improve the accuracy of prion disease diagnoses 
. This center can confirm or refute the presence of vCJD, and also detect other unusual or new prion diseases 
. The prion surveillance efforts further utilizes the laboratory test results in the investigation and identification of CJD among persons <55 years of age. Finally, the use of the SuperMICAR procedure beginning in 1999 (fully in 2003) enables detection of deaths with prion disease even if cause of death on the death certificate is miscoded (e.g., an inappropriate coding rule, a misinterpretation or a misreading of the certificate) 
The occurrence of CJD and vCJD continues to be an international and national concern. Ongoing CJD and vCJD surveillance in many countries of the world, including the United States, remains critical for determining the extent to which the agents of classic and possibly atypical BSE may cause disease in humans 
. Physicians and health care workers in the United States are encouraged to indicate CJD, as appropriate, on death certificates for all their patients who die with CJD or vCJD. In addition, health care workers who provide care to patients with suspected or clinically diagnosed CJD or vCJD should discuss possible options for autopsy with their local and state health department and the NPDPSC. Brain tissue specimens obtained by autopsy from these patients may be submitted to the NPDPSC for further analysis and confirmation of the CJD diagnosis. The ongoing prion disease surveillance and diagnostic testing is important for monitoring the stability of the CJD incidence rates, and detecting possible occurrences of vCJD and other new prion diseases in the United States.