from the national laboratory database , we downloaded laboratory reports to phls  cdsc of cryptosporidium oocysts identified in fecal smears with dates for obtaining specimens between january 1 , 1991 , and december 31 , 2001 . the download was performed on june 20 , 2002 , to ensure that all data for 2001 were complete . the data were aggregated into counts by week the specimen was obtained . to make all weeks exactly 7 days long , we excluded reports with specimen dates on december 31 of every year and on december 30 of every leap year . intervals between date of illness onset , specimen date , and reporting date were reviewed to assess consistency of reporting over time . the exposure interval for the fmd epidemic was defined as weeks 839 of 2001 , which corresponded with the first fmd case on february 20 and last case on september 30 ( 13 ) ( table 1 ) . we plotted the series of cryptosporidium reports for england and wales over time and reviewed data from wales and each region in england individually . the weekly counts of reports were used as the dependent variable in a negative , binomial regression model . explanatory variables were region , season ( weeks 17 , weeks 839 , or weeks 4052 ) , year ( 19912001 ) , fmd interval ( weeks 839 in 2001 ) , and a binary variable for a batch reporting error in 1995 . we used negative binomial regression rather than poisson regression because the variance of the count was not approximately equal to the mean of the count ( 14 ) . an estimate of the reduction in the reports of cryptosporidiosis during the fmd interval was obtained from this model ; this estimate was derived from the ratio of the mean count within weeks 839 in 2001 to the mean count during the same interval in all other years , adjusted for all other explanatory variables . for each year considered , the mean counts a , b , c , and d were summarized in a 2 x 2 table ( table 2 ) . the rate ratio for the fmd interval = ( a / b)/(c / d ) . a similar model was used to estimate a rate ratio for the fmd interval in each region separately and to estimate rate ratios for weeks 839 in 1991 , 1992 , and all other years . fecal specimens positive for cryptosporidium species received by the phls cryptosporidium reference unit in swansea with dates of specimen between january 1 , 2000 , and december 31 , 2001 , were genotyped by using polymerase chain reaction and restriction fragment length polymorphism analysis of a region of the cryptosporidium oocyst wall protein gene ( 15 ) . monthly rainfall data for england and wales were also examined for trends during the surveillance period . the data set comprised 51,322 reports of crytosporidium . we concluded that an outlying count of 387 reports with a specimen date of october 6 , 1995 , was a batch - reporting error of cases from a large waterborne outbreak in torbay , in southwest england , which occurred during july rates were generally < 5/100,000 in london ; in several recent years , the highest rates occurred in northwest england ( table 3 ) . the timing of clearly identifiable spring and autumn peaks of cryptosporidium laboratory reports was fairly consistent from year to year ( figure 1 ) . weeks 839 of each year included all spring peaks and the first half of all autumn peaks . however , spring and autumn peaks were not always clearly identifiable in each year . the year 1993 was notable for the absence of an autumn peak ; in 1996 and 1997 , identifying either a spring or autumn peak was difficult ( figure 1 ) . in 2001 , weekly counts remained lower than in previous years until approximately week 35 , when an autumn peak comparable to previous years was observed ( figure 1 ) . communicable disease surveillance centre , by specimen week , england and wales , 19912001 . reporting artifact on october 6 , 1995 not shown . by modeling counts , a significant decrease in overall seasonal and yearly associations weeks 839 in 1992 showed an estimated 22% decrease , whereas weeks 839 in 2001 , corresponding to the fmd epidemic , showed an estimated 35% ( 95% confidence interval [ ci ] 20% to 47% ) decrease in england and wales as a whole ( p=0.001 ) . we estimated that the fmd epidemic interval was associated with some reduction of human cryptosporidiosis in all regions of england and in wales , but the largest association was in northwest england , which showed decrease of approximately 63% ( 95% ci 31% to 80% ) ( p=0.001 ) ( table 5 ) . the age distribution for persons with reported cryptosporidiosis for weeks 839 in 2001 was similar for the same time of year in each of the preceding 10 years . a history of foreign travel preceding diagnosis was given in 5% of reports and showed a single autumn peak in weeks 3141 of each year , while those reports from case - patients without a history of foreign travel showed both spring and autumn peaks ( figure 2 ) . communicable disease surveillance centre , by specimen week , including reports with and without foreign travel , england and wales , 19912001 . genotyping results were available for approximately half of the reported cases of cryptosporidiosis made to phls  cdsc in 2000 and 2001 . when specimens from persons with a history of recent foreign travel were excluded , the proportion of genotype 2 cases was generally higher in the first half of both years ( figure 3 ) . in weeks 2124 of 2001 ( may 21june 17 ) , the proportion of genotype 2 cases was significantly lower than for the same weeks of 2000 ( figure 3 ) . in weeks 839 as a whole , the proportion of genotype 2 cases was significantly lower in 2001 than in 2000 ( 49% , 338/696 in 2001 ; 63% , 977/1,558 in 2000 ; p<0.00005 ) . laboratory isolates of cryptosporidium species , proportion of genotype 2 , by specimen week , england and wales , 2000 and 2001 . mean rainfall in england and wales between january and june ( to approximately week 26 ) was < 60 mm in 1992 and 1996 ( table 6 ) : in these years , the spring peak in cryptosporidium reporting was also below normal ( figure 1 ) . in contrast , rainfall in the first 6 months of 2001 was above the 19611990 average and showed a pattern more like that that observed in 1998 , 1999 , and 2000 : in these years , a spring peak in cryptosporidium reporting was not conspicuous ( figure 1 ; table 6 ) . including the 19611990 mean . cryptosporidium reports from england and wales decreased substantially during the fmd epidemic in livestock in 2001 , with a marked attenuation of reports in the first half of the year . modeling counts of cryptosporidium reports showed that the observed decrease was unlikely to be explained by seasonal and yearly associations . in none of the previous 10 years were deviations from overall yearly and seasonal associations as large as those estimated for weeks 839 in 2001 . for a number of reasons , this decrease can not be attributed to errors in transmission and entry of reports into the phls  cdsc database . counts returned to normal levels by about week 35 of 2001 , followed by an autumn peak comparable in size to that in most previous years . a similar pattern of decrease was not apparent for salmonella , campylobacter , or giardia reports , which were received and stored in a manner similar to those for cryptosporidium ( phls data ) . lag times between onset of illness , specimen collection , and reporting dates were stable throughout the data set , except for the single batch - reporting artifact in 1995 ( for which adjustment was made in the regression analysis ) . no change or disruption in reporting methods or data storage genotyping of specimens from 2000 and 2001 showed that most submitted were of genotype 2 ( livestock and human strain ) in the first half of each year , whereas most were of genotype 1 ( human - only strain ) in the second half of each year . during the fmd epidemic interval , the proportion of genotype 2 isolates tested was lower than that for the same time of year in 2000 . the presence of an autumn peak in case - patients reported to have recently traveled abroad , coincident with an autumn peak in case - patients not known to have recently traveled abroad , is consistent with substantial underreporting of travel abroad and the association of the autumn peak with such travel . conversely , the absence of a spring peak among case - patients with reported recent travel abroad suggests that the spring peak is predominantly due to exposures occurring within the u.k . the absence of clear spring peaks in human cryptosporidium reports in 1992 and 1996 may have been related to below average rainfall in england and wales in the first 6 months of these years . however , the rainfall levels in the first 6 months of 2001 were similar to those in 1998 , 1999 , and 2000 , years in which spring peaks were conspicuous . therefore , no strong evidence suggests that the exceptionally low number of reports observed during the fmd epidemic interval could be explained by below average rainfall . the low reporting rate for cryptosporidium in london may be explained by the historic shortage of public health laboratories in the capital . cdsc , examine all fecal specimens for cryptosporidium oocysts and report all positive results to phls recent surveys indicate that 20% of laboratories in the northwest region of england and wales and 40% of laboratories in the east and southeast regions of england continue to use varied criteria to select a subset of submitted fecal specimens for examination for cryptosporidium oocysts , and may not always report positive test results ( 17,18 ) : similar variation may be expected in other regions . however , we have no evidence to suggest that these laboratories systematically changed their practices during the study period . the recent comparatively high reporting rates of cryptosporidium infections in the northwest region can not be entirely explained by the fact that a higher proportion of laboratories now have policies requiring the examination of all fecal specimens for cryptosporidium oocysts and the reporting of all positive results . the northwest region of england has experienced comparatively frequent confirmed and suspected waterborne outbreaks of cryptosporidiosis ( 18 ) ; therefore , a genuinely higher incidence is a more likely explanation . throughout england and wales , a substantial decrease in reports was observed , coinciding with the start of the fmd epidemic in 2001 , followed by a return to normal levels by about week 35 . a systematic change in testing and reporting by over 200 laboratories does not explain this observation . on the whole , our results suggest that a decrease in genotype 2 cryptosporidium infection in humans was associated with a decrease in human exposure to reservoirs of infection in livestock in england and wales during the fmd epidemic interval . that the fmd epidemic interval was associated with a decrease in all english regions and wales , including london , may have been because visitors from throughout england and wales had decreased access to affected regions . that contamination of water supplies was decreased through removal of livestock from drinking water catchments by slaughter or containment elsewhere is also plausible . the fmd epidemic was estimated to have had the largest effect in northwest england , which is consistent with the particularly large change in animal husbandry and livestock numbers associated with the fmd epidemic in this region . livestock fecal contamination of an unfiltered surface water reservoir may have decreased in the english lake district , which serves approximately one - third of the population of the northwest region with drinking water ( 19 ) . water companies in england and wales have been required to conduct risk assessments of their water sources for cryptosporidium and to undertake real - time monitoring of treated water for oocysts at high - risk works since april 1 , 2000 ( 20 ) . however , introduction of this regulation was not associated with a decrease in cryptosporidium reporting between april 1 , 2000 , and the beginning of the fmd epidemic on february 20 , 2001 . the surveillance patterns observed suggest that exposure to livestock and their excreta may contribute a substantial fraction of human cryptosporidiosis in england and wales . our observations support continued concern over the presence of cryptosporidium oocysts in public water supplies , especially in northwest england , and suggest that policies and the economics for the management of water catchments and water treatment in england and wales , especially the northwest region , require further review . the impact of the fmd epidemic in livestock on cryptosporidium infection in humans can be characterized as complex . long - lasting changes to farming practices and restructuring of rural economies occurred and will continue . additionally , water companies continue to improve the microbiologic safety of public water supplies , supported by strict legal limits for cryptosporidium oocyst concentrations in treated water . whether the decline in cryptosporidium reporting coincident with the fmd epidemic will be sustained in future years will be interesting to observe . changes in livestock - mediated exposure to cryptosporidium would not correspond precisely with the interval between the first and last confirmed cases of fmd . a delay was expected between the start of the fmd epidemic in livestock and a change in livestock - mediated cryptosporidium exposure in humans and its consequent detection by the national laboratory surveillance system . key components of this delay include the incubation period of cryptosporidium in humans ; the amount of time before seeking medical attention , and the time required for giving a fecal test , examining the specimen , and reporting and entering positive test results into the national database . we expected the degree of change in livestock - mediated cryptosporidium exposure to vary by time and place because of variation in livestock densities , the intensity of animal culling , and differences in the containment of animals from traditional pastures between different areas of the u.k . nonetheless , the coincidence between the fmd epidemic and decline in human cryptosporidiosis is striking and suggests that the fmd epidemic in livestock has changed the ecology between humans , livestock , and cryptosporidium in england and wales . further studies to define the contribution of key components of the fmd epidemic on human cryptosporidium infection may be of value in appropriate geographic areas such as northwest england . such studies could include modeling the independent effect of changes in livestock densities , farm access , and rural access and adjusting for water supply to residences , changes in water treatment , and rainfall .
during the 2001 epidemic of foot and mouth disease ( fmd ) in livestock in england and wales , we discovered a corresponding decrease in laboratory reports of cryptosporidiosis in humans . using a regression model of laboratory reports of cryptosporidiosis 
 , we found an estimated 35% ( 95% confidence interval [ ci ] 20% to 47% ) reduction in reports during the weeks spanning the period from the first and last cases of fmd . 
 the largest reduction occurred in northwest england , where the estimated decrease was 63% ( 95% ci 31% to 80% ) . genotyping 
 a subgroup of human isolates suggested that the proportion of cryptosporidium genotype 2 strain ( animal and human ) was lower during the weeks of the fmd epidemic in 2001 compared with the same weeks in 2000 . 
 our observations are consistent with livestock making a substantial contribution to cryptosporidium infection in humans in england and wales ; our findings have implications for agriculture , visitors to rural areas , water companies , and regulators .