Foot and Mouth Disease in Livestock and Reduced Cryptosporidiosis in Humans, England and Wales

William J. Smerdon, Tom Nichols, Rachel M. Chalmers, Hilary Heine, Mark H. Reacher

Disclosures

Emerging Infectious Diseases. 2003;9(1) 

In This Article

Methods

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 8-39 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 1-7, weeks 8-39, or weeks 40-52), year (1991-2001), FMD interval (weeks 8-39 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 8-39 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 8-39 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]. The proportion of isolates of genotype 2 was examined. Monthly rainfall data for England and Wales were also examined for trends during the surveillance period.

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