Seasonality of Viral GI in the US elderly

Seasonality of viral gastroenteritis has been documented in developing and developed countries. The studies focused primarily on children; data on elderly are lacking. We abstracted hospitalization records from the Centers of Medicare and Medicaid Services, associated with �enteritis due to specified virus� (ICD-9-CM-008.6), infections due to �other organism not elsewhere classified� (ICD008.8), �ill-defined intestinal infections� (ICD009), and �other and unspecified noninfectious gastroenteritis and colitis� (ICD558.9) among patient ≥65 year-old in each state from 1991�2004. Seasonal time to peak was estimated with harmonic regression adapted for Poisson-distributed outcome, presented as daily time series. Of 1.67 millions records of gastrointestinal infections (ICD001�009), 0.31 millions (18.9%) were due to other unclassified organisms, 76,993 cases (4.6%) were due to ill-defined intestinal infections, and only 7,810 cases (0.5%) were due to specified viruses. We found no uniform seasonal pattern in Ill-defined intestinal infections across all states. However, thirty-eight out of 51 states showed significant seasonality in enteritis due to specified virus (ICD008.6). Median time to peak was 56th (IQR: 46th, 77th) calendar day. The south-eastern part of the U.S. viral enteritis peaked the latest. All states showed a significant seasonality in both categories with poorly defined causal pathogens: ICD008.8 (Median & IQR: 33rd, 24th�38th) and ICD558.9 (Median & IQR: 58th, 47th�63rd). Their times to peak, though being three weeks apart, were highly correlated (r=0.57, p<0.001), indicating synchronization of events which are likely to reflect untested and/or undetected causal pathogens. Further study on testing and coding practices for viral infections is recommended.