Seasonality of ill-defined gastrointestinal infections: Can we guess what we do not know

Proper disease classification is indispensable to a functional surveillance system: well-specified outcomes allow health professionals to introduce focused and effective preventions. However, newly emerged diseases may render the current classification obsolete; rare or hard-to-test diseases may not justify the creation of a new code in the system. Such problems may have propagated disease codes labeled as “ill-defined” or “unclassified”. Using the records of the Centers of Medicare and Medicaid Services, we documented the proportion, annual rates, and seasonal peaks of hospitalizations from 1991 through 2004 involving gastrointestinal infections (ICD 001–009) among US elderly. We detected an overwhelmingly high proportion of the gastrointestinal diagnoses with unspecified nature (ICD 008.8 and 009, totally 64%). Attempt was made to decipher their temporal patterns by comparing them with the rest of the gastrointestinal illnesses. Seven out of nine major categories demonstrated significant seasonal oscillation (p<0.001). Gastrointestinal infections involving “other organism not elsewhere classified” (ICD 008.8) peaked at 32nd calendar day (95% CI: 31st–33rd), which was close to the peak of Norwalk virus (ICD 008.63, 28th calendar day, 95% CI: 23rd–35th). Ill-defined intestinal infections (ICD 009), peaked at 137th calendar day (95% CI: 126th–149th), was temporally close to the peaks of “other gram-negative bacteria” (ICD 008.47, 138th calendar day) and “other bacterial food poisoning”(ICD 005, 143rd calendar day). The need of a more comprehensive and descriptive diagnostic coding system should be considered.