To study differences in the presentation of laboratory-confirmed acute viral respiratory infections, especially gastroenteritis symptoms, according to age and causative viral agent during five influenza seasons (1999 to 2004.).

Five influenza seasons (1999-2000 to 2003-2004) were studied. Data from samples taken at 24 primary care sentinel centres for influenza surveillance distributed throughout Catalonia (Spain), which were positive for any of the viruses studied (influenza virus A, [IVA], influenza virus B [IVB], syncytial respiratory virus [SRV], adenovirus [ADV] and parainfluenza 1,2,3 viruses), were analyzed. The following clinical variables were evaluated: sudden onset, fever, respiratory symptoms, sore throat, malaise and gastrointestinal symptoms. These variables were studied for each virus and for two age groups: age 14 years or younger and older than 14 years. Odds ratios (OR) and 95% confidence intervals (CI) were calculated.

Of a total of 1,591 samples tested during the influenza seasons studied, 42.68% (679) samples belonged to subjects aged ≤ 14 years and 57.32% (912) to subjects aged > 14 years. There were 286 IVA (166/286 [58.04%] in subjects aged ≤ 14 years and 120/286 [41.95%] in subjects aged > 14 years), 107 were positive for IVB (78/107 [72.90%] in subjects aged ≤ 14 years and 29/107 [27.10%] in subjects aged > 14 years), 53 SRV (39/53 [73.58%] in subjects aged ≤ 14 years and 12/53 [22.64%] in subjects aged > 14 years) and 40 ADV (36/40 [90%] in subjects aged ≤ 14 years and 4/40 [10%] in subjects aged > 14 years). The differences in symptoms between the two age groups could be compared for IVA: fever (OR = 16.536; 95% CI = 4.893-55.887; p = 0.001) and gastrointestinal symptoms (OR = 4.664; 95% CI = 1.569-13.868; p < 0.03) were significantly more frequent in subjects aged ≤ 14 years, while malaise (OR = 0.048; 95% CI = 0.026-0.089; p = 0.001) was more frequent in subjects aged > 14 years.

The results indicate that acute viral respiratory infections always present with fever in children, whatever the virus, but not in adults. Gastrointestinal symptoms were more strongly associated with IVA and IVB and these symptoms were most frequent in children aged under 5 years.