MINI-SYMPOSIUM: RESPIRATORY VIRUSES – PART I
Epidemiology and seasonality of respiratory tract virus infections in the tropics

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Abstract

Acute viral respiratory tract infections are a significant cause of morbidity worldwide. Information on the epidemiology and seasonality of these infections is important in planning vaccination and treatment strategies. In temperate climes, there are distinct seasonal peaks in the winter months. This paper reviews the seasonal trends of respiratory viral infections in the tropics. Despite the absence of a winter season, consistent seasons of infection, albeit less distinct, have been observed. With few exceptions, respiratory syncytial virus and influenza infections have been observed mainly during the rainy seasons in Asian, African and South American countries.

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INTRODUCTION

Respiratory tract infections (RTI), a large proportion of which are viral, account for the majority of acute illnesses at all ages worldwide.1., 2. They are a significant cause of morbidity and mortality at the extremes of age and in high-risk individuals. In temperate regions of the world, there is a clear seasonal variation in their occurrence, with peaks during the cold winter months. This variation is less apparent in the tropics, where there is less fluctuation in ambient temperature.

SEASONALITY OF RESPIRATORY VIRUSES IN THE TROPICS

One of the earliest studies that suggested a seasonal pattern to respiratory virus infections in the tropics was carried out on an isolated island in the West Indies.5 In that study, the authors examined the incidence of “colds”, which were essentially acute upper respiratory tract infections (URTI). They studied the entire population (about 700 individuals) prospectively over 12 months and found evidence that an increased incidence of URTI was associated with a decrease in environmental

AGE AND SEX DISTRIBUTION OF RTIS IN THE TROPICS

RTIs are common in children, and they decrease with increasing age. This is similar in both temperate and tropical regions. It is clearly reflected in the comprehensive study of inpatients and outpatients with RTI in Taiwan. Of 523 virus-infected children, 32.5% were under the age of 1 year, 37.7% were aged 1–3 years, 17.2% aged 3–6 years, 7.3% aged 6–10 years and only 1.9% were above the age of 10 years.16 In Senegal, 64.1% of patients presenting to seven healthcare units with influenza-like

AETIOLOGY OF RTI IN THE TROPICS

Community-based studies with sampling for viral isolation provide the best information about the aetiology of RTI. Unfortunately, there is a paucity of such studies in the tropics. In the study from North-east Brazil, rhinovirus was found in 45.6% of the virus-positive samples, followed by parainfluenza (16%), enterovirus (15.8%), adenovirus (9.9%) and influenza (5.7%).25 Another study in Thailand32 found RSV, parainfluenza and adenovirus to be the main pathogens in acute RTI. Rhinovirus was

MORTALITY AND MORBIDITY OF RTI IN THE TROPICS

Improvement in healthcare and implementation of vaccination policies have somewhat lessened the scourge of infectious disease in many countries. Regarding viral RTI, it is difficult to assess mortality because of the lack of community-based studies, use of less-sensitive viral-isolation techniques and the presence of superimposed bacterial infection. For RSV infection, mortality in developed countries is estimated to be between 0.3 and 1%.36., 37. It is similarly low in Hong Kong (0.15%)10 and

CONCLUSION

Epidemiological differences exist for RTIs between tropical and temperate regions. There is an obvious seasonal pattern to RSV and influenza infections. The burden of these infections and the advent of new and often expensive treatments such as monoclonal antibodies make it imperative for healthcare providers everywhere to study the impact of respiratory viruses in their country and implement optimal preventive and treatment strategies.

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