THALASSAEMIA IS A TROPICAL DISEASE

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T.R. Kotila

Department of Haematology, University College Hospital, Ibadan.

Abstract

Genes for thalassaemias, sickle cell disorders and Glucose-6- phosphate dehydrogenase (G6PD) deficiency are known to be associated with prevalent malaria infection. The prevalence in the heterozygote state for sickle cell anaemia (SCA), G6PD and alpha thalassaemia is between 25-30% in Nigerians but the prevalence for the beta thalassaemia trait (BTT) is low. Under-diagnosis of BTT may arise from the similarity in its clinical manifestation to that of SCA which is of high prevalence in Nigeria and secondly because the hypochromia and microcytosis associated with it may be misdiagnosed as iron deficiency anaemia. There is therefore the need to review this disorder in the light of the wide use of automation in processing a full blood count which will include red cell indices, a good screening method for the thalassaemias. This expectedly will aid easy and early diagnosis of the disorder.

Keywords: Thalassaemia, malaria, red cell indices, misdiagnosis.

Introduction

Tropical diseases are diseases that are prevalent or unique to the tropical and subtropical regions. Though there are many tropical diseases malaria remains the most common vector-borne disease that is widespread in the tropical and subtropical regions of the world. It is also the strongest known force for evolutionary selection in the recent history of the human genome1. Malaria is also the evolutionary driving force behind sickle cell disease, thalassaemia and Glucose-6- Phosphate Dehydrogenase (G6PD) deficiency. The global distribution of thalassaemia encompasses the major malaria prevalent regions of Africa, Asia and Mediterranean regions where malaria was once common. The distributions of the genes associated with malaria are similar in the heterozygote state among Nigerians. The prevalence for sickle cell disease, G6PD deficiency and alpha () thalassaemia in the heterozygote state is 25-30%^1-3, it is not until recently that the prevalence of beta () thalassaemia trait was found to be similar⁴. It is therefore likely that there is an under-diagnosis of the thalassaemia in the African setting mostly because of the lack of the required diagnostic facilities. This however, should not preclude physicians from entertaining the diagnosis, hence the need to discuss these disorders.

The distribution of á thalassaemia though worldwide is restricted in clinicaly severity. The clinically severe type which is associated with hydrops fetalis and HbH disease is found mostly in South East Asia and occasionally in the Mediterranean region. The deletional form in contrast to those due to point mutation is of lesser clinical severity and has a more globally distribution. The only type of thalassaemia found in Nigerians till date is the -^3.7deletion (5), recently 300 chromosomes were screened for á thalassaemia and only the -^3.7 deletion was detected with 42% being heterozygote and 9% homozygote (in press). The implication of this is that the  thalassaemia present in Nigerians is of little clinical significance. Beta thalassaemia on the other hand is almost always of clinical significance though the severity differs too and it is therefore classified clinically into thalassaemia major, intermedia and minor. If the prevalence of  thalassaemia is as high as recently stated⁴, then the disease and its complications are being misdiagnosed⁶. There is a need therefore to review this subject with a view to understand the subtle ways in which it may be misdiagnosed. This communication will discuss mostly the epidemiology, pathogenesis, clinical presentation and management of  thalassaemia. The diagnosis of the disorder was discussed alongside that of other haemoglobinopathies extensively in an earlier communication⁷

Epidemiology
It is estimated that 1.5% of the world’s population are carriers of  thalassaemia with an estimated 60,000 new carriers born each year⁸. Southeast Asia accounts for about 50% of the world’s carriers while Europe and the Americas jointly account for 10-13% of the world carriers⁹. Beta thalassaemia is widespread throughout the Mediterranean with uneven distribution in Greece and Italy, but it is less common at the western end of the Mediterranean and appears to be little in France except in those of Italian or Spanish descent10. The disorder is however common in the Middle East and west Asia, and it is probably the commonest inherited haemoglobin disorder in India. Sickle cell disease on the other hand is believed to dominate the haemoglobinopathies in south of the Sahara, while  thalassaemia is reported to be between 3-7% in most of North Africa¹⁰.