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AFRICA
CVJAFRICA • Volume 26, No 2, H3Africa Supplement, March/April 2015
Changing global and African epidemiology of
stroke
Stroke has attracted global attention, as one in six people will
develop stroke in their lifetime.
13,14
Stroke is a significant medical
and public health problem, with loss of productivity and burden
on families, caregivers and society.
13-15
The lifetime risk of stroke is
one in five women and one in sixmen.
13
Stroke is the most common
cause of acquired disability and the second most common cause
of death worldwide.
.
The World Health Organisation (WHO)
estimates for 2001 indicated that death from stroke and disability-
adjusted life years (DALYs) due to stroke was at least seven times
higher in low- and middle-income countries (LMIC) than in high-
income countries (HIC).
16
Recent incidence estimates
17,18
indicate
that whereas stroke incidence declined 12% in HIC, it increased
by 12% in LMIC over the last decade.
Africa, in epidemiological transition,
19,20
is currently faced
with an exploding but neglected burden of non-communicable
diseases (NCDs), including hypertension, diabetes mellitus and
dyslipidaemia, which often culminate in stroke. The recent
UnitedNations high-level meeting on the global burden of NCDs
highlighted their disproportionately high burden and stressed the
urgent need to tackle them, particularly in developing countries.
The burgeoning incidence of stroke in Africa is attributable
to rising cardiovascular risk profile, which is in turn driven
by epidemiological transition, an aging population, rapid
urbanisation and accompanying lifestyle changes. Africa, with
a current population of over one billion, has a stroke prevalence
rate of up to 963/100 000 population,
21
an incidence rate of up to
315/100 000 population and a three-year mortality rate as high
as 84%. About 3.2 million Africans develop incident stroke every
year.
15
Recent data from Nigeria, Tanzania and Sudan showed
that stroke was the leading cause of elderly medical admissions,
22
while up to 78% of neurological hospital admissions were due
to stroke.
23
The impact of this on mental capacity, quality of
life and economic productivity portends great danger for the
emerging economies of Africa.
24-26
Peculiarities of stroke in people of African
ancestry
Enhanced predisposition, different pattern of subtypes, worse
severity and often poorer outcome of stroke in people of African
descent is quite well established. According to data from the
INTERSTROKE study, ischaemic stroke accounts for 66%
while haemorrhagic stroke accounts for 34% in Africa, compared
to 91 and 9%, respectively for ischaemic and haemorrhagic
stroke in HIC. Ischaemic stroke subtypes diagnosed in African
populations were small vessel (27%), cardio-embolism (25%),
large vessel (14%), others (20%) and undetermined (14%).
27
Among sub-Saharan Africans, stroke affects a relatively
younger age group and productive workforce than in developed
economies.
27,28
Data from the USA suggest that African Americans
have a higher predisposition, worse severity and often poorer
outcomes compared to Caucasian Americans.
29,30
In a recent report
from the multi-ethnic South London Stroke Registry study, black
stroke survivors had worse cognitive outcome compared to other
racial groups.
31
Although, this may be due to socio-economic
differences, disparities in healthcare-seeking practices and
differential access to healthcare services, the influence of underlying
differences in genetic factors cannot be underestimated.
32,33
Genomics and health disparities
Genetics and genomics research offer insight into disparities
in the risk profile, phenotypes and outcome of diseases among
different populations as a result of accumulated small differences
in common alleles or rare variants, interactions among multiple
genetic loci and interactions between genes and environmental
factors, which may include cultural practices and health-seeking
behaviour.
34,35
The potential of treatment approaches tailored
to individual, unique genomic profiles represents a distinct
potential impact of genomics on improving health disparities.
Also, the globalisation of complex chronic diseases further
suggests that all populations are susceptible, and that variation in
rates may also be explained as a result of differential exposure to
environmental causes, including lifestyles, cultural practices and
health-seeking behaviours.
36
African human genomic variation
African populations present the highest genomic diversity,
the lowest levels and most divergent patterns of linkage
disequilibrium, as well as smaller haplotype block sizes across
human populations.
8,37
Although the human species is believed
to have originated from Africa about 200 000 years ago,
studies of genomic variation in Africa suggest that the present
pattern of variation within and between populations is a
product of several factors. These include demographic history,
population structure, diversities of geographical location,
language classification and different patterns of subsistence,
dietary differences, multiple migrations with accompanying high
levels of genetic admixture and survival related to exposure to
infectious diseases.
38,39
For example, Tishkoff and colleagues
8
identified 14 ancestral
population clusters in Africa with four predominant clusters that
broadly represent populations from major African geographical
regions and the four dominant African language families. These
are Niger-Kordofanian (spoken primarily by agriculturalist
populations located in large contiguous regions of sub-Saharan
Africa from West Africa to eastern and southern Africa), Nilo-
Saharan (spoken predominantly by pastoralist populations in
central and eastern Africa), Afro-Asiatic (spoken predominantly
by agro-pastoralists and pastoralist populations in northern and
eastern Africa), and Khoisan (a language family that contains
click consonants, spoken by hunter–gatherer San populations
in southern Africa as well as the Hadza and Sandawe hunter–
gatherers in Tanzania). The remaining 10 are mainly restricted
to specific geographic regions, languages, or in some cases,
individual populations.
More recently, Shriner and colleagues
9
analysed ancestry data
from 12 global and regional diversity projects with genome-wide
genotype data for 3 528 unrelated individuals from 163 samples
from around the world. They identified 19 ancestral components
with 94.4% of individuals showing mixed ancestry. Furthermore,
they validated the earlier findings of Tishkoff and colleagues
and identified an additional ancestral component in Africa, the
Omotic-speaking peoples of Ethiopia.
Our knowledge of African human genomic variation is
growing. This was previously limited by the small number of
African populations involved in landmark projects such as
the International HapMap project
40
and the more recent 1 000
Genomes project.
41
In these projects, participation was limited