Editorial by sdsdfqw21


									Editorial                                                    when excess cas es are imminent),
Malaria is a m ajor publi c health                           s pecific (so that there are few “fals e

problem         for        m any         countries ,         alarms ”) and tim ely (s o that there is

particula rly         Africa.      While          the        adequate tim e to act).
endemicity         of       malaria             varies
enorm ous ly across regions , many                           Several s tudies have projected that

countries       have districts that are                      global climatic change will increas e

prone to periodic epidem ics.                                future m alaria transm ission in Africa,
                                                             but    the     link    between        malaria
Epidemics typically occur with little or                     epidem ics and clim ate is s till hotly
no warning, and in m os t cases in                           dis puted.
areas where the health sys tem is
unprepared            to    deal         with     the        This      edition     of   Epidemiological

emerging problem . These epidemics                           Comm ents explores this relationship,

are frequently triggered by climate                          with a close look at the findings of a
anom alies . Cons iderable efforts are                       number       of     s tudies   that     have

being made to prom ote, develop and                          inves tigated the ass ociation between
implement         early         warning           and        climate      change        and        malaria
detection       s ys tems          for     malaria           epidem ics .

epidemics in Africa. Ideall y, public
health and vector control workers
would have access to a s ys tem that
alerts      them           when      subs tantial
num bers of cases are expected, and
such alerts would be sens itive (s o
that alerts are reliably generated

Malaria Epide mics-Predicting                      (Patz et al., 2002). Human health is
                                                   expected to be affected by clim ate
the Unexpected
                                                   change,        both         directly      through
1.       Introduction
                                                   increased m ortality from                 extrem e
Hum an malaria, a paras itic dis eas e
                                                   temperatures and weather events ,
transmitted exclusively by bites of
                                                   and also indirectly, through effects
infected fem ale m osquitoes of the
                                                   on morbidity and m ortality related to
genus Anopheles, is one of the
                                                   changes         in     food           production,
world’s m os t serious dis eases . An
                                                   exacerbated                 air          pollution,
es timate     by    the    World     Health
                                                   demographic           dis placement,             and
Organization is that in Africa, malaria
                                                   changes        in     the         distribution     of
mortality in young children almos t
                                                   biological organisms that transm it
doubled from the 1980s to the 1990s
                                                   vector-borne diseas es (McMichael et
(WHO, 2003). The dis ease causes
                                                   al., 1996). Thes e diseases , becaus e
some 3000 deaths each day and
                                                   of the dependence of the vectors
impos es huge losses in economic
                                                   and pathogens on climatic factors ,
productivity (Sachs and Malaney,
                                                   are    expected             to       change        in
                                                   dis tribution and intens ity. Malaria is
Meanwhile,         there     is     growing        one of the vector-borne diseas es
cons ensus         that     anthropogenic          that is expected to be mos t s ensitive
actions     will   change     the    earth’s       to long-term environmental change.
clim ate in the near future and that
this may already be happening. The
                                                   2.     Malaria and climate
Intergovernmental Panel on Climate
                                                   The im portance of climate as a
Change (IPCC, 1996) predicts a 1-
                                                   driving force of malaria transmiss ion
3.5°C ris e in global m ean surface
                                                   has been known s ince the earlies t
tem perature by 2100, and many
                                                   days of research on this devastating
scientists predict that the effects on
                                                   paras itic dis ease. However, it is only
the hydrological cycle will create
                                                   with the advent of effective weather
more extrem e weather events such
                                                   forecas ting        techniques          that     this
as hurricanes, floods and droughts

knowledge can now be im plem ented                    s ince   the   gonotrophic      cycle    is
num erically. The three m ain clim atic               s hortened, (c) mos quitoes survive
factors     that     affect    m ala ria    are       long enough to acquire and transm it
tem perature,         precipitation         and       the      paras ite       (Table         1).
relative hum idity.                                   Temperatures lower than 16 ºC or
                                                      higher than 30ºC have a negative
2.1       Temperature and malaria                     impact    on   the     growth     of    the
Tem perature         and       malaria      are       m osquitoes    (Yang     and    Ferreira,
pos itively correlated. The higher the                2000).
tem perature, (to a certain degree),
the higher the risk and rate of                       The duration of the s porogony cycle
malaria. Tem perature determines the                  in the mid-gut of the mos quito als o
anopheline         m osquito’s         feeding        depends on tem perature and on the
intervals , population intensity, and                 s pecies of the paras ite the m os quito
longevity, as well as the sporogenic                  is carrying. The sporogony cycle on
cycle of the paras ite in the m os quito              average las ts about 10 days , but
(Gillies and De Meillon, 1968).                       s hortens as temperature increas es
                                                      (Table 1).
Once       fem ale     adult        mos quitoes
emerge, they look for a blood meal,
and in the process ingest malaria
paras ites with the blood. The feeding
frequency of mos quitoes increases
with      tem perature,        res ulting    in
increased proportions of infective
mosquitoes . Tem peratures within the
range of 20ºC to 30ºC affect malaria
transmiss ion in s everal ways : (a)
developm ent         of       Anopheles      is
shortened; (b) biting capacity of
fem ale m os quitoes           is    increased

Table 1: The effect of mean tem perature on the duration of the mosquito’s life
cycle and sporogonic cycle and its effect on the am ount of lead time from the
availability of breeding sites to the occurrence of m alaria cases
Weather factors            Stages and duration of mosquito life cycle and sporogony cycle affected
                                                             be w eather factors
        Mean                 Mosquito lifecycle                       Sporogony                      Incubation
      temperature                                                                                     period in
                           Larva            Adult        Adult first bite          infectious        human host
                                   (days)                             bite (days)
           16                       47                                      111
           17                       37                                      56
           18                       31                                      28
           20                       23                                      19
           22                       18                                      7.9                      (10-16 days)
           30                       10                                      5.8
           35                       7.9                                     4.8
           39                       6.7                                     4.8
           40                       6.5                                     4.8
Source: T eklehaimanot et al., 2004a

2.2        Precipitation and malaria                          on the local habits of mos quitoes (Bi
Anopheline mos quitoes breed in                               et     al.,         2003).    For      example,
water habitats, thus requiring jus t the                      Anopheles               funestus,         breeds
right amount of precipitation in order                        wherever             there        is   sufficient
for m osquito breeding to occur. They                         permanent water such as swam ps ,
are us ually found in regions with an                         rice fields , and in s om e cas es it can
annual average rainfall of 1100mm                             be found breeding in wells and
to 7490 mm . However, the effect of                           hous ehold containers (Gillies and De
rainfall        on   the     transmiss ion          of        Meillon, 1968) whereas fres hwater
malaria is very complicated varying                           m embers of the Anopheles gamb iae
with the circums tances of particular                         complex do bes t in small s unlit pools
geographic regions and depending                              (Gillies and De Meillon, 1968).

2.3      Relative    humidity        and       epidem ics        are     caus ed     by
malaria                                        m eteorological     conditio ns ,   which
Relative hum idity determines the life         create temporary epidem ics which
span of the mos quito. High relative           only las t one s eason of transmission.
hum idity lengthens the life of the            Type II epidem ics are caused by
mosquito.     The    lifespan   of   the       lands cape changes that create a
mosquito is an im portant factor in the        new equilib rium level of endem icity
developm ent process of the malaria            and type III epidem ics are caused by
paras ite in the vector. Hence, high           interruptions in measures that were
relative humidity allows the parasite          controlling m alaria .
to complete the necessary life cycle
so that it can transmit the infection to       Figure 1. Malaria risk across sub-
several pers ons (Mouchet et al.,              Saharan Africa

3.       Epidemics
Malaria     epidem ics   are    reported
frequently and often cause high
morbidity and mortality. Figure 1
shows malaria ris k areas across sub-
Saharan Africa. It is es timated that
110 million Africans live in areas
prone to m ala ria epidem ics (WHO,
2003). Thes e epidemics are caused
by a dis turbance of the equilibrium
between hos t, parasite and vector.
Najera et al. (1998) define three
different types of epidemics . Type I

3.1     Studies          of        malaria        devas tating epidem ic of m alaria in
epidemics                                         South Africa in the late 90’s was als o
Over     the   pas t     two      decades ,       linked to high rainfall following years
epidemics of Plasmodium falciparum                of drought (WHO 2001). In 2000, a
malaria, often with high case fatality            total of 64622 cases and 458 deaths
rates , have been comm on in areas                were       reported       to      the   National
of unstable transm ission in Africa.              Departm ent of Health, representing
These uns table areas include thos e              the highes t malaria epidemic the
where transmission is limited by                  country      had         ever     experienced.
rainfall (e.g. Sahel) or by both (e.g.            Although          this      epidem ic      was
the     Southern       African    highland        associated with high rainfall, there
plateau) (WHO, 2001).                             were       several        other     contributing
                                                  factors : (a) the growing res is tance of
Three    s tudies   of    epidemics      of       m alaria     paras ites        to anti-malarial
malaria in different parts of the Eas t           drugs        es pecially            chloroquine
African highlands have found that                 (Hans ford, 1989; Nuwaha, 2001) and
increased malaria incidence was                   (b) the resis tance of mos quitoes to
linked to climatic changes ; although             ins ecticides ,      in     particular     DDT
one of the s tudies als o claimed that            (Figure2).
increasing drug resis tance had an
effect (Githeko, 2001, Zhou et al.,
2004, Malakoti, 1998).           In another
study of m alaria in the African
highlands , Hay et al. (2002) found no
significant change in climate at four
locations where malaria incidence
has been increasing s ince 1976. An
epidemic in Ethiopia was attributed
to higher temperatures , rainfall and
relative hum idity than in previous
years      (Woube,        1997).       The

Figure 2: Annual m alaria notifications from 1990-2005

                     70000                                                                                                                         500

                                                                                                                                                         Number of deaths
   Number of cases

                     60000                                                                                                                         400
                     40000                                                                                                                         300
                     30000                                                                                                                         200
                         0                                                                                                                         0
                                                                                     Year                                     Cases                Deaths

Source: Epidemiology Surveillance Directorate and Malaria Control Programme

There is an increas ing body of                                                               general reduction in malaria in the
literature s ugges ting that weather                                                          Tanzanian Highlands (Lindsay et al.,
associated with El Nino Southern                                                              2000).
Os cillation (ENSO) drives malaria
epidemics . In Venezuela, m alaria                                                            The res ults of all these studies
mortality and m orbidity have been                                                            reveal that although there is a link
reported to increase by 36.5% in the                                                          between clim ate and malaria, more
years following recognized El Nino                                                            research needs to be done on the
events (Barrera et al., 1999). The                                                            causes of epidemics or increas es in
heavy rains ass ociated with the                                                              m alaria          transm iss ion, taking into
1997/8 El Nino res ulted in increased                                                         account all the factors that could be
malaria                      in          the           south-wes tern                         relevant to m alaria. Furtherm ore,
Highlands in Uganda (Kilian et al.,                                                           assess ing the impact of climate on
1998).                   Malaria                  epidemics                  in               m alaria is difficult becaus e of the
Columbia were als o proposed to be                                                            high spatial and tem poral clim ate
associated                         with          ENSO                 cycles                  variability and the lack of long-term
(Poveda et al., 2001). Another s tudy                                                         m alaria data.
however,                     reported                  the      opposite
effect, where high rainfall ass ociated
with the 1997/8 El Nino led to a

3.2       Prevention and control of                    s ufficient    information about          pas t
epidemics                                              events is available, ii) the information
The       mos t     im portant      factor    in       can be quantified num erically and iii)
reducing the impact of an epidemic                     aspects of the pas t pattern are likely
is    a tim ely res ponse in               which       to continue into the future. One of
effective     control        m easures       are       the     obs tacles      in   the    practical
undertaken as s oon as the epis ode                    developm ent of local MWES is a lack
has been detected. Recent res earch                    of     long-term         reliable    malaria
has       found       that     rainfall      and       m orbidity data and repres entative
tem perature es tim ates would be able                 climatic data. Countries s uch as
to provide us eful epidemic early                      Botswana, Swaziland and to a less er
warning information (Teklehaimanot                     extent Madagascar, Namibia and
et al., 2004a and Teklehaimanot et                     Tanzania have success fully begun to
al., 2004b). Cons equently rainfall                    use      rainfall,      tem perature      and
and tem perature are s ome of the                      population vulnerability as indicators
essential          elem ents         for     the       for    early     warning       of    malaria
developm ent          of     an      integrated        epidem ics .
Malaria Early Warning Sys tem for
sub-Saharan Africa, as outlined by                           • Long range forecasting
the       World     Health        Organization,        Long-range forecas ting is based on
(WHO 2001; WHO 2003).                                  ENSO          indices        and       clim ate
                                                       forecas ting. For areas in which future
3.2.1 Malaria              Early     Warning           climates can be predicted, such
Systems (MEWS)                                         m eas urements can broadly forecas t
      •   Early warning                                m alaria epidemic ris k m onths in
Early warning s ys tems attem pt to                    advance (WHO, 2003). In many
predict epidem ics before unusual                      countries      increases       in    malaria
transmiss ion activity begins and are                  transmiss ion occur in cyclic periods
usually           bas ed       on          known       of 2 to 7 years . In Colombia, India,
meteorological factors (WHO, 2003).                    Madagas car and Venezuela, thes e
Prediction is possible only if i)                      changes have been linked to altered

weather conditions during the ENSI                    highlight the importance of other
cycle (Boum a et al.,1997). Practical                 factors s uch as declining health
challenges in the use of long-range                   s ervices , drug resis tance, ins ecticide
clim ate forecas ting for prediction of               resistance     as      well       as       local
malaria epidemics are (a) the need                    environm ental changes (Mouchet et
to identify climate factors that favour               al., 1998; Malakoti et al., 1998).
epidemics and subs equently mark                      Undoubtedly, a m ix of all thes e
out geographic regions where long-                    reasons is behind the ris e in malaria
range forecasting is feas ible and (b)                cases .
the interplay of climatic factors with
other factors s uch as population                     It is agains t this background and
immunity.                                             experiences gained from previous
                                                      epidem ics that res earch efforts into
4.     Discussion and Conclusion                      the des ign of predictive tools that
The expanding res earch interes t in                  can be used operationally by health
the health cons equences of climate                   s ervices to improve m alaria epidem ic
change has res ulted in a number of                   control    strategies         have        been
papers citing this process              as a          heightened. Of critical significance is
significant contributor to increased                  the development MEWS. However,
malaria      transm iss io n.     Increasing          given the com plex nature of malaria
tem peratures       and rainfall      (as    a        epidem ics ,        the           s uccess ful
cons equence of         clim ate change)              implementation      of     this    tool     will
have been argued as the bes t                         depend         on         m ultidis ciplinary
explanatory      variables       ass ociated          cooperative      res earch          involving
with m alaria cas es . However, doubts                researches      from      the      field      of
over   the     importance        of   climate         epidem iology, paras itolo gy, botany,
change as a trigger factor for recent                 m icrobiology and clim atology.
epidemics have been put forward by
others ,      who       challenge           the
assum ptions      behind        the malaria
transmiss ion       models       used       and

                                                      factors .   Tropical     Medicine    and
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