mentintheplasmodiaisespeciallynoticeable),hemorrhages,necroticfocion
obturatedvesselssurroundedbyinflammatoryreactions(Du ̈rckgranulo-
mas).Furtherchangescanbefoundinthespleenandliver(forinstanceswel-
ling,hyperplasiaofphagocytosingcellscontainingplasmodiaandpigment),
heart,lungs,kidneys,andotherorgans.
Resistanceandimmunity.Certainpropertiesofbloodareresponsibleforin-
creasednaturalresistancetomalarialinfection.Forinstance,theintraery-
throcyticdevelopmentofP.falciparumisinhibitedinpersonswithvarious
hemoglobinopathies(HbS,HbE,HbF,HbC),inglucose-6-phosphatedehydro-
genasedeficiency(G6PDD)andb-thalassemia.Ontheotherhand,persons
withG6PDDaremoresensitivetocertainantimalarials(quinine,8-amino-
quinoline).PersonslackingtheDuffybloodgroupantigenareresistantto
P.vivax,butsusceptibletoP.ovale.Amilkdietpartiallyinhibitsthedevelop-
mentofmalarialparasitesintheRBCsbecauseofaresultingreducedsupply
ofp-aminobenzoicacid(vitaminH 1 ).Thisresultsinamildermalarialcourse,
e.g.,ininfants.
Inthecourseofamalariainfection,ahostimmuneresponsedevelops,
which,however,doesnotconfercompleteprotection,butrathermerely
raisesthelevelofresistancetofutureinfections.Accordingly,thecourse
ofmalariainfectionsislessdramaticinpopulationsofendemicareasthan
inpersonsexposedtotheparasiteslessfrequentlyorforthefirsttime.In
thesemalariousareas,childrenarethemainvictimsofthedisease,which
islessfrequentandtakesamildercourseinolderpersons.Infantsofmothers
whohaveovercomemalariausuallydonotbecomeillinthefirstmonthsof
lifeduetodiaplacentalantibodytransmissionandacertainlevelofprotec-
tionfromthemilkdiet.Ontheotherhand,childrenwithoutmaternalanti-
bodiescanbecomeseverelyilliftheycontractmalaria,sincetheirownim-
munedefensesaredevelopinggradually.Nonimmunetravelersfromnonma-
lariousregionsareatspecialriskofinfection.
Theimmunityconferredinhumansbyexposuretoplasmodiadevelops
graduallyandisspecifictothestrainsandstagesthatarecapableofantigen
variation.Aparticularlyimportantpartofthegeneralizedimmuneresponse
appearstobethecomponentinducedbyasexualbloodforms,whichconfers
aprotectiveeffectagainstnewinfections.Thespecialistliteratureshouldbe
consultedformoredetailsonthisaspect.Despitemanyyearsofintensive
effort,adecisivebreakthroughinthedevelopmentofmalariavaccineshas
notyetbeenachieved.
Epidemiology.Constantminimumtemperaturesof 16 – 188 C(optimum: 20 –
308 C)andhighhumidityforseveralweeksarepreconditionsforvectoral
transmissionofmalaria.Furtherrequirementsfortheplasmodialcycleare
anepidemiologicallyrelevantparasitereservoirinthepopulationandthe
presenceofsuitablevectors.530 9 Protozoa9
Kayser, Medical Microbiology © 2005 Thieme