“Negli ultimi 25 anni la malattia è diventata endemica in oltre 100 Paesi. Nell’Africa sub-sahariana, che con il 90 per cento dei decessi detiene il record mondiale, la malaria è la prima causa di morte e il numero dei casi è aumentato del 40 per cento negli ultimi 25 anni. […] Tutti parlano di Aids o di tumore, ma il vero problema mondiale è lui, il Plasmodium falciparum, che nessun vaccino, nonostante gli scienziati ci lavorino da decine di anni, è ancora riuscito a sgominare.” 

Massimo Alberizzi, Corriere della Sera.

La Malaria è una parassitosi, ossia una patologia provocata da un’infezione per mezzo di un parassita. In questo caso i parassiti coinvolti fanno parte della famiglia dei protozoi, appartenenti al genere Plasmodium. La Malaria è una patologia che nell’uomo è causata da quattro specie diverse di Plasmodium: falciparum, vivax, ovale, malariae.

Si tratta di una patologia molto diffusa nelle zone tropicali e subtropicali. Nel bacino Mediterraneo però si sono presentati, nel corso degli ultimi anni, diversi casi autoctoni dovuti al cambiamento climatico, alla migrazione di insetti vettori ed all’ingresso, nei paesi di riferimento, di soggetti contagiati provenienti da zone endemiche. Le diverse specie di Plasmodio che colpiscono l’uomo sono distribuite in maniera differente a livello geografico in base alle loro caratteristiche come ad esempio la capacità di adattarsi a determinati climi piuttosto che ad altri. Nello specifico è interessante la differenza che esiste tra Plasmodium falciparum e Plasmodium vivax: nel primo caso lo sviluppo del parassita è tipico delle zone come America Latina e Sud-Est Asiatico, mentre nel secondo caso il parassita ha la capacità di sopravvivere anche a temperature più miti e quindi di diffondersi in zone differenti.

La trasmissione all’uomo di questi parassiti avviene attraverso la puntura di zanzare femmine del genere Anopheles, solo le femmine infatti sono ematofaghe e si cibano di sangue umano. Questi insetti vengono definiti vettori biologici in quanto, oltre a trasportare il parassita e trasmetterlo ad un altro soggetto, permettono lo sviluppo di una parte del ciclo vitale del parassita al loro interno.

Ciclo vitale del Plasmodio della Malaria.
Ciclo vitale del Plasmodio della Malaria.

La zanzara Anopheles, dopo aver punto un soggetto infetto, diventa vettore del parassita. Nell’intestino dell’insetto infatti avviene la riproduzione del parassita che dopo una serie di fasi di replicazione e sviluppo formano gli sporozoiti che vengono convogliati fino alle ghiandole salivari dell’insetto. Il parassita verrà espulso attraverso la saliva dell’insetto nel momento in cui pungerà l’ospite infettandolo.
Gli sporozoiti, una volta in circolo nel sangue umano, giungono nel fegato e cominciano a replicarsi. A questo punto si forma lo schizonte, ossia l’epatocita ricolmo di Plasmodi della Malaria.

Ci sono alcune specie di Plasmodio che tendono a rimanere latenti all’interno del fegato anche per alcuni anni, in questo caso si parla di ipnozoiti. Una volta giunto il momento propizio, i parassiti vengono rilasciati dal fegato (merozoiti) e raggiungono gli eritrociti (trofozoiti) nei quali andranno a replicarsi nuovamente. La replicazione del parassita all’interno degli eritrociti porta ad un’alterazione strutturale e funzionale della cellula che in primo luogo rilascia i Plasmodi che andranno ad infettare altri globuli rossi causando episodi febbrili e secondariamente, portando ad un ingrossamento splenico.

Rilascio del parassita da parte di eritrociti infetti.
Rilascio del parassita da parte di eritrociti infetti.

Il ciclo vitale del parassita all’interno dell’organismo umano è importantissimo dal punto di vista clinico e diagnostico. Il paziente infatti mostrerà differenti manifestazioni cliniche in base allo stadio del ciclo vitale e alle caratteristiche antigeniche del parassita. Tipicamente i pazienti affetti da Malaria presentano febbre associata a brividi e sudorazione, cefalea, mialgia, vomito, diarrea e prostrazione.

La febbre è una condizione tipica del quadro clinico dei soggetti affetti da Malaria in quanto dovuta alla fuoriuscita del parassita dai globuli rossi. L’episodio febbrile è caratteristico di ogni tipo di Plasmodio. Ad esempio i Plasmodi falciparum, vivax e ovale causano una febbre terzana che si presenta quindi ogni tre giorni, mentre il Plasmodio malariae causa una febbre quartana ossia ogni quattro giorni. Questo è dovuto ai diversi tempi del ciclo vitale di ogni specie considerata.
L’aggressività della patologia è inoltre determinata non solo dallo stadio del ciclo vitale ma anche dalle caratteristiche antigeniche specifiche del parassita che vengono espresse sulla superficie eritrocitaria (VSA o Antigeni Variabili di Superficie). Esistono ad esempio antigeni che permettono l’unione dei globuli rossi tra loro o contro l’endotelio dei vasi causando l’occlusione del microcircolo e tutto ciò che ne consegue come l’insufficienza renale o quella che viene definita Malaria Cerebrale.

Striscio e goccia spessa di sangue periferico.
Striscio e goccia spessa di sangue periferico.

La diagnosi di malaria deve essere fatta nel minor tempo possibile per permettere un trattamento efficace. L’analisi diretta del caso è l’unica diagnosi utile e consiste nella ricerca del parassita attestandone la sua reale presenza allestendo vetrini per analisi parassitologica. Viene eseguito uno striscio di sangue capillare o periferico conservato con EDTA (anticoagulante chelante del calcio) e colorato con Giemsa, il colorante d’elezione per il tessuto ematico. Oltre a questo tipo di ricerca è necessaria l’analisi microscopica su goccia spessa di sangue che consiste nello spargere qualche goccia di sangue sulla superficie del vetrino ottenendo la rottura dei globuli rossi per azione meccanica e quindi l’eventuale rilascio dei parassiti sparsi nel campione.
La diagnosi può inoltre essere fatta anche grazie all’utilizzo di test rapidi che permettono di procedere subito con la terapia migliorando la prognosi del paziente anche se risultano comunque poco sensibili per la ricerca di alcune specie di Plasmodio.

La terapia preventiva e curativa della Malaria si effettua somministrando ad esempio derivati dell’artemisinina o altri farmaci specifici. E’ molto importante anche la profilassi ambientale che prevede il prosciugamento delle paludi, l’impiego di insetticidi, l’eliminazione dei ristagni d’acqua e la profilassi individuale generale e farmacologica soprattutto per i soggetti non immuni che si recano nelle zone endemiche.

Esistono vaccini che agiscono durante la fase pre-eritrocitaria del ciclo vitale di alcune specie di Plasmodio ma con una scarsa efficacia, in quanto l’immunità data sia da infezioni precedenti che dai vaccini è di tipo transitorio e di durata relativamente ridotta.

E’ vitale quindi conoscere i reali rischi di infezione e le problematiche verso le quali si potrebbe incorrere.
Prevenire è meglio che curare.

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