W O R L D H E A L T H O R G A N I Z A T I O N
ORGANISATION M O N D I A L E D E L A SANTÉ
V I R / R C / 7 4 . 2 3 ( A r b o ) E N G L I S H O N L Y
M E E T I N G O F D I R E C T O R S OF WHO R E F E R E N C E CENTRES FOR A R B O V I R U S E S , aiLAMYDIAE AND R I C K E T T S I A E G e n e v a , 1-5 A p r i l 1 9 7 4
E P I D E M I O L O G I C A L TRENDS OF C E R T A I N ARBOVIRUS D I S E A S E S YELLOW F E V E R IN WEST A N D CENTRAL A F R I C A
b y
D r Y. R o b i n , D i r e c t o r , WHO R e g i o n a l R e f e r e n c e C e n t r e f o r A r b o v i r u s e s , I n s t i t u t P a s t e u r , D a k a r , S e n e g a l
1, C e n t r a l A f r i c a 1.1 H i s t o r i c a l r e v i e w
To b e g i n w i t h , i t i s a s t r i k i n g f a c t t h a t t h e r e h a v e n e v e r b e e n i m p o r t a n t e p i d e m i c s o f y e l l o w f e v e r i n C e n t r a l A f r i c a . T h e m o s t t h a t h a s o c c u r r e d h a s b e e n a f e w g r o u p e d c a s e s i n Z a i r e i n 1 9 5 8 .
S i n c e 1 9 7 0 , t h e r e s e e m s t o h a v e b e e n r e s u m e d a c t i v i t y o f t h e v i r u s , a s r e p o r t e d f r o m C a m e r o o n ( 1 c a s e - 1 d e a t h ) a n d E q u a t o r i a l G u i n e a ( 4 c a s e s ) i n 1 9 7 0 , A n g o l a ( 6 5 c a s e s - 42 d e a t h s ) a n d Z a i r e ( 2 c a s e s - 2 d e a t h s ) i n 1 9 7 1 a n d a g a i n C a m e r o o n ( 2 c a s e s - 2 d e a t h s ) i n
1 9 7 2 .
O b v i o u s l y t h e r e a l n u m b e r o f y e l l o w f e v e r c a s e s i s v e r y m u c h h i g h e r t h a n t h e n u m b e r n o t i f i e d , b u t i t w o u l d s e e m t h a t n o i m p o r t a n t e p i d e m i c c o u l d h a v e o c c u r r e d u n n o t i c e d .
1.2 P r e s e n t s i t u a t i o n
D e s p i t e t h e i n s t i t u t i o n o f a s u r v e i l l a n c e s y s t e m i n 1 9 6 8 , n o t v e r y m u c h i s k n o w n a b o u t t h e p r e s e n t s i t u a t i o n .
1.2.1 S e r o l o g i c a l s t u d i e s a m o n g _ t h e _ h u m a n p o p u l a t i o n
S e r o l o g i c a l s u r v e y s h a v e b e e n c a r r i e d o u t i n s e v e r a l c o u n t r i e s o f C e n t r a l A f r i c a :
C e n t r a l A f r i c a n R e p u b l i c , C h a d , C a m e r o o n , a n d G a b o n ^' ^' ' ^' I'*' 1"^' W h i l e t h e s e s u r v e y s h a v e s t i l l n o t made i t p o s s i b l e t o o b t a i n a t r u e i d e a o f t h e c i r c u l a t i o n o f y e l l o w f e v e r v i r u s b e c a u s e o f v a c c i n a t i o n s a n d o t h e r i n f e c t i o n s c a u s e d b y G r o u p B v i r u s e s , t h e y a r e n e v e r t h e l e s s u s e f u l f o r a s s e s s i n g t h e d e g r e e o f r e c e p t i v i t y o f t h e p o p u l a t i o n s . A l t h o u g h t h e r e h a s n e v e r b e e n a r e a l e p i d e m i c , t h e s i z e o f t h e r e c e p t i v e p o p u l a t i o n m a k e s t h e d a n g e r o f o n e o c c u r r i n g e v e r p r e s e n t ^.
1.2.2 S e r o l o g i c a l s t u d ^ 3'''2Di_*ii:Ë_YÎEÎ2^E2*25
M i l l a n e t a l , r a r e l y f o u n d c a r r i e r s o f y e l l o w f e v e r a n t i b o d i e s i n s t u d i e s o f 6 9 4 s e r a f r o m w i l d a n d d o m e s t i c a n i m a l s : a f e w among s q u i r r e l s ( 2 / 6 8 ) a n d m o n k e y s ( 6 / 3 6 ) .
The issue of this document does not constitute formal publication. It should not be reviewed, abstracted or quoted without the agreement of the World Health Organization. Authors alone are responsible for views expressed in signed articles.
Ce document ne constitue pas une publication.
Il ne doit faire l'objet d'aucun compte rendu ou r é s u m é ni d'aucune citation sans l'autorisation de l'Organisation Mondiale de la S a n t é . Les opinions e x p r i m é e s dans les articles s i g n é s n'engagent que leurs auteurs.
V I R / R C / 7 4 . 2 3
( A r b o ) page 2
In t h e C e n t r a l A f r i c a n R e p u b l i c and Chad, D i g o u t t e ^' ^ c a r r i e d out s e r o l o g i c a l s t u d i e s on w i l d a n i m a l s ( b i r d s , r o d e n t s and lai-ge mammals); t h e s e s t u d i e s however, d i d not r e v e a l any a n t i b o d i e s f o r y e l l o w f e v e r on i t s own. On tho o t h e r hand, r e a c t i o n s o f t h e t y p e c a u s e d by m u l t i p l e i n f e c t i o n w i t h Group B v i r u s e s were f r e q u e n t .
1 , 2 . 3 S t u d ^ o f th2_'^£2t2£S
In C e n t r a l A f r i c a , w h i l e s t u d i e s have p r o v i d e d many f a c t s c o n c e r n i n g p o t e n t i a l v e c t o r s i n t h e C e n t r a l A f r i c a n R e p u b l i c , Cameroon and Gabon , t h e r e have been v e r y few s t u d i e s on t h e Congo o r Chad ^,
I t c o u l d be s a i d however, t h a t g e n e r a l l y s p e a k i n g , v e c t o r s a r e p r e s e n t e v e r y w h e r e . A e d e s a e g y p t i has been found e v e r y w h e r e i n towns, most o f t e n as a p e r i d o m e s t i c p e s t , s i n c e t h e p r o b l e m o f w a t e r s t o r a g e does n o t a r i s e i n the humid zones o f C e n t r a l A i r i c a .
A. a f r i c a n u s has been w e l l s t u d i e d , above a l l i n Cameroon and t h e C e n t r a l A f r i c a n R e p u b l i c , I n t h o s e c o u n t r i e s i t i s v e r y a b u n d a n t and v e r y a g g r e s s i v e n e a r t h e g r o u n d ( p a l m g r o v e s , marshy l o w l a n d s , f o r e s t g a l l e r i e s ) . I t b i t e s day and n i g h t , but m o s t l y a t dawn and d u s k ,
A. s i m p s o n i i s a l s o p e r i d o m e s t i c i n i t s e c o l o g y (banana p l a n t a t i o n s ) . I t i s a c t i v e d u r i n g t h e d a y - t i m e b u t becomes l e s s a n t h r o p o p h i l i c from e a s t t o w e s t . Where i t i s
a n t h r o p o p h i l i c , i t may c a u s e t r a n s m i s s i o n between humans and t h u s s t a r t an e p i d e m i c ( E t h i o p i a ,
1 9 6 0 - 1 9 6 2 ) .
The t h r e e v e c t o r s t h e r e f o r e , so f a r r e c o g n i z e d as m a j o r v e c t o r s o f y e l l o w f e v e r , a r e o m n i p r e s e n t i n C e n t r a l A f r i c a .
1 . 3 F u t u r e p r o s p e c t s
S t u d i e s a r e s t i l l needed i n o r d e r t o d e t e r m i n e how y e l l o w f e v e r v i r u s m a i n t a i n s i t s e l f i n i t s s y l v a t i c c y c l e . E f f o r t s must be c o n c e n t r a t e d on r e g i o n s where i t i s s u s p e c t e d t h a t t h e v i r u s i s p e r m a n e n t l y a c t i v e : Cameroon (Ayos R e g i o n ) and t h e C e n t r a l A f r i c a n R e p u b l i c ( S o u t h West R e g i o n ) ^.
2 . West A f r i c a
2 . 1 H i s t o r i c a l r e v i e w
U n l i k e C e n t r a l A f r i c a , West A f r i c a has known v a s t e p i d e m i c s o f y e l l o w f e v e r d u r i n g i t s h i s t o r y .
The r e v i v a l o f y e l l o w f e v e r f i r s t became m a n i f e s t a t D i o u r b e l i n S e n e g a l i n t h e f o r m o f an e p i d e m i c i n 1 9 6 5 . I n 1 9 6 9 , t h e r e was an e p i d e m i c i n f i v e West A f r i c a n c o u n t r i e s : M a l i , Upper V o l t a , N i g e r i a , Togo and Ghana. I n 1 9 7 0 , y e l l o w f e v e r was r e c o r d e d i n Ghana, N i g e r i a and Togo and i n 1 9 7 2 a g a i n i n Ghana. On t h e o t h e r hand, no c a s e o f y e l l o w f e v e r has been r e p o r t e d f r o m I v o r y C o a s t s i n c e 1 9 4 8 , from Dahomey s i n c e 1 9 5 1 , from G u i n e a s i n c e 1 9 5 2 and from S i e r r a Leone s i n c e 1 9 5 5 ,
2 . 1 P r e s e n t s i t u a t i o n
The w h o l e s u r v e i l l a n c e s y s t e m depends i n e f f e c t on t h e i n t e r e s t shown i n t h i s d i s e a s e
b d o c t o r s i n t h e s e n t i n e l h o s p i t a l s and t h e p r e v e n t i v e m e d i c i n e d e p a r t m e n t s . So f a r , t h e
s v s t e m has n o t p r o d u c e d t h e r e s u l t s a n t i c i p a t e d , so much so t h a t a p a r t from s t u d i e s c a r r i e d
o u t by a few teams, k n o w l e d g e o f t h e e p i d e m i o l o g i c a l s i t u a t i o n i n r e g a r d t o y e l l o w f e v e r i n
w e s t A f r i c a has made l i t t l e p r o g r e s s .
V I R / R C / 7 4 . 2 3 ( A r b o ) page 3
2.2.1 S e r o l o g i c a l _ s t u d i e s _ a i n o n g _ t h e _ h u m a n p o p u l a t i o n
S e r o l o g i c a l s u r v e y s among t h e human p o p u l a t i o n have c o v e r e d a l m o s t t h e w h o l e a r e a o f
West A f r i c a . B e c a u s e o f t h e i n t e r f e r e n c e from v a c c i n a t i o n s and h e t e r o l o g o u s G r o u p B r e a c t i o n s , i t i s d i f f i c u l t t o draw any c o n c l u s i o n s from them as r e g a r d s c i r c u l a t i o n o f t h e y e l l o w f e v e r v i r u s . However, s u r v e i l l a n c e o f c h i l d r e n s h o u l d e n a b l e t h e p r o b l e m t o be t a c k l e d . T h i s i s t h e c a s e i n t h e Kedougou r e g i o n , where i n c h i l d r e n who have c e r t a i n l y not been v a c c i n a t e d , s e r o l o g i c a l r e a c t i o n s have been r e c o r d e d w h i c h v e r y s t r o n g l y s u g g e s t r e c e n t a t t a c k by the y e l l o w f e v e r v i r u s . In the E n g l i s h - s p e a k i n g c o u n t r i e s where t h e r e have been no r e g u l a r v a c c i n a t i o n campaigns, i n t e r p r e t a t i o n i s e a s i e r . T h i s i s t h e c a s e w i t h S i e r r a L e o n e where a r e c e n t s u r v e y showed a v e r y low l e v e l o f a c t i v i t y o f t h e y e l l o w f e v e r v i r u s .
In any c a s e , s e r o l o g i c a l s u r v e y s do g i v e some i d e a o f t h e s i z e o f the r e c e p t i v e p o p u l a t i o n . The d a n g e r o f an e p i d e m i c w i l l t h e n depend on the e x i s t e n c e n e a r t h e p o p u l a t i o n c o n c e r n e d o f a s o u r c e o f the v i r u s ( s y l v a t i c c y c l e ) and t h e p r e s e n c e i n s u f f i c i e n t numbers, o f v e c t o r s
c a p a b l e o f b r i n g i n g about t r a n s m i s s i o n between human b e i n g s . 2.2.2 S e r o l o g i c a l s t u d i e s among w i l d v e r t e b r a t e s
I t i s e s s e n t i a l l y s u r v e i l l a n c e o f t h e p r i m a t e s t h a t has been the s u b j e c t o f a j o i n t s t u d y between ORSTOM and t h e I n s t i t u t P a s t e u r , Dakar, w i t h a s s i s t a n c e from WHO. I n t e r p r e t a t i o n o f t h e r e s u l t s i s made d i f f i c u l t by i n t e r f e r e n c e from o t h e r G r o u p B a r b o v i r u s e s , m a i n l y t h e Z i k a v i r u s . I t may however be c o n c l u d e d t h a t t h e zone where y e l l o w f e v e r i s most h i g h l y endemic c o v e r s t h e G u i n e a savanna a r e a s between 9 and 11 N. I t i s t h e r e f o r e i n t h i s zone t h a t p a r t i c u l a r l y s t r i c t s u r v e i l l a n c e s h o u l d be e x e r c i s e d ^,
2.2.3 S t u d y o f t h e v e c t o r s
The l i s t o f known p o t e n t i a l v e c t o r s c o m p r i s e s 11 s p e c i e s a t t h e moment, but i t i s n o t e x h a u s t i v e and l a b o r a t o r y s t u d i e s s h o u l d b r i n g t o l i g h t o t h e r p o t e n t i a l v e c t o r s . M a j o r v e c t o r s a r e p r e s e n t e v e r y w h e r e ^' ^"^.
A. a e g y p t i i s p r e s e n t e v e r y w h e r e . I n S e n e g a l , t h e o n l y zone where d e n s i t y i n d e x e s a r e s t i l l d a n g e r o u s i s t h e e p i d e m i c f o c u s o f 1965 where t h e y a r e o v e r 50 i n a l m o s t a l l t h e v i l l a g e s s u r v e y e d ^. A. a f r i c a n u s r e m a i n s r e s t r i c t e d t o t h e f o r e s t g a l l e r i e s .
A. s i m p s o n i does n o t b i t e man.
On t h e o t h e r hand, A e d e s o f t h e D i c e r o m y i a genus r e a d i l y b i t e i n t h e v i l l a g e s , a t t a c k i n g t h e whole p o p u l a t i o n i n d i s c r i m i n a t e l y and p r o b a b l y b e i n g t h e c a u s e o f t h e i n f e c t i o n s f o u n d i n
2 c h i l d r e n
2.3 F u t u r e p r o s p e c t s
E p i d e m i o l o g i c a l r e s e a r c h on y e l l o w f e v e r i s a l o n g - t e r r a t a s k . The s c a n t y r e s u l t s o b t a i n e d seem out o f p r o p o r t i o n t o t h e amount o f work i n v o l v e d . However, we h a v e had a n o p p o r t u n i t y o f c a r r y i n g out i n c e r t a i n r e s t r i c t e d z o n e s ( E a s t S e n e g a l , t h e K o r h o g o R e g i o n i n I v o r y C o a s t ) c o m b i n e d s t u d i e s on t h e t h r e e l i n k s i n t h e y e l l o w f e v e r v i r u s c y c l e : man, o t h e r v e r t e b r a t e s and t h e v e c t o r s . A l t h o u g h t h e s e programmes may seem a m b i t i o u s , t h e y s h o u l d n o t be abandoned f o r t h e y a r e t h e v e r y f o u n d a t i o n o f a s y s t e m o f e f f e c t i v e s u r v e i l l a n c e .
V I R / R C / 7 4 , 2 3 ( A r b o )
