WORLD H E A L T H ORGANIZATION ORGANISATION M O N D I A L E D E L A SANTÉ EXPERT COMMITTEE ON YELLOW FEVER Entebbe, 9-15 March 1 9 7 1
VIR/YF/71.8
ORIGINAL:
FRENCH
INDEXED
STUDIES AND RESEARCH TO UNDERTAKE CONCERNING YELLOW FEVER by
J . Hamon and M. Cornet M e d i c a l e n t o m o l o g i s t s , ORSTOM
1.
INTRODUCTION
I t i s d i f f i c u l t t o draw up a complete l i s t o f s t u d i e s and r e s e a r c h t o u n d e r t a k e w h i l e t h e v i e w p o i n t s o f the p a r t i c i p a n t s i n the WHO E x p e r t Committee on Y e l l o w F e v e r a r e not y e t known. C o n s e q u e n t l y , t h e f o l l o w i n g e n u m e r a t i o n , a l t h o u g h e x t e n s i v e , cannot be r e g a r d e d as r e s t r i c t i v e . The v a r i o u s s u g g e s t i o n s a r e s u b m i t t e d f o l l o w i n g t h e o r d e r adopted i n t h e d r a f t agenda. 2. EPIDEMIOLOGY AND ECOLOGY 2.1
Vectors
I t i s e x t r e m e l y i m p o r t a n t t o c o m p i l e an a c c u r a t e l i s t o f t h e b l o o d - s u c k i n g a r t h r o p o d s l i a b l e t o harbour y e l l o w f e v e r v i r u s and t r a n s m i t i t by t h e i r b i t e , g i v i n g p r i o r i t y t o s p e c i e s i n n a t u r a l f o c i w i t h c o n s i d e r a b l e l o n g e v i t y o r w h i c h r e t a i n a normal rhythm o f a c t i v i t y i n t h e d r y season. I n p a r t i c u l a r t h e v e c t o r r o l e o f t i c k s s h o u l d be r e a s s e s s e d . I t s h o u l d be noted f u r t h e r m o r e t h a t t h e a b i l i t y t o t r a n s m i t pathogens seems t o be g e n e t i c a l l y d e t e r m i n e d . C o n s e q u e n t l y , most o f the o l d e r n e g a t i v e e x p e r i m e n t s s h o u l d be r e p e a t e d and extended, w o r k i n g w i t h a l a r g e r range o f b o t h v i r u s and v e c t o r s t r a i n s . In r e g a r d t o a l r e a d y known p o t e n t i a l v e c t o r s , many s t u d i e s and i n v e s t i g a t i o n s a r e necessary. 2.1.1
Distribution
The d i s t r i b u t i o n and r e l a t i v e abundance o f Aedes a e g y p t i a r e o n l y v e r y i m p e r f e c t l y known i n some y e l l o w f e v e r endemic a r e a s . They s h o u l d be i n v e s t i g a t e d wherever r e c e n t d a t a a r e not a v a i l a b l e . As c o n c e r n s A f r i c a , t h e s e s t u d i e s s h o u l d d e a l i n j j a i t i c u l a r w i t h t h e c o u n t r i e s of C e n t r a l and E a s t A f r i c a , t h e E n g l i s h - s p e a k i n g c o u n t r i e s o f West A f r i c a ( e x c l u d i n g N o r t h Ghana) and t e r r i t o r i e s under P o r t u g u e s e r u l e . As concerns the A m e r i c a s , t h e s i t u a t i o n as r e g a r d s e r a d i c a t i o n o p e r a t i o n s and r e c e n t r e - i n f e s t a t i o n s s h o u l d be d e t e r m i n e d . S i m i l a r s t u d i e s might be e n v i s a g e d i n c o u n t r i e s where y e l l o w f e v e r c o u l d be i n t r o d u c e d a c c i d e n t a l l y , such as t h e Comoro I s l a n d s and Madagascar. The d i s t r i b u t i o n and r e l e v a n t abundance o f o t h e r y e l l o w f e v e r v e c t o r m o s q u i t o s a r e g e n e r a l l y e x t r e m e l y l i t t l e known, except i n a few l i m i t e d a r e a s . I t would be a d v i s a b l e , b o t h i n A f r i c a and the A m e r i c a s , t o o b t a i n a d d i t i o n a l d a t a on d i s t r i b u t i o n and t o o r g a n i z e s t u d i e s on r e l e v a n t abundance i n t h e d i f f e r e n t b i o c l i m a t i c zones. P r i o r i t y c o u l d be g i v e n to s p e c i e s l i a b l e t o cause e p i d e m i c s . The issue of this document does not constitute formal publication. It should not be reviewed, Authors alone abstracted or quoted without the agreement of the World Health Organization. articles. are responsible for views expressed in signed Ce document ne constitue pas une publication.
Il ne doit faire l'objet d'aucun compte rendu ou résumé ni d'aucune citation sans l'autorisation de l'Organisation Mondiale de la Santé. Les opinions exprimées dans les articles signés n'engagent que leurs auteurs.
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.8
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2 2.1.2 F.cology Ae. and aegypti. c o n t r o l of
So f a r , l i t t l e i s known c o n c e r n i n g t h e e c o l o g y o f t h e v e c t o r s , e v e n o f Research should cover a l l p o i n t s b e a r i n g a r e l a t i o n s h i p to the epidemiology yellow fever. i
The l o c a l i z a t i o n o f r e s t i n g p l a c e s , a v e r a g e f r o m t h e i r p r e - i m a g i n a l b r e e d i n g p l a c e s and f r o m order to o r g a n i z e e f f e c t i v e imagocidal campaigns f l i g h t ranges higher than those u s u a l l y accepted y e l l o w f e v e r v i r u s f r o m one s e p a r a t e wooded a r e a
l o n g e v i t y and t h e s p r e a d i n g p o w e r s o f a d u l t s s i t e s where they f e e d s h o u l d be known i n i n t h e e v e n t o f an e p i d e m i c . Moreover, might e x p l a i n the r a p i d d i s s e m i n a t i o n of to another.
The n a t u r e and p r o d u c t i v e c a p a c i t y o f t h e d i f f e r e n t l a r v a l b r e e d i n g p l a c e s , t h e l e n g t h o f th.' p r e - i m a g i n a l l i f e - c y c l e and t h e e x t e n t o f s e a s o n a l v a r i a t i o n s i n d e n s i t y a r e i m p o r t a n t data i n any p r e v e n t i v e c a m p a i g n w h o s e a i m i s t o e r a d i c a t e o r g r e a t l y r e d u c e t h e n u m b e r s o f p o t e n t i a l vectors. A g o o d k n o w l e d g e o f maximum l o n g e v i t y , p o s s i b i l i t i e s o f s u r v i v a l i n t h e d r y s e a s o n a n d f e e d i n g p r e f e r e n c e s c o n s t i t u t e s , t o g e t h e r w i t h the f a c t o r s a l r e a d y m e n t i o n e d , an e s s e n t i a l e l e m e n t i n any e p i d e m i o l o g i c a l r e s e a r c h p r o g r a m m e . D e t e r m i n a t i o n o f a b s o l u t e d e n s i t i e s w o u l d be and w i t h a v i e w t o b i o l o g i c a l c o n t r o l . 2.2 Vertebrate hosts other than man d e s i r a b l e , both for q u a n t i t a t i v e epidemio-
logy
A l a r g e v a r i e t y of v e r t e b r a t e s have been found t o g i v e p o s i t i v e s e r o l o g i c a l t e s t s f o r yellow fever. A l i m i t e d n u m b e r o f w i l d and d o m e s t i c v e r t e b r a t e s h a v e b e e n e x p e r i m e n t a l l y infected. Whereas tho p r i m a t e s are almost a l l r e c e p t i v e , the r e s p o n s e o f o t h e r v e r t e b r a t e s v a r i e s from s p e c i e s t o s p e c i e s . S u c h s t u d i e s s h o u l d be i n t e n s i f i e d b y l o o k i n g f o r v e r t e b r a t e h o s t s w i t h a l o n g e r p e r i o d of v i r a e m i a , or a t r a n s i e n t immunity, or a b l e to spread the v i r u s over long distances. S u s c e p t i b i l i t y t o y e l l o w f e v e r i n f e c t i o n m u s t a l w a y s be t a k e n i n t o consideration; v e r t e b r a t e s w h i c h d i e f r o m y e l l o w f e v e r may be g o o d " a m p l i f i e r s " o f t h e d i s e a s e , but they have l i t t l e chance of p l a y i n g a p a r t i n m a i n t a i n i n g the v i r u s i n c i r c u l a t i o n in natural foci. The p r a c t i c a l i m p o r t a n c e o f v e r y s l i g h t d e g r e e s o f v i r a e m i a m i g h t b e reassessed. 2.2.1 Distribution and abundance t i l l o f t e n o n l y v e r y p a r t i a l l y known, p a r t i c u on t h i s p o i n t s h o u l d b e e x t e n d e d . Generally i n d i v i d u a l s although, together with population s i g n i f i c a n c e of the d i f f e r e n t s p e c i e s of
The d i s t r i b u t i o n o f v e r t e b r a t e h o s t s i s s l a r l y i n the case of small s p e c i e s . Studies e v e n l e s s i s known r e g a r d i n g t h e a b u n d a n c y o f dynamics, i t governs the r e a l e p i d e m i o l o g i c a l vertebrate host.
f
of
I n an i n i t i a l s t a g e , p r i o r i t y c o u l d b e g i v e n t o v e r t e b r a t e s w h i c h a m p l i f y t h e v i r u s d u r i n g e n z o o t i c s and i n t h e p r e - e p i d e m i c p h a s e . 2.2.2 Ecology
the
circulati
I n f e c t i o n o f v e r t e b r a t e s a n d s p r e a d o f t h e v i r u s by t h e m d e p e n d s o n t h e i r e c o l o g y . xhe c h i e f a s p e c t s t o be c o n s i d e r e d a r e t h e r e s t i n g p l a c e s , f e e d i n g p l a c e s , u s u a l r a n g e o f a c t i v i t y and t h e p o s s i b i l i t y o f l o n g d i s t a n c e d i s p e r s i o n o r m i g r a t i o n . 2.3 It Transmission i s possible that cycles the c y c l e s at present known a r e only amplifying c y c l e s which exist
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m e r e l y d u r i n g t h e e n z o o t i c and p r e - e p i d e m i c p h a s e s a n d t h a t t h e p e r m a n e n t c y c l e i n n a t u r a l f o c i i s of a d i f f e r e n t type, i n v o l v i n g n e i t h e r primates nor m o s q u i t o s . Research along t h e s e l i n e s c a n o n l y be s u c c e s s f u l l y u n d e r t a k e n i f p e r m a n e n t y e l l o w f e v e r f o c i a r e a c c u r a t e l y located.
3.
VIROLOGY AND
IMMUNOLOGY
The m i n i m a l a n t i g e n e t i c d i f f e r e n c e s o b s e r v e d b e t w e e n t h e s t r a i n s seem t o h a v e no p r a c t i c a l i m p l i c a t i o n s and s o f a r no s p e c i a l a d a p t a t i o n h a s b e e n r e p o r t e d o f a g i v e n s t r a i n t o a g i v e n v e c t o r , b u t t h i s a s p e c t m i g h t be s t u d i e d . S t u d y o f c r o s s i m m u n i t y w i t h o t h e r g r o u p B a r b o v i r u s e s h a s b e e n commenced i n m o n k e y s , T h e s e i n v e s t i g a t i o n s s h o u l d be c o n t i n u e d i n o r d e r t o d e t e r m i n e t h e e x t e n t o f t h e p r o t e c t i o n a f f o r d e d i n d i v i d u a l s and c o m m u n i t i e s b y p r e v i o u s i n f e c t i o n w i t h t h e s e a r b o v i r u s e s . A l l r e s e a r c h w h i c h can l e a d t o i n c r e a s e d a c c u r a c y of the i m m u n o l o g i c a l methods employed f o r i d e n t i f y i n g v i r u s s t r a i n s and f o r c h a r a c t e r i z i n g and a s s a y i n g s p e c i f i c a n t i b o d i e s s h o u l d be s t r o n g l y encouraged s i n c e i t c o u l d g r e a t l y f a c i l i t a t e e p i d e m i o l o g i c a l s t u d i e s . A d m i n i s t r a t i o n of immunodepressants to w i l d animals captured i n n a t u r a l f o c i would p e r h a p s make i t p o s s i b l e t o d e t e c t y e l l o w f e v e r v i r u s i n s p e c i e s w h o s e v i r a e m i a l e v e l i s low f o r d e t e c t i o n by t h e u s u a l m e t h o d s . 4. V A C C I N E S AND VACCINATION have a t h e r m o s t a b l e v a c c i n e whose p a t h o g e n i c i t y i s t h e h e a t s t a b i l i t y o f 17 D v a c c i n e o r b y e l i m i n a t i n g v a c c i n e , o r by i s o l a t i n g o t h e r v a c c i n a l s t r a i n s D and t h e D a k a r s t r a i n s .
too
I t w o u l d be e s p e c i a l l y v a l u a b l e t o a s l o w a s p o s s i b l e , e i t h e r by i m p r o v i n g t h e n e u r o t r o p i c p r o p e r t i e s of the Dakar c o m b i n i n g t h e a d v a n t a g e s o f b o t h t h e 17 5. VECTOR CONTROL Research should be carried out
simultaneously insecticides
in five
different
directions: can be added without
s e l e c t i o n and e v a l u a t i o n o f r i s k to d r i n k i n g water, s e l e c t i o n and e v a l u a t i o n o f l a r v a e and a d u l t v e c t o r s ,
other
than Abate which
-
i n s e c t i c i d e s which
can
be
a p p l i e d by
fogging
against
-
i m p r o v e m e n t and e v a l u a t i o n o f e q u i p m e n t and m e t h o d s f o r a p p l i c a t i o n o f i n s e c t i c i d e s i n a v e r y s m a l l v o l u m e i n t h e d i f f e r e n t b i o c l i m a t i c and p l a n t r e g i o n s Of y e l l o w f e v e r e n d e m i c a r e a s , b o t h f r o m t h e g r o u n d and f r o m t h e a i r , of the distribution and characteristics of i n s e c t i c i d e - r e s i s t a n t vector
- determination populations,
- d e v e l o p m e n t and 6.
e v a l u a t i o n of b i o l o g i c a l
or genetic
vector
c o n t r o l methods.
S U R V E I L L A N C E METHODS The i m p r o v e m e n t o f s u r v e i l l a n c e m e t h o d s a s c o n c e r n s v e r t e b r a t e h o s t s d e p e n d s f i r s t l y r e s u l t s of immunological research. P r o g r e s s c o u l d a l s o d e f i n i t e l y be made i n r e g a r d sampling of w i l d v e r t e b r a t e p o p u l a t i o n s . be the on to
the the
W i t h r e s p e c t t o t h e s u r v e i l l a n c e o f v e c t o r s , one o f t h e b i g g e s t a d v a n c e s w o u l d d e v e l o p m e n t o f s i m p l e and e f f e c t i v e t r a p s g i v i n g r e p r o d u c i b l e r e s u l t s .