WORLD H E A L T H ORGANIZATION W ORGANISATION M O N D I A L E D E L A SANTÉ
VIR/YF/71,6 ORIGINAL: FRENCH
—>^s— INDEXED
EXPERT COMMITTEE ON YELLOW FEVER Entebbe, 9-15 March 1971 METHODS OF SURVEILLANCE FOR YELLOW FEVER - SEROLOGICAL STUDIES IN WILD AND SENTINEL ANIMALS by Dr Y. Robin I n s t i t u t P a s t e u r , Dakar,
Senegal
S u c c e s s i v e v a c c i n a t i o n campaigns, b o t h i n A f r i c a and t h e Americas, have made any i n t e r p r e t a t i o n o f human s e r o l o g i c a l s u r v e y s almost i m p o s s i b l e as c o n c e r n s y e l l o w f e v e r . At p r e s e n t i t i s g e n e r a l l y t h e n e g a t i v e r e s u l t s w h i c h a r e most v a l u a b l e s i n c e they enable the r e c e p t i v e p o p u l a t i o n t o be e s t i m a t e d and c o n s e q u e n t l y t h e r i s k o f an epidemic t o be e v a l u a t e d i n accordance w i t h o t h e r e p i d e m i o l o g i c a l d a t a , t h e abundance o f t h e v e c t o r i n particular. However, t h e presence o f t h e v i r u s i s n e c e s s a r y b e f o r e t h e r e can be an e p i d e m i c and many q u e s t i o n s a r i s e i n t h i s c o n n e x i o n : where i s the v i r u s m a i n t a i n e d i n n a t u r e , how i s i t propagated, how can the e x t e n t o f i t s s p r e a d be checked? Yellow fever i s p r i m a r i l y a z o o n o s i s : by the s u r v e i l l a n c e o f w i l d v e r t e b r a t e s c a p t u r e d i n g i v e n a r e a s and the study o f s e n t i n e l a n i m a l s p l a c e d i n s e l e c t e d s i t e s n o t o n l y can an i n v e n t o r y o f w i l d p o p u l a t i o n s l i a b l e to be a f f e c t e d by y e l l o w f e v e r v i r u s be drawn up, but a l s o an i d e a formed o f t h e i n t e n s i t y o f the c i r c u l a t i o n o f t h e v i r u s . T h i s s t u d y has two a s p e c t s : a t t e m p t s t o i s o l a t e t h e v i r u s , and s e r o l o g i c a l study o f exposed a n i m a l s . I t i s t h e l a t t e r a s p e c t w h i c h w i l l be c o n s i d e r e d here. Equipment and methods The s e r o l o g i c a l study o f w i l d fauna and o f s e n t i n e l a n i m a l s poses c e r t a i n s p e c i a l problems
Wild vertebrates As a c c u r a t e a l i s t as p o s s i b l e o f t h e a n i m a l s p e c i e s i n a g i v e n r e g i o n must be e s t a b l i s h e d S u b s e q u e n t l y , s y s t e m a t i c c a p t u r e s s h o u l d be made i n o r d e r t o d e t e r m i n e t h e d e n s i t y o f t h e d i f f e r e n t animal p o p u l a t i o n s . The a n i m a l s c o u l d be s h o t , but t h e e f f e c t o f t h i s type o f " c a p t u r e " on t h e presence o f n o n - s p e c i f i c n e u t r a l i z i n g s u b s t a n c e s i n t h e serum o f b i r d s o b t a i n e d i n t h i s way i s w e l l known ( S o b e r e r e t a l . 1964; Hardy e t a l . 1964). The i d e a l s o l u t i o n i s t o use non-wounding t r a p s . By t h i s means t h e a n i m a l s c a n be c a p t u r e d , b l e d , and then marked and r e l e a s e d . By t a k i n g s u c c e s s i v e samples from t h e same a n i m a l , such a study o f t h e w i l d fauna i n i t s n a t u r a l h a b i t a t g i v e s v e r y u s e f u l a d d i t i o n a l i n f o r m a t i o n on t h e s e a s o n a l a c t i v i t y o f t h e v i r u s and on t h e d u r a t i o n o f a n t i b o d i e s i n the h o s t .
The Issue of this document d o e s not formal publication. It should not be
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document ne constitue pas une publication.
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abstracted or quoted without the agreement of the World Health are articles. Organization. Authors responsible for views e x p r e s s e d in signed
V I R / Y F / 7 1 .6
page 2 •Sentinel anima] s Monkfys aïo . s t i l l the .sentinel a n i m a l s o l choictj f o r y e l l o w f e v e r s i n c e by t h e i r u s e l ) o s s i b l e infection.', can be d e t e c t e d i n the f o r e s t canopy. The mosquitos l i a b l e t o t r a n s m i t the viru.s at t h i s i;:;ht aii-', i n f a c t , not much a t t r a c t e d by l a b o r a t o r y mice, t h e o t h e r v e r y f r e q u e n t l y used s e n t i i K ; ! a n i m a l s . C h i c k e n s , r a b b i t s , g u i n e a - p i g s and hamsters, w h i c h may have a s e l . T C t i v i ' a t t r a c t i o n f o r c e r t a i n v e c t o r a r t h r o p o d s , (^an a l s o be employed. Serum co 1 1 etc t i on Blood s a m p l i n g t e c h n i q u e s v a r y a c c o r d i n g to the method o f c a p t u r e and t h e z o o l o g i c a l group. In thp case of l a r g e a n i m a l s killf.'d or wounded by gun-shot the b l o o d i s t a k e n by c a r d i a c [lunoture w i t h tho minimum d e l a y i n o r d e r to a v o i d as f a r as p o s s i b l e t h e a p p e a r a n c e of n o n - s i ) e c i f i c i n h i b i t i n g s u b s t a n c e s . In the case o f s m a l l e r a n i m a l s c a p t u r e d a l i v e s e v e r a l mctliocis may be employed: c a r d i a c puncture, c a r o t i d puncture, o r b i t a l s i n u s p u n c t u r e , s e c t i o n of the a x i l l a r y a r t e r y ( r o d e n t s ) , s e c t i o n o f the c a r o t i d a r t e r y ( b a t s ) , o r i n c i s i o n o f t h e humeral v e i n ( b i r d s , b a t s ) . Ihe serum c o l l e c t e d a l t e r c o a g u l a t i o n i s s t o r e d at -20'C. When v e r y s m a l l amounts a r e sampled paper f i l t e r d i s c s can be used. However the drawbacks o f t h i s t e c h n i q u e a r e w e l l known: l o s s o f s e n s i t i v i t y , d e t e r i o r a t i o n o f g l o b u l i n s (Bond et a l . 1969; Cohen e t a l . 1969) So as t o a v o i d c o n t a m i n a t i o n o f the s e r a as f a r as p o s s i b l e , which i s e s s e n t i a l t o p r e s e r v e t h e i r a n t i c o m p l e m e n t a r y power, c o n c e n t r a t e d a n t i b i o t i c s o l u t i o n s s h o u l d be u s e d during sampling. Sero]ogical methods employed
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It I S d e s i r a b l e f o r a i l l a b o r a t o r i e s to employ s t a n d a r d i z e d t e c h n i q u e s f o r h a e m a g g l u t i n a i n h i b i t i o n , complement f i x a t i o n and n e u t r a l i z a t i o n .
As o n l y l i m i t e d amounts o f serum a r e i n v o l v e d , i t is n e c e s s a r y t o use m i c r o methods. A l t h o u g h w e l l adapted to h a e m a g g l u t i n a t i o n i n h i b i t i o n and complement f i x a t i o n ( S e v e r , 1962) t h ( y a r e not y e t v e r y w e l l d e v e l o p e d as c o n c e r n s n e u t r a l i z a t i o n . H a e m a g g l u t i n a t i o n i n h i b i t i o n (HI) remains the s i m p l e s t s c r e e n i n g method. Although t h e r e i s not y e t any u n i v e r s a l l y employed system o f serum treatment, acetone t r e a t m e n t g i v e s the b e s t r e s u l t s (Semenov e t a l , 1968). Complement f i x a t i o n (CF) does not p r e s e n t any t e c h n i c a l d i f f i c u l t y p r o v i d e d u n c o n t a m i n a t e d s e r a a r e used, whence t h e n e c e s s i t y t o c o l l e c t t h e b l o o d o v e r a n t i b i o t i c s ( o r a n t i s e p t i c s ) By u s i n g t h i s r e a c t i o n r e c e n t i n f e c t i o n s c a n be d e t e c t e d . The n e u t r a l i z a t i o n t e s t (NT) i s s t i l l c a r r i e d out f o r the most p a r t on mice. u r g e n t l y n e c e s s a r y t o d e v e l o p a t i s s u e c u l t u r e m i c r o t e c h n i q u e which can be adopted laboratories. It i s by a l l
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Ge] i m m u n o d i f f u s i o n can p r o v i d e a r a p i d and cheap method f o r the d e t e c t i o n o f a n t i b o d i e s w i l d v e r t e b r a t e s o r s e n t i n e l a n i m a l s (Papadopoulos et a l . 1970). o f tlie results i n t e r p r e t a t i o n must be based on the study o f a s i n g l e serum i n group the t h r e e most commonly used specimen tests
Interpretation
Foi- the most p a r t , por a n i m a l . interpretation (1(1, CF, NT). sliould
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VIR/YF/TI.S
page 3 In the case of y e l l o w f e v e r by i t s e l f , i n t e r p r e t a t i o n w i n be easy and i t w i l l be p o s s i b l e t o d i s t i n g u i s h between r e c e n t a t t a c k s ( p o s i t i v e CF) and o l d e r ones ( n e g a t i v e C F ) . I f s e v e r a l a n t i g e n s are i n v o l v e d , then i n t e r p r e t a t i o n becomes v e r y d i f f i c u l t . The t i t r e c a n n o t be used as a g u i d e , s i n c e h e t e r o l o g o u s r e a c t i o n s can e x c e e d the homologous response i n i n t e n s i t y , not o n l y as r e g a r d s the HI a n t i b o d i e s , but a l s o v e r y o f t e n the CF and even the NT antibodies, Another f a c t o r i n v o l v e d i n i n t e r p r e t a t i o n of a n i m a l s e r o l o g i c a l t e s t s i s the d u r a t i o n of t h e a n t i b o d y i n the a n i m a l a f t e r an i n i t i a l i n f e c t i o n : t h i s d u r a t i o n i s sometimes v e r y s h o r t e v e n as concerns HI a n t i b o d i e s which n o r m a l l y appear at an e a r l y s t a g e and p e r s i s t f o r a l o n g time. S e n t i n e l animals s h o u l d be examined b e f o r e b e i n g exposed i n o r d e r t o make s u r e t h a t they h a v e no immunity. Subsequently they s h o u l d be checked a t r e g u l a r i n t e r v a l s . If this i s d o n e i n t e r p r e t a t i o n of the s e r o l o g i c a l r e s u l t s i s not too d i f f i c u l t . Conclusions The i d e a l i n r e g a r d to y e l l o w f e v e r s u r v e i l l a n c e would be t o d e t e c t the danger of a human e p i d e m i c at the moment when t h e r e i s a r e c r u d e s c e n c e of the v i r u s i n w i l d fauna. Such fauna i s varied. Monkeys, a l t h o u g h they are s t i l l the best c o n t r o l s f o r c i r c u l a t i o n o f the v i r u s i n n a t u r e a r e no l o n g e r the o n l y a n i m a l s i n v o l v e d . O u t s i d e the l a r g e f o r e s t a r e a s , s i m i a n p o p u l a t i o n s do not seem numerous or p r o l i f i c enough t o e n s u r e c o n t i n u o u s c i r c u l a t i o n of the virus. Moreover they a c q u i r e l o n g - l a s t i n g immunity v e r y r e a d i l y . The r e s u l t s of s e r o l o g i c a l s u r v e y s and e x p e r i m e n t a l i n f e c t i o n s have l e d to the s u s p i c i o n t h a t o t h e r v e r t e b r a t e s are c o n c e r n e d i n the maintenance of y e l l o w f e v e r v i r u s d u r i n g i n t e r - e p i d e m i c p e r i o d s , e.g. hedgehogs, c a r n i v o r e s , b a t s and b i r d s . A l t h o u g h we have no p r o o f of t h e i r d i r e c t i n v o l v e m e n t i n t h e f o r e s t c y c l e o f y e l l o w f e v e r , s t u d i e s o f these v e r t e b r a t e s s h o u l d c o n t i n u e s i n c e so f a r we do not y e t know how y e l l o w f e v e r v i r u s i s maintained i n nature i n A f r i c a . REFERENCES Bond, J . 0. et a l . (1969) F i l t e r paper d i s c method of c o l l e c t i n g whole b l o o d s f o r a r b o v i r u s s e r o l o g i c s u r v e y s i n a d u l t s , H l t h Lab. S c i . , 6 ^ , 168-172 Cohen, A. B., H a t g i , J . N. & Wisseman, C. L. (1969) S t o r a g e s t a b i l i t y of d i f f e r e n t a n t i b o d y s p e c i e s a g a i n s t a r b o v i r u s and r i c k e t t s i a l a n t i g e n s i n b l o o d d r i e d on f i l t e r paper d i s c s , Amer. J . Epidem., 89, 345 Hardy, J . L., S c h e r e r , W. F. & Warner, D. W. (1964) A r b o v i r u s n e u t r a l i z i n g s u b s t a n c e s i n a v i a n plasmas. 2. S t u d i e s on t h e i r mechanism o f r e l e a s e and s p e c i f i c i t y a f t e r c o l l e c t i o n of plasmas by s h o o t i n g and c a r d i a c p u n c t u r e of b i r d s . Amer. J . t r o p . Med. Hyg., 13, 867-875 Papadopoulos, 0., Anslow, R. 0. k Hanson, R. P. (1970) A p p l i c a t i o n of the i m m u n o d i f f u s i o n p r o c e d u r e t o r a p i d i d e n t i f i c a t i o n of a r b o v i r u s e s and t o d e t e c t i o n of s p e c i f i c a n t i b o d y i n s e n t i n e l a n i m a l s . Amer. J . Epidem., 92, 145-150 S c h e r e r , W. F. et a l . (1964) A r b o v i r u s n e u t r a l i z i n g s u b s t a n c e s i n a v i a n plasmas. h i g h e r p r e v a l e n c e a f t e r c o l l e c t i o n of b i r d s by s h o o t i n g and c a r d i a c p u n c t u r e n e t t i n g and j u g u l a r v e n i p u n c t u r e . Amer. J . t r o p . Med. Hyg., 13, 859-866 Semenov, B. F. et a l . (1968) Non s p e c i f i c i n h i b i t o r s of h e m a g g l u t i n a t i n g a c t i v i t y a r b o v i r u s e s i n s e r a of r o d e n t s and b i r d s , Vop. V i r u s . , 13, 200-202 1. T h e i r than a f t e r
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