r j - ^

WORLD HEALTH ORGANIZATION

ORGANISATION MONDIALE DE LA SANTÉ

MEETING OF DIRECTORS OF WHO COLLABORATING CENTRES FOR VIRUS REFERENCE AND RESEARCH

ENGLISH ONLY

IN.

Geneva, 2 - 6 November 1981

Draft agenda item 7 . 1

ARBOVIRUSES IN AFRICA

by

M. Kalunda, Virus Research I n s t i t u t e , Entebbe, Uganda

I w i l l discuss arboviruses i n A f r i c a i n three categories , namely, those which are of in terna t ional s i g n i f i c a n c e , those which are associated with fevers of undetermined o r i g i n and

those which might par t i c ipa te i n the e t io logy of B u r k i t t ' s lymphoma.

A f r i c a i s the home of a large number of arboviruses which pose a great danger to the

world i n general and to A f r i c a i n p a r t i c u l a r . These viruses include yellow f e v e r . Lassa, Marburg, Ebola , R i f t Val ley fever and to a lesser extent Congo-Crimean haemorrhagic fever . These diseases have occurred mainly i n t r o p i c a l A f r i c a n countries with a large number of mammalian fauna and arthropods.

Yellow fever

Outbreaks of yellow fever v i rus in fec t ions have occurred i n West A f r i c a since 1977 and i n Ghana alone since 1 9 6 9 . 1 8 , 1 9 A large epidemic occurred also i n Senegal i n 1965 with several thousand cases and several hundred dea ths . ^ In Eastern A f r i c a the recent epidemics of yellow fever occurred i n the Sudan and western Ethiopia i n 1959^ and again i n south-west Ethiopia i n 1 9 6 0 - 1 9 6 2 producing enormous numbers of cases and between 15 000 and 30 000 dea ths .10 ,11 These epidemics show that yellow fever s t i l l ex is t s i n most t r o p i c a l A f r i c a n countries and therefore demand a continued su rve i l l ance .

The Table below indicates the o f f i c i a l n o t i f i c a t i o n s of this disease in A f r i c a since 1 9 7 7 . I t shows an increase i n both the number of cases and deaths, as we l l as the spread to three other countries between 1977 and 1 9 7 9 . However, these o f f i c i a l n o t i f i c a t i o n s seem to underestimate the true f i g u r e s . For instance 30 cases, a l l of them f a t a l , were n o t i f i e d i n the Gambia i n 1 9 7 8 , but re t rospect ive c l i n i c a l survey revealed 271 suspected cases and 63 deaths. The retrospect ive se ro log ica l survey increased these f igures to 5000-8000 cases and 1 0 0 0 - 1 7 0 0 deaths.18>19 xhe d i f fe rence in these f igures i s accounted fo r by d i f f i c u l t i e s i n i d e n t i f y i n g a typ i ca l forms of yellow fever by c l i n i c a l examination without laboratory t e s t s . A l s o , some people, e spec ia l ly those l i v i n g i n ru ra l areas, do not always seek medical treatment or report a l l i l l ne s se s and deaths. Hence, the e f f e c t of yellow fever , l i k e that of other diseases discussed i n th is report , i s by f a r greater than that determined from o f f i c i a l n o t i f i c a t i o n s .

Lassa, Marburg and Ebola

These three diseases are some of the most important v i r a l diseases in A f r i c a because of the severe epidemics they have produced and the potent ia l threat of s imi la r epidemics appearing outside A f r i c a .

Lassa v i r u s , an arenavirus, caused an epidemic i n Niger ia i n 1969.'^ Since then, sero-epidemiological surveys have established that this v i rus i s ac t ive i n a l l West A f r i c a n

countries between Senegal and Za i r e . In S ie r ra Leone alone this disease accounts f o r 107» of a l l f e b r i l e i l l n e s s e s and fo r 1.7% of the general death rate.^ Recent se ro log ica l surveys

The issue of this document does not constitute Ce document ne constitue pas une publication, formal publication. It should not be reviewed. II ne doit faire l'objet d'aucun compte rendu ou abstracted or quoted without the agreement of résumé ni d'aucune citation sans l'autorisation de the World Health Organization. Authors alone l'Organisation Mondiale de la Santé. Les opinions are responsible for views expressed in signed exprimées dans les articles signés n'engagent articles. que leurs auteurs.

VIR/CC/81 .18

page 2

have e s t a b l i s h e d Lassa v i r u s a n t i b o d i e s i n 4.81 of 433 human samples from L i b e r i a . More

r e c e n t l y , 27.37, of 44 specimens from Sudanese m i l i t a r y personnel were found to have IFA

a n t i b o d i e s to p o l y v a l e n t US-MBG-EBO a n t i g e n s . In S i e r r a Leone and Guinea, Lassa f e v e r was

found as a disease of f o r e s t and savanna areas. However, no evidence of t h i s d i s e a s e was

found i n man i n the C e n t r a l A f r i c a n Republic or nort h e r n Z a i r e . 6 Mastomys n a t a l e n s i s has

been i d e n t i f i e d as the rodent r e s e r v o i r host of Lassa v i r u s . I t s u f f e r s no i l l e f f e c t and

develops no immune r e s p o n s e . ^

Marburg v i r u s was f i r s t r e p o r t e d i n 1967 i n West Germany from an outbreak of a f a t a l

haemorrhagic d i s e a s e among l a b o r a t o r y workers who had con t a c t w i t h t i s s u e s of the A f r i c a n

green monkey (Cercopithecus aethiops) from Uganda. R e t r o s p e c t i v e f i e l d and l a b o r a t o r y

s t u d i e s conducted i n Uganda f a i l e d to show any c l i n i c a l evidence of a s i m i l a r d i s e a s e i n

monkeys or.humans. However, HI a n t i b o d i e s were demonstrated i n monkeys, i n d i c a t i n g t h a t

Marburg or a c l o s e l y r e l a t e d v i r u s was a c t i v e i n Ugandan monkeys, some of which were sent t o

West Germany.'' Subsequently, Marburg v i r u s reappeared i n South A f r i c a i n 1975 and i n Kenya i n 1980.6 S e r o l o g i c a l surveys i n the human p o p u l a t i o n have demonstrated s p e c i f i c a n t i b o d i e s

i n 1.4X of 499 samples from the C e n t r a l A f r i c a n R epublic and i n 27o of 433 samples from

L i b e r i a . More r e c e n t l y , 4.67. of 237 human serum samples from Kenya had a n t i b o d i e s to t h i s

v i r u s . 6

Ebola v i r u s caused two epidemics of haemorrhagic d i s e a s e tn Z a i r e and the Sudan i n 1976 with a t o t a l of 537 n o t i f i e d cases and 362 d e a t h s . ^ ^ > , 17 xhe disease reappeared i n the

Sudan i n 1979 producing 34 confirmed cases and 22 deaths.^ S e r o l o g i c a l surveys have e s t a b ­

l i s h e d the presence of Ebola v i r u s i n 11% of 433 persons i n L i b e r i a and i n 67o of Sudanese. More r e c e n t l y , 8.47» of 237 persons i n Kenya were found to be i n f e c t e d . However, evidence o f

t h i s v i r u s was not found i n Zairean monkeys, s q u i r r e l s , A f r i c a n porcupines, bats or rodents. 6

In summary, s e r o l o g i c a l surveys found Ebola v i r u s i n r a i n f o r e s t s i n West and C e n t r a l

A f r i c a and Marburg v i r u s i n C e n t r a l , East and Southern A f r i c a . The presence of a n t i b o d i e s t o

these three v i r u s e s i n many persons who gave no h i s t o r y of i l l n e s s or contact w i t h known c a s e s

suggests that these v i r u s e s u s u a l l y cause m i l d or even asymptomatic i n f e c t i o n i n man. Once

these v i r u s e s have been t r a n s m i t t e d to man through contact w i t h t h e i r r e s e r v o i r h o s t s , they

become capable of adapting to man-to-man t r a n s m i s s i o n , hence the great danger of episodes of

these d i s e a s e s i n non-endemic c o u n t r i e s

R i f t V a l l e y fever

U n t i l r e c e n t l y . R i v e r V a l l e y f e v e r has not been an important human disease because most

outbreaks a f f e c t e d domestic animals and a small number of people working c l o s e l y w i t h these

animals.^'^ Human death had never been d i r e c t l y a s c r i b e d to R i f t V a l l e y f e v e r . However, the

f i r s t severe human epidemic of R i f t V a l l e y f e v e r occurred i n South A f r i c a i n the e p i z o o t i c o f

1 9 5 0 - 1 9 5 1 , where R i f t V a l l e y f e v e r v i r u s was i s o l a t e d from f i v e p e r s o n s . A subsequent

e p i d e m i o l o g i c a l survey estimated 20 000 human cases.^ A g a i n , i n 1975 , another epidemic of

R i f t V a l l e y fever appeared i n South A f r i c a i n which at l e a s t four persons d i e d . ^ ^

The epidemic i n Egypt i n 1977 was the l a r g e s t and the most f a t a l human epidemic of

R i f t V a l l e y f e v e r . The o f f i c i a l n o t i f i c a t i o n gave 18 000 human cases and 598 deaths. However, estimates of c l i n i c a l cases range between 20 000 and 200 000 .8 The high number of

human cases and deaths were a t t r i b u t e d to the in c r e a s e d v i r u l e n c e of the v i r u s and perhaps the

high s u s c e p t i b i l i t y of the human p o p u l a t i o n i n Egypt due to endemic l i v e r d i s e a s e s , such as

s c h i s t o s o m i a s i s . 8

Although the source of t h i s epidemic was not e s t a b l i s h e d , the occurrence of a R i f t V a l l e y

f e v e r e p i z o o t i c i n the Sudan i n 1976 s t r o n g l y suggests that the v i r u s was introduced i n t o

Egypt from the Sudan by i n f e c t e d camels. Egypt imports 50 0 0 0 - 1 0 0 000 Sudanese camels every

year. 5 The appearance of R i f t V a l l e y f e v e r i n Egypt i n 1977 and i t s reappearance i n 1978 and 1979 as w e l l as the p o t e n t i a l continued spread i n Egypt and other Mediterranean c o u n t r i e s ,

r a i s e s concern f o r p u b l i c h e a l t h i n t h i s r e g i o n and the world at l a r g e .

V I R / C C / 8 1 . 1 8

page 3

A r b o v i r u s e s a s s o c i a t e d with f e v e r s of undetermined o r i g i n

The r o l e of ar b o v i r u s e s i n fe v e r s of undetermined o r i g i n i s not w e l l e s t a b l i s h e d i n

A f r i c a because some of the e f f e c t s of these v i r u s e s are a t t r i b u t e d to m a l a r i a . Most f e b r i l e

p a t i e n t s are th e r e f o r e t r e a t e d f o r m a l a r i a before the d i a g n o s i s i s confirmed by l a b o r a t o r y

t e s t s , which are not always conducted due to lack of f a c i l i t i e s or manpower. V i r a l i n f e c ­

t i o n s are t h e r e f o r e suspected only a f t e r the fevers f a i l to respond to a n t i b i o t i c s and

a n t i m a l a r i a l drugs. Specimens taken f o r v i r u s i s o l a t i o n at t h i s l a t e stage u s u a l l y y i e l d

negative r e s u l t s . Hence, v i r a l I n f e c t i o n s , p a r t i c u l a r l y those causing sporadic cases, are

not normally c a t e r e d f o r i n p u b l i c h e a l t h .

The importance of arb o v i r u s e s i n p u b l i c h e a l t h can be shown by s t u d i e s conducted i n my

I n s t i t u t e . Attempted v i r u s i s o l a t i o n i s conducted on acute blood samples from f e b r i l e

p a t i e n t s with suspected v i r a l i n f e c t i o n s by s u c k l i n g mice i n o c u l a t i o n . In the past three

y e a r s , 1 0 4 6 specimens were examined and these y i e l d e d 2 0 ( 1 . 9 7 o ) v i r a l i s o l a t e s . These

i s o l a t e s have been i d e n t i f i e d as f o l l o w s : 7 CHIK, 3 Congo, 2 Z i k a , 1 Dugbe and 1 WSL. Si x remaining i s o l a t e s are i n t e r r e l a t e d and are a l s o d i s t a n t l y r e l a t e d to Yogue v i r u s from

Senegal. The importance of ar b o v i r u s e s i n p u b l i c h e a l t h can a l s o be emphasized by the

f i n d i n g given above that Lassa fever accounts f o r 1 0 7 o of a l l f e b r i l e i l l n e s s e s i n S i e r r a

Leone.

The r o l e of ar b o v i r u s e s i n the pro d u c t i o n of B u r k i t t ' s lymphoma , .

B u r k i t t ' s lymphoma i s suspected to be caused by an i n t e r a c t i o n between E p s t e i n - B a r r v i r u s

and other agents. This suggestion was made a f t e r f i n d i n g t h a t , although E p s t e i n - B a r r v i r u s

i s cosmopolitan, B u r k i t t ' s lymphoma cases are co n f i n e d to t r o p i c a l A f r i c a where m a l a r i a i s

p r e v a l e n t . This o b s e r v a t i o n prompted researchers to conduct an experiment i n Tanzania to

f i n d out whether chemotherapeutic suppression of m a l a r i a r e s u l t s i n a r e d u c t i o n of B u r k i t t ' s

lymphoma cases. 2 The r e s u l t s are not yet known. Since c l i m a t i c c o n d i t i o n s f a v o u r a b l e to

ve c t o r s of m a l a r i a are a l s o favourable to vectors of a r b o v i r u s e s , the e t i o l o g y of B u r k i t t ' s

lymphoma can best be e s t a b l i s h e d by conducting s t u d i e s on the B u r k i t t ' s lymphoma t r i a d :

E p s t e i n - B a r r v i r u s , m a l a r i a and a r b o v i r u s e s .

RECOMMENDATIONS

Yellow f e v e r

1 . V i r u s and s e r o e p i d e m i o l o g i c a l s t u d i e s should be s t a r t e d or continued i n monkeys and

rodents to determine the a c t i v i t i e s of j u n g l e y e l l o w f e v e r i n endemic c o u n t r i e s so t h a t

a p p r o p r i a t e p r e v e n t i v e measures can be taken to avert epidemics.

2 . The r o l e of va r i o u s species of mosquitos, p a r t i c u l a r l y Ae. a e g y p t i , Ae. a f r i c a n u s and

Ae. simpsoni, found i n v a r i o u s endemic regions should be d e f i n e d i n r e l a t i o n s h i p to e r u p t i o n s

of epidemics by conducting s t u d i e s such as p o p u l a t i o n d e n s i t i e s , f e e d i n g h a b i t s and s u s c e p t i ­

b i l i t y to y e l l o w f e v e r v i r u s .

3 . There i s a need to determine f a c t o r s which have prevented the occurrence of y e l l o w f e v e r

epidemics i n most Eastern and C e n t r a l A f r i c a n c o u n t r i e s f o r many years d e s p i t e the f a c t t h a t

y e l l o w f e v e r i s endemic i n these c o u n t r i e s and v a c c i n a t i o n has been l i m i t e d to i n f a n t s and

overseas t r a v e l l e r s .

4 . P r e c a u t i o n a r y measures a g a i n s t episodes of y e l l o w f e v e r should i n c l u d e v a c c i n a t i o n

campaigns every 1 0 years i n a l l c o u n t r i e s v^ere y e l l o w f e v e r i s endemic. F u r t h e r , persons

who are at high r i s k , namely, those working i n f o r e s t s as w e l l as medical and l a b o r a t o r y

personnel i n v o l v e d i n y e l l o w f e v e r s t u d i e s , should r e c e i v e booster doses of y e l l o w f e v e r

vaccine whenever necessary.

5. V i s i t o r s from non-endemic c o u n t r i e s to be v a c c i n a t e d a g a i n s t y e l l o w f e v e r r e g a r d l e s s of the p e r i o d of t h e i r stay i n y e l l o w f e v e r endemic c o u n t r i e s .

!

V I R / C C / 8 1 . 1 8

page 4

R i f t V a l l e y f e v e r

1. Seroepidemiological surveys to be conducted i n a l l A f r i c a n States t o determine the

d i s t r i b u t i o n of R i f t V a l l e y f e v e r i n man and l o w e r animals.

2. S u r v e i l l a n c e of R i f t V a l l e y f e v e r v i r u s c o n d u c t e d i n endemic c o u n t r i e s t o i n c l u d e the

t r a c i n g o f t h e s o u r c e o f i n f e c t i o n and t h e s u r v e i l l a n c e o f i m p o r t e d a n i m a l s and a n i m a l p r o d u c t s

o f a l l t y p e s .

3. The r o l e played by imported and smuggled animals and animal products i s to be d e f i n e d i n

r e l a t i o n s h i p to the dissemination of R i f t V a l l e y f e v e r .

4. Large q u a n t i t i e s of R i f t V a l l e y fever vaccine of recognized q u a l i t y , safety and e f f i c a c y

to be prepared and made a v a i l a b l e for emergency use i n man and domestic animals.

Other arboviruses

1. The importance of a r b o v i r u s e s , p a r t i c u l a r l y those causing v i r a l haemorrhagic f e v e r (VHF)

to be emphasized by encouraging specimen c o l l e c t i o n from acute cases o f suspected v i r a l

i n f e c t i o n s .

2. WHO to c o l l a b o r a t e with n a t i o n a l governments to c i r c u l a t e g u i d e l i n e s to medical

p r a c t i t i o n e r s and v e t e r i n a r i a n s on the c l i n i c a l and epidemiological manifestations of v i r a l

d i s e a s e s , specimens to be taken and a set of actions to be followed i n future episodes, s i m i l a r

to what was given f o r E b o l a - l i k e diseases.16

3. WHO to recommend c e r t a i n laboratory techniques to be used i n c o l l a b o r a t i n g centres f o r

e f f i c i e n t v i r u s i s o l a t i o n and r a p i d i d e n t i f i c a t i o n . New s e r o l o g i c a l t e s t s such as i n d i r e c t

immunofluorescence, radioimmunoassay and enzyme-linked immunoadsorbent assay to be a p p r a i s e d

f o r use i n c o l l a b o r a t i n g c e n t r e s . Where necessary, an adequate supply of d i a g n o s t i c

substances to be provided f o r implementation of these recommendations.

4. To t r a i n researchers who are d i r e c t l y engaged i n arbovirus studies i n the use of the

recommended procedures to ensure r a p i d diagnosis of v i r a l i n f e c t i o n s and proper e v a l u a t i o n o f

e p i d e m i o l o g i c a l data from various l a b o r a t o r i e s . S i m i l a r l y , workshops to be organized f o r

t h i s purpose.

5. WHO to increase the number of c o l l a b o r a t i n g centres i n A f r i c a and sponsor more r e s e a r c h

p r o j e c t s on arbovirus d i s e a s e s .

6. WHO to i n i t i a t e and promote j o i n t research p r o j e c t s on v i r a l diseases by a l l c o u n t r i e s i n

each region f o r economic reasons and s c a r c i t y of manpower and also because the a c t i v i t i e s o f

v i r u s e s and t h e i r vectors are not r e s t r i c t e d by geographical boundaries. S i m i l a r l y , WHO

should encourage i n d i v i d u a l c o u n t r i e s to disseminate t h e i r f i n d i n g s to other countries i n the

same or d i f f e r e n t regions f o r i n f o n n a t i o n and a c t i o n .

Lassa, Marburg and Ebola

1. A t e c h n i c a l team of experts to be formed f o r working out a m u l t i d i s c i p l i n a r y approach i n

planning research p r o j e c t s and a d v i s i n g on c o n t r o l measures, i n c l u d i n g vaccine production

against t h i s group of d i s e a s e s .

2. A thorough s e r o e p i d e m i o l o g i c a l survey to be conducted i n a l l A f r i c a n States to determine

the geographical d i s t r i b u t i o n , r e s e r v o i r and amplifying hosts as well as p o s s i b l e vectors

i n v o l v e d . - '

3. Safety m a t e r i a l s and equipment, such as gowns, gloves, masks and caps and also immune

g l o b u l i n s a g a i n s t these v i r u s e s to be supplied to c o l l a b o r a t i n g centres as precautionary

measures against another epidemic. \!

i

V I R / C C / 8 1 . 1 8

page 5

4. S p e c i a l containment h o s p i t a l s and l a b o r a t o r i e s to be set up i n each endemic r e g i o n to minimize r i s k s i n v o l v e d i n sending overseas suspected or confirmed cases f o r treatment.

5. To prevent an alarm and the expenses i n v o l v e d tn i n s t i t u t i n g p u b l i c h e a l t h measures ag a i n s t suspected VHF cases, a l l overseas persons v i s i t i n g any sub-Saharan A f r i c a n country should take adequate m a l a r i a p r o p h y l a x i s . 3

6. U n t i l adequate i n f o r m a t i o n i s a v a i l a b l e on the r o l e of v a r i o u s species of mammals, b i r d s and r e p t i l e s i n the t r a n s m i s s i o n of VHF, WHO should order that any of these animals be exported from A f r i c a only a f t e r being t e s t e d by s p e c i f i e d procedures and found f r e e from VHF v i r u s e s and a n t i b o d i e s .

YELLOW FEVER I N AFRICA: 19 7 7-1979

Country 19 7 7 1978 1979

Country Case Death Case Death Case Death

Gambia

Ghana

N i g e r i a

Senegal

110 33

30

219

30

40 494

11

1

120

T o t a l 110 33 249 70 506 120

Réf.: WHO Wkly epidem. Rec. , 1980, _55 , 345-351

VIR/CC/81.18

page 6

REFERENCES

1. Berdonneau, R., Série, C. , Pathier, R. , Hannoun, C., Papaionnou, S. G. & Georgieff, p. (1961) B u l l . Soc. Path, exot., 54, 267-283

2. de-The, G., Geser, A., Day, N. E., Tukei, P. M., William, E. H., B e r i , D. P., Smith, P. G_ Dean, A. G., Bornkamm, G. W., Feorino, P. &Henle, W. (1978) Epidemiological e v i d e n c e ' for causal relationship between Epstein-Barr virus and Burkitt's lymphoma from Ugandan prospective study. Nature, 274, 756-761

3. Edmond, R. T. D., Smith, H. & Welsby, P. (1978) Assessment of patients with suspected v i r a l haemorrhagic fever, B r i t , med. J . , j ^ , 966-967

4. Frame, J . D., Baldwin, J . M., j r , Cache, D. J. & Troup, J . M. (1970) Lassa fever, a new virus disease of man from West A f r i c a . 1. C l i n i c a l description and p a t h o l o g i c a l findings, Amer. J . trop. Med. Hyg., 19, 670-696

5. Hoogstraal, H., Meegan, J . M., K h a l i l , G. M. & Adham, F. M. (1979) The R i f t Valley f e v e r epizootic i n Egypt 1977-1979. 2. Ecological and entomological studies, Trans, rnv Soc. trop. Med. Hyg. , 73, 624-629 ' ^'

6. Johnson, K. M. , McCormick, J . B., Webb, P. A., Keley, M. P., Regnery, R. C. & Morens , D. ^ (1980) Report from Special Pathogen Branch, Center for Disease Control, United S t a t e s ' of America, Arthropod-Borne Information Exchange No. 39, pp. 7-18

7. Kafuko, G. W., Henderson, B. E., Williams, M. C. & K i s s l i n g , R. E. (1970) Investigationg i n Uganda r e l a t i n g to Marburg agent. In: Infections and immunosuppression i n sub­human primates, Munksgaard & Copenhagen, Balner & Beveridge, pp. 45-48

8. Meegan, J . M. (1979) The R i f t Valley fever epizootic i n Egypt 1977-1979. 1. D e s c r i p t i o n of the epizootic and v i r o l o g i c a l studies. Trans, roy. Soc. trop. Med. Hy^. . 73, 618-623 *

9. Sehulz, K. H. (1951) R i f t Valley fever i n South A f r i c a . Special Report No. 5 / 5 I , Union Department of Health - Plague Research Laboratory, pp. 1-26

10. Série, C. , Andral, L., Lindrec, A. & Neri, P. (1964) Epidémie de fièvre jaune en E t h i o p i e (1959-1962), B u l l . Wld Hlth Org., 30, 299-319

U. Simpson, D. 1. F. (1978) V i r a l haemorrhagic fevers of man. B u l l . Wld Hlth Org., 56, 819-832 '

12. Soubert, J. D. S., Feguson, A. A. & Gear, J. (1951) R i f t Valley fever i n South A f r i c a . 2. The occurrence of human cases in the Orange Free State, the Northwestern Cape Province, the Western and Southern Transvaal. Epidemiological and c l i n i c a l f i n d i n s S. A f r . med. J . , 25, 890-891 '

13. Van Velden, D. J . J . , Meyer, J. D., Oli v e r , J . , Gear, J . H. S. & Mcintosh, B. (1977) R i f t

Valley fever a f f e c t i n g humans i n South A f r i c a , S. A f r . med. J . , 51, 867-871

14. Weiss, K. E. (1957) R i f t Valley fever - a review. B u l l , epizoot. Dis. A f r . . 5, 431-458

15. World Health Organization (1977) Wkly epidem. Rec.. 52, 177-180

16. World Health Organization (1977) Wkly epidem. Rec.. 52̂ , 185-192

17. World Health Organization (1977) Offset publication No. 36

18. World Health Organization (1980) Wkly epidem. Rec., _55, 345-351

19. World Health Organization (1980) Wkly epidem. Rec., 55, 355-360