D8854.PDF

Rev. sci. tech. Off. int. Epiz.,
1985, 4 (4), 795-801.
Bluetongue in the Sudan
E . M . E . A B U ELZEIN*
Summary: The epidemiological features of bluetongue (BT) in the Sudan are
discussed with reference to future control policies to be implemented. BT
virus type 5 and epizootic haemorrhagic disease of deer (EHD) virus type 2 are
the only BT and BT-related viruses recorded so far in the country. Culicoides
kingi and C. imicola act as vectors of the virus, and cattle are its reservoir.
Although a high incidence of BT antibodies has been detected among
cattle, sheep, goats and camels in the Sudan, the clinical disease has been seen
only in sheep.
No vaccination against the disease is practised in the country.
K E Y - W O R D S : Bluetongue virus - Camel - Cattle - Culicoides - Disease con­
trol - Epidemiology - Goat - Serotypes - Sheep - Sheep disea­
ses - Sudan.
INTRODUCTION
Bluetongue (BT) is an infectious, non-contagious disease of sheep, cattle, goats,
and other ruminants. It is caused by an orbivirus of the family Reoviridae. Twentyfour virus serotypes are known. They share a c o m m o n antigen which cross-reacts in
the agar gel immunodiffusion (AGID), complement fixation (CF), and fluorescent
antibody (FAT) tests. Specific virus serotyping is achieved by using the classical
serum neutralisation test (SNT), the micro SNT, plaque reduction (PR), plaque
inhibition (PI) and heamagglutination inhibition (HI) tests using highly purified
virus (16).
BT and related diseases are worldwide in distribution.
The first record of BT in the Sudan was made in 1953 (6) when samples from
suspected sheep were confirmed by the Onderstepoort Laboratory to contain BT
virus. In later years, BT virus isolations and antibody surveys of Sudanese animals
revealed that the disease was widespread (9, 10, 1, 2, 3, 4, 5). In spite of this, n o
vaccination is practised against the disease in the country.
No reports regarding the introduction of BT into the Sudan are available.
S U D A N E S E STRAINS OF B L U E T O N G U E VIRUS
In the Sudan, strains of BT virus have been isolated so far from diseased sheep,
apparently healthy cattle and the Culicoides vectors.
* Central Veterinary Research Institute, Alamarat, P.O. Box 8067, Khartoum, Sudan.
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BT virus isolation from diseased sheep
The first record of BT virus isolation from sick sheep was made when samples
from the Blue Nile Province were confirmed by the Onderstepoort Laboratory to
contain BT virus (6). Subsequently, the virus was isolated from outbreaks involving
sheep in Western Sudan (10) and from several outbreaks in lambs in Khartoum
Province (2, 5).
BT is widespread in sheep in the Sudan, but the disease is often confused with
other clinically similar diseases, and samples are rarely sent for virological investi­
gation.
During an outbreak of the disease in sheep in Khartoum Province, BT virusprecipitating antigen was detected in serum and plasma from affected animals.
Later, BT virus was isolated and identified from them.
BT virus isolation from apparently healthy cattle
In collaboration with the Animal Virus Research Institute at Pirbright, UK, and
our laboratory at Soba, Suda, two strains of BT virus were isolated from the blood
of apparently healthy cattle from the Sudan (Mr. K. Herniman, personal communi­
cation). These are being serotyped.
In a serological survey of apparently healthy Sudanese cattle, BT virusprecipitating antigens were detected in serum from some of these cattle. BT virus
was later isolated and identified from them (3). That virus isolate was found to be
pathogenic for sheep (to be published).
Several other BT virus isolates have been recovered from the blood of appa­
rently healthy Sudanese cattle (3).
BT virus isolation from the Culicoides
vectors
A comprehensive study by Boorman and Mellor (7) has shown that the Colicoi­
des vectors of BT do exist in the Sudan, although they failed to isolate virus from
them. In a further work, Mellor et al. (14) succeeded in isolating BT virus of
serotype 5 and epizootic haemorrhagic disease of deer (EHD) virus of serotype 2
from C. imicola and C. kingi (respectively), suggesting that they could be involved
in the biological transmission of BT virus in the Sudan.
ANTIBODY SURVEYS IN S U D A N E S E LIVESTOCK
Serological surveys of BT precipitating antibodies have been performed only on
domestic animals so far (10, 8, 1, 4). Such antibodies were found to be widespread
all over the country, with high incidence in sheep, cattle, goats, and relatively low
levels in camels (4).
Attempts to use the micro SNT to detect neutralising antibodies against the spe­
cific serotypes of BT virus in the Sudanese sera were unsuccessful due to crossreactions between the virus serotypes (Mr. K. Herniman, personal communication).
This was attributed to repeated infection of animals by the different virus serotypes
in the field.
In a recent serological survey (2), it was found that viraemic animals could be
detected serologically by using the A G I D test. Such animals do not contain BT anti­
bodies in their serum and thus could have been judged as negative despite the fact
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that they harboured infectious BT virus (3). Detection of such animals in a serologi­
cal survey will undoubtedly increase the number of animals with evidence of expo­
sure to BT infection (11).
N A T U R A L
CLINICAL
B L U E T O N G U E
A N I M A L
IN
SUDANESE
SPECIES
Sheep
The two forms of the disease seen in sheep in the Sudan are acute and mild.
1. Acute
form
This is usually seen in lambs (3-12 m o n t h s old) with almost 100% morbidity rate
and around 4 0 % mortality rate (5). It is manifested by pyrexia, inappetence, saliva­
tion, nasal discharge, conjunctivitis, hyperaemia. of the buccal cavity and nasal
mucosae, cyanosis of the tongue, ulceration of the dental pad and the tongue epi­
thelium, swelling of the knee joints and lameness, which is usually followed by
recumbency and death.
2. Mild
form
This is mostly seen in adult sheep. It is manifested by high fever, nasal discharge
and lacrimation. Some pregnant animals may abort.
Cattle
Despite repeated isolations of virus from apparently healthy cattle and the
detection of precipitating antigens and antibodies to BT virus in their serum, the
natural disease has never been seen in cattle. However, M o h a m e d et al. (15) believe
they saw clinical BT in a calf in K h a r t o u m Province. Unfortunately, they failed to
isolate the virus and to reproduce the disease in experimentally inoculated lambs or
calves.
Other animal species
The natural disease has never been seen in any other wild or domestic animal
species in the Sudan.
LABORATORY
DIAGNOSIS
Prompt serological detection of viral antigen in sick animals
In earlier years, early serological detection of BT virus was attempted on tissues
from sick animals. Recently (2, 5), it was found that blood plasma and serum from
naturally and experimentally infected lambs in the Sudan contain high titres of BT
virus, and this may be employed for early serological diagnosis of the disease in sick
animals. The procedure consists of collecting whole blood in edetic acid (EDTA)
and centrifuging at 1000 rpm for 10 minutes to separate the plasma, which is
collected and tested against a k n o w n group-specific BT antiserum in the A G I D test
(2). Precipitin lines are usually seen after 18 hours. This technique has been used to
diagnose several outbreaks in sheep in the S u d a n . Virus isolation from plasma of
infected animals has always correlated well with the results of the A G I D test (3).
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Isolation of virus from animals
Early attempts to isolate virus depended on the inoculation of mice, the yolk sac
of embryonated hens' eggs or tissue culture with citrated whole blood or other mate­
rial. Later it was found that sonicated blood inoculated intravascularly into chick
embryos gave the best results (5). This was usually followed by further passages in
baby hamster kidney cells, primary bovine kidney, or chick-embryo fibroblasts (5).
Concentration of the isolated virus by ultracentrifugation (5, 3) provided good
yields of virus suitable for the rapid A G I D test. This gave better results than unconcentrated virus from cell culture material (Dr. M. Jeggo, personal communication).
CONTROL OF BLUETONGUE IN T H E S U D A N
Neither vaccination nor any other control measure is employed in the Sudan.
ECONOMIC CONSIDERATIONS
Many range areas in the Sudan have suffered considerably from the drought of
the past 3-4 years, which has been aggravated by encroaching of desertification. As
a result, several thousands of sheep have been lost. Such losses are undoubtedly
significant in an African country like the Sudan, where the contribution of animal
production is vital to combat the shadow of famine in the area.
In an attempt to p r o m o t e sheep production, several intensive sheep fattening
and breeding farms have been established along the Nile banks in Khartoum Pro­
vince and elsewhere in the country. Production flourished at first, but then the
annual lamb crop was depleted by continuous outbreaks of BT, which have caused
immense economic losses (5). Adult sheep were also affected, resulting in lowered
feed conversion, abortion, and other ill-effects.
There has not yet been any detailed study of the economic effects of BT on
cattle and other animals species in the Sudan.
DISCUSSION
In earlier years it was thought that BT was of little significance among the disea­
ses of animals in the Sudan. Such a belief was founded solely on clinical observa­
tions without supporting virological evidence.
With the introduction of virological research in this field in the mid-seventies
(9), BT was recognised as a real threat to Sudanese sheep, and the serious losses
which were once attributed to other causes were discovered to be due to BT (9, 10,
1, 2, 3, 4, 5).
Although there has been little investigation of the reservoir(s) of BT virus in the
Sudan, cattle have conclusively been shown to harbour the virus without showing
signs of the disease (2, 3; Dr. M . Jeggo, personal communication).
Recent information has shown that susceptible calves in the Sudan can be expe­
rimentally infected with BT virus, contracting a severe form of the disease
(Dr. Faysa A. Omer, personal communication). So, with the introduction of highly
susceptible foreign breeds of cattle into the Sudan, it is expected that BT may infect
them, bearing in mind that most of the dairy farms having foreign cattle are establish­
ed in irrigated lands, where insect vectors of BT are available throughout the year.
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Detection of BT virus in plasma or serum permits a rapid diagnosis of BT. This
is because the blood of infected animals is the richest source of the BT virus (13).
However, blood cannot be used directly for detecting the virus by means of the
rapid A G I D test, because haemolysis of the erythrocytes and the release of their
pigments into the gel make reading of the results difficult or impossible. Instead,
blood plasma a n d / o r serum, which are equally rich in the virus (13) are used in the
AGID test for the serodiagnosis of the BT virus (2, 5).
Despite the isolation of an appreciable number of strains of BT virus from sick
and apparently healthy animals in the Sudan, few of them have been serotyped.
The provision of an efficient BT serotyping unit in the Sudan would provide rapid
information on the types of virus isolated from animals and insect vectors in the
country. This would be of great value for international epidemiology and for any
future vaccination scheme in the country. Until such a unit is established, virus iso­
lates should be sent to reference laboratories abroad for serotyping.
There is little information on the insect vectors of BT in the Sudan. A conti­
nuous, long-term study of the vectors, their habitat and ecology would be vital for
future control p r o g r a m m e s .
Although results of BT antibody surveys conducted on Sudanese animals show
that the incidence is highest in Southern Sudan and in other parts of the country
where the insect vectors are a b u n d a n t , incidence is still high in areas where insuffi­
cient water is available for the breeding of the vectors. In such a situation, infected
midges might have been blown by prevailing winds to such areas, resulting in infec­
tion or seroconversion in animals. T h u s , meteorological considerations should be
taken into account when planning for the control of BT in the Sudan.
Animals found to be serologically negative during a survey should be tested for
the presence of precipitating BT antigen. This is because such animals may have
been viraemic at the time of bleeding (11, 2) and before BT antibodies have appear­
ed in their serum, and thus could be overlooked as BT-exposed animals.
With all the information currently available, it is felt that a well-implemented
control policy for BT is essential in the Sudan. This could entail the following:
1. Reduction of the populations of insect vectors.
2. Isolation and serotyping of as many strains of BT virus as possible from sick
and apparently healthy animals, and from the insect vectors.
3. Formulation of the most appropriate vaccine, taking into account the latest
theories concerning the composition of BT vaccines (12).
4. Vaccination of susceptible sheep in breeding and fattening centres, sheep at
risk, and vaccination in the face of outbreaks.
5. Vaccination of foreign and cross-bred cattle.
6. Priority to vaccination in areas where the disease is endemic and where highly
productive animals are kept. Such areas are Khartoum Province and the Central
Region.
CONCLUSION
Bluetongue is highly endemic in the Sudan. Overt clinical disease has been seen
only in sheep so far, causing great economic losses. Cattle h a r b o u r the virus
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without necessarily showing symptoms, and thus can spread infection to sheep. BT
virus type 5 and epizootic haemorrhagic disease of deer virus serotype 2 have been
isolated from Culicoides midges. C. kingi and C. imicola are potential vectors of
the disease in the Sudan. No vaccination or other control measures are practised in
the country.
*
* *
LA FIÈVRE C A T A R R H A L E D U MOUTON A U S O U D A N . — E.M.E. Abu Elzein.
Résumé : L'auteur étudie les caractéristiques épidémiologiques de la fièvre
catarrhale du mouton ou bluetongue (BT) au Soudan sous l'angle des actions
futures de prophylaxie à mettre en œuvre. Les seuls types de virus de la fièvre
catarrhale ou apparentés reconnus jusqu'à présent dans le pays sont le virus
BT de type 5 et le virus de la fièvre hémorragique épizootique (EHD) de type
2. Le virus BT a pour vecteurs Culicoides kingi et C. imicola, et pour réservoir
les bovins.
Bien qu'une incidence élevée d'anticorps vis-à-vis du virus BT ait été décelée parmi les bovins, les ovins, les caprins et les chameaux au Soudan, la maladie clinique n 'a été constatée que chez les ovins.
Aucune vaccination n'est pratiquée contre la maladie dans le pays.
M O T S - C L É S : Bovins - Chameau - Chèvre - Culicoides - Epidémiologie Maladies des ovins - Ovins - Prophylaxie - Virus de la fièvre
catarrhale ovine - Sérotypes - Soudan.
*
* *
LA LENGUA AZUL EN S U D Á N . — E.M.E. Abu Elzein.
Resumen : Estudia el autor las características epidemiológicas de la lengua
azul o bluetongue (BT) en Sudán desde la óptica de las actuaciones futuras de
control que se hayan de emprender. Los únicos tipos de virus de la lengua azul
o emparentados reconocidos hasta la fecha en el país son el virus BT de tipo 5
y el virus de la fiebre hemorrágica epizoótica (EHD) de tipo 2. El virus BT
tiene por vectores Culicoides kingi y C. imicola, y por reservorio a los bovinos.
Aunque se haya descubierto una elevada incidencia de anticuerpos frente
al virus BT entre bovinos, ovinos, caprinos y camellos en Sudán, únicamente
se ha verificado la enfermedad clínica en los ovinos.
No se ha practicado ninguna vacunación contra la enfermedad en el país.
P A L A B R A S C L A V E : Bovinos - Cabra - Camello - Control - Culicoides Enfermedades ovinas - Epidemiologéña - Ovinos Serotipos - Sudán - Virus de la lengua azul.
*
* *
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and plasma of camels, sheep a n d cattle in t h e S u d a n , using the gel immunodiffusion test.
Arch. Virol., 79, 131-134.
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