African Swine Fever (ASF) is a highly contagious and fatal viral disease of
animals in the pig family (Geering et al., 2001).
ASF virus is hardy and it belongs to the family Asfaviridae and genus Asfivirus
(Wilkinson and Denyer, 1992). This virus will survive
for 15 weeks in putrefied blood, 70 days in blood on wooden boards, 11 days
in faeces at room temperature, 3 h at 50°C, 18 months in pig blood at 4°C,
150 days in boned meat at 39°C and 140 days in salted dried hams (Valleeet
Domestic pigs are highly susceptible to infection and African warts and bush
pigs serve as reservoir to the aetiologic agent (Vallee et
al., 2001). There is no human risk associated with the disease but it
has several factors that make it a potential biological weapon which includes
its high mortality and morbidity in infected animals. The virus is also highly
communicable both directly and indirectly. There is no treatment and vaccines
for the disease (OIE, 2000).
Outbreaks of the disease have not been common for many years. But in 1998-99, a pandemic of ASF swept through many countries in Africa, causing high mortalitities in pigs and heavy economic losses to farmers. The lesions of many of the Nigerian outbreaks were studied to compare the pathotype involved in that pandemic with existing pathotypes.
Clinical report, histopathology and diagnosis: Twelve outbreaks of ASF were studied in Nsukka and Enugu areas of Enugu state, Nigeria. Clinical signs presented were anorexia, shallow and rapid respiration of 100-160 min-1, sway gait and depression. Body temperature ranged between 39 and 41.1°C. There was extensive erythema of the skin, especially over the ears, flanks, back, ventral areas of the thorax and abdomen. Treatments with antibiotics and trypanocides did not appear to be of any benefit. Death occurred within 5-6 days of onset of clinical signs.
Necropsy of the dead animals showed haemorrhages on the pleural and peritoneal membranes. Spleen and lymph nodes were enlarged and dark red. Lung was swollen and contained excess fluid and blood. The sub-capsular surface of the kidney and the mucosa of the urinary bladder were petechiated. Samples of the liver, spleen, kidney, mesenteric lymph node and lungs were fixed, processed, embedded in paraffin wax, sectioned, stained with Haematoxylin and Eosin (H and E) and studied under the light microscope.
There were areas of infarction (Fig. 1), karyorrhexis and
depletion of lymphocytes (Fig. 2). The mesenteric lymph node
also showed severe lymphocytic depletion and necrosis (Fig. 3).
There was severe flooding of the pulmonary alveoli and bronchioles by eosinophilic
exudates (Fig. 4, 5). Tiny round eosinophilic
intranuclear inclusion bodies 2-4 nucleus-1 were observed in the
hepatocytes (Fig. 6).
||Spleen: histological section showing haemorrhagic infarct.
||Spleen: histological section showing Karyorrhexis and depletion
of lymphocytes. HE x200
||Lymph nodes: histological section showing Karyorrhexis and
depletion of lymphocytes. HE x200
The hepatic sinusoids were distended (Fig. 6) while the portal
canal was oedematous and often infiltrated by lymphocytes. The hepatic arterioles
showed proliferation and thickening of the endothelium with severe congestion
and dilatation of the hepatic veins.
||Lungs: histological section showing eosinophilic exudates
in and outside the pulmonary bronchioles. HE x200
||Lungs: histological section showing massive flooding of the
pulmonary alveoli by exudates. HE x200
||Lungs: histological section showing tiny intranuclear inclusions
in the hepatocytes and dilated sinusoids. HE x200
Hyaline casts and pyknosis of renal epithelial cells were observed in the
tubules and ducts. Congestion and haemorrhages were common in allthe organs
studied. Serum samples were assayed for ASF antibody using the Indirect-enzyme
Linked Immunosorbent Assay (I-ELISA) method, outlined by Office of International
Epizootics (OIE, 2000).
|| Detection of antigens in tissues and antibody in sera of
The biologicals and reagents were supplied by Instituto National de Invevstigaciones
Agrarias-centro de Investigacion en Sanidad Animal (INIA-CISA) (Madrid, Spain).
The diluted ASF positive antigen was dispensed into the wells of the microtitre
plate and incubated overnight at 4°C. The plates were washed with buffer.
The diluted serum samples known ASF positive and negative sera were dispensed
into the wells and incubated on a shaker for 1 h at 37°C. The plate was
washed and the diluted protein A horse radish peroxidase conjugate was added
and incubated at 37°C for 1 h. The plate was washed and the substrate solution
was added and incubated at room temperature for 15 min. The reaction was terminated
with stopper solution and titres were read in an ELISA plate reader connected
to an IBM computer at 492 nM wave length or filter. The samples gave positive
results for ASF antibody (Table 1). The impression smears
of the mesenteric lymph nodes were also assayed for ASF antigen using direct
immunofluorescent method described by OIE (2000). The
biologicals and reagents were supplied by INIA-CISA (Madrid, Spain). Impression
smears of the tissue samples known ASF positive and negative tissues were made
on a clean glass slides and air-dried. The smears were fixed in acetone at room
temperature for 10 min.
The slides were washed in Phosphate Buffered Saline (PBS) for 3 min and air dried. The smears were encircled with wax pencil and ASF antibody conjugated to fluorescein isothiocyanate was added and incubated for 1 h at 37°C. The slides were washed, air dried, mounted with cover slips in 10% PBS-glycerol and studied under the fluorescent microscope. The samples gave positive fluorescence for ASF antigens (Table 1).
The above observations are in agreement generally with the clinical and pathological
changes earlier described for ASF (Smith et al., 1974;
Timonery et al., 1992; Seifert,
1996; Rossiter, 1998; Vallee et
al., 2001). However, the intranuclear inclusions described in the report
do not appear to have been observed earlier in ASF. Rossiter
(1998), Timonery et al. (1992) and Seifert
(1996) reported no inclusion body. But Smith et al.
(1974) described intranuclear inclusions in the neurons only. Nervous involvement
was not seen in the cases researchers studied. Necropsy showed no evidence of
parasitic infection. The role played by other possible secondary or pre-existing
infections in the severity of the outbreaks was not clear. However, the fact
that treatment with antibiotics and trypanocides did not produce beneficial
effect gave an indication that bacterial and trypanosome secondary infections
were either absent or minimal.
From this study, it could be founded that the virus involved in the pandemic is similar to the existing pathotypes. The need for development of vaccines for control of this disease cannot be over emphasized even though the farmers were paid compensation by the Government of the Federal republic of Nigeria.
Farmers should adhere strictly to biosecurity measures which includes thorough decontamination of infected premises and proper disposal of carcasses and wastes. There should be designation of infected zone with control of pig movement. All suspected cases must be reported to the appropriate authorities for immediate investigation. Contacts between domestic pigs and wild pigs should be avoided and wild pigs to be sold by hunters should be inspected and tested for ASF. Detailed epidemiological investigation with tracing of possible sources (upstream) and possible spread (downstream) of infection must be carried out.