Pig lymphnode lesions and an immunohistochemical analysis on selected abattoir-derived lymphnode samples for the presence of African Swine fever virus

Abstract

The economy of Uganda is largely dependent on agriculture, and both crop and animal production contribute up to 29.9% of the national Gross Domestic Product (GDP). Although pig products constitute an important source of affordable animal protein for humans, swine diseases are a great limiting factor in the achievement of these goals. ASF has been reported in Uganda almost annually in sporadic form. This research was designed to investigate pig lymphnode lesions and an immunohistochemistry analysis on selected abattoir derived lymphnode samples for the presence of African Swine Fever (ASF) antigen. A total of 258 samples were collected for the study, with examination of collected lymphnode samples for gross and histopathological lesions. Ninety highly suspect lymphnode samples were then subjected to Immunohistochemistry (IHC) analysis for presence of African Swine Fever Virus antigen using polyclonal antibodies. The major gross lesion manifested was haemorrhagic lymphnodes 94 (36.4%). Others 17 (6.6%) presented with haemorrhage and edema, lymphnode necrosis 76 (29.4%) and atrophy 32 (12.4%) The least observed lesion was plain enlargement 6 (2.3%) whereas 156 (60.4%) without a significant lesion. On histopathology, the most observed lesion was parenchymal haemorrhage 131(50.8%). Others observed were subcapsular haemorrhage 97 (37.6%), and medullary haemorrhage 46 (17.8%). Follicular necrosis 126 (48.8%), follicular hyperplasia 124 (48.1%), lymphoid depletion 22 (8.5%), lymphoid proliferation 86 (33.3%) and eosinophil accumulation 48 (18.6%). The ninety (90) samples highly suspicious for African swine fever virus infection were subjected to IHC investigation. Twenty two (24.4%) samples presented positive for the viral antigen of interest. The origin and number of the pigs from which positive samples were identified on IHC was:- Jinja 3 (0.033%), Sembabule 3 (0.033%), Kiboga 3 (0.033%), Kayunga 2 (0.022%), Soroti 5 (0.05%), Masaka 4 (0.04%)and Luwero 2 (0.022%), Soroti and Masaka districts registering the highest number of cases amongst the sampled animals. These districts occasionally report ASF out breaks although confirmatory diagnosis is not usually carried out. Based on the findings of the study, it’s observed that there were lymphnode samples containing the ASFV antigen, an indication of subclinical or chronic form of the disease as clinical signs were not manifested in the live animals; implying that African swine fever is an endemic disease of pigs within Uganda. However, much as a number of lymphnodes presented with gross and microscopic lesions highly suggestive for the ASF virus infection and yet were negative on IHC, confirmatory diagnosis is the best option to ascertain the status of the case rather than rely on the elementary tools of diagnosis i.e. gross and histopathological lesions plus clinical signs. Further research involving field studies other than abattoir sites would provide an insight on the ASF strains responsible for the outbreaks of the disease in the country with possible goal of developing a vaccine for ASFV infection as treatment can`t do much.