Pathogenesis and pathology of heartwater (cowdriosis) in sheep and goats
Heartwater is a rickettsial disease of ruminants primarily cattle, sheep and goats transmitted by several species of Amblyomma ticks. It is prevalent throughout Africa south of the Sahara wherever the vector ticks are found in association with livestock. The disease syndrome is characterised by fever, nervous, intestinal and pulmonary disorders. The mechanism of .i.mmun i,ty of heartwater has not been clearly understood. Cowdry (1925) identified Cowdria ruminantium as the causative agent of heartwater. He observed it In the endothelial cells of the ruminant host and in the epithelial gut cells of the vector tick Amblyomma habreum. He then observed that c. ruminantium replicated exclusively in the endothelial cells of ruminant host. This observation was supported by Jackson and Neitz (1932) and Pienaar (1970). Du Plessis (1910, 1975) however, added another dimension to the replication of c. ruminantium when he reported that the c.ruminant lymph nodes were the primary site of replication and that the ruminant endothelial cells were the secondary site. Be also found that the organism as it occurred in lymph nodes did nothave,a limiting membrane compared to the form occurring in vascular endothelial cells. A comprehensive study on the clinical pathology has been done by Ilemobade and Blotkamp (1978). The ultrastructural studies on the organism will broaden our understanding of the pathogenesis of the disease heartwater. The alm of the present study was to elucidate further the ultra-structural appearance of the organism c. ruminantium in the two sites of replication. The findings would then help in the understanding of the growth of the organism and may be useful to the immunological studies of heartwater disease. Attempts to confirm the organism in a blood smear have been unsuccessful. In vitro attempts a.t the culturing of ruminantium has to date been difficult. Twenty sheep and twenty goats were used in this study. They were bought from Kiambu which is known to be a heartwater free area, brought to the Faculty of Veterinary Medicine Kabete and housed In groups of four. They were then allowed to adjust to the stall feeding for two weeks, ear tagged, dewormed and then bled for baseline values for another week. The stabilate of C. ruminantium (in 10 ml aliquots) was obtained from Veterinary Research Laboratories at Kabete where it had been stored in liquid nitrogen (-196oC). The stabilate was thawed immediately in a water ba t h (37oC) and inoculated into three test animals. Subsequent passage was carried out from one animal to another by intravenous inoculation of whole blood with the donor and recipient animals standing side by side. Clinical examinations were done in the morning between 7-8 a.m., noting demeanour, rectal temperature, heart and respiratory rates, digestive system, appearance of mucous membranes and changes in locomotor and nervous systems. The average incubation period was found to be eight days for both sheep and goats following artificial inoculation. The disease was 100% fatal in goats while mortality in sheep was much lower (40%). The disease syndrome wa acute to peracute in goats and subacute to acute in sheep. Respiratory and nervous disorders were the two most marked signs seen terminally. Blood for haematology was obtained for red cells counts (RBC), white cell counts (WBC) both total and differential, haemoglobin (HB) and packed cell volume (PCV). The erythrocyte indices were later calculated to determine the mean corpuscular volume (MCV) a.n.dmean corpuscular haemoglobin concentration (MCBC) . Blood for serum biochemistry was obtained for Aspartate amino transferase (AST), Blood urea nitrogen (BUN) total protein content (TP), albumin and globulins. MCBC was found to have dropped significantly in test goats compared to controls with the maximum drop coinciding with the period of peak fever reaction. MCHC in sheep and lymphocytes in both species were found to have insignificant difference in both controls and test animals. WBC, RBC, HB, PCV and MCV were found to be significantly higher in controls of both specles compared to the test animals. AST, BUN and TP were founa to be significantly higher in test animals compared to controls during the same period of study. Blood was also processed for study using the light and electron microscopes to confirm the presence of the organism. Nine animals were sacrificed at various stages of the disease to obtain organ tissues for electron microscopic studies. Nine other animals were allowed to run the heartwater disease syndrone to death , Standard necropsy procedures were used for all the animals sacrificed or dying after the full course of the disease syndrome. The histopathological sections, including brain squash smears and impression smears were prepared routinely. Histopathological sections were stained with haematoxylin and eosin (H&E) while squash and impression smears were stained with Giemsa. The maln gross lesions were pulmonary oedema, petechial haemorrhages in various organs, hydropericardium and hydrothorax. Histopathology mainly revealed, cellular infiltration in various organs, congestion and haemorrhages. Little gross pathology was observed in the brain except for mild congestion and oedema. Impression smears of the lungs, various internal lymph nodes and spleen showed cytoplasmic, intranuclear and perinuclear presence of the organism. Free forms of the highly pleomorphic organism were also observed. Brain squash smears revealed the typical c. ruminantium mbrulae in endothelial cells and were used for confirmation of the disease in all cases. Electron microscopy showed perinuclear attachment of the organism in the lymphocytes and reticular cells of lymph nodes. The forms found in these cells did not have limiting membranes and seemed to be budding from the nucleus. The buffy coat showed possible organisms which were phagocytosed by circulating - macrophages. The endothelial cells revealed forms previously reported which were bound in a limiting membrane and which seemed to divide by binary fission. The most important finding which has not been reported before was the observation of intranuclear and perinuclear replication of the organism. From this study it was concluded that previous attempts to culture the organism in vitro had been difficult because of the two stage life cycle, the first of which requires the host cell nucleic material.