Clinical and pathological observations in Kenyan donkeys experimentally infected with trypanosoma Congolense.
The study was conducted to establish the type and severity of Trypanasoma congolense infection in donkeys, particularly the clinical picture, and to correlate this with the effects on draught power and the role these animals play in the perpetuation of the disease. Five mature donkeys were subcutaneously inoculated with 7.5 x 106 blood stream forms of T. congolense, strain IL 3575. Three donkeys in adjacent fly proof stalls were kept as controls. The donkeys were monitored clinically on daily basis for a period of three months. Blood was collected from the jugular vein twice a week for haematology and once a fortnight for serum biochemistry. Blood from the marginal ear vein was drawn into heparinized capillary tubes for diagnostic purpose. Thin buffy coat smears were made, stained in 1:5 Giemsa for 15 - 20 minutes and observed under oil-immersion for the presence or absence of trypanosomes. Subinoculations into mice and sheep were done to verify whether the trypanosomes observed in donkeys were viable and patho~enic. The degree of anaemia in sheep and the rapidity at which mice died was the measure of pathogenicity. Donkeys were euthanized after three months and thorough post mortem examinations done. Samples were collected from the lymphoid organs, heart, lungs, skeletal muscles, kidneys, gastrointestinal-tract, brain and glandular organs whether showing gross lesions or not. They were fixed in ten per cent formalin, embedded in paraffin wax, sectioned at thickness of 6pm and stained with haematoxylin and eosin ( H & E). The infection had a prepatent period of 29 - 41 days in donkeys. The clinical picture indicated a subclinical infection with no signs but a positive diagnosis was made using thin stained buffy coat smears. Pyrexia peaks accompanying parasitaemic phases in ruminants and horses were lacking. The respiratory and heart rates were elevated during the third month of infection when pounding heart beats were picked on auscultation, and the mucus membranes were pale. Attempts to quantify the parasites in blood using a haemocytometer were unsuccessful because they were low in numbers and appeared sporadically. These trypanosomes were morphologically similar to those inoculated into the donkeys. The results showed red blood cells to decrease by 46.7% ; the packed cell volume by 41.6% haemoglobin concentration by 41.4% in the infected donkeys. In the control group, the red blood cells decreased by 28.6%; the packed cell volume by 22.2% and the haemoglobin concentration by 26.8%. This indicated that a decrease of 18.1%, 19.4% and 14.6% in the red blood cell count, packed cell volume and haemoglobin concentration, respectively in infected animals was attributed to T. congolense infection. Mean corpuscular volume, mean corpuscular haemoglobin and mean corpuscular haemoglobin concentration increased over pre-infection values by 15.8%, 30.1% and 6.3%, respectively in infected animals. Thus the anaemia observed could morphologically be classified as normochromic, macrocytic. The leukocytes counts showed minimal changes although a relative decrease in neutrophils and a relative increase in lymphocytes was observed in infected animals. Monocytes counts were higher in infected animals. This depicts an efficient regulatory mechanism(s). The fall in total plasma proteins plus a rise in eosinophil count in both groups of animals suggested a helminth infection. This was confirmed at post-mortem when helminth parasites were observed in the gastro-intestinal tract from the stomach, intestines to the colon. The parasites were mainly the small and large strongyles. This concurrent helminthiasis contributed to the decrease of 22.2% in packed cell volume in the two groups. The total bilirubin rose to 0.8mg% from 0.2 - 0.5mg% while blood urea nitrogen increased to 89mg% from 12 - 40mg%. These values were all within the ranges provided by other workers. At necropsy, the infected donkey carcasses were in fair body condition but moderate amounts of straw - coloured fluid was found in all the body cavities. The liver was slightly enlarged while the spleen displayed prominent Malphigian corpuscles. Microscopically, the lymph nodes had follicular hyperplasia characterized by many prominent follicles. Medullary cords were enlarged and had increased number of plasma cells as well as macrophages. The spleen had follicular hyperplasia of the Malphigian corpuscles, the peri arteriolar lymphatic sheaths were expanded while the marginal zones closely resembled the white pulp. Macrophages and plasma cells were encountered in the red pulp while mitotic figures of medium-sized lymphocytes were occasionally observed in the follicles. The lungs had patches of atelectasis, kidneys displayed areas of interstitial nephritis while the liver showed hydropic degeneration and occasional helminth larval migratory tracts. The mice peaked in parasitaemia 10 - 15 days after inoculation. Most of these died within 30 days although a few of them survived for more than 60 days. In sheep, a parasitaemia appeared 7 - 17 days after inoculation and persisted in transient peaks to the end of the experiment. A decrease of 17.5% in packed cell volume was attributed to trypanosomiasis. These observations suggests that donkeys may be potential reservoirs of T. congolense infections in endemic areas.