A clinico-pathological study of equine trypanosomiasis.

In view of the paucity of information on the pathogenesis, pathology and diagnosis of equine trypanosomiasis, an outbreak which occurred in a herd of 36 horses on a farm at Thika, Kenya was investigated. In this outbreak, 6 horses were initially found to be infected with trypanosomes, two of them twice, over a period of 7 months. In the investigation that followed, the specific objectives were :- (1) To establish the disease prevalence and incidence; (2) To identify and characterise the species of trypanosomes involved; (3) To institute chemotherapy and monitor the response of the horses to treatment; (4) To confirm whether a trypanosome isolate from the infected horses on the farm would produce a similar disease in experimentally infected horses, and to observe the clinical course of infection; (5) To advise on the best method for control of the disease on the farm. The horses on the Del Monte farm were examined at least once a month during the period May, 1990 to November, 1991 (19 months). A total of 25 out of 36 horses succumbed to trypanosome infection, including the six initially found to be infected. The disease prevalence was therefore high (69.4 %) during the approximately 2 year period from the onset of the outbreak. The incidence of infection at the beginning of the farm visits was also high with 16 cases detected in the first 2 months, but thereafter dropped and became low and seasonal. The majority of horses that became infected were those used to patrol the farm, in contrast to the breeding and young animals which were confined to a stable area. Clinically, the infected horses exhibited poor body condition, weakness with lethargy, fever, a high respiratory and heart rate, as well as icteric or pale mucous membranes, often with petechiae. Some of them also had oedema of the lower parts of the body, particularly the hind limbs. The parasitological tests employed, which included microscopic examination of Giemsa stained blood and buffy coat smears, the buffy coat technique, and mouse inoculation, all detected I. congolense only, on all the farm visits. Using antigen ELISA, however, the animals were shown to have T. congolense, T. vivax or T. brucei antigens in the blood circulation. Mixed trypanosome infections were evident in 58% of the cases but with T. congolense predominating. On haematological examination, a decrease in the blood packed cell volume (PCV) was noted. This was particularly marked in the animals with mixed infections involving T. congolense and T. brucei as well as those infected with all 3 species, in contrast to those with single trypanosome infections. The infected horses were subsequently treated using diminazene aceturate at a dose range of 3.5-7 mg/kg. The majority of the animals recovered as evidenced by aparasitaemia, a decline in "antigenaemia" and an improved clinical condition. However, a relapse infection or re-infection was observed in 40% of the animals, 2 or more months later. Hence additional treatment at 6 or 7 mg/kg diminazene aceturate was instituted to effect full cure. One of the infected animals, nevertheless, died despite chemotherapy. The ante-mortem and post-mortem findings revealed that it died due to severe anaemia and probably renal failure too. To identify the vector of the disease, ten odour baited NG2G (NGU) traps were set up in different areas of the farm. The traps were checked for tsetse and other biting flies at 2-4 week intervals for a period of 14 months. A total of 609 Glossina pallidipes were trapped in 8 different locations. Biting flies such as Stomoxys, Tabanus, Haematopota and Philoliche species, were also trapped. The numbers of tsetse and other flies trapped were greater during and immediately after the rainy season. This correlated with the incidence of trypanosomiasis in the horses which was also higher during this period. Hence, G. pallidipes and to some extent other biting flies, were most likely responsible for transmission of the parasites to the horses. A few (18) tsetse flies found live were dissected but no trypanosomes were detected. The failure to detect trypanosomes in Glossina was most probably due to a low infection rate in the tsetse population and the small number of flies dissected. During the field study, 18 trypanosome isolates were obtained from the blood of the infected horses for characterization. All the isolates were identified morphologically as belonging to the subgenus Nannomonas. The majority grew in mice which is characteristic of T. congolense in contrast to the other species of this sub-genus, T. simiae, which does not. Using recombinant DNA hybridization probes, the isolates were further identified as the "savannah type" genotype (or subspecies) of T. congolense. There were 5 different chromosome profiles displayed among 6 isolates examined by orthogonal field alternation gel electrophoresis (OFAGE), indicating a diversity of serodemes (strains). In order to confirm that the T. congolense isolates obtained from the infected Del Monte farm horses were the cause of the disease observed, an experimental study was conducted using 8 horses. Six of the horses were infected by allowing experimentally infected G. pallidipes to feed upon them. The remaining 2 horses were fed upon by uninfected tsetse and were used as negative controls. The horses were examined daily for a period of 12 weeks and the clinical course of the disease observed. Blood collected in EDTA from each animal was taken daily for haematological investigations and for monitoring the course of parasitaemia. In addition, plasma samples were tested for circulating antigens and anti-trypanosomal antibodies using the antigen and antibody ELISA techniques. Serum samples were also obtained for bilirubin assay. The clinical picture of disease observed in the experimentally infected horses was similar to that observed in the field. This was irrespective of the fact that the field cases involved mixed trypanosome infections whereas the experimentally infected horses had T. congolense only. The blood tests revealed the development of anaemia with a reduction in the packed cell volume, haemoglobin concentration and red blood cell count, as well as thrombocytopenia. In addition, some of the T. congolense infected horses displayed a leucocytosis, while all the animals had intermittent hyperbilirubinaemia. A fluctuating parasitaemia and "antigenaemia" were observed. The anti-trypanosomal antibodies also varied in a similar manner. The course of the disease in the experimentally infected horses was noted to be variable. In 2 horses it was acute, terminating in death in one of them. Subacute and chronic infections were also observed in one and three horses respectively. Twelve weeks post-infection, the surviving experimentally infected horses were treated with diminazene aceturate at a dose rate of 7 mg/kg. The animals apparently recovered as described earlier. At 8 and 10 weeks post treatment, however, 2 of the horses developed a relapse infection. They were treated, this time using 1% Isometamidium chloride at a dose rate of 0.5 mg/kg, and were cured. In summary, trypanosomiasis was a severe disease in the horses and it had a high prevalence and incidence on the farm. Mixed infections involving T. congolense, T. vivax and T. brucei were demonstrated in the infected horses, but T. congolense appeared to play the major role in the pathogenesis of the disease. The T. congolense stocks isolated from the horses on the farm were of the "savannah type" and comprised several serodemes or strains. The course of T. congolense infection in experimentally infected horses compared well with the clinical picture observed in naturally infected horses on the farm. The infections were treatable using Diminazene aceturate, or by the sanative pair Diminazene aceturate and Isometamidium chloride. Finally, in view of the small number of horses at risk, the extensive acreage patrolled, and the wide distribution of the vectors, it was recommended that trypanosomiasis on the Del Monte farm would best be managed by diagnosis and treatment of infected animal