TRYPANOSOMA VIVAX FROM TSETSE INFESTED AND NON-TSETSE INFESTED AREAS OF NORTHWEST ETHIOPIA: INVESTIGATION ON PATHOGENICITY AND DRUG RESISTANCE
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Date
2015-02
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Abstract
Animal production in Ethiopia accounts for 15-17% of total Gross Domestic Product (GDP) and 35-49% of agricultural GDP. Unfortunately, the development and intensification of livestock productivity is hampered by devastating diseases, including African animal trypanosomosis. The most important trypanosomes, in terms of economic loss in domestic livestock, include: Trypanosoma vivax, T. congolense and T. b. brucei. Trypanosomes are often described as tsetse transmitted, but this is not always the case, with several species evolved for non-vector transmission or mechanical transmission by other flies. In Ethiopia, particularly the Northwest region is affected by both tsetse and non-tsetse transmitted T. vivax, with expected huge impact on livestock productivity. The control of trypanosomosis in Ethiopia relies on vector suppression and trypanocidal drugs, diminazene aceturate (DA) and isometamidium chloride (ISM) being the drugs most frequently employed. Consequently, drug resistance has become a common issue, especially for T. congolense in several areas. Although fragmented reports are available on the prevalence of the disease, attempts have not been done to demonstrate the relative importance of tsetse and non-tsetse transmitted T. vivax in terms of pathogenicity and drug resistance. This is important not only for evaluating treatment responses and to understanding host resistance/tolerance to the infections but also in the design of future vaccines and trypanocides, as well as the selection of hosts for tolerance or resistance to the parasites.
Therefore, our study started with a research questionnaire survey, a prevalence study and experimental studies were conducted to assess trypanocidal drug utilization practices, estimate prevalence of trypanosomosis and the consequent pathogenesis and drug resistance status of T. vivax isolates from tsetse-infested and non-tsetse infested areas of Northwest Ethiopia. The questionnaire survey showed trypanosomosis was a significant animal health constraint for 84% (n=84) and 100% (n=100) of the farmers questioned from non-tsetse and tsetse infested areas respectively. Responses on trypanocidal drug utilization indicated that risk factors for the development of drug resistance such as frequent treatment, poor handling and administration of trypanocides are prevalent and treatment failures are common. The investigation of bovine trypanosomosis carried out in the wet season of 2011 and 2012 showed the prevalence varied from 17.59% in 2011 to 25% in 2012 in tsetse infested areas, and the difference was (P < 0.001) significant. Similarly, in non-tsetse infested areas the prevalence varied from 3.85% in 2011 to 5.93% in 2012, but in this case the rise was not significant. T. congolense (75 %) was the most prevalent species followed by T. vivax (20.58%), and mixed infections (4.41%) in tsetse infested areas while in non-tsetse infested areas only T. vivax was detected.
For the investigation of pathogenicity and drug resistance against diminazene aceturate (DA) and isometamidium chloride (ISM), two consecutive experimental studies were performed on five different T. vivax isolates which we isolated from the field; three from a non-tsetse infested area and two from a tsetse infested area; in each case young Zebu cattle were experimentally infected with the isolates. Firstly one isolate from tsetse infested area (TT-ETBS1) and one isolate from non-tsetse infested area (NT-ETBD1) were tested. Secondly one isolate from tsetse infested area (TT-ETBS2) and two isolates from non-tsetse infested area (NT-ETBD2 and NT-
VI – Summary
ETBD3) were tested. In total 94 young Zebu cattle of which 34 for pathogenicity and 60 for drug resistance studies were used to conduct four different experiments in two consecutive experiments.
Parameters measured for the investigation of pathogenicity included clinical, haematological, biochemical, cytokine, and gross and histopathological changes over the course of an infection. The result shows that both TT and NT T. vivax parasites caused clinical infection, with parasites appearing in circulation earlier in NT than TT infected cattle. The infections were characterized by reduced feed intake, intermittent pyrexia, undulating parasitaemia, rough hair coat, enlarged superficial lymph nodes, pale mucus membrane, lacrimation, emaciation and dehydration. Less frequently, diarrhea, mandibular oedema, nervous signs and recumbence were observed in both groups of infected animals. The mean packed cell volume (PCV), haemoglobin, total red blood cell and white blood cell counts were significantly lower and mean corpuscular volume (MCV) was higher in infected animals and consequently the type of anaemia observed was macrocytic normochromic in nature. The extent of changes in serum biochemical parameters, except for few cases of differences, infection with T. vivax derived from both tsetse and non-tsetse infested areas initiate similar biochemical changes indicative of significant pathology and there is no difference between parasite origins and the two experimental infections. Upon euthanasia, infected animals showed enlarged and haemorrhagic spleen; swollen and edematous lymph nodes; pneumonic and emphysematous lung; enlarged liver and haemorrhagic lesions on the brain and intestine. Histopathology revealed significant abnormalities characterized by lymphoid hyperplasia of the spleen, interstitial pneumonia, hepatic necrosis, tubulointerstitial nephritis, meningoencephalitis, and lymphoid hyperplasia in lymph nodes.
With respect to the cytokine analysis, significant increases in immune cytokine (IFN-, TNF-α, IL-12 and IL-10) secretions were demonstrated in the first experiment among groups infected by both tsetse and non-tsetse T. vivax isolates, as compared with the non-infected group, but no statistical difference was observed between the groups infected with the two parasite types. However, in the second experiment, tsetse adapted T. vivax elicited a stronger immunological response than the T. vivax from the mechanically transmitted population. Concomitant with this observation is that blood parasite load remained low throughout the experimental period in the group TT compared to the NT groups.
The drug resistance studies revealed that treatment of infected cattle with the recommend doses of trypanocides showed incomplete parasite clearance, consistent with the occurrence of resistant strains and adding to growing evidence that such resistance may be a problem. Furthermore at higher doses, one isolate from non-tsetse infested area (NT-ETBD2) was confirmed resistant and another one isolate (NT-ETBD3) was suspected for resistance to DA while one isolate from tsetse infersted area (TT-ETBS2) was suspected for resistance to ISM.
Summary - VII
Generally, T. vivax from the two locations is almost equally pathogenic, parasite appearance in blood is faster and immune cytokine responses for the most part are lower in case of NT isolate whereas trypanocidal drug resistance is prevalent in both areas. Accordingly the following recommendations are forwarded; a) The present findings, reminded us that the impact of T. vivax should not be neglected in both tsetse and non-tsetse infested areas, b) Frequent administration, poor utilization and handling of trypanocidals could be sources of the problem for drug resistance and hence, appropriate corrective measures be taken in both tsetse and non-tsetse infested areas. In addition, further studies on the mechanisms of resistance for T. vivax is essential to monitor drug resistance and identify potential targets for new drugs, c) The early appearance of parasites in blood and lower cytokine responses in non-tsetse T. vivax infected groups needs further investigation from the parasite, vector and host angles.
Description
PhD Dissertation
Keywords
Trypanosomosis, Africa, Nagana