Trypanotolerance in East African Orma Boran cattle

Breeding pure and crossbreeds of European cattle breeds for milk production improvement under tropical climate conditions-a review

The aim of this review was to present the breeding status of pure or crossbreeds of European cattle with African indigenous cattle in order to improve milk yield in African tropical conditions. As the African indigenous breeds are characterized by small size, low body weight (300-450 kg) and low production performances (less than 1000 kg per lactation), short lactation length (205-240 days), etc., the crossbreeding with pure European cattle was done in order to ameliorate production performances especially milk and beef production. As results, it was seen that F1 offspring improved output productions than their parents of tropical origin. Also, it was seen that F1 offspring showed disease and heat resistance than their parents of temperate climate origin. However, at different degrees of crossbreeding, it was seen that the offspring can demonstrate bad production performances than their parents and therefore, the crossbreeding has to respect a certain degree. Other consequences are that the crossbreeding of local and pure European cattle breeds has resulted in calving complications in African local cattle. In African indigenous cattle, the selection as an improvement tool was given less emphasis as a solution to improve output production. That is why, the implementation of a well-prepared selection programs could be practiced in African indigenous cattle in order to preserve African cattle genotypes, as they are endangered of extinction due to various crossbreeding with exotic cattle breeds.

Leveraging Available Resources and Stakeholder Involvement for Improved Productivity of African Livestock in the Era of Genomic Breeding

The African continent is home to diverse populations of livestock breeds adapted to harsh environmental conditions with more than 70% under traditional systems of management. Animal productivity is less than optimal in most cases and is faced with numerous challenges including limited access to adequate nutrition and disease management, poor institutional capacities and lack of adequate government policies and funding to develop the livestock sector. Africa is home to about 1.3 billion people and with increasing demand for animal proteins by an ever growing human population, the current state of livestock productivity creates a significant yield gap for animal products. Although a greater section of the population, especially those living in rural areas depend largely on livestock for their livelihoods; the potential of the sector remains underutilized and therefore unable to contribute significantly to economic development and social wellbeing of the people. With current advances in livestock management practices, breeding technologies and health management, and with inclusion of all stakeholders, African livestock populations can be sustainably developed to close the animal protein gap that exists in the continent. In particular, advances in gene technologies, and application of genomic breeding in many Western countries has resulted in tremendous gains in traits like milk production with the potential that, implementation of genomic selection and other improved practices (nutrition, healthcare, etc.) can lead to rapid improvement in traits of economic importance in African livestock populations. The African livestock populations in the context of this review are limited to cattle, goat, pig, poultry, and sheep, which are mainly exploited for meat, milk, and eggs. This review examines the current state of livestock productivity in Africa, the main challenges faced by the sector, the role of various stakeholders and discusses in-depth strategies that can enable the application of genomic technologies for rapid improvement of livestock traits of economic importance.

African Indigenous Cattle: Unique Genetic Resources in a Rapidly Changing World

At least 150 indigenous African cattle breeds have been named, but the majority of African cattle populations remain largely uncharacterized. As cattle breeds and populations in Africa adapted to various local environmental conditions, they acquired unique features. We know now that the history of African cattle was particularly complex and while several of its episodes remain debated, there is no doubt that African cattle population evolved dramatically over time. Today, we find a mosaic of genetically diverse population from the purest Bos taurus to the nearly pure Bos indicus. African cattle are now found all across the continent, with the exception of the Sahara and the river Congo basin. They are found on the rift valley highlands as well as below sea level in the Afar depression. These unique livestock genetic resources are in danger to disappear rapidly following uncontrolled crossbreeding and breed replacements with exotic breeds. Breeding improvement programs of African indigenous livestock remain too few while paradoxically the demand of livestock products is continually increasing. Many African indigenous breeds are endangered now, and their unique adaptive traits may be lost forever. This paper reviews the unique known characteristics of indigenous African cattle populations while describing the opportunities, the necessity and urgency to understand and utilize these resources to respond to the needs of the people of the continent and to the benefit of African farmers.

Trypanotolerance in N'Dama x Boran crosses under natural trypanosome challenge: effect of test-year environment, gender, and breed composition

Background

Trypanosomosis, a protozoal disease affecting livestock, transmitted by Glossina (tsetse) flies is a major constraint to agricultural production in Sub-Saharan Africa. It is accepted that utilization of the native trypanotolerance exhibited in some of the African cattle breeds to improve trypanotolerance of more productive but susceptible breeds, will offer a cost effective and sustainable solution to the problem. The success of this approach is based on the premise that quantitative trait loci previously identified under relatively controlled situations confer useful trypanotolerance under natural field situations. As part of a study to authenticate this hypothesis, a population of 192 cattle, consisting of six batches of N’Dama and Kenya-Boran backcross animals [(N’Dama x Kenya-Boran) x Kenya-Boran] born over the period 2002 to 2006 was constructed. Some of the batches also included pure Kenya-Boran cattle, or N’Dama x Kenya- Boran F1 animals. Each batch was exposed as yearlings to natural field trypanosomosis challenge over a period of about one year; the entire challenge period extending from December 2003 to June 2007. Performance of the animals was evaluated by weekly or biweekly measurements of body weight, packed blood cell volume (PCV), parasitemia score, and number of trypanocide treatments. From these basic data, 49 phenotypes were constructed reflecting dynamics of body weight, packed cell volume (PCV) and parasitemia under challenge.

Results

Females were distinctly more trypanotolerant than males. F1, backcross and pure Kenya- Boran animals ranked in that order with respect to trypanotolerance. Overall batch effects were highly significant (p<0.001) for most traits, and were generally more significant than the gender or genetic type effects. The superior trypanotolerance of the F1 animals was expressed in all three components of animal defense strategies against pathogens: Avoidance resistance, and tolerance.

Conclusions

The results show that trypanotolerance derived from the N’Dama is expressed under field conditions; and that the trait is primarily additive in nature, being expressed in heterozygous condition and in a three-quarters Boran genetic background. The results further, underscore the complexity of the trait in the field manifesting all three host disease-control strategies, and show the importance of gender and local environmental conditions in determining response to challenge.

Trypanotolerance in N'Dama x Boran crosses under natural trypanosome challenge: effect of test-year environment, gender, and breed composition

BACKGROUND

Trypanosomosis, a protozoal disease affecting livestock, transmitted by Glossina (tsetse) flies is a major constraint to agricultural production in Sub-Saharan Africa. It is accepted that utilization of the native trypanotolerance exhibited in some of the African cattle breeds to improve trypanotolerance of more productive but susceptible breeds, will offer a cost effective and sustainable solution to the problem. The success of this approach is based on the premise that quantitative trait loci previously identified under relatively controlled situations confer useful trypanotolerance under natural field situations. As part of a study to authenticate this hypothesis, a population of 192 cattle, consisting of six batches of N'Dama and Kenya-Boran backcross animals [(N'Dama x Kenya-Boran) x Kenya-Boran] born over the period 2002 to 2006 was constructed. Some of the batches also included pure Kenya-Boran cattle, or N'Dama x Kenya- Boran F1 animals. Each batch was exposed as yearlings to natural field trypanosomosis challenge over a period of about one year; the entire challenge period extending from December 2003 to June 2007. Performance of the animals was evaluated by weekly or biweekly measurements of body weight, packed blood cell volume (PCV), parasitemia score, and number of trypanocide treatments. From these basic data, 49 phenotypes were constructed reflecting dynamics of body weight, packed cell volume (PCV) and parasitemia under challenge.

RESULTS

Females were distinctly more trypanotolerant than males. F1, backcross and pure Kenya- Boran animals ranked in that order with respect to trypanotolerance. Overall batch effects were highly significant (p<0.001) for most traits, and were generally more significant than the gender or genetic type effects. The superior trypanotolerance of the F1 animals was expressed in all three components of animal defense strategies against pathogens: Avoidance resistance, and tolerance.

CONCLUSIONS

The results show that trypanotolerance derived from the N'Dama is expressed under field conditions; and that the trait is primarily additive in nature, being expressed in heterozygous condition and in a three-quarters Boran genetic background. The results further, underscore the complexity of the trait in the field manifesting all three host disease-control strategies, and show the importance of gender and local environmental conditions in determining response to challenge.

Response of four indigenous cattle breeds to natural tsetse and trypanosomosis challenge in the Ghibe valley of Ethiopia

A comparative study on the response of four indigenous cattle breeds of Ethiopia, namely Abigar, Horro, Sheko and Gurage, to natural challenge of trypanosomosis in the Tolley-Gullele area of the Ghibe valley has been undertaken from August 2000 until August 2004. Fifty female yearlings each of Horro, Sheko and Abigar and 31 of the Gurage were purchased from their natural habitats and introduced in to medium to high tsetse-trypanosomosis challenge area of the Ghibe valley. While the natural habitats of first three breeds are naturally infested with tsetse flies and trypanosomosis, that of the Gurage is known to be very minimal, if any, and hence the Gurage breed was used in this study as the known susceptible breed. During the study animal health, production performance and tsetse fly situation were monitored monthly. The Sheko breed has manifested very significantly (p<0.001) high overall average packed cell volume (PCV) values (25%) compared to that of Abigar (24%), Horro (23%) and Gurage (22%). It also had the lowest mean trypanosome prevalence rate of 9% against 23% of Horro, 26% of Abigar and 27% of Gurage, and the least number of Berenil treatments (1.36) compared to Abigar (4.0), Horro (4.6) and Gurage (6.7). While the Abigar manifested high sensitivity and frequent death to PCV depression, the Horro showed strong resilience to PCV depression and better response to Berenil treatment assistance. At this stage the Sheko breed was also found to be equal to the other breeds in its reproductive performance. These results need to be substantiated with further in-depth investigation including immune response, animal behavior and environmental influences.

The incidence of calf morbidity and mortality due to vector-borne infections in smallholder dairy farms in Kwale District, Kenya

An observational longitudinal study was carried out on 92 randomly selected smallholder farms in two coastal lowland zones of Kwale District in Kenya between December 1997 and November 1999. The objective was to estimate the incidence of the main vector-transmitted diseases in pre-weaned calves. From an initial 41 pure or cross-bred Bos taurus calves which were less than 2 months and whose birth and disease histories were known, study calves were recruited progressively and monitored until they were weaned at around 146 days. Overall, 130 calves in 67 farms were monitored and these contributed a total risk period of 30,062 days. Disease parameters were analysed and compared as true annual and age-specific incidence rates. The incidences of East Coast fever (ECF) (23.1%) and trypanosomosis (29.1%) were the highest among the vector-borne diseases. The corresponding mortality incidence rates of ECF and trypanosomosis were 10.9 and 3.6%, respectively. The annual incidence rates of anaplasmosis and babesiosis were 10.9 and 1.2%, respectively. There was no mortality arising specifically from anaplasmosis or babesiosis. Analysis of survival times to natural infection indicated that the field challenge resulting to cases of trypanosomosis was much higher compared to the risk of either ECF or anaplasmosis. It was concluded that these vector-borne diseases constrain production of replacement stock in this coastal lowlands region of Kenya.

Differences in susceptibility to trypanosome infection between Nkedi Zebu and Ankole cattle, under field conditions in Uganda

A cross-sectional study was conducted in tsetse-infested areas of Soroti district in Uganda, with the aim of assessing the response of the Nkedi Zebu and Ankole breeds of cattle to trypanosome infection. Overall, 1215 Nkedi Zebu and 260 Ankole cattle kept under similar levels of tsetse challenge were examined for trypanosome infection, using the Buffy-coat technique and haematocrit centrifugation, and had their packed-cell volumes (PCV) measured. As expected, the infected cattle, whether of the Nkedi Zebu (26.7% v. 29.6%; P < 0.05) or Ankole breeds (24.9% v. 29.1%; P < 0.05), had significantly lower mean PCV than the uninfected. In the Nkedi Zebu cattle, the prevalence of trypanosome infection was lower (7.9% v. 10.8%; P > 0.05) and the overall mean PCV was significantly higher (29.4% v. 28.7%; P < 0.05) than in the Ankole. Compared with the Ankole, Nkedi Zebu cattle appear to be less susceptible to (detectable) trypanosome infection and to the trypanosome-attributable lowering of their PCV.

Immunological approaches to the control of animal trypanosomiasis

Control of African trypanosomiasis relies heavily on attempts to eliminate the tsetse fly vectors, and the use of trypanocidal drugs which are not entirely satisfactory. But the prospects for immunological control have a long history - since the last century, explorers and settlers in Africa have progressively exposed their horses and cattle to tsetse bites (a process known as 'salting') in an attempt to build up some degree of immunity to the disease. More recently, immunologists and biochemists have used modern techniques to explore the possibility of suitable vaccines. In this article, Vinand Nantuyla discusses the problems and possible contributions of the immunological approach.

Genetics and trypanotolerance

Genetic resistance to disease and its use in domestic livestock usually ranks last, if at all, amongst preferred disease control measures - usually preceded by measures such as chemotherapy, vector control and vaccination. Thus, interest in genetic resistance is often a reflection of dissatisfaction with other control strategies, and the current emphasis on trypanotolerant cattle in Africa is just such a case. Eighty years of tsetse fly eradication programmes have had little impact on tsetse distribution, although recent research with odour baited targets impregnated with insecticide brings hope for the future. The search for a vaccine has proved more arduous than anticipated and the number of drugs available for therapy and prophylaxis is limited. In the search for alternative solutions to the problem of African trypanosomiasis, attention has recently focused on genetic resistance - a subject normally covered by immunologists or veterinarians(3-7). In this article, Rosemary Dolan discusses the concept from the geneticist's viewpoint.

Trypanosomiasis-induced infertility in dromedary (Camelus dromedarius) bulls: changes in plasma steroids concentration and semen characteristics

The effect of Trypanosomiasis on concentrations of plasma steroids and semen characteristics was studied in 24 dromedary bulls. Based upon the parasitological and serological diagnosis, 18 bulls were found infected with Trypanosoma evansi (Group 2) and six were found to be free from infection and served as controls (Group 1). The infected animals exhibited signs of anaemia indicated by the decrease of packed cell volume (PCV) and haemoglobin concentration (Hb), pale mucus membranes, weight loss, lethargy, weakness and dullness. However, five animals (27.8%) of the infected group revealed elevated rectal temperatures and three animals (16.7%) revealed testicular degeneration upon palpation of their scrotal contents. Concentrations of plasma oestradiol-17beta (86.5 +/- 8.6 pg/ml versus 232.5 +/- 74.4 pg/ml) and testosterone (4.8 +/- 0.7 ng/ml versus 2.7 +/- 1.5 ng/ml) were significantly different (P < 0.05) between the control and infected bulls. Evaluation of the semen collected by electroejaculation and evaluated by a computerized cell motion analyzer revealed normal semen characteristics in the control animals compared to deteriorated ones in the infected bulls. There were highly significant (P < 0.01) decreases in sperm count (12.2 +/- 1.3/ml versus 6.5 +/- 4.9 x 10(6)/ml), motility percentage (68.2 +/- 6.7% versus 27.4 +/-15.6%), percentage of live spermatozoa (73.2 +/- 8.3% versus 35.8 +/- 8.2%) and increases in percentage of morphological abnormalities (3.3 +/- 0.6% versus 15.9 +/- 1.0%) in the infected group. An examination of the plasma hormonal profiles and semen characteristics in the infected bulls indicated that altered Sertoli cell function due to formation of immune complexes in four bulls (Group 2A), pituitary dysfunction in six bulls (Group 2B), testicular degeneration in three bulls (Group 2C) and finally trypanotolerancy in five bulls (Group 2D) are possible factors responsible for poor semen characteristics and infertility induced by T. evansi infection in dromedary bulls.

Mapping of quantitative trait loci controlling trypanotolerance in a cross of tolerant West African N'Dama and susceptible East African Boran cattle

Trypanosomosis, or sleeping sickness, is a major disease constraint on livestock productivity in sub-Saharan Africa. To identify quantitative trait loci (QTL) controlling resistance to trypanosomosis in cattle, an experimental cross was made between trypanotolerant African N'Dama (Bos taurus) and trypanosusceptible improved Kenya Boran (Bos indicus) cattle. Sixteen phenotypic traits were defined describing anemia, body weight, and parasitemia. One hundred seventy-seven F2 animals and their parents and grandparents were genotyped at 477 molecular marker loci covering all 29 cattle autosomes. Total genome coverage was 82%. Putative QTL were mapped to 18 autosomes at a genomewise false discovery rate of <0.20. The results are consistent with a single QTL on 17 chromosomes and two QTL on BTA16. Individual QTL effects ranged from approximately 6% to 20% of the phenotypic variance of the trait. Excluding chromosomes with ambiguous or nontrypanotolerance effects, the allele for resistance to trypanosomosis originated from the N'Dama parent at nine QTL and from the Kenya Boran at five QTL, and at four QTL there is evidence of an overdominant mode of inheritance. These results suggest that selection for trypanotolerance within an F2 cross between N'Dama and Boran cattle could produce a synthetic breed with higher trypanotolerance levels than currently exist in the parental breeds.

Rinderpest vaccination and the incidence and development of trypanosomosis in cattle

An investigation was made into whether recent vaccination of cattle with tissue culture rinderpest virus would cause immunosuppression and lead to more frequent or more severe infection with trypanosomes in animals grazing in tsetse-infested areas. Herds of cattle on Galana Ranch in Kenya were divided, with approximately half of each herd being vaccinated with tissue culture rinderpest virus strain Kabete 'O', while the rest remained unvaccinated. The herds were then exposed to the risk of natural infection with trypanosomes on the ranch. Three experiments were performed during different seasons. Infections with Trypanosoma congolense and Trypanosoma vivax were frequently detected but there was no evidence that vaccinated animals were more likely to acquire trypanosome infections or to show a more severe disease than unvaccinated cattle. It is concluded that tissue culture rinderpest vaccine does not cause immunosuppression and can safely be used in cattle likely to be exposed to tsetse flies and trypanosomosis.

Susceptibility to trypanosomosis of three Bos indicus cattle breeds in areas of differing tsetse fly challenge

Studies to assess the differences in susceptibility to trypanosomosis among Bos indicus cattle breeds (Maasai Zebu, Orma Boran and Galana Boran) were conducted under conditions of varying tsetse fly challenge at the Nguruman escarpment in south-western Kenya, for a period of 1 year. It was found that under tsetse challenge quantified as high, Maasai Zebu and Orma Boran were less susceptible than Galana Boran to trypanosome infections, as judged by the significantly lower incidence of infection, development of less severe anaemia, fewer requirements for trypanocidal drug treatments, higher growth rates and fewer mortalities. In the area where tsetse challenge was considered low as a result of a tsetse fly control operation using odour-baited traps, only the Maasai Zebu and Orma Boran were compared. No significant differences in the incidence of infection, degree of anaemia or growth rates were observed between the two breeds, but all were significantly different from their counterparts in the high tsetse challenge area. These results suggest that there is variation in resistance to trypanosomosis among Bos indicus cattle breeds that could be exploited as part of the integrated trypanosomosis control programmes in East Africa.

Studies on host resistance to tick infestations among trypanotolerant Bos indicus cattle breeds in east Africa

Recent epidemiological studies carried out in East Africa have indicated that some Bos indicus cattle breeds such as the Orma Boran and Maasai Zebu have a degree of trypanotolerance worth exploitation by their introduction into trypanosomosis endemic areas where other cattle breeds cannot survive. However, in most areas of East Africa, trypanosomosis, ticks, and tick-borne diseases occur together. It is therefore important to obtain information on the susceptibility of these breeds to tick infestation and tick-borne diseases. This study was therefore designed to determine the susceptibility of these cattle breeds to tick infestations. They were compared with the Galana Boran (trypanosusceptible) and the Friesian (susceptible to tick infestations, tick-borne diseases, and trypanosomosis). The four breeds of cattle were exposed to natural tick challenge for a period of seven months and whole body weekly tick counts were done on each animal. Significant differences to tick infestations among the four breeds were observed. For both Rhipicephalus appendiculatus and Boophilus decoloratus, susceptibility to infestation increased in the order, Maasai Zebu, Orma Boran, Galana Boran and Friesian. The results generated by this pilot study so far suggest that variation in susceptibility to tick infestations exists among the four breeds. The Orma Boran and Maasai Zebu showed greater resistance to tick-infestations than the Galana Boran and Friesian. This suggests that utilization of these trypanotolerant cattle breeds could be feasible even in the face of tick challenge and should therefore be considered when planning integrated trypanosomosis and tick control strategies.

Integrated control of tropical animal parasitoses

In the past, the control of parasites of domestic animals has relied mainly on the use of drugs and pesticides. Although these compounds are still of great importance in the prevention and treatment of parasitic diseases, the accent has shifted in recent years to a more flexible approach, integrating various other control measures. The main reasons for this change are: development of resistance of the parasites against the compounds used; reduced development of new compounds to overcome resistance (increasingly stringent regulations on toxicity and residues, resulting in very high research and development costs, insufficient return for industry because of short life-span of new products due to resistance, and limited and poor markets for compounds against parasites in developing countries); increasing cost of new products for the user; problems associated with toxicity, environmental pollution and residues in animal products. Integrated parasite management makes use, where possible, of biological and mechanical control methods, of acquired and innate host resistance, and of genetical, ecological, sanitary and regulatory procedures, although chemical control can seldom be entirely abolished. Cost-effectiveness and sustainability in all respects are of primary importance.

The effect of trypanosoma vivax infection on late pregnancy and postpartum return to cyclicity in boran cattle

A study was designed to examine the effect of infection with Trypanosoma vivax KETRI 2501 on the maintenance of pregnancy and postpartum return to reproductive function in susceptible Galana (n = 6) and trypano-tolerant Orma Boran (n = 6) heifers during the third trimester of pregnancy. Of the 12 study animals, 3 Galana and 3 Orma Boran heifers served as controls. One of 3 Galana heifers calved prematurely with subsequent perinatal loss. Of the 2 heifers that produced live calves, 1 calf died shortly after birth, while the other survived. Two of 3 Orma heifers calved prematurely and all 3 calves died shortly after birth. The 6 control heifers produced live calves at term, all of which survived. Infection with T. vivax during the third trimester of pregnancy delayed the resumption of ovarian activity after calving, with the Ormas taking a significantly (P < 0.05) shorter time from calving to ovulation. There was no clear evidence that premature birth was associated with pathological changes in reproductive organs. Results from this study demonstrated that infection with pathogenic T. vivax during late pregnancy influenced the outcome of pregnancy in both susceptible Galana and trypano-tolerant Orma Boran heifers, resulting in premature births, perinatal loss, retained placentae, low birth weights and a prolonged period to the onset of postpartum ovarian activity.

The effect of trypanosomosis on pregnancy in trypanotolerant Orma Boran cattle

A study was undertaken to investigate the influence of trypanosomosis on the outcome of pregnancy in trypanotolerant Orma Boran (Bos indicus) exposed to natural tsetse challenge in an area of Kenya infested predominantly with Glossina pallidipes. Of 73 pregnant Orma heifers, 58 (79.5%) produced live calves at term, 13 (17.8%) aborted and 2 (2.7%) died of trypanosomosis. Of the 71 surviving animals, 22 (31%) were infected with Trypanosoma vivax , 21 (29.6%) T. congolense, and 26 (36.6%) had mixed infections with T. vivax and T. congolense. These results suggest that in areas of high trypanosomosis risk reproductive function is affected even in trypanotolerant cattle, and that both T. vivax and T. congolense can be responsible for the abortions observed in the field. It is suggested that maintenance of pregnancy in the face of trypanosome challenge was dependent on individual variation among the Orma cattle, but as challenge increased beyond the limits of effectiveness of trypanotolerance, disruption of pregnancy occurred.

The effect of experimental infection of Boran cattle in early and mid-pregnancy with Trypanosoma vivax

Six susceptible Galana and five trypanotolerant Orma Boran (Bos indicus) cattle were infected experimentally with Trypanosoma vivax KETRI 2501 by cyclical transmission using Glossina morsitans during early and mid-pregnancy. Four pregnant animals, two of each Boran type were used as controls and remained uninfected throughout the study period. Three out of the six infected susceptible Galana Borans aborted, whilst one had a stillborn calf. None of the trypanotolerant Orma Boran cattle aborted and all carried their pregnancies to term. All control animals produced live calves at term. The mechanisms leading to disruption of reproductive function in susceptible Boran cattle were not clear but could involve a number of factors, including anaemia, weight loss and post-infection decline of plasma progesterone levels. It is concluded that infection with T. vivax disrupts maintenance of pregnancy in susceptible Galana Borans but does not affect maintenance of pregnancy in the Orma Boran, demonstrating their tolerance to infection with T. vivax.

Pharmacology of diminazene: a review

Chemotherapy for trypanosomiasis in domestic livestock depends on only a few compounds, of which several are chemically closely related. Of these compounds, the most widely used therapeutic agent in cattle, sheep and goats is diminazene aceturate. Diminazene was first described in 1955. Subsequently, a substantial body of data has been generated on various pharmacological aspects of the compound. In this review, we consider the current status of knowledge concerning the therapeutic spectrum of diminazene, resistance to diminazene in trypanosomes, and combination therapeutic regimens in which diminazene has been administered together with other compounds. Analytical techniques for diminazene, the pharmacokinetics of diminazene, data concerning diminazene's toxicity, and the different molecular mechanisms by which diminazene may exhibit trypanocidal action are also considered.

Failure of chemoprophylaxis against bovine trypanosomiasis on Galana Ranch in Kenya

The duration of prophylaxis provided by 1 mg kg-1 bodyweight of homidium bromide was compared with that provided by 1 mg kg-1 bodyweight of isometamidium chloride in a 12 month field trial involving 90 Boran cattle exposed to trypanosome challenge on Galana Ranch in Kenya. Weekly trypanosome prevalences of over 30% were observed during 4 of the 12 months. During these periods of heavy challenge, parasites were detected 2-3 weeks after administration of both homidium bromide and isometamidium chloride. Both prophylactic drugs were administered, on a group basis, eight times over the 12 month trial and in addition individual infections were also treated with diminazene aceturate. Isometamidium chloride provided slightly longer periods of prophylaxis than homidium bromide, 28.4 days compared with 25.4 days. There was a highly significant difference in the productivity of the two groups during a period of poor grazing. 27% of the isometamidium chloride herd died from a severe wasting condition with substantial liver damage evident on post mortem. The condition was not observed in the homidium bromide herd. The surviving animals in the isometamidium chloride herd had a mean annual weight gain of 24 kg less than that recorded in the homidium bromide herd.

Susceptibility of N'Dama and Boran cattle to sequential challenges with tsetse-transmitted clones of Trypanosoma congolense

The susceptibility of N'Dama cattle (Bos taurus) to four consecutive infections with different tsetse-transmitted clones of Trypanosoma congolense was compared with that of Borans (Bos indicus). All animals were aged 13 months at the start of the study and had been born and raised free from trypanosomiasis under the same management and nutritional conditions, thereby limiting environmental factors that could have influenced susceptibility. While cattle of both breeds were equally susceptible to the establishment of trypanosome infections, the N'Damas exhibited superior resistance. Despite infection with virulent parasites, the N'Damas gained weight at the same rate as uninfected control animals, they did not develop anaemia to the extent that trypanocidal drug treatment was required, and all made a spontaneous recovery to normal haematological values within two to four months. In contrast, all the Borans needed treatment during the course of the four infections because of severe anaemia and showed markedly reduced liveweight gains. These clinical differences in the N'Damas were associated with two repeatable characteristics, namely, the ability to control parasitaemia and to 'resist' anaemia, processes that did not appear to be linked. Also in contrast to the Borans, the N'Damas were able to mount accelerated haemopoietic responses, resulting in the reduced severity of anaemia following a primary infection. These findings pose the question as to whether the ability to control parasitaemia and to 'resist' anaemia could be used as criteria for identifying resistant or trypanotolerant cattle.

Homidium bromide as a chemoprophylactic for cattle trypanosomiasis in Kenya

Homidium bromide was used in a strategic chemoprophylactic regime to control trypanosomiasis in Boran cattle in Kenya. Trypanosome infection rates in cattle receiving homidium bromide prophylaxis were compared with those in control cattle which received no prophylaxis but were treated with diminazene aceturate when infected. Homidium bromide was administered twice during the year after which no infections were detected for periods of nineteen weeks and seventeen weeks respectively. The drug sensitivity of the infecting trypanosomes is believed to be a major factor in determining the duration of prophylaxis.