Transition models to assess risk factors for new and persistent trypanosome infections in cattle-analysis of longitudinal data from the Ghibe Valley, southwest Ethiopia.

The objective of this study was to apply transition models to distinguish between factors associated with both incident and persistent trypanosome infections. Data collected from 1561 cattle were analyzed from a long-term study involving 8 herds in which both trypanosome infections (a total of 56,931 cattle sampling-months) and tsetse (Glossina spp.) challenge were monitored monthly from March 1986 to March 1998. Both pour-on and insecticide-target tsetse control programs and mass treatment with diminazene aceturate before tsetse control were associated with significant decreases in both incidence and persistence of trypanosome infection relative to noncontrol periods, as were seasonal and sex effects. The magnitudes of the effects were, however, often different for new and persistent infections. For persistence of infection, there were 2 trends. In general, the duration of infection increased during the study, despite the regular treatment with diminazene aceturate. The transition model had 2 major benefits. The first was to identify an increasing duration of infections with time, taking into account other factors associated with increasing infection risk. The second was to highlight different patterns in the effects of certain factors on new and persistent trypanosome infections.

The incidence of new and the prevalence and persistence of recurrent trypanosome infections in cattle in southwest Ethiopia exposed to a high challenge with drug-resistant parasites.

A method is described for calculating new infection incidence from monthly field data collected between April 1994 and February 1998 from an average of 770 Ethiopian Highland Zebu cattle maintained under traditional management in the Ghibe valley, southwest Ethiopia and exposed to a high challenge with drug-resistant parasites. Each month cattle with a packed cell volume (PCV)<26% and detected as parasitaemic, or cattle showing clinical signs of trypanosomosis, were treated with diminazene aceturate at 3.5 mg/kg body weight. An infection was defined as a new infection if it was preceded by 2 previous months in which both samples had a PCV>/=26% and were not detected with trypanosomes. Using this definition the average monthly incidence of infections of Trypanosoma congolense in cattle over 36 months of age was 13.3%, and the prevalence of recurrent infections 13.0%. Assuming that an animal had recovered from infection when PCV again returned to 26% without parasites being detected, mean persistence of infection was 3.8 months (median between 2 and 3 months). In contrast, T. vivax infections were susceptible to diminazene. The incidence of T. vivax infections in adults was lower than for T. congolense (2.8%), and they were less pathogenic (mean reduction of 3.1% units of PCV due to T. vivax infection compared with 4.6% units for T. congolense). Also, fewer cases were treated. Calves were first detected parasitaemic at an average age of 8.8 months and their infections persisted longer than those in adults. The effect of age on incidence of infection was not significant beyond 15 months of age. Adult male cattle appeared to be more susceptible to T. congolense infection than adult female cattle.

Use of deltamethrin 'pour-on' insecticide for the control of cattle trypanosomosis in the presence of high tsetse invasion.

A deltamethrin 'pour-on' insecticide was applied monthly to over 2000 cattle exposed to a high challenge of drug-resistant trypanosomes and high tsetse re-invasion pressure in the Ghibe valley, south-west Ethiopia. Blood samples were taken monthly from an average of 760 cattle for determination of PCV and presence of trypanosomes. The area of the valley is approximately 350 km2 and the cattle grazed in roughly four locations covering about a quarter to half of the area. Two years before the trial commenced, Glossina morsitans submorsitans Newstead (Diptera: Glossinidae) began to invade the valley. Despite the use of the pour-on the mean apparent density of G. m. submorsitans continued to rise, and, during the 4 years of tsetse control, was more than three-fold higher than that recorded during the previous 18 months. Over the same period there was little change in the apparent density of Glossina pallidipes Austen (Diptera: Glossinidae). By contrast, the mean monthly prevalence of trypanosome infections in cattle over 36 months of age decreased from 38.3 to 29.0%, the incidence of new infections decreased from 26.6 to 16.0% (a reduction of 40%), and packed cell volume in cattle increased from 21.7 to 24.1%. Evidence of a change in apparent parasite transmission rate was demonstrated by regression of infection incidence in cattle on the logarithm of apparent density of G. m. submorsitans. Before the trial started the regression coefficient was 45.8 +/- 6.3 and this reduced to 9.2 +/- 2.5% incidence per log(e) (flies/trap/day) during the period of tsetse control. It was concluded that this indicated reductions in tsetse numbers in the immediate vicinities of cattle in a way that was not reflected in overall tsetse catches. Nevertheless, the comparatively high levels of trypanosome prevalence that persisted in the cattle demonstrates that, where invasion prevalence is high, treatment of small pockets of cattle will not eradicate tsetse. To achieve more significant reduction in trypanosome prevalence in cattle, integrated methods of control utilizing target barriers in the major routes of invasion will be needed.

A statistically derived index for classifying East Coast fever reactions in cattle challenged with Theileria parva under experimental conditions.

A statistically derived disease reaction index based on parasitological, clinical and haematological measurements observed in 309 5 to 8-month-old Boran cattle following laboratory challenge with Theileria parva is described. Principal component analysis was applied to 13 measures including first appearance of schizonts, first appearance of piroplasms and first occurrence of pyrexia, together with the duration and severity of these symptoms, and white blood cell count. The first principal component, which was based on approximately equal contributions of the 13 variables, provided the definition for the disease reaction index, defined on a scale of 0-10. As well as providing a more objective measure of the severity of the reaction, the continuous nature of the index score enables more powerful statistical analysis of the data compared with that which has been previously possible through clinically derived categories of non-, mild, moderate and severe reactions.

Estimating the effects of tsetse control on livestock productivity--a case study in southwest Ethiopia.

A tsetse control campaign was started in January 1991 using a synthetic pyrethroid cypermethrin 'pour-on' applied monthly to cattle exposed to high levels of drug-resistant trypanosomes in the Ghibe valley of southwest Ethiopia. In December 1992, a cost-recovery scheme was introduced, and thereafter farmers paid for treatment. An average of 100 village Ethiopian Highland Zebu cattle were monitored monthly from March 1986 to February 1997. Individual animals in this herd were treated with diminazene aceturate (Berenil, Hoechst, Germany) at 3.5 mg/kg when trypanosomes were detected and their packed cell volume was less than 26%. Superimposed on this systematic trypanocidal chemotherapy, control of tsetse resulted in average reductions from 1992 to 1996 of 95% and 75% in the mean relative densities of tsetse and biting flies, respectively, and of 63% in the prevalence of trypanosomal infections in cattle. Despite these reductions, there was no significant increase in the body weight of the cows, calving rate or the mean body weight of calves at 12 months of age. There was, however, an average decrease of 57% in calf mortality (including still births) by 12 months of age, an increase of 49% in the ratio of live calves under 12 months of age to cows over 36 months of age, and an increase of 8% in the body weight of adult males.

Population dynamics of ticks on indigenous cattle in a pastoral dry to semi-arid rangeland zone of Uganda.

Studies on seasonality and population dynamics of ticks on indigenous cattle and their crosses (calves) were carried out in Buruli Ranching Scheme, Nabiswera, Luwero district of Uganda on three treatment groups of animals: group 1 (twice a week dipping), group 2 (once a month dipping) and group 3 (no tick control). During this study, four major species of ticks of economic importance were recorded in decreasing order of abundance: Rhipicephalus appendiculatus, Rhipicephalus evertsi evertsi, Amblyomma variegatum and Boophilus decoloratus. Of these ticks, a seasonal pattern of activity was only observed in R. appendiculatus ticks, with peak activities occurring during rainy seasons. Greater numbers of ticks were recorded on cows than calves in the three treatment groups, with the exception of A. variegatum where the reverse occurred. The mean numbers of ticks per animal were highly significantly different (p < 0.01) when group 1 animals were compared with group 2 and 3 animals. However, no significant differences (p > 0.05) were observed in mean tick numbers between group 2 and 3 animals. Highly significant differences (p < 0.01) were observed in mean tick numbers on cows and calves (more than 12 months old) in different calving seasons. The state of lactation only affected tick counts on cows in group 1; significantly more ticks (p < 0.01) were observed in lactating than non-lactating cows. Furthermore, significantly greater (p < 0.05) numbers of ticks (with the exception of B. decoloratus) were recorded during the second year of study (March 1992-May 1993) than the first year (January 1991-February 1992), despite lower rainfall during the former period.

Variations in packed red cell volume and trypanosome prevalence and relationships with reproductive traits in east African Zebu cows exposed to drug-resistant trypanosomes.

Approximately 320 East African Zebu cows over 36 months of age were monitored monthly from 1986 to 1992 in nine village herds in an area of high trypanosomiasis risk in southwest Ethiopia where there was resistance to all available trypanocidal drugs. Cows were individually treated with diminazene aceturate, either when they were detected parasitaemic and their packed red cell volume (PCV) decreased below 26%, or when they showed clinical signs of trypanosomiasis. The average annual monthly trypanosome prevalence was 24% and the number of treatments of diminazene aceturate per cow per year was 3.1, both of which increased with age. Mean PCV decreased and mean trypanosome prevalence increased during lactation. There was a significant linear association between the time detected parasitaemic during the first 150 days of lactation and calving interval. When corrected for frequency of parasitaemia and treatment there was also an average reduction of 8.4 +/- 2.6 days in calving interval per % unit increase in PCV. Age at first calving decreased by 0.44 +/- 0.26 months per % unit increase in mean PCV maintained between 24 and 30 months corrected for parasitaemia and treatment. The percentage of pregnancies terminating in abortions significantly increased from 6.8 +/- 1.0% to 10.4 +/- 1.3% when cows detected parasitaemic at least once during the last three months of pregnancy were compared with cows not detected parasitaemic. The largest increase to 19.4 +/- 4.3% was in cows with low mean PCVs < 22%. It was concluded that cows which were able to maintain higher than average PCVs when parasitaemic showed superior reproductive performance than those with lower than average PCVs.

Epidemiology of bovine trypanosomiasis in the Ghibe valley, southwest Ethiopia. 2. Factors associated with variations in trypanosome prevalence, incidence of new infections and prevalence of recurrent infections.

An average of 840 East African Zebu cattle from nine herds in the Ghibe valley, southwest Ethiopia were monitored from January 1986 to April 1990. Each month blood samples were collected for analysis of packed red cell volume (PCV) and detection of trypanosomes. Animals found to be parasitaemic and with a PCV less than 26% were treated with diminazene aceturate at a dose of 3.5 mg/kg body weight. The majority of infections were associated with Trypanosoma congolense (84% of infections in adult cattle and 71% in cattle less than 24 months of age), and the mean percentage of adult animals detected parasitaemic 1 month after treatment of an infection with T. congolense was 27%. In order to assess possible existence of drug resistance, a model was applied which allowed monthly incidences of new infections to be distinguished from recurrent infections. This model showed that the monthly incidence of new infections of T. congolense in adult cattle increased significantly from 11% in 1986 to 24% in 1989 following a concomitant increase in the tsetse challenge. The corresponding increase in overall prevalence of T. congolense was from 17% to 38% and the mean prevalence of recurrent infections increased significantly from 6% to 14%. These findings ruled out the possibility that the high prevalence of trypanosome infections in cattle was due only to a high tsetse challenge and pointed to the existence of T. congolense populations which expressed resistance to diminazene. There were variations associated with season, herd, age and sex in the incidence of new infections, prevalence of recurrent infections and relapse to treatment.