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Repeated cross-sectional skin testing for bovine tuberculosis in cattle kept in a traditional husbandry system in Ethiopia
  1. R. Tschopp, DVM, MSc, PhD1,
  2. E. Schelling, DVM, PhD1,
  3. J. Hattendorf, MSc, PhD1,
  4. D. Young, BSc, PhD, FMedSci3,
  5. A. Aseffa, MD, PhD2 and
  6. J. Zinsstag, DVM, PhD1
  1. 1Swiss Tropical Institute, CH-4002 Basel, Switzerland
  2. 2Armauer Hansen Research Institute (AHRI/ALERT), PO Box 1005, Addis Ababa, Ethiopia
  3. 3Department of Microbiology, Imperial College, London SW7 2AZ
  1. E-mail for correspondence: rea.tschopp{at}unibas.ch

Abstract

Representative repeated cross-sectional skin testing for bovine tuberculosis (TB) was conducted over a period of three years in a total of 5377 cattle in three randomly selected woredas (districts) in Ethiopia (Meskan, Woldia and Bako-Gazer) that had never previously been tested for TB. Almost all (99 per cent) of the animals included local zebus kept in traditional husbandry systems. The comparative intradermal tuberculin test with two diagnostic thresholds were used to define positive test results, one according to the World Organisation for Animal Health (OIE) recommended cut-off of more than 4 mm, and the other with a cut-off of >2 mm. Data analysis was performed using a logistic regression model with a random effect at the village level. Applying the OIE definition, the overall representative apparent prevalence of bovine TB in skin test-positive local zebus was 0.9 per cent (95 per cent confidence interval [CI] 0.6 to 1.3 per cent). Using a cut-off of more than 2 mm the overall representative prevalence increased to 4 per cent (95 per cent CI 2.4 to 4.8 per cent). Due to the low apparent prevalence, the true prevalence could be calculated only in Meskan (4.5 per cent) and Bako-Gazer (2.4 per cent) for the more than 2 mm cutoff. With the exception of Meskan, prevalence by woreda did not change significantly over the years. Mycobacterium avium reactor animals were found at all study sites, but there were significant geographical variations. Overall, bulls and oxen were more at risk of being positive reactors (odds ratio [OR] 1.6, 95 per cent CI 1.1 to 2.3; OR 2, 95 per cent CI 1.4 to 2.6, respectively), as were animals in good body condition (OR 2, 95 per cent CI 1.5 to 2.9). Similar results were found at woreda level with the exception of Woldia, where none of the analysed variables was significantly associated with a positive test result.

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BOVINE tuberculosis (TB) is a chronic debilitating disease caused by Mycobacterium bovis. The causative agent belongs to the Mycobacterium tuberculosis complex, which comprises the phylogenetically closely related Mycobacterium tuberculosis, Mycobacterium africanum, Mycobacterium microti, M bovis, bacillus Calmette-Guérin (BCG) and Mycobacterium canetti (Mostowy and others 2005).

Bovine TB has been eradicated or at least controlled in most parts of the developed world, but remains prevalent in many sub-Saharan countries. The disease causes not only a potential zoonotic risk through the consumption of contaminated raw animal products (such as milk) and/or close contact with infected animals, but is also an economic burden to the livestock sector and thus to society, although these problems have not yet been quantified in Africa (Zinsstag and others 2006). The exact epidemiology of bovine TB in Africa is still largely unknown, and national control strategies in livestock are rare or non-existent.

In developed countries, tuberculin skin testing is universally recognised as a method for the diagnosis of bovine TB in live cattle, and forms the basis for national test and slaughter programmes (Ayele and others 2004). The use of a cut-off point for the comparative intradermal tuberculin test (CIDT) of more than 4 mm, as recommended by the World Organisation for Animal Health (OIE), is used worldwide for standard skin test result interpretation. However, this standard cut-off has been questioned as it might not be adaptable to the situation in Africa, where the cattle breeds, prevailing prevalence and epidemiology of the disease, and official goals of disease control differ from those of industrialised countries. In a recent study in central Ethiopia, Ameni and others (2008) suggested the use of a cut-off of more than 2 mm (meaning a difference in skin thickness between the reaction to bovine and avian tuberculin of greater than 2 mm) as being more appropriate for skin testing cattle in central Ethiopia, as it increases the sensitivity of the test without affecting its specificity. Similarly, a recent study in Chad showed a better performance of the single intradermal comparative cervical test with a cut-off value of 2 mm (Ngandolo and others 2009).

Bovine TB has been shown to be endemic in cattle in Ethiopia, with the prevalence ranging between 7.9 per cent and 49 per cent (Ameni and others 2003,Regassa and others 2010). However, most field and abattoir prevalence studies have focused on central Ethiopia, which is characterised by urban and peri-urban management settings and/or herds of exotic cattle breeds or their crossbreeds, which are more productive than traditional zebu breeds (Ameni and others 2003, Asseged and others 2004, Teklul and others 2004, Ameni and others 2007). The central highlands are also the major area of milk supply for Addis Ababa, a fast-growing capital city of 3.1 million people (Central Statistical Agency [CSA] 2008). This region is subject to more research regarding the burden of bovine TB than other parts of Ethiopia. However, over 80 per cent of the Ethiopian population is rural and directly dependent on livestock for their livelihood. In these more remote rural areas, Ethiopian farmers keep mainly zebu breeds under traditional management systems for crop farming, where they are used as draught animals. The prevalence of bovine TB in these areas was, until the present study, largely unknown.

This paper presents the findings of a repeated cross-sectional study of skin testing in cattle conducted over three years in three rural regions of Ethiopia that had not been tested previously for bovine TB: from the northern highlands, across the Rift Valley, and down to the southern part of the country. The primary goal was to assess the representative prevalence of tuberculin positivity in cattle kept in the traditional husbandry system and to follow trends over time in view of a future cost study of disease. The use of the different CIDT cut-offs is discussed, as well as the role of Mycobacterium avium in the testing procedure.

Materials and methods

Study areas

A repeated cross-sectional study was conducted from 2005 to 2008 in two regional zones of Ethiopia: Amhara, and the Southern Nations, Nationalities and People Region, between the latitudes of 5.1°N and 11.5°N and the longitudes of 36.1°E and 40.1°E. Within these regions, three woredas (districts) were randomly selected (see below) according to the further requirements of a larger programme on bovine TB in Ethiopia (such as the presence of abattoirs and hospitals in the same areas). The three woredas were Woldia (in the northern highlands), Meskan Mareko (in the Rift Valley), and Bako-Gazer (in the southern middle lands). The altitude of the study sites ranged from 1300 m to 3500 m above sea level. The Woldia site had two distinct ecological zones, the lowlands (<2000 m) and the highlands (>2000 m). All farmers were sedentary smallholders with a mixed livestock/crop farming system and cattle husbandry practice.

Study design

Cattle were selected by stratified cluster sampling, proportional to the size of the cattle population in a kebele (administrative units within the woredas), and villages were considered as clusters based on calculated intra-class correlation coefficients. Cattle belonging to individual owners in the study areas were kept communally during the day; therefore, herds belonging to individual owners were taken into consideration, but animals belonging to all owners were regrouped into one single ‘village’ herd for each village.

Lists of kebeles and villages within the kebeles were obtained from each woreda agricultural office. Kebeles within the woreda were selected randomly using random numbers generated in Excel (Microsoft). Villages were selected randomly and proportionally to their number within a particular kebele. Approximately 30 animals were selected per village. Cattle younger than 6 months of age, late pregnant cows and clinically sick animals were not included in the study.

The selected cattle from a village were gathered together for testing and subsequent reading of the test. If animals were not present on the day of reading, a house-to-house visit was conducted. As compensation and incentive for the farmers’ participation, all tested cattle were dewormed on the reading day with albendazole boluses (Ashialben 2500; Ashish Life Science).

The cross-sectional study was repeated three times on a yearly basis in the same villages for all three woredas; animals were newly selected at each visit. To avoid the effects of season (for example, drought or the rainy season) on the test results, cattle were tested in each woreda in the same month during the study period.

Calculation of sample size was done using the formulas of Bennett and others (1991) as described by Tschopp and others (2009). The intra-class correlation coefficient rho (ρ) was taken as 0.2 and obtained a design effect (D) of 6.8. Choosing 30 animals (b) per cluster (c), with a disease prevalence of 5 per cent and 17 clusters, gave an estimate of the standard error or precision of 0.025. The total sample size (n) is the product of the number of clusters (c) and the number of samples per cluster (b) (n=b x c), thus equalling 510 animals per woreda, which gave a total number of required animals of 2040.

Skin testing of cattle

Skin testing was carried out by the same person to avoid technical bias related to the testing method. The authors applied the CIDT in all cattle using avian and bovine purified protein derivative (PPD) supplied by Veterinary Laboratories Agency – Weybridge. Intradermal injections of 0.1 ml (2500 iu/ml) bovine PPD and 0.1 ml (2500 iu/ml) avian PPD were applied at two shaved sites, 12 cm apart, in the middle neck region, after having recorded the skin thickness at each site with a calliper. Skin thickness was measured again at both injection sites after 72 hours. The reaction at each site was calculated as the difference between the skin thickness after 72 hours and the thickness before the injection. The data were analysed using two different diagnostic thresholds to define positive test results: first, an animal was considered positive if the bovine PPD reaction minus the M avium PPD reaction was greater than 2 mm (Ameni and others 2008, Bongo and others 2009); and secondly, an animal was considered positive if the bovine PPD reaction minus the M avium PPD reaction was greater than 4 mm (the OIE definition). A village herd was considered positive if at least one positive reactor was present.

To assess the prevalence of M avium, M avium complex-positive reactors were arbitrarily defined by assessing the skin reaction at the M avium PPD site alone; animals with a difference greater than 4 mm between the 72-hour reaction and the skin thickness before the M avium PPD injection were defined as positive.

Cattle data

In addition to the CIDT result, the following information was collected for each tested animal: sex, breed, age and body condition score. Animals were categorised into four age groups: calves younger than one year, juveniles between one and three years, reproductive animals between three and 10 years, and animals older than 10 years. The body condition score of the animals was assessed using a scale from 1 to 5 as described by Msangi and others (1999), but the scores were grouped into three categories that better reflected the assessment in the field: emaciated to thin, normal, and muscular to fat. Cattle were differentiated into three different breed types: traditional zebus, crossbreeds and exotic breeds. Within the traditional zebu breeds, the animals were, if possible, further categorised on the basis of phenotypic characteristics into three breeds: the Kola/Danakil breed, which belongs to the Sanga group and is found in the north-eastern part of Ethiopia; the Ethiopian Boran; and a breed found in Bako-Gazer traditionally called Male. In order to avoid recall bias, farmers were asked on the testing day about the work stamina of their oxen (castrated bulls used as draft animals) and if they had noticed any changes such as weakness or a reluctance to work in the field during the past year.

Statistical analysis

Data were double-entered into Access (Microsoft), validated with EpiInfo (version 3.3.2) and analysed with the software package STATA 10.1 (StataCorp). A variance component analysis was performed for the different levels of the multicluster sampling (woreda, kebele and village) using Generalized Linear Mixed Models with binary outcome and logit link function (GLLAMM add-on). The analysis indicated that most variance was associated with village and woreda. Village was therefore included as random effect in the logistic regression model.

Data on prevalence and cattle characteristics were analysed using a logistic regression with village as random effect. Prevalence was calculated as the overall prevalence, as prevalence stratified by woreda and by year, as well as an overall prevalence by woreda combining all three years of the study. Apparent prevalence was calculated using both the official (>4 mm) cut-off and other (>2 mm) cut-off value. Estimates of the true prevalence and 95 per cent confidence intervals (CI) were obtained using the Rogan-Gladen estimator as described by Greiner and Gardner (2000), based on the proportion of test-positive cattle (apparent prevalence) and the sensitivity and specificity of the test of 69 per cent and 97 per cent, respectively, as described by Ameni and others (2008). Univariable and multivariable analyses of risk factors inherent to the animal (age, breed, sex and body condition) and altitude were performed with the more than 2 mm cut-off only, using logistic regression models, as mentioned above. The results were expressed as odds ratio (OR), 95 per cent CI for the OR, and P values (significance set at P<0.05).

Results

Prevalence of bovine TB in cattle

The prevalences of bovine TB at individual animal level, stratified by woreda and by year, is shown in Table 1. Overall, a total of 5377 cattle were tested with the CIDT in the three woredas over a period of three years. Each woreda had a unique temporal pattern of prevalence over the years, but prevalence by woreda did not change significantly over the years, with the exception of the second year in Meskan, where the peak prevalence of 7.9 per cent (cut-off >2 mm) differed significantly from the two other years with lower prevalences (P=0.013).

The lowest prevalence was found in Woldia (0.3 per cent and 2.2 per cent when using the more than 4 mm and more than 2 mm cut-off, respectively). The prevalence with the more than 2 mm cut-off was 3.3 times higher than with the more than 4 mm cut-off in Bako-Gazer, 4.6 times higher in Meskan, and 7.3 times higher in Woldia. The prevalence in the lowlands of Woldia was lower (1.7 per cent) than in the highland zone (2.9 per cent), but the results were not statistically significant (OR 0.6, 95 per cent CI 0.1 to 3.1; cut-off >2 mm). True prevalence could be estimated only when using the more than 2 mm cut-off for Meskan and Bako-Gazer, which had an overall prevalence of 4.5 per cent (95 per cent CI 2.9 to 6.1) and 2.4 per cent (95 per cent CI 0.9 to 3.9), respectively. True prevalence could not be estimated for Woldia due to the low apparent prevalence and low sensitivity of the tuberculin test (65 per cent).

Herd prevalence, when using the >4 mm cut-off, was lowest in Woldia (six of 22 herds positive [27.3 per cent]) and highest in Bako-Gazer (13 of 19 herds positive [68.4 per cent]). In Meskan, 14 of 21 herds were positive (66.7 per cent). When using the more than 2 mm cut-off, herd prevalence reached 100 per cent in Meskan and Bako-Gazer, whereas 20 of 22 villages were positive (91 per cent) in Woldia.

Risk factors for a positive CIDT

Because only three Borans and three Holsteins were tested that were negative by the CIDT, these animals were excluded from the univariable and multivariable analyses. The results of the univariable analysis are shown in Table 2. Sex was significant in the univariable analysis including all three woredas, with bulls and oxen showing an OR of 1.6 (95 per cent CI 1.1 to 2.3) and 2 (95 per cent CI 1.4 to 2.6), respectively, when compared with females. Animals with muscular to fat body condition were more at risk for a positive CIDT (OR 2, 95 per cent CI 1.5 to 2.9) than animals with a normal body condition. Juvenile cattle aged one to two years had an OR of less than 1 for bovine TB positivity (overall OR 0.5, 95 per cent CI 0.3 to 0.8) when compared with adult breeding cattle aged three or more years to 10 years. Oxen accounted for 1268 (32.8 per cent) of the tested cattle; they were older than five years, which is normally the earliest age that cattle are castrated in Ethiopia, and 90.1 per cent of the animals were in good body condition (normal or muscular to fat).

The analysis of the different breed categories had to be conducted separately for each woreda, as Male cattle were found only in Bako-Gazer and Kola cattle only in Woldia. In Woldia, 36.2 per cent of the tested cattle were Kola. The proportion of positive animals in Woldia was 1.6 per cent for Kola compared with 3.2 per cent for unclassified zebus (OR 0.5, 95 per cent CI 0.2 to 1.0). The proportion of adults (animals older than three years) was higher in the Kola cattle (91 per cent) than in unclassified zebus (75.3 per cent). The sex composition was similar in the Kola group and in the unclassified zebu group. Male cattle accounted for 4.8 per cent of the tested cattle in Bako-Gazer. The proportion of positive cattle was higher in the Male cattle (five of 78 animals) than in the unclassified zebus (72 of 1528), but this different was not statistically significant. The proportion of adult animals was higher in Male cattle (97.3 per cent) than in unclassified zebus, with animals older than four years accounting for 51.3 per cent of the Male animals. Male cattle had a higher proportion of females (74.4 per cent) than unclassified zebus (50.3 per cent). Crossbreeds were present in all woredas, but in such low numbers (58 animals overall, or 1.1 per cent of the total number of animals sampled) that no analysis was performed for these cattle.

The multivariable analysis confirmed in general the findings of the univariable analyses. The results are shown in Table 3. Across all three woredas, oxen and animals in good body condition were significantly more likely to be positive reactors for bovine TB. This was also the case in Meskan. In contrast, none of the variables was significantly associated with a positive CIDT in Woldia, and in Bako-Gazer only oxen were associated with positive reactors.

M avium reaction

Positive reactions to M avium PPD were found in all three woredas (188 animals overall), but the numbers differed between regions. No M avium reaction was found in calves younger than one year. In Woldia, the prevalence of a reaction to M avium was approximately one-third of that in Meskan (data not shown). The highest prevalence was found in Meskan (4.7 per cent, 95 per cent CI 3.4 to 6.3 per cent), followed by Bako-Gazer (3.6 per cent, 95 per cent CI 2.8 to 4.8 per cent) and Woldia (1.7 per cent,95 per cent CI 1.1 to 2.6 per cent).

The univariable analysis (Table 4) showed that year, sex, age and breed were not significantly associated with positive M avium reactions. However, body condition was shown to influence positivity, with animals in good condition having an OR of 1.6 (95 per cent CI 1.0 to 2.3) when compared with animals in normal body condition, as was the case with M bovis PPD positivity. In the multivariable analysis, only animals older than 10 years were significantly associated with M avium positivity (OR 0.4, 95 per cent CI 0.2 to 0.7) compared with younger animals.

Mixed PPD reactions

A total of 109 animals (2 per cent) showed swellings greater than 4 mm at the avian PPD injection sites as well as at the bovine PPD injection sites. Of these animals, five were classified as positive bovine TB reactors when the OIE recommended cut-off (>4 mm) was applied.

Draft animals (oxen)

Overall, 60 per cent (34 of 57 cases) of all bovine TB-positive (>4 mm cut-off) and 38 per cent (72 of 188) of all M avium-positive animals were oxen.

Thirty-three animals, all oxen, were classified as draught animals; additional information on their stamina was obtained by interview. Twenty-two of the oxen (67 per cent) were bovine TB-positive using the more than 2 mm cut-off. Eleven of these animals were reported to be weaker and less able to do field work during the past year compared with previous years. In contrast, only one of the 11 PPD-negative oxen was reported to be weak. There was a significant difference between the stamina of draught animals with different PPD responses (P=0.02).

Discussion

In order to embark on an efficient national bovine TB control and/or elimination programme, infected cattle need to be identified accurately and in the early stages of the disease. Failure to achieve this will allow continuing transmission of the disease. The detection of bovine TB in live animals using the CIDT is recognised as an official test in most countries. This test uses PPD, a cocktail of mycobacterial antigens with suboptimal specificity. Ameni and others (2008) re-evaluated the OIE recommended cut-off of more than 4 mm and suggested that the more than 2 mm cut-off would be more appropriate for Ethiopia. In the present study, the authors found an overall prevalence of less than 1 per cent when using the more than 4 mm cut-off, much lower than in the findings in previous studies in central Ethiopia, where prevalences in zebu of between 7.9 per cent and 11.6 per cent were described when using the same cut-off (Ameni and others 2003, 2007). Those studies, however, were carried out in an urban/peri-urban region with higher numbers of exotic breeds (Holstein cows and their crossbreeds), more intensive livestock-keeping systems, and a higher burden of bovine TB than the present study population. In central Ethiopia, the prevalence of bovine TB was shown to be between 22 per cent and 49 per cent in exotic breeds (Ameni and others 2007, Regassa and others 2010). In contrast, the present study included only rural areas characterised by local zebu breeds and traditional smallholder mixed farming practice. The differences in M bovis prevalence between rural and peri-urban areas, and between breeds, in Africa are not new, and were described in the first half of the 20th century (Von Ostertag and Kulenkampff 1941).

Considering the absence of effective control in most of sub-Saharan Africa, the present observations show the extraordinary persistence of low-level transmission of bovine TB between animals in rural areas, despite the wide distribution of the disease in the woredas (as indicated by the high herd-village prevalence). Abattoir studies have been carried out in the same study areas and confirmed the low prevalence found in the present study: out of 37,000 inspected cattle, only 4 per cent showed tuberculous-like lesions, and M bovis was isolated from 58 of 171 cultures of acid-fast bacilli (Berg and others 2009). With the given data the authors could not detect a significant temporal trend, although variations over the years were observed. Within a woreda, the same villages were always tested during the same month of each year to avoid seasonal bias, and the same person performed all the tests to avoid bias related to the testing method. The PPD reagents were all from the same manufacturer, but from different batches. Batches could have been influenced by transport, temperature and storage. Also, the variations could have simply been explained by sampling variations. However, seasonality could have influenced the variation between the three different woredas to some extent, since each woreda was tested during different times of the year.

The present observations indicate endemic stable transmission of bovine TB at a comparatively low prevalence, similar to the observations made by Cleaveland and others (2007) in Tanzania. Other authors, however, have found a high prevalence in zebu cattle in other African countries (Bongo and others 2009). In Meskan woreda, the authors observed a general increase in numbers of Holstein cattle over the years, originating from Addis Ababa and the vicinity; this highlights the danger of potential spread of the disease from urban areas characterised by a high prevalence of bovine TB into rural areas with a low prevalence, either by the movement of live animals of exotic breeds or, potentially, through transport of their semen for artificial insemination (Niyaz and others 1999).

The apparent small variation in prevalence of bovine TB over the years in Bako-Gazer woreda, although shown to not be statistically significant, could be explained by a number of factors varying over the years and acting on an already low existing prevalence. These factors could include the removal of positive animals and/or the presence of immunosuppressive factors (which would impair the immunological response to PPD) such as disease epidemics, heavy parasitism, nutritional factors, and involvement of M avium complex agents (Lepper and others 1977, Doherty and others 1996, Dunn and others 2005). However, although there was a geographical pattern of M avium positivity, the data suggested that the prevalence of positive M avium reactions did not vary significantly during the study; therefore, it was unlikely to have influenced the variation in bovine TB prevalence over the years. Studies have shown that coinfection with M avium subspecies paratuberculosis compromises TB skin test results by drastically decreasing the sensitivity of the test (Walravens and others 2002, Aranaz and others 2006, Alvarez and others 2008). Similarly, Amadori and others (2002) showed in a trial that cattle sensitised by mycobacteria of the M avium/intracellulare group concealed M bovis for a period of time. The CIDT might also fail to diagnose M bovis infection because M avium has been shown to provide a certain degree of immunity against M bovis (Hope and others 2005). No studies have been done so far on the prevalence of paratuberculosis (Johne’s disease) in Ethiopian cattle, and it is not know to what extent this disease could compromise the detection of bovine TB in rural areas of Ethiopia using the CIDT, thus leading to false-negative results. It is also likely that other coinfections with organisms such as Fasciola species may affect the CIDT response (Flynn and others 2007). More research is needed to assess the CIDT response in relation to coinfections in Ethiopian cattle before coming to definitive conclusions regarding the prevalence of bovine TB. Coinfections and poor general health (impairing general immunity) of animals could be a major source of false-negative results, indicating the need to interpret survey results with caution.

Animals in good body condition showed more CIDT positive responses than animals in emaciated to thin body condition (OR 2, 95 per cent CI 1.5 to 2.9), a finding that has been reported previously (Delafosse and others 1995). This suggests that animals in poor body condition may have a compromised immune response to PPD injections. Positive M avium reactions were significantly associated with positive M bovis reactors (P=0.006). Mixed M avium and M bovis skin reactions greater than 4 mm each were found in 104 of the 5371 animals tested in the present study; however, these animals were classified as bovine TB-negative according to the authors’ definition of positivity, since both injection sites reacted equally strongly to the PPDs. The question arises whether this phenomenon reflects a true mixed infection with both agents, in which case the animals should be classified as bovine TB-positive, or whether they have a generally sensitised, non-specific immune reaction unrelated to any mycobacterial infection. The isolation and characterisation of mycobacterial strains from animals showing mixed skin reactions would be required to answer this question.

Using the more than 2 mm cut-off increased the apparent prevalence by three- to sevenfold, depending on the region. Reducing the cut-off increases the number of animals judged to be positive to the test. If these animals are removed from the herd in the absence of any compensation, as is likely to be the case in resource-poor countries, the farmer must bear the losses. The question remains in these countries as to who should pay for a control scheme involving the culling of positive animals, a strategy that has been shown to be very effective in industrialised countries where considerable amounts of money have been invested in compensation schemes. The cost of a false-negative diagnosis, however, likely outweighs the cost of a false-positive diagnosis, as undetected sick animals would contribute to the spread of the disease (Bongo and others 2009). Furthermore, the accuracy of a more than 2 mm skin result difference is questioned. There might be biases related to the reading of skin thickness using a cut-off as small as 2 mm; for example, nicks or cuts sustained when the injection sites are shaved could lead to minor skin inflammation and thus increase the skin thickness; animals might not stand still during reading, or pressure put on the caliper by the person reading the skin thickness might lead to an inaccurate measurement.

The authors observed that, overall, oxen were at highest risk for being positive reactors (OR 2, 95 per cent CI 1.4 to 2.6). Farmers usually took extra care of their oxen by feeding them better than the cows and bulls, because oxen are an essential workforce for ploughing, threshing and harvesting. This study could not explain why these risk factors varied between woredas, for example, why neither sex or body condition score influenced the CIDT results in Woldia. There may be an environmental or husbandry component. If oxen are more at risk for bovine TB, the whole agricultural system, particularly crop farming, could be affected by a reduced workforce. It is therefore important to assess the impact of the disease on the daily working capacity of draught animals. In the present study, bovine TB appeared to have a notable effect on the stamina of oxen, which are central to crop farming in Ethiopia and thus to the national economy. However, the chosen sample size was too small to draw final conclusions, and further research is needed in this particular area.

The authors assessed the effect of altitude on bovine TB prevalence in Woldia, where cattle in the highlands (>2000 m above sea level) were more frequently tuberculin-positive than cattle from regions at lower altitudes, but the difference was not significant. There is evidence that TB in human beings is less prevalent at higher altitudes (Saito and others 2006). Although known to be linked with changes in alveolar oxygen pressure, the exact mechanism behind the effect of higher altitude on TB is not fully established. Most extrapulmonary TB cases recorded in the Direct Observation of Treatment, Short-course (DOTS) programme (a World Health Organization [WHO] TB control strategy) at the hospital of Woldia came from the lowlands (R. Tschopp, unpublished data). More research is needed to assess the effect of altitude on TB in cattle and to assess whether the same mechanisms apply to bovine and human TB (while taking into consideration cattle husbandry practices, which may vary between the lowlands and highlands).

Knowing the individual and herd/village prevalence of bovine TB in rural areas could contribute to the choice of a future control programme for the disease in Ethiopia. In light of the present results, the question arises whether the CIDT alone is accurate enough to detect infected animals in rural areas that seem to be characterised by a very low bovine TB prevalence, a high herd/village prevalence, and a high burden of other infectious diseases and parasitism, and also, which cut-off would be more suitable in such a situation. It is questioned whether the standard skin test should be coupled with blood tests such as the IFN-γ test to increase the sensitivity of detection of positive animals. Ethiopia has the largest national cattle herd (43 million animals) in Africa (CSA 2008). Therefore, a mass vaccination programme – as was done, for example, in Malawi (Ellwood and Waddington 1972) – or a test and slaughter programme of all herds is not feasible financially and logistically, for as long as the state is unable to compensate farmers for culled animals. However, a test and slaughter programme might be economically feasible in some regions with very low bovine TB prevalence. It is therefore essential to know the bovine TB status of each region. Furthermore, by defining TB-free or -positive zones, movement restrictions could help minimise the prevalence of bovine TB in regions that are at low risk. A future challenge will be to avoid the spread of the disease into rural areas with a currently low prevalence by the import of exotic breeds (most commonly Holsteins) or their semen, in order to improve the production of dairy cattle, from central Ethiopia, which has a high bovine TB prevalence.

M tuberculosis has been isolated from abattoir samples from Ethiopian cattle, indicating transmission between human beings and animals (Berg and others 2009). Considering the high prevalence of human TB in Ethiopia (WHO 2008), and the observed transmission to cattle, the question arises to what extent M tuberculosis infection in cattle might affect the CIDT. This question requires further research and attention, especially when considering the zoonotic potential of both M bovis and M tuberculosis.

In conclusion, this study showed a low apparent prevalence of bovine TB in cattle in areas of rural Ethiopia, in contrast to a higher prevalence in central Ethiopia. The CIDT results were likely to be influenced by many factors, such as the testing method, pre-sensitisation with non-tuberculous mycobacterial infections, and other infectious diseases. The results should therefore be interpreted with caution. Inconsistency of results regarding risk factors for positive reactors inherent to animals in different regions has highlighted the danger of making generalised statements, and indicate that results should be interpreted regionally within Ethiopia. Finally, this study suggests a possible role for M avium complex agents in the general epidemiology of TB, which should be investigated further.

Acknowledgments

The authors are grateful to the Wellcome Trust (UK) for funding this study. They thank AHRI/ALERT (Addis Ababa) for logistical support. They also thank Nesredin Hussein, Mohamed Sanni, Habtamu Tadelle, Tesfaye Erenso, Mesgebu Asmro, Bamlaku Tilahun and Alemayehu Kifle for their valuable help and support during fieldwork, and Brian Robertson for commenting on the manuscript.

References

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Footnotes

  • Dr Tschopp is also at AHRI/ALERT, PO Box 1005, Addis Ababa, Ethiopia

  • Provenance: not commissioned; externally peer reviewed

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