High prevalence and increased severity of pathology of bovine tuberculosis in Holsteins compared to zebu breeds under field cattle husbandry in central Ethiopia

Abstract

A comparative study on the prevalence and pathology of bovine tuberculosis (TB) was conducted on 5,424 cattle (2,578 zebus, 1,921 crosses, and 925 Holsteins), which were kept on pasture in the central highlands of Ethiopia, using a comparative intradermal tuberculin test, postmortem examination, and bacteriology. The overall prevalence of bovine TB was 13.5%; prevalence was higher in Holsteins than either zebus (22.2% versus 11.6%, chi(2) = 61.8; P < 0.001) or crosses (22.2% versus 11.9%, chi(2) = 50.7; P < 0.001). Moreover, the severity of pathology in Holsteins (mean +/- standard error of the mean [SEM], 6.84 +/- 0.79) was significantly higher (P = 0.018) than the severity of pathology in zebus (5.21 +/- 0.30). In addition, the risk of TB in Holsteins was more than twice (odds ratio [OR] = 2.32; 95% confidence interval [CI] = 1.89, 2.85) that in zebus. Animals between 5 and 9 years of age were at higher (OR = 2.37; 95% CI = 1.80, 3.12) risk of bovine TB than those 2 years of age or below. A significant difference (chi(2) = 351; P < 0.001) in the occurrence of TB lesions in lymph nodes was recorded; the mesenteric lymph node (mean pathology score +/- SEM, 1.95 +/- 0.08) was most severely affected, followed by the retropharyngeal (0.80 +/- 0.05) and caudal mediastinal (0.8 +/- 0.06) lymph nodes. Fifty-six percent (n = 145) of the animals with gross TB lesions were culture positive; the lowest culture positivity was recorded in the skin lesions (27.3%) and the lesions of the mesenteric lymph node (31.5%). Both the skin test response and the postmortem findings suggested a higher susceptibility to bovine TB in Holsteins than zebus under identical field husbandry conditions (on pasture). In the light of increased numbers of Holstein cattle introduced into this area to raise milk production to satisfy the needs of Addis Ababa's growing population, these findings highlight the need for a control program in these herds.

FIG. 1.

Map of the study area. Selalle and Holeta are known for their dairy production. Farmers in these areas keep three major breeds of cattle (Holstein, zebu, and Holstein × zebu) under similar conditions. Unlike the majority of Ethiopian farmers, who use milk for home consumption, farmers in Holeta and Selalle sell milk, and the two areas are the major sources of milk for Addis Ababa.

FIG. 2.

Mean pathology scores of lymph nodes. Pathology scores were determined as defined by Vordermeier et al. (43). Results are expressed as the mean score for each lymph node ± SEM.

FIG. 3.

Mean pathology score in skin test-positive cattle assessed in this study. These animals were selected from among skin test-positive animals on the basis of the size of skin indurations and willingness of the owners to sell. The pathology was significantly severe (P = 0.018, Mann-Whitney test) in Holsteins (n = 50) than in zebus (n = 73) under identical husbandry conditions.

FIG. 4.

Rate of culture positivity of suspicious tissues obtained from 145 animals with gross tuberculous lesions, as confirmed by the observation of acid-fast bacilli in the colonies. Culture positivity did not differ (χ² = 0.13, P = 0.72) between Holsteins (54%, n = 50) and zebus (51%, n = 73) kept under identical field husbandry conditions.