Appraisal of interpretation criteria for the single intra-dermal comparative cervical tuberculin test for the diagnosis of tuberculosis in dromedary camels in Ethiopia

Tuberculosis caused by Mycobacterium caprae in a camel (Camelus dromedarius)

BACKGROUND

Animal tuberculosis (TB) is distributed worldwide and has a wide range of wild and domestic reservoirs. Few studies concerning TB in camelids have been published in the last decade, particularly as regards Old World Camelids (OWC), but the increase in reports of TB outbreaks in these species in recent years suggests a high susceptibility to the infection.

CASE PRESENTATION

We studied a dromedary camel (Camelus dromedarius) herd (n = 24) in which a Mycobacterium caprae infection was detected. The TB infection was confirmed in one animal at necropsy through the detection of TB lesions, mainly in the abdominal organs, and the subsequent isolation of M. caprae (SB0157 spoligotype). The whole herd was additionally tested using cellular and humoral based diagnostic techniques. The intradermal tuberculin test results were compared with those obtained using P22 ELISA for the detection of specific antibodies against the M. tuberculosis complex. The TB infected animal was a positive reactor to both the intradermal tuberculin tests and P22 ELISA, while the others were negative to all the diagnostic tests.

CONCLUSION

The present study found M. caprae infection in OWC. This is the first report of M. caprae infection in an OWC not living in a zoo. Since the animal was born in the herd and fed with goat's milk, this practice was suspected to be the potential source of TB infection, which was not confirmed in the other animals present in the herd. Moreover, our results highlight that the intradermal tuberculin test and the P22 ELISA could be valuable tools for the diagnosis of TB in OWC.

Tuberculosis in small ruminants and dromedary camels in Ethiopia: A systematic review and meta-analysis

Tuberculosis (TB) is a chronic infectious disease of livestock with serious economic and public health impact in Ethiopia. The disease is reported from cattle, small ruminants and dromedary camels in the country. However, there is no organized summary report on the magnitude and distribution pattern of TB in small ruminants and dromedary camels, unlike that of bovine TB. Consequent to this gap, this review was organized to provide pooled prevalence estimates, and examine level of heterogeneity among studies at national level. In addition, it attempts to illustrate the spatial distribution patterns along the three livestock species based on available reports. Tuberculosis articles on the aforesaid livestock species were searched online using PubMed, CAB direct, Web of Science and AJOL databases. Eighteen articles published from January 2000 to May 15, 2020, written in the English language that fulfill the selection quality criteria were considered for the review. Altogether, 50 district based observational studies conducted on 10,371 goats, 6262 dromedary camels, and 1457 sheep were used for analysis. Accordingly, the pooled prevalence estimates of TB, in a random effect model were 2.3 % (95 % CI: 1.7, 3.1) for goats, 0.8 % (95 % CI: 0.5, 1.4) for sheep and 8.2 % (95 % CI: 6.6, 10.2) for dromedary camels. The subgroup analysis revealed presence of statistically significant differences (p < 0.001) in pooled prevalence estimates among the three species. In multivariable meta-regression model, diagnostic methods used for screening (single intra-dermal comparative cervical tuberculin test (SICCTT)> 2mm, SICCTT > 4mm and detailed postmortem inspection) were the only predictors identified to show statistically significant difference (p<0.001) and explained 68.6 % of the explainable heterogeneity (R² =0.686) in goat TB studies. In general, study reports on small ruminant and dromedary camel TB are limited throughout the country. The most significant data gaps were in Gambella, and Benshangul-Gumuz regional states, where no single report could be retrieved on small ruminant TB. Limitation of study reports and lack of comparable categories constrained further investigation on other predictors in sheep and camel studies. Thus, the authors would like to emphasize the need for more representative studies in the species of concern in all regions of the country. Meanwhile, the relatively higher proportion of TB in dromedary camels demands special attention in arid and semiarid parts of the country, as it is the leading livestock species on which agropastoralist and pastoralists livelihoods depend.