DISCUSSION
M. bovis strains identified in the present case, classified as spoligotypes SB0134 and SB0133, suggesting mixed infection. Although mixed infection has not been frequently described in Ethiopia, it is more likely to happen in herds which have established their stocks from different farms, as was the case for the present herd. Hence, the mixed infection observed in this calf could be due to transmission from more than one reactor, possibly recruited from different herds without prior testing for bTB.
The time-frame for development of tubercle lesions in the investigated calf is comparable with artificially infected calves via respiratory route under experimental conditions.12 However, under such conditions; the extent of pathology could vary depending on the infective dose and factors specific to the animals and husbandry systems. Interestingly, the gross pathological lesions in the present calf were found only in specific lobes of the lungs.
Evidences have shown that the route of transmission of M. boviscould be inferred from lesions distribution.13 In the present calf, tubercle lesions were localized in the lungs and no tubercle lesions were observed in other tissues and organs, which could suggest inhalation of infectious droplets as a possible route of infection while ingestion of infected colostrum or milk might seem less likely. Other possibilities of in-utero transmission through hematogenous route and/or aspiration of M. bovis contaminated amniotic fluid 14, 15 could not be justified as there was no lesion suggestive of bTB in other vital organs, such as liver. Tuberculosis transmitted hematogenously is typically characterised by presence of hepatic tubercles or more disseminated form of the disease,16 while through aspiration of infected amniotic fluid, the primary tubercle could be in the lungs and/or the gut.
The molecular test result imply that the dam was infected with M. bovis even though it was not a reactor to the SICCT test (the increase at the PPD-B injection site was 3.2 mm while at the PPD-A site it was 2.9 mm). This could potentially be due to a development of anergic status, in which the dam would either had been be in a late disease stage or immunosuppressed attributable to the hormonal effects during pre- and post-parturition. An animal with a chronic stage of the disease is known to be highly infectious and would likely serve as a good source of infection to a neonate by transmission in several possible ways.17, 18 Therefore, it is highly likely that the calf in this observed case got infected in its first week of life, while suckling the colostrum by inhalation of aerosol primarily generated from its dam, although the possibility of transmission from other infected animals could not be ruled out as all reactors in this farm shared the same housing and husbandry system as the calf. In fact, the latter could explain why strains of the two spoligotype patterns were isolated from its lungs.
In conclusion, although this is a single case, it illustrates that the progression of bTB in calves can be rapid under natural conditions, contributing to transmission within herds. This has important implications in the appropriate structure of models used to estimate the rate of transmission within herds. 19 Calf can respond to the tuberculin test at a very early age, and thus should be considered in the routine bTB screening to reduce transmission within herds.