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.