Life without TB no doubt would make farming a happier, more profitable experience for many of us. Saying this some have adapted within their areas and founded their businesses due to the disease, but we still need to aim to reduce the total number herds under restriction.
‘Bovine Tuberculosis (TB) is an infectious disease of cattle. It is caused by the bacterium Mycobacterium bovis (M. bovis) which can also infect and cause disease in many other mammals including humans, deer, goats, pigs, cats, dogs and badgers.’
In England overall, the herd incidence rate for the 12 months to end December 2021 was 8.8, a decrease of 0.6 from the previous 12 months. Scotland, which has had officially TB-free (OTF) status since 2009, herd incidence and herd prevalence remain very low and stable. In Wales herd incidence increased by 0.5 compared to the previous 12 months, and herd prevalence increased slightly between December 2020 and December 2021.
A 2018 paper from a Michigan dairy herd investigated the route of infection and how it correlates with the anatomic distribution of lesions in cattle and cats. All adult cattle and calves down to 1 day of age were tested using the caudal fold test, 451 in total. There were 80 suspects, (18% reactor prevalence); 56 adult cattle and 24 suspect calves.
Confirmatory gamma interferon testing was performed on all 80 suspect cattle. The initial submission of nine adult cattle that were strong caudal fold test reactors (caudal fold swellings of several cm in diameter) was made to the Diagnostic Centre for Population and Animal Health (DCPAH; Lansing, Michigan), where they were euthanized and subjected to full autopsies.
The remaining 47 adult cattle that were classified as reactors were shipped directly to slaughter facilities, where they were examined by U.S. Department of Agriculture veterinarians similar to the autopsy protocol performed on the initial nine cattle.
24 strong caudal fold test reactor calves were subsequently submitted to and autopsied at DCPAH following protocols identical to those described for the adult cattle.
21 domestic cats were collected on the farm, euthanized, and submitted to DCPAH for autopsy. In a similar fashion to the cattle, head, thoracic, and abdominal lymph nodes were collected from the cats and submitted for histologic examination, mycobacterial culture, and PCR testing at DCPAH.
The differences in anatomic distribution between the adult cattle versus the calves and cats from this facility were striking. In the adult cattle, gross anatomic, histologic, and culture-positive results were predominantly within their thoracic lymph nodes, whereas both calves and cats had a statistically significant higher prevalence of culture positives in their abdominal lymph nodes.
This difference in anatomic distribution appears to correlate well with the likely routes of transmission of TB in this case. It would be suggested the adult cows would have contracted the disease through airborne transmission, as they would have been housed in the same shed, whereas the calves and cats that were fed waste milk (colostrum and antibiotic–contaminated milk) most likely caught the disease by oral ingestion. In addition, the 46% prevalence of thoracic culture positives in calves is most likely related to the lateral transmission of tuberculosis by the aerosol route. These calves started out in individual hutches, but after 3-6 weeks are raised in group pens.
The aerosol route of infection is believed to be responsible or 80–90% of all tuberculosis infections in cattle.
It is important to note that when considering oral transmission of tuberculosis, it requires 16–18 million times the infective dose compared to respiratory inoculation, so would be more of a risk if the milk is fed for a prolonged length of time. In this herd, at least two adult milking cows were shedding M. bovis into their milk, and because only 19 of >300 adult milking cattle had their milk tested, there were likely additional cows shedding M. bovis. In this research waste milk was routinely fed to calves for up to six months and to cats continuously, this provided ample opportunity to expose these animals by ingestion.
I fear TB is often a taboo topic and something I appreciate we are all trying to eradicate, let’s not forget the routes of transmission and ensure we are minimising risk where possible:
- Via respiratory route
- Via infected milk
- Before birth through placenta
- Indirectly through environmental contamination.
