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Bovine TB Pathogen Genotypes Trace Disease History in Mozambique

NEW YORK (GenomeWeb) – A genetic study of Mycobacterium bovis samples collected from cattle in Mozambique has teased out bovine tuberculosis (TB) transmission patterns in the area. 

The disease, which can be transmitted between animals and humans, and infects both cattle and wildlife, persists because the pathogen survives in animal reservoirs. It is of particular concern in Africa, where raw milk and meat from infected animals are frequently consumed and where HIV-infected individuals are especially susceptible to the disease.

Researchers from Portugal, Sweden, Mozambique, and South Africa genotyped nearly 200 M. bovis isolates from cattle from small and large commercial herds in Mozambique. As reported today in PLOS Neglected Tropical Diseases, they detected pronounced genetic diversity in the bovine TB-causing pathogen, which broadly tracked with sampling sites across the eight provinces considered.

"[T]here is a strong association between M. bovis genetic characteristics and geography," senior author Margarida Correia-Neves, a researcher at the University of Minho in Portugal and the Karolinska Institute in Sweden, and her colleagues wrote. "This implies that the diversity of M. bovis isolates in Mozambique does not seem to be caused by recent introduction to the territory, but is probably maintained within reservoirs in each particular region."

For their study, the researchers used 228 cattle samples collected between 2007 and 2013 at three small- or large-scale cattle operations in each of 10 districts in Mozambique, a southeast African country where bovine TB has previously been detected in more than one-tenth of cattle. In addition, they looked at 220 samples from dead animal that had been sent to a veterinary laboratory because of suspected bovine tuberculosis.

Using a combination of spacer oligonucleotide typing (known as spoligotyping), PCR-based 'regions of difference' (RD analysis), and Mycobacterial interspersed repetitive units-variable number tandem repeat (MIRU-VNTR) at two dozen loci, the team successfully genotyped 178 of the isolates, representing samples collected at 113 southern, 47 central, and 18 northern Mozambique sites.

The investigators noted that the most commonly identified spoligotype, found in more than half of the isolates, had a so-called BCG-like profile resembling a bovine TB vaccine strain. The RD data provided a look at clonal complexes in the region, while MIRU-VNTR patterns for 59 of the M. bovis isolates helped to tease out relationships within specific spoligotypes.

More than three dozen isolates belonged to a clonal complex that's been described in the UK, former British colonies, and parts of the New World, but is relatively rare in many parts of Africa, the authors noted. That M. bovis clonal complex was particularly common in southern Mozambique, prompting them to speculate that it may have reached the country via commercial farms receiving animals from South Africa.

Indeed, by folding in genotypes from other African isolates genotyped for past studies, the team saw phylogenetic evidence for at least one M. bovis introduction from South Africa. Even so, a distinct clade appeared to reflect a long local history for the pathogen in Mozambique, and the authors did not detect isolates from a clonal complex that has previously been identified in East African countries such as Ethiopia and Tanzania.

"Although Tanzania borders Mozambique, our data suggest that cattle movement between the two countries is limited," the researchers explained. "However, sample size from the North of Mozambique, where there is a shared border with Tanzania, was very small and one needs to consider that this signal might be present at low prevalence and was not seen in this study."

Understanding the transmission routes of M. bovis in Mozambique and other countries, they concluded, "is essential in order to focus public health and veterinary resources to contain bovine tuberculosis."