This is the first study investigating the presence of TTSuV1 and 2 in domestic pigs in Uganda, and indeed on the African continent. Available studies from many parts of the world demonstrate that these viruses are widespread among pig populations [3, 17], and it is therefore not surprising to see the same pattern in Ugandan pigs: our study showed that around half of the investigated domestic pigs were infected with either of the TTSuV species, indicating that TTSuV infection is common in Uganda. TTSuV1 was detected in fewer pigs compared to TTSuV2, and in most (13/16) of the samples positive for TTSuV1, TTSuV2 was also detected. Whereas TTSuV2 was detected in similar frequencies as in previous comparable studies [12, 18, 19], the detected prevalence of TTSuV1 positive pigs was lower compared to what has been reported elsewhere [11, 12, 19, 20].
The genetic similarity of the sequenced Ugandan TTSuV UTR regions was higher for TTSuV1 than for TTSuV2 (TTSuV1: 90-100%; TTSuV2: 63-100%). The sequence identity to sequences from other parts of the world displayed a similar pattern (TTSuV1: 90-98%; TTSuV2: 64-99%). Previous studies have shown that TTSuV lacks an apparent geographical clustering when studying the UTR region . The phylogenetic analysis of TTSuV1 (Figure 2) displays the Ugandan sequences evenly distributed among the isolates from other parts of the world. TTSuV2 also grouped evenly together with the sequences from other parts of the world (Figure 3). A grouping of the sequences into two clades was due to an 11-12-nucleotide long region present in a number of sequences (a similar result was obtained by using Maximum Likelihood based on the Tamura-Nei model (data not shown)). An additional 20-nucleotide long region was present in three of the Ugandan isolates and these therefore formed a well-supported sub-clade. Overall, the bootstrap values within the clades were generally low for both genotypes, meaning that branching patterns of these are not always highly confident. Using the ORF1 capsid gene as a marker, which has been suggested previously , could possibly give a better idea of the evolutionary origin of the sequences, since the capsid gene presumably is under purifying selection. Also using the complete genome would be valuable for phylogenetic analysis [9, 10] to investigate possible episodes of viral introduction(s) and relations to epidemiological events.
The potential pathogenicity of TTSuV is unclear, but it is speculated that these viruses can be directly or indirectly involved in disease development. Several studies in pigs concern the role of co-infection of TTSuV1 and 2 with known pathogenic viruses like PCV-2, the causative agent of PMWS [13, 19, 21]. Although two of these studies indicate a somewhat higher co-infection rate of TTSuV in diseased pigs further studies are needed to prove the involvement of TTSuV in PMWS pathogenesis. 18 out of the 95 samples in the present study had previously been tested positive for African swine fever virus, but the proportion of TTSuV1 and 2 positives among these 18 samples did not differ significantly from that of the entire sample (data not shown).