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Streptococcus suis is a zoonotic pathogen causing substantial economic losses to the pig industry, as well as being a human health burden due to infections worldwide, especially in Southeast Asia. In Thailand, there was high cumulative incidence in humans during 1987–2021, mostly in males. At least five large outbreaks have been documented after the largest outbreak in China in 2005, which was related to the consumption of raw pork or dishes containing pig’s blood. The major clinical features are sepsis or meningitis, with hearing loss a major complication of S. suis disease. Thai human S. suis isolates have shown diversity in serotypes and sequence types (STs), with serotype 2 and STs 1 and 104 being major genotypes. β-Lactam antibiotics can be used in empirical treatment for human S. suis infections; however, intermediate resistance to penicillin has been reported. Reducing S. suis incidence in Thailand requires a multidimensional approach, with combined efforts from the government and public health sectors through policy, regulations, education, and active surveillance.
As shown in Table 1, for S. suis isolated from patients in Thailand, serotype 2 (93.4%) was dominant, followed by serotypes 14 (5.2%), 24 (0.6%), 5 (0.4%), 4 (0.1%), 9 (0.1%), 31 (0.1%), and unencapsulated (0.1%), respectively [17][20][21][22][23]. MLST classified serotype 2 into five CCs: CC1, CC25, CC28, CC104, and CC233/379. Of these, CC1 is a major CC of human S. suis infection in this country and ST1 is the main ST in CC1 [21], while serotype 14 was classified to only CC1, with ST105 predominant [21][23]. For serotype 2, ST104, ST25, ST28, and ST233 were the main STs in CC104, CC25, CC28, and CC233/379, respectively. Notably, STs 1 and 104 for serotype 2 are the predominant STs in Thai human infections, and CC104, CC233/379, and CC221/234 are found exclusively in Thailand [20][21][24].
Serotype | Clonal Complex | Sequence Type | Reference |
---|---|---|---|
2 | 1 | 1, 11, 105, 126, 144, 298, 337 | [17][18][19][20][21][22][23][25][26][27][28][29] https://pubmlst.org/organisms/streptococcus-suis (accessed on 5 October 2022) |
25 | 25, 102, 103, 380, 381, 395, 515, 516 | ||
28 | 28, 382 | ||
104 | 101, 104, 391, 392, 393, 512, 513, 514 | ||
233/379 | 233, 379, 1656, 1713 | ||
1687/1688 | 1687, 1688 | ||
Singleton | 232, 236 | ||
4 | 94 | 94 | |
5 | 221/234 | 221 | |
Singleton | 181, 235 | ||
9 | 16 | 16 | |
14 | 1 | 11, 105, 127 | |
24 | 221/234 | 221, 234 | |
31 (Unencapsulated) |
221/234 | 221 | |
Unencapsulated serotype 2 or 1/2 |
28 | 28 |
Other studies have revealed that Thai S. suis isolates were susceptible to penicillin [13]. This contrasted with Nakaranurack et al. (2017), who reported that 6 out of 11 Thai S. suis isolates were intermediately resistance to penicillin, whereas cefotaxime and vancomycin were completely susceptible [33]. However, a study in 2021 demonstrated that 448 S. suis isolates recovered from human infections in Thailand had 8.2% intermediate resistance to penicillin, while they were all susceptible to cefepime and ceftriaxone [34].
Resistance to tetracycline (98.2%), clindamycin (94%), erythromycin (92.4%), and azithromycin (82.6%) with the resistance genes tet(O) and ermB were the predominant determinant genes of tetracycline and erythromycin (also macrolide-lincosamide–streptogramin B (MLSB)) resistance detected in 448 S. suis isolates [34]. Resistance to tetracycline appeared common in S. suis from human infections worldwide [35][36][37][38][39], whereas the resistance rates to erythromycin were low in Poland, Hong Kong, and Vietnam [35][36][40]. Although tet(O) is prevalent in human S. suis in Thailand, the tet(M) gene is the most widespread in human S. suis in China and Vietnam [36][38]. The ermB gene was predominant in human isolates in Vietnam and Thailand, while mefA was present in Hong Kong [34][36]. This may indicate differences in the local spread of tetracycline and erythromycin-resistance genes among human S. suis isolates in different countries or geographical regions.