Parasitology · 3 authors, 4 centres
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This study found that 95.3% of tsetse flies in eastern Zambia carried Sodalis or Wolbachia symbionts, while 25.5% were infected with trypanosomes. No statistically significant association was observed between symbiont presence and trypanosome infection, suggesting symbionts may not influence vector competence in this region. The detection of Trypanosoma brucei rhodesiense confirms ongoing risk of human sleeping sickness in the Luangwa valley.
**Background:** African trypanosomiasis, caused by Trypanosoma protozoa and transmitted by tsetse flies (Glossinidae), is a fatal disease affecting both humans (sleeping sickness) and livestock (Nagana) in sub-Saharan Africa. Tsetse flies harbor three endogenous symbionts—Wigglesworthia glossinidia, Sodalis glossinidius, and Wolbachia—which influence host nutrition, reproduction, and potentially vector competence. Previous studies have yielded conflicting results on whether symbionts affect trypanosome transmission. This study aimed to establish the prevalence of Sodalis and Wolbachia in tsetse species from eastern Zambia's Luangwa valley and determine their relationship with trypanosome infection.
**Methods:** Tsetse flies were collected between 2019 and 2020 from seven villages (Katemo, Ncheka, Nsefu, Chilanga, Chinzombo, Malama, Chikowa) in Mambwe district, eastern Zambia, using Epsilon traps baited with 3-n-prophyphenol and 1-octe-3-nol. Supplementary captures from moving vehicles were used in low-density areas. Total genomic DNA was extracted from individual flies (wings and legs removed) using QIAamp DNA mini kits. Symbiont presence was determined by species-specific PCR: Sodalis was detected via amplification of a 650-bp fragment of the hemolysin gene (primers HemF/HemR), and Wolbachia via a 610-bp fragment of the wsp gene (primers 81F/691R). Trypanosome species were identified using ITS-PCR targeting the internal transcribed spacer 1 of ribosomal RNA, producing different-sized products for different species. T. b. rhodesiense was confirmed by SRA gene PCR. Statistical analysis used Epi-info 7.2 with chi-square or Fisher's exact tests, odds ratios, and Pearson's correlation.
**Key Results:** A total of 278 tsetse flies were captured: 237 (85.3%) Glossina pallidipes and 41 (14.8%) Glossina morsitans morsitans. The combined prevalence of Sodalis and Wolbachia was 95.3% (n=278, 95% CI 92.8–97.8). Overall trypanosome prevalence was 25.5% (n=278, 95% CI 20.4–30.7), comprising T. brucei 10.8% (n=30, 95% CI 7.1–14.4), T. congolense 1.4% (n=4, 95% CI 0.0–2.8), T. vivax 1.4% (n=4, 95% CI 0.0–2.8), and T. b. rhodesiense 0.7% (n=2, 95% CI −0.3–1.7). In G. pallidipes, symbiont prevalence was 94.9% (n=225, 95% CI 92.2–97.7) and trypanosome prevalence 26.6% (n=63, 95% CI 21.0–32.2). In G. m. morsitans, symbiont prevalence was 97.6% (n=40, 95% CI 92.8–102.3) and trypanosome prevalence 19.5% (n=8, 95% CI 7.4–31.6). No significant difference was observed between species for symbionts (P=0.46) or trypanosomes (P=0.34). Female flies had higher odds of harboring Sodalis (OR=1.9, 95% CI 0.8–4.4) and Wolbachia (OR=1.3, 95% CI 0.7–2.5) than males. Among Sodalis-positive flies (n=240), T. brucei prevalence was 12.9% (95% CI 8.7–17.2), T. congolense 1.7% (95% CI 0.1–3.3), T. vivax 1.3% (95% CI −0.2–2.7), and T. b. rhodesiense 0.8% (95% CI −0.3–2.0). Among Wolbachia-positive flies (n=220), T. brucei prevalence was 12.7% (95% CI 8.3–17.1), T. congolense 1.8% (95% CI 0.1–3.6), T. vivax 1.4% (95% CI −0.2–2.9), and T. b. rhodesiense 0.9% (95% CI −0.4–2.2). Odds ratios for T. brucei infection were 1.3 (95% CI 0.5–3.2) with Wolbachia and 1.7 (95% CI 0.5–6.0) with Sodalis. Pearson's correlation coefficients between symbionts and trypanosomes were all near zero (range −0.04 to 0.06), with no statistically significant correlations.
**Clinical Implications:** The high prevalence of trypanosomes (25.5%) in tsetse flies, including the human-infective T. b. rhodesiense, confirms ongoing transmission risk for both animal and human trypanosomiasis in eastern Zambia. The absence of a significant association between symbiont presence and trypanosome infection suggests that Sodalis and Wolbachia do not play a major role in modulating vector competence in this region, contrary to some previous studies. This finding indicates that symbiont-based control strategies may not be effective in the Luangwa valley ecosystem. The high prevalence of Wolbachia in female flies warrants further investigation as a potential basis for population control through cytoplasmic incompatibility. The study highlights the need for continued surveillance and integrated control approaches, as trypanocide resistance threatens current management of Nagana.