Trypanosomiasis is one of the major endemic diseases in sub-Saharan Africa. It is caused by trypanosomes, an extracellular flagellated protozoan parasite of the genus Trypanosoma. The disease exists in two forms: The human African trypanosomiasis or sleeping sickness and the animal African trypanosomiasis. In humans, the disease is caused by two subspecies of Trypanosoma brucei: Trypanosoma brucei rhodesiense responsible for the acute form of the disease in eastern and southern Africa, while Trypanosoma brucei gambiense is responsible for the chronic form of the disease in western and central Africa [1]. Approximately 56 million people are estimated to be at different levels of risk of contracting HAT and more than a surface of 1.18 million Km2 are still at risk of T.b. gambiense infection [2]. In 2019 there were 992 cases recorded in Africa [1]. The animal form of the disease, animal African trypanosomiasis (AAT) is caused by several species and subspecies of trypanosomes including Trypanosoma congolense, Trypanosoma vivax, Trypanosoma simiae, Trypanosoma uniforme, Trypanosoma godfreyi, Trypanosoma brucei brucei and Trypanosoma grayi. They cause pathogenic infections in cattle, sheep, goats, pigs, dogs, camels and horses [3–5]. The disease is one of the major constraints to agricultural development on the continent.
Trypanosomes are cyclically transmitted between different vertebrate hosts by tsetse flies (Diptera: Glossinidae). Thirty-one species and subspecies of tsetse flies have been described. They can be grouped into three groups or subgenera based on common characteristics and morphology due to bio-ecological and genetic similarities: the riverine Palpalis, the savannah Morsitans and the forest Fusca [6, 7]. They acquire trypanosomes when feeding on an infected mammalian host. The trypanosomes undergo a series of transformations and multiplication in their gut and give rise to infective forms which will be inoculated into a new host during the feeding [8].
Tsetse gut harbours a diversity of bacteria acquired from the environment or maternally transmitted [9]. Previous studies have shown that these bacterial populations vary considerably depending both on the tsetse species or sub-species and the geographic origin of the flies [10]. This microbial community influences several aspects of tsetse’s physiology, including nutrition, fecundity development and maturation of the innate immune system and vector competence [11, 12].
Tsetse flies have established long-term associations with four vertically transmitted endosymbiotic bacteria including Wigglesworthia glossinidia, Sodalis glossinidius, Wolbachia sp, and Spiroplasma sp that was recently established as the fourth tsetse symbiont in G. fuscipes fuscipes, G. tachinoides, and G. palpalis palpalis [13]. They show different types of relation with their host.
All tsetse flies house Wiggleworthia glossinidia, the primary and obligate endosymbiont. It resides intracellularly within the bacteriome in the anterior midgut and also found extracellularly in milk gland of the fly [11]. Wiggleworthia glossinidia provides dietary supplements absent from the fly’s vertebrate blood-restricted diet, supports larval development and contributes to maturation of the adult immune system [12, 14].
Wolbachia endosymbionts are obligatory intracellular bacteria belonging to the Order Rickettsiales. Wolbachia infects a broad range of arthropod and filarial nematode species and is probably the most prevalent endosymbiont found in insect germlines [11]. Within the genus Wolbachia, 17 supergroups (A to Q) are currently recognized based on sequences of the five conserved genes fbpA, coxA, ftsZ, gatB, coxA, and hcpA and the amino acid sequences of the four hypervariable regions of the Wsp protein [11, 15]. The majority of insect infections fall into supergroups A and B [16].
In tsetse, Wolbachia resides mainly in reproductive tissues and is maternally transmitted from generation to generation by trans-ovarian transmission. They are also transferred horizontally among arthropods. Wolbachia infection in the tsetse host results in a variety of reproductive abnormalities such as parthenogenesis, male killing, feminization and cytoplasmic incompatibility [16–18].
Cytoplasmic incompatibility results in embryonic mortality in the progeny derived from matings between insects with different Wolbachia infection status: when an infected male mates with an uninfected female or a female infected with a different strain of the bacterium [16]. The presence of this symbiont in the tsetse fly Glossina morsitans has been associated with the induction of cytoplasmic incompatibility [17]. This effect confers indirect reproductive advantages to the infected females and is considered as a potential vector control alternative.
Some studies have shown that Wolbachia infections limited mosquito-transmitted pathogens including dengue virus, chikungunya virus, Plasmodium parasites, yellow fever virus, Zika virus and filarial nematodes [19–22].
The tsetse’s secondary and facultative symbiont is the commensal Sodalis glossinidius. It is a gram-negative organism belonging to the Enterobacteriaceae family. It is widely spread in numerous tissues of the fly (midgut, fat body, milk gland, salivary glands and reproductive system) and can be found both intracellularly and extracellularly [23]. The Sodalis genome consists of one circular chromosome of 4.17 Mbp, three extrachromosomal plasmids designated pSG1, pSG2, and pSG3, as well as a phage, ФSG1. However, its genome sequence shows reduced coding capacity with a large number of pseudogenes [24]. Sodalis glossinidius can be transmitted maternally via haemolymph, milk gland secretions, and horizontally during mating [11, 25]. In tsetse flies, the specific role of this symbiont is still not clear. However, it has been shown to affect host longevity and has been suspected to play a role in potentiating susceptibility to trypanosome infection in tsetse by influencing the efficacy of the tsetse immune system possibly through lectin-inhibitory activity [26].
The genus Spiroplasma belongs to the Mollicutes class, and the Tenericutes phylum. Spiroplasma are found abundantly in insects either in the gut or haemolymph where they have developed a large variety of interactions with the host that can be classified as commensal, pathogenic or mutualist [13].
It has been shown that Spiroplasma confers protection against pathogens e.g Drosophila neotestacea from a nematode [27], the pea aphid against fungi [28] and Drosophila hydei against a parasitoid wasp Leptopilina heterotoma [29]. However, reproductive alterations such as cytoplasmic incompatibility, male-killing and sex determination have been related to numerous species of Spiroplasma [13].
Recently, Spiroplasma has been established as a new class of tsetse symbiont in G. fuscipes fuscipes, G. tachinoides, and G. palpalis palpalis. The interactions between Spiroplasma and Glossina seem to be beneficial because of its ability to extend lifespan and reduce the vector competence for Trypanosoma [11, 30].
Current control measures against trypanosomiasis are mainly based on chemotherapy. In the absence of effective vaccine and to address the limitations associated with chemotherapy, disruption of trypanosomes transmission through vector control is crucial. Transmission of pathogens by vector depends on its vector competence, which can be affected by several factors, including vector endosymbionts [31]. Due to their importance, interactions between the symbionts and their hosts are being harnessed toward the development of novel approaches for vector and disease control [32–34].
The present study aims to investigate the presence of Sodalis, Spiroplasma and Wolbachia in wild population of tsetse flies from Cameroon, Nigeria and Chad.