Need of entomological criteria to assess zero transmission of gambiense hat

Abstract

Human African trypanosomiasis, also known as sleeping sickness, is a neglected tropical disease due to trypanosomes (Kinetoplastidae) transmitted to humans by a vector, the tsetse fly (Diptera: Glossinidae), that only occurs in Africa. There are 2 forms of the disease: Rhodesiense HAT (r-HAT) is due toTrypanosomabruceirhodesiense and occurs East and South of Africa. Gambiense HAT (g-HAT) is due toTrypanosomabruceigambiense, occurs in West and Centre of Africa, and is considered as the chronic form of the disease. Only g-HAT will be considered in this paper, since it is the only one that is targeted for “elimination as interruption of transmission” by WHO in the 2030 WHO roadmap for NTDs. The disease is lethal without treatment with some exceptions, and there is no vaccine. Control efforts have been usually implemented through diagnosis and treatment. The diagnosis involves a complex chain composed of clinical examination, a serological test, followed by a parasitological confirmation, and treatment is usually given only to people in whom trypanosomes have been detected. A diagnosis of the neurological stage still relies on lumbar puncture. Treatment has for long been based on arsenical derivatives showing severe side effects, then has improved through the NECT (Nifurtimox-Eflornithine combination therapy; [1]), and even better recently with the arrival of an oral treatment that can treat the 2 stages, the fexinidazole [2,3]. Combining medical activities with tsetse control in g-HAT foci can accelerate HAT control, as observed in Guinea [4], Chad [5], and D. R. Congo [6]. In 2018, less than 1,000 cases of HAT were reported [7], thanks to this combination of medical and vector control tools. It can be estimated that, currently, this has allowed decreasing and preventing HAT transmission in an area of more than 7,000 km2, protecting more than 1 million people living in these foci of 5 countries, Guinea, Cote d’Ivoire, Chad, DRC, and Uganda [8]. With this spectacular decrease in number of cases observed, some countries are in the process of submitting their dossier to WHO for the validation of elimination as public health problem, and some even are in the process of interrupting transmission. Within the context of “elimination as interruption of transmission,” a key question now becomes: what criteria should be used to assess this interruption of transmission? Here we will review how elimination efforts are nowadays being followed up in g-HAT, how this compares to other vectorborne NTDs (VB-NTDs), and we will review the entomological tools available that may be used for interruption of transmission.