Table 2. Outbreak investigations of fatal autochthonous malaria cases, Belgium, 2020 (n = 2).

Hypothesis	Case investigation	Conclusion
1. Travel-related malaria, acquired in endemic zones	No recent travel history: none of the patients had travelled to an endemic area for more than 50 years. No travel of another member of the family to an endemic area nor visitors from abroad. No relapsing malaria as the causative pathogen of these cases was Plasmodium falciparum.	Can be excluded
2. Induced or not mosquito-transmitted malaria	No blood transfusion or organ transplantation in the last 5 years. Nosocomial transmission could be excluded, as the cases were not admitted to any hospital wards in the last 3 years.	Very unlikely
3. Introduced malaria	Based on postal codes of patients with imported P. falciparum infections in August and September 2020 (n=15), no travel-related cases could be identified that could have been the source of infection of a native vector species. Anopheles claviger and Anopheles daciae were found inside the house and in a 500 m buffer zone around the house, respectively; these species are not known as vectors of P. falciparum.	Very unlikely
4. Odyssean malaria or acquired through imported infectious exotic mosquito	The cases lived at 5 km from the international airport Brussels and the military airport Melsbroek. An imported exotic Anopheles infected could have flown from one of these places to the cases' house.	Most likely explanation