Odyssean malaria remains a rare event but has potential serious implications

Malaria is mainly a travel-associated infection in Belgium where between 2016 and 2019, 327 to 420 cases were reported annually by the National Reference Laboratory. Local transmission is only sporadically reported.

In their paper published in Eurosurveillance, van Bortel et al. investigate various hypotheses regarding possible sources and ways of infection of the two patients. They conclude that the most likely route of transmission was through an infectious exotic Anopheles mosquito that was possibly imported via the international airport of Brussels or the military airport Melsbroek. The two people who died lived close to both airports (at a distance of 5 km) and based on genomic analysis of the parasites identified in the two cases, the most likely origin of the Plasmodium was Gabon or Cameroon.

No travel history, no suggested local transmission

During the second COVID-19 wave in Belgium, a person in their 80s with obesity and arterial hypertension presented to the emergency ward of a peripheral hospital with acute-onset diarrhea and dyspnoea for one day. On admission on 29 September 2020, the patient had fever (38.7 °C), was conscious and oriented. Despite antibiotic therapy and supportive treatment, the patient rapidly deteriorated and died of refractory shock on 30 September 2020.

Diagnosis of severe malaria caused by P. falciparum was made retrospectively when malaria was diagnosed in the second hospitalised case – the first patient's spouse who was admitted to hospital a day later. Both tested negative for SARS-CoV-2.

A diagnosis of severe malaria was made, based on dyspnea, hyper-parasitemia and lactic acidosis.

Neither of the two patients had a recent travel history so an entomological investigation followed to search for presence of Anopheles mosquitoes as potential vectors in the couple's house and garden as well as in a wider area around their house to assess possible routes of local transmission.

A single dead female mosquito was collected in the couple's house and identified as Anopheles claviger but was tested negative for the presence of the malaria parasite (Plasmodium spp.). In addition, the investigation did not reveal any travel-associated malaria case that could have been identified as index case. The authors conclude that "it is very unlikely that local Anopheles mosquitoes were responsible for the two local malaria cases."

A case of Odyssean malaria?

During the potential transmission time beginning of September 2020, the temperatures in Belgium were quite high, which, according to the authors, could have favored the survival of introduced exotic Anopheles mosquitoes. The authors argue that given that the proximity of the couple's residence to the international airport Brussels and the military airport Melsbroek, an infectious exotic Anopheles mosquito could have survived and reached the house as import of exotic Anopheles species has been previously reported in Belgium.

So-called Odyssean malaria, also referred to as airport, port or suitcase malaria, is the result of a mosquito bite from an imported infectious exotic mosquito. "The conclusion of this possible route of infection is often based on excluding other hypothesis, since a direct observation of this event is very unlikely", state van Bortel et al. who acknowledge that "Odyssean malaria remains a rare event. Yet, it has significant implications, particularly for the patient, as delayed or missed diagnosis of the cause of illness often results in high rates of complications and mortality".

The authors advocate for awareness among health care practitioners working near airports as they should consider malaria as a differential diagnosis when laboratory and/or clinical features such as recurrent and unexplained fever or severe thrombocytopenia are observed even without obvious exposure abroad.