Overview

Malaria is a tropical protozoal infection caused by the Plasmodium parasite, and is transmitted via the female Anopheles mosquito in endemic areas. It is the most commonly imported tropical disease in the UK, and is a medical emergency.

Epidemiology

· Approximately 250 million cases per year and 600,000 deaths per year globally
· 1000 – 2000 imported cases per year in the UK
· Endemic throughout Africa, Asia, and South America
· For country-specific malaria risk, check https://travelhealthpro.org.uk/
The travellers at highest risk are those visiting friends and relatives, as they are less likely to attend pre-travel health checks, and may believe they are not at risk of acquiring malaria

Malaria global risk map https://www.cdc.gov/malaria/about/distribution.html

Malaria parasites

There are five species of malaria which are clinically relevant. Plasmodium falciparum is the most severe and most common, particularly in patients who have travelled to Africa, followed by Plasmodium vivax (more common in Asia and the Americas). Less common species are P malariae, P ovale, and P knowlesi.

Malaria parasites are transmitted as sporozoites by mosquito bites. The parasites initially infect the liver, then multiply and infect red blood cells (this is the stage that causes the clinical symptoms of malaria). In P vivax and P ovale infections, parasites can remain dormant in the liver as hyponozoites and cause relapsing infection weeks to years after the initial phase.

Clinical history

It is essential to take a full travel history including all countries visited, length of time, and areas visited within the country (rural vs urban areas)
All febrile travellers should undergo a viral haemorrhagic fever risk assessment as well: https://assets.publishing.service.gov.uk/media/5a807f14ed915d74e33fac3d/VHF_Algo.pdf
History of mosquito exposure is important: any bites, use of DEET-based insect repellent, mosquito nets, and appropriate clothing to decrease bite risk. However, patients do not always notice mosquito bites.

Malaria prophylaxis can decrease risk but is not 100% effective. It is important to check:

• Was prophylaxis taken?
• Were any doses missed?
• Was this continued after leaving the country? (generally prophylaxis is recommended to continue 1 – 4 weeks after leaving the endemic region depending on which agent is used)

Clinical Features

Symptoms of malaria can be non-specific – you should consider malaria in any febrile traveller returning from an endemic area. Symptoms typically begin 1-2 weeks after exposure but can occur months later, or even over a year after exposure in non-falciparum malaria. Symptoms include:

• Fever
• Rigors
• Headache
• Malaise
• Myalgia
• Nausea and vomiting

Severe cases can present in single- or multi-organ failure.

Examination findings may include fever, pallor due to anaemia, hepatosplenomegaly (rare) and jaundice.

Investigations

EDTA blood (ie an FBC bottle) should be sent urgently for thick and thin films
Most laboratories will also perform a rapid diagnostic test (lateral flow), which has quite good sensitivity for P falciparum and reasonable sensitivity for P vivax

If the rapid test is negative, consider alternative differentials (e.g. dengue fever, typhoid), but false negatives do occur, so discuss with local infection specialist if your suspicion is high

Blood film interpretation requires significant skill and experience, and may not be available in all labs
Thick films have a high sensitivity for detection of parasitaemia
Thin films have a high specificity – allows quantification of parasitaemia and determination of species
Malaria is not ruled out until three blood films are negative

Plasmodium falciparum identified on thin film (from https://www.cdc.gov/dpdx/malaria/index.html)
Routine bloods should be requested including U+Es, FBC, glucose, coag profile, LFTs, VBG, HIV test and blood cultures (secondary bacterial infections can be present). Findings may include:

• Leukopaenia
• Thrombocytopaenia (nearly always present, and not predictive of severity)
• Anaemia
• Severe malaria is defined as any of the following:
• Shock (BP <90/60)
• AKI, oliguria, or haemoglobinuria (“Blackwater fever”)
• Reduced GCS/coma/seizure
• Acidosis
• Hypoglycaemia
• Hb <80g/L
• DIC/spontaneous bleeding
• Pulmonary oedema/ARDS on CXR
• Parasite count >2% on a thin blood film

Management

Discuss cases of suspected malaria with your local ID service urgently, as treatment should be started promptly
Patients with P falciparum malaria should generally be admitted as there is potential for rapid deterioration, unless your local ID unit has a specific ambulatory pathway in place.

First line treatment for uncomplicated malaria is artemisinin combination therapy (ACT) e.g. oral artemether/lumefantrine (Riamet; see BNF for complicated dosing schedule)

Severe cases of P falciparum should be treated with IV artesunate (discuss with your nearest ID unit if not available on site; in dire straits IV quinine may be used instead)

In cases of P vivax/ovale, primaquine must be added to eliminate hypnozoites from the liver to prevent relapse
Check G6PD status (or add onto your FBC sample) before commencing primaquine

Malaria Prevention

People travelling to malaria-endemic areas are advised to have a travel health consultation before departure and to take malaria prophylaxis. The 3 main options are Malarone™️, doxycycline and mefloquine, all of which should be taken during travel and for a period after leaving the endemic area.

Prevention of mosquito bites with DEET-based repellent and mosquito nets is also important.

There is currently one malaria vaccine (RTS,S) which is used for children in endemic areas, but there are no vaccines currently available for travellers. 

References

TravelHealthPro https://travelhealthpro.org.uk/factsheet/52/malaria

BMJ Best Practice https://bestpractice.bmj.com/topics/en-gb/161

UK national guidelines: https://www.journalofinfection.com/article/S0163-4453(16)00047-5/fulltext

Written by: Dr Hermaleigh Townsley – IMT2 Infectious Diseases
Reviewed by: Dr Hugh Adler ID/MM – SpR