Supplement

Canadian Recommendations for the Prevention and Treatment of Malaria Among International Travellers

7. Malaria Diagnosis

It is imperative that a travel history be obtained from all patients with a history of fever, and that thick and thin blood films for malaria be requested urgently for all individuals who have travelled to or through a malaria-endemic area. P. falciparum malaria usually presents within 3 months of last exposure; however, it may be delayed in patients who have taken chemoprophylaxis. In addition, other types of malaria, especially that caused by P. vivax, may occur months and occasionally up to 5 years after travel in endemic areas.

The treatment of malaria depends upon the species of parasite and the level of parasitemia; therefore, every effort should be made to determine these parameters on an urgent basis. Since malaria is a reportable disease in all provinces/territories, physicians are required to report all cases to the local public health authority.

Occasionally, a single blood film examination may be falsely negative for malaria parasites. Repeat blood films over 48 hours (e.g., every 12 hours x 3) may be required to exclude the possibility of malaria.

The examination of thick and thin blood films by an experienced microscopist is essential for the diagnosis of malaria. The clinical presentation (history and physical examination) of malaria is often non-specific. When malaria is a consideration, especially when the patient may be at risk of P. falciparum infection (whether chloroquine-sensitive or not), the laboratory diagnosis and quantification of the level of parasitemia must be considered a medical emergency and performed as soon as possible (< 24-hour turnaround time).

Not all laboratories are proficient in the diagnosis and speciation of malaria. If appropriate expertise cannot be ensured, then the patient should be treated empirically for chloroquine-resistant falciparum malaria and an immediate referral of the patient or the specimen should be made to a specialized facility. These facilities can be identified through the Canadian Malaria Network Centres, listed in Appendix VI.

While rapid diagnostic tests (RDTs) that use dipstick techniques for the diagnosis of malaria are currently being evaluated in the research setting, none is currently licensed for use in Canada. These RDTs are based on antigen detection of trophozoites and are targeted primarily at P. falciparum infections. Some tests differentiate between infections with other species or between falciparum and non-falciparum infection. They are simple to perform and do not require special equipment. They are rapid to interpret and require minimal training to operate. On the other hand, they may remain positive for up to 2 weeks after microscopic clearance, they are relatively expensive compared with microscopy, and they are not quantitative.

A WHO working group has reviewed the issues surrounding these test kits and identified further research required and possible scenarios for their use. One such scenario would be for self-treatment by travellers to remote areas. Research to date would suggest that this is not feasible, as interpretation by lay people is inaccurate (D II - evidence-based medicine, see Appendix II). There are no data available on self-diagnosis and self-treatment in the long-term traveller or expatriate population. Without training, there is no reason to believe that the efficacy of these interventions will be any better than that demonstrated in the general travel population. However, given that long-term travellers and expatriates represent a reasonably homogeneous group, training in diagnosis and self-treatment (see Section 6), including the use of rapid diagnostic tests for malaria, may prove to be helpful in this population when access to reliable, formal medical care is inadequate. Self-diagnostic kits that require refrigeration will limit access to this technology in some regions.

Polymerase chain reaction (PCR) techniques are also rapidly emerging as a definitive diagnostic tool and can demonstrate impressive sensitivity and specificity (B I - evidence-based medicine). They are, however, limited to laboratories that have the expertise and equipment to conduct these analyses and are still primarily research tools. They are useful as an adjunct to microscopy to confirm cases with low parasitemia and uncertain species. This is particularly useful in Canada, where the incidence of disease is quite low. The Canadian Malaria Network Centres, identified in Appendix VI, can direct clinicians to sites where this technology is available.

Recommendations

1. The diagnosis of malaria in a suspected case is a medical emergency and requires accurate laboratory testing within a maximum of 24 hours (A I - evidence-based medicine).
2. Microscopy of Giemsa stained thick and thin smears is the current gold standard for the laboratory diagnosis of malaria (A I - evidence-based medicine). Wright's stained thick and thin smears are used in some laboratories but may miss parasite details that assist in speciation.
3. PCR has a role in the confirmation of diagnosis but is not accessible widely in a timely fashion, as of yet.
4. RDTs are of limited utility in the Canadian setting and should not be used as a primary diagnostic tool (DI - evidence-based medicine).

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