Image: Malaria parasite in human blood (Photo courtesy of Shutterstock).
The use of saliva in the diagnosis of infectious diseases is an attractive alternative to procedures that involve blood drawing. It promises to reduce risks associated with accidental needle pricks and improve patient compliance particularly in malaria survey and drug efficacy studies.
Quantification of malaria parasitemia is useful in establishing severity of disease and in assessing individual patient response to treatment. In current practice, microscopy is the recommended technique, despite its limitations. The adoption of malaria rapid diagnostic test (MRDT) has greatly improved access to diagnosis.
A team of tropical medicine specialists at the Kenya Medical Research Institute Centre for Biotechnology (KEMRI, Nairobi, Kenya) screened patients aged between 2 and 65 years presenting with clinical symptoms of malaria using the MRDT SD Malaria Ag Pf/Pan. The infection status of patients found to be positive by RDT was confirmed by microscopy. A matched saliva sample was collected from patients confirmed to be Plasmodium falciparum malaria (mono-infection).
Saliva samples were collected using saliva collection aid and immediately stored at − 80 °C. P. falciparum lactate dehydrogenase (PfLDH) in saliva was detected and quantified using the Standard Diagnostics Inc commercial malaria antigen enzyme-linked immunosorbent assay (ELISA) kit. The difference in concentration of PfLDH in saliva and blood was determined using this commercial malaria antigen ELISA kit.
The team found 175 patient samples were positive for malaria by microscopy. Of these, 62 (35%) tested positive for PfLDH in saliva, 113 (65%) were false negatives. For those the 53 (85%) that tested positive, were from patients with moderate to high parasitemia while nine (15%) were from patients with low parasitemia. A correlation co-efficient of 0.18 indicated a weak positive relationship between the concentration of PfLDH in saliva and blood parasitemia. There was a marginal difference between levels of PfLDH in saliva of patients with moderate to high parasitemia and those with low parasitemia.
The authors concluded that there is a weak correlation between the levels of PfLDH in saliva and blood parasitemia in malaria-positive patients. For the assay to be useful, the sensitivity of the test has to be improved. In this study, the sensitivity was limited due to use of a commercial assay designed for plasma samples. Future work should be focused on the development of a customized saliva assay that would take into consideration the complex saliva matrix and mitigate the effects of proteases and mucins that may have contributed to low sensitivity. The study was published on January 5, 2018, in the Malaria Journal.
Kenya Medical Research Institute Centre for Biotechnology