The silent threat: asymptomatic parasitemia and malaria transmission

Lindblade, Kim A.; Steinhardt, Laura C.; Samuels, Aaron M.; Kachur, S. Patrick; Slutsker, Laurence

doi:10.1586/eri.13.45

Cited by 463 publications

(552 citation statements)

References 119 publications

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“…30 It is worth noting that an elimination strategy based on microscopic mass screening and treatment would miss the majority of infected individuals, confirming our previous observations in Pailin Province. 31 Similarly, at the village level, more than a third of villages which appear free of malaria based on microscopy alone actually harbor infection.…”

Section: 29supporting

confidence: 88%

National Malaria Prevalence in Cambodia: Microscopy Versus Polymerase Chain Reaction Estimates

Lek¹,

Popovici²,

Ariey³

et al. 2016

The American Society of Tropical Medicine and Hygiene

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Abstract. Accurate information regarding malaria prevalence at national level is required to design and assess malaria control/elimination efforts. Although many comparisons of microscopy and polymerase chain reaction (PCR)-based methods have been conducted, there is little published literature covering such comparisons in southeast Asia especially at the national level. Both microscopic examination and PCR detection were performed on blood films and dried blood spots samples collected from 8,067 individuals enrolled in a nationwide, stratified, multistage, cluster sampling malaria prevalence survey conducted in Cambodia in 2007. The overall malaria prevalence and prevalence rates of Plasmodium falciparum, Plasmodium vivax, and Plasmodium malariae infections estimated by microscopy (N ¼ 8,067) were 2.74% (95% confidence interval [CI]: 2.39-3.12%), 1.81% (95% CI: 1.53-2.13%), 1.14% (95% CI: 0.92-1.40%), and 0.01% (95% CI: 0.003-0.07%), respectively. The overall malaria prevalence based on PCR detection (N ¼ 7,718) was almost 2.5-fold higher (6.31%, 95% CI: 5.76-6.89%, P < 0.00001). This difference was significantly more pronounced for P. falciparum (4.40%, 95% CI: 3.95-4.90%, P < 0.00001) compared with P. vivax (1.89%, 95% CI: 1.60-2.22%, P < 0.001) and P. malariae infections (0.22%, 95% CI: 0.13-0.35%, P < 0.0001). The significant proportion of microscopy-negative but PCR-positive individuals (289/7,491, 3.85%) suggest microscopic examination frequently underestimated malaria infections and that active case detection based on microscopy may miss a significant reservoir of infection, especially in low-transmission settings. BACKGROUND

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Section: 29supporting

confidence: 88%

National Malaria Prevalence in Cambodia: Microscopy Versus Polymerase Chain Reaction Estimates

Lek¹,

Popovici²,

Ariey³

et al. 2016

The American Society of Tropical Medicine and Hygiene

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“…Conversely, more asymptomatic infections would be expected in areas of high malaria transmission where individuals can rapidly acquire immunity that partly restricts parasite growth. Such a correlation between high endemicity, and thus transmission intensity, and high asymptomatic prevalence has been reported in several African countries, ranging from 20% to 97% in areas of high transmission compared with less than 10% in areas of low transmission [2,42]. At a microscale, several studies also show a higher prevalence of asymptomatic carriers in villages of high compared with low malaria incidence [88].…”

Section: Transmission Intensity Submicroscopic Infections and Asympmentioning

confidence: 75%

“…This cyclical nature of the emergence of clinical symptoms is what led to the original nomenclature given to the fevers associated with malaria infections ('tertian' or 'quartan' fever) well before its life cycle was understood. However, asymptomatic malaria infections are common in humans living in endemic areas [2], even in low-transmission settings [72], with prevalences four to five times higher than clinically patent infections. Although asymptomatic cases generally correlate with lower-density infections [73] and lower rates of infecting mosquitoes [74], the extended longevity of asymptomatic parasitemia expands the risk of transmission during much of this time and overall it constitutes a high proportion of transmission [74], possibly greater than that derived from clinical malaria cases [75].…”

Section: Box 1 Asymptomatic Malariamentioning

confidence: 99%

Malaria Parasites in the Asymptomatic: Looking for the Hay in the Haystack

Galatas

Bassat

Mayor

2016

Trends in Parasitology

116

122

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With malaria elimination back on the international agenda, programs face the challenge of targeting all Plasmodium infections, not only symptomatic cases. As asymptomatic individuals are unlikely to seek treatment, they are missed by passive surveillance while remaining infectious to mosquitoes, thus acting as silent reservoirs of transmission. To estimate the risk of asymptomatic infections in various phases of malaria elimination, we need a deeper understanding of the underlying mechanisms favoring carriage over disease, which may involve both pathogen and host factors. Here we review our current knowledge on the determinants leading to Plasmodium falciparum symptomless infections. Understanding the host-pathogen interactions that are most likely to affect transitions between malaria disease states could guide the development of tools to tackle asymptomatic carriers in elimination settings. Being AsymptomaticPeaceful coexistence with the infected host, rarely causing clinical symptoms, is a common but poorly understood phenomenon that occurs for many human pathogens [1]. Because such asymptomatic cases of infection occur without eventual overt symptoms, they do not come to clinical attention, thus representing a large hidden reservoir of active infection that permits their persistence and eventual spread to other human hosts. This is the case for many malaria infections in semi-immune individuals from endemic areas, which commonly cause a mild febrile illness or no apparent symptoms at all, while keeping parasite numbers at low densities [2] (Box 1).Carriage of asymptomatic malaria (see Glossary) infections, being at the same time difficult to detect and to manage, has considerable implications for the design and use of malaria elimination strategies. Symptomless infections that persist during the dry season in areas of seasonal transmission have been suggested to seed the malaria outbreaks after annual rains when mosquitoes reappear [3]. Similarly, asymptomatic parasitemia may potentially contribute to persistence of transmission in low-transmission settings [4]. However, the precise contribution of asymptomatic malaria to transmission in areas that have achieved substantial reductions in the malaria burden remains a matter of debate [5], as is the assumption that detecting and targeting these individuals for radical treatment, as opposed to using mass drug administration approaches, would be an efficient and effective tactic. Moreover, eradication in Europe, North America, and other parts of the world using no better diagnostics than microscopy [6] suggest that key determinants of success might not be finding the last parasite. From a practical point of view, the proportion of asymptomatic infections in certain situations and phases of malaria elimination may impose the need for modifications of detection methods (active versus passive case detection) [7] if, for example, symptom-based surveillance at health facilities is insufficient and mass blood surveys are necessary to inform the design of elimination i...

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“…Asymptomatic patients far outnumber the symptomatic infections (Lindblade et al, 2013;Alves et al, 2002;Lin et al, 2014) and are likely not to come to hospital. These infections may last from weeks to months and to an average of 194 days (Males et al, 2008;Bottius et al, 1996;Felger et al, 2012).…”

mentioning

confidence: 99%

Malaria among relatives escorting sick patients during the dry season to Karume Health Centre, Mwanza, Northwestern Tanzania

Kamugisha¹,

Marwa²,

Lund³

et al. 2018

J. Public Health Epidemiol.

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Malaria is still a public health problem in the world. It accounted for an estimated 214 million cases and 438,000 deaths in the year 2015. During the dry season, most people are likely to be asymptomatic and therefore fail to be diagnosed with malaria. This study established the proportion of people who came to health facility, escorting sick relatives and had detectable malaria parasites. This was a cross sectional study. All relatives who escorted sick patients to Karume Health Centre between August and December 2013 were screened for malaria using malaria rapid diagnostic test (mRDT) and single round PCR targeting mitochondrial DNA. A total of 400 relatives were screened for malaria using two methods. Prevalence of malaria was 14.5 and 16.8% by mRDT and polymerase chain reaction (PCR), respectively. The prevalence of malaria was higher among febrile patients by methods, mRDT (17.8%) and PCR (17.1%), respectively. The prevalence of asymptomatic malaria was 16.4 and 16.5% by mRDT and PCR, respectively. The overall agreement between the two tests was 87.1% with positive agreement of 63.8% and negative agreement of 91.2%. There were a substantial proportion of patients with malaria who visited the health facilities during the dry season. mRDT and single round PCR targeting mitochondrial DNA had a good agreement and can be used for detection of both symptomatic and asymptomatic malaria. Provider initiated screening can help to improve malaria detection during the dry season, as we move towards reduced malaria prevalence and elimination phase.

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The silent threat: asymptomatic parasitemia and malaria transmission

Cited by 463 publications

References 119 publications

National Malaria Prevalence in Cambodia: Microscopy Versus Polymerase Chain Reaction Estimates

National Malaria Prevalence in Cambodia: Microscopy Versus Polymerase Chain Reaction Estimates

Malaria Parasites in the Asymptomatic: Looking for the Hay in the Haystack

Malaria among relatives escorting sick patients during the dry season to Karume Health Centre, Mwanza, Northwestern Tanzania

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