Clinically silent Plasmodium vivax infections in native Amazonians of northwestern Brazil: acquired immunity or low parasite virulence?

BACKGROUND: Malaria remains common among native Amazonians, challenging Brazil's elimination efforts. OBJECTIVES: We examined the epidemiology of malaria in riverine populations of the country's main hotspot - the upper Juruá Valley in Acre state, close to the Brazil-Peru border, where Plasmodium vivax accounts for > 80% of cases. METHODS: Participants (n = 262) from 10 villages along the Azul River were screened for malaria parasites by microscopy and genus-specific, cytochrome b (cytb) gene-based polymerase chain reaction. Positive samples were further tested with quantitative TaqMan assays targeting P. vivax- and P. falciparum-specific cytb domains. We used multiple logistic regression analysis to identify independent correlates of P. vivax infection. FINDINGS: Microscopy detected only one P. vivax and two P. falciparum infections. TaqMan assays detected 33 P. vivax infections (prevalence, 11.1%), 78.1% of which asymptomatic, with a median parasitaemia of 34/mL. Increasing age, male sex and use of insecticide-treated bed nets were significant predictors of elevated P. vivax malaria risk. Children and adults were similarly likely to remain asymptomatic once infected. MAIN CONCLUSIONS: Our findings are at odds with the hypothesis of age-related clinical immunity in native Amazonians. The low virulence of local parasites is suggested as an alternative explanation for subclinical infections in isolated populations.

Clinically silent Plasmodium vivax infections in native Amazonians of northwestern Brazil: acquired immunity or low parasite virulence?

BACKGROUND Malaria remains common among native Amazonians, challenging Brazil′s elimination efforts. OBJECTIVES We examined the epidemiology of malaria in riverine populations of the country′s main hotspot - the upper Juruá Valley in Acre state, close to the Brazil-Peru border, where Plasmodium vivax accounts for > 80% of cases. METHODS Participants (n = 262) from 10 villages along the Azul River were screened for malaria parasites by microscopy and genus-specific, cytochrome b (cytb) gene-based polymerase chain reaction. Positive samples were further tested with quantitative TaqMan assays targeting P. vivax- and P. falciparum-specific cytb domains. We used multiple logistic regression analysis to identify independent correlates of P. vivax infection. FINDINGS Microscopy detected only one P. vivax and two P. falciparum infections. TaqMan assays detected 33 P. vivax infections (prevalence, 11.1%), 78.1% of which asymptomatic, with a median parasitaemia of 34/mL. Increasing age, male sex and use of insecticide-treated bed nets were significant predictors of elevated P. vivax malaria risk. Children and adults were similarly likely to remain asymptomatic once infected. MAIN CONCLUSIONS Our findings are at odds with the hypothesis of age-related clinical immunity in native Amazonians. The low virulence of local parasites is suggested as an alternative explanation for subclinical infections in isolated populations.

Molecular and morphological characterization of Hepatozoon spp. in Brazilian snakes

The genus Hepatozoon represents one of six genera in the hemogregarine group. Some studies in snakes indicated effects in the host, from slight influences on fitness to severe effects on growth rate, reproduction and offspring survival rates. Diagnosis and identification are usually through blood smear analyses; but not all infected animals show parasitemia. Based on this, the present study established an adapted molecular protocol to identify Hepatozoon spp. to be used as a complementary test for routine diagnoses at the Clinical Analysis Laboratory at São Paulo Zoological Park Foundation. The study was conducted with 113 individuals. Microscopical analysis and molecular techniques were used to identify the parasite. Microscopic analyses showed 13.3% of the samples to be positive. The first pair of primers, targeting 18S rRNA gene, amplified parasite DNA in 6.3% of the samples. The second pair of primers, targeting Apicoplast fragment, were used only on samples that were identified microscopically as being positive, detecting the presence of parasite DNA in 93.3% of these. Phylogenetic analysis of the resulting sequences found five clusters for the 18S gene and five clusters for the Apicoplast fragment. Studies involving Hepatozoon spp. are still scarce and limited, mainly in snakes and the impacts of this parasite on the vertebrate host, so diagnostic studies are essential for wildlife conservation, especially in ex situ work.

Molecular and morphological characterization of Hepatozoon spp. in Brazilian snakes

The genus Hepatozoon represents one of six genera in the hemogregarine group. Some studies in snakes indicated effects in the host, from slight influences on fitness to severe effects on growth rate, reproduction and offspring survival rates. Diagnosis and identification are usually through blood smear analyses; but not all infected animals show parasitemia. Based on this, the present study established an adapted molecular protocol to identify Hepatozoon spp. to be used as a complementary test for routine diagnoses at the Clinical Analysis Laboratory at São Paulo Zoological Park Foundation. The study was conducted with 113 individuals. Microscopical analysis and molecular techniques were used to identify the parasite. Microscopic analyses showed 13.3% of the samples to be positive. The first pair of primers, targeting 18S rRNA gene, amplified parasite DNA in 6.3% of the samples. The second pair of primers, targeting Apicoplast fragment, were used only on samples that were identified microscopically as being positive, detecting the presence of parasite DNA in 93.3% of these. Phylogenetic analysis of the resulting sequences found five clusters for the 18S gene and five clusters for the Apicoplast fragment. Studies involving Hepatozoon spp. are still scarce and limited, mainly in snakes and the impacts of this parasite on the vertebrate host, so diagnostic studies are essential for wildlife conservation, especially in ex situ work.