Marcelo U. Ferreira

Human toxocariasis: diagnosis, worldwide seroprevalences and clinical expression of the systemic and ocular forms

Abstract Although human toxocariasis ranks among the most common zoonotic infections worldwide, it remains relatively unknown to the public. The causal agents are the nematode parasites Toxocara canis and T. cati, whose definitive hosts are dogs and cats, respectively. When embryonated eggs are accidentally ingested by humans, larvae hatch in the small intestine, penetrate the intestinal wall and migrate, via the bloodstream, to the liver, lungs, muscles, eye and central nervous system. Although most human infections are asymptomatic, two well-defined clinical syndromes are classically recognised: visceral larva migrans (a systemic disease caused by larval migration through major organs) and ocular larva migrans (a disease limited to the eyes and optic nerves). Two less-severe syndromes have recently been described, one mainly in children (covert toxocariasis) and the other mainly in adults (common toxocariasis). Here, the current laboratory diagnosis, epidemiology and main clinical features of both the systemic and ocular forms of human toxocariasis are reviewed. New developments in serological diagnosis are described, the available seroprevalence data are analysed, and the results of relevant clinical studies that have been published over the last decade are explored, to provide an updated overview of this neglected but highly prevalent human infection.

Mosaic organization and heterogeneity in frequency of allelic recombination of the Plasmodium vivax merozoite surface protein-1 locus

The organization and allelic recombination of the merozoite surface protein-1 gene of Plasmodium vivax (PvMsp-1), the most widely prevalent human malaria parasite, were evaluated in complete nucleotide sequences of 40 isolates from various geographic areas. Alignment of 31 distinct alleles revealed the mosaic organization of PvMsp-1, consisting of seven interallele conserved blocks flanked by six variable blocks. The variable blocks showed extensive variation in repeats and nonrepeat unique sequences. Numerous recombination sites were distributed throughout PvMsp-1, in both conserved blocks and variable block unique sequences, and the distribution was not uniform. Heterozygosity of PvMsp-1 alleles was higher in Asia (0.953 ± 0.009) than in Brazil (0.813 ± 0.047). No identical alleles were shared between Asia and Brazil, whereas all but one variable block nonrepeat sequence found in Brazil occurred in Asia. These observations suggest that P. vivax populations in Asia are ancestral to Brazilian populations, and that PvMsp-1 has heterogeneity in frequency of allelic recombination events. Recurrent origins of new PvMsp-1 alleles by repeated recombination events were supported by a rapid decline in linkage disequilibrium between pairs of synonymous sites with increasing nucleotide distance, with little linkage disequilibrium at a distance of over 3 kb in a P. vivax population from Thailand, evidence for an effectively high recombination rate of the parasite. Meanwhile, highly reduced nucleotide diversity was noted in a region encoding the 19-kDa C-terminal epidermal growth factor-like domain of merozoite surface protein-1, a vaccine candidate.

Population structure and transmission dynamics of Plasmodium vivax in rural Amazonia.

Understanding the genetic structure of malaria parasites is essential to predict how fast some phenotypes of interest originate and spread in populations. In the present study, we used highly polymorphic microsatellite markers to analyze 74 Plasmodium vivax isolates, which we collected in cross-sectional and longitudinal surveys performed in an area of low malaria endemicity in Brazilian Amazonia, and to explore the transmission dynamics of genetically diverse haplotypes or strains. P. vivax populations are more diverse and more frequently comprise multiple-clone infections than do sympatric Plasmodium falciparum isolates, but these features paradoxically coexist with high levels of inbreeding, leading to significant multilocus linkage disequilibrium. Moreover, the high rates of microsatellite haplotype replacement that we found during 15 months of follow-up most likely do not result from strong diversifying selection. We conclude that the small-area genetic diversity in P. vivax populations under low-level transmission is not severely constrained by the low rates of effective meiotic recombination, with clear public health implications.

Extensive microsatellite diversity in the human malaria parasite Plasmodium vivax

The population structure of Plasmodium vivax remains elusive. The markers of choice for large-scale population genetic studies of eukaryotes, short tandem repeats known as microsatellites, have been recently reported to be less polymorphic in P. vivax. Here we investigate the microsatellite diversity and geographic structure in P. vivax, at both local and global levels, using 14 new markers consisting of tri- or tetranucleotide repeats. The local-level analysis, which involved 50 field isolates from Sri Lanka, revealed unexpectedly high diversity (average virtual heterozygosity [H(E)], 0.807) and significant multilocus linkage disequilibrium in this region of low malaria endemicity. Multiple-clone infections occurred in 60% of isolates sampled in 2005. The global-level analysis of field isolates or monkey-adapted strains identified 150 unique haplotypes among 164 parasites from four continents. Individual P. vivax isolates could not be unambiguously assigned to geographic populations. For example, we found relatively low divergence among parasites from Central America, Africa, Southeast Asia and Oceania, but substantial differentiation between parasites from the same continent (South Asia and Southeast Asia) or even from the same country (Brazil). Parasite relapses, which may extend the duration of P. vivax carriage in humans, are suggested to facilitate the spread of strains across continents, breaking down any pre-existing geographic structure.

Genome-wide SNP genotyping highlights the role of natural selection in Plasmodium falciparum population divergence

BackgroundThe malaria parasite Plasmodium falciparum exhibits abundant genetic diversity, and this diversity is key to its success as a pathogen. Previous efforts to study genetic diversity in P. falciparum have begun to elucidate the demographic history of the species, as well as patterns of population structure and patterns of linkage disequilibrium within its genome. Such studies will be greatly enhanced by new genomic tools and recent large-scale efforts to map genomic variation. To that end, we have developed a high throughput single nucleotide polymorphism (SNP) genotyping platform for P. falciparum.ResultsUsing an Affymetrix 3,000 SNP assay array, we found roughly half the assays (1,638) yielded high quality, 100% accurate genotyping calls for both major and minor SNP alleles. Genotype data from 76 global isolates confirm significant genetic differentiation among continental populations and varying levels of SNP diversity and linkage disequilibrium according to geographic location and local epidemiological factors. We further discovered that nonsynonymous and silent (synonymous or noncoding) SNPs differ with respect to within-population diversity, inter-population differentiation, and the degree to which allele frequencies are correlated between populations.ConclusionsThe distinct population profile of nonsynonymous variants indicates that natural selection has a significant influence on genomic diversity in P. falciparum, and that many of these changes may reflect functional variants deserving of follow-up study. Our analysis demonstrates the potential for new high-throughput genotyping technologies to enhance studies of population structure, natural selection, and ultimately enable genome-wide association studies in P. falciparum to find genes underlying key phenotypic traits.

Plasmodium falciparum Accompanied the Human Expansion out of Africa

Plasmodium falciparum is distributed throughout the tropics and is responsible for an estimated 230 million cases of malaria every year, with a further 1.4 billion people at risk of infection. Little is known about the genetic makeup of P. falciparum populations, despite variation in genetic diversity being a key factor in morbidity, mortality, and the success of malaria control initiatives. Here we analyze a worldwide sample of 519 P. falciparum isolates sequenced for two housekeeping genes (63 single nucleotide polymorphisms from around 5000 nucleotides per isolate). We observe a strong negative correlation between within-population genetic diversity and geographic distance from sub-Saharan Africa (R(2) = 0.95) over Africa, Asia, and Oceania. In contrast, regional variation in transmission intensity seems to have had a negligible impact on the distribution of genetic diversity. The striking geographic patterns of isolation by distance observed in P. falciparum mirror the ones previously documented in humans and point to a joint sub-Saharan African origin between the parasite and its host. Age estimates for the expansion of P. falciparum further support that anatomically modern humans were infected prior to their exit out of Africa and carried the parasite along during their colonization of the world.

Antigenic diversity and immune evasion by malaria parasites.

Malaria parasites are major human pathogens annually associated with 300 million to 500 million clinical cases worldwide and 0.5 million to 3 million deaths, mostly among children under the age of 5 years living in sub-Saharan Africa ([50][1]). Human malaria is caused by four species of parasitic

Sequence diversity and evolution of the malaria vaccine candidate merozoite surface protein-1 (MSP-1) of Plasmodium falciparum.

The merozoite surface protein-1 (MSP-1) of the malaria parasite Plasmodium falciparum is a major blood-stage antigen containing highly polymorphic tripeptide repeats in the domain known as block 2 and several non-repetitive domains that are essentially dimorphic. We have analyzed sequence variation in block 2 repeats and in non-repetitive block 17, as well as other polymorphisms within the MSP-1 gene, in clinical isolates of P. falciparum. Repeat haplotypes were defined as unique combinations of repeat motifs within block 2, whereas block 17 haplotypes were defined as unique combinations of single nucleotide replacements in this domain. A new block 17 haplotype, E-TNG-L, was found in one isolate from Vietnam. MSP-1 alleles, defined as unique combinations of haplotypes in blocks 2 and 17 and other polymorphisms within the molecule, were characterized in 60 isolates from hypoendemic Brazil and 37 isolates from mesoendemic Vietnam. Extensive diversity has been created in block 2 and elsewhere in the molecule, while maintaining significant linkage disequilibrium between polymorphisms across the non-telomeric MSP-1 locus separated by a map distance of more than 4 kb, suggesting that low meiotic recombination rates occur in both parasite populations. These results indicate a role for non-homologous recombination, such as strand-slippage mispairing during mitosis and gene conversion, in creating variation in a malarial antigen under strong diversifying selection.

High prevalence of Plasmodium malariae and Plasmodium ovale in malaria patients along the Thai-Myanmar border, as revealed by acridine orange staining and PCR-based diagnoses

The prevalence of the four human malaria parasites was investigated among malaria patients at northern, central and southern towns in Thailand along the border with Myanmar between September 1995 and May 1996. Thin smears obtained from 548 Thai and Burmese patients were reviewed by an acridine orange staining method, and many mixed infections with two to four species, including P. malariae and P. ovale, were detected. These diagnostic results were compared with those by two PCR‐based diagnoses, microtitre plate hybridization (MPH) and a nested PCR method, both of which targets the same, species‐specific regions in the 18S rRNA genes. In both PCR diagnoses, many P. malariae and P. ovale infections were also detected. Detection sensitivity of P. malariae infection was higher in nested PCR than MPH, and a total prevalence of P. malariae infection estimated by nested PCR reached 24.3% (133/548). In 16 of them, the size of PCR products amplified by the P. malariae‐specific primer was about 20‐bp shorter than the expected size of 115‐bp. Four of 16 possessed two different bands with normal and shorter sizes, suggesting that P. malariae isolates may be separated into two types, and that those with shorter products may be new variant form (s) with a nucleotide deletion in the target region. On the other hand, 21 P. ovale infections (3.8%) were detected by nested PCR, but four of them were MPH‐negative because of the sequence variation at the probe region. These results indicated that the prevalence of P. malariae and P. ovale along the Thai‐Myanmar border may be substantially higher than previously reported.

Tendência secular das parasitoses intestinais na infância na cidade de São Paulo (1984-1996)

OBJETIVO: Estimar a prevalencia e a distribuicao social das parasitoses intestinais na infância, estabelecer a tendencia secular dessas enfermidades e analisar sua determinacao, com base em dois inqueritos domiciliares, realizados na cidade de Sao Paulo, SP, em 1984/85 e 1995/96. METODOS: Os inqueritos estudaram amostras probabilisticas da populacao residente na cidade com idades entre zero e 59 meses (1.016 em 1984/85 e 1.280 em 1995/96). Amostras de fezes foram coletadas nos dois inqueritos e submetidas a exame parasitologico pela tecnica de sedimentacao, realizando-se leituras de preparacoes simples e de preparacoes coradas com lugol para exame de cistos de protozoarios. O estudo da distribuicao social das parasitoses levou em conta tercis da renda familiar per capita em cada um dos inqueritos. A estrategia analitica para estudar os determinantes da evolucao da prevalencia das parasitoses na populacao empregou modelos hierarquicos de causalidade, analises multivariadas de regressao e procedimentos analogos aos utilizados para calcular riscos atribuiveis populacionais. RESULTADOS/CONCLUSOES: Houve entre os inqueritos reducoes expressivas na prevalencia das parasitoses em geral (de 30,9% para 10,7%), das helmintoses (22,3% para 4,8%), da giardiase (14,5% para 5,5%) e do poliparasitismo intestinal (13,1% para 0,5%). Embora declinios intensos tenham sido observados em todos os estratos sociais, manteve-se inalterada no periodo a forte relacao inversa entre nivel de renda e ocorrencia de parasitismo. Mudancas positivas em determinantes distais (renda familiar e escolaridade materna) e intermediarios (moradia, saneamento do meio e acesso a servicos de saude) das helmintoses, justificaram parte substancial da reducao de sua prevalencia. A reducao da giardiase foi atribuida a melhorias na escolaridade materna e nas condicoes de moradia e saneamento. A duplicacao da frequencia a creches refreou o declinio da giardiase.OBJETIVO: Estimar a prevalencia e a distribuicao social das parasitoses intestinais na infância, estabelecer a tendencia secular dessas enfermidades e analisar sua determinacao, com base em dois inqueritos domiciliares, realizados na cidade de Sao Paulo, SP, em 1984/85 e 1995/96. METODOS: Os inqueritos estudaram amostras probabilisticas da populacao residente na cidade com idades entre zero e 59 meses (1.016 em 1984/85 e 1.280 em 1995/96). Amostras de fezes foram coletadas nos dois inqueritos e submetidas a exame parasitologico pela tecnica de sedimentacao, realizando-se leituras de preparacoes simples e de preparacoes coradas com lugol para exame de cistos de protozoarios. O estudo da distribuicao social das parasitoses levou em conta tercis da renda familiar per capita em cada um dos inqueritos. A estrategia analitica para estudar os determinantes da evolucao da prevalencia das parasitoses na populacao empregou modelos hierarquicos de causalidade, analises multivariadas de regressao e procedimentos analogos aos utilizados para calcular riscos atribuiveis populacionais. RESULTADOS/CONCLUSOES: Houve entre os inqueritos reducoes expressivas na prevalencia das parasitoses em geral (de 30,9% para 10,7%), das helmintoses (22,3% para 4,8%), da giardiase (14,5% para 5,5%) e do poliparasitismo intestinal (13,1% para 0,5%). Embora declinios intensos tenham sido observados em todos os estratos sociais, manteve-se inalterada no periodo a forte relacao inversa entre nivel de renda e ocorrencia de parasitismo. Mudancas positivas em determinantes distais (renda familiar e escolaridade materna) e intermediarios (moradia, saneamento do meio e acesso a servicos de saude) das helmintoses, justificaram parte substancial da reducao de sua prevalencia. A reducao da giardiase foi atribuida a melhorias na escolaridade materna e nas condicoes de moradia e saneamento. A duplicacao da frequencia a creches refreou o declinio da giardiase.