Mice infected with the rodent malaria parasite Plasmodium berghei exhibit ultrastructural changes of the blood-brain barrier during the course of infection. Firm adherence including cellular interdigitation of infected cells or leucocytes and even clusters of cells to the vascular-endothelial lining is repeatedly observed early during infection. Ghosts and membrane remnants can be found engulfed in the surface of the endothelial cells. Frequently leucocytes migrate between endothelial cells and even cause a lift off and degeneration of these cells. In addition, endothelial cells exhibit increased pinocytotic activity, many irregular cytoplasmic extensions and even phagocytic activity. These changes are associated with degenerative changes in the basement membrane. Swelling and deposition of collagen-like fibres and even loss of fragments of basement membrane is observed. In some places fingerlike extensions of pericytes passed through the basement membrane and contacted or even bulged into the cytoplasm of endothelial cells. Ballooning and even coalescence of perivascular astrocytes was observed and contributed to the appearance of a perivascular oedematous space. The observed changes indicate a progressive deterioration of the blood-brain barrier eventually leading to endothelial lesions and hemorrhage.
Yaws (framboesia tropica), caused by Treponema pallidum subspecies pertenue, is a chronic infectious disease, found in tropical rural regions. As a result of mass treatment campaigns conducted in the 1950s and 1960s the prevalence of yaws and the other endemic treponematoses (pinta and endemic syphilis) has decreased greatly. However, in several tropical regions in the world resurgence of yaws was reported in the 1980s. In this article we present the results of a small survey on yaws in six health centres in the Pariaman region in West Sumatra, Indonesia. A general conclusion from this small survey is that yaws is far from being eradicated and that in this region the disease is on the increase again. Some major findings of T. pertenue research are presented here.
The present study was carried out between December 1986 and January 1988. A series of 3,048 individuals (46% males and 54% females) were examined in 56 villages; onchocercal dermatitis and lymphadenitis were seen. The crude prevalence of nodules and of microfilariae amounted to 14% and 43%, respectively. The prevalence of nodules was significantly higher at the femoral trochanter than at other anatomical sites. The overall prevalence of nodules, and of microfilariae, was significantly higher in males than in females. The age-adjusted prevalence and the geometric mean microfilarial load were significantly higher in the cohort of males older than 15 years than in their female counterparts. The overall geometric mean microfilarial load was 18 microfilariae per skin snip. The correlation between prevalence and intensity of infection was poor, but was perfect between mean microfilarial density per skin snip and mean microfilarial density per milligram skin. The ratio of mean microfilarial density per skin snip to mean microfilarial density per milligram skin was 1:6. The correlation between prevalence of onchocercomata and microfilarial prevalence or microfilarial density was weak. Both of the latter criteria identified the Rokel/Seli as the river with the highest proportion of hyperendemic villages. The implications of the present findings for the epidemiology of onchocerciasis are discussed.
A two-year longitudinal study carried out in five villages in a forest region of Gorama Chiefdom, Kono District, Sierra Leone, revealed that infection with Onchocerca volvulus was hyperendemic, the overall rate of infection being 61.6%. Prevalence rose from 28.6% in the 1-5-year age group, peaking among the 21-30-year age group (82.5%) and then levelling off. The intensity of infection though low, peaked in the 31-40-year-old males and in the 41-50-year-old female cohorts. The iliac crest was more sensitive for microfilarial (mf) recovery than the shoulder and the outer canthus. The majority of the nodules were located in the pelvic region. Microfilariae invasion of the eye was low, with low anterior chamber lesions but high posterior chamber lesions comprising mainly optic atrophy and choroidoretinitis. The rate of blindness was 1.6%, ocular onchocerciasis being the leading cause of blindness followed by cornea opacities.
Leptospirosis is caused by different leptospiral variants. Analysis by cross agglutination absorption tests (CAAT) led to the definition of entities called serovars to distinguish between leptospires on sub-species level, and to the designation of reference strains representing serovars. For decades CAAT has been used to classify leptospires and now approximately 200 serovars have been recognized. In the last few years, it has become increasingly more clear that the serovar concept is no longer fully satisfactory as it may fail to adequately define epidemiologically important entities. In addition, CAAT is too cumbersome and time-consuming for routine typing. Various methods have been developed based on antigenic or genetic analysis with the purpose to supplement or to replace the CAAT. Most of these methods are still in an experimental state. It is to be expected that a typing method based on genomic analysis will eventually become most important. Such a new method should have considerable advantages in order to be acceptable for the development of a new classification system replacing the system based on serovars, which is widely accepted and in many respects still satisfactory. From the new methods, analysis of leptospiral DNA fragment length after digestion with restriction enzymes (REA) has been widely used and proven to be useful for typing. Pending the development of new typing methods that have clear advantages and may lead to a new classification system, it is suggested that the classification system based on serovars is maintained and that REA is added to each description of a new serovar.
P. falciparum lacks a functional citric acid cycle. Unlike most tissues of the mammalian host, it is totally dependent on glycolysis for energy generation. A compound which selectively inhibits the parasite's ATP-generating machinery is therefore a potential antimalarial agent. Such a drug may interact in two ways: a) by inhibiting the activity of an enzyme or b) by disturbing the micro-organization of consecutive enzymes in a metabolic pathway. In mammalian tissues the glycolytic pathway involves the cytoskeleton as a matrix to keep phosphofructokinase, aldolase and glyceraldehyde-3-phosphate dehydrogenase in an optimal sterical position for rapid substrate conversion. For instance, these three enzymes bind to the band 3 protein in erythrocytes or to actin in muscle cells. P. falciparum aldolase binds with very high affinity to the band 3 protein of human erythrocyte ghosts. However, the true in vivo site of association is believed to be actin II of P. falciparum. This actin has a sequence element which is almost identical to that of the band 3 aldolase binding site. We therefore suppose that plasmodia exploit a similar matrix organization. If true, the association of these enzymes with the cytoskeleton is a target for novel antimalarials. In contrast to all vertebrate aldolases, P. falciparum and P. berghei aldolases have two neighbouring lysine residues near the carboxy-terminus. We show here that mutagenesis of these basic residues has an effect on the catalytic constants Vmax and KM and moreover, the ability to bind to band 3 is reduced.(ABSTRACT TRUNCATED AT 250 WORDS)
A 235 kD rhoptry protein produced by the malaria parasite, Plasmodium yoelii is the target of antibodies that protect mice against blood-stage challenge with the virulent YM strain. In the protected animals the parasites are confined to reticulocytes and the course of parasitaemia is reminiscent of an avirulent 17X strain infection. The DNA coding for the rhoptry protein has been identified as a multigene family containing at least four members. Sequence analysis of short DNA clones has identified the C-terminus of the protein; a preliminary analysis of longer clones confirms that the genes are polymorphic. The possible implications of these findings for the biology of the parasite are discussed.
It is clear from both laboratory and clinical studies that the blood-stage malaria parasite does not itself directly cause most of the serious complications of the disease, with the possible exception of anaemia. For example, T cell- deprived mice with lethal infections survive longer and mice can be protected against early death by vaccines that appear not to affect parasitaemia. In certain cases antibodies to TNF have the same effect. Clinically it has been known for over 50 years that children in endemic areas develop immunity to the serious toxic aspects of malaria several years before their parasitaemias start to fall. Recent work on the induction of cytokines such as tumour necrosis factor (TNF) by exoantigens of the blood-stage parasite and on the role of cytokines in this and other toxic diseases suggests that an appropriate vaccine might induce antibody that blocks the effect of the exoantigens, thus conferring on young children the anti-disease immunity that normally takes years to appear. Such vaccines might be less hampered by the antigenic variation that makes anti-parasite immunity slow to develop. Characterisation of the molecules involved is a high priority.
By definition, the biology of a living organism must be characterized before its molecular biology can be interpreted. Malariologists are fortunate in that the malaria parasite was used as a well-controlled therapy for tens of thousands of hospital patients. During many of these treatments the opportunity was taken to study malaria and the behaviour of the parasite in detail. From these, and similar studies on volunteers, together with numerous epidemiological surveys, the operational characteristics of immunity to malaria in man have been well defined. Unfortunately this information, which exists in some detail in the older literature, does not seem to have been available to many investigators. This situation has led to interpretations of molecular data which are inconsistent with the known biology of the parasites and human-parasite relationships. This article considers how the structure of one of the best studied antigens, MSP1, can be viewed in the context of the host-parasite relationship. It postulates some testable hypotheses which aim to reconcile the molecular characteristics of the antigen with the biology and immunology of the asexual erythrocytic stage of the parasite.
There are several mechanisms responsible for the extensive antigenic diversity found in the asexual blood stages of Plasmodium falciparum. Failure to express antigens is a feature of many isolates cultured in vitro but probably is not a major cause of antigenic diversity in vivo. Numerous point mutations occur in allelic forms of asexual blood stage antigens and are assumed to contribute to antigenic diversity but as yet few such mutations have been mapped to antigenic epitopes. A major cause of antigenic diversity is the expression of different repetitive sequences in allelic forms of several antigens including the S-antigen and the two merozoite surface antigens, MSA-1 and MSA-2. The sequencing data indicates that S-antigen genes fall into many allelic families whereas both MSA-1 and MSA-2 are dimorphic. Further diversity has arisen as a result of intragenic recombinations between the dimorphic forms of both MSA-1 and MSA-2. In addition to this diversity reflecting the expression of different allelic genes, asexual blood stages of malaria parasites undergo antigenic variation in that clonal parasite populations can vary the form of an antigen on the surface of infected erythrocytes. Antibodies or DNA probes directed against variable repeat sequences can be used to distinguish different isolates of P. falciparum. The use of antibodies to S-antigen repeats has been particularly useful for typing the parasites causing infections. The application of S-antigen typing to field studies in Papua New Guinea has demonstrated marked diversity in the parasites causing infections in one area.
Chloroquine resistance in Plasmodium falciparum bears a striking similarity to the multi-drug resistance (MDR) phenotype of mammalian tumour cells which is mediated by P-glycoprotein. P. falciparum has two mdr-like genes (pfmdr 1 and pfmdr 2) and pfmdr 1 has been linked to the chloroquine resistance phenotype. We show that pfmdr 1 encodes a protein of 160,000 Daltons that is expressed at higher levels in a chloroquine resistant cloned isolate. The pfmdr 2 gene is located on chromosome 14 and it is in equal copy number in chloroquine resistant and sensitive isolates. Therefore amplification of pfmdr 2 is not linked to chloroquine resistance. This is in contrast to the pfmdr 1 gene which has been shown to be amplified in some chloroquine resistant isolates.
Using minisatellite DNA probes and various Southern blots containing DNA samples from rodent malaria parasites it was shown that minisatellite-like sequences occur in the genome of these Plasmodium species. In contrast to the high copy number as observed in higher eukaryotes, the use of fingerprinting techniques on DNA from these parasites reveals that minisatellite sequences are only present at a small number of loci. When parasite lines which differ in biological parameters are compared, no frequent restriction fragment length polymorphism (RFLP) is observed. Screening data banks revealed the presence of repeated copies of one of the probes, which has the monomer sequence CAGGTGG, in the DNA encoding the immunodominant peptide repeat region of the circumsporozoite (CS) protein from a Plasmodium cynomolgi strain. Comparing the sequences for the CS protein from a number of strains of P. cynomolgi revealed that the core region of the repeats, though subject to limited variation, shares homology to the bacterial recombination signal sequence Chi (GCTGGTGG). The implications of the above findings for genetic variation in malaria parasites and evolution of minisatellite sequences will be discussed.
The rapid divergence of tandemly repeated families among Plasmodium species is in striking contrast with the extraordinary fidelity of the repeat copies present in a given genome. Close examination of the arrays of tandem repeats reveals an unsuspected regularity in the distribution of base substitutions, and clear relations between sets of repeats of different length, often coexisting in the same repeated region. The regular distribution of defects in the linear lattice gives rise to a hierarchy of periodicities. A model is proposed which accounts simultaneously for the rapid and concerted evolution of the repeats, and for the generation of supra-periodicities. The implications of this model with respect to repeated-pattern evolution are discussed.
