Genetic complexity of Plasmodium vivax parasites in individual human infections analyzed with monoclonal antibodies against variant epitopes on a single parasite protein

Monoclonal antibodies against variant epitopes of a highly polymorphic protein (PV200) in schizonts of Plasmodium vivax have been used to analyze the variety of genetically distinct populations of parasites present in the peripheral blood of individual P. vivax infections in Sri Lanka. In 9 out of 10 isolates of freshly drawn P. vivax infected blood from different individuals, parasites of only 1 PV200 serotype was found within each individual infection, even though parasites were serotypically distinct between individuals. In 1 isolate parasite population, 3 distinct PV200 serotypes were identified. Thus, most P. vivax infections appeared to consist of a single genetically homogeneous population of parasites within the detection limits of the technique. The prevalence of P. vivax infections in an area of malaria transmission in southern Sri Lanka and the densities of oocysts in mosquitoes fed on P. vivax infected individuals indicated that parasite populations would be transmitted many times before encountering parasites of other origins, and that individual populations would tend to reduce to genetic homogeneity during transmission. These expectations are consistent with the high proportion of genetically homogeneous P. vivax isolates observed.

Cutaneous leishmaniasis in Sri Lanka. An imported disease linked to the Middle East and African employment boom

Cutaneous leishmaniasis acquired by two Sri Lankan nationals while they were employed in Iraq and Northern Nigeria respectively constitutes examples of an imported disease related to the 'Middle East and African employment boom'. In both cases the diagnoses were confirmed by demonstrating the parasites in smears from the lesions and in tissue sections, and by culturing the parasites in vitro. Since leishmaniasis, neither visceral nor cutaneous is prevalent in Sri Lanka the risks of 'introduced' diseases is discussed here in the context of these two cases.

Uninfected erythrocytes form "rosettes" around Plasmodium falciparum infected erythrocytes

The human malaria parasite, P. falciparum, exhibits cytoadherence properties whereby infected erythrocytes containing mature parasite stages bind to endothelial cells both in vivo and in vitro. Another property of cytoadherence, "rosetting," or the binding of uninfected erythrocytes around an infected erythrocyte, has been demonstrated with a simian malaria parasite P. fragile which is sequestered in vivo in its natural host, Macaca sinica. In the present study we demonstrate that rosetting occurs in P. falciparum. Rosetting in P. falciparum is abolished by protease treatment and reappears on further parasite growth indicating that, as in P. fragile, it is mediated by parasite induced molecules which are protein in nature. P. vivax and P. cynomolgi, which are not sequestered in the host, did not exhibit rosetting. Rosetting thus appears to be a specific property of cytoadherence in malaria parasites.

Plasmodium vivax: cloning and expression of a major blood-stage surface antigen

Plasmodium vivax is a highly prevalent malaria pathogen of man; the following report is the first to describe the cloning and expression of a major asexual erythrocytic stage antigen of this species. The screening of a genomic DNA expression library with a monoclonal antibody directed against a 200-kDa surface component (Pv200) of the more mature schizonts of P. vivax led to the selection of a recombinant bacterial clone which produced a fusion protein. Mouse and rabbit immune sera raised against the purified fusion protein recognized the 200-kDa parasite antigen on Western blots and reacted with the surface of segmenters by immunofluorescence. Sequencing of the 1.9-kb P. vivax DNA insert coding for this fusion protein revealed a 45-47% homology at the nucleotide level with the P. falciparum gene of a parasite surface antigen, Pf195, which has been shown to be a promising candidate for a malaria vaccine in primates and in man.

Plasmodium vivax: karyotype polymorphism of field isolates

Pulse-field gradient electrophoresis (PFG) has been applied to the karyotype analysis of Plasmodium vivax isolates obtained directly from infected patients in Sri Lanka. Detection of separated chromosomes was performed either by ethidium bromide staining of gels or by hybridization with a telomer specific probe. Each of the 15 different isolates examined exhibited a different chromosome migration pattern, indicating that a high level of polymorphism prevailed in wild populations of P. vivax. Chromosome size variation was further confirmed using a P. vivax chromosome-specific probe which also demonstrated that, in each isolate, the parasite population appeared to be homogeneous. These observations were made directly on parasites from infected blood, without the necessity for culture amplification, indicating that PFG can be used on a large scale for the epidemiological analysis of wild parasite populations.

Modulation of human malaria transmission by anti-gamete transmission blocking immunity

Natural Plasmodium vivax malaria infections in man evoke anti-gamete transmission blocking antibodies which influence the infectivity of malaria patients to the vector mosquito. In this study, entomological, immunological and parasitological data obtained through the monitoring of an epidemic of human vivax malaria in Sri Lanka were used in a mathematical simulation to assess the effect of naturally induced transmission blocking immunity on malaria transmission. A mathematical model to describe malaria transmission accounting for transmission blocking immunity was developed from the basic differential equations originally stated by R. Ross and the epidemic was simulated using the available data. An attempt was made to predict the monthly malaria incidence by means of the mathematical simulation, with and without accounting for transmission blocking immunity. A plausible mathematical solution of the epidemic could be obtained when transmission blocking immunity was accounted for, and it was not possible to obtain such a plausible solution in the absence of immunity. Thus, the postulated occurrence of transmission blocking immunity was essential to describe adequately this malaria epidemic, indicating that, at least in epidemic situations, naturally occurring transmission blocking immunity has a controlling influence on malaria incidence.

Boosting of transmission-blocking immunity during natural Plasmodium vivax infections in humans depends upon frequent reinfection

The infectivity to mosquitoes of 31 acute Plasmodium vivax patients was measured by permitting mosquitoes to feed directly on the patients. The infectivity of these patients correlated closely with titers of antibodies in their serum as measured by indirect immunofluorescence against air-dried female gametes of P. vivax. Infectivity by direct feeding was also closely parallel to the transmission-blocking activity of the sera of patients as measured by the suppression of infectivity of parasitized blood by autologous serum relative to normal (nonmalarial) human serum when fed to mosquitoes through a membrane. These results are consistent with serum antibodies in human P. vivax infections as major factors determining the infectivity of an infected individual to mosquitoes. It was further noted that individuals having a second attack of P. vivax within less than 4 months were considerably less infectious to mosquitoes than first-attack patients were. This "boosting" of transmission-blocking immunity was much less if longer intervals intervened between attacks. We discuss the immunological implications and possible epidemiological significance of this short-term boosting of transmission-blocking immunity by successive P. vivax infections.

Monoclonal and polyclonal antibodies both block and enhance transmission of human Plasmodium vivax malaria

Antibodies against gametes of the malarial parasite inhibit the development of the parasite in the mosquito and curtail the transmission of malaria. We now report that a monoclonal antibody against gametes of the human malaria pathogen Plasmodium vivax and antibodies induced during natural infections of P. vivax in humans which suppress infectivity of the parasites to the vector at high concentrations can, at lower concentrations, have the opposite effect and enhance the level of malaria infection in the mosquitoes. Infectivity enhancing effects of up to 12-fold were demonstrated when a transmission blocking monoclonal antibody and immune human sera were diluted, in some undiluted immune human sera, and in the sera of vivax malaria patients during convalescence after drug cure.

Immunoelectron microscopy of Schüffner's dots in Plasmodium vivax-infected human erythrocytes

Plasmodium vivax induces morphologic alterations in infected host erythrocytes that are visible by light microscopy in Romanovsky-stained blood smears as multiple brick-red dots. These morphologic changes, referred to as Schüffner's dots, are important in the identification of this species of malarial parasite and have been associated by electron microscopy with caveolavesicle complexes along the erythrocyte plasmalemma. We have produced a monoclonal antibody (MAb A 20) that identifies an antigen in Plasmodium vivax-infected erythrocytes that is associated with the caveola-vesicle complexes of the parasitized host cell. This monoclonal antibody reacts with air-dried P vivax-infected erythrocytes to produce a pattern by the indirect immunofluorescence test (IFT) that is evocative of Schüffner's dots. Immunoelectron microscopy of P vivax-infected human erythrocytes using MAb A 20 confirmed the location of this antigen within vesicles of caveola-vesicle complexes. On Western blots MAb A 20 recognized four polypeptides of 54, 64, 72, and 86 kd. MAb A 20 reacted by IFT with 90% of Sri Lankan isolates and with a Brazilian P vivax isolate, which indicates that the epitope identified by this monoclonal is conserved.

Rosetting: a new cytoadherence property of malaria-infected erythrocytes

Plasmodium fragile infection of the toque monkey is a natural host-parasite association in which parasite sequestration occurs as during P. falciparum infection of humans. We have studied parasite sequestration of P. fragile and demonstrated the existence of a new property of cytoadherence of infected erythrocytes, "rosetting," which is defined as the agglutination of uninfected erythrocytes around parasitized erythrocytes. Rosetting in vitro and sequestration in vivo appear simultaneously as the parasite matures. The spleen plays a role in modulating cytoadherence; both sequestration and rosetting, which occur with cloned parasites from spleen-intact animals, are markedly reduced in splenectomized animals infected with parasites derived from the same clone. Sequestration and rosetting can be reversed by immune serum. Protease treatment of infected blood abolishes rosetting; however, if treatment is performed at an early stage of schizogony, rosetting reappears if parasites are allowed to further develop in the absence of protease. These results indicate that with P. fragile in its natural primate host, rosetting and sequestration are related to the presence on the infected erythrocyte surface of a parasite-derived antigenic component, the expression of which is modulated by the spleen.

Plasmodium vivax malaria: parasite biology defines potential targets for vaccine development

Although progress in the development of an antimalarial vaccine has been mostly obtained through the study of P. falciparum, significant advances have recently been made in the study of P. vivax, the other major human malarial parasite. Antigens which have been shown to be important in P. falciparum have been characterized and in some cases cloned in P. vivax. Other studies have examined some of the more specific biological characteristics of P. vivax. Among these are studies on components present in caveolae--vesicle complexes of the infected erythrocyte, on the occurrence of delayed hepatic development leading to relapse, or on the Duffy erythrocyte antigen as a key receptor for parasite invasion. Although progress has been made in the short-term in vitro maintenance of P. vivax, the inability to maintain the parasite in continuous culture led to the investigation of wild parasite populations in patients; occurrence of extensive antigenic and karyotype polymorphism was detected in this way, as was a double-blocking and enhancing activity of human antibodies on parasite development in the vector. The association of monoclonal antibodies with DNA recombinant technology allowed the characterization of a number of P. vivax antigens to be made. Among these, an antigen shared between sexual and asexual stages was shown to constitute a target for transmission-blocking immunity. The cloning of an antigen involved in transmission-blocking immunity, along with that of the surface antigen of the sporozoite (CSP) and of a major surface antigen of the invasive merozoite (PV200) constitutes a significant step towards the development of a multivalent recombinant vaccine against P. vivax.

Diversity of Plasmodium vivax-induced antigens on the surface of infected human erythrocytes

Antigens were demonstrated on the surface of Plasmodium vivax schizont-infected erythrocytes by an indirect immunofluorescence test using fresh unfixed infected erythrocytes from acute vivax malaria patients. Surface immunofluorescence was used to show that sera of P. vivax-infected individuals contain antibodies directed against these surface antigens. Thirteen different isolates were screened for reactivity of surface antigens with a panel of 8 heterologous human immune sera and an immune rabbit serum. Surface immunofluorescence was detected in several isolates with some but not all the human sera, and not all sera reacted with the "positive" isolates. These results indicate a high degree of polymorphism of the surface antigens of different P. vivax isolates. Sera from patients who had suffered multiple malaria attacks and the immune rabbit serum (which was raised by immunizing with 7 different isolates) recognized surface antigens on more isolates than sera from patients who had experienced only one attack of malaria, indicating that repeated exposure to the disease confers immunity against a spectrum of variants of a polymorphic malarial antigen(s) prevalent in nature.

Demonstration of antigenic polymorphism in Plasmodium vivax malaria with a panel of 30 monoclonal antibodies

A panel of 30 monoclonal antibodies was established against asexual erythrocytic stages of Plasmodium vivax and used to investigate the antigenic composition of the parasite. At least 38 different antigenic polypeptides of P. vivax were characterized by the Western blot technique. The possible location of these antigens, as well as their stage and species specificity, was determined on the basis of the staining patterns produced by these antibodies on air-dried parasites in the indirect immunofluorescence test. Immunofluorescence performed with 30 different monoclonal antibodies on 50 different isolates of P. vivax obtained from patients showed that a high level of antigenic polymorphism prevailed in P. vivax. Only six monoclonal antibodies reacted with epitopes that were represented in more than 80% of parasite isolates, and therefore, appeared to be relatively conserved among different isolates. The other 24 monoclonal antibodies reacted with only 20 to 70% of parasite isolates.

Antigenic variation of cloned Plasmodium fragile in its natural host Macaca sinica. Sequential appearance of successive variant antigenic types

The course of infection of Plasmodium fragile in its natural host, the toque monkey Macaca sinica, consists of a primary peak of parasitemia followed by several distinct, successive peaks of lower parasitemia. In the S+ host, the late intraerythrocytic asexual developmental stages of P. fragile induce the expression of antigens on the surface of infected erythrocytes, which could be detected using the technique of surface immunofluorescence. Immunofluorescence using unfixed erythrocytes in suspension has shown that antigens are recognized by immune serum on the surface of the erythrocytes infected with more mature stages of the parasite. These antigens undergo variation, each successive peak of parasitemia being characterized by a different variant antigenic type (VAT). The appearance of the successive VATs occurs in a sequential manner, following the same order in different sets of animals. This constitutes the first example of a sequential expression of antigens in a malaria parasite; it indicates that, in P. fragile, antigenic variation is not the result of random mutations selected by antibody. Parasite-induced antigens on the surface of infected erythrocytes could not be detected in the S- host. However, when nonexpressing parasites from the S- host were transferred by blood passage into a naive S+ animal, they began to express antigens on the surface of infected erythrocytes within two erythrocytic cycles. We have demonstrated that the ability of S- parasites to switch to a particular VAT when passaged into a S+ animal changes during the course of an infection in the S- animal, indicating that, although surface antigens are not expressed, the processes leading to antigenic variation occurs even in the S- host. Antibodies directed against these surface antigens inhibit the growth of intra-erythrocytic parasites. The growth inhibition effects of antibodies are also variant specific, indicating that these variant surface antigens are functionally important for parasite survival.

Malaria transmission-blocking immunity induced by natural infections of Plasmodium vivax in humans

Immunity to malarial infections in human populations is known to affect the development of the asexual blood stages of the parasites in the human host and to be capable of conferring significant protection against morbidity and mortality due to the disease. In this study we show that during acute infection with Plasmodium vivax malaria, one of the two main malarial pathogens of humans, most individuals also develop immunity that suppresses the infectivity of the sexual stages of the parasite to mosquitoes. The immunity is antibody mediated and is directed against the parasites in the mosquito midgut shortly after ingestion of blood by a mosquito. This immunity could be expected to have significant effects on the natural transmission of P. vivax malaria.

A metropolitan hospital in a non-endemic area provides a sampling pool for epidemiological studies on vivax malaria in Sri Lanka

An analysis of records of 494 malaria patients admitted to the General Hospital in Colombo (the capital of Sri Lanka where malaria transmission is not known to occur) from 1981 to 1984 is presented and compared with national malaria data from the entire country. The incidence of predominantly Plasmodium vivax malaria rose sharply over the 3 years; its species distribution and seasonal variation in patients in the General Hospital, Colombo (GHC) generally reflected the disease pattern in the country as a whole. The disease had spread from mainly the endemic dry zone to the non-endemic wet zone. Malaria patients at the GHC were mainly residents of Colombo who had acquired malaria during brief visits to endemic areas, and we have demonstrated how information from them can be used as a sampling method to obtain almost immediate epidemiological information from the whole country. Based on the histories of selected patients we deduced the incubation periods and possible relapse patterns of P. vivax infections in Sri Lanka. This study also provided an insight to the epidemiology of the disease in the city.

Plasmodium vivax: isolation of mature asexual stages and gametocytes from infected human blood by colloidal silica (Percoll) gradient centrifugation

The densities of human erythrocytes infected with P. vivax obtained from infected patients were determined by isopycnic centrifugation in continuous gradients of Percoll. The approximate densities of erythrocytes infected with rings were 1.086 to 1.1, trophozoites (amoeboid stages) 1.053 to 1.086, schizonts and gametocytes 1.053 to 1.056 and of the other cellular elements of blood, uninfected erythrocytes, 1.086 to 1.1, polymorphonuclear leucocytes 1.073 to 1.086 and mononuclear cells 1.062 to 1.073 g/ml. Based on these values, a one-step gradient of 47% Percoll was devised to separate erythrocytes infected with the more mature stages (trophozoites, schizonts and gametocytes) from uninfected erythrocytes. By this method it is possible to obtain parasitaemias of 88% to 98% from blood with starting parasitaemias of less than 0.7%. This method is therefore being routinely used for immunological, biochemical and molecular biological studies on P. vivax.

Anti-gamete antibodies block transmission of human vivax malaria to mosquitoes

Antibodies were raised in rabbits by immunizing against fresh unfixed or cryopreserved female gametes of the human malaria pathogen Plasmodium vivax. The antibodies were shown to react with the surface of gametes by the indirect immunofluorescent test. When parasite isolates from P. vivax infected individuals were fed through a membrane to Anopheles tessellatus mosquitoes in the presence of immune rabbit sera, they completely blocked the infectivity of the parasite isolates to the vector. Immunoglobulins separated from these sera also blocked infectivity to the same extent as did the immune sera indicating that antibodies were responsible for the transmission blocking effect of the sera. This study indicated that P. vivax like other malaria parasites is highly susceptible to anti gamete transmission blocking immunity.

Immunity to malarial antigens on the surface of Plasmodium falciparum-infected erythrocytes

An indirect immunofluorescence test with fresh non-fixed infected blood as antigen was used to show that antibody in human sera from the Gambia recognized antigens on the surface of Plasmodium falciparum-infected human erythrocytes. Surface immunofluorescence was detected on 90% of erythrocytes infected with trophozoites and schizonts produced in continuous culture of isolates from the Gambia (FCR 3/K+), Brazil and Thailand. Fluorescence was equally strong with a Gambian parasite clone (FCR 3/K-) that lacked knobs, an ultrastructural modification of the erythrocyte membrane associated with parasite sequestration. Immunofluorescence could not be detected with an isolate from Uganda. The surface antigenicity of parasitized erythrocytes was eliminated by chymotrypsin and trypsin treatment. Fluorescence was specific for the surface of trophozoite- and schizont-infected cells on the condition that fresh erythrocytes were added to cultures every 4-5 days (subculture); if fresh erythrocytes were not added for over 2 weeks, a large percentage of non-infected erythrocytes also bound antibody. Normal erythrocytes incubated with media from these cultures also gave positive surface immunofluorescence. Thus, there are two types of antigenicity on erythrocytes: one expressed on infected erythrocytes and another passively absorbed from media to normal erythrocytes when parasites are not subcultured for long periods.

Vaccination to prevent transmission of Plasmodium yoelii malaria

It was possible to block the transmission of infection of the rodent malaria parasite Plasmodium yoelii nigeriensis to Anopheles stephensi mosquitoes by immunizing mice with a vaccine containing formalin-fixed gametes. Both intramuscular and intravenous routes were effective, immunity was achieved with a single dose and the immunity persisted for 6 months at least. Transmission-blocking immunity was found to reside in a serum factor, probably antibody, and to be directed against extracellular gametes, acting on them in the gut of the mosquito, while gametocytes in the circulation of the vertebrate host remained unaffected. The gamete vaccine afforded partial protection against the disease, but immunization with asexual parasites alone showed that this protection was due largely to the presence of asexual forms as contaminants and that anti-gamete immunity is stage specific.

Congenital malaria due to Plasmodium vivax: a case report from Sri Lanka

A case of Plasmodium vivax malaria in an eight-week-old infant in Colombo is documented, with epidemiological and circumstantial evidence which strongly supports a transplacental route of infection. The malarial antibody levels detected by the indirect fluorescent antibody technique in both mother and child are discussed in terms of the present epidemiological pattern of malaria in the country. We also comment on the species incidence of congenital malaria, this case being the first caused by P. vivax in Sri Lanka, despite this species being more prevalent than P. falciparum which has been reported in six previous cases of congenital malaria in Sri Lanka.

Filarial infection of the breast

The breast is a common site of filarial infection in females in Sri Lanka. We report our experience with 13 cases of filarial breast nodules, 12 containing adult worms and the other only microfilariae. In five of these cases the species was identified as Wuchereria bancrofti.

Enterobius vermicularis in ectopic sites

We document six cases in which tissues were invaded by Enterobius vermicularis. These cases illustrate several mechanisms whereby the worms form granulomata in ectopic sites. In three cases, the worms passed through pre-existing breaches in the intestinal mucosa. In one case, a gravid worm migrated via the female genital tract to ther peritoneal cavity. In two other cases, male worms were found on the outer surface of the intestine, suggesting active penetration of the intestinal wall.