Malaria remains a major cause of morbidity and mortality in the tropics, with Plasmodium falciparum responsible for the majority of the disease burden and P. vivax being the geographically most widely distributed cause of malaria. Gametocytes are the sexual-stage parasites that infect Anopheles mosquitoes and mediate the onward transmission of the disease. Gametocytes are poorly studied despite this crucial role, but with a recent resurgence of interest in malaria elimination, the study of gametocytes is in vogue. This review highlights the current state of knowledge with regard to the development and longevity of P. falciparum and P. vivax gametocytes in the human host and the factors influencing their distribution within endemic populations. The evidence for immune responses, antimalarial drugs, and drug resistance influencing infectiousness to mosquitoes is reviewed. We discuss how the application of molecular techniques has led to the identification of submicroscopic gametocyte carriage and to a reassessment of the human infectious reservoir. These components are drawn together to show how control measures that aim to reduce malaria transmission, such as mass drug administration and a transmission-blocking vaccine, might better be deployed.

Epidemiology and Infectivity of Plasmodium falciparum and Plasmodium vivax Gametocytes in Relation to Malaria Control and Elimination

Background Plasmodium falciparum and Plasmodium vivax are responsible for most of the global burden of malaria. Although the accentuated pathogenicity of P. falciparum occurs because of sequestration of the mature erythrocytic forms in the microvasculature, this phenomenon has not yet been noted in P. vivax. The increasing number of severe manifestations of P. vivax infections, similar to those observed for severe falciparum malaria, suggests that key pathogenic mechanisms (eg, cytoadherence) might be shared by the 2 parasites. Methods Mature P. vivax-infected erythrocytes (Pv-iEs) were isolated from blood samples collected from 34 infected patients. Pv-iEs enriched on Percoll gradients were used in cytoadhesion assays with human lung endothelial cells, Saimiri brain endothelial cells, and placental cryosections. Results Pv-iEs were able to cytoadhere under static and flow conditions to cells expressing endothelial receptors known to mediate the cytoadhesion of P. falciparum. Although Pv-iE cytoadhesion levels were 10-fold lower than those observed for P. falciparum-infected erythrocytes, the strength of the interaction was similar. Cytoadhesion of Pv-iEs was in part mediated by VIR proteins, encoded by P. vivax variant genes (vir), given that specific antisera inhibited the Pv-iE-endothelial cell interaction. Conclusions These observations prompt a modification of the current paradigms of the pathogenesis of malaria and clear the way to investigate the pathophysiology of P. vivax infections.

On the Cytoadhesion of Plasmodium vivax–Infected Erythrocytes

The larvae of Megaselia scalaris (Diptera: Phoridae) exploit a broad spectrum of larval pabula. Consequently, the species features in a range of situations that affect the resources or well-being of humans. Furthermore, M. scalaris is readily cultured in the laboratory and is therefore favored as an experimental species for genetic, developmental, and bioassay studies. However, the extensive but widely scattered literature, reviewed herein, is frequently ignored by specialists, which gives rise to the unwitting repetition of previous work as well as insupportable proposals.

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Natural History of the Scuttle Fly, Megaselia scalaris

Molecular approaches are being used increasingly for epidemiological studies of visceral and cutaneous leishmaniases Several molecular markers resolving genetic differences between Leishmania parasites at species and strain levels have been developed to address key epidemiological and population genetic questions The current gold standard, multilocus enzyme typing (MLEE), needs cultured parasites and lacks discriminatory power PCR assays identifying species directly with clinical samples have proven useful in numerous field studies Multilocus sequence typing (MLST) is potentially the most powerful phylogenetic approach and will, most probably, replace MLEE in the future Multilocus microsatellite typing (MLMT) is able to discriminate below the zymodeme level and seems to be the best candidate for becoming the gold standard for distinction of strains Population genetic studies by MLMT revealed geographical and hierarchic population structure in L tropica, L major and the L donovani complex The existence of hybrids and gene flow between Leishmania populations suggests that sexual recombination is more frequent than previously thought However, typing and analytical tools need to be further improved Accessible databases should be created and sustained for integrating data obtained by different researchers This would allow for global analyses and help to avoid biases in analyses due to small sample sizes

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Molecular approaches for a better understanding of the epidemiology and population genetics of Leishmania

Dirofilaria repens is a nematode affecting domestic and wild canids, transmitted by several species of mosquitoes. It usually causes a non-pathogenic subcutaneous infection in dogs and is the principal agent of human dirofilariosis in the Old World. In the last decades, D. repens has increased in prevalence in areas where it has already been reported and its distribution range has expanded into new areas of Europe, representing a paradigmatic example of an emergent pathogen. Despite its emergence and zoonotic impact, D. repens has received less attention by scientists compared to Dirofilaria immitis. In this review we report the recent advances of D. repens infection in dogs and humans, and transmission by vectors, and discuss possible factors that influence the spread and increase of this zoonotic parasite in Europe. There is evidence that D. repens has spread faster than D. immitis from the endemic areas of southern Europe to northern Europe. Climate change affecting mosquito vectors and the facilitation of pet travel seem to have contributed to this expansion; however, in the authors’ opinion, the major factor is likely the rate of undiagnosed dogs continuing to perpetuate the life-cycle of D. repens. Many infected dogs remain undetected due to the subclinical nature of the disease, the lack of rapid and reliable diagnostic tools and the poor knowledge and still low awareness of D. repens in non-endemic areas. Improved diagnostic tools are warranted to bring D. repens diagnosis to the state of D. immitis diagnosis, as well as improved screening of imported dogs and promotion of preventative measures among veterinarians and dog owners. For vector-borne diseases involving pets, veterinarians play a significant role in prevention and should be more aware of their responsibility in reducing the impact of the zoonotic agents. In addition, they should enhance multisectorial collaboration with medical entomologists and the public health experts, under the concept and the actions of One Health-One Medicine.

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Recent advances on Dirofilaria repens in dogs and humans in Europe

Sri Lankan cutaneous leishmaniasis (CL), once considered sporadic, is fairly widespread in some parts of the country. Identification of 5 isolates from 4 CL patients by enzyme analysis during 2002 showed that they were all Leishmania donovani zymodeme MON-37, the parasite which also causes visceral leishmaniasis in India and East Africa.

Sri Lankan cutaneous leishmaniasis is caused by Leishmania donovani zymodeme MON-37

Background  and objectives Cutaneous myiasis (CM) due to Diptera fly larvae shows different patterns in different regions. Many modalities of treatment have been described. The objectives of our study were to identify the species causing CM in Sri Lanka, the common sites of infestation, and the contributory factors, and to assess some treatment modalities, in particular mineral turpentine and certain herbal preparations.

Methods  All patients with CM admitted or referred to the Dermatology Unit at the General Hospital, Kalutara, over 18 months starting from July 1997, and all patients with CM from the orthopedic and surgical wards of the National Hospital of Sri Lanka in Colombo over 6 months from July 1997, were studied. Details of the history and examination were recorded on specially designed forms. Maggots extracted were identified at the Department of Parasitology, Faculty of Medicine, University of Colombo. The modalities of treatment employed in the patients were recorded. In the Department of Parasitology, a colony of Chrysomya megacephala was maintained. Homogenized leaf extracts of Azadirachta indica (neem) and Pongamia pinnata (Indian beech) and mineral turpentine (active ingredient–-low aromatic white spirits) were tested for efficacy in killing C. megacephala larvae in vitro. Leaf extracts were not used directly on patients.

Results  There were 16 patients (10 males and five females; the sex of one patient was not recorded). The mean age was 58.5 years (range, 11–94 years). Identification of larvae revealed C. bezziana in 14 (87.5%) and C. megacephala in two (12.5%) patients. The foot was affected in 15 (93.7%) and the scalp in one patient. The immediate predisposing factor for CM in dermatology patients was infected dermatitis. The other relevant associated factors were: diabetes mellitus, psychiatric illness, leprosy, and mental subnormality. Turpentine was a useful adjunct in the removal of maggots manually. There were no side-effects to turpentine. In the in vitro testing, turpentine was 100% effective in killing maggots. Some patients required surgical removal under anesthesia. Indian beech and neem leaf extracts were not effective against Chrysomya larvae in vitro.

Conclusions  All cases of CM were due to larvae of Chrysomya species. The commonest was C. bezziana. C. megacephala larvae causing CM have been reported for the first time in Sri Lanka. The foot was the site of predilection. Dermatitis, psychiatric illness, leprosy, diabetes, and mental subnormality were some contributory factors. Topically instilled mineral turpentine, followed by manual removal of maggots, was effective in most cases. The plant extracts tested in vitro were ineffective. As C. bezziana is an obligatory parasite capable of penetrating deeply, the importance of preventive measures is emphasized.

A study of cutaneous myiasis in Sri Lanka.

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.

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

Cutaneous leishmaniasis is an established disease in Sri Lanka. The majority of cases have been reported from the North-Central Province, with the southern parts of the country considered less affected. However, during 2004, when the services of a dermatologist were available, a considerable number of patients were referred from the Southern Province, which formed the basis for this study. The clinical pattern, detailed geographical distribution within the Southern Province and periodicity of the cases were studied over a period of 12 months. Of the 113 patients diagnosed, the highest number was within the 10-19 years age group. Most patients were from densely populated rural areas around Matara, a large town within this province. There was a notable increase in the number of cases presenting during February-March and August-September, which are periods following monsoonal rains. Exposed areas of the skin were commonly affected, with the majority of patients having single lesions. Females and males were equally affected. This is in contrast to our previous findings in the North-Central Province where the majority of patients were male soldiers with multiple lesions.

R. L. Ihalamulla

Papers

Sri Lankan cutaneous leishmaniasis is caused by Leishmania donovani zymodeme MON-37

A study of cutaneous myiasis in Sri Lanka.

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

Cutaneous leishmaniasis in southern Sri Lanka

First report of mucosal tissue localisation of leishmaniasis in Sri Lanka.