Seroconversion of sentinel chickens as a biomarker for monitoring exposure to visceral leishmaniasis

An overview of the sand fly salivary proteins in vaccine development against leishmaniases

Leishmaniases are a group of vector-borne parasitic diseases transmitted through the infected sand flies. Leishmania parasites are inoculated into the host skin along with sand fly saliva. The sand fly saliva consists of biologically active molecules with anticoagulant, anti-inflammatory, and immunomodulatory properties. Such properties help the parasite circumvent the host's immune responses. The salivary compounds support the survival and multiplication of the parasite and facilitate the disease progression. It is documented that frequent exposure to uninfected sand fly bites produces neutralizing antibodies against specific salivary proteins and further activates the cellular mechanisms to prevent the establishment of the disease. The immune responses due to sand fly saliva are highly specific and depend on the composition of the salivary molecules. Hence, thorough knowledge of these compounds in different sand fly species and information about their antigenicity are paramount to designing an effective vaccine. Herein, we review the composition of the sand fly saliva, immunomodulatory properties of some of its components, immune responses to its proteins, and potential vaccine candidates against leishmaniases.

Vaccination in Leishmaniasis: A Review Article

Leishmaniasis is caused by protozoan Leishmania parasites that are transmitted through female sandfly bites. The disease is predominantly endemic to the tropics and semi-tropics and has been reported in more than 98 countries. Due to the side effects of anti-Leishmania drugs and the emergence of drug-resistant isolates, there is currently no encouraging prospect of introducing an effective therapy for the disease. Hence, it seems that the key to disease control management is the introduction of an effective vaccine, particularly against its cutaneous form. Advances in understanding underlying immune mechanisms are feasibale using a variety of candidate antigens, including attenuated live parasites, crude antigens, pure or recombinant Leishmania proteins, Leishmania genes encoding protective proteins, as well as immune system activators from the saliva of parasite vectors. However, there is still no vaccine against different types of human leishmaniasis. In this study, we review the works conducted or being performed in this field.

Human antibody reaction against recombinant salivary proteins of Phlebotomus orientalis in Eastern Africa

Background

Phlebotomus orientalis is a vector of Leishmania donovani, the causative agent of life threatening visceral leishmaniasis spread in Eastern Africa. During blood-feeding, sand fly females salivate into the skin of the host. Sand fly saliva contains a large variety of proteins, some of which elicit specific antibody responses in the bitten hosts. To evaluate the exposure to sand fly bites in human populations from disease endemic areas, we tested the antibody reactions of volunteers' sera against recombinant P. orientalis salivary antigens.

Methodology/Principal findings

Recombinant proteins derived from sequence data on P. orientalis secreted salivary proteins, were produced using either bacterial (five proteins) or mammalian (four proteins) expression systems and tested as antigens applicable for detection of anti-P. orientalis IgG in human sera. Using these recombinant proteins, human sera from Sudan and Ethiopia, countries endemic for visceral leishmaniasis, were screened by ELISA and immunoblotting to identify the potential markers of exposure to P. orientalis bites. Two recombinant proteins; mAG5 and mYEL1, were identified as the most promising antigens showing high correlation coefficients as well as good specificity in comparison to the whole sand fly salivary gland homogenate. Combination of both proteins led to a further increase of correlation coefficients as well as both positive and negative predictive values of P. orientalis exposure.

Conclusions/Significance

This is the first report of screening human sera for anti-P. orientalis antibodies using recombinant salivary proteins. The recombinant salivary proteins mYEL1 and mAG5 proved to be valid antigens for screening human sera from both Sudan and Ethiopia for exposure to P. orientalis bites. The utilization of equal amounts of these two proteins significantly increased the capability to detect anti-P. orientalis antibody responses.

Hosts repeatedly bitten by phlebotomine sand flies develop species-specific antibody responses against certain sand fly salivary antigens. Salivary gland homogenate (SGH) is frequently used to evaluate the levels of this antibody response in host. However, SGH is less suitable for large-scale studies, since obtaining sufficient numbers of salivary glands is labor intensive and requires expertise in dissection. To replace SGH as antigen to screen for exposure to sand fly bites, specific recombinant salivary antigens were utilized. Our study assessed the human antibody reactions against recombinant salivary proteins of Phlebotomus orientalis. This sand fly species is a vector of Leishmania donovani, the causative agent of severe visceral leishmaniasis in Eastern Africa. To identify valid markers of exposure to P. orientalis in humans, we screened for anti-P. orientalis antibody responses in serum samples from individuals residing in Sudan and Ethiopia. We tested nine recombinant salivary antigens and found a combination of yellow-related protein (mYEL1) and antigen 5-related protein (mAG5) the best marker of exposure, accurately correlating with the levels of exposure to P. orientalis bites as determined using SGH. Thus the combination mYEL1+ mAG5 can comprise a useful epidemiological tool to determine levels of exposure to P. orientalis in populations living in endemic areas of Eastern Africa, which could help in monitoring the distribution of P. orientalis and therefore assessing suitable anti-vector campaigns.

Biomarkers for Zoonotic Visceral Leishmaniasis in Latin America

In Latin America, zoonotic visceral leishmaniasis (ZVL) arising from infection by L. infantum is primarily transmitted by Lutzomyia longipalpis sand flies. Dogs, which are chronic reservoirs of L. infantum, are considered a significant risk factor for acquisition of ZVL due to their close proximity to humans. In addition, as a vector-borne disease the intensity of exposure to vector sand flies can also enhance the risk of developing ZVL. Traditionally, IFN-γ and IL-10 are considered as the two main cytokines which determine the outcome of visceral leishmaniasis. However, more recently, the literature has demonstrated that different mediators, such as lipid mediators (PGE-2, PGF-2 alfa, LTB-4, resolvins) and other important inflammatory and anti-inflammatory cytokines are also involved in the pathogenicity of ZVL. Analysis of a greater number of mediators allows for a more complete view of disease immunopathogenesis. Additionally, our knowledge has expanded to encompass different biomarkers associated to disease severity and healing after specific treatments. These parameters can also be used to better define new potential targets for vaccines and chemotherapy for ZVL. Here, we will provide an overview of ZVL biomarkers identified for both humans and dogs and discuss their merits and shortcomings. We will also discuss biomarkers of vector exposure as an additional tool in our arsenal to combat ZVL.

Effectiveness of dog collars impregnated with 4% deltamethrin in controlling visceral leishmaniasis in Lutzomyia longipalpis (Diptera: Psychodidade: Phlebotominae) populations

BACKGROUND

There is little information on the effect of using deltamethrin-impregnated dog collars for the control of canine visceral leishmaniasis.

OBJECTIVES

The objective of this study was to evaluate the effectiveness of the use of 4% deltamethrin-impregnated collars (Scalibor®) in controlling visceral leishmaniasis in Lutzomyia longipalpis by comparing populations in intervention and non-intervention areas.

METHODS

Phlebotomine flies were captured over 30 months in four neighbourhoods with intense visceral leishmaniasis transmission in Fortaleza and Montes Claros. We calculated the rates of domicile infestation, relative abundance of Lu. longipalpis, and Lu. longipalpis distribution in each site, capture location (intra- and peridomestic locations) and area (intervention and non-intervention areas).

FINDINGS

In the control area in Fortaleza, the relative abundance of Lu. longipalpis was 415 specimens at each capture site, whereas in the intervention area it was 159.25; in Montes Claros, the relative abundance was 5,660 specimens per capture site in the control area, whereas in the intervention area it was 2,499.4. The use of dog collars was associated with a reduction in captured insects of 15% (p = 0.004) and 60% (p < 0.001) in Montes Claros and Fortaleza, respectively.

MAIN CONCLUSIONS

We observed a lower vector abundance in the intervention areas, suggesting an effect of the insecticide-impregnated collars.

Antibody response to sand fly saliva is a marker of transmission intensity but not disease progression in dogs naturally infected with Leishmania infantum

BACKGROUND

Antibody responses to sand fly saliva have been suggested to be a useful marker of exposure to sand fly bites and Leishmania infection and a potential tool to monitor the effectiveness of entomological interventions. Exposure to sand fly bites before infection has also been suggested to modulate the severity of the infection. Here, we test these hypotheses by quantifying the anti-saliva IgG response in a cohort study of dogs exposed to natural infection with Leishmania infantum in Brazil.

METHODS

IgG responses to crude salivary antigens of the sand fly Lutzomyia longipalpis were measured by ELISA in longitudinal serum samples from 47 previously unexposed sentinel dogs and 11 initially uninfected resident dogs for up to 2 years. Antibody responses were compared to the intensity of transmission, assessed by variation in the incidence of infection between seasons and between dogs. Antibody responses before patent infection were then compared with the severity of infection, assessed using tissue parasite loads and clinical symptoms.

RESULTS

Previously unexposed dogs acquired anti-saliva antibody responses within 2 months, and the rate of acquisition increased with the intensity of seasonal transmission. Over the following 2 years, antibody responses varied with seasonal transmission and sand fly numbers, declining rapidly in periods of low transmission. Antibody responses varied greatly between dogs and correlated with the intensity of transmission experienced by individual dogs, measured by the number of days in the field before patent infection. After infection, anti-saliva antibody responses were positively correlated with anti-parasite antibody responses. However, there was no evidence that the degree of exposure to sand fly bites before infection affected the severity of the infection.

CONCLUSIONS

Anti-saliva antibody responses are a marker of current transmission intensity in dogs exposed to natural infection with Leishmania infantum, but are not associated with the outcome of infection.

More than the eyes can see: The worrying scenario of canine leishmaniasis in the Brazilian side of the triple border

A cross-sectional epidemiological study in the extreme-west of the state of Paraná was carried out to access the prevalence, distribution, and risk variables of canine Visceral Leishmaniases (cVL). This study was conducted in three areas, two cities of far west of Parana state: Foz do Iguaçu (FI) and Santa Terezinha de Itaipu (STI), and along two transects between these two municipalities. To sample the entire urban area, the cities (FI and STI) were divided into a grid of squares of 400 m2 (patch). Among the 526 patches, 123 in FI, 40 in the transects and 33 in STI were selected according to the 'worst scenario' criterion. In the transect areas, in each 0.86 km five dogs from houses were surveyed to leishmaniasis. In each patch, blood of five dogs from houses (and from neighborhood when necessary) in the areas that seemed to be the most appropriate for the proliferation of vector were surveyed. The infection of the dogs by cVL were assessed using two serological tests were used (cELISA and TR-DPP®), and, for those seropositive for both methods, the PCR method were used. Moreover, dogs presenting clinical signs or cutaneous lesions were sampled to PCR. The identification of Leishmania species was confirmed using PCR-RFLP followed by DNA sequencing. Micro, meso and macro scale environmental variables were also surveyed and statistically analyzed. The prevalence rate Leishmania infantum was 23.8% in FI, 4.7% in STI and 9.1% in the transects areas. Among the extrinsic variables analysed, the number of vectors and the presence of infected dogs in neighbouring were positively correlated with the occurrence of infected dogs. Dog size was positively correlated with cVL infection, while the quality of the dog's nutrition affected cVL negatively. As for cutaneous leishmaniasis (CL), the first registry of dogs infected with L. braziliensis in the region shows that there is potential for transmission in peri-urban areas, since environmental conditions allow the proliferation of vectors capable of transmitting this species of parasite. cVL is widely spread in FI, with high prevalence. This supports the hypothesis that the parasite has been present in the region for longer than previously believed, despite the fact that the presence of leishmaniais in the region has only been recognized recently. It is important to control the population of dogs infected with L. infantum (parasite and non-antibodies) to prevent the spread of the disease to other dogs and also to people in the region.

Insights into the sand fly saliva: Blood-feeding and immune interactions between sand flies, hosts, and Leishmania

Background

Leishmaniases are parasitic diseases present worldwide that are transmitted to the vertebrate host by the bite of an infected sand fly during a blood feeding. Phlebotomine sand flies inoculate into the mammalian host Leishmania parasites embedded in promastigote secretory gel (PSG) with saliva, which is composed of a diverse group of molecules with pharmacological and immunomodulatory properties.

Methods and findings

In this review, we focus on 3 main aspects of sand fly salivary molecules: (1) structure and composition of salivary glands, including the properties of salivary molecules related to hemostasis and blood feeding, (2) immunomodulatory properties of salivary molecules and the diverse impacts of these molecules on leishmaniasis, ranging from disease exacerbation to vaccine development, and (3) use of salivary molecules for field applications, including monitoring host exposure to sand flies and the risk of Leishmania transmission. Studies showed interesting differences between salivary proteins of Phlebotomus and Lutzomyia species, however, no data were ever published on salivary proteins of Sergentomyia species.

Conclusions

In the last 15 years, numerous studies have characterized sand fly salivary proteins and, in parallel, have addressed the impact of such molecules on the biology of the host–sand fly–parasite interaction. The results obtained shall pave the way for the development of field-application tools that could contribute to the management of leishmaniasis in endemic areas.

Lutzomyia longipalpis urbanisation and control

Since the description of Lutzomyia longipalpis by Lutz and Neiva more than 100 years ago, much has been written in the scientific literature about this phlebotomine species. Soares and Turco (2003) and Lainson and Rangel (2005) have written extensive reviews focused on vector-host-parasite interactions and American visceral leishmaniasis ecology. However, during the last two decades, the success of Lu. longipalpis in colonising urban environments and its simultaneous geographical spreading have led to new theoretical and operational questions. Therefore, this review updates the general information about this species and notes the more challenging topics regarding the new scenario of urbanisation-spreading and its control in America. Here, we summarise the literature on these issues and the remaining unsolved questions, which pose recommendations for operational research.

Impact of Insect Salivary Proteins in Blood Feeding, Host Immunity, Disease, and in the Development of Biomarkers for Vector Exposure

Functional genomic approaches based on expression of recombinant proteins linked to biochemical and disease model approaches resulted in the discovery of novel biological activities and the role some of these proteins play in disease transmission. Importantly, the expression of salivary proteins was recently shown to be affected by environmental factors and by the presence of the pathogen in the salivary gland. A practical application resulting from insect saliva research is the use of insect antigenic salivary protein as biomarkers of vector exposure in humans and animal reservoirs, an approach that is yielding interesting results in the field.

Phlebotomine fauna, natural infection rate and feeding habits of Lutzomyia cruzi in Jaciara, state of Mato Grosso, Brazil

Visceral leishmaniasis (VL) in Brazil is transmitted by the phlebotomine Lutzomyia longipalpis and in some midwestern regions by Lutzomyia cruzi. Studies of the phlebotomine fauna, feeding habits and natural infection rate by Leishmania contribute to increased understanding of the epidemiological chain of leishmaniases and their vectorial capacity. Collections were performed in Jaciara, state of Mato Grosso from 2010-2013, during which time 2,011 phlebotomines (23 species) were captured (68.70% Lu. cruzi and 20.52% Lutzomyia whitmani). Lu. cruzi females were identified by observing the shapes of the cibarium (a portion of the mouthpart) and spermatheca, from which samples were obtained for polymerase chain reaction to determine the rates of natural infection. Engorged phlebotomines were assessed to identify the blood-meal host by ELISA. A moderate correlation was discovered between the number of Lu. cruzi and the temperature and the minimum rate of infection was 6.10%. Twenty-two females were reactive to the antisera of bird (28%), dog (3.30%) and skunk (1.60%). We conclude that Lu. cruzi and Lu. whitmani have adapted to the urban environment in this region and that Lu. cruzi is the most likely vector of VL in Jaciara. Moreover, maintenance of Leishmania in the environment is likely aided by the presence of birds and domestic and synanthropic animals.

What's behind a sand fly bite? The profound effect of sand fly saliva on host hemostasis, inflammation and immunity

Sand flies are blood-feeding insects and vectors of the Leishmania parasite. For many years, saliva of these insects has represented a gold mine for the discovery of molecules with anti-hemostatic and immuno-modulatory activities. Furthermore, proteins in sand fly saliva have been shown to be a potential vaccine against leishmaniasis and also markers of vector exposure. A bottleneck to progress in these areas of research has been the identification of molecules responsible for the observed activities and properties of saliva. Over the past decade, rapid advances in transcriptomics and proteomics resulted in the completion of a number of sialomes (salivary gland transcriptomes) and the expression of several recombinant salivary proteins from different species of sand fly vectors. This review will provide readers with a comprehensive update of recent advances in the characterization of these salivary molecules and their biological activities and offer insights pertaining to their protective effect against leishmaniasis and their potential as markers of vector exposure.

Sand-fly saliva-leishmania-man: the trigger trio

Leishmaniases are worldwide diseases transmitted to the vertebrate host by the bite of an infected sand-fly. Sand-fly biting and parasite inoculation are accompanied by the injection of salivary molecules, whose immunomodulatory properties are actively being studied. This mini review focuses on how the interactions between sand-fly saliva and the immune system may shape the outcome of infection, given its immunomodulatory properties, in experimental models and in the endemic area. Additionally, we approach the recent contributions regarding the identification of individual salivary components and how these are currently being considered as additional components of a vaccine against leishmaniasis.