Perspectives Aquatic Insects and Mycobacterium ulcerans: An Association Relevant to Buruli Ulcer Control? Manuel T. Silva*, Françoise Portaels, Jorge Pedrosa

ycobacterium ulcerans infection, Arthropods can be vectors of many the host immune response) [11–14]. which can cause Buruli ulcer, infectious agents. The hypothesis Pre-exposure of mice to these salivary M is the third most common that arthropods were involved in the antigens induces protective immunity human mycobacteriosis worldwide, transmission of M. leprae to humans was against pathogen transmission by after tuberculosis and leprosy. Buruli originally put forward at the end of the neutralizing the immunosuppressive ulcer occurs predominantly in 19th century [8]. This hypothesis was activity [11,12,14]. Furthermore, humid tropical areas of Asia, Latin intermittently considered and tested salivary molecules can adsorb to the America, and, mainly, Africa, where until the early 1990s, but it was never pathogen [13,15]), a binding that can the incidence has been increasing, consistently demonstrated. cause the microorganism to become surpassing tuberculosis and leprosy in The hypothesis that predatory aquatic an innocent bystander of the host’s some regions [1]. insects, including those in the families antisalivary immunity—again leading Buruli ulcer is a devastating, Naucoridae and Belostomatidae to protection against pathogen necrotizing, “skin-eating” disease of the (order Hemiptera) (Figure 1), were transmission [12]. poor, sometimes producing massive, transmitters of M. ulcerans from aquatic Since efforts to develop vaccines disfi guring ulcers, with a huge social niches to humans was advanced targeting antigens of arthropod- impact [1,2]. Furthermore, both in 1999 [9]. The hypothesis was borne pathogens have been largely Buruli ulcer and its pathogen have later reinforced by Marsollier and disappointing, targeting arthropod high scientifi c interest, with unique, colleagues on the basis that [10]: (1) salivary components provides a enigmatic, and controversial features the salivary glands of Naucoris cimicoides potential alternative mechanism to [1–4]. However, research on Buruli are colonised with M. ulcerans upon block the transmission of arthropod- ulcer has been limited, although interest feeding on grubs containing the borne diseases [11,12,14]. Indeed, has grown since 1998, when the World pathogen; (2) M. ulcerans-infected N. vaccines have been described that Health Organization established the cimicoides transmit the pathogen to protect mice against leishmaniasis by Global Buruli Ulcer Initiative, and in mice upon biting; and (3) N. cimicoides 2004 called for urgent action to control in Buruli ulcer–endemic areas can be targeting sand fl y salivary proteins [14]. the disease and to increase research. naturally colonised by M. ulcerans; this The above results indicate that There is no vaccine against Buruli ulcer colonisation may occur through feeding immunity to salivary antigens of vectors and treatment remains diffi cult [1,2,5]. on aquatic snails and fi sh, which take can be an epidemiological biomarker A detailed description of M. ulcerans up M. ulcerans from water, mud, and of insect biting and of the consequent infection, including its clinical aspects, aquatic plants [6,7]. is available at http:⁄⁄www.afi p.org/ These results have reawakened Funding: The authors received no specifi c funding for this article. Departments/infectious/bu/. previous interest in the association A new area of research is the between arthropods and human Competing Interests: The authors have declared association between arthropods and M. mycobacterioses, and have opened a that no competing interests exist. ulcerans. And now a new study in PLoS new area in Buruli ulcer research. Citation: Silva MT, Portaels F, Pedrosa J (2007) Medicine, by Marsollier and colleagues, Aquatic insects and Mycobacterium ulcerans: An takes our understanding of this Immunity to Vector Antigens association relevant to Buruli ulcer control? PLoS Med 4(2): e63. doi:10.1371/journal.pmed.0040063 association further. in Arthropod-Borne Diseases Examples of arthropod-borne diseases Copyright: © 2007 Silva et al. This is an open-access Arthropods and Mycobacteria article distributed under the terms of the Creative are leishmaniasis and Lyme disease, Commons Attribution License, which permits There is evidence that M. ulcerans is not transmitted by sand fl ies and by ticks, unrestricted use, distribution, and reproduction in transmitted person-to-person but is an any medium, provided the original author and source respectively. These haematophagous are credited. environmental pathogen transmitted vectors bite the host’s skin to take to humans from its aquatic niches a blood meal. The bite introduces Manuel T. Silva is at the Institute for Molecular and Cell Biology (IBMC), University of Porto, [6,7]. However, it is not clear how this the pathogen along with saliva that Porto, Portugal. Françoise Portaels is at the transmission occurs [6,7]. profoundly alters the skin by molecules Mycobacteriology Unit, Department of Microbiology, Institute of Tropical Medicine, Antwerp, Belgium. with antihemostatic activity (which Jorge Pedrosa is at the Life and Health Sciences enable the vector to take an effective Research Institute (ICVS), School of Health Sciences, The Perspectives section is for experts to discuss the meal) and immunosuppressive activity University of Minho, Braga, Portugal. clinical practice or public health implications of a published article that is freely available online. (which enhances the infectivity of the *To whom correspondence should be addressed. injected pathogen by counteracting E-mail: [email protected]

PLoS Medicine | www.plosmedicine.org 0229 February 2007 | Volume 4 | Issue 2 | e63 Limitations of the New Study There are three major limitations to the new study. The fi rst is that the study did not analyse whether the antibodies against insect salivary proteins in mice protected by prior biting by uninfected N. cimicoides are effectors of protection or only biomarkers of the protective status. Additional studies are, thus, necessary to clarify the roles of humoral and cell-mediated immunity in such protection. Second, studies by Marsollier and others have been centered on Naucoridae, but it is possible that Belostomatidae and other predatory aquatic insects are also transmitters of M. ulcerans [9,16], which could complicate the investigation by extending the list of relevant insect salivary proteins. Another source of complication could be the occurrence of extensive polymorphisms in the relevant salivary proteins; the extent of these polymorphisms should be studied using doi:10.1371/journal.pmed.0040063.g001 specimens of aquatic insects collected in Figure 1. Semiaquatic Hemiptera That Have Tested Positive for M. ulcerans different geographical areas. The top two images show Macrocoris sp. 1.0 cm in body length (family Naucoridae), and the Third, a general lack of knowledge bottom two images show Appasus sp., about 2.5 cm in body length (family Belostomatidae). The about the transmission of M. ulcerans ventral and dorsal views are in the left and right photographs, respectively. Photo from [2]. is pertinent to this study—the overall relevance and contribution of biting immunoprotection against arthropod- would bind to M. ulcerans during its by M. ulcerans-colonised aquatic borne diseases. stay in the insect’s salivary glands, and insects to the transmission of Buruli These observations prompted would coat the bacilli when they are ulcer is unknown. While sand fl ies Marsollier and colleagues to search transmitted by biting. and ticks are haematophagous, and for immune signatures that could be The new study did not analyse the biting is indispensable both for correlates of protection against M. mechanism of this protection, but one vector survival and for transmission ulcerans and to investigate the relevance possibility is that M. ulcerans transmitted of the disease [11], Naucoridae and of vector salivary antigen–based vaccine by N. cimicoides to a sensitised host Belostomatidae are carnivorous insects strategies for Buruli ulcer. would become the innocent bystander and only accidentally bite humans [7]. target of the immune response against Therefore, other forms of transmission The New Study the adsorbed salivary proteins, resulting of M. ulcerans to humans, including Marsollier and colleagues’ new study in host protection [12]. These results skin trauma, have also been considered in PLoS Medicine [16] extends the suggest that inhabitants of Buruli [1,5–7,19]. authors’ previous observations [10] ulcer–endemic areas could become and shows that repeated biting by M. naturally immunised if repeatedly Implications ulcerans-free N. cimicoides renders mice bitten by uninfected N. cimicoides, a If future work supports the notion that more resistant to the infection obtained mechanism for protection that has aquatic insects are important in the through biting by the insects carrying also been suggested for leishmaniasis transmission of the causative organism M. ulcerans. The researchers also found [12,14,17] and Lyme disease [18]. in Buruli ulcer, as some data suggest that subcutaneous immunization The observation that unaffected, [9,10,16], the results now published of mice with salivary extracts of M. possibly resistant people exposed in PLoS Medicine could have important ulcerans-free N. cimicoides protected to aquatic environments in Buruli public health implications. against infection following injection of ulcer–endemic areas have higher titers First, following the work with murine M. ulcerans, but only when the bacilli of antibodies reacting with Naucoridae leishmaniasis [14], attempting to were fi rst coated with salivary proteins. and Belostomatidae salivary proteins develop a Buruli ulcer vaccine that This suggests that the protection than do patients with Buruli ulcer in targets N. cimicoides salivary antigens conferred to mice by previous N. the same areas [16] is consistent with would be justifi able. (It must be kept cimicoides biting is associated with this hypothesis. This observation also in mind, however, that the progress antibodies (detected in the blood of suggests that biting of humans by with vaccines against arthropod-borne repeatedly bitten mice) reacting with N. cimicoides and immunity to those diseases based on salivary proteins proteins of N. cimicoides saliva that proteins occur in a natural setting. of vectors has been slow. Because

PLoS Medicine | www.plosmedicine.org 0230 February 2007 | Volume 4 | Issue 2 | e63 of scientifi c, technical, and safety 5. Sizaire V, Nackers F, Comte E, Portaels F possible to develop pan-arthropod vaccines? (2006) Mycobacterium ulcerans infection: Trends Parasitol 22: 367–370. problems [12,14], such vaccines still Control, diagnosis, and treatment. Lancet 13. Ramamoorthi N, Narasimhan S, Pal U, Bao F, remain unavailable for human use.) In Infect Dis 6: 288–296. Yang XF, et al. (2005) The Lyme disease agent addition, the presence of antibodies 6. Duker AA, Portaels F, Hale M (2006) Pathways exploits a tick protein to infect the mammalian of Mycobacterium ulcerans infection: A review. host. Nature 436: 573–577. against salivary antigens of aquatic Environ Int 32: 567–573. 14. Titus RG, Bishop JV, Mejia JS (2006) The insects may be an important biomarker 7. Merritt RW, Benbow ME, Small PLC (2005) immunomodulatory factors of arthropod of protective status against Buruli ulcer, Unraveling an emerging disease associated with saliva and the potential for these factors to disturbed aquatic environments: The case of serve as vaccine targets to prevent pathogen with epidemiological relevance in the Buruli ulcer. Front Ecol Environ 3: 323–331. transmission. Parasite Immunol 28: 131–141. study of populations at risk in endemic 8. Benchimol JL, Romero Sa M (2003) 15. Mejia JS, Moreno F, Muskus C, Vélez ID, Titus areas of the disease.
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