IS A VACCINE NEEDED FOR PREVENTING SCABIES INFECTION?

Vaccination is one of the greatest achievements against human infectious diseases. Deadly, or chronic infectious diseases that have significant impact on human society, such as those caused by polio virus or hepatitis B virus have been or are now being controlled after the introduction of vaccines (Nossal, Reference Nossal2003). However, comparatively, scabies only causes minor skin disease in most people (Walton et al. Reference Walton, McBroom, Mathews, Kemp and Currie1999, Reference Walton, Holt, Currie and Kemp2004, Reference Walton, Beroukas, Roberts-Thomson and Currie2008; Walton and Currie, Reference Walton and Currie2007; Mounsey et al. Reference Mounsey, Pasay, Arlian, Morgan, Holt, Currie, Walton and McCarthy2010; Walton, Reference Walton2010). Is there a need for a vaccine against this disease?

Scabies is endemic among people living in low-resource communities with overcrowded housing, such as Aboriginal people living in Northern Australia and those in developing countries (Walton et al. Reference Walton, Holt, Currie and Kemp2004). Scabies is also often associated with secondary infection by S. pyogenes, which is a major precipitant of acute post-streptococcal glomerulonephritis, and possibly rheumatic fever (McCarthy et al. Reference McCarthy, Kemp, Walton and Currie2004). Although community education, disease reporting and improved drug supply can improve the control of scabies, these measures can be difficult to implement (La Vincente et al. Reference La Vincente, Kearns, Connors, Cameron, Carapetis and Andrews2009). An effective vaccine has the potential to protect these groups of people more efficiently, with lower economic costs. Moreover, the intensive use of acaricides leads to the development of drug resistance in scabies mites. Clinical and in vitro resistance of S. scabiei to ivermectin has been reported in human crusted scabies (Currie et al. Reference Currie, Harumal, McKinnon and Walton2004) and also in dogs infested with S. scabiei var. canis (Terada et al. Reference Terada, Murayama, Ikemura, Morita and Nagata2010). Permethrin resistance has been observed in a rabbit model of S. scabiei var. canis (Pasay et al. Reference Pasay, Walton, Fischer, Holt and McCarthy2006). In the event of continued emergence of acaricide resistance in human scabies, vaccine or immunotherapy may provide another way to overcome this problem.

Crusted scabies, the severe form of scabies infection, responds poorly to treatment and the death rate of crusted scabies remains high (Walton et al. Reference Walton, McBroom, Mathews, Kemp and Currie1999, Reference Walton, Beroukas, Roberts-Thomson and Currie2008, Reference Walton, Pizzutto, Slender, Viberg, Holt, Hales, Kemp, Currie, Rolland and O'Hehir2010; Roberts et al. Reference Roberts, Huffam, Walton and Currie2005). Even in the event of treatment, crusted scabies patients are often highly susceptible to re-infection from community contacts, and recrudescence is common. Some patients have several episodes of crusted scabies per year, requiring quarantine and hospitalization, severely impacting on quality of life. Immune suppressed patients, including patients with acquired immunodeficiency syndrome (AIDS) and organ transplant patients, tend to develop atypical scabies manifestations and crusted scabies (Orkin, Reference Orkin1993b ; Taplin and Meinking, Reference Taplin and Meinking1997). Interestingly however, a large subset of patients with crusted scabies has no known risk factor or immune deficit, and the immune mechanisms underlying the development of crusted scabies remain poorly understood (Walton, Reference Walton2010; Walton et al. Reference Walton, Pizzutto, Slender, Viberg, Holt, Hales, Kemp, Currie, Rolland and O'Hehir2010). Subsequent bacterial infection can result in life-threatening conditions. As such, effective immunotherapy against early scabies infection would be very helpful to patients that tend to develop into crusted scabies.

Institutional outbreaks of scabies in childcare centres, aged care centres, prisons and hospital wards are not uncommon in developed countries. Current life expectancy in Australia reaches 80 years old for males and 84 for females. The likelihood of older people living in aged care centres will increase in future, and scabies is a common problem in aged care facilities (Paules et al. Reference Paules, Levisohn and Heffron1993; Makigami et al. Reference Makigami, Ohtaki, Ishii, Tamashiro, Yoshida and Yasumura2011, Reference Makigami, Ohtaki and Yasumura2012; Ariza et al. Reference Ariza, Walter, Worth, Brockmann, Weber and Feldmeier2012). In a survey conducted in Japan to investigate risk factors of scabies in psychiatric and long-term care hospitals, 44·9% had one or more scabies cases in a year, with both patients and medical professionals affected (Makigami et al. Reference Makigami, Ohtaki, Ishii, Tamashiro, Yoshida and Yasumura2011). Scabies tends to manifest atypically in older people, which complicates diagnosis, and some even progress to crusted scabies. Therefore, older people living in aged care centres and people working in the aged care centres would benefit from a vaccine against scabies infection, which could prevent problematic outbreaks in these settings. Similarly, more and more children are now attending childcare centres in developed countries (Churchill and Pickering, Reference Churchill and Pickering1997). As infants and young children commonly carry the highest burden of scabies within a community (Clucas et al. Reference Clucas, Carville, Connors, Currie, Carapetis and Andrews2008), outbreaks of scabies in childcare centres may cause significant problems to families, similar to those caused by head lice. Vaccination to children attending childcare centres or child carers may prevent the disease transmission among them.

In summary, an effective vaccine, either prophylactic or therapeutic, is needed for the control and treatment of this neglected skin disease in defined populations, and would benefit multiple groups in the community (Hengge et al. Reference Hengge, Currie, Jager, Lupi and Schwartz2006).

PROTECTIVE IMMUNE RESPONSES IN SCABIES

A vaccine against scabies has yet to be developed, and research in this area has been extremely limited. It is clear however that the development of a scabies vaccine is unlikely to be successful without an understanding of the protective arm of immune responses against S. scabiei infection and modulating mechanisms utilized by the parasite to survive in human skin.

Protective immune responses develop after scabies infection. In human scabies, the immune response is usually able to control mite numbers and in some cases clear the infestation. People with a previous infestation of ordinary scabies are generally infested with fewer mites and more rapid development of symptoms after re-infection (Mellanby, Reference Mellanby1977), suggesting that protective immune responses do develop after infestation with scabies mites (Walton et al. Reference Walton, McBroom, Mathews, Kemp and Currie1999, Reference Walton, Beroukas, Roberts-Thomson and Currie2008, Reference Walton, Pizzutto, Slender, Viberg, Holt, Hales, Kemp, Currie, Rolland and O'Hehir2010). In animal studies, dogs previously infested with scabies developed protective immunity to subsequent infestation (Arlian et al. Reference Arlian, Morgan, Rapp and Vyszenski-Moher1996). Goats challenged experimentally with mites were shown to be resistant to reinfestation (Tarigan and Huntley, Reference Tarigan and Huntley2005). Sheep following the secondary challenge, developed a smaller area of mange lesion than that seen following primary challenge and live S. scabiei mites were not detected in skin samples (Rodriguez-Cadenas et al. Reference Rodriguez-Cadenas, Carbajal-Gonzalez, Fregeneda-Grandes, Aller-Gancedo and Rojo-Vazquez2010a ). However, the exact immune protective mechanisms against scabies infection have not been demonstrated.

Humoral immune responses

Data from both human and animal studies demonstrate that antibody levels are increased after S. scabiei infection (Roberts et al. Reference Roberts, Huffam, Walton and Currie2005; Walton et al. Reference Walton, Beroukas, Roberts-Thomson and Currie2008). In humans, some reports suggest that there are no differences in scabies-specific IgM, IgG antibody responses between ordinary scabies patients and controls, but only a limited number of antigens have been tested (Walton et al. Reference Walton, Pizzutto, Slender, Viberg, Holt, Hales, Kemp, Currie, Rolland and O'Hehir2010).

Crusted scabies patients develop stronger antibody responses than those of ordinary scabies (Salo et al. Reference Salo, Reunala, Kalimo and Rantanen1982; Walton et al. Reference Walton, Beroukas, Roberts-Thomson and Currie2008). In one study, it was demonstrated that sera from all of the crusted scabies patients tested showed strong IgE binding to 11–21, and IgG binding to 1–7 scabies proteins. In contrast, only three of the seven patients with ordinary scabies showed IgE binding to 1–6 scabies proteins, and their antibody binding was much weaker, although the authors tested reactivity to a var. canis mite extract (Arlian et al. Reference Arlian, Morgan, Estes, Walton, Kemp and Currie2004). Serum IgA levels usually decrease in ordinary scabies compared with controls (Salo et al. Reference Salo, Reunala, Kalimo and Rantanen1982; Walton et al. Reference Walton, Pizzutto, Slender, Viberg, Holt, Hales, Kemp, Currie, Rolland and O'Hehir2010), while in crusted scabies, IgA levels were elevated in 64% of patients (Roberts et al. Reference Roberts, Huffam, Walton and Currie2005).

There are limited studies demonstrating whether antibody responses to scabies antigens are protective. In a goat model of scabies, vaccination with soluble proteins of S. scabiei invoked high levels of scabies-specific IgG in the serum of all animals but failed to induce specific IgE (Table 1). These immunized goats were not protected against mite challenge. In contrast, goats challenged experimentally with mites developed strong serum IgE and IgG antibody responses to scabies antigens and were shown to be resistant to reinfestation (Tarigan and Huntley, Reference Tarigan and Huntley2005). Recently, these observations have been confirmed in a similar study, where sheep were challenged with S. scabiei var. ovis. The challenged sheep developed both IgG and IgE responses to mites. Following the secondary challenge, sheep developed a smaller area of mange lesion than that seen following primary challenge and live S. scabiei mites were not detected in skin samples (Rodriguez-Cadenas et al. Reference Rodriguez-Cadenas, Carbajal-Gonzalez, Fregeneda-Grandes, Aller-Gancedo and Rojo-Vazquez2010a ). Interestingly, it has been shown that host antibody can be detected in mite gut (Rapp et al. Reference Rapp, Morgan and Arlian2006).

Table 1.

Vaccination studies against scabies

Taken together, the above results indicate that scabies infection evokes antibody responses to S. scabiei antigens. Sarcoptes scabiei-specific IgE may serve a marker for the development of protective immunity against the mite infection, at least in the above animal studies. However, high levels of IgE are characteristic of crusted scabies, where protective immunity does not develop (Walton, Reference Walton2010). Therefore, it is still not known whether antibody against selected scabies specific antigens, especially scabies-specific IgE, may have a protective role against scabies infection.

CURRENT STATUS OF SCABIES VACCINE DEVELOPMENT

TickGARD and GAVAC are marketed commercial vaccines against the cattle tick Boophilus microplus. These vaccines are based on a B. microplus derived glycoprotein Bm86 that is expressed on the surface of mid-gut cells, which is not exposed to the host immune system (Odongo et al. Reference Odongo, Kamau, Skilton, Mwaura, Nitsch, Musoke, Taracha, Daubenberger and Bishop2007; Parizi et al. Reference Parizi, Githaka, Logullo, Konnai, Masuda, Ohashi and da Silva Vaz2012a , Reference Parizi, Reck, Oldiges, Guizzo, Seixas, Logullo, de Oliveira, Termignoni, Martins and da Silva Vaz b ). Bm86 specific antibody binds to Bm86 when the tick sucks blood from immunized cattle, damages the digestive system of a tick and subsequently causes parasite death by antibody mediated killing. Since both ticks and scabies mites are ectoparasites, the success of tick vaccines is encouraging for the development of a vaccine against scabies. Although S. scabiei does not directly feed on blood, it feeds on serum that seeps into the burrow. Immunohistochemistry demonstrates that host immunoglobulins are present in the mite gut (Rapp et al. Reference Rapp, Morgan and Arlian2006; Willis et al. Reference Willis, Fischer, Walton, Currie and Kemp2006). Although there is little evidence that natural antibodies play a direct role in protective immunity to scabies, vaccine-induced cellular immune responses to scabies, or antibodies directed to selected antigens are more likely to provide the ultimate protective role.

As described above, in animal studies, vaccination with soluble proteins of S. scabiei invoked high levels of scabies-specific IgG in the serum of all animals but failed to induce specific IgE, and immunized animals were not protected (Tarigan and Huntley, Reference Tarigan and Huntley2005). Recently, it was demonstrated that hydrophobic, but not hydrophilic antigens from S. scabiei reacted with sera from infected pigs (Hejduk et al. Reference Hejduk, Hofstatter, Lowenstein, Peschke, Miller and Joachim2011), indicating these antigens are more immunogenic than soluble antigens that have previously been tested. Rabbits immunized with dust mite extract were resistant to infection by S. scabiei var. canis. Interestingly, resistant hosts exhibited increased cell infiltration, significantly lower scabies-specific immunoglobulin titres and produced antibody to fewer scabies mite antigens than did non-resistant hosts (Arlian et al. Reference Arlian, Rapp and Morgan1995). The same has been observed in naïve rabbits infested with S. scabiei var. canis with less severe clinical symptoms (‘resistant individuals’) (Arlian et al. Reference Arlian, Morgan, Vyszenski-Moher and Stemmer1994a , Reference Arlian, Rapp, Vyszenski-Moher and Morgan b ). In a recent report, sera from scabies-infected patients reacted with scabies mite recombinant proteins with sequence homology to house dust mite allergens, and sera from crusted scabies patients had stronger immune responses against these allergens (Walton et al. Reference Walton, Pizzutto, Slender, Viberg, Holt, Hales, Kemp, Currie, Rolland and O'Hehir2010). The above results suggest that the host can be protected from scabies infection through vaccination; cross reactivity to antigens from dust mite may protect the host from pathological scabies infection. Finally, protection is achieved through generating immune responses to unidentified antigen/antigens.

One of the key obstacles currently preventing advances in scabies vaccine development is the knowledge and selection of protective antigen or antigens. Both exposed and/or concealed antigens of scabies may provide protective responses, if these are important proteins for parasite survival or/and pathogenesis. The construction of comprehensive S. scabiei cDNA libraries has enabled the identification of several candidate antigens. Recombinant apolipoprotein (Harumal et al. Reference Harumal, Morgan, Walton, Holt, Rode, Arlian, Currie and Kemp2003), glutathione S transferases (Pettersson et al. Reference Pettersson, Ljunggren, Morrison and Mattsson2005), serine proteases (Holt et al. Reference Holt, Fischer, Allen, Wilson, Wilson, Slade, Currie, Walton and Kemp2003; Beckham et al. Reference Beckham, Boyd, Reynolds, Willis, Johnstone, Mika, Simerska, Wijeyewickrema, Smith, Kemp, Pike and Fischer2009), cysteine proteases (Walton et al. Reference Walton, Pizzutto, Slender, Viberg, Holt, Hales, Kemp, Currie, Rolland and O'Hehir2010) and serine protease inhibitors (Mika et al. Reference Mika, Reynolds, Mohlin, Willis, Swe, Pickering, Halilovic, Wijeyewickrema, Pike, Blom, Kemp and Fischer2012) have been expressed and purified in vitro, and their location in mites and human skin determined. However, immunogenicity of these antigens has yet to be characterized in detail. The scabies mite itself inhibits Th1 responses and mite proteins such as glutathione S transferases have immunomodulatory functions (Ouaissi et al. Reference Ouaissi, Ouaissi and Sereno2002; Lalli et al. Reference Lalli, Morgan and Arlian2004; Arlian et al. Reference Arlian, Morgan and Paul2006), Their influences on the efficacy of vaccine, especially therapeutic vaccine, has to be carefully considered.

In summary, vaccination using dust mite extracts protected immunized animals from mites challenge, although whether protection is mediated by a specific protective antibody, or by induced cellular immune responses, is not clear. Protective antigens are also not known. It is therefore necessary to investigate how the immune system controls scabies, and identify the antigen or antigens that elicit protective immune responses. Increased understanding of the immune-pathogenesis of crusted scabies will also lead to the development of specific immunotherapy for this group of patients.

FUTURE DIRECTIONS: GENERATING A ROBUST PROTECTIVE IMMUNE RESPONSE AGAINST SELECTED S. SCABIEI ANTIGEN/ANTIGENs AND LESSONS FROM OTHER PARASITIC DISEASES

Vaccination with cysteine proteinases has been shown to be effective against a number of parasites, and in some cases the efficacy of the vaccine is also associated with the inhibition of cysteine proteinase activity by IgG from the immune sera (Nisbet and Huntley, Reference Nisbet and Huntley2006). Glutathione S-transferases also have immunomodulatory functions and seem a promising vaccine candidate in human schistosomiasis and other parasite infections including scabies (Ouaissi et al. Reference Ouaissi, Ouaissi and Sereno2002). The genomes of schistosomes are now available and have helped researchers to identify new antigens for schistosomiasis vaccine development (Kupferschmidt, Reference Kupferschmidt2013). Hence, selection of appropriate antigens may not be possible until the entire genome of scabies mite is sequenced, although preliminary sequencing has commenced (Mounsey et al. Reference Mounsey, McCarthy and Walton2012). Other ‘next generation’ approaches, such as systematic and functional genomics, reverse vaccinology and selection of proper expression systems are likely required to identify the right scabies vaccine antigen candidates (Geldhof et al. Reference Geldhof, De Maere, Vercruysse and Claerebout2007; Maritz-Olivier et al. Reference Maritz-Olivier, van Zyl and Stutzer2012).

Pre-existing immune responses to selected antigen in endemic areas also need careful evaluation. Recently, a hookworm vaccine candidate, based on the protein of Ancylostoma Secreted Protein 2 of Necator americanus (Na-ASP-2), was suspended from phase I clinical trial, because of severe adverse events caused by pre-existing IgE antibody to the antigen before vaccination (Schneider et al. Reference Schneider, Jariwala, Periago, Gazzinelli, Bose, Hotez, Diemert and Bethony2011). Pre-existing immune responses to antigen incorporated in vaccine has also been shown to inhibit T cell responses, a phenomenon called ‘original antigenic sin’ (Liu et al. Reference Liu, Xu, Hardy, Khammanivong, Zhao, Fernando, Leggatt and Frazer2003, Reference Liu, Dyer, Leggatt, Fernando, Zhong, Thomas and Frazer2006). As scabies infection rates in endemic areas reaches 40–80% in some high-risk groups in Africa, indigenous communities in Australia and New Zealand, the influence of pre-existing circulating antibody is a critical consideration (Heukelbach and Feldmeier, Reference Heukelbach and Feldmeier2006).

Traditionally, a vaccine utilizes antigens plus an adjuvant to induce immune responses. Currently, the strategy has been extended to firstly optimize antigenicity of an antigen, and secondly to push the vaccine-induced immune response to higher quantity and quality by using a synergistic combination of cytokines, Toll-like receptor ligands and co-stimulatory molecules, and finally by removing the braking mechanisms mediated by regulatory T cells, NKT cells and suppressive molecules such as CTLA-4, PD-1 and TGF-β (Berzofsky, Reference Berzofsky2012). Previously, it was demonstrated that blockage of IL-10 signalling at the time of immunization generated robust Th1 and CD8+T cell responses (Liu et al. Reference Liu, Dyer, Leggatt, Fernando, Zhong, Thomas and Frazer2006, Reference Liu, Leerberg, MacDonald, Leggatt and Frazer2009; Chen et al. Reference Chen, Ni and Liu2011a ). IL-10 signalling blockage protects the host from infection with viral (Brooks et al. Reference Brooks, Trifilo, Edelmann, Teyton, McGavern and Oldstone2006), bacterial (Pitt et al. Reference Pitt, Stavropoulos, Redford, Beebe, Bancroft, Young and O'Garra2012) and parasitic infection (Fairfax et al. Reference Fairfax, Amiel, King, Freitas, Mohrs and Pearce2012), and has therapeutic effects against tumours (Berezhnoy et al. Reference Berezhnoy, Stewart, McNamara, Thiel, Giangrande, Trinchieri and Gilboa2012). It is likely that careful selection of proper antigens, together with administration of Toll-like receptor ligands and blockage of negative immune-signalling, may protect vaccinated people from scabies infection and may have therapeutic effects against crusted scabies, which is hard to manage using current modalities.

As scabies is generally not life threatening, vaccination compliance is a limiting factor if it is used to control scabies in a community. Nanotechnology provides some exciting ways in developing novel application-friendly vaccines. Recently, it was shown that oral administration of nanotechnology based vaccine, or needle-free skin vaccination, generated effective immune responses against cancer and viral infection (Chen et al. Reference Chen, Fernando, Crichton, Flaim, Yukiko, Fairmaid, Corbett, Primiero, Ansaldo, Frazer, Brown and Kendall2011b ; Zhu et al. Reference Zhu, Talton, Zhang, Cunningham, Wang, Waters, Kirk, Eppler, Klinman, Sui, Gagnon, Belyakov, Mumper and Berzofsky2012).

Taken together, a scabies vaccine would be useful for controlling infection in both developing and developed countries. Understanding the immuno-pathogenesis of scabies and host immunological protective mechanisms, and combining new concepts and technology in vaccination will promote the generation of an effective vaccine against scabies infection.