Review article Vaccination as a Means of Control of Parasitic Diseases. Adeniyi DS Infectious Diseases Unit-APIN Centre Laboratory, Jos University Teaching Hospital, Jos, Plateau State, Nigeria. Correspondence : E-mail: daveaden@yahoo.com, Phone: +2348036261703 ________________________________________________________________________________________________ Abstract : Parasitic infections are responsible for the aetiology of many pathological conditions in both humans and animals globally. The human producti vity and animal production losses incurred as a result of these varied disease conditions are hard to adequately quantify; and this is despite the use of the traditional approaches at controlling and mitigating the effects of parasitic agents. The use of vaccine as a one-step approach in the control of parasitic infections is very economical and highly sustainable, and it’s also very efficient and highly viable. Many animal vaccines are already in use, and many more are still being researched. However, researches and clinical trials are still ongoing towards the successful development of a viable human parasite vaccine. Also, many emerging and novel approaches are being explored towards the successful development of both human and animal vaccines. This review identifies the use of vaccines as a one-step solution to the emerging inadequacies of the traditional methods of parasite control; and the potential possibilities provided through molecular advances in vaccine development. Keywords: Parasite vaccines, Chemotherapeutic agents, Zoonotic diseases, Genomics technology. ______________________________________________________________________________________ INTRODUCTION Parasitic infections are responsible for many acute and chronic disease conditions in humans and animals. These infections are associated with huge economic losses both in terms of animal production and in terms of human productivity. Controlling parasitic infections therefore requires efficient, economic, and sustainable control methods and approaches. The traditional integrated approaches of controlling parasitic infections which includes grazing management, biological control, strategic chemotherapy, and breeding for genetic resistance [1], does not seem cost effective and or efficient enough at controlling these parasitic infections; hence, the pressing need for the development of successful and efficient parasite vaccines. The reasons necessitating the urgency for the development of effective parasite vaccines are not far fetched as many of these pathogenic parasites are now beginning to developing resistance to the traditional chemotherapeutic agents used against them [2]. Also, since the rate at which these parasites are developing resistance is not in tandem with the rate at which new and improved chemotherapeutic agents against parasites are being developed. More so, the presence of drug residue in milk, milk products, and meat [3] which are parts of the unfavorable outcomes in the use of chemical agents for the control of parasitic infections, have greatly necessitated the need for the development and use of vaccines as the most preferable one-step option to interrupting parasitic infections. PARASITE VACCINES The universal use of anti-parasitic drugs for the control of parasitic diseases have been very effective for many years; however, recent developments have shown that some of these parasites are beginning to developing resistance against these commonly used chemical agents[2]. This is aside the deleterious effects of these chemical agents on humans and animals, the environment, human and animal food chain respectively. These challenges have now shifted our focus at finding better alternatives which vaccines and vaccination provides. The following therefore, are the currently and possible prospective vaccines that can and may be used against pathogenic parasitic agents. 1. Protozoan Vaccines: Protozoans are single celled organisms which live in wide varieties of wet and moist environments. Protozoan infections in animals are related to significant production losses and many of these animals are also responsible for zoonotic diseases due to their very close relationship to humans; hence, their significance as reservoirs for diseases in humans cannot be overemphasized. Researches into the production of human protozoan vaccines are still underway [4]; thus vaccines for human protozoon are not yet available, but a number of veterinary protozoan vaccines are being formulated and are available [5]. It should however be noted that the antigenic diversity of haemo-protozoon is part of what has made the development of vaccines against them very difficult [6]. Hence, the various types of animal protozoan vaccines available are as follows: (a) Live Protozoan Vaccine These vaccines are based on the use of live organisms that stimulate an immune response or reaction in the host; thus mimicking the natural infection. These types of vaccines have been employed in the control of avian Eimeria acervulina, Eimeria maxima, Eimeria mitis, Eimeria tenella, and Eimeria mivati infections. Also, pathogenic protozoans such as Theileria parva, Theileria annulata, Toxoplasma gondii, Babesia bovis, Babesia bigemina, and Anaplasma species have been effectively managed in animals with the aid of live vaccines [1]. (b) Killed and sub-Unit Protozoan Vaccines Several inactivated vaccines consisting of crude whole organisms or defined antigenic structures are used in this type of vaccine. In general, these vaccines are not as effective as live organism’s vaccines, but can go a long way in helping to mitigate diseases or transmissions to variable extents. These types of vaccines have been used in the management of Neospora caninum, Giardia intestinalis, Sarcocystis neurona, Tritrichomas foetus, Theileria parva, Theileria annulatum, and Leishmania infantum [7]. 2. Helminthes Vaccines: Helminthes are multicellular pathogens which infects a wide range of animal and human hosts. They cause adverse acute and chronic diseases which may result in morbidity and subsequent mortality if the condition is not promptly and adequately controlled. In the development of vaccines for these parasites, a thorough understanding of the immunology of the host-parasite relationship is most crucial and most essential to avoiding triggering an ineffective immune mechanism which may pose the great danger of severe and adverse immunopathologic responses [8]. There are three families of helminthes which are nematodes (round worms), cestodes (tape worms), and trematodes (flat worms). The various types of vaccines available for the control of these families of pathogenic parasites are as follows: Vaccines against Nematodes: The two main strategy for vaccine development against Gastro-intestinal (GI) nematodes are attenuated vaccines and hidden antigen approach [9]. (a) Attenuated Vaccines: These are the most effective vaccines against lung worms in animals. Most of these vaccines are irradiated larval vaccines which have been mostly successful in the control of such parasites as Dictyocaulus viviparous, Dictyocaulus filarial, and Ancylostoma caninum [10]. (b) Hidden Antigen Vaccines: Hidden antigens are integral membrane proteins that are associated with the gut of GI nematodes. Haemonchus contortus which has hidden gut antigens H11 and H-gal-GP, cysteine protease, and enolase[9] have been successfully managed using this type of vaccine. Vaccines against Cestodes: These vaccines are recombinant vaccines which are based on antigens of the parasite stage that adhere to the gut wall and hence, induce immune responses that interfere with successful attachment. These Onchosphere antigenic proteins have been successfully used to interfere with the attachment of some cestodes to the gut wall. These types of vaccine have been developed against parasitic cestodes such as Teania ovis, Taenia saginata, Taenia solium, and Echinococcus granulosus [11] [12]. Vaccines against Trematodes: Fasciola hepatica and Fasciola gigantica appears to be the two most widely spread parasitic trematodes of farm animals which are usually highly associated with severe economic losses [13]. Other trematodes of critical importance especially to humans are Schistosoma species and Paragonimus species. The successful development of viable vaccines against these parasites is very difficult because of the variable efficacy of these vaccines in different animal species [13]. Also, the ability of some of these parasites to mimic the host’s defensive pathways has conferred on them the innocuous ability to manipulating and evading the host’s immune system [13]. All vaccines that have been so far developed against trematodes have less than 80% level of efficacy [13] ; which though encouraging, is still much below the desired target of 100% viability and efficacy. So far, only Leucine amino-peptidase (LAP) which is present in bacteria, plants, and animals – a novel target for drug and vaccine candidates in parasitic infections, have shown 89% protection; with a cocktail of LAP, cathepsin L proteinases CL-1 and CL-2 giving a 78% protection respectively[14]. These vaccines have been employed in the management of animal infections with Fasciola species. 3. Ectoparasitic Vaccines Ectoparasites live on or in the skin but not within the host’s body; therefore, developing vaccines against ectoparasitic vectors are the most challenging. This is so because these parasites spend their entire life cycle outside the internal environment of the host; and this is also coupled with the fact that they are large and complex multicellular organis ms. These obligate vectors which serve as intermediate hosts (IH) in the transmission of parasitic diseases sometimes produce toxins that trigger severe host’s immunological and hypersensitivity reactions. Some recombinant vaccines have been successfully developed against such ectoparasites like the cattle tick Rhipicephalus (Boophilus) microplus and Rhipicephalus (Boophilus) annulatus by using the recombinant Bm86 gut antigenic protein which is found at the surface of the gut wall of the tick Rhipicephalus microplus [15] [16]. 4. Advance Approaches in Vaccine Development against Parasitic Infections The complex nature of parasitic infections in both humans and animals, and the varied mechanisms of host-parasite relationships, coupled with the advanced mechanisms of evading the host defense systems, all requires that some novel approaches be employed in the development of vaccines to combating the erratic and virulent evolvement of these parasitic agents. The dynamic applications of DNA and genomics technology are now proving an effective tool in the development of vaccines against these rancid pathogenic agents. Molecular Biology and DNA Vaccines: Advances in genomics and molecular biology have given new insights into possible areas of development in the fields of chemotherapeutics and vaccine researches in parasitic diseases. The sequencing of the full genomics of some of the tropical pathogenic parasites such as Trypanosoma species, Leishmania species, Wuchereria bancrofti [17], etc; has opened up a host of protein antigen development. Hence, this will make for possible drug-receptor targets and possible vaccine-receptor targets. DNA vaccines involve the introduction of recombinant plasmids containing an antigenic protein from the parasite which is injected into the patient via an injection [17]. The main purpose of this is to have the plasmid picked up by the host’s immune cells, and then have antibodies produced by the cell mediated defense system; thus, creating an immune response in the host. This response is however dependent upon the rate of uptake of the recombinant plasmids by the immune cells. Edible Vaccines: This is a novel approach for the sustainable development of vaccines in the foreseeable future. Some plants which are consumed by humans and animals are capable of expressing vaccine genes [18] [19]. Plants are now being considered as a major alternative for the manufacturing of active parasite antigens [20]. Plants such as lettuce, potatoes, maize, rice, banana, tobacco, wheat, carrots, peanuts, soybeans, etc, are all potential candidates for this novel approach in vaccine production. Edible vaccines are much cheaper in terms of production and they can be easily stored for future use. The major challenges of edible vaccines however include the poor immunogenecity of some of the vaccines in the host and the genetic and batch to batch variations of some of the genetically vaccine-labeled crops. CONCLUSION As pathogenic parasites have successfully developed effective strategies to escaping the host’s defense mechanisms and their new found abilities to developing resistance to the available chemotherapeutic agents used for their control, the most economical, efficient, and sustainable future approach to the control of parasitic diseases is through the use of vaccines. The urgent need for the development of effective vaccines in the control of these parasitic agents cannot be overemphasized as the common traditional integrated methods of control of parasites now seems no longer adequate; hence, the need for more novel approaches favoring the use of one-step vaccine control methods which has the ability to conferring life-long immunity in both humans and animals. Various types of vaccines are available for the control of parasitic infections especially in animals; the challenge however abound in the development of vaccines against trematodes and the inability to find a protozoan vaccine for man till date – which is a major cause for concern. The future prospects of vaccine development for the control of parasitic infections and diseases for both humans and animals is however bright and hopeful. Some vaccines, especially protozoan vaccines which are meant for animal use have been registered and licensed [7] [21]. And it should be noted that the advances in molecular biology, genomics, genetic engineering, and recombinant DNA technologies have now thrown more light and thus paving new ways for the effective and successful development of economically viable and sustainable vaccines against parasitic agents [22]. Disclosure: The author declares that there is no conflict of interest regarding the publication of this paper. 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