Plant Extracts: A Potential Tool for Controlling Animal Parasitic Nematodes

Many plants play a crucial role in maintaining animal and human life in a natural balance with a tendency to establish an environmental armory among the different biosphere inhabitants. During evolution of living organisms in the biosphere biological interactions with other organisms are established and they affect each other in many ways. Different types of relationships are involved among organisms including parasitism. Heritable strategies of biological adaptation are developed by living organisms to overcome adverse environmental conditions. Plants have developed biochemical mechanisms to defend themselves from biological antagonists that act as their natural enemies (Ryan and Jagendorft, 1995). This principle has led scientists to search for bio-active compounds produced by plants against pathogens (Sheludko, 2010). Since long a number of plants and their metabolites are evaluated against diseases of importance not only in public health (Shah et al., 1987); but also in animal and agricultural production (Githiori et al., 2006). In the present chapter, the importance of using plant extracts as an alternative method of control of animal parasitic nematodes is reviewed from a broad perspective.


Introduction
Many plants play a crucial role in maintaining animal and human life in a natural balance with a tendency to establish an environmental armory among the different biosphere inhabitants. During evolution of living organisms in the biosphere biological interactions with other organisms are established and they affect each other in many ways. Different types of relationships are involved among organisms including parasitism. Heritable strategies of biological adaptation are developed by living organisms to overcome adverse environmental conditions. Plants have developed biochemical mechanisms to defend themselves from biological antagonists that act as their natural enemies (Ryan and Jagendorft, 1995). This principle has led scientists to search for bio-active compounds produced by plants against pathogens (Sheludko, 2010). Since long a number of plants and their metabolites are evaluated against diseases of importance not only in public health (Shah et al., 1987); but also in animal and agricultural production (Githiori et al., 2006). In the present chapter, the importance of using plant extracts as an alternative method of control of animal parasitic nematodes is reviewed from a broad perspective.

Use of plants as a source of phyto-medicines
Ancestral cultures worldwide developed, over many centuries, several cures and remedies from plants and plant extracts against many diseases affecting human populations and a traditional medicinal system based on empiric knowledge was established and was improved through time (Hillier and Jewel, 1983). Some devastating infectious diseases ie., malaria, responsible for deaths of thousands of people can be overcome with traditional herbal anti-malarian drugs obtained from South America, Africa and Asia ie., Cinchona (Cinchona sp.), Qing hao (Artemisa annua), Changshan (Dichroa febrifuga), Neem (Azadirachta indica), Cryptolepsis sanguinolenta) and other plants (Willcox et al., 2005). Researchers around the world have scientifically explored the real effect of many plants used as medicines

Parasites of veterinary importance
Livestock industry worldwide is severely affected by a number of infectious diseases caused by different kinds of parasites. The present chapter focuses on the use of plant extracts against the group of internal parasites and particularly to helminths known as Gastrointestimal Parasitic Nematodes (GIN); considered to be one of the most economically important group of parasites affecting the animal productivity around the world ( ). In this group of parasites the nematodes have a remarkable status as the main pathogens causing severe damage to their hosts. Haemonchus contortus and other genera/species of nematodes belonging to the group of trichostrongylids are of major concern because its blood-sucking feeding habits cause anemia that can be so severe resulting in the death of the animals (Macedo Barragán et al., 2009). This group of parasites is widespread in almost all tropical and sub-tropical countries and is considered responsible for deteriorating animal health and productivity.

Chemotherapy as the unique method of control
The most common method used to control ruminant helminthiasis is the use of chemical compounds commercially available as anti-helmintic drugs that are regularly administered to animals for deworming; the method is considered simple, safe and cheap (Jackson, 2009). There are several disadvantages in the use of such products such as their adverse effect against beneficial microorganisms in soil once they are eliminated with the feces (Martínez and Cruz, 2009). On the other hand, some anthelmintic compounds can remain as contaminants in animal products destined for human consumption ie., meat, milk, etc. (FAO, 2002). One of the main concerns in the use of anthelmintic drugs for controlling www.intechopen.com Plant Extracts: A Potential Tool for Controlling Animal Parasitic Nematodes 121 ruminant parasites is the development of anthelmintic resistance in the parasites that decreases the efficacy of the drugs (Sutherland and Leathwick, 2011; Torres-Acosta et al., 2011) and threatens economical sustainability of sheep production (Sargison, 2011). The anthelmintic resistance can reach enormous proportions when parasites develop mutations in their genome against different groups of anthelmintic drugs. Such phenomenon is known as "Multiple anthelmintic resistance" and it is a real threat to the inefficacy of commercially available anthelmintics (Taylor et al., 2009;Saeed et al., 2010). Such situation has motivated workers around the world to look for alternatives to control these parasites. Searching for plant bio-active compounds with medical properties against parasites has gained great interest in order to at least partially replace the use of chemical drugs.  (Tables 1 and 2). Some forage have been evaluated searching for potential bio-active compounds against sheep and goat parasitic nematodes with variable results. However studies must be intensified; since some individual limitations in application have been noticed; ie., toxicity, metabolic disorders and inappropriate applications can cause severe damage and even the death of treated animals (Rahmann and Seip, 2008). Other plants are being investigated as bio-active forages in the control of Haemonchus contortus in lambs with good/moderate results. For instance Wormwood (Artemisia absinthium) which was offered to lambs for voluntary intake, parasitic burden was reduced almost in 50%. Additionally, faecal egg excretion expressed on a dry matter basis was also reduced by 73% in animals fed with the selected plant (Valderrábano et al., 2010). On the other hand, other plant/plant extracts ie., Melia azedarach (Chinaberrry tree, Indian Lilac) have shown promising results in trials that confirmed not only a very good anthelmintic efficiency, but also no side-effects (Akhtar and Riffat, 1984 (Lorimer et al., 1996); as well as bio-active enzymes such as cystein protease and secondary metabolites such as alkaloids, glycosides and tannins (Athanasiadou and Kvriazakis, 2004). Further in-depth studies need to be undertaken since even though anti-parasitic properties are being demonstrated, negative effects such as reduction in food intake by animals have been identified and this should be considered before establishing their use as an alternative method of control (Githiori et al., 2006).

Condensed tannin-rich plants
In recent studies, researchers are reaching beyond the general knowledge about lethal in vitro activity of plants and bio-active compounds derived from selected plants against the most important nematode parasites of ruminants. New efforts are being carried out to find practical applications of plants or plant products in the control of ruminant parasitic nematodes; including ways and means of overcoming limitations in applications to animals (Rahmann and Seipa, 2007). Recently in Laos, reduction in appearance of nematode eggs on goat feces with the Cassava foliage supplement has been demonstrated (Phengvichith and Preston, 2011).

Conclusions
The use of chemical anthelmintic drugs for controlling animal parasitic nematodes is rapidly loosing popularity due to a number of disadvantages. Anthelmintic resistance in the parasites is spreading and the inefficacy of chemical anti-parasitic compounds is threatening animal health. New plants with medicinal properties against parasites of ruminants are being investigated around the world with promising results. In the near future natural products obtained from plants extracts seems that likely will become a viable alternative of control of parasitizes of veterinary importance. When plant/plant extracts are being selected for use as anti-parasitic drugs in sheep particular attention should be given to the fact that the bio-active compound could be found in stems, roots, leaves, flowers, fruits or even in the entire plant. This means that obtaining plant extracts is a laborious and complex process. Also, the mode of extraction and the solvent used can determine the success in isolating the expected bioactive compounds; since a wide variety of compounds can be hidden into the structural parts of the plants and the only way they could be isolated is through exploring the use of a range of organic solvents. On the other hand, a rigorous effort to identify possible side effects due to the administration of plant extracts should be established before carrying in vivo assays. It is remarkablly important to consider that using plant/plant extracts as a unique method of control is insufficient to control itself the parasitosis in the animals. So, an alternated or combined method with other methods of control should be considered as an integrated method which would lead to reduce the use of chemical anthelmintic drugs.

Acknowledgments
Authors whish to express their gratitude to Dr. Felipe Torres Acosta (Autonomous University of Yucatan, Mexico) for his valuable comments on this chapter.