Anthelmintic resistance & control of Parasitic infections
Serious clinical diseases and welfare problems are corelated with parasitic worm infections which ultimately leads to loss of production in farm animals.
The threat of parasitic infection increases with increase in production in farm animals. Gastrointestinal parasitic infections are less prevalent in cattle as compared to other farm animals, however, the trend of infection increasing with time. On contrast, the goat industry, is at greater threat as compared to other farm animals.
Anthelmintic control of Parasites
A number of effective anthelmintic drugs have been manufactured since last four decades and made accessible to farmers to make sure the effective control of parasitic infection. Moreover, to meet the criteria of intensive farm production first, effective broad spectrum anthelmintic was discovered in 1960’s.
Depending upon chemical structure and mode of action, there are three main categories of anthelmintic used in domestic animals to treat and prevent gut worm infections:
- Benzimidazole (BZ) also known as white drench, disturb energy metabolism in parasites by binding to a soluble protein component named tubulin
- Levamisole (LM) also includes morantel, induces persistent muscle contractions, followed by depolarizing barricades causing paralysis.
- Macrocyclic lactones (ML), it includes doramectin, ivermectin and moxidectin, causes paralysis of pharyngeal muscles.
These anthelmintic are used extensively to get the goals of good production, however, the extensive use of these anthelmintic leads to threat of drug resistance. which is considered by some to be the biggest treat to livestock farming throughout the world, particularly the sheep sector. To this end, a survey was conducted in Scotland in 2003, 64% sheep flocks had evidence of resistance to BZ drugs.
However, LM and ML drugs resistance was not as much prevalent, except levamisole, as drug resistance was reported against levamisole in mid 1990s. No drug can be guaranteed to kill 100 percent worms, but most of the drugs marketed as very effective initially. However, progeny of parasites left behind have more chances to survive and are in greater proportion in next generation.
Basically, the parasites survive have the heterozygous (Bb) resistance genome or homozygous dominant resistance genes (BB), while, all homozygous recessive (bb) genomic parasites will be killed, which have large number in first generation. Whereas, the number of homozygous recessive (bb) organism will be decreased and resistant organism will be increased in progeny.
What is of greatest concern is the fact that multiple resistance (i.e. resistance to all three groups of drugs) can occur and when it does, it can render farm animal production unsustainable. This has already occurred in many parts of South Africa, Australia, North America and particularly South America.
Anthelmintics are/were effectively used for decades to control parasites successfully including nematodes, cestodes and trematodes infestation in farm animals. The effectiveness of these drugs dependent on the economical as compared to production losses and their usage is quite easy at farm level.
However, the resistance factor invites some tools/ideas to avoid the infestation of these parasites, which can match the criteria of effectiveness according to cast and efficacy by adopting the managemental practices.
This goal can be achieved by using some conventional methods such as varying forages or using some predacious micro-fungi. Moreover, the over drug residues in food is not satisfactory for human consumption. Following practices mostly used at farm level are discussed one by one.
Rotational grazing between different grazing specie can reduces the pasture contamination. The chances of disease can be reduced by breaking the life cycle, that hinders the availability of definitive host for infestation.
There are two main strategies to avoid the infestation in extensive grazing system: a ‘standard’ strategy comprising wethers maintained on the one pasture and given anthelmintic treatments in early and late summer, and a ‘new’ strategy in which intensive grazing for 1 month after each treatment and setting aside the grazing pasture for 1 or 2 months was integrated with the anthelmintic treatments.
The first line of defense in living system is the immunity of animals against any parasitic infection, by boosting immunity we can avoid the infection. Immunity, directly dependent on the immuno globolins, which are proteins in nature. The expression of immunity can be enhanced by provision of supplements with diets of animals at farm.
This boosted immunity results as hindrance in establishment of infested larvae as well as the established larvae survival will be reduced. Similarly, an experiment was conducted in 1998 by considering the urea supplementation in sheep and high red blood pack cell volume and high albumin was observed supplemented animals.
It is desirable to produce an effective vaccine against gastrointestinal parasites and There are many fundamental requirements for production of a good vaccine, which can be used at farm level. An ideal vaccine must be effective to numerous parasites at a time, cost effective and must not repeat in single season.
Mainly the approach for production of vaccines dependent on antigen that stimulates the host immunity, but in case of parasites this single antigen approach not so successful because no single antigen has been identified which mediates protective immunity.
An effective vaccine had been developed against Haemonchus contortus by using excretory/secretory products and had shown good results in sheep population. A number of vaccines had been proposed recently including DNA based vaccines. DNA based vaccines required the gene of that particular antigen, which is required for vaccine production.
Vaccines can be made more effective by adding vehicles as membrane-bound thiol Sepharosebinding fraction (TSBP) of adult H. contortus with enriched cysteine proteinase activity had led to significant decreased in fecal egg count and worm burden in sheep population.
Use of predacious micro-fungi
Microfungi can be used to control the parasitic infection, as these fungi revealed as predators of nematodes, trematodes and cestodes. In mycological investigations it was revealed that Duddingtonia flagrans had potential to restrict the growth of nematodes.
However, some initial trails had shown the effectiveness of this trail, while, this kind of biological control is still at its initial stages.
Authors: Arsalan Zafar, Muhammad Kasib Khan, Zaheer Abbas, Abdullah Khalid Chatha