Prevention and control of parasitism in livestock
- Part 3 -

Kalyan Sarma *

7. Use the only recommended dose

Anthelmintic resistance in gastrointestinal (GI) nematode populations is now widely recognised as a major problem in small ruminants and threatens the sustainability of many production systems throughout the world. Although there are several parasitological and management factors which may influence the rate of development of resistance, such as treatment frequency and under dosing, and suggested that the proportion of the parasite population in refugia could be the most important factor determining the rate of development of resistance and should be considered, above all else, throughout the development and implementation of any control strategies.

Several methods have been used to reduce drug resistance:

1. Dilution of resistant with susceptible parasites: Once anthelmintic resistance has been identified to a particular drug on a farm it may be possible to substitute the resistant parasite community for a susceptible one.

2. Targeted treatments: Targeted treatments can be defined as whole flock treatments given at the most appropriate times. The targeted treatment approach serves to reduce the numbers of anthelmintic treatments given to a flock and thus minimize pasture contamination with resistant genotypes. Recent examples of the use of targeted treatments include studies conducted in Wester Australia and Italy where treatments were stopped during summer months when few parasites were present on pasture and given instead at other times when more parasites were present in refugia.

3. Targeted selective treatments (TST): Targeted treatments can be further enhanced by selective treatment of only those animals that will most benefit from treatment, leaving the rest of the flock untreated.

Selective treatments should be directed towards those animals that are disease susceptible (non-resistant and/or non-resilient) or those that most contaminate pasture, however, this requires the ability to identify these individuals within a flock. Targeted selective treatments (TST) exploit both epidemiologically appropriate treatment times and treat only those animals that would benefit from treatment.

The main proof-of-concept studies for the use of TST were conducted in South Africa, where the FAMACHA method was devised and evaluated. FAMACHA assesses the colour of the conjunctival mucosal membrane, on a five-point scale, to estimate the degree of anaemia present as caused by the blood-feeding nematode, H. contortus. Anaemic animals, scoring 4 or 5 on the FAMACHA scale, are at risk of disease and are treated.

Botanical Dewormer

Use of plant origin anthelmintic are increasing day by day in modern medication. These included various extracts from jallop, quassia, areca nut, cloves, aloes, garlic, cucurbit seeds, castor oil, male fern and Chenopodium. In general, these were hazardous concoctions with low anthelmintic efficacy, especially in ruminant species, and they rapidly disappeared from veterinary use with the synthetic anthelmintic compounds.

Although there is a large and diverse range of herbal de-wormers used throughout the word, particularly in the Asian and African countries. The possible use of specialised crops to control nematode infections in grazing ruminants has attracted considerable research interest in recent years. Bioactive plats or forages with secondary metabolites, particu- larly legumes with a high content of proanthocyani-dins (condensed tannins) e.g. Hedysarum-coronarium and Lotus pedunculatus have been reported to reduce worm burdens in grazing lambs by up to 50%.

An in vivo anthelmintic effect has also been observed using quebracho, a condensed tannins extract, a a single high dose against sheep nematodes and the capacity of purified condensed tannins from legumes grown in Denmark to kill nematode larvae in vitro has been demonstrated. However, in several field studies, it has been difficult to relate anti-parasitic effects to the actual amounts of condensed tannins.

Tannins may have a direct anthelmintic effect on resident worm popula-tions in animals and thirdly, tannins and/or metabolites in dung may have a direct effect on the viability of the free-living stages. The enthno-veterinary and medicinal plant knowledge offers a range of plant materisl to be evaluated for their acaricidal properties.

Plant materials are known to possess insecticidal, growth-inhibiting, anti-moulting and repellent activities. Number of reports are available on the effect of different plant materials on tick species such as Jatropha curcas; Annona squamosa; Polyalthialongi-folia, Ageratum conyzoides, Tageteserrecta, Tagetes-minuta, Cymbopogan spp., Mentha piperita, Ocimum sanctum, Dalbergiasisoo- Roxb., Azadirachtaindica, Eucalyptus maculate, Citrus spp., Ferroniaelephantum, Solanum nigrum Lantana camara.

Vaccines

Considerable resources have been and still are being allocated to research into the effector's mechanisms of naturally acquired immunity to gastrointestinal helminths infections of sheep and cattle, to facilitate the development of vaccines.

However, the situation is complex, involving a combination of local hypersensitivity, in addition to the cell, mediated, antibody and inflammatory responses, and is complicated further by the natural unresponsiveness which exists in the young lamb or calf, and the dam around parturition.

Using the successful development of the irradiated larval vaccine against the bovine lungworm, Dictyocaulusviviparus as a model, attempts have been made to produce vaccines against gut parasites in ruminants. Early attempts to immunize ruminants against gastrointestinal helminths, either with crude worm homogenate antigen or by ectopic infection, met with little or no success.

Several vaccines using "hidden" antigens were developed for H. contortus in sheep, and these provided 94 per cent protection about EPG and their efficacy reached 90 per cent when worm burdens were studied.

The glutathione-S-transferases (GST) of Fasciola hepatica, which was chosen as candidate vaccine antigens because homologous protein from Schistosoma mansoni and S. japonicum was protective in laboratory animal models of infection. Sheep and cattle immunized with native GSTs isolated from F. hepatica, have been protected on average by 49 and 29 per cent respectively, although the results from individual trials have been quite variable.

The expectation was that for any vaccine to be acceptable, it had to compete favourably with modern anthelmintic not only in terms of cost but also with regards to spectrum and levels of efficacy. Effective vaccine candidates have been identified and tested, in native form, from:

i. H. contortus: - H11 (Newton & Munn, 1999,
ii. Ostertagiaostertagi: - sub-fractions from parasite excretory- secretory products (Vercauteren et al., 2004)
iii. Fasciola hepatica: - cathepsin Ls and haemoglobin (Dalton, 1996)

A recombinant subunit vaccine against Theileria spp. is probably shortly. The vaccine against the cattle tick, B. microplus, is a recombinant vaccine based on a protein (abbreviated as Bm86) found in the tick at the surface of the gut wall.

The vaccine effectively suppresses the population of tick larvae available for infestation, rather than protecting individual cattle, with a chemical control being applied if tick numbers rise above acceptable limits. Vaccinating cattle with the recombinant B. microplus vaccine induces almost total immunity to B. annulatus, demonstrating immunological cross-protection. This immunity is sufficiently strong to inhibit Babesia transmission.

Conclusion

It is important to realize that each region of the country will have different parasite problems and potentially different prevention/treatment programs.

Therefore, it is important to involve a local veterinarian in all parasite control programs. Proper nutrition is of extreme importance in the control of the effects of parasitism.

Animals in good condition and receiving adequate feed are often able to establish some resistance to internal parasites. Poorly fed animals are unable to cope with parasitism, and death losses are often great. Parasitic disease problems increase with the intensification of production and lack of attention to strict sanitation.

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Concluded...