Contd from previous issue
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.
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.
The writer is from College of Veterinary Sciences and Animal Husbandry, Central Agricultural University, Aizawl, Mizoram
For further details contact: - Public Relations& Media Management Cell, CAU, Imphal. Email: [email protected]