New approaches to footrot control and eradication using outbreak-specific vaccination
Richard J. Whittington, Om P. Dhungyel and Andrew S. McPherson
Farm Animal and Veterinary Public Health, Faculty of Veterinary Science, The University of Sydney, 425 Werombi Road, Camden 2570, NSW, Australia
Virulent footrot is a highly contagious bacterial disease of the feet of sheep, and is one of the primary causes of lameness in the Australian sheep industry. In some regions of Australia, particularly those with long dry periods, traditional methods with foot-bathing and foot-paring have been used to reduce the prevalence of virulent footrot considerably. However, in regions with more uniform rainfall patterns, virulent footrot can become endemic, requiring frequent foot-bathing and sometimes antibiotic therapy to manage; economic costs can be very significant.
The essential transmitting agent of virulent footrot is a slow-growing, anaerobic bacterium called Dichelobacter nodosus (D. nodosus), which lives exclusively on the feet of infected animals. Virulent footrot is a complex disease, resulting from interactions between D. nodosus and other microbial species present on the foot. Mild ambient temperatures and consistent rainfall are also required for the disease to be fully expressed. D. nodosus possesses hair-like structures on its surface called fimbriae which enable the bacterium to move and to attach to the skin surface. Differences in the genetic and physical properties of the fimbriae of D. nodosus have been used to classify D. nodosus into 10 major groups (A,B,C,D,E,F,G,H,I and M). Fimbriae stimulate an animal’s immune system, and are the key component of footrot vaccines. Unfortunately, the immune response of an animal to D. nodosus is specific, and immunity to one serogroup of D. nodosus does not provide protection against the others.
By 1972, three vaccines were commercially available; they were monovalent vaccines targeting only one D. nodosus strain and while they provided some protection against footrot, they did not provide complete protection. Field trials demonstrated the efficacy of these vaccines to be poor, providing only partial protection for a maximum of 8 to 10 weeks, and they were withdrawn from the market in 1976. A multivalent vaccine targeting all 10 D. nodosus strains (Footvax®) was released commercially in 1986 and was evaluated in Australia. As with previous vaccines, Footvax® was shown to provide only limited, short-term protection against virulent footrot, despite targeting all 10 strains. The failure of Footvax® to provide complete protection was attributed to a phenomenon called “antigenic competition”, in which the immune response to one strain modifies or suppresses the immune response to the other strains included in the vaccine. Footvax® continues to be used as a control measure in a number of countries, however this vaccine has been withdrawn from the Australian market for quarantine reasons.
In recent years, outbreak-specific footrot vaccination has been trialled with very good success. Rather than targeting all 10 strains simultaneously, outbreak-specific vaccination is an approach targeting only those strains that are present in a particular flock. Where three or more strains are present in a flock, sequential administration of bivalent or monovalent vaccines is employed, with an inter-vaccination interval of 3 months shown to avoid antigenic competition. To date, researchers from The University of Sydney have conducted numerous successful field trials of outbreak-specific footrot vaccination. Early success was achieved in Nepal, where specific footrot vaccination using a bivalent vaccine was used to eradicate virulent footrot from 40 flocks of sheep and goats that had a 25-year history of the disease. Similar success was later achieved in a flock in Bhutan that had an eight-year history of virulent footrot. Between 2005 and 2013, further field trials of outbreak-specific vaccination were initiated in southeast Australia. The results of these trials were equally promising, and indicated outbreak-specific footrot vaccination to be an effective means of controlling and in eradicating virulent footrot especially from flocks infected with only one or two strains.
A local vaccine company Treidlia Biovet Pty Ltd, under a commercial agreement with the University of Sydney, has been granted a permit by the Australian Pesticides and Veterinary Medicines Authority (APVMA). Under this permit the company is working towards making vaccines on demand and it is anticipated that the vaccines will be commercially available in the near future.
For further information:
Dr Om Dhungyel
P: (02) 9351 1606
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