Vaccines can be used to treat sheep that have footrot, and also to prevent, control and eradicate footrot.
Footrot is caused by the bacterium Dichelobacter nodosus (Beveridge, 1941). There are many different types of this bacterium associated with sheep. In Australia, D. nodosus strains are classified into 10 different serogroups (A, B, C, D, E, F, G, H, I and M) based on the type of protein present in hair-like appendages on the bacterial surface that are known as pili or fimbriae (Claxton, 1989; Claxton et al., 1983; Ghimire et al., 1998). Up to seven serogroups have been detected in a single flock, but usually only 1 to 4 serogroups are present (Claxton et al., 1983; Dhungyel et al., 2013). Vaccines for footrot are composed of the relevant type of pili.
Research on ovine footrot vaccines, and their application for control, treatment, and eradication of the disease in sheep has a long history (Dhungyel et al., 2014).
- Early 1970’s: The first commercial footrot vaccines were patented by the CSIRO. Three mono-valent vaccines were produced, each targeting a single D. nodosus serogroup. These vaccines were shown to be ineffective in field trials and were withdrawn from the market in 1976. The poor efficacy of these vaccines was most likely due to the adjuvant formulation and the presence in flocks of serogroups that had not yet been discovered.
- 1986: A multivalent commercial vaccine targeting 9 serogroups, Footvax®, was released commercially but provided only partial protection for up to 10 weeks under severe field challenge. Footvax® continues to be used in some countries as a control measure. However, due to biosecurity concerns it is no longer available in Australia.
- 1990’s: The poor efficacy of multivalent vaccines was attributed to the phenomenon of antigenic competition (Hunt et al., 1994; Raadsma et al., 1994). The mechanism is poorly understood, however, it is thought that the immune system is unable to process several similar antigens simultaneously.
- 2002: Outbreak-specific vaccination, which involves vaccination against one or two identified serogroups at a time with mono- or bivalent vaccine, was shown to be very effective in a trial in a transhumance system in Nepal (Egerton et al., 2002). Subsequent trials in Bhutan and Australia confirmed this (Gurung et al., 2006; Dhungyel et al., 2008; Dhungyel et al., 2013).
- 2009: Antigenic competition was shown to be time-limited. Where more than 2 serogroups are present in a flock, antigenic competition can be overcome by sequentially targeting the relevant serogroups with a different bivalent vaccine at 3 month intervals (Dhungyel and Whittington, 2009). Using this approach, up to 8 serogroups can be targeted within a 12-month period.
- 2016: Monovalent and bivalent vaccine became commercially available in Australia, (Tréidlia Biovet, Sydney). Ongoing use of this vaccine has shown that footrot can be successfully controlled and eradicated by outbreak-break specific vaccination.
Recommended strategy for treatment, control of footrot using outbreak-specific vaccination
An accurate diagnosis of virulent footrot is required in order to justify control and eradication using vaccines. There is no economic benefit in attempting to control benign footrot. However, there is a diagnostic grey zone with intermediate footrot, where there may be some economic losses and separately in some regions where producers do become concerned about mild, temporary, seasonal lameness, particularly in young sheep; this syndrome is a form of footrot of uncertain economic impact that requires detailed research (see pages on intermediate footrot).
The following information has been extracted from a AWI research report by Whittington and Dhungyel (2010).
In general, a control program would commence with the specific intention of proceeding with eradication once the severity of disease in the flock was brought down to a more manageable level.
Where there are only a few serogroups of D. nodosus present it will generally be easier to vaccinate than to footbath. Where there are multiple serogroups present and the severity of the problem is affecting production and negatively impacting animal welfare, foot bathing or antibiotic treatment may be required to reduce the severity of the disease, until several different vaccines have been applied to control the various serogroups.
If there are only one or two serogroups present, specific vaccination should be applied, all feet of all sheep should be inspected, and non-responders should be identified and culled. Non-responders are sheep that have score 1 or worse lesions in one or more feet during a foot inspection between 6 weeks and 12 weeks after the last dose of vaccine. If the prevalence of sheep with residual lesions is sufficiently low, they should be isolated from the flock, quarantined and culled as soon as possible. If the prevalence remains high, the situation must be investigated using diagnostic tests to determine whether other serogroups are present. If so, a further round of vaccination is necessary. This can be done 3 months after administration of the first vaccine.
If there are 3 or more serogroups present, several rounds of vaccination will be required at 3 month intervals. It is possible to provide protection against up to 8 serogroups in 12 months. Inspection of the entire flock should be timed for between 6 weeks and 12 weeks after the last planned dose of vaccine. If the prevalence of sheep with residual lesions is sufficiently low, they should be isolated from the flock, quarantined and culled as soon as possible. If the prevalence is higher, the situation must be investigated using diagnostic tests to determine whether other serogroups are present. If so, a further round of vaccination is necessary.
It is important to cease any other treatments after the last planned vaccine has been given, so as to be able to assess the number of residual footrot cases. This allows diagnostic tests to be applied, and facilitates culling of non-responders.
It is critical that sheep which have not responded to vaccination are identified and eliminated as they may act as a reservoir of infection in the flock. The stage at which culling of sheep with residual footrot lesions can be contemplated will vary from farm to farm, depending on the value of sheep, the cost of replacement sheep, the need for culling for reasons other than footrot, the cost of vaccine and other treatments for footrot, and other factors.
When it is clear that footrot is no longer active, and no cases can be detected, the flock should be monitored until such time as there has been a period suitable for expression and transmission of footrot. This is generally a period with sufficient rainfall and warmth to allow good pasture growth. All feet of all sheep then need to be inspected. Cases need to be identified and the diagnosis confirmed using laboratory tests. It is possible at this time that cases of benign footrot may be identified. These do not warrant treatment or control measures. If virulent footrot is no longer present, the disease is considered to have been eradicated. The next challenge is to prevent its reintroduction.
During an eradication program using specific vaccination it is important that secure fencing be maintained to prevent straying of sheep from neighbours. If possible the flock should be closed. If this is not possible, sheep should be obtained under vendor declaration and should be inspected for footrot prior to purchase. Newly arrived sheep, including rams, should be quarantined and inspected. Ideally they would be examined again after a footrot transmission period to ensure that they are free of footrot, before introducing them to the main flock. Conventional eradication programs have been enhanced by producers from a district forming an interest or self-help group, supported by professionals with experience in footrot, and the same would apply to specific vaccination.
The University of Sydney has produced guidelines for laboratory diagnosis when outbreak specific vaccination is being considered as a control option. Please click on this link: Footrot Fact Sheet 1 Diagnosis for specific vaccination 270417
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Important disclaimer: The advice contained on this website is of a general nature. Please consult your veterinarian or government district veterinarian, animal health or biosecurity officer for an accurate diagnosis if you suspect footrot, and for specific advice on the best course of action to prevent, control and eradicate footrot.
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