Laboratory Diagnosis adds rigour to footrot program in SA

Thursday, February 13, 2020

Changes have been introduced to the way footrot is diagnosed in South Australia under the state’s Footrot Management Program (Footrot Program).

Caused by the bacteria Dichelobacter nodosus, footrot is a complex bacterial disease which can vary significantly in clinical expression due to variations in host, agent and environmental factors.

In South Australia, sheep producing areas range from 900mm plus average annual rainfall to less than 150mm. Having adequate moisture on the ground is one of the main contributing factors that aids footrot spread and development. With such vast variations in rainfall in sheep production areas, gaining a consistent clinical diagnosis from one flock to another has always been problematic.

It's a common myth that footrot doesn’t occur in sheep flocks from lower rainfall areas such as the Mid North, Yorke and Eyre peninsulas.. In drier areas the footrot bacteria can survive in a sheep’s hoof for years without causing any lameness and hence can “fly under the radar” of most sheep producers. For footrot to express fully and cause debilitating lameness, three factors are required:

  • adequate rainfall in the preceding months (combined with reasonable pasture length),
  • ambient temperatures above 10C
  • susceptible sheep or goats.

Importantly, some sheep within a flock are far more resistant to picking up a footrot infection than others. That is why, even in flocks with severe virulent footrot there are often a number of sheep with no footrot lesions at all.

Relying on detection of severe footrot lesions at a particular point in time is a very blunt diagnostic technique and can result in misdiagnosis and poor management of footrot. Sheep inspected in drier times or in areas that have lower rainfall will often not fully express the severity of footrot that is carried by the flock. Producers then feel a false sense of security, that the infection they are dealing with isn’t that significant and hence do not implement treatment and control measures.

To combat these difficulties, the state’s Footrot Program (with the support of the SA Sheep Industry) has elected to use the Elastase virulence test to diagnose footrot and its potential severity (virulence capability).

The Elastase test for footrot, has been shown to align well with the potential severity of footrot infections on South Australian farms. The test aids in determining potential virulence where environmental and host factors do not allow for full lesion expression. The Elastase test requires that successful cultures of D. nodosus are detected from multiple lesions in sheep from an infected flock.

Elastase is an enzyme produced by some strains of D. nodosus. It enables the bacteria to break down elastin, a component of the tissue connecting the hoof horn to the rest of the foot.

In this test, the speed at which the bacterium breaks down elastin indicates its virulence. Test plates are checked every 4 days for up to 28 days to determine the results.

  • Virulent strains of D. nodosus bacteria take 4 – 12 days.
  • Intermediate to virulent strains take 16 days.
  • Benign strains take 17 – 28 days to clear the plates. Note, some benign isolates do not produce enough elastase to clear the plates at all.

The benefit to the Footrot Program of utilising the Elastase test is three fold:

  • It allows for predictive diagnosis of the potential severity of Footrot infection both in areas and flocks where full expression is limited as well as during the traditional “non-spread” period when it is too cold or dry for spread and lesion expression.
  • It will assist in the implementation of control measures in flocks previously classified as being infected with “benign” footrot and thus limit spread of disease to uninfected flocks.
  • It allows for a flock diagnosis to be achieved through samples submitted by private veterinary practitioners, thus making the program more efficient and effective.

If you have previously been advised that your flock is infected with Benign Footrot (often referred to as Footscald) it is important that you have your flock reinspected to achieve a laboratory diagnosis using the Elastase test. This inspection can be performed by an experienced private veterinary practitioner or Biosecurity SA officers.

The current version of the National Sheep Health Declaration (NSHD) is compulsory for all movements of sheep within and into South Australia. The NSHD requires that sheep producers now declare flocks infected with both virulent and benign footrot. This is particularly important as imported sheep may have only been expressing benign footrot on the source property, however when moved to a different environment or exposed to sheep in a different flock may well develop into a more virulent form.

A revised South Australian version of the Footrot ute guide funded by Australian Wool Innovations is in development and will be released in 2020. This guide will provide sheep producers with easy to access and up-to-date footrot information relevant to producers in South Australia.

Please direct enquiries to Chris van Dissel, Manager Footrot Management Program, Biosecurity SA.
Phone: 08 8568 6415