Footrot in sheep

Description

Virulent footrot is caused by virulent strains of Dichelobacter nodosus (formerly Bacteroides nodosus).  Outbreaks of severe lameness can occur under favourable environmental conditions. Carrier sheep may remain a long-term source of infection for the flock, even after dry periods, and spread may occur when moist conditions return.

Early lesions resemble benign footrot, but can rapidly progress to underrunning of the sole and hoof wall. There is little or no purulent discharge, but a black-grey slime like substance with a characteristic odour may be present. Many sheep may be affected within the flock, showing lesions of varying severity, with more than one foot usually affected per animal. Chronically-infected sheep may not appear lame, but often have obvious deformities of the affected hoof.

Benign footrot is caused by infection with benign strains of D nodosus. It is seen in sheep grazing wet pastures under mild weather conditions and usually affects a number of sheep in the flock, of all age groups.

Benign footrot can cause marked lameness, particularly in rams and heavy sheep. Affected sheep show necrosis and inflammation of the interdigital skin, involving part or all of the soft horn of the axial wall with a small percentage of affected sheep with some underrunning of the soft horn of the heel. The lesions resolve quickly if sheep are moved to dry pasture or if treated with foot-baths. It cannot be differentiated from early virulent footrot, but does not progress to extensive underrunning of the hard horn of the hoof.

In ovine footrot, a distinction between virulent and benign can be made provided there are no recent treatments or environmental conditions which would mask the full expression of disease due to the infecting strain of D nodosus.

Examination of sheep after a period of spread (wet pasture conditions/mild temperature) is more reliable to assess flock status. However, additional factors will also influence disease spread and expression, including host resistance and stocking rate.

In all circumstances where lameness is occurring, it is recommended that sufficient sheep be examined to ensure that virulent footrot is not present. In most mobs, this will involve the examination of at least 100 sheep. In some mobs, it may require examination of more sheep to detect a sufficient number of affected sheep (score 2 or greater) to assist in the diagnosis.

Footrot scoring system

Descriptions of benign and virulent footrot are also provided on page 2 of the Primefact: Footrot in Sheep and Goats.

Lesions are illustrated on pages 5 and 6 of the Primefact.

Diagnosis and tests available

Diagnosis

Suspect cases should be notified to a district veterinarian or other authorised officer within one working day. Diagnosis is based on field investigations including flock history, environmental assessment and clinical examination of a sufficient number of sheep in the flock. In some flocks, examination of other mobs and/or second examination 10 to 14 days later will be necessary to check for progression of the disease, and to differentiate benign from virulent footrot.

Tests available

¹ Turnaround times are provided as a guide only. For specific information about your submission please contact Customer Service.
² This test is not NATA accredited.

Specimen requirements

Interpretation of Laboratory results

A diagnosis of virulent footrot is made by the field veterinarian investigating the disease in accordance with Footrot policies and procedures (see link below). Laboratory results must be interpreted in conjunction with clinical findings. Laboratory results may provide useful ancillary information to support diagnosis; however, no laboratory test for footrot has 100% sensitivity or specificity. A laboratory test cannot be used on its own to establish a diagnosis of virulent footrot.

Culture and virulence testing of Dichelobacter nodosus is expensive and labour intensive, with cultures requiring on average 8-18 days to isolate the organism in sufficient purity for characterisation.

The elastase test is currently used as the standard laboratory test to assess virulence of isolates. Elastase is a protease produced by some strains of D. nodosus and is detected using the elastase test which detects hydrolysis of elastin particles in agar. The plates are read approximately every three/four days for up to 28 days. Under environmental conditions suitable for expression of the disease: Strains that clear elastase within 4 to 11 days are usually capable of causing severe virulent footrot. Strains that have not cleared the elastase media at 28 days are usually benign. Strains that clear elastase between 12 and 28 days are usually capable of causing virulent footrot of lesser severity.

As a minimum, at least 100 randomly selected sheep should be examined in the mob and the lesions in individual sheep recorded, before specimens are collected for laboratory testing. Suitable samples from a minimum of 5 representative affected animals should be forwarded together with an adequate description of lesions in the sampled group. This will allow a minimum of 5 (up to a maximum of 20) isolates to be examined in the laboratory.

The following information must be included in the Footrot Specimen Advice Form (included in the footrot sampling kit):

• number of sheep examined
• foot scores of affected sheep
• pasture conditions and rainfall over the past 3-4 weeks
• previous footrot history of the flock
• history of recent introductions
• history of recent treatments
• age, breed and sex of the sheep

Further Information