An anamnestic serological test for ovine footrot

Diagnosis of footrot in goats: application of ELISA tests for response to antigens of Dichelobacter nodosus

Goats are an important natural host for footrot and are infected with Dichelobacter nodosus that have virulence characteristics similar to those of sheep strains. However, the humoral response of goats to D. nodosus antigens and the possibility of a serological diagnosis of footrot in goats have not been studied. With the aim of evaluating a diagnostic ELISA test, we investigated the primary immune response of goats to experimental and natural infection, the memory response in recovered animals, and the transfer and persistence of colostral antibodies in kids. Footrot stimulated the goat's immune system and, as in sheep, under-running lesions were the primary stimulus for production of anti-D. nodosus antibodies. The immune response could be detected in ELISA using either fimbrial or outer membrane protein (KSCN) antigens of D. nodosus. Antibody titres resulting from infection declined quickly after recovery and reached pre-infection levels within 3-4 months. Previously affected animals, however, mounted a memory response when injected with purified D. nodosus antigens. Antibody levels attained after anamnestic challenge were correlated with the maximum levels attained during infection, and were therefore indicative of the infection status. Anti-D. nodosus antibodies were also transferred to kids via colostrum, but these antibodies did not persist and therefore were unlikely to interfere with the diagnostic ELISA after 3 months of age. Though these ELISA tests were highly specific, their sensitivity was rather low. Therefore, they are only suitable for a herd diagnosis of footrot in goats and are dependent on the development of advanced under-running infections in a proportion of affected goats.

Pilus ELISA and an anamnestic test for the diagnosis of virulent ovine footrot and its application in a disease control program in Nepal

The immunological memory (anamnestic) responses in sheep recovered from virulent footrot (VFR) can be aroused by subcutaneous injection of outer membrane protein (OMP) antigens of Dichelobacter nodosus. The magnitude of this response is directly correlated to the highest antibody response attained during infection and memory lasts at least a year after recovery from VFR. However, some older animals show non-specific responses to OMP antigens. In this study an evaluation of D. nodosus pilus antigen for the anamnestic diagnosis of footrot in sheep was undertaken. The results indicated that the primary and anamnestic responses to pilus were similar in character to OMP antigen but were highly specific. The sensitivity of the procedure for detection of sheep with a history of VFR was approximately 80%. A low proportion of sheep with mild lesions due to virulent strains of D. nodosus reacted to anamnestic challenge. Anamnestic challenge with 10 microg pilus was used in a VFR surveillance program in migratory sheep flocks in Nepal. Conventional diagnostic methods could not be applied during the disease transmission periods in these flocks because of their migration to alpine pastures far away from human habitation. The results supported clinical and bacteriological findings suggesting that virulent strains of D. nodosus have apparently been eliminated from these flocks in Nepal.

Antigens for serological diagnosis of ovine footrot

An antigen extracted from Dichelobacter nodosus with potassium thiocyanate (KSCN) is currently used in enzyme-linked immunosorbent assay (ELISA) for serological diagnosis of ovine footrot, but the test lacks specificity in mature sheep. Other antigens were therefore evaluated for use in this test. Structural components of the cell envelope of D. nodosus including outer membrane, cytoplasmic membrane, lipopolysaccharide and pilus and extracellular proteases were purified from cultured D. nodosus while recombinant membrane proteins, protease and pilus antigens were also evaluated. Many antigenic components of D. nodosus participated in reactions in ELISA that were not specific for infection with D. nodosus and apart from pilus, none of the antigens resulted in improved specificity of the ELISA. Using a positive-negative cut-off to yield sensitivity of 70%, ELISA using pili from cultured D. nodosus serogroup A had a specificity of 98.3% compared with 89.7% for the ELISA with KSCN-extract as antigen (P < 0.001). Recombinant pili morphogenetically expressed in Pseudomonas aeruginosa were unsuitable for use in ELISA due to copurification of Pseudomonas antigens to which apparently healthy sheep directed antibodies. The application of ELISA with D. nodosus pilus as antigen in footrot control programs is discussed.

Immunoblot and ultrastructural analysis of antigens extracted from Dichelobacter nodosus with potassium thiocyanate

The antigens extracted from Dichelobacter nodosus with potassium thiocyanate were analyzed by western blotting with sera from footrot-free sheep and from sheep infected with D. nodosus to identify components specific for infection. Several components with molecular mass < 33 kD were associated with infection, particularly bands of 32.5 kD, 30.5 kD, and 28 kD. Components with molecular mass > 56 kD may be responsible for false-positive reactions observed when sera of footrot-free sheep react with the potassium thiocyanate extract in solid phase enzyme-linked immunosorbent assays (ELISAs). Bands of 62.5 kD, 56 kD, 40 kD, and 19 kD were recognized for most sheep regardless of their disease status or ELISA reactivity and therefore do not appear to be detected by the ELISA. No components of the potassium thiocyanate extract were completely specific for infection. Antigens in the extract were identified primarily on the cell envelope by immunogold electron microscopy. Labeling was concentrated in the periplasm, with minor labeling of the cell surface. The extract consisted of tangled strands of particles with electron-dense cores, and few vesicular structures were observed.

Further observations on the primary and anamnestic humoral responses to Dichelobacter nodosus in sheep in relation to the diagnosis of footrot

An anamnestic serological test for ovine footrot was evaluated. Footrot-free lambs were infected with Dichelobacter nodosus and treated four, six or eight weeks later. There were strong linear correlations between the severity of the lesions and both the primary response and the anamnestic response evoked by the subcutaneous injection of an antigen from D nodosus 16 weeks after the treatment of the lambs; the latter correlation was stronger than the correlations reported elsewhere in mature sheep. Similar anamnestic responses were elicited six and 12 months after the treatment of mature sheep which had had severe lesions. Natural anamnestic responses were demonstrable in sheep which had had recurrent clinical episodes of virulent footrot. The non-specific humoral responses after the anamnestic challenge of footrot-free sheep increased with age and did not depend on the dose of the antigen between 10 and 200 micrograms. Using the pooled data from sheep of all ages and a positive-negative cut-off which was selected to obtain a sensitivity of 75 per cent, the specificity of the anamnestic test was 90 per cent, similar to that reported for the primary response when it was used to diagnose footrot. The anamnestic test can be applied to determine the presence and severity of footrot in young sheep.

Effects of the severity and duration of lesions on the primary and anamnestic humoral responses of sheep to Dichelobacter nodosus and observations of natural resistance to footrot

In a flock of 137 sheep naturally infected with Dichelobacter nodosus the severity of the lesions was the principal factor associated with the humoral response early in the period of spread of D. nodosus, underrun lesions having the greatest effect. However, after five to six weeks, the duration of underrun lesions rather than their severity or number primarily influenced the response. Sheep first affected late in the period of spread had fewer affected feet, milder lesions and a lower humoral response than those affected earlier. An anamnestic humoral response was stimulated by injecting membrane-protein antigen of D nodosus subcutaneously 18 weeks after the sheep had been treated parenterally with antibiotics and antiseptic footbathing. The anamnestic response was related to the antibody level reached during the infection phase, and hence with the duration and severity of the lesions, and with the residual antibody level at the time of the anamnestic challenge, suggesting that the population of memory B cells specific for D nodosus was proportional to the size of the originally activated B cell population. Even after allowing for differences between the duration and severity of the lesions differential responses were detectable among the sheep. Primary and anamnestic responses of a non-specific nature occurred in control sheep.

Application of ELISA to the serological diagnosis of virulent ovine footrot

The sensitivity and specificity of an ELISA that detects antibody in the serum of sheep against Dichelobacter nodosus was investigated. The sensitivity of the assay was determined in two trials in which sheep free of virulent footrot were exposed to a virulent strain of D. nodosus. In the first trial up to 96.6% of sheep that developed severe foot lesions were seropositive 2 weeks later. The proportion of seropositive sheep depended on the interval between exposure to D. nodosus and collection of blood samples and reached a maximum 6 weeks after exposure in the first trial and plateaued from 8 weeks after exposure in the second trial. Of sheep with severe lesions present for only 1 week, 40.0% became seropositive, while 98.3% of sheep with severe lesions present for 3 or more weeks became seropositive in Trial 1. Of the unexposed control sheep from the same flock, 93.3% remained seronegative. At least 75% of sheep that were vaccinated with commercial footrot vaccines remained seropositive for at least 24 months after last vaccination and 58.5% remained seropositive for at least 36 months. The specificity of the assay in flocks of sheep from different geographical regions was determined also. Overall, 95.2% of 2978 sheep free of virulent footrot were seronegative. However the rate of false positive serological reactions ranged from 1.6% to 11.3% between flocks and was higher in older sheep (7.0%) compared with sheep less than 1.5 years old (2.7%). Passively acquired colostral antibodies against D. nodosus were measured in 22 lambs and did not persist beyond 55 days mean age. Non-specific reactions with cross reactive antigens were proposed to explain age related changes in serological results. Applications of this ELISA in footrot control programmes are discussed.