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This article was published in 1987
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Serological Testing for Leptospirosis in the Tamworth Pastures Protection Board

by E.S.G. SERGEANT, V.I. Tamworth

INTRODUCTION

This report details the results of serological testing for Bovine Leptospirosis within the Tamworth Pastures Protection Board area during the period June 1985 to June 1986 inclusive. The results include all submissions to the Regional Veterinary Laboratory, Armidale that were tested for Leptospirosis during that period. Because of the diversity of submissions and the fact that a few properties may have had repeat samplings, possibly of the same cows, no detailed statistical analysis has been attempted.

SPECIMEN SUBMISSION

Generally two types of submissions were involved:

1. Samples collected by Veterinarians as part of a disease investigation, mainly abortions or infertility. In these cases usually only 1-4 samples were submitted for testing.

2. Samples submitted on an ad hoc basis in association with Brucellosis eradication testing. In this case sample sizes ranged from 1 to 81 and testing was usually requested at the whim of the Veterinary Inspector, the owner, or of Brucellosis staff. There was no disease history collected with any of these submissions.

INCIDENCE OF CLINICAL DISEASE

Unfortunately there was insufficient history associated with the submissions to allow any conclusions, to be reached associating serological results with clinical disease. As mentioned previously, some submissions were associated with abortions or infertility, but no detailed history was available, and the majority of submissions had no history whatsoever. For the same reasons there was no vaccination history available on any of the submissions, although some may have been previously vaccinated.

SAMPLE TESTING

All samples submitted were tested for both Lepto. pomona and Lepto. hardjo at the Regional Veterinary Laboratory at Armidale. The test used was the MAT test, carried out according to standard laboratory procedures.

RESULTS

All initial results shown interpret any titre of 100 or more on MAT as being serologically positive (100% specificity). Some results will also be shown using interpretations based on reduced specificity

Analysis of results for all submissions is shown in Table I.

EFFECT OF CATTLE TYPE

Analysis showed that only 6 submissions (26 samples) were made from dairy cattle, mostly related to disease investigation. This was felt to be too few samples to provide any real comparison between beef and dairy cattle.

GEOGRAPHIC DISTRIBUTION

Table II shows analysis of results broken down into Pastures Protection Board divisions. Generally speaking Divisions B and D and parts of C would be regarded as climatically and geographically more suitable for the spread of Leptospirosis.

SEASONAL DISTRIBUTION

Table III shows analysis of results broken down into seasons of the year.

EFFECTS OF SAMPLE SIZE

Table IV shows the effect of sample size on herd prevalence.

TEST SPECIFICITY

Tables V and VI show the effect on herd prevalence of varying the assumed specificity of the test. For example an assumed specificity of 90% assumes that up to 10% of animals tested may show a false positive titre of 100. Therefore positive herds would have either one or more titres of 300 or more, or at least 10% of animals sampled had a titre of 100.

DISCUSSION

As mentioned previously, no detailed statistical analysis has been carried out because of probable biases caused by the sampling methods used.

Despite this bias and lack of statistical support, there appears to be a very high serological prevalence of Leptospirosis in the Tamworth Pastures Protection Board, with 50% of animals tested having positive titres, and 85% of submissions having at least one positive result. These results did vary with both season and location, however this variation was only by about 10% either way.

A Division, which is largely farming country, had the lowest herd prevalence of the 4 divisions, at 76.5%. This is not surprising as the area is lighter stocked and conditions are less favourable for the spread of Leptospirosis than in the other divisions.

On a seasonal basis prevalence was also lowest in Autumn, at 74%. Again this is not surprising considering the very dry conditions experienced during the summer and autumn of 1985/6: This seasonal pattern would probably be quite different in a 'normal year'.

The current recommendations for testing 10-15 animals or more in a herd when Leptospirosis is suspected is supported by the analysis of herd prevalence when related to sample size. The overall prevalence of 85.8% jumped to 97.5% when only submissions of 10 or more samples were considered. This compares extremely favourably with a prevalence of only 71.5% when submission of less than 5 samples were considered. Most of this increase would be associated with the increased detection of herds with a relatively low infection rate.

One other interesting observation was an apparent relationship between infection with the two strains being tested for. Whilst only 3.7% of herds were positive for L. pomona only, some 31.5% of herds were positive for both L. Pomona and L. hardjo. The explanation for this finding is unclear. The two strains are apparently quite distinct serologically and are unlikely to cross react (R. Whittington, pers. comm.). This lack of cross reaction between strains is supported by the fact that only 4% of individual animals had titres to both L. hardjo and L. pomona. It would appear that some management factors may be involved so that L. pomona mainly occurs under conditions where L. hardjo is a common occurence.

Finally, it would appear that the serological test used has a reasonably high specificity, and infection rates within herds are generally high, since allowing for up to 20% of false positive reactions in a herd only varied herd prevalence by about 10%. This indicates that the majority of submissions with positive serological reactions had at least 20% of animals with a titre of 100 or at least one titre of 300 or more.

CONCLUSION

It would appear that there is a very high prevalence of Leptospirosis, particularly L. hardjo, throughout the Tamworth Pastures Protection Board area. Further testing is necessary to accurately define this prevalence and to determine any relationship between herd infection and clinical disease.

Table I.

Results for all submissions:

No. of submissions:		133
Cattle tested:		1602
Individual cattle titres (%):
Pomona=100	Pomona >100	Hardjo=100	Hardjo >100	Both <100
7.3	3.1	26.3	17.1	50.3
Herd (submission) analysis (%):
+ve Pomona	+ve Hardjo	+ve Both	-ve Both	+ve 1/Both
3.7	50.3	31.5	14.2	85.8

Table II.

Geographic distribution of results:

Division	Cattle	Cattle results (%)		Herd results (%)
	/Herds	Pom. >=100	Har. >=100	+Pomona	+ Hardjo	+Both	-Both
A	225/17	15.1	47.1	5.8	41.1	29.4	23.5
B	251/19	11.0	26.6	5.2	36.8	47.3	10.5
C	501/39	8.0	50.4	0.0	64.1	28.2	7.6
D	590/55	10.6	44.7	5.4	47.2	29.0	18.1
TOTAL	1602/133	10.4	43.4	3.7	50.3	31.5	14.2

Table III.

Seasonal distribution of results:

Season	Cattle	Cattle results (%)		Herd results (%)
	/Herds	Pom. >=100	Har. >=100	+Pomona	+ Hardjo	+Both	-Both
Summer	214/24	12.1	56.4	4.1	41.6	41.6	12.5
Autumn	210/23	13.3	37.0	4.3	34.7	34.7	26.0
Winter	684/51	9.4	41.6	3.9	52.9	31.3	11.7
Spring	494/35	9.8	43.1	2.8	62.8	22.8	11.4
YEAR	1602/133	10.4	43.4	3.7	50.3	31.5	14.2

Table IV.

Effect of sample size on results:

Sample size	Cattle	Cattle results (%)		Herd results (%)
	/Herds	Pom. >=100	Har. >=100	+Pomona	+ Hardjo	+Both	-Both
<5	80/35	13.7	42.5	11.4	48.5	11.4	28.5
>=5	1522/98	10.2	43.4	1.0	52.0	37.7	9.1
>=10	1390/77	10.5	44.0	1.2	53.2	42.8	2.5
>=20	801/28	9.7	38.9	0.0	53.5	42.8	3.5
>=1	1602/133	10.4	43.4	3.7	50.3	31.5	14.2

Table V.

Effect of varying test specificity (all tests):

Assumed specificity	Herd results (%)
	+Pomona	+ Hardjo	+Both	-Both	+1/Both
80%	5.2	57.8	17.2	19.5	80.5
85%	5.2	56.3	21.0	17.2	82.8
90%	4.5	55.6	24.0	15.7	84.3
95%	4.5	52.6	27.8	15.0	85.0
100%	3.7	50.3	31.5	14.2	85.5

1602 Cattle tested on 133 submissions.

Table VI.

Effect of varying test specificity (>=10 tested):

Assumed specificity	Herd results (%)
	+Pomona	+ Hardjo	+Both	-Both	+1/Both
80%	2.5	64.9	22.0	10.3	89.7
85%	2.5	62.3	27.2	7.7	92.3
90%	2.5	62.3	29.8	5.1	94.9
95%	2.5	57.1	36.3	3.8	96.2
100%	1.2	53.2	42.8	2.5	97.5