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This article was published in 1980
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Distribution of Brucellosis Complement Fixation Titres in Vaccinated and Unvaccinated Herds

C. Gee, B.V.Sc., Veterinary Inspector, Goulburn

Aim

The purpose of the study was to assess the significance of Strain 19 residual titres in the eradication effort in the Goulburn District.

Methods

There is no pretence that this study is objective or that any of the results are statistically significant. It is merely a subjective look at the situation in a selection of Goulburn herds.

The population studied consisted of all herds of over 100 eligible cattle in the Goulburn district. These herds were classified into three categories:—

(a) Vaccinated Clean Herds,

(b) Unvaccinated Infected Herds, and

(c) Vaccinated Infected Herds.

A C.F.T. Histogram was prepared for the total of all herds in each category. Different vertical scales are used in the different categories because of the sharp differences in the reactor rate. To further illustrate the C.F.T. distribution pattern, a typical herd of each category was selected. The histograms of individual herds are merely samples and no significance is claimed for them.

All results quoted were derived from the first full herd test of each herd.

The allocation of the herds to the three categories was on this basis:-

(a) Vaccinated Clean Herds

1. No C.F.T. Over 16 recorded at any time for that herd.

2. No other evidence in recent years of infection in that herd.

3. Over 50% of eligible cattle recorded as vaccinated at the first herd test OR over 10% of R.B.T. positive, C.F.T. negative females at the first herd test.

(b) Unvaccinated Infected Herds

1. One or more C.F.T. over 4 in unvaccinated beast.

2. Assessed unvaccinated using same guidelines as in Category a.

(c) Vaccinated Infected Herds

1. Any C.F.T. over 16 in an eligible beast at any time.

2. Any other clear evidence of infection in recent years.

3. Assessed as vaccinated as in Category a.

Results

The results are listed in Table 1.

The distribution of the C.F.T. reaction levels in the Vaccinated Clean Herds is shown in Figure 1. The mode is obviously at the left of the chart at C.F.T. 4. Figure 2 is an example of this distribution in a particular herd.

In the Unvaccinated Infected Herds (Figure 3) the mode is to the right of the chart at C.F.T. 64. Figure 4 shows the distribution in a very heavily infected herd which mirrors the aggregate distribution.

The histogram of the Vaccinated Infected Herds shows modes on the far right and far left at C.F.T. 4 and C.F.T. 64 (Figure 5).

Figure 6 is the chart for a herd which happens to conform to the general pattern. It is my suggestion that the mode to the right represents infection. The mode to the left represents a sum of infection and residual vaccination. titres.

Discussion

The allocation of herds with an eligible beast showing a C.F.T. of 16 to the clean herd category is certain to be questioned. It is possible that some of these cows may have, in fact, been infected. However, the general pattern of the low-grade reactors in vaccinated, clean herds suggests that residual strain 19 titres as high as 16 occur occasionally. To date none of the herds classed as clean on this basis have reverted to infected.

It must be stressed that the type of sorting into the specific categories discussed here had no place in the eradication procedure. A herd with a C.F.T. 16 reactor would have been treated as an infected herd but may have been placed in the clean category in this study.

The detection of significant numbers of residual strain 19 reactors in eligible cattle was expected. However, we should be realistic in our thinking. We must accept that some low grade reactors in both clean and infected herds will be showing only residual vaccination titres. We don't want to be going on epidemiological witch hunts for organisms that aren't there.

Summary

The frequency distributions of brucellosis C.F.T. titres in herds of over 100 eligible cattle in the Goulburn district are studied. The herds are divided into three categories according to Strain 19 vaccination and proven infection. The results suggest a significant contribution to the reactor rate due to persistent vaccination titres.

TABLE 1

Distribution of CFT Reaction in Three Herd Categories

CFT Level	CFT	CFT	CFT	CFT	CFT	CFT
Herd Status / Total Number	4	8	16	32	≥64	≥4
Vaccinated Clean Herds 13,363 head	36 ex 13,363 0.27%	16 0.12%	19 0.14%	-		71 0.53%
Unvaccinated Infected Herds 4,786 head	63 1.31%	56 1.17%	79 1.65%	81 1.69%	216 4.51%	495 10.3%
Vaccinated Infected Herds 9,873 head	79 0.8%	57 0.58%	47 0.48%	47 0.48%	83 0.84%	313 3.17%