Volume 3, Number 39 - December 24, 2003
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Strain 19 vs. RB51
A great deal of energy has been invested into dismay and criticism over the required use of RB51 brucellosis vaccine rather than Strain 19 brucellosis vaccine. Frustrated, impassioned, and sometimes inflammatory speeches are often delivered on the topic.
An assertion that is often made is that Strain 19 had proven itself over many years to provide protection from the disease. True. There is no question that Strain 19 has been extremely important in the success of the brucellosis eradication program. It has been a great tool and it has established a great track record. Nonetheless, allegiance to Strain 19 finds people asserting that the situation "wasn't broken." The assertion is in error.
Perhaps the most important drawback of Strain 19 is its telltale immune signature. The manner in which Strain 19 creates immunity from the disease leaves the potential of confusing a high vaccine titer with disease titers which would indicate presence of the disease. False positives do occur with Strain 19 vaccinates. Once a fairly common occurrence, the frequency of false positives is falling quickly as the cattle herd becomes predominantly RB51 vaccinates.
False positives are the "broken" component of Strain 19; the Achilles heel which has been largely fixed with the implementation of RB51. Those who have experienced a traceback from a false positive Strain 19 vaccinate will testify that the initial reaction of the agencies to a false positive is precisely the same as if there were discovered real case of the disease: 1) The freedom of the herd's movement would be restricted. 2) The disease detectives would perform their function to ascertain the origin of the test reaction. 3) If the test titer was high enough and the donor animal unavailable, there would be whole herd testing conducted.
In other words, the actions which would occur upon discovery of a Strain 19 false positive reaction mimic exactly the first stages which occurred in this current incident. In the past, tracebacks, restrictions and testing occurred fairly frequently. In Wyoming, it was reported that the agencies handled nearly a dozen of these cases a year. RB51 is on its way to eliminating this stressful situation.
There is considerable skepticism over the efficacy of RB51. Critics point out the newness of the vaccine, the odd way in which it works, the tardy final approval of the vaccine, and the lack of long term efficacy studies on the vaccine.
The fact is, none of these are particularly damning characteristics of the vaccine, while some of them are positive. For example, it's mode of action saves us from the likelihood of false positives.
Though there are no long term efficacy studies on RB51, neither are there any on Strain 19. All we have going for us are empirical observations of success. We don't know if RB51's ability to protect against the disease decreases with animal age. Unfortunately, we don't know that about Strain 19 either. To date, the results appear to be identical between Strain 19 and RB51.
The testing results which have come from the Boulder infection will no doubt find themselves in the scientific journals. They are very interesting. According to the investigation and test results, the cattle in the infected herd were made up of 211 RB51 vaccinates and 76 Strain 19 vaccinates. Of the 31 animals found to have the disease, 22 were RB51 vaccinates and seven were Strain 19 vaccinates. The immunity failure percentages are 9.2 percent for Strain 19 and 10.4 percent for RB51. These numbers are remarkably close - and certainly too close to call any advantage.
Those anxious to find a difference in these numbers will point out that the Strain 19 vaccinates would be older cows. If you are to assume that the vaccine effectiveness decreases with age, then surely the Strain 19 provides better protection. Unfortunately, we don't know if efficacy decreases with age. We do know that in all other observations, older females seem to be less susceptible to the disease. Thus, there is no advantage to either vaccine in this examination of the evidence.
Another test employed in searching for differences in these numbers is the assertion that the group of younger cows appears to be greater. As noted earlier, the numbers are remarkably close and certainly the difference isn't likely to be discernable with any statistical test. One can say however, that since younger females are more susceptible to the disease, one would predict that there should be a higher incidence in younger females given equal vaccine efficacy. Since there is not any appreciable difference, perhaps there is merit in the argument that RB51 is performing better than its predecessor.
However, the sobering element to the entire efficacy argument is the fact that no immune system's protection is invulnerable to the disease, given a large enough challenge. Even a small ingestion of material from a freshly expelled brucellosis abortion or aftermath will likely overwhelm any animal's immune system protection.
Finally, one must realize that the efficacy of either vaccine (or any vaccine) is much lower than one would expect. The real protections that the cattle industry has from the disease are the management practices that go hand-in-hand with a vaccine program. They include testing procedures to identify presence of the disease, vaccination programs, separation in time and/or space of susceptible animals from infection risks, sanitation practices, and many others. Each of these management practices is effective at delivering some level of protection. While the level of protection offered by vaccine may be less than 75 percent, each of these management practices compounds the level of protection.
Each ranch's number will vary, but to suggest an example with hypothetical numbers: 70 percent immunity protection, compounded by 90 percent protection provided by spatial separation, compounded by 50 percent protection offered by timing separation, compounded by 60 percent protection as a result of sanitation, and perhaps another 25 percent protection as a result of other practices finally yields 99.6 percent protection when the entire protection system is considered. The protection afforded by the entire management system is the reason for the remarkable success of the shield that we have in place against brucellosis in western Wyoming. Arguments over the vaccine are focused on factors of very small importance in the overall picture.
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