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January 2009

Effectiveness of Test and Remove for Bovine Tuberculosis Eradication in Herds

In the past, herd depopulation has been the primary technique for combatting bovine tuberculosis (bTB), but larger herd sizes, decreased indemnity funding, and animal welfare concerns make test and remove (T & R) methods worth reconsidering. This article examines a herd that underwent T & R to demonstrate that T & R provides an effective, practical and economical way to handle herds infected with bTB, and that policies that do not discourage T & R procedures are needed.

Phil Durst
Extension Dairy Educator
Northeast Michigan

Bovine tuberculosis (bTB) was a significant risk to the health of both people and cattle through the early 1900s in this country. In 1917, the Cooperative State-Federal Tuberculosis Eradication Program began and through it, bTB prevalence was brought to very low levels by the 1990s. Since then, however, we have seen sporadic cases of bTB and currently four states; California, New Mexico, Minnesota and Michigan are or have zones that have a classification that is less than TB-Free.

In the efforts to eradicate bTB in the US, herd depopulation has been favored because it eliminates the uncertainty of determining which animals are infected and therefore the possibility that some infected animals survive and will be able to spread the disease. Diseases caused by Mycobacterium species such as bTB and Johne’s Disease are difficult to detect with high reliability in individual animals.

Yet, as herds became larger, and as indemnity dollars became more limited, questions have been raised about whether whole-herd depopulation is a wise use of tax dollars, whether it can withstand animal welfare questions, and whether it is more effective than removal of test-responsive animals from a herd in which bTB has been diagnosed.

Study of Test and Remove Herds

At the 2007 meeting of the United States Animal Health Association (USAHA), the author made a presentation to the TB Committee on Michigan’s experience with Test and Remove (T & R) herds. He detailed the outcomes of five dairy herds in northeast Michigan that either had or were undergoing a T & R protocol for bTB. As a result of that presentation, a subcommittee was established by the chair of the USAHA TB committee on Test and Remove Policy Assessment and the author was named the chair of that subcommittee.

The subcommittee is composed of regulatory and university veterinarians and researchers and producer representatives from all states directly impacted by bTB. The subcommittee compiled data from all US herds that had gone through T & R protocols for bTB since 1985, the year of the outbreak in the El Paso, Texas area. While more herds were diagnosed as bTB infected during this period (in the period FY 1990-2008, there have been 77 affected beef herds and 44 affected dairy herds) only 15 herds in the US have undergone T & R since 1985, some of which were subsequently depopulated in the El Paso Milkshed Buyout (EPMB). The list of those herds, the year of initial diagnosis, approximate number of cattle and the number of head confirmed with the disease initially and totally is shown in Table 1.

An important observation is that the initial prevalence level of bTB in these herds is very low— less than 1 percent in herds greater than 200 head and less than 3 infected animals in herds of fewer than 200 head.

Issue of Within-herd Transmission

In 8 of the 15 herds, all bTB positive animals ever identified in the herd were diagnosed in the initial diagnostic test. That is, in just over half the T & R herds, subsequent testing did not reveal any evidence of within-herd transmission (WHT) or latent, undisclosed positive animals. All of these herds were test-negative in up to 18 whole herd tests over periods from 5 to 14 years thereafter (Table 2). Some of these herds are intact and continue to be tested annually.

In 7 of the 15 herds, bTB positive animals were subsequently diagnosed upon continued testing as part of the T & R program. Of those, six herds had animals detected within 4 years of the initial diagnosis. This is within the current quarantine period. Therefore, if all these herds had been under a herd plan that called for a minimum four year quarantine, almost all would have been detected prior to release of quarantine.

It is true that in the late 1980s, five herds in the T & R program had cattle diagnosed positive after they were released from quarantine. However, quarantine at that time, following 1980s Uniform Methods and Rules (UM&R), was as short as 14 months. Three of those herds had repeated diagnoses of positive cattle (cows and heifers) of the same DNA strain as the original infection over a period of up to 12 years. However, the total prevalence rate was still very low (less than 2%) in these herds and following the last positive animal detected, they went years and many negative WHTs without additional positive animals found. These herds were depopulated in the EPMB.

The key issue is detection of infected animals. There is still not a single test that has high enough sensitivity and specificity to identify all positive cattle. However, there have been changes in the T & R testing protocols that increase the probability of detecting any additional positive animals by using multiple tests, such as using gamma interferon testing in parallel with caudal fold testing (CFT) with the removal of any responders.

Effectiveness of Test & Remove

The data show that of the 15 herds that went through a T & R protocol, 6 have been released from quarantine and are still clear of bTB. Three of those have gone over 6 years since the last diagnosed animal. Two herds remain under quarantine and, pending a negative whole herd test in 2009, are scheduled for release from quarantine. The disease is believed to have been eliminated from these herds.

The other seven herds were depopulated in the EPMB. Before those herds were depopulated, all had been released from quarantine. Four of those Texas dairies had gone over 6 years without further diagnosed animals. It may be inferred that bTB was eliminated in these herds through T & R. Table 2 shows the number of years and number of whole herd tests that were negative since the last positive animal in these herds. While it cannot be said with 100% confidence that any of these herds were or are completely free of bTB, the confidence in their freedom is high and increases with time when testing is continued.

Risk of Spread of bTB from Test & Remove Herds

The January, 2005 UM&R prescribes a T & R program that keeps the herd quarantined for at least 4 years after initial diagnosis. When any subsequent animal is diagnosed, the clock resets on the quarantine period and the testing protocol. Herd plans prescribe rules for additions to herds as well as the removal of animals and products from the farm. A good herd plan should limit risks to other herds specific to the herd operation. It has not been shown that any other herd has been infected by bTB from these herds in T & R protocol whether during or off quarantine.

Relative Cost of Depopulation vs. Test & Remove

The costs of whole-herd depopulation have become prohibitive. Using actual expense figures over the period of 2003-2008 from New Mexico for T & R in a herd of 1500 cows where one positive animal was diagnosed in 2002 and another in 2005, the costs of T & R can be compared to what the indemnity alone would have been had this herd been depopulated. The cost of T & R can be estimated at $512,918 based on data from 2003-2008 including state and federal costs for caudal fold and gamma interferon testing, indemnity (190 head), and personnel. Conversely, a proposed herd depopulation offer (including indemnity only, no other costs such as personnel) amounts to $3,750,000. Those figures show that depopulation would have cost the government seven times more, while eliminating tremendous farm income at the same time. At the 2008 USAHA meeting, John Clifford, Deputy Administrator of USDA APHIS, said that the government can no longer afford to depopulate large herds, especially when only a few positive animals are found.

Strengthening Test & Remove

Test and Remove may be economically better for governments as well as economically better for the community, but is it safe enough for the cattle industry within a state and for neighboring states and trading partners? In addition, one must ask whether it is safe for people and for wildlife.

The subcommittee members believe that the answer lies in developing a strong herd plan with the owners of affected herds and then monitoring the compliance with the plan throughout the entire quarantine. While strong, the herd plan must also be practical and feasible in order to be credible and subsequently implemented by the producer. The subcommittee submitted a herd plan template to the USAHA TB Committee.

The actions required to reduce risk will depend on the nature of the risk to the herd. A thorough epidemiological investigation should be conducted to determine the risks. Every herd that is diagnosed with bTB suffered a breakdown of biosecurity that allowed the cattle to be exposed to the bacteria. Therefore, every dairy operator and herdsman in a T & R program needs to evaluate and strengthen their biosecurity practices.

Subcommittee Conclusions

Though only 15 herds have undergone T & R since 1985, the data examined are from more than 22,000 head of cattle in three states with 257 whole herd tests beginning with diagnosis. There were 405,659 individual animal CFTs in these herds over a period of 23 years.

Based on their examination of the data, the T & R subcommittee concluded that: 1) T & R can be effective in eliminating bTB from herds; 2) as time increases since positive animals are diagnosed in a T& R herd, confidence in having eliminated the disease increases; and, 3) safeguards can be built into herd plans to protect the industry, public and wildlife.

However, current USDA rules create a strong disincentive for states to allow T & R of infected herds. Herds are counted as affected herds throughout the 4.5-year period of quarantine. The count of affected herds is part of the definition for state/zone status and by allowing herds to go through T & R, states have put their status at risk of being downgraded. At the 2008 USAHA meeting, a policy change proposal presented by the T & R subcommittee that would reduce that disincentive was adopted by the TB Committee and the USAHA board. Clifford has indicated that USDA is seeking input for changes that would create greater flexibility in meeting program goals. USDA Listening Sessions in December, 2008 were an opportunity to hear from stakeholders.

Policies which create greater potential for T & R would not increase the risk to other herds or states, nor would it deter the US from reaching its goal of being bTB free. Rather, T & R provides an effective, practical and economical way to handle herds infected with bTB.

 

 

 

 

 

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