Discussion Paper on the risks posed by FMD carriers occurring amongst vaccinated cattle
.... in Uruguay recurrent disease did not occur even though the extensive outbreak in 2001 was controlled by vaccination of cattle only, sharing the pastures with young cattle, pigs and millions of unvaccinated sheep.
Thus, although all available evidence indicates that the probability of transmission of FMD by vaccinated carriers to susceptible livestock is extremely low – in fact there are no well-documented cases - still this risk is perceived as serious and justifies a longer ban on export trade than when outbreaks are not controlled by stamping-out.
It is this perception that in its consequences has led to the killing of millions of cattle and sheep (in the U.K.) and to the killing of hundreds of thousands healthy vaccinated livestock (in The Netherlands).
However, levels of risk – not perceived risks - must be considered when developing regulations and guidelines for the international movement and trade in animals and animal products. It must also be the most important consideration when deciding on the various options to deal with an outbreak.
Export restrictions of animals and animal products place a heavy penalty on the use of FMD vaccine. As a consequence countries with an important export of livestock or livestock products have - for the earliest regaining of their FMD free status - omitted the use of vaccine after an incursion of FMD. If vaccination had been used, healthy vaccinated cattle were slaughtered in order to avoid long export bans. It is curious that vaccinated pigs also must be destroyed even though pigs have not been shown to become carriers.
However, in all those cases where outbreaks of FMD were controlled by vaccination without slaughter of the vaccinated animals no recurrent FMD occurred caused by vaccinated carriers (Barteling and Sutmoller 2002). Also, products from these animals went into the normal consumption circuit without causing new outbreaks. Sutmoller & Casas, 2003 concluded that with adequate protocols products from vaccinated animals represent an acceptable, “close to zero” animal health risk.
By further analysis of the outbreak in Uruguay (Sutmoller et al., 2003) we can try to get some insight in the size of the risk represented by carriers.
In short, during 2001 Uruguay suffered an extensive outbreak of FMD with over 2057 infected farms among total of 47,057 livestock farms of which 28.795 had cattle and sheep. Only in the first cases stamping-out was applied (total of about 7000 animals were killed) with an immediate movement control of all livestock. At the height of the epidemic there were 40-60 new cases per day with some 20.000 animals. Vaccination was started within a week after the start of the outbreak, first as ring vaccination of the cattle population, but soon vaccination was extended to whole cattle population. Sheep were not included in these vaccinations, even though cattle and sheep used the same pastures. After the first vaccination round the movement restrictions were relaxed, followed by a revaccination of cattle. The epidemic lasted for a total of 8 months.
(For graph see original pdf file http://www.fao.org/AG/AGAInfo/commissions/docs/greece04/App63.pdf)
According several authors (a.o.: Salt 1993, Sutmoller & Gaggero, Sutmoller et al. 1967) it is most likely that about 50- 90% of the susceptible cattle that became clinically diseased on the 2057 infected farms developed the carrier state. In addition there must have been contacts with the clinically affected animals, both cattle and sheep that also resulted in carriers. This means that we can safely assume that during the epidemic some 50.000 carriers have been generated. These carriers used common pastures with unvaccinated sheep (often the sheep/cattle ratio is 1:1, but often higher) and in several districts roaming pig are numerous. Also, in any vaccinated population there are calves with decreasing maternal antibody levels becoming susceptible for the disease.Still there was no recurrent disease in the year following the last case. From that we can conclude that one carrier animal represents a risk of transmitting disease in Uruguay was less than 1: 50.000. Of course this risk decreases with time.
Systematic vaccination of the cattle population as done in Uruguay also will accelerate the decrease of the number of carriers.
From all the other outbreaks occurring globally during the past decades that were controlled by vaccination - all with no incidents of recurrent disease - we may conclude that likely this risk is even less
In a relatively limited outbreak situation - like the one in The Netherlands – that is rapidly controlled the number of vaccinated carriers will be small when by stamping out of diseased herds is being applied together with immediate stand-still of all livestock movements and ring or zonal vaccination. Elbers et al. (2003) made a sero-survey of wildlife in the epidemic area. For that survey samples were tested from suspect wild deer and farmed deer thought to be at risk or possibly exposed to FMD virus. All samples were negative, but there still the existence of a small number of carriers in the cattle population can not be excluded.
If in the Netherlands the vaccinated animals (and - for arguments’ sake – 100 carriers) had been left alive - then the likelihood of carriers causing a recurrence of FMD would be 1:500 (one outbreak as a consequence of 500 epidemics of the size of the 2001 outbreak). However, in view of the findings of Elbers et al. (2003) the total risk may be much smaller – how much smaller we don’t know.
This risk could have been further reduced by screening for potential carriers by an a-NSP test and removing potential carriers from the herds. The remaining carrier risk would have been magnitudes smaller and of the order of other accepted FMD risks. Therefore, the use of vaccine should not be hampered by additional trade restrictions, a conclusion that was already made by other arguments
(Barteling and Sutmoller, 2002). Also, if rules for export trade had been based on risk evaluation, in the Netherlands the destruction of some 250,000 healthy vaccinated cattle would not have been necessary
In addition, whatever timeframe for the waiting periods to regain the FMD-free status would be agreed upon, it is be crucial that after any outbreak, the veterinary service should - to the satisfaction of the international trading community - show the absence of FMD virus circulation, before normal export can be resumed. However, trade in animal products can be resumed as soon as the survey is satisfactorily completed and the absence of FMD virus circulation has bee proven.
Conclusions and Recommendations
The subjects that have been considered in this discussion paper can be summarized as follows:
•Science-based risk assessments should be made to compare the different risks associated with different eradication methods trad in animal and animal products.
•For illegally imported meat Europe may run a risk of one outbreak within 4 to 100 years with a most likely probability of one outbreak occurring in 13 years.
•For the importation of 250,000 ton of meat from South America by the EU, produced in accordance with OIE rules and guidelines, the probability is extremely low (less than 1:100,000) that this volume contains meat from at least one infected herd. Carriers do not contribute to that risk.
•One carrier animal represents a risk of transmitting FMD to susceptible contact animals that is likely much less than 1:50,000 (there are no well documented cases of transmission).
•Considering that the risk posed by vaccinated carriers is extremely low and considering the levels of risks that Europe of illegally and illegally imported meat has accepted, the policy of restrictions on export trade when part of the livestock population is vaccinated for controlling outbreaks should be reconsidered.
•If serological surveys after an outbreak are carried out rapidly and efficiently, it shows the international community, that veterinary and laboratory services are efficient and well-organized.
Because this is one of the most important risk reduction factors within the risk equation, this element should be fundamental for recovery or obtaining the FMD-free status. Export restrictions should be lifted as soon as the tests have been carried out with negative results.
If the vaccination option is used at an early stage, it will cause the least disruption of social and economic life. If outbreaks are kept limited, e.g. by vaccination, with limited consequences for export trade, it will stimulate veterinary services and stakeholders to notify the disease and to take proper action in the earliest stage possible. This will be another element to safeguard the international community from spread of this disease.
Also, we strongly recommend to investigate whether carrier bulls are able to transmit disease by the sexual route.
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