«Summary Outbreaks of FMD can either be controlled by stamping-out or (circle) culling, or by (ring-) vaccination around the outbreak area, or by a ...»
Culling versus vaccination: challenging a dogma in veterinary (FMD)
Simon J. Barteling¹ and Paul Sutmoller²
¹Consultant Veterinary Vaccines. address: Nieuwe Keizersgracht 438, 1018 VG, Amsterdam, The
Netherlands. E-mail: firstname.lastname@example.org
²Animal Health Consultant, former chief of Laboratories of the Panamerican Foot and Mouth Disease
Center PAHO/WHO. Present address:1502 Largo Road #101, Richmond, Virginia 23233, United
States of America E-mail: email@example.com Summary Outbreaks of FMD can either be controlled by stamping-out or (circle) culling, or by (ring-) vaccination around the outbreak area, or by a combination of the two methods. The pros and cons of the two methods are discussed. A major draw-back – next to many other disadvantages - of the massive circle culling is its contribution to spreading disease.
We challenge the dogma that “vaccination against FMD will prevent the symptoms but will not eradicate the disease”. Where outbreaks were controlled by consistent vaccination with a qualified vaccine the disease did not re-occur. Also, there are no documented cases where cattle vaccinated with a qualified vaccine, caused new outbreaks. Therefore, the risks posed by vaccinated carriers must be an acceptable, “close to zero” risk. Certainly, if used in combination with an anti-NSP test, vaccination should become the major tool in controlling outbreaks of FMD, without additional negative consequences for export trade.
Introduction The oldest way of controlling outbreaks of FMD is by stamping-out, the killing and disposal of all susceptible livestock on infected farms. Already in 1892 a United Kingdom Act of Parliament stipulated that all susceptible animals must be killed and also, that the farmers would be financially compensated.
Although from 1940 onwards vaccines have been developed that protect against the disease, so far the U.K., Ireland, Scandinavian countries, the USA, Canada and some other countries have always stuck to the stamping out policy. Circle culling – invented by epidemiologists/computer modellers – and applied during the 2001 outbreaks, can be considered an extension of the stamping-out approach.
Another method of controlling outbreaks is by (ring-) vaccination. However, in 2001 vaccination had considerable drawbacks for export trade unless vaccination was declared “suppressive” and the vaccinated animals were successively killed.
Here, we first consider the effectiveness of the two methods in controlling outbreaks. Then we will discuss risks of eventual carriers remaining in the vaccinated population and, what became almost a veterinary dogma that “vaccination against FMD will prevent the symptoms but will not eradicate the disease”. We will also discuss whether it is justified to punish – in terms of export trade - the use of (limited) ring- vaccination for controlling FMD outbreaks.
113 Stamping-out Traditionally, in FMD free countries, stamping-out has been the first option to eradicate the disease. As a first line of defense it is often quite successful, at least if the disease has not yet spread too widely and if the density of livestock in the area is relatively low. Also, during the first days of an outbreak a proper vaccine might not be available. The choice of the stampingout option should depend as well on the possibility of tracing dangerous contacts, political will and available resources.
In general, stamping-out consists also of the killing and disposal of all susceptible livestock of the outbreak farm and of “contact” farms that are most likely infected, followed by thorough cleaning and disinfection. If the outbreak farm is located at some distance from other farms and without intensive contacts, the slaughter of only infected premises - with surveillance of neighboring farms - might be adequate. In general, however, one must be “ahead” of the disease and, to that end, also slaughter “dangerous contact farms”. However, the latter are difficult to define and decisions can create feelings of arbitrariness and unfairness.
In the U.K. and in some other countries in the past the carcasses were burned or buried, however, this creates environmental problems and, therefore, rendering is preferred.
If stamping-out of the disease succeeds in a relative short period of time it may be the most economical way of dealing with the outbreak.
If the disease is already widespread or occurring in an area with a high-density livestock population it becomes increasingly difficult to bring an outbreak under control and “classical”
stamping-out has the following drawbacks:
- Staff must be available for detecting where the virus came from and where it might have spread in the mean time. Active and intensive surveillance is required in order to be “ahead of the disease” and to detect infection at an early stage. Such surveillance in itself represents a risk of spreading disease.
- Heavy equipment used in these operations is difficult to decontaminate and might be a source of infection e.g. by contamination of roads when being driven to another job or back home.
- Large numbers of contractors are involved who are not trained in disease containment.
Most of them will be from rural areas and might even live next door to farmers. By having social contact in the farming community they can themselves spread disease.
- Disposal of cadavers also presents a risk since virus in lesions, excrements and excretions is not rapidly destroyed and might be disseminated by transport of cadavers, by pyres, at burial sites or rendering plants. To our knowledge, neither existing transport systems for carcasses, nor the handling of the carcasses at the rendering plants are bio-secure (Barteling 2002).
- High speed of operations is needed with logistics that are very much dependent on local circumstances. This can hardly be foreseen in contingency plans and, therefore, in the beginning of outbreaks it will take time before the required speed is reached. This will matter more if during a number of years (e.g. ten years or more) a country had no experience of outbreaks or, like in the U.K. the disease is already widespread when it is detected.
Ring- (circle) culling So-called circle culling around infected premises (I.P.) as has been applied in the UK (and in the Netherlands) can be considered as an extension of usual stamping-out procedures. It has been “invented” by epidemiologists-modellers who make use of computer models on how 114 (fast) the disease spreads (Ferguson et al. 2001; Keeling et al. 2001). The aim of the circle is to eliminate incubating infections that may have spread from the outbreak farm(s) and to create a “fire break” zone around the outbreak. The diameter of the circle was based on the analysis of spread of FMD during the outbreak using computer models. However, the calculated distance of spread must include spread due largely to the culling process itself (see above). Also, important parameters such as virus dissemination by the involved species, number of animals involved, important parameters for the farms at risk (see below) and other factors are not incorporated.
The advantage is that the decision-making is simple and circle culling reduces much of the need for “tracing and anticipating” and surveillance. When the outbreaks are successfully brought under control, the current (economic) advantage is that the FMD-free OIE status can be obtained shortly after completion of the operation.
However, there are major draw-backs with respect to rapid disease control and other
- Even more than for “classical” stamping-out a military organization is needed to implement all the logistics for the required speed.
- For the creation of a fire-break zone most of the culled farms will not be infected.
However, some will be in the (undetected) pre-clinical infection stage. Because of the massive nature of the culling these farms and the equipment used for the hauling and destruction will not get the necessary attention for disinfecting with all the risks of further spread of disease.
- Most culled farms within the circle are not infected and do not represent a risk of further spread of the disease and, therefore, are culled unnecessarily. Consequently, one outbreak may cause the unnecessary deaths of large numbers of animals. For example, in The Netherlands there was an outbreak on a calf-fattening farm just outside the main infection zone. Although there was no single indication of virus spread, not even on the farm itself (approximately 500 calves), within the 2 km zone 70,000 animals had to be killed.
- The strict application of circle culling poses a threat to zoological collections and valuable (rare) breeding stock as well.
- The small risk represented by hobby farms and smallholdings is not taken into account.
- Massive killing and destruction of livestock is usually not done with adequate respect for animal welfare and bio-ethical principles.
- In contrast to control by vaccination (see below), in the outbreak area (s) the duration of a campaign cannot be predicted. A long drawn-out campaign is very disruptive for the rural society as a whole and includes also sectors like tourism. The rural community may fear the control measures more than the disease, and, because their animals are not protected by vaccination, will live under this fear for several months after the last case.
- An enormous serological surveillance exercise is often required to detect residual infection since new cases could easily restart the epidemic at its tail-end, particularly if movement controls are prematurely lifted.
- Last but not least, many culls represent a human tragedy and a traumatic experience not only for farmers and their families, but for many veterinarians and others in the rural societies as well. The risk-avoidance behaviour of farmers leads to social isolation and breakdown of the social-economic and trading patterns of rural communities.
115 If stamping-out is the method of choice, it should be based on the evaluation of how virus spreads (known risk factors). Certainly, the disease will not spread in a circular manner. The
principal routes of virus spread to be considered are by:
– animal movements from the farm, during the pre-clinical phase;
– estimated virus aerosol production (species and number of animals involved) and weather conditions (temperature, humidity, wind direction) on the days preceding the outbreak (Donaldson et al. 2001);
– animal proximity at farm boundaries, e.g. grazing in adjacent fields;
– size and animal species of neighbouring holdings e.g. a large cattle holding ( large air sampling volume) represents a greater risk of inhaling a minimal infectious dose” (Sellers
1971) and becoming a new generator of disease than a few sheep or goat of a nearby hobby farmer;
– people (veterinarians, inseminators, visiting farmers, cleaning and disinfecting crews, etc.) – vehicles (e.g. animal transport, machinery) contaminating roads that are used by the farmer’s family and associates;
Because, in principle, every culling may represent a human and animal welfare tragedy each farm should be judged on its own (risk) parameters. If possible, it would be very helpful if epidemiological factors and associated risks could be incorporated into a computer model to enable rapid risk assessment and decision making for each farm Ring vaccination Instead of killing all susceptible livestock around an I.P. a “fire break zone” can be created by vaccination.
The factors determining expected spread (as mentioned above) also should determine the size of the “ring” or area around an IP. In addition, to get “ahead” of the disease one must anticipate the natural spread over the period that is needed to organize the vaccination. In general, this will be a few days plus 4-5 days needed before animals become protected by the induced immunity.
In comparison to culling the method is relatively cheap and, therefore larger areas can be vaccinated in a relatively short period of time.
The logistics of a vaccination campaign are rather simple. It can be carried out on a large scale by a limited number of (trained) staff under full bio-safety conditions or by farmers and trained farm hands as is common practice in South America. The latter method has the advantage that there will be no risk of cross-contamination between farms caused by the visiting vaccinators.
When a potent vaccine is used, outbreaks in the vaccinated zone/ring will usually cease within a week when effective herd immunity is reached.
Therefore, ring-vaccinations should be performed without delay and should include all susceptible species. Preferably, the vaccination should be carried out from the outside of the “ring” towards the centre, the I.P. To protect the most endangered farms as soon as possible, vaccination should proceed simultaneously from the I.P. towards the outside. In the immediate vicinity of the outbreak farm, the large (cattle) holdings should be vaccinated first 116 because potentially, those are the largest “aerosol collectors” and, therefore, will be most at risk.
Ring/emergency vaccinations should be included in any contingency plan:
- to stop the disease from spreading;
- to avoid all of the above mentioned disadvantages of stamping-out;
- to prevent the suffering of animals as much as possible;
- to ensure that a few weeks after vaccination life in the affected area can resume its normal course, with minimal socio-economic consequences.
Concerns have been raised with regard to human consumption of meat or other products from vaccinated animals. However, there is not any reason to object to human consumption of the meat of vaccinated animals. In Europe meat from vaccinated animals has been consumed for over fifty years, including the meat originating from South America.