Control of the Deliberate Spread of Foot-and-Mouth Disease Virus

Abstract

Foot-and-mouth disease (FMD) is one of the most feared of transboundary animal diseases. Accidental or deliberate release of the causative agent can have both direct and indirect effects that result in massive economic losses and disruption. The direct effects of an FMD outbreak include immediate losses to agricultural production and disruption of local economies, while the indirect effects are mainly related to disease control measures such as restriction of market access at local and global levels and the high costs of disease control. To improve the capacity of the European Union (EU) to counter animal bioterrorism threats, AniBioThreat was launched with a special focus on threats to living animals, feed, and food of animal origin. As part of this project, several zoonotic or animal pathogenic agents are considered from different perspectives. FMD virus was selected as one agent to be scrutinized because it is highly contagious and an outbreak can have a severe economic impact. Ways to fight a deliberate outbreak can be demonstrated through the example of FMD. In this article, the virology and epidemiology of FMD virus are discussed with special attention to the related law enforcement aspects.

The threat of bioterrorism is a real one that must be taken seriously, at least in part because it is clandestine. So-called overt acts of terrorism, such as bombings, hostage takings, and assassinations, are more familiar to law enforcement agencies, enabling them to respond in traditional ways to such events. Biological agents used by terrorists are likely to be applied covertly, which poses a unique challenge: Biological agents are generally not visible, and the time from exposure to onset of illness may vary from days to a few weeks. This gives the perpetrators time to leave the scene and makes it difficult to collect evidence or establish a chain of custody.

Bioterrorism is defined as the intentional release of biological agents or toxins for the purpose of harming or killing humans, animals, or plants. In this article, we review bioterrorism targeted at animals. Attacks against animal health and animal production in general can cause economic losses that may intimidate or coerce a government or a civilian population and may be thought to further the political or social objectives of the terrorists.

Foot-and-mouth disease (FMD) is one of the most feared of transboundary animal diseases; it has the potential to be used by bioterrorists because it is highly contagious and an outbreak can have a great economic impact. The direct effects of an FMD outbreak include immediate losses to agricultural production and disruption of local economies, while the indirect effects are mainly related to disease control measures such as restriction of market access at local and global levels, the high costs of disease control, and a decline in tourism. Most economic loss is caused by mandatory epidemiologic countermeasures, such as the need to cull large numbers of healthy but susceptible animals that may be potentially infected by FMD virus (FMDV).

The economic significance of FMD is underlined by some recent examples. The estimated costs of natural FMD outbreaks in the UK in 2001 were approximately £3.8 million (US$5.186 million).^1,2 Even the threat of FMDV without an actual outbreak may be costly. The government of New Zealand was threatened by hoaxes that claimed that FMDV had been deliberately released on the island of Waiheke. New Zealand is FMD free without vaccination, so the threat was taken seriously. The reactions resulted in costs between 1.5 and 2 million NZD, which included a 14-day-long operation in which 12,000 animals were located and monitored by more than 100 experts from the Ministry of Agriculture and Forestry, the New Zealand Food Safety Authority, and operational staff from other organizations. ³

Efficient countermeasures against a deliberate spread of FMDV need a comprehensive, multidisciplinary, and multisectoral approach that includes private and government veterinary services, law enforcement agencies, and crisis management organizations. This kind of cooperation is necessary because an appropriate reaction to bioterrorism requires diverse competencies and expertise including counterterrorism, animal health protection, consequence management, laboratory security, and scientific and academic research. Veterinary and law enforcement authorities share a common interest as both are needed to clarify the reasons behind an outbreak, reduce economic losses, and prevent further outbreaks. This kind of cooperation is made easier when police are present at an FMD outbreak to ensure support for establishing epidemiologic zones.

In this article we focus on the biological and epidemiologic features of FMDV, and related information that could be useful to law enforcement agencies is discussed from the veterinary perspective.

Detection and Containment of the Virus

FMDV belongs to the family Picornaviridae, genus Aphthovirus. The virion is nonenveloped with a diameter of 22-30 nm. The genome is a positive single-stranded RNA of about 8,400 nucleotides, which encodes for 4 structural proteins, such as VP1, that form the virion, as well as for nonstructural proteins (NSPs) such as the polyprotein 3ABC and 3D. FMDV has 7 immunologically distinct serotypes: A, O, C, SAT1, SAT2, SAT3, and Asia1. More than 60 subtypes are classified in the 7 serotypes, and new subtypes occasionally arise spontaneously.^4-6

Viruses belonging to different serotypes do not trigger cross-immunity against each other; for this reason, there is no universal vaccine that can confer protection against all serotypes and subtypes. All domestic and wild cloven-hoofed animals are susceptible to FMDV, but rats, mice, and guinea pigs have also been demonstrated to be susceptible. Camelidae (camels, dromedaries, llamas, and vicunas) have low susceptibility.^7,8 Infections in humans are very rare and of minor clinical significance. ³ The virus can be transmitted in various ways including direct or indirect contact (droplets); passive vectors, both animate (eg, humans) and inanimate (eg, vehicles, implements); and by air, especially in temperate zones (up to 60 km overland and 300 km by sea). ⁸ Depending on the affected animal species, the biological features of FMDV (ie, incubation time, detectability, persistence) may vary broadly, and as a consequence, different control measures should be applied. The incubation period of FMD depends on the infected animal species: 2 to 14 days in cattle, 3 to 8 days in sheep, and 1 to 4 days in pigs.

For cattle, the infectious period with virus shedding occurs only in the acute stage of the disease. Carriers of FMDV can occur, but their low virus excretion over a prolonged infectious period does not contribute to an in-herd outbreak. ⁹

Sheep and goats have a longer infectious period, ¹⁰ a lower reproduction number, ¹¹ and a higher probability of subclinical infection ¹¹ compared to cattle, which may lead to delayed detection. In addition to the longer incubation time, infected sheep show only a few lesions; as a consequence, the infection may persist in a sheep herd, and the number of clinical cases may remain below the detection limit. ¹²

Pigs have a shorter latent and incubation period than cattle, but pigs can produce approximately 3,000 times more airborne FMDV particles than do cattle or sheep; thus, swine can considerably boost the disease. Nevertheless, the aerosol production of infectious FMDV by pigs differs strain by strain.^9,13,14

Clinical symptoms of the disease can include fever, blisters on the feet and on the mucosa of the mouth, loss of appetite, drooling, and lameness. ⁸ Surveillance for clinical signs of FMD still plays a significant role. If an experienced person examines large numbers of susceptible animals, this can serve as a solid basis for the identification of the disease. FMD can be easily identified in cattle and pigs, but it is especially hard to recognize in small ruminants because of the subtle and nonspecific symptoms.

In the case of a covert bioterrorist act, the reaction may start with the veterinary authority, as would be the case in a natural outbreak. It would be beneficial to conduct a joint investigation by veterinary authorities and law enforcement bodies. Joint interviews of farmers and stakeholders could have several advantages, including reducing collection and documentation of conflicting information, simultaneous access to information, a multidisciplinary interview perspective, and fewer opportunities for misunderstandings. In addition, the veterinary authority alone might not be able to prove that the FMDV had been spread deliberately. Intentional release of infectious agents can be traced back by investigating the circumstances of the disease spread, including how the outbreak started and whether the outbreak can be attributed to infected imported animals, animal products, or a breach of animal health regulations. Suspicions may be raised if a disease appears in a geographical location where it is not usually seen or at a time of year when it would not be expected. Law enforcement agencies should have a list of infectious agents that have bioterrorism potential, and investigation and joint interviews can be started immediately when a disease involving one of these agents is diagnosed. One task of the AniBioThreat project is to compile such a list of agents with appropriate supporting data and provide it to law enforcement bodies.

Clinical suspicions should be confirmed by laboratory testing. A relatively long time may elapse between clinical findings and laboratory confirmation of the disease because of the logistics of sending samples to central laboratories; in most cases, regional laboratories do not have the expertise, equipment, facilities, or even the legal right to diagnose exotic diseases. Based on the observations from the 2001 FMD outbreak in the Netherlands and the UK, ¹⁵ the time between infection and detection in a cattle herd was about 8 days. Another possible reason that laboratory test results take time is because of the significance of FMDV and its grave consequences: The results of laboratory tests must be sound and well-confirmed, proving unequivocally that infectious, intact FMD virions are circulating in a herd. It is of paramount importance that academic researchers develop state-of-the-art diagnostic methods that can reduce the time between the first detection and disease confirmation.

FMDV can be shed by breath, saliva, feces, urine, milk, and semen for up to 4 days before clinical signs appear. Tests to detect the early stages of infection could prevent the virus from spreading undetected, but such tests are unavailable or in short supply. The role of differential diagnosis is essential, as FMD may manifest clinical signs similar to vesicular stomatitis, swine vesicular disease, and vesicular exanthema of swine. ¹⁶ The conventional detection of the virus is based on either direct (ie, virus isolation, mice or guinea pig inoculation, ELISA, and RT-PCR) or indirect methods (ie, virus neutralization and ELISA).

A specific diagnostic task is the differentiation of vaccinated animals from infected ones. This approach is called DIVA (differentiating infected from vaccinated animals). The concept of measuring antibodies targeted to NSP as a means of distinguishing FMDV-infected from vaccinated animals is almost 20 years old, ¹⁷ and the presence of an internal antigen of the FMD virion like NSPs was revealed more than 40 years ago. ¹⁸

Viral replication during infection results in the production of a number of NSPs, some of which are antigenic. Available ELISA techniques are used to detect specific NSPs. The so-called 3ABC-ELISAs show relatively high sensitivity and specificity. The sensitivity is close to 100% for infected animals, provided more than 8 to 15 days have passed after infection. However, in a worst case scenario, in which vaccinated animals have been exposed to the virus and become infected without developing clinical signs, the sensitivity will be lower—approximately 90%. ¹⁹ The 3ABC-ELISA should then be used for testing animals at the herd or flock level rather than the individual animal level. ²⁰ Furthermore, although the 3ABC-ELISA has been validated in some laboratories, additional technical improvements are required so that its use can be extended to regional laboratories, as first responders could be essential in early detection.

It is also important to note that the collection of traditional forensic evidence may be difficult, as items of interest may be contaminated and successful decontamination may not be possible without destroying critical evidence. For proper disinfection, the particular features of the virus have to be considered carefully. Depending on the material, such as feces, hay, or wool, the survival capacities for FMDV in the environment are estimated to be up to 3 months. Low temperatures and the protection of the virus from drying and sunlight can increase the survival time. ²¹ The virus is pH-sensitive at pH below 6 or above 9 to 11.^8,21,22 Accordingly, most of the recommended chemical disinfectants are acids, such as citric acid (0.2%-3%), peracetic acid (0.4%-2%), and formic acid (4%),^8,21-24 or alkalis like sodium hydroxide (2%) and sodium carbonate (4%).^6,19-21 In addition to pH modifiers, oxidizing agents such as Virkon^® or sodium hypochlorite (3%) are also recommended.^8,21,23,24 The use of formalin or formaldehyde gas is advised for special applications such as the decontamination of electrical equipment,^23,24 straw and forage, ²⁵ or wool and hair. ⁸ Appropriate contact times for the disinfectants depend on the substance used, the contaminated material, and the stage of the decontamination process. For the final surface disinfection, contact times of up to 2 hours should be sufficient.^8,21,23

FMDV is easily transmitted by air, which means that cleaning actions bear the risk of a further spread of the virus by reaerosolization. Therefore, a preliminary disinfection prior to cleaning and final disinfection is recommended. Because of the decreased efficacy of disinfectants on polluted surfaces, prolonged contact times are required for a preliminary disinfection (eg, 4 h or 24 h).^22,24 Materials such as bedding, manure, and slurry need special treatment for the inactivation of FMDV. Solid components should be stacked to heat under the addition of 100 kg granulated quick lime on 1 m³ material for at least 42 days.^22,25 Slurry can be treated with 40% lime milk (60 l/m³), formalin (15 kg/m³), or 50% sodium hydroxide (30 l/m³) for a minimum of 4 days or stored for a period of at least 42 days up to 6 months.^22,25

Control Strategies for an FMD Epidemic

Current international norms ²⁶ classify countries as FMD-free, FMD-free with vaccination, or FMD-infected. Despite the fact that 65 countries are FMD-free without vaccination, FMD still remains one of the most widespread transborder animal diseases; more than 100 countries have not been designated as FMD-free by the World Organisation for Animal Health (OIE). ²⁶ These countries are mainly located in Africa, Asia, and the Middle East, and many of them are listed by the US Department of State as countries that harbor terrorist organizations or offer direct or indirect support to terrorist organizations. ²⁷ These groups may have easier access to this deadly pathogen as a result of natural outbreaks.

Currently, EU member states are classified as FMD-free without vaccination, but there is always the risk that FMD may be introduced again in the EU. ⁹ Trade connections or even infected food products brought in by tourists to European harbors or airports may pose the threat of reintroducing FMDV. Feeding pigs with contaminated meat product scraps may also play a significant role in bringing FMDV into a country, as was the case in the 2001 outbreak in the United Kingdom.

Compulsory prophylactic vaccination of cattle against FMD ceased in the EU during 1990-91 according to Council Directive 85/511/EEC as amended by Council Directive 90/423/EEC, Commission Decision 93/590/EEC, and Council Decision 91/666/EEC. ²⁵ Subsequently the incidence of FMD in Europe was reduced below the critical limit due to vaccination campaigns.

In order to control potential outbreaks and eliminate the possibility that the virus could be reintroduced into the EU, administrative countermeasures were adopted that can be classified as preventive and reactive. The preventive measures include import policy, quarantine rules, feed and swill treatments, action plans, training sessions, and regular meetings of expert committees. The reactive approach comprises the “stamping-out” policy, which is based on the strategy of killing infected herds, appropriately disposing of potentially infected material, and controlling the movements of live animals, meat, meat products, milk, milk products, animal by-products, people, vehicles, farm fomites, and any other substance liable to transmit the virus. In addition to slaughtering animals, the premises should be cleaned, disinfected, and not restocked with susceptible animals for a defined period. ⁸ The stamping-out policy was also supported by the fact that vaccines used at that time occasionally had incomplete inactivation, which led to dissemination of the disease caused by vaccination. A further fear was that vaccinated animal groups might play a role in disease spread because vaccinated cattle could become virus carriers (pigs cannot become carriers). For this reason “FMD-free without vaccination” status is superior in the OIE classification to “FMD-free with vaccination” status in terms of allowing these countries to export live animals or animal products to any country in the world, while countries that are FMD-free with vaccination may face certain trade restrictions. Stamping out is the best way for a country to rapidly regain its disease-free status according to international norms established by the OIE. ⁷

To prevent the direct and indirect economic losses caused by the disease, different vaccination strategies can be applied. There are 2 main vaccination strategies: prophylactic (preventive) vaccination and protective (reactive) vaccination.

Prophylactic vaccination is carried out in, for example, countries such as those in southern Africa or South America where FMDV is enzootic. With prophylactic vaccination, buffer (or barrier) and ring vaccinations can be subgrouped. Buffer vaccination can be applied against an outbreak of disease in a neighboring country or region along the border of a country or region at risk. For instance, on several occasions since the 1960s, parts of Bulgaria and Greece bordering on Turkey and in the Turkish Thrace were buffer vaccinated against the threat of both endemic and exotic FMD spread across the Bosporus from Asia into Europe. ²⁸

Protective vaccination includes the subgroups of ring vaccination and suppressive vaccination. Ring vaccination is usually applied in an FMDV-free country, which does not normally use vaccination and where the aim of ring vaccination is to localize the disease. To accomplish this goal, a vaccination zone is established in which animals are vaccinated surrounding the focus of an outbreak and including a surveillance zone 10 km in radius.

A special type of emergency vaccination is the suppressive (or dampening down or “vaccinate to kill”) vaccination. It is usually applied in the outbreak region with the aim to reduce the amount of circulating virus. Vaccinated animals must be registered and permanently marked. Subsequent to the onset of the fully developed immunity induced by vaccination, which usually takes approximately 2 weeks, animals should be slaughtered in slaughterhouses located in the vaccination zone. This type of emergency vaccination was applied during the outbreak of FMD in the Netherlands in 2001. ²⁹

Regardless of the vaccination strategy used, it should be noted that, during the 14 days following the vaccination of cattle and 7 days following the vaccination of pigs, virus transmission can occur from those species to susceptible animals that come in contact with them.^30,31

To carry out a vaccination campaign against FMDV, it is necessary to have the proper amount of ready-to-use vaccine batches available in reserve. The storage of conventionally formulated FMD vaccines in a strategic reserve is expensive, because the vaccine must be replaced every 18 months due to its limited shelf-life. An alternative is the well-established indefinite storage of concentrated, inactivated FMDV antigen at ultralow temperatures over liquid nitrogen, which can be formulated rapidly into vaccine when required. Further advantages of the antigen bank are that antigens can be formulated in the most suitable adjuvant based on the application field and the quality of the antigens can be ensured by regular monitoring and testing. Consequently, many countries have decided to maintain a strategic stock of unformulated FMDV antigens in an antigen bank.

In 1982, the United States, Canada, and Mexico signed a collaborative agreement establishing the North American FMD Vaccine Bank. In 1985 Finland, Ireland, Norway, Sweden, the United Kingdom, Australia, and New Zealand established the International Vaccine Bank (IVB), which is located at the Institute for Animal Health (IAH), Pirbright, UK. In 1995 Malta joined the IVB as an associate member. The European Union (EU) Directorate General for Health and Consumers established the EU vaccine bank in 1992, and 26 countries have arrangements with this bank for the supply of vaccine or antigen. As a strategic supplement to the EU bank, many European countries have their own national antigen stocks produced by a private company on a contractual basis and located in Pirbright. ³²

All of these banks can offer rapid access to efficient and potent inactivated vaccines in case of emergency. The EU vaccine bank is able to formulate oil-adjuvanted vaccines within 3 days of receiving an inquiry; AL(OH)₄ adjuvanted vaccine can be produced in 4 days. It is intended that, in case of an FMDV outbreak, an authorized vaccine should be used in the EU if possible. This approach is clearly reflected in the Committee for Medical Products for Veterinary Use (CVMP) guideline dealing with the requirements of multistrain dossiers. A multistrain dossier comprises a wide range of vaccine strains combining the specific information of the strains with the general information of the other aspects. This enables a rapid vaccine manufacture using an already authorized formulation as a “frame” together with a vaccine strain, which is selected in the current and particular disease situation. ³³ To obtain the maximum protection, the selected antigens of FMDV serotypes for the development of vaccines should regularly be reevaluated based on the current global FMD situation, since dominance, emergence, and decline of certain FMDV serotypes may occur.

The Decision to Implement Emergency Vaccination

In the EU the manufacture, sale, and control of FMD vaccines and vaccination policy are comprehensively regulated by Council Directive 2003/85/EC ²⁵ and the Strategy for Emergency Vaccination Against Foot and Mouth Disease (FMD). ³⁴ In addition to the advantages of the nonvaccination policy in the EU, stamping out requires enormous technical and logistical capacities, which may be overtaken by the rapid spread of the disease, as was the case in the UK epidemic in 2001. In a similar case, vaccination may be the most appropriate and practical approach to curbing the epidemic. Therefore, Council Directive 90/423 (art. 13.3) permits the use of emergency vaccination as an adjunct to the control and eradication measures. The rapid and objective assessment of the determining parameters is crucial for the decision to commence a vaccination program. Thus, experts have to consider and carefully weigh several factors.

The decision regarding vaccination must be based on the serious evaluation of the following factors:

• size of affected area;

• affected species and number of animals;

• results of epidemiologic analysis of the way the disease was introduced, the strain(s) of FMDV, and potential directions of disease spread; and

• expert opinion of a meteorologist about the expected weather conditions and direction of wind to assess the significance of airborne spread.

The size and shape of the vaccination zone should be established according to the outbreak conditions. In general, when other influencing factors have not surfaced, a circle 8 km in radius should be established as a vaccination zone, the size and shape of which will be influenced by the progress of the disease; thus, the final size of the vaccination zone usually cannot be estimated at the beginning of the outbreak. The border of the vaccination zone can be a river, lake, railway line, or other physical barrier. It is essential that vaccination in the established zone be carried out within 5 days.

In addition, logistical conditions and human and material resources for diagnostics, slaughtering, decontamination, transport, and rendering must be considered along with the location of these capacities (Table 1). The criteria for emergency vaccination should be assessed on a case-by-case basis. It is essential to have proper communication (red lines or red numbers) among the stakeholders, but farmers and the public must also be equally informed about steps needed to combat the FMD outbreak and about compensation, closed roads, and affected traffic. All these measures serve to maintain public support and cooperation and to reduce losses.

Table 1.

Criteria for Consideration in Decision Making Related to Protective Emergency Vaccination

Criteria	For Vaccination	Against Vaccination
Population density of susceptible animals a	High	Low
Clinically affected species	Significant number of pigs involved	Predominantly ruminants
Movement of potentially infected animals or products out of the protection zone b	Evidence	No evidence
Predicted airborne spread of virus from infected premises	High	Low or absent
Suitable vaccine c	Available	Not available
Origin of outbreaks (traceability)	Unknown	Known
Incidence slope of outbreaks d	Rising rapidly	Shallow or slow rise
Distribution of outbreaks a	Widespread	Restricted
Public reaction to total stamping out policy	Strong	Weak
Acceptance of regionalization after vaccination	Yes	No

a

See Report of the Scientific Veterinary Committee of 14 November 1996, “The feasibility of identifying densely populated livestock areas in the Community that pose a particular high risk of major disease epidemics” doc ref VI/1986/96 final.

b

The presence of susceptible wildlife in the area should be taken into account.

c

High potency for all species and containing appropriate type/strain antigen.

d

An analytic expression (formula) that provides insight into the contribution of different parameters to the effectiveness of the control measures has been proposed. ³⁵ The analysis is based on the “basic reproduction ratio,” R₀ (the average number of animals/herds infected by 1 infectious animal/herd), for which the following threshold condition holds: when R₀ >1, the infection can spread through the population; when R₀ <1, infection will eventually die out. Basic reproduction ratios typical of foot-and-mouth disease epidemics in the UK have been reported. ³⁶

It is advisable for veterinary authorities to take the lead in using social media. They may already have a presence on social media sites, and civilians can also use this communication channel to disseminate information rapidly. Cooperation among several bodies—including the meteorological service to analyze the chance of airborne spread, veterinary authorities and police for securing the vaccination zone, and local vets for the actual vaccination—can profoundly increase the chances of effectively combating FMD. Personal relationships among experts from different authorities clearly improve cooperation. For this reason simulation exercises and contingency plans are essential to organizing and practicing the required complex, multidisciplinary collaboration. Since 2002, the OIE has periodically published information about simulation exercises carried out in OIE member states.

Conclusion

To combat bioterrorism efficiently, various agencies should be involved that have diverse expertise and competencies, such as counterterrorism, animal health protection, consequence management, laboratory security, and scientific and academic research. “To prevent and to be prepared” means that, in order to have effective collaboration, well-established common practice, regular simulation exercises, contingency plans, cooperation agreements, and the legal framework to act should be established in advance. This kind of partnership among agencies is essential. For example, police should consult appropriate authorities to determine safety or personal protective equipment needs prior to handling evidence. It would also be advisable for police to have a list of diseases with bioterrorism potential; a diagnosis of such a disease may automatically trigger a police investigation to clarify the circumstances of the outbreak.

As quarantine enforcement is the responsibility of law enforcement services, they usually are present on site at outbreaks. If the outbreak is deemed to be deliberate, a law enforcement investigation should be opened and planning should begin on how to collect evidence in a contaminated environment. This activity should be carried out parallel to veterinary epidemiologic countermeasures as all authorities have a responsibility and interest in protecting the rest of the animals and reducing further losses. Both veterinary authorities and law enforcement agencies are interested in reducing further damage. Thus, stamping out policy is used in the EU to combat FMD, but vaccination may be possible if the epidemiology indicates such a countermeasure. An in-depth and objective analysis of determining parameters should be carried out before commencing a vaccination program. If the overall analysis of these parameters supports a program of protective emergency vaccination, then the program must be implemented without delay. Any delay in decision making and use of countermeasures may lead to a wider spread of the disease.