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Foot-and-Mouth Disease in Pigs: Current Epidemiological Situation

6 April 2012, at 12:00am

A review of the aetiology, transmission, diagnosis, prevention and control of foot and mouth disease (FMD) were outlined by Emilio A. León at the Institute of Pathobiology at Castelar, Argentina, at the International Symposium on Emerging and Re-Emerging Diseases in Pigs in 2011.

Introduction

Foot and mouth disease (FMD) is a global problem, affecting productivity and trade where it is endemic, and requiring strong investments in preventing measures and surveillance where it has been eradicated.

According to the Manual of Diagnostic Tests and Vaccines for Terrestrial Animals of OIE (2010) FMD is the most contagious disease of mammals and has a great potential for causing severe economic loss in susceptible animals.

Aetiology

FMD is caused by FMD virus (FMDV), which belongs to the genus Ahtovirus, family Picornaviridae. There are seven serotypes of FMDV namely, O, A, C, SAT 1, SAT 2, SAT 3 and Asia 1. Infection with one serotype does not confer immunity against another.

Some FMDV strains have a pronounced predilection for one livestock species or another.

Susceptible species

All members of the order Artiodactyla (cloven-hooved mammals) can be infected by FMDV, as well as a few species in other orders.

Cattle are normally the most frequently involved in epidemics. Small ruminants, deer and buffalos may be naturally infected. However, they show little or none clinical signs.

The disease is generally most severe in cattle and pigs. Under certain conditions, pigs may play an important role on the transmission of the agent.

Transmission

Pigs may become infected with FMDV by eating contaminated products, by direct contact with another infected animal, or by being placed in a heavily contaminated environment.

Probably the respiratory route is the more usual portal of entry for pigs, even if they may require as much as 600 times more than the exposure to aerosol virus required by a bovine or an ovine, to cause infection. On the other hand, pigs are much more susceptible to infection by the oral route than are ruminants.

Excretion of the virus can begin up to four days before the onset of clinical signs. This is of great epidemiological significance. The agent is excreted in large quantities in expired air, in all secretions and excretions and from ruptured vesicles. Pigs may liberate vast quantities of airborne virus in their expired breath, about 3,000 times as much as cattle.

Once infection is established within a pig herd, transmission by direct contact between infected and susceptible animals can be very rapid, and many routes of viral entry may be involved. Maximum excretion of virus coincides with development of clinical disease and lesions on the snout, tongue and feet, and declines over the following three to five days as the antibody response develops.

Unlike ruminants that have recovered from FMD infection, pigs do not become carriers, and there is no evidence of viral ribonucleic acid persisting in infected pigs after three or four weeks of becoming infected.

Clinical Signs and Lesions

Incubation period in natural conditions varies with virus strain, the exposure dose and the route of entry. It may vary from 24 hours to 11-14 days.

The disease is characterised by fever and vesicles on the feet, in and around the mouth, and on the mammary gland. In pigs the more severe lesions usually occur in the feet. Vesicles develop on the coronary band and in the interdigital space. Lesions at other sites are less frequent and less severe. Young pigs up to 14 weeks may die suddenly due to heart failure; piglets are particularly susceptible.

Incidence of disease in not immunised populations can be as high as 100 per cent. Mortality rate in adult animals is usually negligible but it can be extremely high in suckling piglets.

Diagnosis

In farms presenting high mortality of piglets and a significant proportion of pigs showing lameness, fever and vesicular lesions, FMD should be strongly suspected. Actions should be taken immediately to secure a definitive diagnosis and prevent further spread on the agent.

In the laboratory, FMD can be diagnosed by virus isolation, detection of viral antigens, and serology. Pigs may be affected by other vesicular diseases, which are clinically indistinguishable from FMD. Therefore, laboratory diagnosis of any suspected FMD case in pigs is therefore a matter of urgency.

Other pig vesicular diseases are:

  • Vesicular stomatitis (VS): may affect horses and cattle as well (sheep and goats may be experimentally infected). Two distinct immunological classes of VS virus have been recognised: New Jersey and Indiana. It is endemic in northern parts of South America and all Central America, being less frequent in USA. It is not present in other continents.

  • Swine vesicular disease (SVD): does not affect other species. It can be a subclinical, mild or severe vesicular condition depending on the strain of virus involved, the route and dose of infection, and the husbandry conditions under which the pigs are kept. Recent outbreaks of SVD have been characterised by less severe or no clinical signs; infection has been detected when samples are tested for a serosurveillance programme or for export certification. The last report to OIE from a European country was from Portugal in 2007: one farm was affected, with a population of 1,800 pigs. All were eliminated by stamping out. The origin of the outbreak remains unknown.

  • Vesicular exanthema (VES): does not affect other species. It was originated in California and became widespread in the USA during the 1950s but a vigorous campaign to eradicate the disease was successful. In 1959, the USA was declared free of VES, and the disease was designated a foreign animal disease. It has never been reported as a natural infection of pigs in any other part of the world.

World Epidemiological Situation

During 2010, a total of 716 outbreaks of FMD were notified to OIE, affecting 21 countries from two continents: Africa and Asia. These records are the result of immediate notifications, meaning that cases occurring in endemic countries are not included.

Some data concerning those 716 outbreaks are presented in Table 1, which shows the number of susceptible animals exposed to the virus, diseased, dead, destroyed and slaughtered animals. The source is OIE World Animal Health Information System, OIE (WAHIS), 2010.

Table 1. Data of 716 FMD outbreaks from 2010 notified to OIE and recorded in WAHIS


Table 1. Data of 716 FMD outbreaks from 2010 notified to OIE and recorded in WAHIS

Almost 14,000 pigs were directly affected, producing more than 8,000 deaths (estimated mortality rate 60 per cent).

The 716 outbreaks were due to the following FMDV serotype: A: 14 (two per cent); O: 665 (93 per cent); SAT1: 6 (one per cent); SAT2: 16 (two per cent) and Asia1: 15 (two per cent).

As a consequence of these outbreaks, near 300,000 pigs were destroyed. These are the direct cost of the disease. Economical losses in terms of international trade interruption and costs of control measures should be added to the direct costs in order to have a rough estimation of general losses.

The role of pigs in the epidemiology of FMD is not the same everywhere. Firstly, it strongly depends on the production system, in countries or areas having high density of pig farms the role of this species may be crucial. Secondly, it depends on the host tropism of the FMDV strain; for instance, the 1997 FMD epidemic that took place in Taiwan was due to type O/ Taiwan/97, which produced high morbidity and mortality in pigs, but did not affect cattle.

Measures of Prevention

In FMD–free areas or countries, import control including quarantine, is the first line of defence.

Import quarantine policy should include pre-export testing and quarantine, animal heath certification and any necessary post-arrival inspection testing and quarantine. These policies should be based on the results of risk analyses. Border inspection posts (in airports, seaports and borders) should be able to properly intercept all risk products and animals.

The swill feeding to pigs is a major way of introducing FMD into a country. Therefore, consideration should be given to banning swill feeding or at least implementing practices that will make it safe.

Control Measures

In both, endemic areas and areas that suffered reintroduction of FMDV, the control measures have the objective of reducing the reproductive rate (the average number of new cases produced by an infected individual in a time period). As for any other transmissible disease, those measures include three possible independent strategies:

  1. Eliminating sources of infection: firstly, the sources of infection should be identified. Sources of infection are infected animals, contaminated premises, tools, vehicles, wild animals and others. Once identified, the sources of infection have to be eliminated by means of stamping out, slaughtering, disinfection, biocontention measures.
  2. Interrupting contact between infected and susceptible individuals: movement interdictions, sanitary barriers, zoning, bioexclusion measures.
  3. Decreasing proportion of susceptible animals: vaccination programmes. None of these strategies is perfect. For that reason the simultaneous implementation of more than one strategy is convenient.

Both free and infected areas need to have in place an effective programme of early detection and rapid response.

For early detection it is necessary:

  • all those involved in the animal production chain must be aware of FMD clinical signs and the procedures for notification
  • members of the official veterinary services have to be regularly trained, including simulation exercises
  • having a specialist FMD diagnostic team
  • laboratory diagnostic capabilities for rapid and certain diagnosis
  • access to a network of international reference laboratories
  • implementation of passive and active epidemiological surveillance programmes.

Due to the high transmissibility of FMDV, the control measures taken in endemic areas will have little probability of success if they are taken individually by a country or area. The control policies have to be undertaken at regional basis. For that, strong collaboration and transparency between parts are needed.

References

1. OIE. 2012. Manual of Diagnostic Tests and Vaccines for Terrestrial Animals - Chapter 2.1.5. Foot and Mouth Disease.

2. Kitching, R.P. and Alexandersen, S. 2002. Rev. Sci. Tech. Off. Int. Epiz., 21(3):513-518.

3. Geering, W.A. and Lubroth, J. 2002. FAO Animal Health Manual No. 16. Preparation of foot-and-mouth disease contingency plans. ISSN 1020-5187.

4. Orvid Spickler, A., Roth, J.A., Gaylon, J., Lofstedt, J. 2010. Emerging and Exotic Diseases of Animals, 4th Ed. Iowa State University. ISBN 978-0-9745525-8-3.

Further Reading

- You can view the full report by clicking here.


Further Reading

- Find out more information on foot and mouth disease (FMD) by clicking here.


April 2012