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PREVENTION AND CONTROL OF FOOT AND MOUTH DISEASE
V. A. Srinivasan

Courtesy : Festschrift - Dr. S. Ramachandran

Foot and Mouth Disease (FMD) is caused by a highly contagious apthovirus in the family picornaviridae. The disease is also referred as ‘Political disease’ as it affects trade between countries. The virus infects many cloven-footed domestic animals such as cattle, buffaloes, sheep, goats and pigs. In addition to farm animals, FMD affects more than 30 species of wild ruminants. The disease causes severe economic losses in endemic countries. 

In India, the total annual loss, both milk and non-milk losses, has been estimated to be about Rs.3463 crores annually at prices and population for the year 1990. The average annual loss was calculated as about Rs.125 per animal. 

The loss is the highest at Rs.854 in crossbred cows, at Rs.250 in buffaloes at Rs.166 in drought/work animals, at Rs.143 in indigenous cows and the lowest at Rs.8 in young stock (1,2). 
The cost of control of the disease in FMD-free countries during the outbreak is enormous. 

Epidemiology
Recent outbreaks in a few FMD-free countries have changed the world distribution of FMD. 
Europe: Many countries have experienced FMD outbreaks since the cessation of vaccination in the European Union in 1991. (Table 1)

Table 1 : FMD outbreaks in European countries during 1991-2001

FMD virus type 0 was observed in most of the outbreaks. Greece reported Asia-I during the year 2000. 
South America: Chile, Guyana, Surinam and French Guiana have remained free from FMD during the past ten years. Colombia, Paraguay and Uruguay reported outbreak of FMD during 2000 and Argentina reported the occurrence of FMD in 2001. FMD virus types 0 and A are prevalent. 

Africa: FMD is endemic in most of African countries. FMD virus types 0, A, SAT 1, SAT 2 and SAT 3 were reported in African countries (Table 2) 

Table 2 : Serotype prevalence in Africa

Kenya reported outbreaks due to FMD serotypes, 0, A, C, SAT I and SAT 2. No other country in the world has reported such a wide range serotype circulation.(3). 

Middle East: FMD is endemic in most of the Middle Eastern countries and sero types 0, A and Asia 1 circulate in the region. (Table 3)

Table 3 : Serotype prevalence in Middle East

Asia: All countries in Asia except Indonesia and Singapore are FMD endemic. FMD virus serotypes 0, A, C and Asia I are reported to be present. Taiwan experienced a FMD outbreak in 1998. Japan and Mongolia reported FMD virus serotype 0 during 2000 after remaining free from FMD for more than 3-6 decades. 

India: FMD is endemic in India and occurs throughout the year. Serotypes 0, A, C and Asia I are prevalent, but 85% of FMD outbreaks in lndia are due to type 0. No incidences of FMD due to type C have been recorded since 1995. 

Cattle, buffaloes, sheep, goats and pigs suffer from FMD. The disease is transmitted from cattle, buffaloes, sheep and goats to pigs. Pigs do not pose major problem in initiation and transmission of FMD in India contrary to the observations in many European and South East Asian countries for the following reasons: 

1. Pig rearing and organised pig farms are not popular in India and the pig population is insignificant. 
2. Major pig population remains with poorer sections of the society and they are not stall-fed. 
3. Pigs are rarely fed with swill. 
Other species of animals transmit the virus due to large scale movement over long distances. 

Appearance of FMD in FMD-free countries: During the last 21 months (January 2000-September 2001) many FMD-free countries reported FMD (Table 4). Identification of serotype SAT 2 in Kuwait, Saudi Arabia and Asia I in Greece during the year 2000 also caused great concern. Serotype C has not been recorded in the world since 1997.

Table 4 

FMD outbreaks in countries which remained free: Cattle, buffaloes, sheep, goats and pigs are highly susceptible to FMD. Young animals such as calves, piglets, lambs and kids are severely affected with high mortality. FMD outbreaks in France in 1979 were characterized by early mortality in piglets. High mortality (50,000) in lambs was noticed in 1989 Tunisian FMD outbreak. High piglet mortality was reported during FMD outbreak in Taiwan in 1997. Recent Mediterranean FMD outbreak witnessed lamb mortality to the extent of 94 percent in individual farms (4).

Transmission

The spread of FMD virus during an outbreak is rapid as several factors favour the transmission. The multiple susceptible species of animals, quantity of virus liberated, range of excretions and secretions containing the virus and the virus stability in environment are responsible for the high transmissibility of the disease. 

The common mechanism of spread of virus is by direct contact of susceptible animals with infected animals or inhalation of droplets containing the virus. Young calves get the infection by drinking contaminated milk. Affected adult animals recover from acute disease within a month. The effect is pronounced in high yielding dairy cattle and they seldom recover their full productivity. Milk yield during the current lactation and subsequent lactation is depressed. FMD virus induces myocarditis in young animals resulting in death. The mortality rate varies in young animals and the immune system clears the infection within 1 to 3 weeks in pigs, (5,6). Bright sunlight and cloudfree conditions favour dispersal of virus. Lower humidity (below 60%) arising out of such atmospheric conditions causes desiccation and inactivation of virus. Cleaning of animal sheds where FMD infected animals are identified is very important. Such cleaning procedures create aerosols favouring spread of infection. Air-borne FMD virus may get carried away for more than 16 km in one hour under normal wind speed. (7).

The virus produces acute infection in all domestic ruminants, which may result in the establishment of a carrier status except in pigs. Recovered animals can become carriers and the virus can persist in sheep for nine months and in cattle for at least 3 years after recovery. FMD virus transmission experiments from recovered animals to susceptible animals by direct contact have been unsuccessful. However, there is considerable field evidence that recovered carrier animal may play a role in new outbreaks of disease (8). Carrier status has been observed in both vaccinated and unvaccinated animals that have contact with live FMD virus. Carrier animals eliminate the virus and the changes that occur in the immune carrier animal to overcome the persistent infection is not clearly understood. International trade prohibits the movement of FMD antibody positive ruminants, as they are considered potentially infected. This trade embargo is considered prudent / wise even though great majority of ruminants derived from FMD endemic areas are unlikely to be carrying live virus. The development of a reliable serological test for differentiation of persistently infected and vaccinated animals will help in safe animal trade between countries (3).

Cattle and sheep have the highest risk of airborne FMD virus spread from infected pigs while pigs are relatively resistant to infection by airborne FMD virus. The virus replicates rapidly and to high titres in pigs upon inoculation of as low doses as 800 TCID50 by heel bulb inoculation while 104 to 105 TCID50 doses by oral route is likely to infect the pigs. This indicates low susceptibility of pigs through oral route (9). Pigs are potent emitters of FMD virus. However, pigs require high doses of airborne virus to infect them and the minimum dose of airborne virus required to infect pigs is more than 800 TCID50 while cattle and sheep require 10 TCID50 to be infected. One hundred affected pigs could transmit sufficient airborne virus to infect cattle up to 2 kilometers away. Air-borne spread of FMD virus between pig herds over a distance greater than 0.1 km would require more than 100 infected pigs in the source farm (10)

Diagnosis
FMD is characterized in cattle and buffaloes by fever, depression, anorexia and drop in milk yield. Other clinical signs include vesicles on the tongue, gums, udders and feet. The vesicles rupture leading to formation of ulcers. There is excessive salivation and lameness. Several diseases produce clinical signs similar to FMD and hence there is a need for differential diagnosis in cattle from the diseases such as vesicular stomatitis (VS), bovine viral diarrhoea (BVD), infectious bovine rhinotracheitis (IBR), mucosal disease, rinderpest, bovine papular stomatitis and calf diphtheria. In sheep, rinderpest, PPR, blue tongue and foot rot and in pigs swine vesicular disease (SVD) and vesicular exanthema of swine need to be differentiated from FMD. 
Clinical diagnosis of FMD in cattle, buffaloes and pigs is not difficult, but the mild clinical signs in sheep and goats pose problem in early diagnosis. The clinical findings in other vesicular diseases such as swine vesicular disease (SVD) and vesicular stomatitis are often indistinguishable from FMD virus infection. FMD is characterized by rapid spread of the infection and hence it is necessary that diagnosis must be quick, sensitive and specific. 

Clinical diagnosis of FMD in small ruminants such as sheep and goats is difficult to establish, as signs in adult animals are often mild and transient. Lameness, pyrexia and mortality may provide a clue to the diagnosis. Spread of disease in sheep is related to the management conditions. Sheep maintained in intensive conditions favour the spread of infection due to close contact. 

The importance of early diagnosis of clinical infection in small ruminants such as sheep and goats need not be overemphasized. This is well illustrated by the recent FMD outbreak in the United Kingdom where the main reservoir was sheep and the infection was wide spread before it was detected. Moreover clinical diagnosis in sheep and dispersed structure of the livestock herding posed serious problem for control of FMD. 
Confirmation of diagnosis of FMD in sheep assumes importance as oral lesions in sheep are mild and collection of sufficient epithelium for laboratory diagnosis is often difficult. This results in discrepancies in clinical signs and laboratory diagnosis. The viraemia starts before clinical signs and later high antibody titres are detected in blood within 3 to 4 days after viraemia. Hence, both virus and antibody can be seen in blood. It is therefore, essential that appropriate samples be collected for establishing a definite diagnosis. 
Definitive diagnosis of FMD is based on the demonstration of FMD virus or antigen in tissue samples. Identification of viral antigens in vesicular samples submitted from animals in early stages of disease is of paramount importance. This helps in diagnosis without recourse to isolation of virus in cell culture which is time-consuming and laborious. The epithelium from ruptured vesicle is the sample of choice. Epithelium from foot lesions can also be submitted for diagnosis but the bacterial contamination often complicates the diagnosis. In sheep, goats and buffaloes foot epithelium can be sent for diagnosis in the absence of buccal lesions. The Complement Fixation test (CFT) has long been the test of choice for FMD virus antigen detection, but its disadvantages limited the use of the test. It also provided an opportunity for development of a specific and sensitive test. In recent yeas ELISA procedures have become the standard diagnostic tests (11). 
In addition to tests such as CFT and ELISA, RT-PCR assay for serotyping of FMD virus has been published, but the protocols are labour intensive (12,13). A combination of aqueous phase hybridization step in conjunction with RT-PCR and an ELISA readout was reported to be fast and efficient in identifying the serotype of FMD virus (14). A chromatographic strip test has been developed for diagnosis of FMD virus antigen in nasal swabs, epithelial suspension and probang samples from clinical samples from the field, from experimentally infected samples and infected cell culture fluids. This can be used as a pen-side test, which will reduce the necessity of transmission of samples to the laboratory and help in rapid diagnosis of the disease in the field (15). 
Molecular epidemiology provides clues to the possible origin and spread of FMD virus. This involves the study of the nucleotide and protein sequences of FMD virus isolates originating from different parts of the world. 
Initial studies by Bachrach et al. (16) showed many types of phenotype variants in FMD virus. Persistent infections in ruminants are not uncommon after recovery from an acute infection and this leads to high mutation frequencies. The use of molecular techniques has revolutionized the study of epidemiology of FMD as they facilitate characterization of individual strains of virus (17). The genetic relationship of FMD virus can be calculated by nucleotide sequence of different isolates originating from different countries and expressed as percentage of nucleotide differences. The nucleotide sequence comparisons are based on the study of the gene coding for the VPI structural protein, which is also referred as ID gene. Most of the studies include 160-nucleotide sequence of ID gene, as comparison of whole FMD virus genotype is not feasible. The selection of the 160 nucleotide is arbitrary (3). The nucleotide sequence relationship is represented as dendrograms and helps to identify the evolutionary origin of the isolates. It is observed that FMD genes can reach an evolutionary rate of 7 x 10-2 mutation per nucleotide sites per year (18). Evolution of a new Pan Asia type 0 topotype has appeared, these genotypes did not exist at the time of report of first FMD phylogenies fifteen years ago. This topotype had spread rapidly displacing the other pre-existing topotypes (19). 

Detailed information on Indian FMD virus type 0 isolates obtained during a nine-year period (1987-1995) has been reported. Type 0 viruses were grouped into 2 genetic lineages, one lineage included twenty-five of the twenty seven isolates and the other lineage consisted of 2 viruses, one originating from northeastern States and the other a vaccine virus. Results of the nucleotide sequencing of the isolates showed 1.2 to 19.4% variation. European type 0 isolates formed a distinct group (20). 

Another study (21) included ninety Indian type 0 FMD viruses originating from different parts of the country from 1993-1999. These isolates were grouped into four distinct genotypes. Genotype I formed the major group, which included isolates from Bangladesh, Egypt, Israel, Saudi Arabia, Syria, Turkey and China. Indian type 0 isolates originating from different parts of the country showed 0-8.8% nucleotide sequence variation indicating the spread of virus from one part of the country to another part due to large scale livestock movement. 

Vaccines
FMD losses in Europe were controlled by quarantining the infected animals and often deliberately spreading the disease in infected herds before 1920. This was referred to as ‘aphthisation’ which involved rubbing the tongue of healthy cattle with a rough towel contaminated with virus from naturally infected cattle. The reasoning behind such a process was to shorten the disease outbreak period and to induce convalescent immunity. In 1925, the first report on the successful immunization of calves with a formalized emulsion of vesicular epithelium was published. The first FMD vaccine was produced in 1938 using tongue epithelium harvested from cattle deliberately infected with FMD virus. Formalin was used to inactivate the virus and aluminium hydroxide was used as an adjuvant. In 1951, Frenkel described the production of vaccine using tongue epithelium obtained from slaughterhouse. The disadvantage of this technique was the requirement of large constantly available supply of bovine tongues and associated bacterial contamination. 

A major advance was the development in early 1960s, of the production of FMD virus in vitro, using continuous cell line (22). The virus is produced in large quantities in BHK suspension cultures. Aziridine compounds have replaced formalin as they produce first order inactivation kinetics and assure proper and complete inactivation (23). Oil adjuvants, single and double emulsion are used to produce vaccine for immunization of all species of animals including pigs. A well-managed intensive vaccination programme using the inactivated vaccines has helped to control FMD in different regions of the world. Best examples include elimination of FMD from Chile, Denmark and Indonesia. Current research focuses attempt to develop DNA vaccines and plant vaccines as they are likely to eliminate the use of live virus for vaccine production and will also help in differentiating vaccinated and infected animals. (24,25) 

Control
Countries in different regions of the world adopt FMD control policies depending on the epidemiology of the disease. 

a) Countries free of FMD 
b) Countries where certain regions are FMD-free 
c) FMD endemic countries 

Many countries have achieved FMD-free status by the slaughter of all infected and susceptible in-contact animals, strict animal and animal product import regulation and animal movement restrictions within the country. FMD-free countries do not use vaccination policy for control of FMD for the following reasons: 

1. Cattle, sheep and goats may remain carriers regardless of the development of either clinical or sub-clinical disease. 
2. Vaccinated animals can still become carriers of live virus as a result of contact with field virus during the outbreak.
3. It is difficult to distinguish vaccinated in-contact and convalescent animals and no serological test has been developed to differentiate the infected and vaccinated animals. 
4. Moreover, a two-year period without outbreak is required after the cessation of vaccination before countries can be accepted as FMD-free after an outbreak. 

However, if a country adopts restricted ring vaccination around the outbreak, FMD-free status can be accepted in 3 months period after cessation of vaccination provided all vaccinated animals are slaughtered, absence of further outbreaks is reported and serological survey shows freedom from antibody-free animals. 
A few countries report some regions as FMD-free provided they meet certain criteria mentioned above. However, the regions will lose the FMD-free status if outbreaks are reported in the FMD-free regions. 
FMD endemic countries do not follow stamping out policy and use only vaccination as a measure of control. Many Asian, Middle Eastern and African countries adopt vaccination as a preventive measure. Large livestock population, socio-economic and financial constraints and unlimited movement of animals make the efforts to control FMD difficult. 

FMD control in India 
Control of FMD in India is attempted by vaccination of susceptible productive cattle. The National Dairy Development Board (NDDB) attempted to study the usefulness of regular vaccinations in control of FMD outbreaks and also to understand the difficulties encountered in large scale vaccination programme. A pilot scheme was undertaken by NDDB with the assistance of Overseas Development Administration of U.K. in Nilgiris district of Tamil Nadu in 1982. All the susceptible animal population such as cattle, buffaloes, sheep and goats were regularly vaccinated. The coverage of over 80% vaccinations was achieved which reduced the disease to an insignificant level. 

This experience has imparted confidence to take up the mass vaccination programmes on a larger scale in a phased manner. Accordingly NDDB had undertaken a massive FMD control programme in four phases in 29 districts in three Southern States namely Tamil Nadu, Kerala and Karnataka during 1987-1994 (Table 5). The phase I was taken up in seven neighbouring districts of Nilgiris. The vaccinations were conducted free of cost covering at least 80% of the susceptible population. The local governments supported the programme with suitable legislations for compulsory vaccinations. Each phase consisted of three, six monthly vaccinations of all the susceptible population. At the end of each phase, the next phase was undertaken in the neighbouring districts. A border area vaccination programme covering about 15 kms. was continued in case the control project districts were open to non-control project areas. This helped to create an immune belt and prevent the introduction of FMD virus into the control project area. The vaccinations were also continued along the cattle migratory routes 5 kms. on each side.

The project utilized 36.65 million doses of FMD quadrivalent vaccine (Table 6). Disease reporting system was developed to report all outbreaks so that immediate ring vaccinations could be undertaken to confine the disease within the infected premises. The other measures included ear tagging to identify the vaccinated animals, implementation of cattle disease act to restrict the movement of animals in the infected area. 
At the end of the programme NDDB requested the local state governments to continue the project as it had significantly reduced the number of outbreaks. The project did not continue due to various financial and administrative reasons. As an alternative, tapering grant scheme was introduced in most of the Dairy co-operative unions. Under this scheme the farmers were offered subsidies on a tapering basis so that the additional burden on the farmer was increased slowly over a long time. The subsidies were 50% for the first year and gradually decreased to 20% over a period of four years. The scheme was successful only in selected pockets where the livestock industry was popular and crossbred populations were high. Hence it achieved limited success in controlling the Foot and Mouth Disease. 

Table 5 : FMD Control Project Districts taken up phase wise

Table 6 : Vaccinations during 1987-1994 in FMD control project

After these projects the Foot and Mouth Disease control programmes were left to be implemented to the local authorities such as the state governments and the dairy cooperatives. The success rate differed depending on the local conditions. Some states like Kerala where the vaccination coverage was very good, could control the disease for longer periods but due to other problems like uncontrolled movement of animals the disease could not be controlled over a long time. Currently the FMD programme is implemented by a few States through Central / State Government assistance. The quantity of vaccine used during the past 5 years is furnished in (Table 7).

Table 7 : FMD vaccine utilization in India from 1995 to 2000

It is clear from the data that only 2% of the animal population is vaccinated. This favours the spread of infection rapidly during the outbreak. 

FMD control programme implementation should be a coordinated effort between the Animal Husbandry Department of various State Governments, Dairy Cooperatives and various voluntary organizations involved in the livestock development. This kind of cooperation can lead to control of foot and mouth disease as demonstrated by NDDB in the pilot FMD Control Project. 

FMD control in Israel 
Israel follows a strict policy of vaccination and movement control. The primary aim is to keep their high yielding cattle, consisting of 350,000 cows, disease-free. Young cattle are vaccinated twice, while adult cattle receive annual vaccination with a trivalent vaccine. The vaccination is carried out by State Veterinary personnel. Sheep are identified as groups and vaccinated only once with a monovalent vaccine. Movement of animals are controlled and permitted only by license issued by the State veterinarian (26). 

Preventive measures in organized herds 
Spread of infection within a farm can be controlled effectively by certain practices (27). Since intra-herd spread is mainly through contact spread, all efforts should concentrate on reducing contact spread. They suggest a few measures to reduce the contact spread. 

1. Stocking fewer animals in each pen 
2. Improving the number and distance between pens. 
3. Placing highly susceptible stock in the center of the farm far away from roads and entrance where vehicle movements are high. The susceptible stock pens should be surrounded by pens with highly protected stock.

Prevention of FMD spread 
The increased threat of FMD spread to FMD-free countries appears to be related to ever increasing international trade and also massive movement of animals over long distances. The following points require attention: 

1. Careful consideration of movement of animals within endemic countries as well as between FMD endemic and free countries. 
2. Rapid diagnosis of the disease in small ruminants. 
3. Development of a specific and sensitive serological test for differentiation of vaccinated and infected animals. 
4. There is a need to evaluate the usefulness of ring vaccination and culling method for effective control of the disease. 

Culling helps to diminish the infected susceptible population thereby reduces the transmission of disease. (28).

Recent FMD outbreak in the United Kingdom provided several lessons in dealing with massive outbreaks in FMD-free countries.

1. FMD being highly contagious requires rapid diagnosis. 
2. Spread of the disease is accelerated in the presence of large number of major host species and where movement is frequent. 
3. Effective surveillance and destruction of animals where infection is identified. 

Conclusions
Foot-and-Mouth Disease (FMD) is an acute viral disease, which affects many domestic animals and wild ungulates. The disease is characterized by formation of vesicles in mouth, feet, udder which rupture to form ulcers. The disease causes severe economic losses and it is estimated that the loss due to FMD in India is approximately Rs.3463 crores per annum. Acute form of the disease with severe lesions is noticed in cattle, buffaloes and pigs while in sheep and goats, the clinical signs are mild. Cattle, buffaloes, sheep and goats can become carriers after infection. Early diagnosis is of paramount importance in sheep as the disease runs a mild course and poses problems in clinical diagnosis in the field. ELISA is used commonly for typing of virus and RT-PCR helps in arriving at early and accurate diagnosis. FMD caused panic in many countries in the world during the past 18 months as the year 2000 and 2001 witnessed outbreaks in several FMD-free countries such as Japan, Mongolia, United Kingdom, France, Netherlands and South Africa.

FMD-free countries adopted strict quarantine and animal, animal product imports to prevent the introduction of the disease. FMD was controlled in countries like U.K., Japan, France and Netherlands by stamping out method. FMD endemic countries adopt vaccination for prevention and spread of the disease. Control of FMD in India is difficult due to large livestock population, unlimited movement of animals and socio-economic conditions. Systematic control of FMD in India was attempted in 29 districts of Tamil Nadu, Kerala and Karnataka by NDDB during 1987-1994. The experience gave indications that the disease incidence can be reduced to a great extent by mass vaccination of animals. International and regional coordination are required to control FMD in endemic countries, which will reduce the risk of spread of infection from endemic to FMD-free countries. 

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Authors Corresponding address: 

Dr. V.A. Srinivasan
Executive Director,
Indian Immunologicals Limited, Gachibowli, Hyderabad - 500 019,India

The views expressed in this article are of the author(s), and any clarifications can be obtained from the author(s).