Foot-and-mouth disease and other infectious vesicular diseases

Foot-and-mouth disease (FMD) is a highly contagious viral disease of domestic ruminants and pigs, a number of wild ungulates and hedgehogs. FMD and the other infectious vesicular diseases are characterised by vesicle formation in the epidermis of the feet and in the mouth, labia and muzzle or snout. It is not usually fatal though some calves may die as a result of myocarditis. Recovery is often prolonged and it is this, which causes the enormous economic losses that result from epidemics of the disease.

Because of the nature of FMD and the economic importance of the ruminant industries to New Zealand economy, an outbreak here would have severe direct economic effects. In addition there would be restrictions or prohibitions by trading countries on importation of New Zealand animals and animal products.

Geographic distribution

New Zealand has never had a case of FMD.

Areas of the world which have remained free from FMD in recent years include Australia, North and Central America, Indonesia and the Pacific islands.

The disease continues to occur in much of Africa and Asia, and parts of South America.

Western Europe has had recent outbreaks, which have all been successfully controlled. These include the 2001 outbreak in the UK, which spread to Ireland, France and the Netherlands, and separate outbreaks in Italy and Greece. Japan has also recently eradicated outbreaks. Sporadic outbreaks occur in Eastern Europe.

Cause

FMD is caused by an Aphthovirus of the family Picornaviridae. There are seven distinct serotypes. Within the basic serotypes there are a number of subtypes. There is no cross-immunity between serotypes.

The virus is stable between pH 7 and 9. Beyond those values, inactivation occurs. Thisallows for a range of disinfection procedures to be used, either acids or alkalis. Formalin is also effective. By contrast phenolic and quaternary ammonium compounds have little effect. Heat, UV and gamma irradiation have been used to treat animal products.

Host range

FMD affects cloven-hoofed animals. Domestic ruminants and pigs are susceptible. African buffalo, impala, kudu, warthogs and bush pigs are natural hosts. Elephants, coypu, agouti and hedgehogs are also affected.

Transmission

The virus is naturally transmitted by the oropharyngeal route and direct contact with animal products. Significant spread can be effected by mechanical means, such as vehicles, fomites, non-susceptible animals and by human activity. Infected animals excrete virus in exhaled air and in their secretions and excretions as well as from clinical lesions. Ruminants and especially pigs excrete large numbers of virus particles as an aerosol. In certain conditions of wind and humidity these aerosols can carry infection to susceptible animals many kilometres away. Cattle are at greater risk of succumbing to such aerosols because of their greater tidal respiratory volumes and the lower infective dose for cattle compared to other species.

Excretion of the virus may occur for up to 4 days prior to the development of clinical disease.

Clinically inapparent infections can occur, particularly in cattle and sheep. Such animals excrete virus and contribute to the spread of the disease. Post infection carrier states occur in cattle, sheep and goats in a high proportion of animals and can persist for up to 3 years or more in cattle, with lesser periods in sheep and goats.

Clinical signs

After an incubation period of 2 to 8 days, fever, depression and anorexia are noted with formation of vesicles in and around the mouth and in the bulbs of the heels, interdigital cleft and coronary bands of the feet. Vesicles may also be observed on the teats of lactating animals.

Attack rates are very high, though mortality is low. Most deaths are in calves as a result of myocarditis. However production losses are high including permanent reductions in milk production and reproductive performance. Chronic illness, of uncertain pathogenesis often results also.

Differential diagnosis

The clinical signs of FMD and vesicular stomatitis are identical in ruminants and pigs. In pigs, swine vesicular disease (SVD) and vesicular exanthema (VE) closely mimic FMD. It is therefore essential that any occurrence of vesicular disease in ruminants or pigs be regarded seriously and no attempt made to differentiate them on clinical grounds. Horses are affectedby vesicular stomatitis, but not FMD, SVD or VE.

Virus recovery from tissue specimens or exudates is the basis for differentiation between the vesicular diseases.

Diagnosis

Clinical diagnosis of an acute vesicular disease, including FMD, are usually made readily enough. However, if an index case is of some standing when examined it may be difficult to diagnose, because of ulceration and the presence of secondary infections or healing. Most probably though more typical lesions can be found in herd-mates that have been more recently infected. It is therefore important to examine in-contact animals whenever a vesicular disease is suspected.

Confirmation of diagnosis is based on recovery of virus from vesicular fluids and epithelial tissue associated with lesions or detection of antigen using an ELISA or CFT. It is important for epidemiological investigations that virus be recovered for typing purposes.

Risk of introduction

The most likely means of introduction of FMD or SVD is by illegal or accidental importation of infected animal products or material contaminated with virus. However, such items would have to be presented to susceptible animals in some suitable way to establish infection. Border security reduces the ingress of these, and the end use of many illegal or accidental imports would not create infection opportunities for livestock.

Feeding garbage to pigs is often considered a possible means of introducing FMD and the other infectious vesicular diseases, on the premise that illegal meat imports might be fed. However, most illegal imports are small in volume (imported by individual people or in mail) and destined for home consumption. Any waste material is more likely to end up in urban waste disposal rather than with garbage-fed pigs. However, feeding of garbage to pigs is regulated and urban dumps are generally well managed, so any 'leakages' of illegal importations of meat etc is unlikely to reach susceptible species in an infective form.

One estimate made of the likelihood of FMD introduction into New Zealand, is once in every 53 years. A separate analysis of risks posed by informal importations by incoming passengers or mail is that the disease might be imported two or three times per century. Historically, the demonstrable risk has been less than once every 150 years.

Effects of introduction

The occurrence of FMD would have very serious effects on that part of the economy that depends on the production of livestock, and the processing of their products. As that constitutes some seven billion dollars (about 10% of the nation's GDP) the effect on the national economy would be severe. If uncontrolled, the losses of production and the restrictions that would be placed on New Zealand product by market countries would be greater and more long-lasting than the considerable cost of control.

A 2002 Reserve Bank study of a senario involving 50 affected farms estimated an 8% drop in exports of goods and services and a $6 billion loss of gross domestic product in the first year.

Control

Any outbreak of FMD or other infectious vesicular disease would be dealt with in the first instance by traditional 'stamping out' procedures. This involves the quarantine of infected properties, and destruction of affected herds. A controlled area would be declared that would ban movement of susceptible livestock in the area around any infected property. There would be intense monitoring of herds and flocks in the area to detect any spread of disease. Premises that have been contaminated by infected animals would be cleaned and disinfected and left depopulated for a period of time until it was judged safe to re-introduce new animals.

In a larger area around this monitoring zone, movement of susceptible livestock and their products would be subject to control. Gatherings of livestock, for instance at saleyards, would be banned.

Product which was derived from infected or possibly infected stock would be suitably treated, if possible, or otherwise destroyed. Product generated in the controlled areas would be held at secure premises until markets that will accept it are identified.

In a major epidemic the likely demand on resources necessary to control the disease may exceed that which can be provided or afforded. There are two contingencies against such a situation. MAF has an arrangement with counterpart organisations of some other nations for overseas veterinarians to assist the local profession. If the nature of the epidemic is such that tactical vaccination of susceptible animals in an area is a necessary adjunct to control of the disease, MAF has access to a vaccine 'bank' for immediately available supplies.