Foot and Mouth Disease

Filed Under: Cows, Diseases, Pigs

Foot and Mouth Disease (FMD) is a highly infections viral disease of cattle, pigs, sheep, goats, buffalo, deer, antelope, elephant and giraffe. Old world camels are resistant to natural infection while new world camelids such as alpacas and llamas are susceptible to infection. Other animals that may become infected include nutrias, capybaras, hedgehogs, rats, mice and armadillos.

FMD is caused by an aphthovirus of which there are seven serotypes. FMD is endemic in the Middle East, sub-Saharan Africa, India, and Southeast Asia. Europe is generally considered to be FMD free until recent reports of it resurfacing in the United Kingdom. The 2007 outbreak in the UK was believed to be due to a biosecurity breach at one of two research facilities located near the farm from where the outbreak originated. The virus was believed to have been transferred by human movement or perhaps tainted water. The outbreak was quickly brought under control. The United States last reported outbreak of FMD was in 1929.

FMD is present in all body secretions and excretions from infected animals. Transmission usually occurs by aerosolized virus between susceptible and infected individuals. The virus may also be obtained through ingestion, typically through the contamination of feedstuffs and animal by-products. Virus may be present in milk and semen for up to four days before the appearance of clinical signs. Man, dogs, cats and birds may serve as mechanical vectors in the transfer of the virus. Transmission may occur by artificial insemination to a susceptible cow when semen from an infected bull is used.

Ruminants that have recovered from infection as well as vaccinated ruminants have been shown to carry the virus in the pharyngeal region for up to 3.5 years in cattle, and even longer in African buffalo.

The primary site of infection and viral replication is the mucosa of the pharynx. The virus may also enter through skin abrasions or the GI tract.

The incubation period for FMD is 2 - 14 days and is dependent on the infecting dose, the susceptibility of the host and the strain of the virus. Cattle and pigs are usually more severely affected clinically than are smaller ruminants.

Clinical signs include loss of appetite (anorexia), excessive salivation (ptyalism) and elevated temperatures of up to 106°F with a two to three day duration.

The characteristic lesion of FMD is the development of vesicles. Vesicles are blister-like lesions that are elevations of the epidermis up to 1 cm in diameter and are filled with clear fluid. Vesicles are typically fragile and transient in appearance, usually giving way to erosions. Erosions are shallow epidermal defects or ulcers that expose the underlying dermis. Vesicles develop on the tongue, dental pad, gums, lips, coronary band, and interdigital cleft of the feet. Vesicles may also develop on teats and on the udder, particularly in lactating cows and sows. Areas of skin subject to pressure and trauma such as the legs of pigs housed on concrete may also develop vesicles. Most vesicles heal within seven days, although those on the tongue often take a little longer. Due to the fragile nature of vesicles, erosions may be the more commonly seen lesion.

Secondary infections are common especially of the mouth, mammary glands, and interdigital areas resulting in stomatitis, mastitis, and chronic lameness respectively. The lameness may become so extensive that the horny tissue of the hoof may slough. Affected pigs may prefer to lie down or may squeal loudly and have a painful hobbled gait when walking. Milk production drops dramatically in lactating animals. All animals exhibit a loss of condition and a decrease in the growth rate which may persist following recovery. Deer may shed their antlers. Rarely is the disease fatal except in young animals. Young animals may die before the onset of vesicle formation because the virus damages developing cells of the myocardium (heart).

Clinical signs of FMD are indistinguishable clinically from those of vesicular stomatitis, swine vesicular disease, and vesicular exanthema. Samples of vesicular epithelium or vesicular fluid should be sent in phosphate-buffered saline (pH 7.4) to a national laboratory responsible for the diagnosis of FMD in nonendemic areas. Samples may also be tested at the World Reference Laboratory for FMD which is located in Pirbright, UK by prior arrangement. Strict regulations exist concerning the shipment of pathologic and hazardous materials.

The finding of FMD in countries deemed previously free of disease may have major economic implications. Many FMD free countries have strict policies in dealing with an outbreak, demanding the slaughter of all affected and in-contact susceptible animals. These carcasses are then either burned or buried on or close to the premises, and the area is thoroughly washed and disinfected. Countries free of FMD will immediately close their markets to the export of products from the country in which an outbreak originates, resulting in the additional loss of export markets.

In endemic areas, control of FMD is through vaccination and quarantine. The vaccine contains killed virus and the protection offered is of short duration, usually four to six months. Immunity is better provided by vaccines containing strains similar to those present in a particular endemic area. Immunity to one serotype of FMD may not protect against the other subtypes of the virus.

The virus is quickly inactivated outside the pH range of 6.0 to 9.0 therefore weak acids and alkalis will inactivate the virus. The virus is also very susceptible to desiccation and temperatures of greater than 56°C. The FMD virus may survive for prolonged periods when associated with animal protein. Reportedly a proportion of FMD virus in infected milk will survive pasteurization at 72°C for up to 15 seconds.

References:

Kahn, Cynthia Editor: The Merck Veterinary Manual. 9th Edition. p.507-510.

www.APHIS.USDA.gov

Sheep Veterinary Society, Division of the British Veterinary Association.

Blood, D.C. and Radostits O.M., et all. Veterinary Medicine. 5th Edition. Lea and Febiger, Philadelphia. Pp.614-620.

Nolen, Scott. "Investigation of FMD Outbreak focuses on Research Complex". JAVMA. Vol 231, No. 6/News 9/15/07.

Topics: foot and mouth disease, viruses

Symptoms: fever, hypersalivation, loss of appetite

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