Foot & Mouth Disease & Ulcerative/Vesicular Rule-outs: Challenges Encountered in Recent Outbreaks

Foot and mouth disease (FMD) is a highly infectious and contagious viral disease affecting bovidae (cattle, zebus, domestic buffaloes, yaks), sheep, goats, swine, all wild ruminants and suidae. Camelidae (camels, dromedaries, llamas, vicunas) have low susceptibility. Foot and mouth disease is caused by a RNS virus of the family Picornaviridae, genus Aphthovirus. There are seven immunologically distinct serotypes: A, O, C, SAT1, SAT2, SAT3, Asia 1. Foot and mouth disease causes significant economic loss both to countries who manage it as an endemic disease (with or without vaccination), as well as those FMD free countries which may become infected. The mortality rate is low in adult animals, but often higher in young due to myocarditis. Foot and mouth disease is endemic in parts of Asia, Africa, the Middle East and South America (sporadic outbreaks in free areas). The Office of International Epizootics (OIE), also referred to the World Organization for Animal Health maintains an official list of free countries and zones.1 The OIE Terrestrial Code (Chapter 2.2.10) provides detailed information on the categories of freedom that can be allocated to a country as well as guidelines for the surveillance for foot and mouth disease (Appendix 3.8.7). In short, countries may be completely free of FMD, free with vaccination or infected with foot and mouth disease virus (FMDV). Source of FMDV include incubating and clinically affected animals with virus present in breath, saliva, faeces, urine, milk and semen. In experimental settings virus has been detected in milk several days before the onset of clinical signs2. Additional sources of virus are meat and by-products in which pH has remained above 6.0 as well as persistently infected carrier animals. Carrier animals may include cattle and water buffalo; convalescent animals and exposed vaccinates (virus persists in the oropharynx for up to 30 months in cattle or longer in buffalo, 9 months in sheep). Pigs do not become carriers. It has been shown that the African Cape buffalo are the major maintenance host of SAT serotypes. FMDV transmission can occur by either direct or indirect contact. Indirect transmission can occur via contaminated animate vectors (humans, etc.), inanimate vectors (vehicles, implements) or airborne transmission. Indirect disease transmission via animate or inanimate vectors can play a major role in disease transmission. Good biosecurity can significantly reduce this type of transmission. Airborne transmission is often debated and is known to be serotype and species specific as well as require specific environmental conditions to occur. Airborne transmission is favored in temperate zones and has been postulated to occur over distances of up to 60 km overland and 300 km by sea. Foot and mouth disease virus is an unenveloped virus which is preserved by refrigeration and freezing and progressively inactivated by temperatures above 50 C. FMDV is highly sensitive to pH change and is inactivated by pH < 6.0 or > 9.0. There are many disinfectants which are effective against FMDV including sodium hydroxide (2%), sodium carbonate (4%), and citric acid (0.2%). FMDV is resistant to iodophores, quaternary ammonium compounds, hypochlorite and phenol, especially in the presence of organic matter. The virus can survive in lymph nodes and bone marrow at neutral pH, but is destroyed in muscle when is pH < 6.0 i.e. after rigor mortis. FMDV can persist in contaminated feed/commodities and the environment for over to 1 month, depending on the temperature and pH conditions. The incubation period for FMD is 2-14 days. Animals transition through latent (infected but not infectious), subclinically infected (infectious but lacking clinical signs) clinically infected and recovered disease states. In cattle clinical signs include pyrexia, reluctance to eat, bruxism, drooling, lameness, treading or stamping of the feet and decreased milk production. Most clinical signs are related to the development and subsequent rupturing of vesicles at the coronary band and in the oral cavity. Vesicles and ulcerations can also occur on the mammary gland. Recovery in adult animals usually occurs in 8-15 days. Clinical signs for most serotypes are less dramatic in sheep and goats. Swine can develop very severe coronary band lesions and high mortality in piglets has been observed. One of the challenges of diagnosing FMD is that it may be clinically similar to several other vesicular or ulcerative diseases. FMD is clinically indistinguishable from Vesicular stomatitis, Swine vesicular disease and Vesicular exanthema of swine. It may also resemble Bovine viral diarrhea, Mucosal disease, Infectious bovine rhinotracheitis, Bluetongue, Bovine papular stomatitis, Bovine mammillitis and Rinderpest