Foot and Mouth Disease: A Highly Contagious Threat to Livestock 🦠🐄

Foot and Mouth Disease: A Highly Contagious Threat to Livestock 🦠🐄

Foot-and-mouth disease (FMD) is an extremely contagious disease affecting domesticated and wild ungulates. It is characterized by the presence of vesicles in the mouth and on the feet. Surprisingly, even hedgehogs and humans can become infected, albeit rarely. 🚫🦔🙅‍♂️

Aetiology 🧪

FMD is caused by a virus belonging to the aphthovirus genus in the Picornaviridae family. There are seven distinct types of FMD virus, namely A, O, C, SAT 1, SAT 2, SAT 3, and Asia 1. Each type encompasses numerous strains, ranging from closely related ones to those that differ significantly in their antigenic properties.

Distribution 🌍

FMD is endemic throughout sub-Saharan Africa, extending as far south as Tanzania. It is also prevalent in Equador, Bolivia, Peru, parts of Brazil, Columbia, and Venezuela in South America, as well as in most of the Middle East and Far East. However, countries like Canada, Central and North America, Australia, New Zealand, Japan, Argentina, Chile, and South Korea remain free of FMD. Although most of Europe is also free from the disease, occasional outbreaks occur despite strict import regulations. In Southern Africa, FMD is primarily confined to wildlife in game parks, with occasional spillover into neighboring cattle areas.

The most widespread FMD virus types are O and A, prevalent in South America, the Middle East, and Asia. SAT 1, SAT 2, and SAT 3 are usually restricted to Africa but have sporadically spread to the Middle East. Asia 1 occurs in the Far East and India, with occasional incursions into the Middle East. Type C, on the other hand, rarely causes outbreaks in Asia and has nearly disappeared.

Epidemiology 🌬️🐮

FMD is highly contagious, and a mere ten infectious units can initiate the disease in a bovine through the respiratory route. The virus can survive in dry fecal material for up to 14 days in summer and in slurry for up to six months in winter. It can persist in urine for 39 days and on soil for three days in summer and up to 28 days in winter. FMD virus is susceptible to inactivation by extreme pH levels—below 6.0 and above 10.0—while remaining stable between pH 7.2 and 7.6. At 4°C, the virus can survive up to a year in suitable media, but higher temperatures reduce its viability to weeks or even minutes.

The disease spreads primarily through the movement of infected animals, with sheep, goats, and wild ungulates playing a significant role due to their ability to carry the virus with mild or no clinical signs. Pigs, on the other hand, are highly contagious and can excrete up to 400 million infectious units per day—3000 times more than infected bovines, sheep, or goats. Infected cattle can also transmit the virus through milk products, semen, and even before the appearance of clinical signs. Lorries, fomites, and stockmen can become contaminated with the virus from infected carcasses, although the acidic conditions following rigor mortis are usually sufficient to inactivate the virus in meat.

Windborne spread of FMD virus is a significant concern, particularly in regions with favorable climates for virus survival. Evidence suggests that the virus can travel over long distances—up to 250km over the sea and 60km over land. Spread through the wind depends on various factors, including virus production by infected animals, weather conditions, topography, and the susceptibility of animals exposed to the airborne virus. Cattle, with their large respiratory volume, are particularly susceptible to infection through inhalation of low quantities of the virus, making them highly vulnerable to airborne transmission.

Cattle that have recovered from FMD or have been vaccinated can harbor the virus in their pharyngeal region for several months, resulting in a carrier state. Carrier animals, despite being difficult to transmit the disease to susceptible animals under experimental conditions, are believed to have initiated outbreaks based on circumstantial evidence and sequencing of outbreak strains.

Transmission and Pathogenesis🦠📊

Cattle are most susceptible to FMD through intradermal inoculation into the tongue. However, natural infection occurs primarily through inhalation of droplets containing the virus or ingestion of contaminated materials. Just one infectious unit can cause infection via intradermolingual inoculation, while inhalation may require 10 to 100 infectious units. Ingestion of the virus typically demands a higher quantity, although young calves can be infected with lower doses through insufflation of infected milk.

The primary site of viral replication after inhalation is the pharynx and lymphoid tissues in the upper respiratory tract. From there, the FMD virus enters the bloodstream, disseminates throughout the body, and replicates in other glandular tissues. The virus appears in various body fluids, including milk, urine, respiratory secretions, and semen, even before the onset of clinical signs. However, the majority of virus shedding occurs during the early vesicular stage of the disease. An infected bovine can excrete large numbers of infectious units, posing a significant risk to uninfected cattle in the herd and potentially overcoming waning vaccine-induced immunity.

The incubation period for FMD can range from two to 14 days, depending on factors such as the route of infection, virus dose, strain virulence, and host susceptibility. When susceptible cattle come into contact with an infected animal, the incubation period is typically two to four days.

Clinical Signs🤒👅

Following an incubation period of two to 14 days, cattle infected with FMD exhibit various clinical signs. Initially, they experience pyrexia (fever) reaching around 40°C (104°F), which lasts for one to two days. Vesicles then develop on the tongue, hard palate, dental pad, lips, muzzle, coronary band, and interdigital space. Lactating cows may also have vesicles on their teats. In young calves, the virus can invade and destroy developing heart muscle cells, causing mortality before vesicles develop. The vesicles in the mouth rupture within one to two days, leaving shallow ulcers surrounded by fragments of epithelium. On the tongue, the vesicles often merge, resulting in a substantial loss of dorsal epithelium. Vesicles on the feet persist for two to three days before rupturing, depending on the terrain or flooring in the cattle's environment.

Mouth lesions typically heal rapidly, filling with fibrin and transforming into pink, fibrous tissue by the tenth day after vesicle formation. However, normal tongue papillae do not fully regenerate at this stage. Foot lesions take longer to heal and are prone to secondary bacterial infections. Under-run heels may occur due to the initial vesicles and subsequent bacterial invasion.

Infected cattle salivate excessively, develop nasal discharge that progresses from mucoid to mucopurulent, and stamp their feet to alleviate discomfort. They may prefer lying down and resist attempts to stand. Cattle with teat lesions become difficult to milk, and the damaged teats are prone to secondary mastitis.

Affected cattle quickly deteriorate, experiencing weight loss and a dramatic drop in milk production that cannot be recovered during the remaining lactation period. Some animals never fully regain their previous condition due to the development of thyroid gland lesions, resulting in a condition known as "hairy panters."

An outbreak of FMD can have devastating economic consequences, especially in intensively farmed regions. However, in extensive husbandry systems found in South America and Africa, where cattle productivity expectations are low, FMD may seem less significant compared to other prevalent diseases like clostridial, haemoparasitic, and deficiency diseases. This perspective can hinder efforts to control FMD effectively or introduce more intensive farming practices or a dairy industry.

Pathology🧪🔬

During FMD infection, the epithelial cells in the stratum spinosum of the skin undergo ballooning degeneration, leading to the development of vesicles and bullae—characteristic features of the disease. Squamous epithelial cells in the rumen, reticulum, and omasum can also be affected. In young animals, the virus invades myocardial cells, resulting in macroscopic grey lesions, particularly in the left ventricular wall, which gives it a striped appearance, resembling a "tiger heart." Skeletal muscle cells may also undergo hyaline degeneration.

Diagnosis 👩‍⚕️🔍

The initial diagnosis of FMD is typically based on clinical signs and may involve assessing contact between the affected herd and infected animals or reports of FMD in the vicinity. In fully susceptible herds, the clinical signs are often severe and pathognomonic. However, in endemic regions with partial natural or vaccine-induced immunity, clinical signs may be mild and easily overlooked. It is crucial to investigate all vesicular lesions in cattle as potential cases of FMD.

Laboratory confirmation of FMD requires submitting adequate samples under appropriate conditions. A minimum of 2 square cm of epithelium from a ruptured vesicle, suspended in a mixture of glycerine and buffered phosphate, should be sent to a designated laboratory equipped to handle FMD virus and perform type-specific tests.

In countries where FMD is controlled through vaccination, outbreak strains must be related to existing vaccine strains. This can be achieved through microneutralization tests using mixtures of field virus and antisera to a vaccine virus. Serum titers are measured to determine the antigenic relationship between field and vaccine strains, influencing the effectiveness of the vaccine in controlling the outbreak.

Various diagnostic techniques are available, including ELISA to detect virus antigen, virus isolation using cell cultures, PCR for genome detection, and serological tests such as virus neutralization and ELISA to measure antibody levels in vaccinated animals or those exposed to FMD.

Control and Economic Impact💰🛡️

Controlling FMD involves preventing virus introduction, minimizing stock infection, and halting virus spread from infected animals. Each country adopts control strategies based on economic and practical considerations.

Quantifying the economic impact of FMD is challenging. Direct costs, such as vaccination, culling infected animals, movement restrictions, and market closures, can be measured. However, indirect costs, like the loss of potential export markets, are more significant yet difficult to estimate accurately.

Countries striving to prevent or eliminate FMD face two options: routine vaccination of all cattle (and possibly other livestock) or refraining from vaccination and relying on slaughter during outbreaks. The most economical approach depends on critical point analysis or determining the point at which costs for each policy become equal. Factors to consider include the cost of vaccines and their administration or storage as a strategic reserve. Additionally, the cost of controlling an outbreak, including ring vaccination, slaughtered animals, production loss, and trade interruptions, must be calculated and multiplied by the estimated number of outbreaks.

Assessing the economic significance of FMD control requires a well-functioning veterinary infrastructure capable of diagnosing and managing the disease. Without such support, estimating the true cost of FMD becomes an academic exercise.

🐄🚫🦠 Protecting Livestock: The Battle Against Foot-and-Mouth Disease 🦠🚫🐄

Prevention and control measures 🛡️🔒

Preventing the introduction of FMD virus is crucial in controlling the disease. Countries implement strict biosecurity measures at borders and regulate the movement of animals and animal products to minimize the risk of virus transmission. Quarantine protocols and monitoring of livestock farms and markets play a significant role in preventing the spread of FMD.

Vaccination is another essential tool in controlling FMD. Vaccines are developed using inactivated or attenuated strains of the virus and administered to susceptible animals. Vaccination can help reduce the severity of the disease, limit virus shedding, and protect livestock populations from outbreaks. However, the effectiveness of vaccination programs depends on factors such as vaccine quality, coverage, and the matching of vaccine strains to circulating field strains.

In the event of an outbreak, rapid response is crucial. Infected animals are typically culled to prevent further spread of the virus. Surrounding areas may be placed under movement restrictions, and strict biosecurity measures are enforced to contain the disease. Comprehensive surveillance, including clinical monitoring and laboratory testing, helps identify new cases and track the spread of the virus.

International cooperation is vital in combating FMD, especially in regions where the disease is endemic or where cross-border movements of animals occur. Collaborative efforts involve sharing information, harmonizing control strategies, and implementing coordinated vaccination programs. Organizations such as the World Organisation for Animal Health (OIE) play a crucial role in facilitating global cooperation and providing guidelines for FMD control.

Economic impact of FMD💰💔

The economic consequences of FMD can be devastating for affected countries. The direct costs of control measures, including vaccination, surveillance, culling, and compensation to farmers, can be substantial. Additionally, trade restrictions imposed by importing countries following an outbreak can result in significant losses for livestock producers and related industries. The negative impact on export markets, tourism, and overall economic stability can be long-lasting.

In regions heavily reliant on livestock production, such as rural communities, the socioeconomic consequences of FMD outbreaks can be particularly severe. Farmers may suffer substantial financial losses, and the loss of productive animals can disrupt the local economy. Employment opportunities, market access, and food security can be jeopardized, affecting the livelihoods of numerous individuals and communities.

Efforts to control and eradicate FMD require substantial investment in veterinary infrastructure, research and development, surveillance systems, and public awareness campaigns. Governments and international organizations must allocate resources to support these initiatives and strengthen veterinary services to effectively combat the disease.