Foot-and-Mouth Disease

Foot-and-mouth disease represents one of the most economically devastating and highly contagious viral diseases affecting cloven-hoofed animals worldwide, with swine serving as significant amplifying hosts capable of producing and excreting massive quantities of the virus. This acute vesicular disease is caused by an aphthovirus belonging to the Picornaviridae family, with seven distinct serotypes and over sixty subtypes identified globally, each capable of causing severe disease outbreaks that can devastate entire swine industries within weeks of introduction. The disease remains endemic in parts of Africa, Asia, South America, and the Middle East, while countries like the United States, Canada, Australia, and much of Western Europe maintain FMD-free status through rigorous surveillance, import restrictions, and immediate eradication protocols upon detection.

Swine are particularly important in foot-and-mouth disease epidemiology because they can excrete up to 3,000 times more virus through respiratory aerosols than infected cattle, making them extraordinary amplifiers of the pathogen within affected regions. All breeds of domestic pigs are equally susceptible to infection, and the disease affects swine of all ages, though clinical signs may vary in severity depending on the viral strain, infectious dose, and immune status of the animals. Wild boar and feral swine populations present additional challenges for disease control, as they can serve as reservoir hosts and complicate eradication efforts in affected areas.

The economic impact of foot-and-mouth disease extends far beyond direct animal losses, encompassing trade restrictions, export market closures, eradication costs, and long-term industry recovery expenses that can reach billions of dollars during major outbreaks. The 2001 FMD outbreak in the United Kingdom resulted in the culling of over six million animals and economic losses exceeding eight billion pounds, demonstrating the catastrophic potential of this disease when introduced into naive populations. For swine producers, an FMD outbreak typically means mandatory depopulation of affected and contact herds, extended quarantine periods, and potential permanent loss of valuable genetic lines and breeding stock.

Foot-and-mouth disease is classified as a reportable disease in virtually all countries, meaning that any suspicion of infection must be immediately reported to veterinary authorities who will implement emergency response protocols. Treatment of affected animals is not permitted in FMD-free countries because allowing infected animals to recover would perpetuate viral circulation and undermine eradication efforts designed to restore disease-free status as quickly as possible. Early detection and rapid response remain the cornerstones of FMD control, making producer awareness of clinical signs and immediate reporting obligations absolutely critical for protecting national swine herds from this devastating foreign animal disease.

Foot-and-mouth disease is caused by infection with the FMD virus, an extremely small and environmentally resilient RNA virus belonging to the genus Aphthovirus within the Picornaviridae family. Seven distinct serotypes of the virus exist globally, designated as O, A, C, Asia 1, and Southern African Territories types 1, 2, and 3, with serotype O being the most prevalent worldwide and responsible for the majority of outbreaks affecting swine populations. Critically, infection or vaccination against one serotype does not confer protection against other serotypes, meaning that animals in endemic regions may experience repeated infections with different viral strains throughout their lives. Within each serotype, numerous subtypes and topotypes have evolved, creating a complex global landscape of viral diversity that complicates vaccine development and outbreak response efforts.

The FMD virus demonstrates remarkable abilities to survive in the environment and spread through multiple transmission routes, making it one of the most contagious pathogens known to affect livestock. Direct contact between infected and susceptible animals represents the most efficient transmission route, with the virus shed in enormous quantities through expired air, saliva, nasal secretions, milk, semen, feces, and vesicular fluid from ruptured blisters. Swine are particularly efficient viral amplifiers, capable of excreting up to 400 million infectious doses per day through respiratory aerosols during the acute phase of infection, creating dense viral clouds that can spread to nearby premises through air currents under favorable weather conditions.

Indirect transmission plays a crucial role in FMD spread and accounts for many of the long-distance movements of the virus between previously unconnected premises. Contaminated vehicles, equipment, footwear, clothing, and personnel can mechanically transport the virus between farms, while feeding of improperly cooked swill or garbage containing infected meat products has historically been responsible for initiating outbreaks in FMD-free countries. The virus can survive for extended periods in meat products, particularly in lymph nodes and bone marrow where pH changes during rigor mortis do not reach the acidic levels necessary for viral inactivation, making illegal importation of animal products a constant threat to disease-free countries.

Environmental factors significantly influence FMD virus survival and transmission potential, with the pathogen remaining viable for weeks to months under cool, moist conditions while being rapidly inactivated by heat, dryness, and pH extremes. High relative humidity and overcast conditions favor aerosol transmission, with documented cases of airborne spread occurring over distances exceeding sixty kilometers over water and ten kilometers over land under optimal atmospheric conditions. Seasonal patterns in endemic countries often show increased FMD incidence during cooler, wetter months when environmental conditions favor viral persistence and aerosol transmission between farms.

Risk factors for FMD introduction and spread within swine operations include proximity to international borders, ports, or airports where illegal animal product importation may occur, as well as purchase of animals from unknown sources, use of shared equipment or transport vehicles, and feeding of food waste without proper heat treatment. Swine operations with poor biosecurity practices, multiple animal entry points, and frequent visitor traffic face substantially higher risks of disease introduction should FMD reach their geographic region. The movement of clinically normal but infected animals during the incubation period represents a particular challenge, as swine may shed virus for up to four days before developing obvious clinical signs.

Clinical signs of foot-and-mouth disease in swine typically appear following an incubation period of two to fourteen days, though most animals develop symptoms within three to five days after exposure to the virus. The initial presentation often includes sudden onset of high fever reaching 104 to 106 degrees Fahrenheit, pronounced depression, reluctance to move, and complete loss of appetite as the virus replicates systemically before localizing to epithelial tissues. During this prodromal phase, affected pigs may appear dull and lethargic, preferring to lie down rather than compete for feed or engage in normal social behaviors, and observant caretakers may notice subtle changes in behavior before more obvious lesions develop.

The hallmark vesicular lesions of FMD develop rapidly once the virus localizes to epithelial tissues, initially appearing as small blanched areas that quickly progress to fluid-filled blisters ranging from a few millimeters to several centimeters in diameter. In swine, these vesicles most commonly form on the snout, lips, tongue, dental pad, and within the oral cavity, causing severe pain that results in excessive salivation, reluctance to eat, and characteristic lip smacking or chomping movements as animals attempt to cope with mouth discomfort. The vesicles rupture within twenty-four to forty-eight hours, leaving painful eroded areas with ragged epithelial tags that may become secondarily infected with bacteria, leading to more severe tissue damage and prolonged healing times.

Foot lesions represent a particularly significant manifestation of FMD in swine, with vesicles forming at the coronary band junction between the skin and hoof, in the interdigital cleft between the toes, and on the heel bulbs and accessory digits. These lesions cause intense lameness that may be so severe that affected pigs refuse to stand or walk, often remaining in sternal recumbency and moving only when absolutely necessary. As vesicles rupture and erosions develop, secondary bacterial infections frequently complicate recovery and may lead to deeper tissue involvement, hoof wall separation, and in severe cases, complete sloughing of the hoof capsule. Observing a group of pigs with FMD often reveals multiple animals lying down, unwilling to rise even for feeding, with visible foot lesions and pronounced lameness in those few animals that do attempt to walk.

Sows and boars may develop vesicles on the teats, mammary glands, prepuce, and vulvar tissues, creating additional welfare concerns and potential complications for breeding animals. Nursing sows with mammary lesions often refuse to allow piglets to suckle due to pain, leading to piglet starvation, reduced growth rates, and increased pre-weaning mortality even in piglets that do not themselves develop severe disease. Infected boars may experience reduced libido and reluctance to mount due to foot pain, while vesicular lesions on the prepuce can temporarily impair breeding function.

Young piglets, particularly those under two weeks of age, may die suddenly from acute myocarditis without developing the characteristic vesicular lesions seen in older animals, a phenomenon sometimes called "tiger heart" due to the striped appearance of the affected cardiac muscle at necropsy. This peracute form of FMD results from viral replication within cardiac muscle cells, causing myocardial necrosis and acute heart failure that can kill piglets within hours of infection. Mortality rates in young piglets during FMD outbreaks may exceed fifty percent, even when adult animals experience relatively mild disease, making this age group particularly vulnerable to severe outcomes.

As the disease progresses, affected swine typically show gradual improvement in appetite and demeanor once fever subsides, though foot lesions may continue to cause significant lameness for several weeks during the healing process. However, some animals develop chronic lameness due to permanent hoof damage, secondary osteomyelitis, or tendon and ligament injuries sustained while compensating for painful feet. Weight loss during acute illness can be substantial, with finishing pigs losing days to weeks of growth performance that may never be fully recovered, while breeding animals may experience extended periods of reproductive failure following infection.

Diagnosis of foot-and-mouth disease requires immediate involvement of regulatory veterinary authorities due to the reportable nature of this foreign animal disease, and any practicing veterinarian who observes vesicular lesions in swine is legally obligated to report their findings before conducting further investigations. Initial clinical suspicion arises when swine present with the classic combination of fever, salivation, lameness, and vesicular lesions on the snout, mouth, and feet, though the similarity of these signs to other vesicular diseases means that definitive diagnosis always requires laboratory confirmation. Foreign animal disease diagnosticians from state and federal veterinary services will conduct thorough clinical examinations, collect appropriate samples, and initiate preliminary containment measures while awaiting laboratory results that typically take twenty-four to seventy-two hours depending on the tests employed.

Laboratory diagnosis of FMD relies on detection of the virus or viral components in clinical samples collected from affected animals, with vesicular fluid, epithelial tissue from unruptured or freshly ruptured vesicles, and blood samples providing the most reliable diagnostic material. Enzyme-linked immunosorbent assay testing for viral antigen allows rapid identification of FMD virus and preliminary serotype determination within hours of sample receipt at qualified laboratories, while reverse transcriptase polymerase chain reaction provides highly sensitive detection of viral genetic material even in samples with degraded virus. Virus isolation in cell culture remains the gold standard for confirming viable virus presence and allows detailed characterization of the outbreak strain, though this technique requires several days to complete and is typically performed in parallel with more rapid molecular methods.

Serological testing for antibodies against FMD virus plays an important role in outbreak investigations, surveillance programs, and determination of disease-free status following eradication efforts. Animals develop detectable antibodies within seven to fourteen days after infection, and serotype-specific tests can confirm which viral strain caused infection in recovered or vaccinated animals. Differentiating infected from vaccinated animals represents a significant diagnostic challenge addressed through specialized DIVA testing strategies that detect antibodies against non-structural proteins present only following actual infection, not vaccination with purified vaccines.

Differential diagnosis of vesicular lesions in swine must consider several other conditions that produce clinically indistinguishable disease, including swine vesicular disease, vesicular stomatitis, vesicular exanthema, and Senecavirus A infection. Because these diseases cannot be reliably differentiated based on clinical examination alone, all vesicular conditions in swine must be treated as potential FMD until laboratory testing proves otherwise. Chemical and thermal burns, contact irritation from certain bedding materials or flooring, and severe secondary bacterial infections of the feet may occasionally mimic vesicular disease and must also be considered in the differential diagnosis. The inability to distinguish FMD from other vesicular diseases clinically underscores the critical importance of immediate regulatory notification whenever vesicular lesions are observed in swine, regardless of the producer's or veterinarian's preliminary assessment of the most likely cause.

Treatment of foot-and-mouth disease in swine is not permitted in countries that maintain FMD-free status, as the fundamental disease control strategy relies on rapid detection, containment, and eradication rather than management of infected animals that would continue shedding virus and potentially establish endemic circulation. When FMD is confirmed or strongly suspected in an FMD-free country, regulatory authorities implement immediate quarantine of affected premises, establish movement restrictions within defined control zones, and initiate stamping-out procedures that involve humane destruction of all susceptible animals on infected and dangerous contact premises. This approach, while economically and emotionally devastating for affected producers, represents the only proven method for rapidly eliminating the disease and restoring disease-free trading status that allows continued access to international export markets.

The stamping-out strategy typically involves depopulation of all cloven-hoofed animals on confirmed infected premises, followed by thorough cleaning and disinfection of all facilities, equipment, and vehicles before the quarantine period can begin counting down toward eventual restocking. Dangerous contacts, defined as premises with direct or indirect epidemiological links to infected farms, may also require precautionary depopulation depending on the assessed risk level and results of clinical surveillance and diagnostic testing. Movement restrictions within surveillance and control zones may extend for weeks to months, affecting all farms within designated areas regardless of their individual disease status, and creating substantial economic hardship for producers who cannot market animals or receive normal inputs during the restriction period.

Vaccination represents an alternative or supplementary control measure that may be employed during large outbreaks when stamping-out alone cannot contain viral spread rapidly enough to prevent catastrophic losses across an entire region or country. Emergency vaccination creates a buffer of immune animals around infected areas, reducing viral amplification and slowing geographic spread while stamping-out operations continue on confirmed infected premises. However, vaccination complicates the process of demonstrating disease-free status to trading partners, and vaccinated animals may still require eventual slaughter under some international trade agreements, meaning that vaccination decisions involve complex economic and political considerations beyond purely epidemiological factors.

In countries where FMD is endemic and eradication is not currently feasible, supportive treatment may be provided to valuable breeding stock to minimize animal welfare impacts and preserve genetic resources during outbreak periods. Supportive care focuses on maintaining hydration and nutrition through provision of soft, palatable feeds that animals with mouth lesions can consume, along with anti-inflammatory medications to reduce pain and fever. Foot lesions require careful management including clean, dry bedding, foot baths with mild antiseptic solutions, and in some cases topical treatments to prevent secondary bacterial infections that can permanently damage hoof structures.

Antimicrobial therapy has no effect on the FMD virus itself but may be warranted to control secondary bacterial infections that commonly complicate vesicular lesions, particularly when deep tissue involvement threatens to cause permanent lameness or systemic illness. In endemic settings where treatment is practiced, veterinarians must carefully consider withdrawal times for any medications administered to food-producing animals, ensuring that treated animals do not enter the food chain until appropriate clearance periods have elapsed. Economic considerations heavily influence treatment decisions in endemic regions, with producers weighing the costs of medications and extended care against the animal's residual productive value and market price.

Regardless of geographic location and regulatory framework, all swine producers should have established relationships with veterinarians familiar with foreign animal disease recognition and reporting procedures, ensuring that any suspicious illness triggers immediate notification rather than delayed response that allows additional viral spread. Biosecurity investments that prevent disease introduction in the first place represent far more cost-effective protection than any treatment or control measures implemented after infection has occurred.

Recovery from foot-and-mouth disease in individual swine typically occurs over a period of two to three weeks following the acute phase of illness, though the timeline varies considerably depending on the severity of lesions, presence of secondary complications, and overall health status of affected animals. Fever usually resolves within five to seven days as the immune response brings viral replication under control, and appetite gradually returns as oral lesions begin healing and mouth pain diminishes. However, foot lesions often heal more slowly than oral lesions due to continuous contact with bedding, flooring, and fecal contamination, and some animals develop chronic lameness that persists for months or becomes permanent due to structural damage to hooves and supporting tissues.

Animals recovering from FMD require careful management to optimize healing and minimize long-term complications that could affect future productivity and welfare. Soft, clean, dry bedding reduces pressure on damaged feet and minimizes contamination of healing lesions, while non-slip flooring prevents additional injuries to animals already compromised by painful foot conditions. Nutritional support through provision of highly palatable, easily consumed feeds helps recovering animals regain condition lost during the acute illness phase, though severely affected individuals may require extended recovery periods before returning to normal growth rates or reproductive function.

The prognosis for individual animal recovery varies significantly based on lesion severity, secondary complications, and the animal's role in the production system. Finishing pigs with uncomplicated disease typically recover fully and can eventually reach market weight, though lost growth during illness may never be completely compensated and extends time to market by weeks to months. Breeding animals face more variable outcomes, with some returning to normal reproductive function within one to two estrous cycles while others experience prolonged infertility, reduced conception rates, or permanent damage to mammary tissue that impairs future lactation performance.

Return to production considerations in endemic regions where recovery is permitted involve careful assessment of each animal's long-term productive potential balanced against the costs of extended recovery care and the economic value of the animal once recovered. Some producers elect to cull animals with severe hoof damage or prolonged recovery courses rather than investing in extended care, particularly when market prices are favorable or when the animal's genetic value does not justify substantial treatment investments. Biosecurity measures must remain stringent throughout the recovery period and afterward, as recovered animals may shed virus intermittently for extended periods and represent ongoing transmission risks to naive herd mates or neighboring premises.

Prevention of foot-and-mouth disease in FMD-free countries relies primarily on maintaining robust import controls, surveillance systems, and producer biosecurity practices that collectively minimize the risk of viral introduction from endemic regions. Import restrictions prohibit or heavily regulate the movement of live cloven-hoofed animals, animal products, and contaminated materials from countries where FMD circulates, with particular attention to illegal smuggling of meat products that has historically been responsible for initiating outbreaks in previously disease-free areas. International travelers returning from FMD-endemic regions should avoid contact with livestock for several days and ensure thorough cleaning of footwear, clothing, and equipment that may have contacted animals or animal facilities abroad.

Vaccination against FMD is generally not practiced in disease-free countries because it interferes with surveillance programs designed to detect viral introduction, complicates international trade negotiations, and may mask clinical signs that would otherwise trigger rapid detection and response. However, vaccine banks maintained by national governments and international organizations allow rapid deployment of emergency vaccination programs should an outbreak occur and stamping-out measures prove insufficient for containing viral spread. In endemic countries, vaccination programs using inactivated vaccines matched to circulating viral strains represent the primary prevention strategy, though the existence of multiple serotypes and frequent antigenic drift means that vaccine formulations require regular updating to maintain protective efficacy.

Biosecurity measures at the farm level provide critical protection against FMD introduction, regardless of the national disease status or vaccination policy in effect. Effective biosecurity programs establish controlled access points where vehicles, visitors, and personnel undergo cleaning and disinfection before entering animal areas, minimizing the risk of mechanical virus introduction on contaminated fomites. Dedicated clothing and footwear for use only within swine facilities, combined with shower-in and shower-out protocols for personnel, provides additional protection against inadvertent virus introduction by farm workers who may have contact with other livestock operations or potentially contaminated environments.

Swine producers should source replacement animals only from known suppliers with documented health status and should implement quarantine protocols for all incoming stock, isolating new arrivals for a minimum of thirty days while observing for any signs of illness before allowing contact with the resident herd. Feed security measures that prevent access by wildlife and exclude potentially contaminated ingredients reduce another potential introduction route, while proper disposal of food waste and prohibition of swill feeding eliminates the garbage-feeding pathway that has initiated multiple historical outbreaks. Participation in industry-organized health monitoring programs and prompt reporting of any unusual illness patterns supports early detection efforts that benefit the entire swine industry.

Emergency preparedness planning ensures that individual farms and the broader industry can respond rapidly and effectively should FMD be detected, minimizing spread during the critical early days of an outbreak when containment is still achievable. Producers should maintain current inventories of animals and contacts, establish relationships with veterinarians trained in foreign animal disease recognition, and understand their legal obligations for reporting suspicious diseases to regulatory authorities. Industry-wide exercises that simulate outbreak scenarios help identify gaps in response capabilities and build the collaborative relationships between producers, veterinarians, and government agencies that prove essential during actual emergency responses.

Daily management of swine operations in the context of foot-and-mouth disease prevention requires constant attention to biosecurity protocols and health monitoring practices that collectively minimize disease introduction risk while enabling early detection should prevention measures fail. Farm managers should implement structured daily observation routines that include careful examination of animals for any signs of lameness, excessive salivation, reduced feed consumption, or visible lesions on accessible body parts, with immediate isolation and veterinary consultation for any animals showing suspicious clinical signs. Training all personnel in recognition of vesicular disease symptoms ensures that multiple observers are alert to potential problems and understand the critical importance of immediate reporting rather than delayed investigation of concerning findings.

Housing and environmental management for FMD prevention focuses on controlling access to swine facilities and maintaining conditions that would support rapid response should disease occur. Perimeter fencing that excludes unauthorized visitors, wildlife, and stray domestic animals provides fundamental protection against direct contact transmission routes, while controlled entry points allow enforcement of cleaning, disinfection, and personal protective equipment requirements for all personnel and vehicles entering animal areas. Facility design should allow for rapid depopulation and thorough cleaning if required, with consideration given to carcass disposal capabilities, drainage systems that prevent environmental contamination, and equipment that can be effectively disinfected or disposed of during emergency response operations.

Herd health programs in swine operations should incorporate FMD awareness within their broader disease prevention and surveillance frameworks, ensuring that veterinary visits include assessment of biosecurity compliance and discussion of any industry disease alerts or regional concerns. Record-keeping systems must maintain accurate inventories of all animals on the premises, document all animal movements both onto and off of the farm, and track all visitors and service providers who enter animal areas, creating the contact tracing foundation essential for effective outbreak investigation. Electronic identification systems facilitate rapid inventory assessment and movement tracking, proving invaluable during disease emergencies when regulatory authorities require detailed premises information within hours of disease confirmation.

Monitoring programs should extend beyond individual animal health to encompass population-level indicators that might suggest disease introduction before obvious clinical cases develop. Sudden increases in mortality rates, drops in feed consumption across multiple groups, or clusters of lameness cases warrant immediate investigation and should trigger consultation with veterinary professionals experienced in differential diagnosis of swine diseases. Production records that track growth rates, reproductive performance, and mortality trends provide baseline data against which emerging problems can be detected, potentially identifying disease introduction during early stages when intervention has the greatest chance of success.

Economic considerations for FMD preparedness include insurance coverage for potential losses, financial reserves sufficient to survive extended quarantine periods, and business continuity planning that addresses scenarios ranging from temporary movement restrictions affecting the local area to complete depopulation of the operation following disease confirmation. Producers should understand the compensation programs available through government indemnity schemes and private insurance policies, recognizing that these payments typically cover only a portion of actual losses incurred during disease outbreaks. Diversification of income streams, maintenance of strong lender relationships, and conservative financial management provide resilience that helps operations survive the catastrophic but unpredictable events that FMD outbreaks represent.

All breeds of domestic swine demonstrate equal susceptibility to foot-and-mouth disease infection, as the virus exploits cellular receptors and replication machinery common to pigs regardless of their genetic background or breeding purpose. Commercial breeds including Yorkshire, Landrace, Duroc, Hampshire, and Berkshire show no meaningful differences in infection rates or clinical disease severity when exposed to FMD virus under equivalent conditions. Similarly, heritage and rare breeds, miniature pigs kept as companion animals, and wild boar and feral swine populations all readily develop infection when exposed to the virus, making breed selection an ineffective strategy for FMD risk management.

Production system characteristics influence disease impact and spread potential more significantly than breed genetics, with intensive confinement operations facing different risk profiles compared to extensive outdoor or pasture-based systems. Large commercial facilities housing thousands of animals in close proximity may experience explosive within-herd transmission once virus is introduced, potentially generating enormous quantities of infectious aerosol that threatens neighboring premises across considerable distances. Conversely, outdoor operations may have reduced animal-to-animal contact intensity but face greater challenges in implementing biosecurity controls that prevent wildlife contact and environmental contamination from infected premises in the surrounding area.

Genetic selection and breeding programs offer no current options for enhancing FMD resistance in swine populations, as no natural resistance genes have been identified that could be selected for or introgressed into commercial breeding lines. Research into potential genetic engineering approaches for creating FMD-resistant livestock continues in academic settings, but practical application of such technologies remains years to decades away and would face substantial regulatory and market acceptance hurdles before commercial implementation could occur. For the foreseeable future, prevention through biosecurity, vaccination where practiced, and rapid response to incursions remains the only viable approach to protecting swine populations from this devastating disease.

Several other vesicular diseases produce clinical presentations in swine that are indistinguishable from foot-and-mouth disease, requiring laboratory testing for definitive differentiation and creating diagnostic emergencies whenever vesicular lesions are observed. Swine vesicular disease, caused by an enterovirus closely related to human coxsackievirus B5, produces identical vesicular lesions on the feet and snout of infected pigs and was historically confused with FMD before reliable diagnostic tests became available. Vesicular stomatitis, a rhabdoviral disease that also affects horses and cattle, periodically occurs in North American swine populations and must be differentiated from FMD through laboratory testing despite causing clinically similar vesicular lesions.

Senecavirus A infection has emerged in recent years as an increasingly recognized cause of vesicular disease in swine, producing foot and snout lesions along with neonatal mortality that closely mimics FMD presentation. This picornavirus circulates in North American swine populations and has caused diagnostic confusion and regulatory concern when affected herds initially present with vesicular lesions requiring foreign animal disease investigation. While Senecavirus A infection typically causes milder disease than FMD and does not trigger the same regulatory response once confirmed, the clinical similarity underscores the impossibility of differentiating vesicular diseases based on physical examination alone.

Secondary complications of FMD include bacterial infections of damaged epithelial tissues that may progress to deeper structures, potentially causing osteomyelitis, septic arthritis, or systemic bacteremia in severely affected animals. Myocarditis in young piglets represents a serious direct complication of FMD virus infection, causing sudden death without typical vesicular lesion development and potentially devastating preweaning mortality rates during herd outbreaks. Long-term sequelae in recovered animals include chronic lameness due to permanent hoof damage, reduced reproductive performance in breeding stock, and persistent growth deficits in animals that experienced severe acute disease during critical developmental periods.