Orf is a viral skin disease categorized as a zoonosis, meaning it is naturally transmissible from animals to humans. The infection is primarily associated with small ruminants, such as sheep and goats, and is known as Contagious Ecthyma or “sore mouth” in livestock. When transmitted to humans, the disease causes a characteristic skin lesion that is typically localized and self-resolving. Orf is found globally, particularly where sheep and goat farming is common.
The Causative Agent and Primary Animal Hosts
The agent responsible for Orf infection is a large DNA virus belonging to the genus Parapoxvirus. This virus is resilient and can survive for extended periods, sometimes months to years, within dried scabs shed from infected animals or in contaminated environments like pastures and sheds. The virus’s durability contributes to its wide distribution and persistence in livestock herds worldwide.
The primary natural hosts are domestic sheep and goats, with young animals, like lambs and kids, being the most commonly and severely affected. Infection manifests as blister-like lesions that quickly develop into thick, crusty scabs. These scabs typically form around the mouth, muzzle, and nostrils, but can also spread to the udder and teats of nursing females.
The infection’s location often prevents young animals from suckling or eating, leading to weight loss and failure to thrive. This reduction in productivity causes substantial economic losses for the farming industry. Because the animal vaccine used to control the disease is a live virus, recently vaccinated animals can also pose an infection risk to humans and other livestock.
Stages of Human Infection and Symptom Progression
The virus typically enters the human body through a break in the skin, resulting in a single lesion, most frequently located on the fingers, hands, or forearms. After an incubation period of three to seven days, the lesion progresses through six clinical stages, each lasting approximately one week. The entire process typically resolves spontaneously within four to eight weeks in individuals with a healthy immune system.
The first phase is the maculopapular stage, where a small, firm, red or reddish-blue bump appears at the site of inoculation. This progresses into the targetoid stage, where a nodule forms, often exhibiting an iris-like structure with a red center, a white middle ring, and a red border. The acute stage follows as the nodule starts to weep fluid.
The regenerative stage is marked by the nodule becoming firmer and developing a thin, dry crust, often revealing tiny black dots beneath the surface. This is succeeded by the papillomatous stage, in which the lesion takes on a dry, wart-like appearance. The final phase is the regression stage, where the lesion gradually shrinks, dries out, and resolves, usually without leaving a scar. Systemic symptoms like fever or widespread body aches are uncommon.
Routes of Transmission and High-Risk Exposure
Transmission to humans occurs primarily through direct contact with an infected animal or contaminated materials. This often happens when a person has a minor cut or abrasion that allows the virus to enter the skin. Direct contact with active lesions on the animal’s muzzle, mouth, or udder is a common source of inoculation.
Contact with contaminated objects, known as fomites, also serves as a transmission route. These include items such as animal feed buckets, shears, bedding, fencing, or raw wool. Accidental inoculation can also occur from handling the live virus used in the animal vaccine.
Individuals whose professions involve regular interaction with sheep and goats face the highest risk of exposure. This includes:
- Agricultural workers (farmers, shepherds, and shearers)
- Veterinarians and their assistants
- Butchers and meat porters who handle carcasses
- People involved in cultural or religious practices requiring animal preparation
Management of Infection and Prevention Strategies
Since Orf infection is self-limiting, the typical approach is supportive management focused on symptom relief and preventing secondary complications. There is no specific antiviral treatment widely available for immunocompetent individuals, as the immune system clears the virus naturally.
The most important management step is keeping the lesion clean, dry, and covered with a waterproof dressing. This practice protects the lesion from trauma and prevents a secondary bacterial infection, which would necessitate treatment with antibiotics. In rare instances where the lesion is large, persistent, or occurs in an immunocompromised individual, a healthcare provider may consider treatments such as cryotherapy or topical antiviral agents.
Preventing infection relies on strict hygiene and barrier protection when working with susceptible animals. Individuals should wear non-porous gloves, such as latex or rubber, when handling sheep and goats, especially if the animals show signs of “sore mouth.” Thorough handwashing with soap and warm water immediately after any animal contact is a highly effective preventative measure. While animal vaccination helps control the disease in herds, it does not prevent human infection and can sometimes be a source of it.