October 01, 2008
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Orf virus may cause contagious ecthyma in animals or humans

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Ab Klink, the Dutch Minister of Health, was recently asked by Marianne Thieme, a member of the Dutch Parliament, to address the issue of “a mysterious neurological disease affecting slaughterhouse personnel.” This related to the U.S.-derived reports on progressive inflammatory neuropathy in pork plant workers, an emerging syndrome reported in 2007/2008 from Minnesota and Indiana.

Following his reply, in which Klink informed the Dutch Parliament that compressed air to blow pig brains out of skulls is not applied in the Netherlands and therefore the described condition is not present, Thieme asked the minister to provide a full list of the known animal diseases that could infect humans.

Klink explained that the most extensive compilation of pathogens that may cause disease in humans was carried out in 2001, identifying 1,415 organisms, 868 of which were zoonotic. Klink further provided a list of 93 selected zoonotic agents, which, according to RIVM (National Institute for Healthcare and the Environment), “are of relevance for the Netherlands”.

Arnon Shimshony, DVM
Arnon Shimshony

This month’s column will address Orf virus, the 59th disease on the alphabetically-arranged Dutch list, a disease most probably included in similar lists elsewhere.

Orf virus, a member of the genus Parapoxvirus in the family Poxviridae, is the cause of contagious ecthyma (CE), a highly contagious zoonotic viral skin disease that affects sheep, goats and some other ruminants. CE has been found worldwide in all countries that raise sheep. In the United States, this disease is seen most often in the Western states.

Most infections in humans are localized and heal spontaneously; however, large poorly healing lesions can occur in people who are immunosuppressed.

In animals

CE, also called Orf, sore mouth or scabby mouth, occurs mainly in sheep and goats. It can also occur in alpacas, camels, reindeer, musk oxen, bighorn sheep, deer, prong-horn antelope and wapiti.

The incubation period in sheep and goats is two to three days.

In animals, CE causes skin lesions which are painful and often occur on the mouth and muzzle (the so-called buccal form), where they can cause anorexia or even starvation. The initial signs are papules, pustules and vesicles, found on the lips, nose, ears and/or eyelids and sometimes on the feet or perineal region. Lesions can also occur inside the mouth, particularly in young lambs. The skin lesions eventually develop into thick, brown, rapidly growing scabs over areas of granulation, inflammation and ulceration. The scabs are often friable and bleed easily.

Nursing lambs can transmit the virus to their dam, resulting in lesions on the teats and udder (genital form). Young animals may refuse to nurse. Foot lesions (podal form), the least common presentation of CE, are usually located on the pain-sensitive coronary bands causing transient lameness. Uncomplicated infections usually resolve in one to four weeks.

Secondary bacterial infections can occur and, in rare cases, the lesions may extend into the internal organs. Although CE usually resolves spontaneously and the mortality rate is generally low, fatality rates up to 20%, mainly in offspring, have been reported.

The orf virus is thought to enter the skin through cuts and abrasions. This virus can be carried by clinically normal sheep as well as sick animals. The virus remains viable on the wool and hides for approximately one month after the lesions have healed; it is very resistant to inactivation in the environment, recovered from dried crusts after 12 years.

CE vaccines contain live virus prepared from dried scabs or propagated in tissue culture. The duration of immunity after vaccination is controversial; outbreaks have occurred in vaccinated animals, but vaccine breaks may be due to the virulence of the strain. The vaccine can infect humans. Recently vaccinated animals can also transmit infections to humans.

In humans

CE is most common among people who are in close contact with sheep and goats, such as herders, sheep-shearers, veterinarians, butchers and abattoir workers. With the proliferation of petting farms and agrotourism in recent years, a steadily increasing number of infections in patients from urban background, particularly children, have been observed. Incomplete anamnestic inquiry of such patients may lead to misdiagnosis.

The incubation period in humans is three to seven days. Most infected people develop a solitary lesion but generalized infections have also been reported. The initial lesion is a small, firm, red to blue papule at the site of virus penetration, most often a finger, hand or other exposed part of the body. The papule develops into a hemorrhagic pustule or bulla, which may contain a central crust and bleeds easily. In the later stages, the lesion develops into a nodule, which may weep fluid and is sometimes covered by a thin crust. It eventually becomes covered by a thick crust. The skin lesion(s) may be accompanied by a low grade fever that usually lasts only a few days, or by mild lymphadenopathy.

In uncomplicated disease, the lesion heals spontaneously in three to six weeks without scarring. Secondary infections can occur. In most cases, spontaneous recovery, without scarring, occurs in three to six weeks. No deaths have been reported. Human-to-human transmission is nonexistent or very rare.

Large lesions refractory to treatment can occur in people who are immunosuppressed. Unusually large lesions have also been reported in people with atopic dermatitis. Rare cases involving the eye, as well as a generalized vesiculopapular rash on the skin and mucosa, have also been reported. Possible complications include toxic erythema, erythema multiforme and bullous pemphigoid.

Diagnosis and treatment

Contagious ecthyma can be confirmed by electron microscopy of the crust, a small biopsy or fluid from the lesion; however, this technique cannot distinguish the orf virus from other parapoxviruses. Polymerase chain reaction assays can give a definitive diagnosis. Histopathology can also be helpful. Animal inoculation into lambs has been reported. Serology and the detection of viral antigens are not ordinarily used for diagnosis.

In immunocompetent humans, contagious ecthyma is usually self-limiting. Treatment is supportive and typically consists of moist dressings, local antiseptics, finger immobilization and/or antibiotics to treat secondary bacterial infections. According to the literature, large lesions can be removed by surgery, and curettage and electrodesiccation may be used for persistent lesions. Cryotherapy has been reported to hasten recovery. However, according to the undersigned’s experience (as a sheep-disease specialist), the surgical approach deserves to be avoided as far as possible. Supportive, conservative treatment explicitly preferred. I have witnessed throughout the years several cases when, unaware of the true etiology of the condition, medics went over to surgical treatment, aggravating situations that could be resolved by dressings and antiseptics.

Prevention

Abraded or cut skin should not be allowed to contact infected animals, scabs and crusts, wool or hides. Non-porous gloves (rubber or latex) should also be considered when asymptomatic sheep, goats or other susceptible ruminants including deer are handled. This precaution may be particularly advisable when handling an animal’s mouth. The contagious ecthyma vaccines are pathogenic for humans, and gloves should also be used when vaccinating animals.

Any skin that has been exposed should be washed with soap and water. Some sources suggest additional disinfection with 70% isopropyl alcohol after washing. People who are immunosuppressed should avoid contact with infected animals.

For more information:
  • Arnon Shimshony, DVM, is Associate Professor at the Koret School of Veterinary Medicine Hebrew University of Jerusalem, Rehovot, and is the ProMED-mail Animal Diseases and Zoonoses Moderator. Dr. Shimshony was Chief Veterinary Officer, State of Israel, from 1974 to 1999.