Orf infection in sheep and lambs

The disease is caused by a parapox virus that only infects sheep, goats or humans. Occuring worldwide, the orf virus only grows on the surface of the skin and in the mouth and pharynx.

Orf scabs contain millions of virus particles which, when the dry up and drop off the animal, will contaminate the environment.

Scabs can remain infective for a very long time in a dried form – up to 12 years – though the virus will usually not survive an adverse climate (bright sunlight, winter conditions, etc). It can, however, over-winter in buildings, in soil or on carrier animals – infected animals that do not show overt clinical signs.

Young lambs born in an infected area have a specific tendency to develop severe lesions inside the mouth, involving the gums, palate and tongue and, in severe cases, tracking into the back of the mouth, resulting in difficulty of swallow. Due to the localised nature of infection, sheep can only develop a limited immune response.

Hence, sheep that were immune (due to disease or vaccination), can become re-infected and subsequently develop clinical signs once immunity wanes – usually more than a year later. The virus can only cause clinical signs where the skin is damaged – rough grazing or sharp edges on food/water troughs can predispose to infection. Infection pressure can also build up in infected buildings in larger flocks.

A flock can become infected through contaminated bedding, feed or trucks, or by direct contact with infected animals (eg replacements brought onto the farm or contact at marts/shows).

Clinical signs

Although the disease can strike at any time of the year, infections are most commonly seen in young lambs and their ewes in spring time, and in older lambs in the late summer. Typically, disease is characterised by the development of painful scabby lesions around the mouth, lips and nostrils of lambs. The infection may also affect other parts, particularly the teats of ewes and lower legs of lambs. Ewes often develop infection on the udder from infected lambs suckling. Nursing ewes may abandon their lambs, leading to lamb starvation and secondary mastitis.

Additionally, the virus may be spread to other ewes, as hungry, infected lambs try to feed from unaffected dams. Shared facilities, once infected, such as drinking troughs, mineral licks and feeders can facilitate spread.

Disease normally can last for four to six weeks though scabs can become extensive or secondarily infected with bacteria and result in serious disease, and occasionally death.

Treatment of orf

As orf is a viral disease, there is no reliable treatment. Anti-viral drugs have been used in humans successfully to treat infection, though poor results were obtained in sheep using similar products.

Most sheep recover on their own within a few weeks, but you can help prevent the establishment of secondary bacterial infections with the use of topical or, if necessary, injectable antibiotics.

Lambs with severe lesions may have problems feeding and should be fed artificially and treated with injectable antibiotics.

Ewes, which are affected on the teats or udder, should receive special attention to insure that they do not develop mastitis.

Protecting a flock

There are several protective measures that may help reduce the risk of infection. These include:

A lamb receiving an orf vaccine.

Human protection

Humans can also become infected with the orf virus through broken skin, resulting in localised swollen, red areas which can be extremely painful. Those working with sheep can protect themselves by following good hygienic practices and by wearing protective gloves when handling infected animals. Infected people often develop sores on their hands. The sore may be painful and can last for two months. Infected people rarely infect others.

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