Sore mouth disease is a highly contagious disease which mainly affects sheep and goats. Other animals that are able to become infected include dogs, cats, reindeer, and alpacas although the incidence of the disease for these is not as common as in sheep. Sore mouth disease is rarely fatal or severe in its effects although secondary infections can occur. This increases the seriousness of the infection.
The disease orf is caused by a parapoxvirus and in some areas, an infection is also known as sore mouth disease. The pathogen mainly adversely affects the mouth and nostrils of sheep. Orf is a zoonotic disease which means it can be transmitted to humans. Close contact is necessary for this to occur and painful finger lesions are often presented.
All breeds of sheep are susceptible to infection of sore mouth disease, especially those that are used for breeding. Lambs are also particularly susceptible as they have a weaker immune system than their older counterparts. Orf is transmitted generally in areas of rough grazing since the skin must be broken for the virus to enter and infect the sheep. In addition, fomites are also able to act as sources of infection.
The signs and symptoms presented following an infection of orf include scabs and lesions around the animal’s mouth and nose. These wounds often appear to be pustular. Other sites of infection where these lesions may be observed are on young lambs’ lower limbs and the teats of ewes. The oesophagus may also be affected. Vaccinations are available in order to prevent the spread of sore mouth disease.
Transmission of Orf
Transmission of orf results from both direct and indirect contact. In rough grazing where there are risks of skin abrasions resulting from scratches from plants such as thistles or gorse. Bought in sheep may also act as sources of infection. Some sheep are able to act as carrier animals and persistently become infected and may transmit orf to other unaffected sheep. Additionally, contaminated material known as fomites including feed and bedding contribute to the spread of the disease.
The virus is able survive outside the host in infected scabs for years once they have fallen off the sheep into the environment. This increases the spread of the disease, throughout the whole year but particularly in the dry and cool summer months. Following periods of rain the virus becomes less infectious when in the environment.
Signs and Symptoms of Orf
The severity and general signs of orf vary from sheep to sheep. The main signs and symptoms of orf or sore mouth disease are the presentation of lesions around the mouth of sheep and lambs.
Initially, the skin becomes red and swollen. Essentially, blisters form and lead to pustules which can rupture. This results in lesions and scabs. Mouth, eyelids, and feet of ewes may have lesions. Udders can be infected, thus leading to mastitis and the resultant symptoms. In some cases this can halt normal mammary function which can result in death or euthanasia. Ewes may have these lesions on their teats, which mean that they can pass it on to suckling lambs. Lambs with lesions of the mouth will lose condition and weight due to the inability to suckle which can be very painful. This can lead to severe dehydration, emaciation, and death. As the disease progresses, the lesions may form thick crusty layers and can bleed. Sheep are able to become re-infected and often these symptoms are milder and so less severe.
Treatment for Orf Infections
There is no cure as such for this viral disease. Animals infected with orf and without any complications tend to recover before thirty days time. In cases of particular lesions in lambs, surgery may be required. Lambs with painful mouth lesions are sometimes bottle fed. Older sheep with mouth lesions that are preventing them from feeding may be given soft supplementary feed to reduce consequential loss in condition.
Antibiotics are not used in the treatment plan to cure viral diseases. This is because viruses cannot be destroyed by antibiotics. In spite of this, they are used if there is, or if there is a risk of, a secondary bacterial infection. This is also true if the ewes further develop mastitis following lesions on their teats as a result of infection.
Prevention of Sore Mouth Disease
Prevention is more economically worthwhile for areas where orf is of high incidence. Preventative measures include removing course plants which the sheep may come into contact with, such as thistles and gorse. Bought in animals should be isolated for a short period of time before being introduced into the flock to reduce any possible transmission. Overcrowding of sheep should also be avoided.
Vaccines are available for sheep in order to reduce the infection and spread of the pathogen throughout the flock. Ewes can be administered a live and weakened vaccine as young as two months age. In circumstances where the virus has been transmitted throughout the flock or in the nearby area, then lambs are sometimes vaccinated. They can also be vaccinated between six to eight weeks before an expectant outbreak. Pregnant ewes can be given the vaccine eight weeks before lambing to protect their lambs in case of infected teats. Generally, only during a high incidence of infection are vaccines usually ever given.
Previously infected sheep do not become completely immune to orf although if re-infected, the symptoms are usually milder and less severe. This is also true following the inoculation of a vaccine and so it reduces the severity of symptoms.
Diagnosis of Orf
Orf is diagnosed by observing the signs and symptoms presented by the infected animal. Confirmation of the disease is then made by isolating and identifying the virus from samples of the scabs. Electron microscopy is also a method of diagnosis.
Prognosis for Orf
It can be estimated that approximately every one in a hundred sheep can die following an infection of orf. In the cases where ewes can no longer feed their lambs due to damage to their mammary tissues, culling is often the chosen option.