FIG. 1. Goat showing oral vesicles caused by orf virus infection.
Orf (contagious ecthyma, contagious pustular dermatitis, scabby mouth) is a contagious disease of sheep and goats caused by a virus in the family Poxviridae and genus Parapoxvirus. The virus is one of the largest viruses; the brick-shaped 170 to 260 by 300 to 450-nm-diameter capsid contains a linear nonsegmented double-stranded DNA genome of approximately 140 kilobases that is surrounded by a layer of lipid that is not a true envelope (1, 3). The disease is endemic in most countries worldwide.
Orf is transmitted through breaks in the skin and most commonly causes lesions on the mouth, nose (Fig. 1), and teats. Lambs are most susceptible but often infect the mother’s teats when feeding. The resulting lesions on the teats of the mother often cause her to withhold feeding, which causes the lambs to attempt feeding on nonmother ewes, which leads to spread of the disease. The disease is rarely fatal and usually resolves within 2 months. Because the virus does not leave the epidermis, an effective immune response does not occur and subsequently sheep can be reinfected with orf virus throughout their lives. Available vaccines are made from scabs of infected animals, however the immunity is short-lived and the vaccine needs to be readministered to scarified skin yearly several weeks before lambing (2).
Orf is zoonotic and humans with breaks in their skin can contract the disease in the form of a painful red swelling and vesicle at the site of infection; care should be taken when working with sheep suffering from orf virus. The lesions generally take 2 months to heal and may be accompanied by fever (4).
In sheep and goats, orf virus can often be misdiagnosed as bluetongue virus infection, sheep and goat pox virus infection, or peste des petits ruminats, all of which have similar signs and rash. In humans it appears similar to the zoonotic diseases tularemia, cutaneous anthrax, and erysipeloid. Diagnosis is usually based on the signs, serology is rarely used, as the virus is very difficult to isolate and grow. Complement fixation tests are available that detect antibodies in the serum bound to viral antigens and induce the complement cascade, but this test is rarely used (4).
References.
1. Espisito J. J., and F. Fenner. 2001. Poxviruses, p. 2885–2922. In D. M. Knipe and P. M. Howley (ed.), Fields virology. Lippincott Williams and Wilkins, Philadelphia, Pa.
2. Fenner, F., P. A. Bachmann, E. P. J. Gibbs, F. A. Murphy, M. J. Studdert, and D. O. White. 1987. Veterinary virology, p. 397–398. Academic Press, Inc., Orlando, Fla.
3. Moss, B. 2001. Poxviridae: the viruses and their replication, p. 2849–2884. In D. M. Knipe and P. M. Howley (ed.), Fields virology. Lippincott Williams and Wilkins, Philadelphia, Pa.
4. Roberts, W. A., and G. A. Carter. 1976. Essentials of veterinary virology, p. 99. Michigan State University Press, East Lansing, Mich.
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