Contagious Ecthyma/Orf: A Debilitating Disease of Small Ruminants
Gnanavel V1, Sabarinath T1, Monu Karki, Poulinlu G, Aditya Sahoo, Ajayta Rialch and V. Bhanuprakash1
Indian Veterinary Research Institute, Mukteswar 263 138, Nainital District, Uttarakhand, India.
Current address: 1Indian Veterinary Research Institute, Bengaluru campus, Hebbal, Karnataka 560024, India
Introduction:
Orf is a highly contagious and debilitating disease affecting domesticated small ruminants such as sheep and goats and devastating the economy of many developing countries, including all the geographic regions of India. The disease is also known as sore mouth, contagious pustular dermatitis or scabby mouth (and it is usually more severe in goats than in sheep. Orf virus (ORFV) also infects a wide range of wild animals such as the Japanese serow, musk ox, camels, reindeer, seals, and sea lions. The disease is manifested by proliferative lesions on the mouth and muzzle that usually resolved in 1–2 months. Humans contract the infection through direct contact with affected sheep and goats, or fomite contaminated with ORFV and it is considered an occupational zoonosis. Generally, ORFV infection in humans is termed “farmyard pox”. Orf is an English word which means ‘rough’ and the disease was termed so, because of the appearance of the skin after infection with the orf virus.
Virus Characteristics:
The etiological agent, the orf virus (ORFV) is a prototypic member of the genus Parapoxvirus, subfamily Chordopoxvirinae, family Poxviridae. Other important members of the genus are pseudo-cowpox virus (PCPV), bovine papular stomatitis virus (BPSV) of cattle and the parapoxvirus of red deer in New Zealand. The ORFV virion with a size of 260×160 nm shows a well-characterized ovoid structure. Negative staining electron microscopy reveals its unique criss-cross pattern, which is basically due to the superimposed images of spiral tubules surrounding the virion surface. Regarding the physiochemical properties of ORFV, the virus is resistant to heat, ether and other lipid solvents. ORFV can survive for years in dry and extremely cold environmental conditions as the virus is trapped in an infected skin scab. ORFV is a large enveloped dsDNA virus that replicates in the cytoplasm of the infected host cell. The genome size is 135 kbp which encodes approximately 132 proteins. The high GC content (62-65%) of ORFV presents more similarity with the Molluscum Contagiosum virus. The central conserved region of ORFV encodes the proteins essential for viral replication, virion assembly, and morphogenesis. Terminal portions represent 20% of the genome and comprise inverted terminal regions (ITRs) at both ends, which encode virulent factors responsible for immunomodulatory effects of ORFV resulting in short term immunity and re-infection of the target hosts.
Epidemiology:
Contagious ecthyma is more common in late summer, fall and winter on pasture and in feedlots. Lambs and kids are more susceptible to disease than adults. Orf virus enters via broken, scarified or damaged skin and replicates in epidermal cells. The disease is usually transmitted through contact from infected to susceptible animals. Iatrogenic transmission of orf virus may also occur during minor or major surgical intervention, hand contact, drenching and ear tagging. Natural cross infection of orf between sheep and goats can occur. Animal with immune defects and persistently infected animals play an important role in the maintenance of the orf virus in the nature. There is only partial protection following clinical disease or vaccination. Recurrent infections can occur in 1–3 months but are less severe and heal rapidly. Lambs without maternal antibodies vaccinated at the age of 1–4 days of age develop protective immunity against contagious ecthyma.
Pathogenesis:
At the time of grazing, the dried stemmy and spiny feed may abrade the tissues of lips, nostrils, mouth as well as fore stomach. Through such abrasions virus penetrates the skin of mucosa and leads to formation of acanthosis, ballooning degeneration of spinose cells, hyperplasia of basal cells and oedematous and granulomatous inflammation of dermal cells. The virus produces the characteristic lesions in a sequence of papules, vesicles, pustules, scabs and resolution. The pustules develop within a few days. The rupture of pustules results into ulcer formation followed by thick overlaying crust or scab (Fig. 1), which is shed within 3–4 weeks leaving no scar. Though the pathogenesis of orf is simple, it becomes complex from secondary bacterial infection. The most frequent invaders are Staphylococci, alpha hemolytic Streptococci and Corynebacteria spp.
Zoonotic Importance:
Most cases of orf are reported in farmers, veterinarians, and zoo personnel, while shearing, slaughtering, drenching affected animals, as an occupational disease. Lesions in humans usually develop on the fingers, hands or forearms, and are benign in nature. The systemic forms of infection, such as erytema multiformis and generalized lymphadenopathy are very rarely seen. A rare report of a patient with an orf lesion on the nose was documented recently in a Muslim country after a ritual feast. Also, a case of orf infection in a burns patient in Iran was reported, and its phylogenetic relatedness with an Indian isolate was unexpectedly confirmed.cases of human to human transmission are very rare and have not been reported so far. Infection is generally self-limiting, but may create more serious effects in immuno-compromised persons. MNB029/98 is a well-defined human biopsy-derived strain, collected from the owner of a contagious pustular dermatitis infected flock, which can be used for comparative analysis in the case of human infections.
Diagnosis:
The contagious ecthyma can be diagnosed on the basis of characteristic lesions on the anatomic areas of predilection. It should be differentially diagnosed from sheep pox, foot and mouth disease (FMD), staphylococcal dermatitis, dermatophilosis and ulcerative dermatosis. Commonly employed laboratory tests include: electron microscopy (EM), serological tests such as agar gel precipitation test (AGPT), agglutination test, complement fixation test (CFT), enzyme linked immunosorbent assays (ELISAs), serum neutralization test (SNT), and histopathology of affected tissues. Serum neutralization test (SNT) and CFT are usually used for serosurveillance study. A titre of ≥8 and ≥20 are considered positive in SNT and CFT, respectively. Primary lamb testis, lamb kidney, fetal lamb dermis cells, fetal lamb muscle cells, ovine fetal turbinate cells, fetal bovine muscle cells and fetal bovine lung cells as well as cell line MDBK, MDOK, and Vero cells are generally used for isolation of orf virus. After 2–3 blind passages, CPE mainly ballooning, rounding and degeneration of cells are pronounced. Orf virus affected skin tissues on histopathology reveal epidermal hyperplasia with hyperkeratosis, ballooning and degeneration of keratinocytes. EM study of orf lesions in skin from musk ox, Sichuan takin and Shetland sheep reveals the presence of characteristic parapoxvirus virions which appear as crisscross pattern against an electro dense background core. The above mentioned conventional approaches could be of laborious and time consuming leading to subjective of variations. On other hand, PCR and real-time PCR methods have proven to be useful in ORFV rapid diagnosis. Nucleic acid based assays including polymerase chain reaction (PCR) based on B2L or VIR gene and restricted fragment length polymorphism (RFLP) analysis. A semi-nested PCR based on the major enveloped protein B2L gene has been reported to detect low copy number of virus particles from clinical samples. Earlier, several ORFV isolates from sheep, goats, musk ox and ruminant species were characterised by phylogenetic analysis using B2L, an envelope protein gene, to determine the genetic similarities with different countries. Later, highly sensitive, simple and specific isothermal DNA amplification technique called as LAMP have been developed to detect ORFV DNA in clinical samples and this robust assay is found 10-100 times more sensitive than conventional PCR (Fig. 2). Such field oriented nucleic acid detection techniques have been reported by several researchers targeting B2L, F1L and DNA polymerase genes for rapid diagnosis of ORFV in sheep and goats.
Therapeutic management:
Although orf is a self-limited disease, symptomatic treatment with dressings and local antiseptics are very helpful. As secondary bacterial contamination in orf virus infection is not uncommon, therefore topical and systemic antibiotics must be used in treatment schedule. Occasionally levamisole as an immunostimulant is indicated in orf virus infection. Debilitated sheep need to be treated with 10% glucose saline intravenously. Lesions should be washed with 1:100–1:10,000 KMnO4 lotion and application of 1:10 boric acid, mild antiseptic or antibiotic ointment topically with parenteral antibiotic injection is recommended to prevent secondary bacterial complications.Recently several antiviral drugs have been evaluated in therapy of orf infection and found to be highly effective in animal and humans. Antinucleoside phosphonates (ANPs) particularly cidofovir is highly effective in complicated orf virus infection in sheep, goat and human beings.The application of 1% (w/v) cidofovir cream results in milder lesions that resolved more quickly than untreated lesions. Besides the synthetic compounds, few traditional herbal therapies have been attempted in orf virus infections. Among these, plant oils obtained from seasame, castor, juice of Calotropisprocera and Euphorbia spp. have been found effective in the treatment of orf infection in India and African subcontinent whereas in France and Netherlands, Ilex aquifolium is used for curing and preventing contagious ecthyma.
Prevention and Control:
The ORFV is difficult to eradicate once it has entered a flock or herd. Although vaccination is the efficient and cost effective method of preventing the orf virus infections, sanitary measures and disinfection practices should also be implemented along with it. The isolation of infected animals can help prevent the spread of the disease. The new animals should be quarantined before mixing with other animals of a farm to prevent the entry of the orf virus. Necessary measures should be taken to prevent the virus introduction in the farm through equipment and other fomites. Animals should not be allowed to feed on rough straw or vegetation to reduce the risk of cuts in the mouth or on the muzzle as the virus can enter through the cuts. Moreover, the migration of animals from one place to another place particularly the infected animals should be prohibited.
Vaccines:
Vaccination is the only option for the efficient control of orf virus infection, as being a viral disease of sheep and goats, there is no suitable antiviral therapeutic schedule. If a herd is immunized, no new cases may be seen for a few years but when new born unvaccinated animals become dominant, the disease will recur.
Autologous vaccine:
An autologous orf vaccine can be prepared after triturating the scab material in saline followed by addition of penicillin/streptomycin. A drop of the vaccine can be used by scarification in the inner thigh region but not face or legs. Within a few days, there will be a little inflammation, swelling and scabbing and the animal will be immune for life.
Live attenuated tissue culture vaccine:
Live attenuated tissue culture orf vaccine has been found to be effective in reducing the severity of the disease. Vaccines should be used in the farm where infections have occurred in the past and recently vaccinated animals should be isolated from unvaccinated animals. India, primary lamb testes cell (PLT) adapted ORFV strain (Mukteswar) has been attenuated and evaluated for safety, efficacy and potency in goats. The cell culture adapted vaccine is found safe and efficacious by laboratory and limited field trials. However, the duration of immunity after vaccination is limited and recommended for annual vaccination of susceptible sheep and goats above 4 months of age. Outbreaks have occurred in the vaccinated animals due to break down of immunity by the virulent strain of orf virus. However, main disadvantage of this vaccine is that it can disseminate the vaccine virus strain capable of causing the disease and unable to confer solid immunity to reinfection.
Recombinant Vaccinia virus vectored vaccine:
A recombinant vaccine constructed by randomly cloning the orf virus genomic DNA into vaccinia virus vector afforded significant protection against infection with a field isolate of orf virus as compared to control lambs which received only the vaccinia virus as the vector.
Conclusions
The persistency and endemic nature of contagious ecthyma in many parts of the world, its wide host range with zoonotic potential, short-term immunity, and most importantly the unique immune evasion strategy makes it an important virus entity to be studied in detail. Detailed characterization of these virulence genes may unravel the mechanism of immune evasion strategy of ORFV and may provide co-evolutionary process of virus with host in the process of developing the resistance to immunity in same infected hosts in future. Genome wide sequencing and analysis of ORFV genome will provide insight in to genetic evolutionary relationship of circulating ORFV isolates in India and development of novel diagnostics, vaccines and antivirals for ORFV and other emerging parapoxviruses.
Selected References:
Bath, G.F., van Wyk, J.A., Pettey, K.P., 2005. Control measures for some important and unusual goat diseases in southern Africa. Small Rumin. Res. 60, 127–140.
Buddle, B.M., Dellers, R.W., Schurig, G.G., 1984. Contagious ecthyma virusvaccination failures. Am. J. Vet. Res. 45, 263–266.
Delhon, G., Tulman, E.R., Afonso, C.L., Lu, Z., de la Concha-Bermejillo, A., Lehmkuhl, H.D., Piccone, M.E., Kutish, G.F., Rock, D.L., 2004. Genomes of the parapoxviruses ORF virus and bovine papular stomatitis virus. J. Virol. 78, 168–177.
McCabe, D., Weston, B., Storch, G., 2003. Treatment of orf poxvirus lesion with cidofovir cream. Pediatr. Infect. Dis. J. 22, 1027–1028.
Mercante, M.T., Lelli, R., Ronchi, G.F., Fini, A., 2008. Production and efficacy of an attenuated live vaccine against contagious ovine ecthyma. Vet. Ital. 44, 537–542.
Mercer, A.A., Yirrell, D.L., Whelan, E.M., Nettleton, P.F., Pow, I., Gilray, J.A., Reid, H.W., Robinson, A.J., 1997. A novel strategy for determining protective antigens of the parapoxvirus, orf virus. Virology 229, 193–200.
Karki M. 2017. Genetic characterization of virulence genes of Indian orf virus isolates. MVSc thesis submitted at ICAR-Indian Veterinary Research Institute, Bareilly. Pp.1-88.
Rao, B.T., Das, H.J., Sharma, R.D., Singh, S.S., 1994. Some observations on an outbreak of sheep pox in sheep in East Godavari District, Andhra Pradesh. Livest. Advis. 19, 3–6.
Van Linger, R.G., Frank, R.G., Koopman, R.J., Jonkman, M.F., 2006. Human orf complicated by mucous membrane pemphigoid. Clin. Exp. Dermatol. 31, 711–712.
Venkatesan G, Balamurugan V and Bhanuprakash V. 2015. Development and comparative evaluation of Loop mediated Isothermal amplification (LAMP) assay for simple visual detection of orf virus in sheep and goats. Molecular and Cellular Probes 29(3): 193-5.
Venkatesan G, Balamurugan V, Bhanuprakash V. Bora D P, Prabhu M, Yogisharadhya R and Pandey A B. 2012. Rapid detection and quantification of Orf virus from infected scab materials of sheep and goats. Acta Virologica 56(1): 81-3.
Wilson, D.J., Scott, P.R., Sargison, N.D., Bell, G., Rhind, S.M., 2002. Effective treatment of severe facial dermatitis in lambs. Vet. Rec. 150, 45–46.
Fig. 1: Severe orf lesions of proliferative type around mouth region of a lamb in a field case
Fig. 2: Rapid diagnosis of ORFV infection by DNA polymerase gene based PCR (Panel A) showing 214 bp amplicon (Lane 1:50 bp DNA ladder; Lane 2-4: positive control and clinical samples) in agarose gel analysis (AGE), LAMP reaction in AGE (Panel B) showing ladder like pattern (Lane 1: positive sample; Lane 2-4: negative samples) and LAMP reaction identified by SYBR green I dye (Panel C) showing apple green fluorescence in positive sample (Lane 1) and orange brown in negative samples (Lane 2-4).
https://www.pashudhanpraharee.com/contagious-ecthyma-orf-in-small-ruminants/
https://www.sciencedirect.com/topics/medicine-and-dentistry/contagious-ecthyma
