Lumpy skin disease (LSD), caused by the lumpy skin disease virus (LSDV), is an OIE notifiable, vector-borne disease of cattle and Asian water buffalo. After FMD, LSD is one of the most economically important transboundary disease of cattle and Asian water buffaloes since rinderpest was eradicated globally. LSD typically characterised bynodular lesions development in the skin covering the part or whole body causing reduced milk yield, severe emaciation, permanent damage to hides, several secondary complications, months-long chronic debility, and incurs movement or trade bans. In Australia, LSD is considered an exotic animal disease and prohibited matter under the NSW Biosecurity Act 2015.
LSD was first observed in Zambia in 1929. Since then it has spread throughout Africa, the Middle East, South-eastern Europe, Central Asia, and more recently South Asia and China. According to FAO, the disease is endemic in several countries across Africa, parts of the Middle East and Turkey. The first outbreak in South Asia was reported in July 2019, since then it has spread to Bangladesh, India, Nepal, and Bhutan.
THE VIRUS: Lumpy skin disease virus (LSDV) is a large, double-stranded DNA virus belonging to the genus Capripoxvirus within the Poxviridae family. Other members of the family are sheeppox virus (SPPV) and goatpox virus (GTPV). There are high antigenic similarities among LSDV, SPPV and GTPV, while SPPV and GTPV serologically cross-react with LSDV, they do not cause disease in species other than their respective host.
THE HOST: LSD is an infectious disease of cattle and Asian water buffalo (Bubalus bubalis). Breeds of Bos taurus with high milk production are more susceptible than African / Asian indigenous cattle (Bos indicus). The morbidity varies from 2 to 45 per cent and is lower in Asian water buffaloes. In Australia, most of the cattle breeds are Bos taurus meaning they are highly susceptible. Mortality in cattle is usually less than 10 per cent, but can be higher in certain breeds. In wildlife, clinical LSD has been reported in Arabian oryx (Oryx leucoryx), in springbok (Antidorcas marsupialis), and experimental infection produced clinical signs in impala (Aepyceros melampus), giraffe (Giraffa camelopardalis) and Thomson’s gazelle (Eudorcas thomsonii).
SURVIVAL AND INACTIVATION OF THE VIRUS: LSDV is stable in the environment and may remain viable up to three months in dry scabs on skin, at least six months in dirty, shaded pens and infected tissue culture fluid stored at 4°C. Infected animals shed scabs from skin lesions and inside the scabs, virus may remain infectious for several months. LSDV survives in necrotic skin nodules for at least 39 days—even dried out prior to sequestration&emdash;and in air-dried hides at room temperature for at least 18 days. There are no studies published that identify how long it takes for LSDV to lose infectivity in different environments. LSDV survives well within the pH range 6.3-8.3. It is highly susceptible to sunlight, high alkaline or acid pH; can be inactivated at 55°C for 2 hours, 60°C for 1 hour or 65°C for 30 min, or by most detergents such as sodium dodecyl sulphate and detergents containing lipid solvents; 2 per cent Virkon®, 2–3 per cent sodium hypochlorite, 20 per cent chloroform, 2 per cent phenol in 15 min, 1 per cent formalin, 1:33 iodine compounds, and 0.5 per cent quaternary ammonium compounds.
NATURAL COURSE OF THE DISEASE: Characteristic nodular skin lesions appear on the head, neck, chest, abdomen, perineum, genitalia, udder and limbs. The nodular lesion becomes ulcerated followed by scab formation. The natural incubation period of the disease is up to 28 days. Clinical signs in cattle include skin nodules, lachrymation, nasal discharge, high fever (>40.5°C), loss of appetite, enlarged subscapular and prefemoral lymph nodes, sharp drop in milk yield, necrotic plaques in oral and nasal mucous membranes and reduced fertility and occasionally abortion in late pregnancy. Buffaloes may also show skin lesions. Experimentally, one third of the cattle do not show clinical signs but become viraemic and a source of infection through vectors (mosquitoes etc.). Once scabs appear, the virus has probably been circulating within the herd for at least 3–4 weeks. LSDV may be present in the skin lesions and the scabs, blood, nasal, oral and ocular secretions, and semen without visible clinical signs.
TRANSMISSION: LSD is transmitted primarily mechanically by blood-feeding arthropods but is not known to replicate in vectors. Vectors include:
Other routes of spread are iatrogenic, direct or indirect contact and artificial insemination. Flying vectors play an important role in spreading the virus over short distances (less than 50 kilometres) where transporting or walking animals play an important role in the long-distance spread. Ticks may play a role in maintaining LSDV during dry or cold seasons. Each vector has a preferred environmental, temperature, humidity and type of vegetation. In Africa, the Middle East and Europe, there is seasonality in LSD incidence due to vectors being less active during the dry season or cold winters. However, there may be no vector-free season in some Asian countries given prevalent climatic conditions. Therefore, risk factors for spreading or new outbreaks to be considered are cattle movement (short/long distance), herd immunity, farm management, environment, veterinary capacity and climate.
Collecting, recording, and analysing epidemiological data on LSD outbreaks are crucial in order to implement an effective and feasible strategy to control and monitor the impact of activities.
An outbreak investigation should prioritize the following:
a) How long the disease has been present
b) Magnitude of the problem: number of cases, the definition of epidemiological units and the population at risk
c) Possible sources of infection
d) Movements of animals, people, vehicles, or other fomites that may have spread the disease.
Data should also be included in an outbreak investigation:
Preferred sample: Skin lesions and scabs, saliva or nasal swabs, EDTA blood for PCR assay, whole blood for serum samples. Animals presenting a mild case of the disease do not usually have internal lesions, and there is no need to open severely diseased animals as their external lesions are so obvious. So, sampling from live animals would be quicker and more effective. Common tests available for LSD testing are: real-time PCR (detect Capri pox virus DNA), Species-specific PCR (Can differentiate between LSDV, SPPV and GTPV), ELISA (antibody detection), Serum/virus neutralisation test (the gold standard for antibodies to LSDV), Immunoperoxidase Monolayer Assay (IPMA), etc.
Severe cases of LSD are highly characteristic and easy to recognise but early stages of infection and mild cases may be difficult to distinguish even for the most experienced veterinarians, requiring a laboratory confirmation. The diseases may be considered as a differential diagnosis for LSD are:
In case of an outbreak, movements of cattle within the country and across borders should be restricted or totally banned and prophylactic vaccination of the entire cattle population should be carried out well in advance in at-risk areas. Currently, two types of vaccine commercially available are: Homologous vaccine (based on Neethling strain of the LSD virus) and Heterologous vaccine (based on sheep pox or goat pox virus)
Both active and passive surveillance is important for early detection and demonstrates the absence of disease for an LSD-free country like Australia. Surveillance is also important for the evaluation of the distribution and measure effectiveness of a vaccine in a country where LSD is endemic.
According to the OIE Terrestrial Animal Health Code, CHAPTER 11.9 one of the following waiting periods is applicable for regaining LSD free status
|Incidence||Waiting period to regain free status|
|When LSD occurred in a country or zone previously free of LSD and stamping out is applied||• 14 months if clinical, virological and serological surveillance are in place
• 26 months if only clinical surveillance is in place
|When LSD occurred in a country or zone previously free of LSD and stamping out is not applied||• Two years if clinical, virological and serological surveillance are in place
• Three years if only clinical surveillance is in place
|In LSD free country or zone where vaccination is used preventatively||• Eight months if clinical, virological and serological surveillance are in place|
The first known introduction of LSD into South Asia was July 2019 when Bangladesh officially reported an outbreak. In August 2019, the disease appeared in India and western China. In June 2020 LSD was again observed in China. Although there was no official report to the OIE, according to media reports LSD has spread to the southern part of India since January 2020. In Bangladesh, LSD had spread to all divisions by December 2019, despite lack of official reports and had reached the northernmost districts by March 2020. In mid 2020 LSD affected Nepal and Bhutan for the first time.
Issues identified from the Bangladesh outbreak are:
In Australia, LSD is included as a Category 3 emergency animal disease in the Government and Livestock Industry Cost Sharing Deed in Respect of Emergency Animal Disease Responses and considered as one of the top five priority emergency animal diseases. Given the diseases that are considered as differential diagnoses for LSD and some of them are endemic in Australia, LSD should be included in the differential diagnosis list in diseases with skin lesions. For early detection or keeping Australia LSD-free, routine surveillance (active and passive) is the key—where District Veterinarians are the crucial front line in NSW.