Mitek Tarang1, Sachin Tripura2
1Department of Animal Genetics and Breeding ,ICAR-Indian Veterinary Research Institute, Izzatnagar 2Animal Nutrition Division, ICAR-National Dairy Research Institute, Karnal
Introduction:
The African swine fever (ASF) is a contagious, febrile, hemorrhagic & systemic viral disease affecting domestic and wild pigs. The disease outbreak whenever occur can cause substantial morbidity and mortality events in pig and is responsible for huge economic and production losses. The illness is found in regions of Asia, Europe and Africa, and first detected in Kenya in 1909. ASF disease is not a zoonotic disease and it does not cause disease in humans. The clinical signs and symptoms of the disease resembles classical swine fever (CSF) so closely, therefore laboratory tests are required to differentiate between them. Notifying the disease to a veterinarian as soon as any signs arise to ensure the correct quarantine and treatment procedures to control the spreading of outbreak is the earliest measures to be done.
Causative agent
The disease is caused by large, enveloped and complex double stranded DNA virus of Asfarviridae family. Till now 22 known genotypes of the virus have been found. The virus is highly resistant in environment, especially at lower temperatures. The survivability of the virus in different materials and environment are mention below:
• Several days in faeces
• Months in contaminated pens
• Upto 18 months in blood
• Over 140 days in some pork products e.g. salted dry ham
• Years in frozen carcasses
Transmission:
• By direct contact with infected animals, either domestic pig or wild boar usually with the help of their body discharges, faeces and vomits
• Indirect contact via contaminated fomites (human, vehicles , farm utensils and equipments)
• By feeding of contaminated feed and contaminated food waste or uncooked meat from infected animals
• Through the bites of arthropod vectors, particularly soft tick species in the genus Ornithodoros, lice and flies.
Clinical signs:
The symptoms can manifest in multiple ways ranging from death with no signs (peracute, mortality ~100%) to an asymptomatic infection. All age groups of pigs have been found to be equally susceptible to ASFV infection, as opposed to classical swine fever virus (CSFV) where young pigs are much more susceptible.
• Per acute form:
Sudden death occurs with with few lesions
• Acute form:
typically characterized by a high fever, reduced appetite, lethargy, weakness, recumbancy, diarrhea and/or constipation, abdominal pain, hemorrhagic signs, respiratory distress, nasal and conjunctival discharge. Abortion may occur in pregnant females. Death often occurs within 7–10 days after the onset of clinical signs. Result in mortality nearing 100%.
• Sub acute form:
There is high mortality in young animals and much lower mortality in older animals. The clinical signs often include abortion, fever, and transient hemorrhage with death or recovery occurring within 3–4 weeks.
• Chronic form:
Mortality is very low and is characterized by intermittent or low fever, appetite loss, and depression and, in some instances, result in a fatal infection. Some pig develops respiratory problems and swollen joints and arthritis. Animals that remain persistently infected for months, such as survivors or subclinically or chronically infected pigs may play a role in disease persistence in endemic regions.
Postmortem lesion:
• Acute form:
There are numerous hemorrhages of internal organs. Extensive haemorrhages occur in mandibular, renal and gastrohepatic lymph nodes. The spleen enlarges and becomes congested as well as dark red in color; petechiae on the cortical surfaces of kidney can also be seen. Oedema of the lungs, gallbladder and congestion of the brain are found.
• Chronic form:
Focal skin necrosis and skin ulcers are the common chronic lesions. Other lesions include consolidated lungs, caseous pneumonia, fibrinous pericarditis, pleural adhesion and lymph node enlargement.
Diagnosis:
Based on clinical signs and symptoms the disease can be suspected but there is need to send samples to a laboratory specialize in CSF and ASF for confirmatory diagnosis. The spleen, kidney, tonsils, and lymph nodes are the best tissues for virus collection. Enzyme-linked immunosorbent assay (ELISA), immunoblotting, and indirect fluorescent antibody assays are the most common diagnostic techniques. Conventional and real-time PCR have been developed for the detection of ASF virus.
Prevention and control
Currently there is no approved vaccine for the prevention of ASF virus and no treatment available; therefore strict biosecurity measures must be followed for controlling the disease outbreak.
The following points are important for the control and prevention of the disease:
• Prevent direct transmission between infected and susceptible pig
• Isolation of the ill pig
• Prevent contact of the domestic pigs with wild pigs
• Keep away newly acquired pigs separate from the herd for atleast 30 days before mixing with the pigs in the farm
• Do not feed uncooked pork product, swill, garbage, waste to pigs
• Disinfection of vehicles, equipment, footwear, clothing
• Follow appropriate disposal of manure and carcasses
• Control of ticks and other vectors
• Check infected regions before import of goods that could potentially be contaminated
• Advise and educate people on the risks of bringing back pork products from infected regions.
During disease outbreak
Important points listed below should be implemented whenever ASF outbreak occurs in an area:
• Infected and suspected farms must be placed under quarantine
• No movement of pigs or any pig origin should be allowed
• All infected and in contacts pigs must be humanly slaughtered
• Carcasses, animal products and bedding must be burnt or buried deeply
• Vehicle should be disinfected on entering and leaving farms
• There should be strict biosecurity measures followed
• Farms should not be restocked for atleast 40 days after an outbreak following proper disinfection of the farm premises
Recent report of ASF outbreak in India
For the first time the case of African swine fever has been reported in domestic pigs in India. Around 2,800 pigs have died in Assam since February 2020 due to the virus making the state the epicentre of ASF in India. The disease has also been reported in few districts of Arunachal Pradesh bordering Assam. The National Institute of High-Security Animal Diseases (NIHSAD), Bhopal, has confirmed the disease by laboratory diagnosis. The concerned authorities of the states are putting all the efforts to combat the disease spreading.
