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African swine fever

African Swine Fever Vs Classical Swine Fever

The current outbreak of African Swine Fever in India is the first time that the disease has been reported in the country.

 

What is African swine fever?

African swine fever (ASF) is a highly contagious haemorrhagic viral disease of domestic and wild pigs, which is responsible for serious economic and production losses.

It is caused by a large DNA virus of the Asfarviridae family, which also infects ticks of the genus Ornithodoros.

Although signs of ASF and classical swine fever (CSF) may be similar, the ASF virus is unrelated to the CSF virus.

ASF is a disease listed in the World Organisation for Animal Health (OIE) Terrestrial Animal Health Code and must be reported to the OI

Transmission and spread of African Swine Fever

The epidemiology of ASF is complex and varies depending on the environment, types of pig production systems, the presence/absence of competent tick vectors, human behaviour, and the presence/absence of wild pigs.Routes of transmission can include:

  • direct contact with infected domestic or wild pigs;
  • indirect contact, through ingestion of contaminated material (e.g. food waste, feed, or garbage);
  • contaminated fomites, or biological vectors (soft ticks of the genus Ornithodoros) where present.

 

Public health risk

ASF is not a risk to human health.

Clinical signs of African Swine Fever

Clinical signs and mortality rates can vary according to the virulence of the virus and the type/species of pig:

Acute forms of ASF are characterised by high fever, depression, anorexia and loss of appetite, haemorrhages in the skin (redness of skin on ears, abdomen and legs), abortion in pregnant sows, cyanosis, vomiting, diarrhoea and death within 6-13 days (or up to 20 days). Mortality rates may be as high as 100%.

Subacute and chronic forms are caused by moderately or low virulent viruses, which produce less intense clinical signs that can be expressed for much longer periods. Mortality rates are lower, but can still range from 30-70%. Chronic disease symptoms include loss of weight, intermittent fever, respiratory signs, chronic skin ulcers and arthritis.

Different types of pig may have varying susceptibility to ASF virus infection. African wild suids may be infected without showing clinical signs allowing them to act as reservoirs.

Diagnosis of African Swine Fever

ASF may be suspected based on clinical signs but confirmation must be made with laboratory tests, particularly to differentiate it from classical swine fever (CSF).

Prevention and control

Currently there is no approved vaccine for ASF.

Prevention in countries free of the disease depends on implementation of appropriate import policies and biosecurity measures, ensuring that neither infected live pigs nor pork products are introduced into areas free of ASF. This includes ensuring proper disposal of waste food from aircraft, ships or vehicles coming from affected countries and policing illegal imports of live pigs and pork products from affected countries.

During outbreaks and in affected countries, control of ASF can be difficult and must be adapted to the specific epidemiological situation.

Classic sanitary measures may be employed including early detection and humane killing of animals (with proper disposal of carcases and waste); thorough cleansing and disinfection; zoning/compartmentalisation and movement controls; surveillance and detailed epidemiological investigation; strict biosecurity measures on farms.

As observed in Europe and in some regions of Asia, the transmission of ASF seems to depend largely on the wild boar population density and their interaction with low-biosecurity pig production systems. The good knowledge and management of the wild boar population and a good coordination among the Veterinary Services, wildlife and forestry authorities are required to successfully prevent and control ASF.

Depending on the epidemiological situation, the involvement of the soft tick vector should also be considered in the control programme.

 

Geographical distribution

ASF is present in wild and/or domestic pigs in regions of Asia, Europe and Africa,

What is Classical swine fever?

Classical swine fever (CSF), also known as hog cholera, is a contagious viral disease of domestic and wild swine. It is caused by a virus of the genus Pestivirus of the family Flaviviridae, which is closely related to the viruses that cause bovine viral diarrhoea in cattle and border disease in sheep. T

Transmission and spread

The most common method of transmission is through direct contact between healthy swine and those infected with CSF virus. The virus is shed in saliva, nasal secretions, urine, and feces. Contact with contaminated vehicles, pens, feed, or clothing may spread the disease. Animals that are chronic carriers of the disease (persistently infected) may show no clinical signs of illness but may shed the virus in their feces. Offspring of infected sows can become infected in the uterus, and can shed the virus for months.

It has been proven that in parts of Europe, the wild boar population may play a role in the epidemiology of the disease.

The disease has been spread through legal and illegal transport of animals, and by feeding swill containing infective tissues to pigs.

A lapinized CSF vaccine (uses Weybridge strain, UK) has been used in India since 1964 for controlling the disease.

Public health risk

Humans are not affected by this virus. Swine are the only species known to be susceptible.

Clinical signs

The disease has acute and chronic forms, and can range from severe, with high mortality, to mild or even unapparent.

In the acute form of the disease, in all age groups, there is fever, huddling of sick animals, loss of appetite, dullness, weakness, conjunctivitis, constipation followed by diarrhoea, and an unsteady gait. Several days after the onset of clinical signs, the ears, abdomen and inner thighs may show a purple discoloration. Animals with acute disease die within 1-2 weeks. Severe cases of the disease appear very similar to African swine fever.

With low virulence strains, the only expression may be poor reproductive performance and the birth of piglets with neurologic defects such as congenital tremor.

Diagnostic

Because the clinical signs are not exclusive to CSF, and vary widely, laboratory tests are required to detect antibodies or the virus itself.

Prevention and control

Treatment is not attempted. Affected pigs must be slaughtered and the carcases buried or incinerated.

The first barrier to prevent an outbreak of the CSF is to apply strict and rigorous sanitary prophylaxis, as defined in the OIE Terrestrial Animal Health Code. A good communication between veterinary authorities, veterinary practitioners and pig farmers, reliable disease reporting system, and hygiene measures protecting domestic pigs from contact with wild boar are the most effective measures to prevent the disease.

When an outbreak occurs, many actions must be set in place urgently:

  • Slaughter of all pigs on affected farms
  • Safe disposal of carcasses, bedding, etc.
  • Thorough disinfection
  • Designation of infected zone, with control of pig movements
  • Detailed epidemiological investigation, with tracing of possible sources (up-stream) and
  • Surveillance of infected zone, and surrounding area

CSF is found in Central and South America, Europe, and Asia and parts of Africa. North America, Australia and New Zealand are currently free of the disease.

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