Background and history

CSF is one of the most economically-damaging pandemic viral diseases of pigs in the world. Many governments take it very seriously and adopt strict control policies, which include compulsory vaccination or slaughter and eradication policies.

In a susceptible (unvaccinated) herd almost all the pigs are affected. It causes generalised disease, including fever, malaise, lack of appetite, diarrhoea, paralysis, abortion, mummification and the birth of shaking piglets. Mortality is high.

Fortunately, there is only one serotype of the virus and attenuated vaccines are highly effective. Also, it does not spread on the wind or on insects or birds so standard precautions of farm biosecurity should keep it out. However it persists in uncooked and cured meat and these should not be fed to pigs.

Clinical signs

The virus that causes CSF varies in virulence. Some strains are highly virulent and cause acute (i.e. rapid) serious disease. Some strains are of low virulence and cause chronic (i.e. long-lasting) disease, others are intermediate causing sub-acute disease.

Acute disease

• Clinical signs usually appear first in a small number of growing pigs which show non-specific signs of depression, sleepiness, and reluctance to get up or to eat. If you get them up they may wander to the feeder but eat very little or nothing and wander away again to lie down. They walk and stand with their heads down and tails limp. Over the following few days these signs get worse and more pigs become affected.
• Younger piglets may appear chilled, shiver and huddle together.
• Initially affected pigs may appear to be constipated but this generally changes to a yellow-grey diarrhoea as the disease progresses. Early on some of the pigs may develop conjunctivitis (inflammation of the eye surface) with thin discharges. This gets worse, the discharge getting thicker with time until some of the eyelids are completely closed and adhered.
• A constant early sign, which persists throughout the disease until just before death, is a high fever, over 42ºC (107ºF). Check the sick pigs' rectal temperatures. If they are all high suspect CSF.
• As the disease progresses the affected pigs become very thin and weak and develop a staggering walk. Initially this is probably through weakness but later it is due to infection of and damage to the spinal nerves. Partial paralysis of the hind end results in a drunken walk and a tendency to fall to a sitting or lying position. Diarrhoea worsens and some pigs vomit a yellowish bile. The pigs' skins go purple, first over the ears and tail, followed by the snout, lower legs, belly and back. Affected pigs die in 10-20 days. Some pigs go into convulsions before death.

Sub-acute disease

The early signs in growing pigs are similar but they progress more slowly and are less severe. Affected pigs may be ill for up to 30 days before they die.

Chronic and aberrant disease and persistent infection

The virus can cross the placenta and infect the piglets in the sow's uterus. Sows that have been inadequately vaccinated that become infected, or sows which become infected with a virus of low virulence, may appear normal but give birth to shaking piglets many of which die. (Note: there are also other causes of shaking or trembling piglets).

If the virus crosses the placenta before the piglets' immune systems have developed they may be born apparently healthy although possibly weak and may grow on to be persistent carriers without at first showing clinical signs. They shed virus so they are a menace to other pigs. At several weeks or months of age they may develop typical clinical signs but these are likely to be milder, last longer and without the high temperatures.

Virus that infects the piglets in the uterus may cause other effects, namely, death, mummification, abortion or the birth of weak piglets some of which may be deformed. Vaccination of sows during pregnancy with some of the original attenuated virus vaccines resulted in trans-placental infection of unborn piglets with similar adverse results. The newer attenuated vaccines are claimed to be safer.

Low virulence strains of the virus may also multiply in the reproductive tracts of unvaccinated boars or boars which have been inadequately vaccinated. The vaccine virus itself in some of the older attenuated vaccines was thought to do this, resulting in returns to service and abortions.

Diagnosis

In acute or sub-acute outbreaks a presumptive diagnosis can be made on the typical clinical signs and post-mortem lesions but African swine fever and Salmonella choleraesuis infection produce some similar signs and lesions. Salmonella choleraesuis is frequently a concurrent pathological infection with CSF virus, triggered off from its latent state by the CSF virus infection.

In chronic or aberrant cases the clinical signs and lesions are less diagnostic and may only raise a suspicion of CSF.

In all suspected cases laboratory tests should be done to confirm the diagnosis. Investigations are usually carried out by the authorities.

It is best to send whole dead pigs to the diagnostic laboratory so the pathologists can sample what they want. If only samples can be sent the tonsils are best.

Larynx of pig with swine fever, note haemorrhaging (red and dark black areas) Kidneys showing small pinpoint heammorrahges. Haemmorrhaging inside chest cavity.

The tonsils of the pig are very easy to find. Cut away the skin and flesh under and between the lower jaw bone including the tongue. The pair of tonsils are two large red patches each about the size of the end half of your thumb or perhaps slightly bigger. See chapter 15 (Swabbing). Do not freeze them but send them packed with ice.

Virus is present throughout the body. In addition to the tonsils the next best organs to send are the spleen, kidneys and last few inches of the small intestine (before it meets the large intestine).

The laboratory should be able to carry out rapid tests and let you know the diagnosis on the same day they receive the samples.

A quick and fairly accurate test, which has been used for a number of years, is the direct fluorescent antibody test (FAT) carried out on frozen sections of the tonsils or other organs. It can sometimes give false negative results but this can be avoided if a sufficient number of pigs' tonsils are sampled. The CSF virus also cross reacts in the FAT test with bovine virus diarrhoea virus (BVDV) of cattle and the border disease virus (BDV) of sheep, either of which may sometimes infect pigs.

These viruses can cause trans-placental infection of unborn piglets in the sow's uterus resulting in infertility and piglet problems similar to those of CSF. Such congenital infection may also result in newborn piglets which are shedding virus and thus infecting other pigs.

. Haemorrhaging in bladder of pig with swine fever

The false positive results obtained in an FAT may cause awkward problems if there is a control or eradication policy in force. Specific antibodies against specific antigens of the CSF virus differentiate it from BVDV and BDV. Fortunately, BVDV and BDV are relatively rare infections in pigs kept separately from cattle and sheep. One source of infection with BVDV, however, may be the feeding of raw milk by-products to pigs. Another source may be the accidental presence of BVDV in live attenuated aujeszky's disease (pseudorabies) or other vaccines. Recent vaccination (i.e. a few days) of pigs with attenuated CSF vaccine may also cause positive FAT results.

Post-mortem

Normally it requires a trained person to interpret post-mortem examination findings but in the case of CSF an untrained pig person should be able to recognise some of the more marked lesions

If pigs are laid on their backs after death and opened up for examination the picture is striking. There are usually many small haemorrhages throughout the body and larger haemorrhages in some organs such as lymph nodes. Some of these may be bright red and filled with blood. Larger haemorrhages may also be present in the lungs and under the skin.

The kidney surfaces are often described as looking like mottled ducks' eggs in that they are covered with variable sized bloody spots.

The spleen may have dark raised areas of dead tissue. Similar areas of dead tissue occur in other organs (e.g. the tonsils) but are more difficult to find.

The lungs may show severe pneumonia, haemorrhage and pleurisy usually resulting from secondary bacterial infection.

The stomach and gut are usually empty except for scant liquid which may be brightly coloured. Fairly unique lesions to look for are raised so-called 'button ulcers' on the inner lining of the large intestine near its junction with the small intestine.

Causes

Classical swine fever (CSF), otherwise known as hog cholera (HC) or just swine fever, is a specific viral disease of pigs. It affects no other species. It is a notifiable disease in most countries of the world.

Prevention

Vaccination

• In most national CSF eradication programmes and in countries which are free of the CSF virus, vaccination against CSF is not practised and generally is not allowed.
• In enzootic and high risk areas routine vaccination is practised and may be compulsory.
• Inactivated vaccines were in common use but they sometimes contained live virus which resulted in infection. Inactivated vaccines have now largely been replaced by live attenuated vaccines, the most recent of which are relatively safe and effective. Pigs develop protective immunity one week to ten days after vaccination and the immunity lasts two to three years (i.e. the lifetime of many sows and boars). Piglets which are suckled by vaccinated sows receive colostral protection which lasts about 6-8 weeks. During this time they cannot be vaccinated successfully because the maternal antibodies neutralise the vaccine virus before it has had time to stimulate an immunity.
• There is only one mutationally-stable serotype of CSF virus which produces a strong long-lasting immunity. The virus in CSF vaccines was attenuated by frequent passage in rabbits (so-called 'lapinised vaccines' or 'Chinese strain vaccines'). These are still available in some parts of the world but many vaccines now contain viruses that have been attenuated by continual growth in cell cultures.
• In a circumscribed region in which the CSF virus is endemic it is usual to blanket vaccinate all pigs over two weeks of age initially. Piglets born to vaccinated sows would be vaccinated over 8 weeks age. This policy usually results in elimination of the virus from that region.

National eradication programmes

• A number of countries in which CSF was enzootic have successfully eradicated it, most notably Canada and the USA, the UK and most EU countries on mainland Europe.
• Generally, an eradication policy starts with compulsory large scale vaccination over a limited period to reduce the level of virus in the pig population. Vaccination is then stopped and the disease is made notifiable. When CSF is then diagnosed the whole herd and other in contact animals are slaughtered. Theoretically, it would be useful to carry out national serological testing at the same time (as has been done in eradicating aujeszky's disease), but vaccination results in almost life-long positive tests. Also, BVDV and BDV infections cause false positive results.

National preventative programmes

• Countries which are free of the CSF virus prevent re-infection from outside by controlling the importation of pigs and pig meat products, unless they have been well processed, from regions in which the CSF virus is still present. In addition, swill (waste human food) containing meat products must be sterilised by heating in licensed premises.
• The disease is also legally notifiable. If a case does occur the herd is slaughtered, all in-contact pigs are traced and monitored or killed and a standstill order is placed on pig movements in the area around which the case occurred. Attempts are made to find out the source of the infection. This is likely to be illegally imported pig meat scraps which have been given to pigs without first thoroughly cooking them. Other possibilities are the illegal importation of infected semen, the return of unwashed pig lorries from an infected region and contact with wild boar or unprocessed meat.

On-farm precautions

• If you farm in a country where CSF is endemic or where there is a risk of CSF occurring consider routinely vaccinating your herd if vaccination is allowed. This will greatly reduce the possibility of contamination.
• CSF virus does not spread as readily as some other viral infections (e.g. TGE and FMD). Unlike FMD it is not windborne. Thus the conscientious application of simple biosecurity measures should keep it out of the herd.
• If CSF is in your country important precautions include reducing visitors to a minimum, taking precautions against contamination from vehicles, and not allowing pig meat products near any pigs.
• Any replacement pigs coming on to the premises should come from known safe sources and should be quarantined. In some areas the disease has become very mild and spread can go unrecognised.
• Pig buildings should be protected from stray animals, particularly wild pigs and boars.

Treatment

There is no treatment.