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Strategies for Control of Classical Swine Fever

 

Classical Swine Fever is the single most economically important and economically devastating disease of swine. Like all Pestiviruses, the CSF virus has a wide variety of clinical forms and has the tendency to cause persistent infections which are the major cause of the continuance of losses due to the virus.
 
The acute forms of CSF result in high fever and high mortality with CNS signs, diarrhea, cyanosis, and a typical pattern of petechial hemorrhages in a wide variety of organs. Various chronic and atypical forms of the disease exist which on necropsy may display lymphoid necrosis in tonsils, ileum, ileocecal valve, and colon.   The cyanosis and respiratory signs and diarrhea associated with CSF may result in it being confused with other common diseases of swine, and indeed CSF may be complicated by other common swine diseases such as PRRS, PCV-2, PRV, Salmonellosis, or other bacterial causes of respiratory disease.
 
Since the late 1950’s a lapinized CSF vaccine has been in use in China and its use is widespread, with virtually all farms using the vaccine which is administered to both sows and piglets. Sows and gilts are vaccinated on a variable schedule depending upon the prevailing vaccination strategy, and piglets are typically vaccinated at about 35 days of age, although in some situations, pre-colostral vaccination of piglets is practiced.
 
Despite the heavy use of vaccines, CSF continues to be a major source of economic losses. Improper vaccination technique and improper transportation and storage of the vaccine is often blamed for the rather frequent outbreaks in the face of a rigourous vaccination programme.
 
Current practice in many herds is to monitor post-vaccination immune status in piglets to determine if vaccination has been successful. It is not particularly likely that the actual degree of practical immunity to infection can be determined by serology nor is it likely that immunity is in some way proportional to the antibody titer in every pig, since it has been observed that vaccinated pigs without antibody titers can be adequately protected against infection. However, the monitoring of antibody titers likely serves an effective watchdog function inasmuch as the Ab level is a measure of whether or not pigs were properly vaccinated.
 
Some discussion is made as to whether or not tissue culture vaccine derived from rabbit lymphnode and spleen is superior or inferior to vaccine produced in cell culture, and the relative advantages of different products in producing cell mediated immunity vs humoural immunity, etc. A review of the available literature and the technology seems to suggest that either tissue culture or cell culture derived vaccine can be effective, and it does not appear that the many vaccination failures are due to the choice of tissue vs cell culure vaccine. 
 
Immunocompetency of the piglet likewise does not appear to be a major factor, but maternal antibody from colostrum can interfere with vaccination success.
 
The CSF virus does not generally move by the airborne route, unlike PRV, PRRS, SIV, and FMD which readily spread through the air across considerable distances of several kilometers. It is unlikely that CSF would even spread from one side of a barn to another inside a closed air space unless it is carried by the movement of pigs or the movement of contaminated instruments, boots, equipment, etc.
 
Although the tracking of the disease by workers and contaminated equipment and infected pigs certainly is playing a large role in the on-farm spread of the disease, a more important and the most important factor in the spread of Classical Swine Fever is the presence of persistently infected pigs that are transplacentally infected before birth and produce tremendous amounts of virus such that the immunization programme is overcome by the high exposure level.
 
For herds that have good sanitation, are practicing all-in/all-out batch management of pig flow, and have a good vaccination programme for CSF, elimination of CSF by identification and culling of persistently infected sows is a possible strategy. Persistently infected sows can be detected by Antigen-Capture Elisa, immunochemistry, or PCR.    Vaccine virus interference is not an issue if the vaccination date is known because the vaccine virus does not persist in blood longer than 3 weeks after vaccination.