Analysis of Why Pigs Still Contract Classical Swine Fever After Vaccination

Why pigs still contract classical swine fever after being vaccinated against it, causing huge losses to pig production.

The phenomenon of virus-carrying sows in classical swine fever cannot be ignored

The phenomenon of virus-carrying sows is quite common in China, with the proportion of virus-carrying sows among pregnant sows reaching as high as 43%. Virus-carrying sows cause persistent infection of classical swine fever through vertical transmission and horizontal transmission. Piglets congenitally infected with classical swine fever develop acquired immune tolerance, and after repeated vaccination they do not produce antibodies, becoming persistently infected virus-carrying pigs. If such pigs are mistakenly raised as replacement breeding stock, a new virus-carrying breeding herd will be formed, which will lead to the formation of a vicious cycle chain of classical swine fever transmission.

The quality of the vaccine itself and the method of use deserve attention

The quality of the vaccine itself directly affects the immune effect of the vaccine. A large amount of clinical data confirms that tissue vaccines (such as spleen and lymph node vaccines) are superior to cell vaccines, and classical swine fever monovalent vaccines are better than classical swine fever combination vaccines.

The vast majority of vaccines must be stored and transported at low temperatures. For example, classical swine fever vaccine should be stored and transported at -15°C, and repeated freezing and thawing is strictly prohibited. However, most grassroots distributors store it in freezers at 0°C, and the phenomenon of repeated freezing and thawing is also extremely common. In this way, the quality and potency of the vaccine will be greatly reduced.

There are also problems with the method of use. The diluted vaccine cannot be used up within 2 hours, and the vaccine becomes ineffective; disinfecting the needle with iodine tincture or other disinfectants artificially kills the attenuated vaccine; injecting with a large-gauge needle causes the vaccine to leak out; using the flying needle technique results in insufficient immune dose and missed injections; using one needle all the way through turns vaccination into virus-carrying transmission.

The immune dose of classical swine fever deserves in-depth study

Although the lapinized attenuated classical swine fever vaccine currently used in China is still safe and effective against the currently prevalent classical swine fever virus, the evaluation of vaccine immune effects and the formulation of immune standards in the past were usually based on the standard of preventing clinical infection, but there are often cases where the antibody level of some pigs after immunization cannot reach the level required to prevent subclinical infection. Therefore, the vaccine should be based on the standard of being able to prevent subclinical infection.

The subclinical infection state is mainly caused by sows carrying the virus for a long time, being in a state of persistent infection with classical swine fever virus and transmitting the virus to piglets, and the virulence of the virus is at a medium-to-low level. Although no obvious clinical symptoms are shown, lifelong virus carriage, virus shedding and virus transmission make it a major killer on pig farms.

The factors affecting the antibody level after classical swine fever vaccine immunization include the level of maternal antibodies and the antibody level of the pig herd before immunization. Appropriately increasing the immune dose can neutralize maternal antibodies and the antibodies of the pig herd before immunization, reducing their impact on the vaccine, which is conducive to raising the antibody level. When the dose is insufficient, after challenge the vaccine cannot prevent the replication and carriage of virulent virus in the body.

The immune dose recommended in the national classical swine fever immunization program is 1 dose, which is 150 RID (rabbit infective dose), and this dose is far lower than the international standard. Obviously, applying this dose in areas where classical swine fever is unstable is insufficient to cut off subclinical infection of classical swine fever, and in pig production an appropriately increased immune dose should be applied, especially in areas where classical swine fever exists. Although the lapinized attenuated classical swine fever vaccine used on that farm was tested by the laboratory according to procedures and found to be qualified, the original immunization with 1-3 doses per time failed to effectively control subclinical infection with wild-type classical swine fever virus, while switching to an immune dose of 5-10 doses per time after the outbreak proved to have good results.

The widespread existence of immunosuppression is the most important reason for the poor immune effect of classical swine fever vaccine

Regarding the problem of immunosuppression, the main causes of immunosuppression can be summarized in three aspects: first, nutrition; second, drugs; third, disease.

Nutritional factors, that is, whether nutrition is comprehensive, especially fat-soluble vitamins, protein and trace elements, etc. The problem of feed mildew is prominent. Various mycotoxins in moldy feed can cause degeneration and necrosis of liver cells, bleeding and edema of lymph nodes, severely damaging the body's immune organs and causing immunosuppression of the body.

Drug factors, such as improper use of drugs with immunosuppressive effects and so on (such as dexamethasone, bupleurum, sulfonamide drugs, etc.).

Disease factors, such as porcine reproductive and respiratory syndrome, circovirus disease, classical swine fever, streptococcus, etc. These factors cause poor growth and development of pigs, slowed or stagnant growth speed, and directly invade the pig's immune organs and immune cells, causing suppression of cellular immunity and humoral immunity. The phenomenon of “pigs still getting classical swine fever after being vaccinated against it” appears. Among pig herds in China, the infection rates of PRRSV and PCV-2 are very high, and they present as dual infection, with the dual infection rate reaching more than 50%. This shows that respiratory diseases on pig farms are quite serious, secondary infections are common, and reproductive disorder problems will be exposed at any time. These two major immunosuppressive diseases are also the most fundamental reasons why pig diseases in China have become increasingly common in recent years, why epidemics have become increasingly complex, and why pigs have become increasingly difficult to raise.

Low immunity in pigs is the most important reason for the poor vaccine effect. Abandoning the abuse of drugs, vaccines, disinfectants and hormones and using them correctly and reasonably, adopting immune health care technology for the whole herd, and building a good immune population together with corresponding vaccine prophylaxis will produce high-quality immune effects, thereby achieving a very good disease prevention effect.