What are the symptoms of African swine fever and how are infected pigs tested?

When it comes to African swine fever, everyone is familiar with it now, because since it first entered our country in August last year, it has been more than ten months, and from knowing nothing at all to now all breeders have some understanding of it. So whatare the symptoms of African swine fever?How do you detect which pig has become sick?

I. What are the symptoms of African swine fever?

African swine fever is a malignant infectious disease with high fever, high resistance and high mortality. Once an outbreak occurs, the mortality rate of pigs at the affected farm can reach as high as 100%, so the threat to breeders is very great. Since there is currently no effective vaccine for treatment, once a farm has an outbreak there is basically no remedial measure, and the best that can be done is to reduce losses as much as possible, so discovering abnormalities in advance is especially important.

The spread of the African swine fever virus is generally divided into three stages. The first stage is the incubation period. After the virus spreads into the farm and comes into contact with pigs, they do not become sick immediately and are no different from normal pigs. The incubation period is generally 3 to 7 days, and then it enters the second stage.

The second stage is the onset period. The onset period first shows as high fever, followed by loss of appetite, laziness and weakness, and purplish-red patches appearing on the body. This period generally lasts about 3 to 4 days, and then it enters the third stage.

The third stage is the death period of the pigs. Before the pigs die, the area of cyanosis (purple discoloration) further expands, and it is especially obvious behind the ears, on the abdomen and on the back. In the early stage before death, body temperature will drop, and this is the point that differs most from other similar diseases.

II. How do you detect which pig has become sick?

1. If you test the whole herd, that is of course the most accurate, and you can clearly know which one is infected or not. But pay attention to one problem: when sampling the whole herd, the probability of cross-infection is relatively high. Be sure to pay attention not to reuse tools and not to let personnel touch pigs crosswise, especially the mouth and nose. (But in fact, it is quite difficult to achieve absolutely no cross-contamination.)

2. If you are testing only a portion of the pigs (the process must also avoid cross-contamination), three principles are recommended:

① Sample pigs with poor health

② Sample pigs near the doorway and along the walls of the pig house (pigs that easily come into contact with the outside world, such as locations with high frequency of human entry and exit, locations more frequently visited by rats, etc.)

③ Sample a portion from each small area (for example, the same feed trough can be regarded as one small area)

3. Note that if the test result is negative, but related symptoms appear after about 4-5 days, it may be because the pig had just been infected at the time of testing, the virus content in the body was low, and the test was not sensitive. In these cases, retesting is needed. (You may also choose not to test and directly cull this pig as a suspected case, because if there really is infection, then these testing procedures will increase the risk of spread. The entire culling operation process must be disinfected throughout.)

4. Common types of samples we can test

(1) If it is a pathogen testing sample (antigen testing), then it is enough to draw anticoagulated blood from the pig's ear vein or anterior vena cava. Collecting blood from the anterior vena cava is recommended, as it is easier to perform. A total of 5ml (stored in an EDTA anticoagulant tube).

(2) If it is a serological testing sample (antibody testing), then collect separately from pigs in different disease courses and label them well to avoid sample confusion affecting the test results. For the anticoagulated blood collection described above, collect 3~5mL of blood aseptically, leave it at room temperature for 12~24h, collect the serum, place it in a disinfected penicillin vial or sample tube, seal it, label it and send it for testing.

5. Collection methods for other samples:

(1) Collection of nasal fluid: use a sterilized cotton swab to dip the secretions on the nasal mucosa and place them in a sterilized test tube.

(2) Collection of solid organs such as the spleen, lymph nodes, liver and lungs: among these, the spleen is the preferred organ, followed by lymph nodes. Spleens and lymph nodes that show pathological changes visible to the naked eye or show no obvious changes should all be within the scope of sample collection. Using aseptic operation, cut tissue samples about 1cm in diameter, add them to PBS solution containing 100μg/mL penicillin and streptomycin, store and transport them at 4℃ for testing.

(3) In addition, pig manure from sick pigs, piglet carcasses and aborted fetuses, as well as ticks, can also be sent for testing.

Overall, if a pig sick with African swine fever is not carefully identified, it is very easy to confuse it with other diseases. Through clinical necropsy, it is found that African swine fever in pigs is somewhat similar to porcine blue ear disease and acute swine fever. Other pig diseases are very easy to identify through necropsy of the sick pig.