Medoplasma porcine is a chronic respiratory infectious disease in pigs caused by Mycoplasma porcine (MH). Here, let’s first sort out two concepts: Mycoplasma porcine is a disease, which is caused by the pathogen Mycoplasma porcine. Later, I abbreviated as "MH".
The first characteristic of mycoplasma porcine pneumonia is its high incidence and low mortality rate. It is often called pig asthma and pig asthma in my country. The disease is spread all over the world. According to statistics, 30% to 80% of slaughtered pigs have characteristic lesions of the disease. This disease mainly leads to a decrease in pig production performance: daily weight gain decreases by 2.8% to 44.1%, and the difference in feed and meat ratio can reach 22%, resulting in delaying the market age of pork.
1 Economic losses caused by mycoplasma pneumonia
According to statistics, the economic losses caused by simple MH infection are less than US$1/pig. Once mixed infection occurs, the losses caused may exceed US$10/pig. As can be seen from Table 1, the loss caused by pure MH infection is only US$0.63 per pig. However, the loss caused by mixed infection with Porcine Blue Ear Virus (PRRSV) is more than $9 per pig; if MH is mixed infection with Swine Influenza Virus (SIV), the loss caused by more than $10 per pig. In short, infection with MH will cause economic losses, and once mixed infection occurs, it will cause more serious economic losses.
2 Pathogenic characteristics
The pathogen MH of this disease belongs to the genus Mycoplasma and has no cell wall, resulting in its diverse morphology. Observed under a microscope, it is spherical, dot-shaped, rod-shaped and multi-morphological. MH has no cell wall, which means that when choosing antibiotic treatment, antibiotics that act on bacterial cell walls cannot be selected. Penicillins and cephalosporins such as β-lactams are ineffective against MH. How should
choose antibiotics correctly? MH Gram staining is negative, so antibiotics that are effective against Gram-negative bacteria should be selected. Both antibiotics, such as macrolides and lincoamides, are effective against Gram-negative bacteria.
MH is not strong in resistance to external environment and physical and chemical factors, and ordinary disinfectants can be killed.
If the MH is isolated and cultured, it is very slow, so it is generally not isolated in clinical practice, because it is still very complicated and difficult to succeed, and is easily contaminated by other bacteria.
MH did not invade the cells or the blood circulation system. It is colonized on the cilia of respiratory epithelial cells, which can cause cilia damage. From the schematic diagram of MH colonization of cilia (Figure 1), it can be seen that there are bronchial cilia on the epithelial cells, and MH colonization on the cilia may cause damage to the cilia once colonized.
Figure 1 Schematic diagram of MH colonization of cilia
Figure 2 Damage to cilia by MH
Figure 2 The two pictures above Figure 2 are normal cilia tissue and are not infected by MH. The two pictures below show the cilia after being colonized by MH: the cilia become shorter and greatly damaged.
3 Pathogenic mechanism What is the pathogenic mechanism of
MH? Why does colonizing cilia on respiratory epithelial cells cause pig disease? How does it cause pigs to get sick?
First, MH colonizes the cilia of respiratory epithelial cells. This process is very complicated, involving multiple pathogenic proteins (P97 protein, etc.), and also involves the interaction between the pathogen and the host.
Second, once MH is colonized onto the cilia, it will cause damage to the cilia. A normal function of cilia is to remove foreign objects entering the outside world (may include some pathogens) through continuous swing. After the cilia are damaged, the swing function will be weakened, and the removal function will be reduced or even lost, resulting in the external pathogens entering the body very easily. Therefore, Mycoplasma pneumonia is called the "door opener" of pig respiratory disease. It is like a key, opening the first door to the respiratory defense system, so it is sometimes called "key disease".
Third, after MH colonization into cilia, it also causes a large number of lung macrophages and lymphocytes and infiltration. These large amounts of lymphocytes can release inflammatory factors, causing inflammatory responses and causing lung damage.
Fourth, MH can change the function of alveolar macrophages, causing its phagocytosis and clearance functions to decrease, causing continuous infection (254 days), which is very similar to PRRSV. We know that the target cell of PRRSV is alveolar macrophages. PRRSV can cause apoptosis of alveolar macrophages, and also reduce the phagocytosis and clearance functions of macrophages, causing immunosuppression and leading to secondary infection. MH is similar, it can also cause immunosuppression after infection and cause persistent infection.
4 Interaction between MH and other pathogens
Simple MH infection causes no more than US$1/pig, but once mixed infection occurs, it will cause more serious economic losses. Why does this happen? The following is an example of mixed infections of MH and PRRSV, MH and porcine cyclovirus type 2 (PCV2).
First look at the interaction between MH and PRRSV. The picture on the left in Figure 3 shows the lung lesions after 28 days of infection with PRRSV alone, and the lung lesions are not particularly serious at this time; the picture on the middle is the lung lesions after 28 days of infection with MH and PRRSV alone, and it is obvious that lung lesions are much more serious than simple infection. In other words, MH can cause pneumonia caused by PRRSV to be more severe and last longer. MH can attract inflammatory cells (alveolar macrophages, intravascular macrophages), creating an ideal environment for continuous infection of PRRSV. Therefore, mixed infections lead to worsening of lung lesions and greater economic losses.
Figure 3 Interaction of MH with other pathogens
Figure 4 Interaction of MH with PCV2
MH with PCV2 Mixed infection of MH with PCV2 also has similar situations. Once MH and PCV2 are mixed infection, the degree of lung lesions is much more serious than that of pure infection. The reason is that MH can increase the amount of PCV2 virus and prolong the existence time, and can also promote PCV2-related lung and lymphopathy, and promote the incidence of multisystem failure syndrome (PMWS) in weaned piglets (Figure 4). Mycoplasma porcine is an important synergistic factor that causes PMWS.
Through these two studies, it can be found that mixed infection of MH and other pathogens such as PRRSV and PCV2 can lead to more serious economic losses, because the degree of lung lesions after mixed infection is more serious.
5 Epidemiology
Almost all countries in the world have this disease, and the incidence rate of different pig herds is between 30% and 100%. This disease is not seasonal and can occur all year round (most frequent in the cold season of spring and winter). Pigs of different ages, genders and breeds can be infected. MH can be colonized as soon as possible in the delivery room, and clinical symptoms often occur over 9 weeks. The incubation period of the disease varies greatly, some days, some even months. The course of the disease lasts for a long time and can last for several months.
Let’s look at the transmission path. The main source of infection is the bacterial pig, and the transmission methods include vertical transmission and horizontal transmission. Vertical transmission is transmitted from sows to piglets, which often occurs in the delivery room and breastfeeding period. If the sow detoxifies during breastfeeding, the piglets it produces are highly likely to be infected with MH. Horizontal transmission is mainly transmitted through the respiratory tract. In addition, it was also found that it is also worrying that MH can spread through the air, and the distance of air transmission can be as long as 3.2 to 4.7 kilometers. This makes it more difficult to control mycoplasma porcine. It can also be transmitted through the nose contact between poisonous pigs and healthy pigs. The nose-to-nosed pigs can also be transmitted through sneezing and coughing. It is worth noting that the R value of MH infection is only 1.16, and MH infection is slower among different pig herds.
6 Clinical symptoms and pathological changes
6.1 Main symptoms
You can observe this clinical symptoms in pig farms: dry cough. There are very few cases where coughing does not occur. The cough cycle can last for several weeks, or even months.
Under experimental conditions, cough occurred 10 to 14 days after infection with MH, reaching peaks of 4 to 5 weeks, and then gradually disappeared.
In clinical cases, simple MH infection is rare, and mixed infections often occur, resulting in worsening clinical symptoms, fever, dyspnea, and loss of appetite, which will affect production performance, such as the feed-food ratio, daily weight gain, and neatness.
6.2 Pathological changes
If you have dissected a dead pig in a pig farm, you should have seen this lung - fusion bronchopneumonia. Bronchopneumonia mainly occurs in the anterior abdomen of the apical leaf, heart leaf, diaphragmatic leaf and intermediate leaf, and occurs purple-red to gray flesh consolidation (Figure 5), commonly known as "meat-like transformation", with very clear boundaries between the lesion site and normal tissue. Moreover, the lesions are often symmetrical on the left and right, but sometimes they are also asymmetric. The right pointed lobe on the right picture on Figure 5 has lung lesions, and the left pointed lobe has no lesions.
If the above pathological changes are seen when dissecting a dead pig, it often indicates MH infection. This lesion is very indicative and is a characteristic pathological change.
Figure 5 Pathological changes (eye view changes)
Figure 6 Pathological section
Figure 6 The left side is the pathological section of normal lung tissue. You can see the normal alveolar structure. The picture on the right is a typical lung pathological section after MH infection. After MH infection, a large number of inflammatory cells can be infiltrated. It can be seen that after MH infection, a large number of lymphocytes are filled with alveolars and vacuoles. These lymphocytes will release inflammatory factors and cause lung damage. At the same time, the phenomenon of vascular sleeves can also be observed.
7 Summary
In this section, we understand the characteristics of the pathogen (MH) of mycoplasma porcine pneumonia. MH has no cell wall, which tells us to pay attention to choosing antibiotics. Mixed infection with MH, PRRSV and PCV2 makes the lung lesions more severe. We also learned about the epidemiological, clinical symptoms, and characteristic pathological changes of Mycoplasma porcine pneumonia. In short, Mycoplasma porcine pneumonia is the "door opener" of respiratory diseases, also known as "key disease", providing the basis for the occurrence of severe respiratory disease - Pig Respiratory Disease Syndrome (PRDC).
