How to prevent and treat leptospirosis in pigs? Introduction to practical methods
Leptospirosis is a zoonotic and naturally focus-borne infectious disease caused by pathogenic Leptospira. The clinical symptoms of the disease are diverse in manifestation; leptospirosis in pigs is generally a latent infection, but outbreaks also occur from time to time. Acute cases are characterized by fever, hemoglobinuria, anemia, edema, abortion, jaundice, hemorrhagic diathesis, and necrosis of the skin and mucous membranes. The carrier rate and incidence in pigs are relatively high. The disease is distributed worldwide and occurs frequently in tropical and subtropical regions. It occurs and spreads in many provinces and cities in China, with more cases in the Yangtze River basin and various parts of the south. In recent years, the occurrence and spread of leptospirosis in pigs have increased somewhat, and it has been reported in Fujian, Heilongjiang, ** and other places.
Pathogen
The pathogen of leptospirosis belongs to the genus Leptospira, specifically Leptospira interrogans. Leptospira interrogans is pathogenic to humans, livestock and wild animals. Leptospira has many serogroups and serovars; at present, 25 serogroups and at least 190 different serovars of pathogenic Leptospira have been discovered worldwide. The serogroups (serovars) causing leptospirosis in pigs include the Pomona group, the Hebdomadis group, the Autumnalis group, and the Icterohaemorrhagiae group, among which the Pomona group is the most common.
Epidemiology
Pigs of all ages can be infected, but piglets are more commonly affected, especially suckling piglets and weaned piglets, which are the most severely affected; medium and large pigs generally have milder disease, and sows do not become ill. The main sources of infection are diseased pigs and carrier pigs. Leptospira can be shed into the environment through the urine, milk and saliva of carrier and diseased pigs, contaminating it. Pigs shed large amounts of the bacteria and shed for a long period, and they have the most opportunities for contact with humans, thus posing a great threat to humans as well. After humans are infected, they can also carry and shed the bacteria. Complex cross-transmission exists between humans and animals, which is of great significance in epidemiology. Rats and frogs are also very important sources of infection, as they are natural reservoir hosts of the bacterium. Rats can carry the bacteria for life and shed them through urine, causing long-term environmental contamination. Frogs mainly contaminate water sources through urination.
Leptospirosis is transmitted through direct or indirect routes, mainly through the skin, followed by the digestive tract, respiratory tract and reproductive tract mucosa. Bites of blood-sucking insects, artificial insemination and mating can all transmit the disease. The occurrence of the disease has no seasonality, but the peak of prevalence is in the rainy seasons of summer and autumn. The disease often occurs sporadically or as an endemic outbreak.
Clinical symptoms
Clinically, leptospirosis in pigs can be divided into acute, subacute and chronic types.
1. Acute type: Mostly seen in piglets, especially suckling piglets and nursery pigs, occurring as outbreaks or sporadic cases. The incubation period is 1 to 2 weeks. Clinical symptoms include sudden onset, body temperature rising to 40-41°C, remaining high for 3-5 days; diseased pigs are depressed, anorexic, have diarrhea, dry skin, jaundice of the whole body skin and mucous membranes, nervous weakness and tremor of the hind limbs; some cases develop hemoglobinuria with urine the color of strong tea; feces are green with a foul odor, and blood in the feces can be seen in long courses. The mortality rate can reach over 50%.
2. Subacute and chronic types: Mainly characterized by damage to the reproductive system. At the beginning of the disease, body temperature rises to varying degrees, the ocular conjunctiva is congested and edematous, sometimes yellowish, and some have edema of the lower jaw, head, neck and whole body. Sows generally have no obvious clinical symptoms, and sometimes may show fever and agalactia. However, sows less than 4-5 weeks pregnant can abort and have stillbirths 4-7 days after infection with Leptospira, with an abortion rate of 20%-70%. Sows infected in late pregnancy may give birth to weak piglets that cannot stand, cannot suckle, and die within 1-2 days.
Pathological changes
1. Acute type: This type is mainly characterized by septicemia, generalized jaundice, and extensive hemorrhage and necrosis of various organs and tissues. There is yellow staining and varying degrees of hemorrhage in the skin, subcutaneous tissue, serosa and visible mucous membranes, liver, kidneys and bladder and other tissues. The skin is dry and necrotic. There is turbid yellow fluid in the thoracic cavity and pericardium. The spleen is enlarged and congested, and hemorrhagic infarction may sometimes be seen. The liver is enlarged, clay-yellow or brown, friable; the gallbladder is full and congested, and hemorrhagic foci can be seen under the capsule. The kidneys are enlarged, congested and hemorrhagic. The lungs are congested and edematous, with bleeding points on the surface. The bladder contains red or dark yellow urine. The intestines and mesentery are congested, and the mesenteric lymph nodes, inguinal lymph nodes and submandibular lymph nodes are enlarged and grayish-white.
2. Subacute and chronic types: Manifested as tissue edema in various parts of the body, most obvious in the head and neck, abdomen, chest wall and limbs. Hemorrhage in the kidneys, lungs, liver and epicardium is obvious. Excessive yellow fluid and fibrin are often seen in the serous cavities. The liver, spleen and kidneys are enlarged. In chronic cases in adult pigs, renal lesions are the most obvious.
Diagnosis and differentiation
Leptospirosis can only be definitively diagnosed on the basis of clinical symptoms and pathological necropsy, combined with microbiological and immunological diagnosis.
1. Microbiological diagnosis: Before death of the diseased animal, blood and urine can be collected. Postmortem examination must be carried out within 1 h, and no later than 3 h, otherwise most of the bacteria in the tissues will dissolve. Tissues such as the liver, kidney, spleen and brain of dead pigs can be collected; the samples should be processed immediately and examined directly under a dark-field microscope or by immunofluorescence antibody method. The bacteria in pathological tissues can be examined after Giemsa staining or silver impregnation staining. The samples can be used for isolation and culture of the pathogen.
2. Serological diagnosis: Mainly includes the agglutination-lysis test, micro complement fixation test, enzyme-linked immunosorbent assay, charcoal agglutination test, indirect hemagglutination test, indirect fluorescent antibody method and latex agglutination test.
3. Animal test: Samples (blood, urine, tissue suspension) can be inoculated intraperitoneally or subcutaneously into young guinea pigs. If the Leptospira is strongly virulent, the animals may show typical symptoms such as fever, jaundice, refusal to eat and emaciation 3-5 days after inoculation, and finally die. When body temperature rises, heart blood can be collected for culture to detect the pathogen.
4. Molecular biological diagnostic techniques: DNA probe technology and PCR technology can be used to detect the pathogen in samples.
5. Differential diagnosis: Leptospirosis in pigs should be distinguished from porcine eperythrozoonosis, hemolytic anemia of newborn piglets, and so on.
Prevention and control measures
1. Treatment methods
① Diseased pig herds should be isolated and treated promptly, and contaminated environments, utensils and so on should be disinfected promptly.
② 10% florfenicol (0.2 mL per kg body weight, intramuscular injection, once a day for 5 consecutive days) and sulfonamides (sulfa-5-methoxypyrimidine, 0.07 g per kg body weight, intramuscular injection, twice a day for 5 consecutive days) can be used to treat diseased pigs; for severely affected pigs, vitamins and glucose can be used for infusion therapy; streptomycin, oxytetracycline and other tetracycline antibiotics also have certain efficacy.
③ Infected pig herds can be fed oxytetracycline mixed into feed (0.75-1.5 g/kg) for 7 consecutive days, which can prevent and control the spread of the disease. Pregnant sows continuously fed oxytetracycline mixed into feed for 1 month before farrowing can prevent abortion.
2. Preventive measures
① Do a good job in environmental sanitation and disinfection of pig houses.
② Timely detect, cull and dispose of carrier pigs.
③ Do a good job in rodent control to prevent water sources, feed and the environment from being contaminated; keeping dogs, chickens and ducks is prohibited.
④ Pig farms where the disease exists can use inactivated vaccines to immunize the pig herd.