Study on Porcine Parvovirus and Its Differential Diagnosis

Porcine parvovirus (PPV) is one of the most common causes of reproductive failure in pigs worldwide. Since it was reported in Britain in 1967, it has become widespread all over the world. China began to study the disease in the early 1980s, and the Chinese Veterinary Medicine Supervision Institute was the first to isolate the virus in China in 1982. Relevant statistical data show that porcine parvovirus disease has been widespread and epidemic in China.

The economic cost of porcine parvovirus infection is also enormous. According to the United States, the annual reproductive loss caused by this disease amounts to 4.5 million US dollars. Zhao Zhanmin investigated 15 porcine parvovirus positive farms in Beijing, 1288 primiparous sows, mummification and weak fetus rate 59.16%, stillbirth rate 49.17; Guangxi investigated 2024 primiparous sows, abnormal offspring accounted for 42.2% of the total number of births.

Porcine parvovirus infection causes sows, especially primiparous sows, to produce stillbirths, malformed fetuses, mummified fetuses, miscarriages and weak piglets. The disease occurs in susceptible sows infected with the virus during the first 70 days of gestation. Sows returning to estrus, pseudopregnancy (no farrowing), reduced litter size and overall reproductive performance of the herd.

However, recent studies have found that the virus is a co-pathogen of circovirus infection and a cause of the latter epidemic. Circovirus is the main pathogen that causes the decline of pig physique, emaciation, diarrhea, dyspnea and multi-system weakness after weaning.

1 Etiology

Porcine parvovirus is the pathogen of porcine parvovirus, belongs to the family parvovirus, parvovirus genus. The virions are round or hexagonal in shape, unenveloped, 20nm in diameter, and have a single-stranded genome. The virus can grow and reproduce in pig primary cells such as pig kidney, pig testis cells and passage cells, and cell pathological changes can be observed. The virus antigen in cytoplasm can be detected by immunofluorescence technique, and the virus can produce nuclear inclusion bodies in cells. The virus agglutinates red blood cells of human, guinea pig, mouse and chicken.

Parvovirus virulence has strong and weak points,(1) is a virulent strain, seronegative pregnant sow infection will lead to viremia, and through the placenta vertical infection to fetal death;(2) is a cell adapted attenuated strain, pregnant sow infection can not pass through the placenta infection fetus, and is used as attenuated pre-vaccine strain. The presence of interfering defective particles in infected cells can interfere with or attenuate replication, providing sufficient time for the host to establish a resistant immune response, thus preventing viremia and secondary placental infection.

The virus is highly heat-resistant, extremely stable at 4℃, and can be stored for a long time. After heat treatment at 56℃ for 30min, its infectivity and hemagglutination ability did not lose, and it did not lose infectivity and hemagglutination activity at 70℃ for 2h, and it did not lose activity until 85℃ for 5 min.

The virus is highly resistant to various disinfectants. It was insensitive to ether, chloroform and other lipid solvents. Trypsin treatment for a short time not only had no effect on virus suspension, but also increased its infectious potency. Formaldehyde vapor and ultraviolet light also take a long time to kill the virus. PPV can survive for at least 14 weeks under natural conditions in housing and feces. PPV can be killed with 0.5% bleach or sodium hydroxide for 5min, 2% glutaraldehyde for 20 min and 3% formaldehyde for 2 hours.

2 Epidemiology

Pigs are the only known susceptible animals, and domestic pigs and wild boars of different ages and sexes can be infected. It was reported that specific antibodies were also present in the sera of cattle, sheep, cats, guinea pigs, mice and rats. The positive rate of antibodies in mice from diseased farms was higher than that in mice from negative farms.

Pigs infected with PPV are the main source of infection. The virus can be transmitted to fetuses through placenta, forming vertical transmission. At the same time, fetuses infected with PPV in one uterine horn can be transmitted to fetuses in the other uterine horn. In addition to placental infection and mating, artificial insemination infection, sow, fattening pig and boar are mainly infected through infected food, environment through respiratory tract and digestive tract or reproductive tract infection. secretions and excretions of infected pigs can remain infected for several months, contaminated pens are the main storage of PPV, so contaminated pens are still likely to be infected after four and a half months after the sick pigs are removed and cleaned by usual methods, when they are reintroduced into susceptible pigs,

Piglets infected in utero can carry the virus for at least 9 weeks, and some with immune tolerance may carry the virus and shed it for life. Sows infected with the virus produced stillbirths, live fetuses, piglets and uterine secretions contain high titers of the virus.

Viruses can be isolated from sperm cells, spermatic cord, epididymis and accessory gonads of infected boars, which are easily transmitted to susceptible sows during mating.

Fluorescent antibody examination showed that the virus was mainly distributed in some rapidly proliferating tissues in pigs, such as lymphoid germinal center, colon lamina propria, renal interstitial, nasal turbinate membrane, etc.

3 Clinical manifestations

This disease is common in primiparous sows and is generally endemic or sporadic. After the disease occurs, the pig farm may continue to have sow reproductive failure for several years. When sows are infected in early pregnancy, the mortality of embryos and fetuses can be as high as 80%~100%.

Viremia can be produced 1~6 days after acute PPV infection in pigs, virus can be excreted in feces 1~2 weeks, and hemagglutination antibody can be detected in serum 7~9 days after PPV infection.

Acute infections in piglets and sows usually present as subclinical cases, but the virus can be found in many tissues and organs, especially lymphoid tissues. Sows infected in different pregnancy periods can cause different symptoms such as stillbirth, mummified fetus and abortion. Mummy fetuses are the main infection at 30~50 days of pregnancy. Pregnancy 50~60 days infection is more likely to occur stillbirth. Infected sows at 70 days of gestation often have miscarriage symptoms. Transplacental infection can occur in sows infected in the middle and late stages of pregnancy, but the fetus can often survive in utero without obvious clinical symptoms. After 70 days of gestation, most fetuses can develop a meaningful immune response to viral infection and survive, but these piglets often carry antibodies and viruses. When early embryos are infected, transmission of the virus in utero may be less common because the dead embryo can be rapidly absorbed by the mother, effectively eliminating the source of infection in utero.

Previous [1] [2] Next