Ungulate protoparvovirus 1

Species of virus

Porcine parvovirus (PPV), a virus in the species Ungulate protoparvovirus 1 of genus Protoparvovirus in the virus family Parvoviridae, causes reproductive failure of swine characterized by embryonic and fetal infection and death, usually in the absence of outward maternal clinical signs. The disease develops mainly when seronegative dams are exposed oronasally to the virus anytime during about the first half of gestation, and conceptuses are subsequently infected transplacentally before they become immunocompetent. There is no definitive evidence that infection of swine other than during gestation is of any clinical or economic significance. The virus is ubiquitous among swine throughout the world and is enzootic in most herds that have been tested. Diagnostic surveys have indicated that PPV is the major infectious cause of embryonic and fetal death. In addition to its direct causal role in reproductive failure, PPV can potentiate the effects of porcine circovirus type II (PCV2) infection in the clinical course of postweaning multisystemic wasting syndrome (PMWS).

Acute infection of postnatal pigs, including pregnant dams that subsequently develop reproductive failure, is usually subclinical. However, in young pigs and probably in older breeding stock as well, the virus replicates extensively and is found in many tissues and organs with a high mitotic index. Viral antigen is especially concentrated in lymphoid tissues However, there is no experimental evidence to suggest that PPV either replicates extensively in the intestinal crypt epithelium or causes enteric disease as do parvoviruses of several other species. PPV also has been isolated from pigs with lesions described as vesiclelike. The causal role of PPV in such lesions has not been clearly defined. The major and usually only clinical response to infection with PPV is maternal reproductive failure. Pathologic sequelae depend mainly on when exposure occurs during gestation. Dams may return to estrus, fail to farrow despite being anestrus, farrow few pigs per litter, or farrow a large proportion of mummified fetuses. All can reflect embryonic or fetal death or both. The only outward sign may be a decrease in maternal abdominal girth when fetuses die at midgestation or later and their associated fluids are resorbed. Other manifestations of maternal reproductive failure, namely, infertility, abortion, stillbirth, neonatal death, and reduced neonatal vitality, also have been ascribed to infection with PPV. These are normally only a minor component of the disease. The presence of mummified fetuses in a litter can prolong both gestation Either may result in stillbirth of apparently normal littermates, whether or not they are infected. There is no evidence that either fertility or libido of boars is altered by infection with PPV.

PPV is classified in the genus Parvovirus (Latin parvus = small) of the family Parvoviridae. All isolates of PPV that have been compared have been found antigenically similar if not identical. PPV is also antigenically related to several other members of the genus. However, its identity can be established by relatively stringent serologic tests such as virus neutralization (VN) and hemagglutination inhibition (HI).

The biophysical and biochemical properties of PPV have been extensively studied and are summarized as follows. A mature virion has cubic symmetry, two or three capsid proteins, a diameter of approximately 20 nm, 32 capsomeres, no envelope or essential lipids, and a weight of 5.3 × 106 daltons. The viral genome is single stranded deoxyribonucleic acid (DNA) with a molecular weight of 1.4 × 106 (i. e., about 26.5% of the weight of the complete virion). Buoyant densities (g/mL in cesium chloride) of complete infectious virions, incomplete "empty" virions, and extracted virion DNA are 1.38–1.395, 1.30–1.315, and 1.724 respectively. Viral infectivity, hemagglutinating activity, and antigenicity are remarkably resistant to heat, a wide range of hydrogen ion concentrations, and enzymes.

Replication of PPV in vitro is cytocidal and characterized by "rounding up," pyknosis, and lysis of cells (Fig. 1A). Many of the cell fragments often remain attached, eventually giving the affected culture a ragged appearance. Intranuclear inclusions develop Infected cultures may hemadsorb slightly

Primary and secondary cultures of fetal or neonatal porcine kidney cells are most often used for propagation and titration of PPV, although other kinds of cultures are also susceptible. Replication is enhanced by infection of mitotically active cultures. Many cells in such cultures are in the S phase (i. e., the DNA synthesis phase) of their cell cycle, wherein the DNA polymerases of cell origin needed for viral replication are available. If either fetal or adult bovine serum is incorporated in the nutrient medium of cell cultures used to propagate PPV, it should be pretested for viral inhibitors. The same may apply to sera of several other species. Because replication of PPV is affected by mitotic activity, the effect of the serum on the cells is also especially important. In addition, cultures should be pretested for PPV contamination. pigs. Moreover, PPV can be accidentally introduced into cultures in several ways, including the use of contaminated trypsin. If contamination is detected before all cells are infected, the virus can be eliminated by repeatedly subculturing the cells in the presence of nutrient medium containing PPV antiserum. Several investigators have used IF microscopy to follow the development of PPV in cell culture. In general, the sequence of events is as follows. Viral antigen is detected in the cytoplasm of cells soon after infection if the inoculum contains a high titer of virus and viral antigen. Most, if not all, of this early cytoplasmic fluorescence is the result of antigen phagocytized from the inoculum. By sequential examinations, such antigen can be demonstrated first on the external surface of the cytoplasmic membrane and later within the cytoplasm, often relatively concentrated in a juxtanuclear location. The first unequivocal evidence of viral replication is the appearance of nascent viral antigen in the nucleus (Fig. 2A). In at least some infected cells, nascent antigen next appears in the cytoplasm in sufficient quantity that both cytoplasm and nucleus are brightly fluorescent. Infected cells commonly seen in the lung of fetuses that develop a high titer of antibody for PPV probably represent this stage of replication (see Fig. 8C). Affected cells subsequently round up, become pyknotic, and disintegrate with release of virus and viral antigen (Fig. 2B). Other cells in the culture that are not at the appropriate stage to support viral replication continue to phagocytize and accumulate viral antigen in their cytoplasm (Fig. 2C). A second wave of viral replication can be induced if these cells are stimulated to enter the S phase of the cell cycle as, for example, by the addition of fresh culture medium.

PPV agglutinates human, monkey, guinea pig, cat, chicken, rat, and mouse erythrocytes. Erythrocytes of other kinds of animals that have been tested are relatively or completely insensitive, or the results have been equivocal. Several parameters of the hemagglutination (HA) test—such as the temperature of incubation, also have been described.

Tests The HI test is frequently used for detection and quantitation of humoral antibody for PPV. Antibody sometimes can be detected as early as 5 days after swine are exposed to live virus, and it may persist for years. The SN test is occasionally used for detection and quantitation of humoral antibody for PPV. Neutralization of infectivity is usually confirmed by the absence or reduction either of intranuclear inclusions or fluorescent cells in cultures or of viral hemagglutinin in the culture medium. The SN test has been reported to be more sensitive than the HI test. A microtechnique for application of the SN test has been described. a modified direct complement fixation test, also have been used successfully to detect antibody for PPV.

These viruses appear to have evolved ~120 years ago with a rapid increase in their population size within the last 40–60 years. They appear to have evolved initially in wild boars and subsequently spread to domestic pigs. The rate of evolution has been estimated to be 3.86 x 10−4 – 8.23 x 10−4 substitutions per site per year. This rate is similar to other single stranded DNA viruses.

Porcine parvovirus is ubiquitous among swine throughout the world. In major swine producing areas such as the midwestern United States, infection is enzootic in most herds, and with few exceptions sows are immune. In addition, a large proportion of gilts are naturally infected with PPV before they conceive, and as a result they develop an active immunity that probably persists throughout life. Collectively, the seroepidemiological data indicate that exposure to PPV is common. They also emphasize the high risk of infection and reproductive disease among gilts that have not developed immunity before conception. The most common routes of infection for postnatal and prenatal pigs are oronasal and transplacental respectively. Pigs nursing immune dams absorb a high titer of antibody for PPV from colostrum. These titers decrease progressively with time by dilution as pigs grow as well as by biological degradation. They usually reach subdetectable levels in 3–6 months if sera are examined by the HI test. Sometimes passively acquired antibody persists for a longer interval. Moreover, levels of antibody too low to be detected by the HI test may be detected by the SN test. Consequently, some groups of gilts are not fully susceptible to infection and dissemination of virus until either shortly before conception or during early gestation. Contaminated premises are probably major reservoirs of PPV. The virus is thermostable, is resistant to many common disinfectants, and may remain infectious for months in secretions and excretions from acutely infected pigs. It was shown experimentally that although pigs transmitted PPV for only about 2 weeks after exposure, the pens in which they were initially kept remained infectious for at least 4 months. The ubiquity of PPV also raises the possibility that some pigs are persistently infected and at least periodically shed virus. However, shedding beyond the interval of acute infection has not been demonstrated. A possible example of an infected, immunotolerant, sexually active boar was reported. Semen may be contaminated externally, as for example with viruscontaining feces, or within the male reproductive tract. The virus was isolated from a testicle of a boar 5 days after it was injected into the boar's prepuce and from testicles of boars killed 5 and 8 days after they were infected oronasally (Mengeling, unpublished data 1976). Virus was also isolated from scrotal lymph nodes of boars killed 5, 8, 15, 21, and 35 days after oronasal exposure. After day 8, isolation was accomplished by cocultivating lymph node fragments with fetal porcine kidney cells (Mengeling, unpublished data 1976). Irrespective of their immune status, boars can also function as a vehicle for mechanical dissemination of PPV among susceptible females.

Dams are susceptible to PPV induced reproductive failure if infected anytime during about the first half of gestation. This interval of maternal susceptibility is indicated by the collective results of several experimental studies, by in depth epidemiological investigations, and by estimates of the time of death of fetuses collected during epidemiological surveys. or longer, A strain of PPV of slightly greater virulence also has been reported. The usual consequences of infection at different stages of gestation are summarized in Table 1. When only part of a litter is infected transplacentally, as is often the case, one or more littermates are frequently infected by subsequent intrauterine spread of virus. The same would apply if initial infection were through contaminated semen. As a result, any combination or all of the sequelae indicated in Table 1 can develop in the same litter. Intrauterine dissemination is probably less common when early embryos are infected because they are quickly resorbed after death, effectively removing the intrauterine reservoir of virus. and although it apparently cannot penetrate this layer, speculation is that it could pose a threat to the embryo after hatching. In any event, a female infected through semen provides a focus of infection for others. With the possible exception of the uterine changes alluded to in the preceding paragraph, PPV induced reproductive failure is caused by the direct effect of the virus on the conceptus. In the absence of an immune response, the virus replicates extensively throughout these tissues. By the time the conceptus dies, most of its cells contain large quantities of intracytoplasmic viral antigen that can be demonstrated by IF microscopy. The relative lack of nuclear fluorescence at the time of death, compared to earlier stages of the disease, indicates that when the conceptus is severely affected, mitotic activity and the associated conditions necessary for viral replication are suppressed more than phagocytic activity. Death of the conceptus probably results from the collective damage by the virus to a variety of tissues and organs, including the placenta. The mechanism of transplacental infection has been investigated by using IF microscopy to identify infected cells in maternal and fetal tissues at progressively longer intervals after maternal oronasal exposure. Whatever the route, maternal viremia seems a likely prerequisite for transplacental infection. When fetuses were infected earlier in gestation (35, 50, and 60 days) and their dams were killed 7 and 11 days later, the lesions were similar. However, uterine lesions were more severe and also included extensive cuffing of myometrial and endometrial vessels with mononuclear cells. (Fig. 7A). Inflammation stillborn pigs, and neonatal pigs are not recommended for submission unless they are the only samples available. If infected, their tissues will usually contain antibody that interferes with laboratory tests for either virus or viral antigen. [[File:Diseases of Swine 17 6. png|thumb|left|Figure 6. PPV infected fetuses. Bars = 5 cm. (A) Litter of a gilt experimentally infected oronasally on day 47 of gestation and killed 34 days later; fetuses from left (L) and right (R) horn of uterus, numbered 1–4 from cervix toward ovary; fetuses L1 and L4 stunted but alive at necropsy, fetus L3 recently dead, others later stages. (B) Fetuses from litter of a naturally infected gilt, collected at about 114 days of gestation, advanced stage of dehydration (mummification). Identification of viral antigen by IF microscopy is a reliable and sensitive diagnostic procedure. Sections of fetal tissues are prepared with a cryostat microtome and are then reacted with standardized reagents. Tissues are triturated in diluent and then sedimented by centrifugation. The supernatant fluid is tested for agglutinating activity for guinea pig erythrocytes. This test requires a minimum of laboratory equipment and is effective in the absence of antibody. thumb|Figure 7. Tissues of PPV infected fetuses of gilts experimentally infected oronasally. (A) Necrotic focus in liver of live fetus of a gilt infected on day 40 of gestation and killed 42 days later; fetus had numerous macroscopic lesions (H&E; ×400). (B) Perivascular cuffing with mononuclear cells in cerebrum of live fetus, littermate of A; fetus had no macroscopic lesions (H&E; ×320). (Insert) Viral antigen associated with endothelium of cerebral vessel of fetus of a gilt infected on day 46 of gestation and killed 25 days later (IF microscopy; ×312.5). All fetuses were probably infected by intrauterine spread of PPV from transplacentally infected littermates. (Photographs A and B courtesy of T. T. Brown Jr., National Animal Disease Center.) Virus isolation is less suitable as a routine diagnostic procedure than either of the aforementioned tests. Infectivity is slowly but progressively lost after fetal death;

In general, serologic procedures are recommended for diagnosis only when tissues from mummified fetuses are not available for testing as previously described. Results with maternal sera are of value if antibody is not detected, thus excluding PPV as a cause, and if samples collected at intervals reveal seroconversion for PPV coincident with reproductive failure. When serum is not available, body fluids collected from fetuses or their viscera that have been kept in a plastic bag overnight at 4?C have been used successfully to demonstrate antibody. [[File:Diseases of Swine 17 8. png|thumb|left|Figure 8. Cryostat microtome sections of lungs of PPV infected fetuses examined by IF microscopy. (A) Lung of mummified fetus reacted with FA plus nonimmune serum (×312.5). (B) Replicate section reacted with FA plus PPV immune serum (i. e., blocking control) (×312.5). (C) Lung of live fetus with HI antibody titer of 640 reacted with fluorescent antibodies (FA) plus nonimmune serum, two infected cells (arrow) (× 162.5). (Insert) Two similar infected cells in the same section as C (×500). and modified live virus (MLV) vaccines have been developed. An inactivated vaccine has been tested under field conditions, and both types of vaccines were effective when tested under controlled laboratory conditions. These limits may define a very brief interval for effective vaccination of gilts that are bred before 7 months of age. Although inactivated vaccine provides maximum safety, there is experimental evidence that PPV can be sufficiently attenuated so that it is unlikely to cause reproductive failure even if inadvertently administered during gestation. Moreover, it has been shown by transuterine inoculation of both virulent and attenuated virus that a much larger dose of attenuated virus is required to establish infection of fetuses. Duration of immunity following vaccination is unknown; however, in one study antibody titers were maintained for at least 4 months after administration of an inactivated vaccine. Low levels of antibody found to be protective allow speculation that, once the immune system has been primed with PPV, subsequent exposure to virulent virus during gestation is unlikely to result in transplacental infection even if antibody from vaccination is no longer detected. Vaccination is recommended also for seronegative sows and boars. Seronegative sows are usually found only in PPV free herds; in such cases, inactivated vaccine is indicated. Experience has shown that few herds can be expected to remain free of PPV even if access is carefully controlled. Introduction of PPV into a totally susceptible herd can be disastrous. Vaccination of boars should reduce their involvement in dissemination of the virus. Vaccines are used extensively in the United States and in several other countries where PPV has been recognized as an economically important cause of reproductive failure. All federally licensed vaccines marketed in the United States are inactivated.