Absence of evidence of porcine circovirus infection in piglets with congenital tremors

Porcine circoviruses: current status, knowledge gaps and challenges

Circoviruses (CV) include some of the smallest viruses known. They were named after their circularly arranged single-stranded DNA genome with a gene encoding a conserved replicase protein on the sense strand. Circoviruses are widely distributed in mammals, fish, avian species and even insects. In pigs, four different CVs have been identified and named with consecutive numbers based on the order of their discovery: Porcine circovirus 1 (PCV1), Porcine circovirus 2 (PCV2), Porcine circovirus 3 (PCV3) and most recently Porcine circovirus 4 (PCV4). PCVs are ubiquitous in global pig populations and uninfected herds are rarely found. It is generally accepted that PCV1 is non-pathogenic. In contrast, PCV2 is considered an important, economically challenging pathogen on a global scale with comprehensive vaccination schemes in place. The role of PCV3 is still controversial several years after its discovery. Propagation of PCV3 appears to be challenging and only one successful experimental infection model has been published to date. Similarly to PCV2, PCV3 is widespread and found in many pigs regardless of their health history, including high health herds. PCV4 has only recently been discovered and further information on this virus is required to understand its potential impact. This review summarizes current knowledge on CVs in pigs and aims to contrast and compare known facts on PCVs.

Global Status of Porcine circovirus Type 2 and Its Associated Diseases in Sub-Saharan Africa

Globally, Porcine circovirus type 2 (PCV2) is a recognized viral pathogen of great economic value in pig farming. It is the major cause of ravaging postweaning multisystemic wasting syndrome (PMWS) and many other disease syndromes generally regarded as Porcine circovirus associated diseases (PCVAD) in Europe. PCV2 infections, specifically PMWS, had impacted huge economic loss on swine production at different regions of the world. It has been studied and reported at different parts of the globe including: North and South America, Europe, Asia, Oceania, Middle East, and the Caribbean. However, till date, this virus and its associated diseases have been grossly understudied in sub-Sahara African region and the entire continent at large. Two out of forty-nine, representing just about 4% of countries that make up sub-Sahara Africa presently, have limited records on reported cases and occurrence of the viral pathogen despite the ubiquitous nature of the virus. This review presents an overview of the discovery of Porcine circovirus and its associated diseases in global pig herds and emphasizes the latest trends in PCV2 vaccines and antiviral drugs development and the information gaps that exist on the occurrence of this important viral pathogen in swine herds of sub-Saharan Africa countries. This will serve as wake-up call for immediate and relevant actions by stakeholders in the region.

Astrovirus as a possible cause of congenital tremor type AII in piglets?

BACKGROUND

Congenital tremor is associated with demyelination of the brain and spinal cord and is clinically noted as outbreaks of trembling and shaking in newborn piglets during a limited time-period. Six forms of the disease have been described, where form AII may be caused by an, as yet, unidentified viral infection. This study aimed to investigate the presence of astrovirus and circovirus by sequencing and polymerase chain reaction (PCR) analysis and by relating the findings to the occurrence of disease and lesions in the brain, in 4-6 days-old piglets obtained from a clinical outbreak of congenital tremor.

RESULTS

In piglets with congenital tremor, there were mild to moderate vacuolar changes of the white matter in the cerebrum, brain stem and cerebellum. In healthy piglets, less conspicuous vacuolar changes were detected. One healthy and one diseased piglet were positive for porcine circovirus type 2. The nested pan-PCR showed the presence of astrovirus in at least one brain region in all piglets and by sequencing, two different porcine astrovirus lineages were identified.

CONCLUSIONS

The results do not support previous studies identifying porcine circovirus type 2 as the cause of congenital tremor. The demonstration of astrovirus in the brain of piglets suffering from congenital tremor is interesting. However, astrovirus was demonstrated in both healthy and diseased individuals and therefore, further studies are warranted to determine the possible involvement of astrovirus in the pathogenesis of congenital tremor in pigs.

Current State of Knowledge on Porcine Circovirus Type 2-Associated Lesions

Porcine circovirus type 2 (PCV2), a small single-stranded DNA virus, was initially discovered in 1998 and is highly prevalent in the domestic pig population. Disease manifestations associated with PCV2 include postweaning multisystemic wasting disease (PMWS), enteric disease, respiratory disease, porcine dermatitis and nephropathy syndrome (PDNS), and reproductive failure. Although these clinical manifestations involve different organ systems, there is considerable overlap in clinical expression of disease and presence of lesions between pigs and within herds. It is now widely accepted that PCV2 can be further subdivided into different types, of which PCV2a and PCV2b are present worldwide and of greatest importance. This review will focus on PCV2-associated lesions in different organ systems.

Outcome of experimental porcine circovirus type 1 infections in mid-gestational porcine foetuses

BACKGROUND

Porcine circovirus type 1 (PCV1) has been described as a non-cytopathic contaminant of the PK-15 cell line. Several experimental infections with PCV1 failed to reproduce disease in pigs. Therefore, PCV1 is generally accepted as non-pathogenic to pigs. To our knowledge, nothing is known about the outcome of PCV1 infections in porcine foetuses. This was examined in the present study.

RESULTS

Nine foetuses from three sows were inoculated at 55 days of gestation: three with 10(4.3) TCID(50) of the PCV1 cell culture strain ATCC-CCL33, three with 10(4.3) TCID(50) of the PCV1 field strain 3384 and three with cell culture medium (mock-inoculated). At 21 days post-inoculation, all 6 PCV1-inoculated and all 3 mock-inoculated foetuses had a normal external appearance. Microscopic lesions characterized by severe haemorrhages were observed in the lungs of two foetuses inoculated with CCL33. High PCV1 titres (up to 10(4.7) TCID(50)/g tissue) were found in the lungs of the CCL33-inoculated foetuses. All other organs of the CCL33-inoculated foetuses and all the organs of the 3384-inoculated foetuses were negative (< 10(1.7) TCID(50)/g tissue) by virus titration. PCV1-positive cells (up to 121 cells/10 mm(2) in CCL33-inoculated foetuses and up to 13 cells/10 mm(2) in 3384-inoculated foetuses) were found in the heart, lungs, spleen, liver, thymus and tonsils. PCR and DNA sequencing of Rep recovered CCL33 or 3384 sequences from CCL33- or 3384-inoculated foetuses, respectively.

CONCLUSIONS

From this study, it can be concluded that cell culture PCV1 can replicate efficiently and produce pathology in the lungs of porcine foetuses inoculated at 55 days of foetal life.

Effect of porcine circovirus type 2 (PCV2) infection on reproduction: disease, vertical transmission, diagnostics and vaccination

Porcine circovirus type 2 (PCV2) causes great economic losses in growing pigs and there are several reviews on disease manifestations and lesions associated with PCV2 in growing pigs. Reproductive failure in breeding herds, predominately associated with increased numbers of mummies and non-viable piglets at parturition, is one of the disease manifestations of PCV2 infection. Boars shed low amounts of infectious PCV2 in semen for extended time periods, and vertical transmission of PCV2 to fetuses during PCV2 viremia of the dam has been experimentally confirmed. However, intrauterine-infected piglets often are clinically normal. Nevertheless, pigs infected with PCV2 by the intrauterine route can be born viremic, possibly contributing to horizontal spread of PCV2 within the breeding herd and into the nursery. Shedding of PCV2 in semen and prevalence of intrauterine-infected piglets can both be greatly reduced by PCV2 vaccination well ahead of expected PCV2 exposure. This review is a discussion on current knowledge on the effects of PCV2 infection in the dam and inin uterofetuses, including clinical signs, lesions, diagnosis and prevention through vaccination. Infection of boars with PCV2, the potential for PCV2 transmission via semen and prevention of PCV2 shedding are also discussed.

Intranuclear detection of African swine fever virus DNA in several cell types from formalin-fixed and paraffin-embedded tissues using a new in situ hybridisation protocol

In this study, a new in situ hybridisation (ISH) protocol has been developed to identify African swine fever virus (ASFV) genome in formalin-fixed, paraffin-embedded tissues. Different digoxigenin labelled ASFV-probes were tested, including single ASFV-specific oligonucleotides, an 18.5kb restriction fragment from the viral genome and the entire ASFV genome. The latter showed the highest sensitivity in all tissues tested, independently of the virus used for challenge: E75L or Ba71L. Although a similar ASFV genome distribution was observed, the number of ISH-positive cells was higher for Ba71L compared to E75L infected tissues. As expected, the monocyte-macrophage cell lineage was the main target cell for ASFV infection. Corresponding with the last stages of infection, ISH-positive signals were also found in other cell types, including endothelial cells, hepatocytes and neutrophils. Furthermore, two unexpected findings were also noticed: the detection of a specific ISH-signal in lymphocytes and a tendency to find the signal in the nucleus of infected cells. In summary, the present findings demonstrate the utility of this new ISH protocol to study ASFV pathogenesis and its potential use as a diagnostic tool.

Characterization of interstitial nephritis in pigs with naturally occurring postweaning multisystemic wasting syndrome

Kidney samples with interstitial nephritis from 26 pigs affected by postweaning multisystemic wasting syndrome (PMWS) were selected. A histologic evaluation was carried out to describe the type of inflammation and its relationship with viral load, as assessed by in situ hybridization (ISH). Of 26 cases, 10 revealed a tubulointerstitial, lymphoplasmacytic nephritis, 11 an interstitial granulomatous nephritis, and 5 both types of inflammation (mixed type). In 4 cases of granulomatous inflammation, the pattern was not classically nodular, and a population of macrophages and lymphocytes was present (interstitial lymphohistiocytic nephritis). ISH confirmed the presence of porcine circovirus type 2 (PCV2) nucleic acid in all cases. The epithelium of the renal tubules was the most constantly ISH-positive structure. In tubulointerstitial nephritis, the higher the number of positive inflammatory cells, the more severe the inflammation. The ISH reaction was more heterogeneous and unpredictable in granulomatous nephritis, with some epithelioid and giant cells positive by ISH. To quantify macrophages distributed in the three patterns of nephritis, immunohistochemical methods using anti-major histocompatibility complex II (anti-MHC-II) and anti-lysozyme antibodies were undertaken, and semiquantitative evaluation was carried out. MHC-II was mainly expressed by lymphocytes in tubulointerstitial nephritis, but did not always stain macrophages in cases of granulomatous (including lymphohistiocytic) nephritis; the anti-lysozyme antibody revealed macrophages when present in tissues. The amount of PCV2 nucleic acid was not apparently associated with the pattern of inflammation (tubulointerstitial or granulomatous). PCV2 load seems to reflect the severity of the lymphoplasmacytic inflammation but not that of granulomatous and lympho histiocytic types.

Porcine circovirus type 2-associated cerebellar vasculitis in postweaning multisystemic wasting syndrome (PMWS)-affected pigs

Porcine circovirus type 2 (PCV2) is associated with several syndromes in growing pigs, including postweaning multisystemic wasting syndrome and porcine dermatitis and nephropathy syndrome. In the present study, a previously undescribed neurovascular disorder associated with a PCV2 infection is described. Sixteen pigs showed clinical signs of wasting and neurologic deficits. Acute hemorrhages and edema of cerebellar meninges and parenchyma due to a necrotizing vasculitis resulted in degeneration and necrosis of the gray and white matter. Few to numerous PCV2 DNA and antigen-bearing endothelial cells were detected in affected areas of the brain using in situ hybridization and immunohistochemistry. Conventional histochemical stains, as well as the detection of caspase 3 activity and DNA strand breaks by the terminal transferase dUTP nick end labeling assay, showed numerous apoptotic endothelial cells in the vascular lesions observed. Sequencing of various brain-derived PCV2-specific amplicons revealed a strong identity between different isolates and an 89 to 100% identity to previous isolates. The phylogenetic tree showed that there was no clustering of isolates correlating to clinical signs or geographic distribution. This previously undescribed PCV2-associated neurologic disease has features of both postweaning multisystemic wasting syndrome and, to a lesser extent, porcine dermatitis and nephropathy syndrome. The available evidence suggests that direct virus-induced apoptosis of endothelial cells plays a role in the pathogenesis of this unusual PCV2-associated cerebellar vasculitis.

Association of myocarditis with high viral load of porcine circovirus type 2 in several tissues in cases of fetal death and high mortality in piglets. A case study

During a period of 1.5 months, a newly established pig herd experienced a high number of mummifications and stillbirths, a high neonatal mortality rate, and many piglets with congenital tremors or hind leg ataxia. After clinical and histological investigations, the submitted animals were divided into 4 groups: mummified or stillborn (N = 6), live born with myocarditis (N = 5) (average age 22.8 days), live born without myocarditis (N = 14) (average age 20.0 days), and control animals from a different herd (N = 5) (newborn). Statistically significant differences were observed in the mean porcine circovirus 2 (PCV2) load among the 4 groups in the liver (P < 0.0001). The presence of PCV2 antigen within the myocardial lesions was confirmed by immunohistochemistry. A high load of PCV2 DNA was observed in myocardium, liver, and spleen from mummified or stillborn piglets (>1 x 10(7) copies per 500 ng DNA), lower in piglets with myocarditis (>1 x 10(5) copies per 500 ng DNA), and even further lower in pigs without myocarditis (<1 x 10(5) copies per 500 ng DNA), whereas no PCV2 DNA was detected in the control animals. Myocardium, liver, and spleen were well suited for routine testing of fetuses and young piglets by quantitative real-time polymerase chain reaction. Neither porcine parvovirus nor encepaholomyocarditis virus was detected. These results indicate that the PCV2 infection might have been of etiological importance for the fetal deaths and piglet mortality observed in this herd.

A new method to identify cell types that support porcine circovirus type 2 replication in formalin-fixed, paraffin-embedded swine tissues

Porcine circovirus type 2 (PCV2) is detected in high amounts within the characteristic microscopic lesions of postweaning multisystemic wasting syndrome (PMWS) affected pigs. In spite of recent advances on disease pathogenesis, the precise cell types that support viral replication are still a major issue of scientific discussion. In this study, a new methodology to detect cell types that support PCV2 replication was designed. For this purpose, two in situ hybridisation (ISH) methods were developed and applied on tissues of PMWS naturally affected pigs using two probes designed from the ORF1 sequence of the virus. While the complementary probe (CP) detected ssDNA, ORF1 mRNA and replicative form (RF) of PCV2, the RF probe (RFP) exclusively hybridised with the RF of the virus, thus, only labelling cells where PCV2 replication is taking place. Both probes demonstrated to be specific and equally sensitive by an in vitro Southern blot hybridisation assay. ISH labelling with the CP was extensive in lymphoid tissues and of variable amount in other non-lymphoid tissues. With this probe, mainly macrophage-like cells were labelled but also other cell types such as hepatocytes and other epithelial cells. Tissues in which RFP labelling was found more frequently were lung, inguinal and mesenteric lymph nodes, tonsil and liver. Labelling with the RFP was always nuclear, and found in the same cell types as with the CP, although in a relatively low proportion of them; labelling of macrophage-like cells was infrequent. Therefore, the results indicate that at least a certain proportion of macrophages may support PCV2 replication, but main cells where PCV2 replicates are of epithelial/endothelial origin. In summary, the present study permitted the study of cell types that support PCV2 replication by the use of ISH on formalin-fixed, paraffin-embedded tissues of PMWS affected pigs.

Porcine circovirus diseases

Porcine circovirus type 2 (PCV2) is a member of the familyCircoviridae, a recently established virus family composed of small, non-enveloped viruses, with a circular, single-stranded DNA genome. PCV2, which is found all over the world in the domestic pig and probably the wild boar, has been recently associated with a number of disease syndromes, which have been collectively named porcine circovirus diseases (PCVD). Postweaning multisystemic wasting syndrome (PMWS), porcine dermatitis and nephropathy syndrome (PDNS) and reproductive disorders are the most relevant ones. Among them, only PMWS is considered to have a severe impact on domestic swine production. PMWS mainly affects nursery and/or fattening pigs; wasting is considered the most representative clinical sign in this disease. Diagnosis of this disease is confirmed by histopathological examination of lymphoid tissues and detection of a moderate to high amount of PCV2 in damaged tissues. Since PMWS is considered a multifactorial disease in which other factors in addition to PCV2 are needed in most cases to trigger the clinical disease, effective control measures have focused on the understanding of the co-factors involved in individual farms and the control or elimination of these triggers. PDNS, an immuno-complex disease characterized by fibrino-necrotizing glomerulonephritis and systemic necrotizing vasculitis, has been linked to PCV2, but a definitive proof of this association is still lacking. PCV2-associated reproductive disease seems to occur very sporadically under field conditions, but it has been characterized by late-term abortions and stillbirths, extensive fibrosing and/or necrotizing myocarditis in fetuses and the presence of moderate to high amounts of PCV2 in these lesions. Taking into account that scientific information on PCV2 and its associated diseases has been markedly expanded in the last 8 years, the objective of this review is to summarize the current state of knowledge of the most relevant aspects of PCV2 biology and PCVD.

Genetic characterisation of Australian strains of porcine circovirus types 1 and 2

OBJECTIVE

As post-weaning multi-systemic wasting syndrome (PMWS) has not been identified within Australia, to determine if the absence of disease was associated with genetic differences between the strains of porcine circovirus (PCV) present in Australia and those from countries in association with PMWS.

DESIGN

Pig tissues were obtained from weaned pigs found dead or presenting with clinical signs of illthrift and also from neonatal pigs with congenital tremors and used as a source of virus DNA for sequence analysis.

PROCEDURE

DNA was extracted from the tissues and PCV detected by polymerase chain reaction (PCR). PCR with PCV type-specific primers was used to amplify the entire genome from selected tissues. The genomes of three strains of PCV1 and seven strains of PCV2 from three Australian states were sequenced and subjected to phylogenetic analysis using standard procedures.

RESULTS

The three Australian PCV1 strains had 98 to 99% nucleotide identity to strains in other countries and the seven Australian PCV2 strains had 94 to 99% identity to PCV2 strains in other countries where PMWS has occurred. Six of the seven Australian PCV2 strains were genetically similar to each other, while the seventh was more distantly related. There were no consistent differences in the predicted amino acid sequence of the Australian strains of PCV2 and strains associated with PMWS in other countries.

CONCLUSION

There were no consistent differences between Australian strains of PCV and those that have been associated with PMWS in other countries and it appears likely that other factors are responsible for the absence of PMWS in Australia.

A review of porcine circovirus 2-associated syndromes and diseases

Clinical expression of porcine circovirus 2 (PCV2) infection in swine may result in several distinct syndromes and diseases including post-weaning multisystemic wasting syndrome (PMWS), porcine dermatitis and nephropathy syndrome (PDNS), reproductive failure, porcine respiratory disease complex, granulomatous enteritis, necrotizing lymphadenitis, and possibly exudative epidermitis. Association of PCV2 with congenital tremor in piglets is still controversial. The extent of the involvement of PCV2 in swine disease other than PMWS is currently poorly understood. This review concentrates on PCV-2-associated syndromes and diseases other than PMWS.