Porcine Circovirus
Lalrinkima*, S. Hemalatha
Department of Veterinary Pathology
Madras Veterinary College, Chennai
Introduction
Porcine circovirus type2 associated disease (PCVAD) causes significant economic losses in swine industry. The disease was first discovered as a picornavirus like contaminant of porcine kidney 15 cells (PK-15 cells) in 1974. Despite the fact that further research on the virus revealed that it was not harmful, the discovery of PCV type 1 (PCV1) triggered worldwide investigations into the frequency of PCV in swine population and led to the discovery of PCV type 2 (PCV2). PCV2 infection is common in pig herds where pigs were kept in close proximity and spreads through fecal oral route. Infection with porcine circovirus type 2 is associated with the onset and progression of porcine circovirus associated diseases (PCVAD).
In India, studies on PCV2 was initiated by Kumar and his co-worker in 2006 where PCV2 was first detected from cases of postweaning multisystemic wasting syndrome (PMWS) associated PCV2 systemic disease. In 2008, Rajkhowa reported PMWS in crossbred pigs in Uttar Pradesh and Uttarakhand. Stillbirth, neonatal mortality and decreased litter size was found to be positive for PCV2 in an organized swine farm in the state of Uttar Pradesh with a history of mummification by Sharma and Saikumar in 2010. In 2016, Karuppannan and his co-worker reported PCV2 infection at a piggery unit in Tamil Nadu, South India, which had a high rate of stillbirths and neonatal mortality.
Etiology
PCV2 is a small, non-enveloped virus of 17nm diameter with a 1.767 kb single stranded circular DNA genome. The Circoviridae family consists of a group of circular DNA genome viruses that mainly infect birds and mammals. The family is divided into two genera: Cyclovirus and Circovirus. The genus Circovirus consists of 22 virus species, including Porcine circovirus 1 (PCV1, type species) and PCV2.
Transmission
Porcine circovirus in pigs spread in a variety of ways. The disease spreads through the body secretions and excretions such as nasal, tonsillar, bronchial and ocular secretions, feces, urine, semen, saliva and milk from both clinically affected and infected but relatively healthy animals. PCV2 had been discovered in semen samples. House flies also act as transmission vectors. Flies carry and transmit the virus because of their close connection with pigs and environment. Mosquitoes such as Culex species act as mechanical vectors for PCV2 transmission.
Clinical signs
Infection can be observed into 4 forms:
PCV2 Systemic disease
PCV2-SD is characterized by wasting or weight loss, pallor of skin, respiratory distress, diarrhea and icterus. Enlarged subcutaneous lymph nodes were a common finding in the early clinical phases of the systemic disease. Morbidity was 4-30% commonly, occasionally 50-60% and mortality was 4-20%.
PCV2 lung disease
The diseases is characterized by respiratory distress and dyspnea. Reduced feed efficiency, reduced growth rate, anorexia, fever, cough and dyspnea were the signs of the porcine respiratory disease complex (PRDC) with other pathogens like Mycoplasma hyopneumoniae, swine influenza, PRRS etc. in 8 to 26 weeks old pigs.
PCV2 Reproductive disease (PCV2-RD)
Late-term abortions, stillbirths and mummification were the clinical manifestations of PCV2 which was similar to the disease produced by porcine parvovirus. PCV2 replicates in embryos which could result in the death of the embryos and a return to estrus. PCV2 had no effect on the development of a small percentage of embryos before the 21st day of pregnancy.
PCV 2 Enteric diseases
The disease was indicated by enteritis. Most of the PCV2 enteric disease cases were from 8 to 16 weeks age group pigs.
Pathology
Grossly, generalized lymphadenopathy and lung lesions were the most consistent findings in PMWS affected pigs. The lung lesions vary from failure to collapse and increased firmness with diffuse mottled areas of consolidation in the anterior ventral pulmonary areas. Pallor, oedema and non-hemorrhagic ulceration of the pars esophageal mucosa of the stomach and fluid-filled, thin-walled intestine particularly the ileum and spiral colon had also been described. Ascites, hydrothorax and hydropericardium were the common findings in fetus. In a fetal heart, cardiac hypertrophy with multifocal areas of discoloration in the myocardium was commonly present.
Microscopically, lungs revealed lymphohistiocytic or granulomatous interstitial pneumonia, peribronchiolar fibrosis and mild to severe necrotizing bronchiolitis. Interstitial lymphoplasmacytic, granulomatous or mixed type nephritis, granulomatous enteritis, lymphohistiocytic hepatitis with hepatic cords disorganization, apoptosis and peri lobular fibrosis were observed in liver. The presence of multinucleated giant cells was a common finding. Macrophages might also have finely defined spherical basophilic intracytoplasmic botryoid inclusion bodies.
Small litter size and the presence of Lungs – Generalized pallor of
two mummified fetuses lobes with areas of emphysema
and collapse
Diagnosis
The most common golden standard techniques to detect PCV2 nucleic acid and antigen in the tissue were insitu hybridization (ISH) and immunohistochemistry (IHC) using monoclonal or polyclonal antibodies against PCV2. Detection of antibodies by ELISA and serum neutralization test. Real time PCR and multiplex nested PCR are important tools for diagnosis of PCV2.
References
Kumar, G.S., R. Sharma and O.P. Paliwal. 2006. Occurrence and pathology of post 493 weaning multisystemic wasting syndrome. Proceedings, the 23rd Annual Conference of Indian Association of Veterinary Pathologists, pp. 83: 495
Mateusen, B., D.G. Maes, A. Van Soom, D. Lefebvre and H.J. Nauwynck. 2007. Effect of a porcine circovirus type 2 infection on embryos during early pregnancy. Theriogenology. 68: 896-90.
Sharma, R and G. Saikumar. 2010. Porcine parvovirus-and porcine circovirus 2associated reproductive failure and neonatal mortality in crossbred Indian pigs. Trop. Anim. Health Prod. 42(3): 515–522.
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