Characterization of the leukogram in PCV2 infected germ-free pigs

Authors: Gauger, Phillip; Lager, Kelly; Cheung, Andrew

Submitted to: North Central Conference of Veterinary Laboratory Diagnosticians
Publication Type: Proceedings
Publication Acceptance Date: 5/8/2007
Publication Date: 6/7/2007
Citation: Gauger, P.C., Lager, K.M., Cheung, A.K. 2007. Characterization of the leukogram in PCV2 infected germ-free pigs. In: Proceedings of the 46th Annual North Central Conference of Veterinary Laboratory Diagnosticians, June 7-8, 2007, Ames, Iowa. p. 39-42.

Interpretive Summary:

Technical Abstract: Porcine Circovirus (PCV) is a non-enveloped, single stranded DNA virus of the Circoviridae family containing a circular genome of approximately 1.76 kb. The first isolate was recovered as a contaminant of a porcine kidney cell line in1974. This isolate, known as PCV type 1, is considered non-pathogenic. Post weaning multisystemic wasting syndrome (PMWS) was a new clinical disease identified in Canadian herds in the early 1990's. Researchers later found an association of PMWS with PCV. This isolate became known as PCV type 2. PMWS is characterized clinically by wasting and respiratory disease in weaned pigs. Pathological lesions consist of lymphoid depletion and histiocytic replacement of lymphoid follicles. A definitive diagnosis of PMWS requires the detection of PCV2 antigen in conjunction with characteristic microscopic lesions. Other clinical syndromes implicating PCV2 include porcine dermatitis and nephropathy syndrome, reproductive failure, and enteritis. The term Porcine Circovirus Diseases (PCVD) was adopted to encompass the previously listed syndromes. PCV1 and PCV2 share a sequence homology of 68-76%. PCV1 isolates contain genomes ranging from 1758-1760 bp and the genome of PCV2 contains 1767-1768 bp. Intragenomic sequences of PCV1 share a 97-99% homology versus 94.6-99% homology among PCV2 isolates. In lieu of the similarities between PCV2 noted thus far, and studies investigating genomic differences between PCV2 isolates from healthy and diseased pigs, it was assumed there was no difference in pathogenicity. This dogma began to change when analysis of the genomes from more recent isolates of PCV2 in several different countries revealed the existence of two main groups. Another recent study further characterized PCV2 into group 1 isolates with 3 clades and PCV2 group 2 isolates containing 5 clades. Previous to these studies, only PCV2 group 2 isolates were found in the U.S. compared to other countries where group 1 and group 2 isolates have occurred concurrently. Recently, though, reports began to surface of a high mortality syndrome in several swine herds within the United States. Severe respiratory disease with listless, unthrifty pigs experiencing severe dyspnea resulted in mortality rates ranging from 5-50%. Examination of these cases found PCV2 as a consistent feature in all submitted specimens, in addition to other pathogens sporadically isolated as well. Interestingly, the PCV2 isolates from the high mortality herds in the U.S. cases were found to contain both group 1 and group 2 genomes after comparison with several PCV2 isolates registered in Genebank. A preliminary animal study in our lab was conducted using infectious clones from both PCV2 group 1 and 2 genomes recovered from the same pig from a case in North Carolina. Group 1 and group 2 PCV2 virus was generated following transfection of a PCV2-free PK-15 cell line with the respective virus. Germ-free pigs were inoculated oronasally at about seven days of age (day 0 of the experiment). An acute onset of respiratory disease and anorexia ensued at 22 days post infection in PCV2 group 1 pigs. By 26 days post infection, all four pigs inoculated with the group 1 clone experienced similar clinical signs and were euthanized for necropsy. Gross lesions included pneumonia with red, congested and consolidated lungs; a clear to cloudy effusion was present in the pleural and peritoneal cavities, lymphadenopathy, slightly icteric liver and edema in the spiral colon and mesentery. In contrast, only one pig of the four infected with PCV2 group 2 clones experienced clinical signs at 35 days post infection. The other pigs in that group were euthanized thirty-five days post infection and no clinical signs were noted. Abnormal leukograms were a characteristic feature of all infected pigs in this study. A leukopenia was noted at sev