Leading researchers from Ohio State University reported some ground-breaking findings at the recent International Pig Veterinary Society Congress in Durban¹ with potential major impact for pig producers worldwide. Steve Krakowka and his team have shown that a new, difficult-to-control organism, Porcine Torque Teno Virus (TTV) can potentiate both PCV2 and PRRSV infections in pigs.

Torque Teno Virus is a very small virus in the family Circoviridae which also includes PCV2 and Chicken Anaemia Virus (CAV). Both PCV2 and CAV have been identified as important causes of disease in pigs and poultry, respectively. In humans, TTVs have not been associated with any disease and are referred to as 'orphan' viruses. However, these researchers have shown that TTV in pigs can have a significant impact on clinical disease.

Steve Krakowka and his team performed their work on gnotobiotic pigs. These are pigs that are derived by specialist caesarean techniques delivered into a sterile environment where they are kept. They receive no colostrum or other milk from their mother and are maintained on a sterile milk diet. This means that they are free from infectious organisms and as such are ideal models for looking at the impact of individual organisms or specific combinations of organisms.

* "TTV in pigs can have a significant impact on clinical disease"

With TTV and PCV2, the researchers showed that gnotobiotic pigs infected with either virus alone did not develop PMWS. However, pigs first infected with TTV and then PVC2 one week later, developed acute onset PMWS with typical wasting followed by death. In another group of pigs first infected with PCV2 and then TTV, PMWS did not occur suggesting that the order of infection with these viruses is important.

The researchers then looked at dual infections of TTV and PRRSV. Pigs infected with the dual infections developed skin and kidney lesions consistent with Porcine Dermatitis and Nephropathy Syndrome (PDNS), while pigs infected with PRRSV or TTV alone did not develop these lesions. This suggests that these viruses together might have a role in the development of PDNS.

These findings indicate that TTV can modify PCV2 and PRRSV infections in pigs. It is known that PCVD is caused by PCV2, but it is unclear why PCV2 causes problems on some farms but not on others. Researchers in North America have identified at least two genetically different types of PCV2 and suggest a difference in virulence between these. The problem is that other researchers have demonstrated that both strains cause severe clinical disease. The alternative theory is that there is another, hereto unidentified, infectious factor which allows PCV2 to cause PCVD. This has been called Factor X. Could TTV be factor X? In discussions after the paper presentation, Steve Krakowka stated that he believed it could be.

Interestingly another group of researchers from the Universitat Autònoma de Barcelona² reported on findings of TTV in conventional pigs suffering from PMWS in Spain. They found that the virus was present in similar tissues to PCV2 and they raised the question of a potential role of TTV, in co-infection with PCV2, in the pathogenesis of PMWS2. This would support Professor Krakowka's beliefs.

* "Could TTV be factor X?"

The findings with PRRS and TTV are equally interesting. The developments of lesions consistent with PDNS suggest a possible cause for this syndrome. They also raise the question of what impact does TTV have on other PRRS infections and could TTV be involved in making PRRS outbreaks more severe?

Based on this work it is going to be important for farms to control TTV. PCV2 vaccines are doing an excellent job in controlling PCVD, and we have a number of effective vaccines and strategies for controlling PRRS. However if there is another virus that is capable of making both PCV2 and PRRS infections more severe then it is essential that we control it. At present, researchers have been unable to culture TTV so development of any vaccine is likely to be slow. Although the virus is common it is likely that the main route of control will be managemental. One of the main aims will be to reduce or remove the level of challenge on a farm or in a production flow. Biosecurity will play a vital part in this, and for this to work a disinfectant with proven efficacy against TTV must be selected.

TTVs cannot be readily cultured in the laboratory. Furthermore, it is well established that the Circoviridae are resistant to most disinfectants. It is therefore vital to select a disinfectant with proven efficacy against Circoviridae. DuPont Animal Health Solutions Virkon® S has proven efficacy against both PCV2 and CAV and so should be considered, especially with its broad spectrum of activity against other infectious agents including PRRSV.

References

1. S. Krakowka, J. Ellis, K. MacIntosh, S. S. Ringler, D. M.Rings, C. Hartunian, Yan Zhang & G. Allan (2008) Porcine genogroup 1 Torque Teno Virus (G1-TTV) Potentiates PCV2 & PRRSV infections in gnotobiotic swine. Proceedings of the International Pig Veterinary Society (Durban) 1, 99.
2. G. Martin-Valls, L. Martínez-Guinó, M. Pérez, T. Kekarainen & J. Segalés (2008) Torque Teno Virus tissue distribution in healthy, Postweaning Multisystemic Wasting Syndrome and Porcine Dermatitis and Nephropathy Syndrome affected pigs by in situ hybridisation. Proceedings of the International Pig Veterinary Society (Durban) 1, 96.

Further Reading

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Find out more information on Post-Weaning Multisystemic Wasting Syndrome PMWS by clicking here.

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Find out more information on Porcine Dermatitis and Nephropathy Syndrome, PDNS by clicking here.

October 2008