Effects of duck circovirus on immune function and secondary infection of Avian Pathogenic Escherichia coli

Abstract

Duck circovirus (DuCV) infection occurs frequently in ducks in China and is generally believed to lead to immunosuppression and secondary infection, though there has been a lack of detailed research and direct evidence. In this study, one-day-old Cherry Valley ducklings were artificially infected with DuCV alone and co-infected with DuCV and Avian Pathogenic Escherichia coli (APEC). The immune indexes at 32 d old were systematically monitored, including immune organ weight, lymphocyte transformation rate, IL-10, IL-12, soluble CD4 (sCD4), soluble CD8 (sCD8), IFN-γ, viral loads in each organ, APEC colonization, and so on. The results showed the development of immune organs in ducklings was affected, resulting in a decrease in the lymphocyte transformation rate (LTR), IL-12, sCD4, sCD8, IFN-γ and an increase in IL-10 content at 8 to 32 d postinfection (dpi). In the detection of virus loads in some organs, it was found that 8 dpi, DuCV existed stably in various organs, suggesting the importance of preventing and controlling the virus in the early stage of culture. The results of exploring the DuCV infection that shows some influence on secondary infection by APEC. The results showed that DuCV infection could significantly enhance the pathogenicity of APEC and the colonization ability of APEC in vivo. DuCV can induce more serious APEC infection in 24 dpi than in 14 dpi. Based on the above results, it can be concluded that DuCV infection will affect the immune system, cause immunosuppression, and lead to more serious secondary infection.

Keywords: Avian pathogenic Escherichia Coli; duck circovirus; immunosuppression; secondary infection.

Figure 1

Effects of DuCV infection on body weight and immune organ index of ducklings. The immune organs were dissected at 8, 16, 24, and 32 dpi to observe the histopathological changes (A), body weight (B), relative thymus index (C), relative spleen index (D) and relative bursa index (E). *P < 0.05.

Figure 2

Effect of DuCV infection on cytokine secretion and LTR. The lymphocytes isolated from duck peripheral blood were collected, the OD value was measured by CCK-8, and the transformation rate (A) was calculated according to the formula. The concentrations of sCD4 molecule (B), sCD8 molecule (C), IFN-γ (D), IL-10 (E) and IL-12 (F) in serum at 8, 16, 24, and 32 dpi were detected by enzyme linked immunosorbent assay (ELISA). *P < 0.05; **P < 0.01;***P < 0.001.

Figure 3

The viral loads of each organ changed after DuCV infection. The viral loads of livers (A), hearts (B), spleens (C), thymus (D) and bursa of Fabricius (E) were detected by qPCR at 8, 16, 24, and 32 dpi, and the virus copy number was calculated according to the standard curve.

Figure 4

The viral loads of each organ changed after DuCV infection and before APEC inoculation. When 8 dpi (A), 14 dpi (B) and 24 dpi (C) were detected by qPCR, the viral loads in thymus, spleens, bursa of Fabricius, hearts and livers were detected, and the virus copy number was calculated according to the standard curve.

Figure 5

The mortality of ducklings secondary to APEC infection and bacterial colonization in different time after DuCV infection. Each group was inoculated with APEC at 14 dpi and 24 dpi, and the death of 14 dpi (A) and 24 dpi (B) and the colonization of APEC at 12 h (C), 24 h (D), and 24 dpi at 12 h (E) and 24 h (F) were recorded. *P < 0.05; **P < 0.01;***P < 0.001.