Introduction

Bluetongue (BT) is an infectious, non contagious arthropod-borne disease of ruminants caused by Bluetongue virus (BTV), prototype species of the genus Orbivirus, within the family Reoviridae. Twenty four serotypes of bluetongue virus have been identified till date (Davies et al., 1992).

India has significant population of domestic and wild ruminants which are known to be susceptible to bluetongue virus infection. Several exotic breeds of sheep were introduced into the country between 1960 and 1970 for the genetic improvements of national flock by cross breeding with native breeds. In India, the total sheep population is 51 million, accounting to 5 percent of world’s sheep population and 123 million goats accounting for 20 per cent of the total global livestock. (FAO, 2003).

In Tamil Nadu, 22 out of 24 districts were reported to be affected by the bluetongue virus. The reported case of bluetongue virus among sheep and goats occurs presumable in an epidemic form during the Southwest monsoon season which favours vector population of bluetongue virus (Ilango, 2006). Although the history of reporting was not continuous, the number of outbreaks, attacks and deaths among ruminants reported is of great concern that needs immediate attention for the protection of livestock and economic growth (Wilson et al., 1997).

The development of laboratory based systems for the investigation of animal disease such as bluetongue is crucial to understand the infectious agent and the disease process globally. As a result, adaptation of bluetongue virus to grow in laboratory conditions, such as cell culture techniques. It has been possible to develop a range of diagnostic tests and vaccines (Bowne et al., 1970). Hence the attempt has been made to cultivation and adaptation of BTV isolates in two different cell lines namely BHK 21 and Vero.

Materials And Methods

Bluetongue Virus Isolation

Cultivation of BTV Isolates in BHK 21 and Vero Cell Lines

Results

The blood samples were collected from the infected sheep for the isolation of BTV. In the four year study period there were 95 field outbreaks from which 165 samples were collected for the isolation purpose. The virus was cultivated in two different cell cultures like BHK21 and Vero to find out its susceptibility and production of high titre. In this study, infectivity virus titration was done in Vero cell line and BHK 21 cell line and their findings are summarized in Table 1., and Fig.1. In BHK 21 and Vero cell lines the BTV isolates produced visible CPE from 36h post infection, infected cells became swollen and well defined. Rounding of cells, syncytia formation, giant cell formation and grouping of cells have also been observed in fifth passage. The affected cells have shown granularity and undergone variable degrees of shrinkage. Later, most of the cells got detached from the glass (Plate I and II).

Figure 1

Table 1. Comparison of infectivity titre value in vero cell line and BHK 21 cell line

Figure 2

Fig.:1. Comparison of virus titration in two cell lines

Figure 3

Fig 1: Cytopathic effect of Bluetongue virus on vero cell line stained with Giemsa

Figure 4

Fig 2: Cytopathic effect of Bluetongue virus on BHK 21 cell line stained with Giemsa

Infectivity titre values of the thirteen isolates ranged between 6.21 and 10.58 and 5.6 and 9.48 from 3rd to 12th passage in the case of Vero cell line and BHK 21 respectively. Between the two cell cultures, the Vero cell line exhibited more titre value than BHK 21 (Table 1). This was significantly noticed in the mean difference values (p=<0.0001). Significant difference was observed right from the 3rd passage to 12th passage in all the thirteen isolates in terms of mean infectivity titre value (Fig.1) and minimum and maximum titre value. It can be statistically concluded that Vero cell line exhibited maximum infectivity titre than BHK21 cell line.

Discussion

Bluetongue has been recognized as an economically significant vector borne, non contagious, viral disease of domestic and wild ruminants in many parts of the world (Jones et al., 1981). Bluetongue continues to be a major disease affecting sheep in all countries in the tropics and subtropics including India. The economic loss due to bluetongue outbreaks in sheep is very high. Control of the disease is very difficult because of the plurality of the serotypes that occur and no single vaccine may be effective (Nachimuthu et al., 1992). In the absence of vaccines available for the control of Bluetongue virus, the other measures are needed to be adopted such as control of the vector population, treatment of animals for secondary bacterial infection and nursing of the animals. These control measures would be worthwhile if early diagnosis of the disease is accomplished (Prasad et al., 1994).

In BHK 21 and Vero cell lines, the BTV isolates produced visible cytopathic effect (CPE) from 36h post infection. Infected cells became swollen and well defined. Rounding of cells, syncytia formation, giant cell formation and grouping of cells have also been observed in fifth passage. The affected cells have shown granularity and undergone variable degrees of shrinkage. Later, most of the cells got detached from the glass. The same type of CPE and changes in the cell morphology was observed by Meenambigai et al., (2006) who made a comparative study of the inclusion bodies in BHK 21 and Vero cells infected with BTV. During initial passages there were more refractile cells in the cell monolayer, with aggregation and clumping of cells in 48 hours post infection in both cell lines. The initial stage of infection of cellular destruction coupled with clumping of cells and formation of polykayotes were noticed. Later, rounding of cells became more prominent with syncytia formation. The pores in the nuclear envelope appeared much enlarged in BHK21 infected cells than Vero infected cells as compared with the control cell culture preparation. This was in accordance with the findings of earlier research workers Nachimuthu et al (1992) and Clavijo et al (2000).

In our study, BHK-21 cells have shown a high sensitivity to BTV infection, producing a distinct cytopathic effect (CPE) at 3–5 days post-infection. The CPE is characterized by the appearance of foci of rounded and refractile cells. These foci rapidly enlarged to the point of rupture, whereupon they detach and float freely in the culture media. However, as reported by Afshar (1994), isolation of BTV from field samples required the amplification of BTV in ECE before tissue culture can be used. If direct inoculation in tissue culture of field samples is required, this should be carried out in parallel with ECE inoculation (Afshar, 1994).

The infectivity titres of BTV isolates in BHK 21 and Vero cell lines were adduced at the end of 3rd, 5th, 7th, 9th and 10th passage levels. A gradual rise in titres was observed with respect to all the isolates as the passage levels increased. At the end of 12th passage the (Log₁₀) tissue cultures infective dose 50 (TCID50) was found to be between 10.3 and 10.8 in Vero cell line and 8.1 and 8.8 in BHK 21 cell line (p< 0.0001). It housed significant difference in the each passage titre value. In the earlier study of Reed and Muench (1938), a titre value of 7.5 and 6.5 was observed in BHK21 and Vero infected cells at 15th passage. These findings are in agreement with the earlier observations made by Howell and Verwoerd (1971) and Scott et al (1978).

The various mammalian cell lines, Baby Hamster Kidney (BHK21), African green monkey kidney (Vero) and an insect cell line (C6/36) derived from Aedes species were the most common types used for BTV propagation (Wechler and Holland, 1988). BHK21 and Vero cells offer a way of growing BTV to comparably high titres, free of cell metabolites (Homan and Yunker, 1998) and hence was chosen in the present study for the adaptation of BTV. Generally the use of cell cultures for the primary isolation of BTV from the field samples is not recommended. Routinely, it is necessary to amplify the virus in ECE before growing in cell culture for eventual identification using serological assays for viral antigen production (Gould et al., 1989). The same principle was followed in the present study and the outcome of the study was found to be successful as the inclusion bodies induced by the virus (intra cytoplasmic eosinophilic inclusions, nuclear fusion with giant cell formation) correlated with the findings of earlier workers (Brown and Jochim 1962). Moreover, it is also evident from the infectivity titre of 9.48 that the virus has got adapted to BHK21 cells to a greater extent than Vero cells as it happened to be an ideal cell line for Bluetongue vaccine production.

The outcome of the study was found to be successful as the inclusion bodies induced by the virus (intra cytoplasmic eosinophilic inclusions, nuclear fusion with giant cell formation) correlated with the findings of earlier workers (Brown and Jochim 1962). Moreover, it is also evident from the infectivity titre of 9.48 that the virus has got adapted to BHK21 cells to a greater extent than Vero cells as it happened to be an ideal cell line for Bluetongue vaccine production. The silent persistent infection of the BHK 21 and Vero cell lines cells with BTV indicates that more stringent screening of the cells used in the production of live vaccines.

Acknowledgment