Tissue distribution, shedding profile and lateral transmission of chicken anaemia virus

Abstract

<p>Chicken anaemia virus (CAV) is a small circular virus that belonging to the genus <i>Gyrovirus</i> in the family <i>Circoviridae</i>, and is the causal agent of chicken infectious anaemia. The virus is distributed worldwide with little genetic and antigenic variability. It is a relatively new disease, first reported around four decades ago, although its economic significance has been documented more recently. Although clinical signs could be induced in young birds, it usually causes subclinical infection in older birds. The primary target cells are haemocytoblasts in the bone marrow and lymphoblasts in the thymic cortex and while mortality induced by CAV in chickens is low, serious complications due to immunosuppression and secondary infections may occur. These effects lead to significant economic loss in the global poultry industry. Surprisingly, a significant basic information about CAV is lacking in the literature, including, the shedding routes and rate of the virus excretion from the host, and routes of lateral infection of flockmates. Traditional diagnostic methods for the disease and/or infection are often not sensitive enough to detect the presence of the virus in the host, in the absence of specific clinical signs. In light of the above, the main objectives of this thesis are to determine the shedding rate of the virus and its lateral transmission routes and to develop a reliable and quick diagnostic and monitoring methods of CAV infection based on quantitative real-time PCR (qPCR). Defining the dynamics of the virus in host tissues is also an objective.</p><p> Four animal experiments were conducted to address the above objectives. The first two experiments were conducted to determine the tissue distribution (in four selected tissues) and shedding profiles of CAV in specific pathogen-free (SPF) and commercial broiler chickens respectively up to 28 days post infection (DPI). The third experiment was conducted to study the dynamics of the virus over a longer period of time (56 DPI) in nine tissues. CAV genome detection and qPCR quantification in dust, litter and faeces were attempted in all these three experiments with variable results. A fourth experiment was carried out to investigate the lateral transmission routes of CAV.</p><p> Investigations into the dynamics of CAV in nine tissue samples (thymus, bone marrow, bursa, spleen, liver, kidney, gonads, skin, and feather shaft) over time up to 56 DPI revealed that all tissue types were positive between 6 and 56 DPI. This result suggests that there is no specific tissue tropism of CAV and virus is present in lymphoid cells distributed in all tissues. The highest load of virus was detected in thymus and bone marrow which contain major aggregations of the target lymphocytes or their precursor cells; therefore, these two tissue samples are preferred samples for diagnosis and monitoring of CAV infection in chickens. Day of collection is not critical because the virus is detectable from days 6 to 56 post-infection in significant levels. The CAV genome was also detected and quantified in dust and faeces but making these non-invasive samples potentially be used for monitoring CAV in chicken flocks. However, detection in litter samplers was less successful. It was demonstrated that the virus shed in faeces are infective and can infect flockmates orally. Virus shed in faeces as early as 5 DPI are infective, but the highest level of faecal shedding occurs around 2-3 weeks post-infection. In the present study, we established that a minimum of 10⁴ genomes in faecal material are required to gain 100 % infection and 10^3.57 is required to orally infect 50 % of chickens at 16 days of age. Airborne transmission of the virus was also demonstrated, although the origin of the virus in air could not be determined; it could be from fragmented faeces, chicken dander or both. Nor could it be ascertained whether the airborne transmission involved primary infection via the ocular, respiratory or digestive epithelia given the anatomical linkages between the three. In the end of this thesis it has been determined the shedding rate of CAV in faeces and profile in dust, confirmed the faecal-oral route of CAV transmission, also demonstrated airborne transmission and developed diagnostic and monitoring technique for CAV infection either from tissue or environmental samples such as poultry dust. Further validation will be required for monitoring based on environmental samples before use in the industry.</p>