Effect of route of infection on serological response in juvenile rabbits experimentally inoculated with RHD : a dissertation submitted in partial fulfilment of the requirements for the Degree of Bachelor of Science with Honours at Lincoln University

Abstract

Rabbit haemorrhagic disease (RHD) was illegally introduced into New Zealand as a biocontrol agent of rabbits in 1997. Since its introduction, kill rates across the country have been inconsistent (Parkes et al.,1998), and populations with high levels of immunity have been recorded (Marchandeau et al., 2000). It is proposed that the route by which juvenile rabbits are infected with the virus affects the rate of seroconversion and therefore immunity in these populations.

Juvenile rabbits less than 8 weeks of age do not generally die when infected with rabbit haemorrhagic disease virus (RHDV), due to innate resistance and the presence of maternal antibody. Following infection they may or may not seroconvert and become immune to RHD. The mechanism that controls this development of immunity is unclear although it has been proposed that the route by which they are infected may affect subsequent immune response, and as a result seroconversion.

This series of trials was conducted in two parts to investigate the role of route of infection on seroconversion in juvenile rabbits; both feral and domestic animals were used. In the first trial, 60 captive bred feral animals housed in outdoor runs were dosed orally, intraocularly or subcutaneously at either 10 or 100 wso with RHDV. Changes in sera antibody levels (as measured by a competition ELISA) were analysed to determine whether the route of infection had an effect on development of immunity. The majority (93%) of the animals were discovered to be seropositive (>50% inhibition at a 1:10 dilution) prior to inoculation with RHDV. None of the animals died and there was no significant treatment effect (p=0.187) between the treatment groups. An additional part to Trial 1 assessed the effect of an extra dose of 1500 wso of RHDV given subcutaneousely 17 days following the first innoculation. No animals died and no significant effect on antibody levels was observed between the treatment and control groups (p=0.291).

The second trial was carried out under quarantine conditions in the Landcare Research quarantine animal rooms to prevent cross infection between animals within and between treatment groups. This trial was divided into two parts, the first used 11 feral rabbits caught in Waipara, and the second used 24 captive bred, domestic animals. The feral rabbits were dosed subcutaneously or orally with 100 LD50. One animal died as a result of RHD infection, the majority of the rest of the animals seroconverted and there was no significant difference (p=0.536 between the subcutaneous and oral treatments.

In the domestic rabbits there was a significant treatment effect (p<0.05), those animals dosed orally with 100LD50 seroconverted and became immune, none of the other animals seroconverted. We concluded that route of infection does have an effect on seroconversion in seronegative domestic rabbits. This was thought to be due to the components of the immune system stimulated by oral infection. A number of possibilities were proposed, including the presence of antibodies in the mucous membranes of the GIT that were not necessarily present in the sera antibody samples measured, possible CMI mechanisms, and genetic differences which could affect virus receptors or major histocompatibility complexes.

It was concluded that a number of possible mechanisms could be playing a part in the development of immunity to RHDV when domestic rabbits are inoculated via an oral route of infection. Subsequent trials would be required to determine the cause of this immunity.