Do coccidian and nematode infections influence immunity against myxomatosis and rabbit haemorrhagic disease in European rabbits?
Wild animals are often infected with several types of parasites whose interactions determine the host’s fitness and the epidemiology of disease. Carlos Rouco and colleagues studied these interactions in rabbits in southern Spain, the original home of rabbits. Over the last 50 years, rabbit populations have declined dramatically on the Iberian Peninsula, due largely to the arrival of myxomatosis (MYXO) in the 1950s and rabbit haemorrhagic disease (RHD) in the late 1980s. The additive effect of these viral diseases has, in some less-favourable areas, led to rabbits becoming extinct. For example, 5 years after the arrival of RHD, Spain’s wild rabbit population halved.
In this study, Carlos and his colleagues tested whether a microparasite (a coccidian) and a macroparasite (a gastrointestinal nematode) influence the ability of rabbits to generate an immunologic response against MYXO and RHD. Infections of MYXO, RHD and coccidia result in a decrease of specialist (Th1cells in the rabbit that generate effective antiviral responses. On the other hand, macroparasites such as nematodes mainly produce a different (Th2) cellular response. Furthermore, the Th1response blocks the Th2response and vice versa, and in theory only one response can happen at any one time. The team hypothesised that individual rabbits with high parasite loads have a reduced ability to sero-convert (the process of developing immunity) against both MYXO and RHD. Additionally, since coccidia and viruses are under the same immunologic regulation, coccidia would have less effect than nematodes on the immunosupression of both viral diseases.
|Restocking plot, predator-proof fence, and collecting blood samples from the study rabbits. Images - Alex Bertó and Carlos Rouco|
The team took advantage of an experimental rabbit restocking programme in Hornachuelos Natural Park (Andalusia, Spain), where rabbits were released in predator-proof 4-ha fenced plots. On each plot, 30 evenly distributed artificial warrens, consisting of piles of stumps and rocks covered with loam and branches, were built above ground (see photo). Near each warren, water and commercial food pellets were freely provided. Rabbit abundance was monitored via counts of their faecal pellets; their antibody status by taking blood samples from 563 rabbits during seasonal captures; and their coccidian and nematode loads from studies of fresh faecal pellets.
Coccidian loads, nematode loads, and season explained the sero-conversion rates for MYXO. The pattern for RHD was less clear as the best model of rabbit abundance, coccidian loads and nematode loads was no better at explaining sero-conversion than using the average rates (null model). Sero-conversion against MYXO was low when coccidian loads were high, suggesting that rabbits with high microparasite loads would be unlikely to mount a good immunological response against MYXO.
Coccidian infections are the main cause of intestinal disease in rabbits. These infections are quite prevalent in rabbit populations and cause high mortality. Furthermore, the growth potential of young rabbits recovering from coccidiosis is severely compromised, which further reduces their immune responses. In spite of this, the trade-offof Th1against Th2may not be the only way the rabbit immune system responds when co-infections occur. Contrary to expectations, nematode load seems to play a minor role as this explained only 1.4% of the variance in the data.
Carlos and his colleagues suggest that coccidiosis plays a key role in rabbits’ immunologic responses to MYXO, and has important implications for disease management aimed at either increasing rabbit populations in areas where they are scarce, or for reducing populations where they are considered a pest.
The work was funded by an IPA-CSIC contract through projects 20070687_5 and 20091418.