Is a deadly rabbit virus affecting fox numbers?

Q: Four years or so ago the local rabbit population crashed. We have had myxomatosis previously but the population always seemed to recover to some extent — this time it has not. Could rabbit haemorrhagic disease virus (RHDV) be causing this? Since foxes that prey on rabbits also seem fewer 
in number recently, could RHDV 
be infecting foxes as well?

A:

Rabbit haemorrhagic disease (RHD) is a highly contagious viral infection that occurs in both wild and domestic rabbits. Contrary to its name, the virus — known as RHDV — causes red blood cells to clump together in the blood, making it clot.

Blood clots become lodged in various vital organs, cutting off the local blood supply. Blood leaking from these affected tissues is the cause of the haemorrhages seen at post-mortem.

Signs of rabbit haemorrhagic disease

These vary and and may include loss of appetite, lack of co-ordination, respiratory difficulties and discharge 
of blood from various orifices. Animals generally show few external signs of infection and it is more likely that an apparently healthy-looking rabbit will simply be found dead. The incubation period is short, between 24 and 72 hours, with death occurring within several hours or up to one or two days later. RHD is difficult to diagnose in the wild.

There are now two main pathogenic strains of the virus. RHDV1 was first seen in the UK in 1992. More recently 
a separate strain, RHDV2, has appeared. There are strain differences; most notably RHDV2 affects young rabbits less than four weeks old. RHDV1 does not and immunity in young rabbits surviving RHDV1 infection was thought to explain how wild populations would recover. RHDV2 infection also tends to be more protracted; rabbits take longer to die and hence provide more time 
and greater opportunity for the virus 
to spread to infect others.

How the virus affects foxes

Fox populations in affected areas can be adversely influenced, not because foxes are infected per se — indeed they can help spread infection by eating infected carcasses and spreading the virus in the environment in their faeces — but because they are affected indirectly. The rabbit represents a primary food source for the fox as well as other predators such as buzzards, so as the rabbit population declines the number of foxes dwindles too.