MANAGEMENT, CONTROL AND REGULATIONS

IN TERRESTRIAL CARNIVORES

The control of rabies has been among the greatest suc­cesses of wildlife disease management. Before the first quarter of the 20th century most recorded cases of rabies were in the domestic dog.

From the 1970s, computer models have been used to simulate rabies spread, primarily in wildlife, and examine control options. After a preliminary field study(²2), the critical threshold density, below which the epidemic would die out, was calculated at between 0.2 and 1.0 fox per km². As survival at this density will be very high, it does not appear possible for fox densities to be sustained below this level, by culling alone, as mortality is compensated by an increase in life expectancy.

In 1969 foxes were first immunized against rabies in the USA by the oral administration of a live attenuated strain of the rabies virus(²3). Field trials using similar strains (SAD strains) started soon after in Europe.

As oral immunization of wildlife is only possible using infectious (live) vaccine, all the above infectious vaccine strains can pose environmental safety problems, which have to be properly assessed before field delivery. Targeting the European red fox resulted in the near complete elimi­nation of rabies from West- and Central Europe. Similar strategies have subsequently been used to control rabies in a range of other carnivores in the USA and Canada¹-²⁴). Unless there are reasons to change the distribution, the protocol for oral fox vaccination should follow the Euro­pean Commission guidelines¹-²⁵). These state that following any re-emergence of disease, emergency vaccination of an area of 20—50 km width and a bait density of 18—20 and 20—30 baits per km² should be followed in areas of low and high fox density, respectively. In areas of particularly high fox density, there may be difficulties in eliminating the disease with vaccination on its own¹-²⁶’²⁷), and the above protocol has not been tested in such circumstances.

I t is worth noting that there are two advantages of culling over vaccination. Firstly, culling may remove infected animals that do not then become rabid. Such individuals cannot be detected while incubating the disease. Secondly, culling reduces the overall productivity of susceptible animals. This latter effect can also be achieved with a fertility control agent, and modelling sug­gests this potentially has the greater effect. As fox culling can also result in increased movement⁽²⁸⁾ and thus may lead to an increase in disease spread, it is likely that future oral rabies developments will incorporate immunofertility agents.

In Eastern Europe, the spread of the raccoon dog has caused complications for rabies control, as rabies could spread more easily into areas of low fox density⁽²⁹⁾. It appears to be a competent host, and contributes about half of the wildlife cases in some Baltic countries. The raccoon dog hibernates in very cold winters, but not in warmer ones, and this may affect the probability of disease spread­ing in a mixed community of foxes and raccoon dogs. Raccoon dogs continue to spread through Europe, and although the oral rabies vaccination appears to work well in this species, the effective increase in host density means that wildlife outbreaks in areas where both species are present need closer monitoring.

IN BATS

Currently, the bat lyssaviruses do not appear to affect the conservation status of the host bat species. In addition, human infection from these diseases in Europe is rare. However, monitoring infection and protecting human health when lyssavirus infection has been detected in a colony may require adaptive measures and informing the inhabitants of the house and neighbours. Sometimes it is practical to separate the bat roost from the rest of the habitation by filling cracks and blocking access of the bats to the rooms. In the case of caves, public access can be prevented by bars. Finally, if no solution can be found, the access of the bats to the roost could be closed at the time of the year when the colony has gone, to prevent re-colonization.

PREVENTION AND CURRENT EU REGULATION

Rabies is listed as a regulated disease in the EU for the purpose of eradication and monitoring programmes. Several decisions have been published following expert committee recommendations¹-¹³’¹⁴’²⁵). The objective of the programme in every member state is to eradicate the disease from wild animals (sylvatic rabies) or from domes­tic animals (urban rabies) if an outbreak occurs.

In addition, each member country is under an obliga­tion to have a rabies surveillance scheme and a set of measures to manage the domestic pet and fur animal cir­culation inside the EU, or coming from third countries (individual identification, minimum age allowed for trav­elling, relevant vaccination, passport, etc.).

PUBLIC HEALTH CONCERN

In Europe, locally there can be an emergency or high alert status situation where an outbreak of wildlife rabies re­emerges or persists (e.g. Italy at the border of Slovenia since autumn 2008). Worldwide, 55,000 human deaths/year are recorded by the WHO, with an associated total of more than a million disability-adjusted life years (DALY) per year (a standardized comparative measure of extended disease impact). Studies carried out in affected continents indicate that the number of human rabies deaths may be up to 100 times higher than that officially reported(3⁰). Human mor­tality from endemic canine rabies can be estimated at between 24,500 and 90,800 deaths, with most occurring in Asia and Africa. Each year, there are about 12—13 million people receiving expensive post- exposure prophylaxis (PEP) in the world. Thus, reoccurrence of an epidemic form of rabies would be important, as the rabies virus is a significant pathogen and 100% lethal in infected individuals. In the EU, three different main threats can be anticipated:

• a former bat lyssavirus evolving to a new rabies variant communicable to terrestrial animals(³¹) ;

• sporadic reintroductions from dog rabies resulting in non-notified outbreaks in domestic carnivores, poten­tially spreading to wildlife and domestic animals;

• spread of fox or raccoon-dog rabies by local contagion across the borders of the EU or malicious/irresponsible translocation of infected wild animals (i.e. foxes, raccoon dogs or raccoons).

Other, less plausible risks, involve a classical rabies strain imported from the Americas by a bat hibernating in timber for instance, or emergence of a new bat lyssavirus strain. In both scenarios the result should be expected only as sporadic events.

Each reintroduction of dog rabies in Europe now is a challenge to public health infrastructure and represents a significant cost in resources, in terms of new preventative programmes, public concern abatement and post-exposure prophylaxis.