ÐÓ°ÉÔ­´´

FOXES foraging in the European countryside this spring have been finding
a surprise awaiting them in the undergrowth: pellets of fat and fishmeal,
with plastic packets hidden inside. As the fox bites into the pellet, live
viruses contained in the packets flood its mouth. The viruses immunise the
fox against rabies. And as the foxes in the region become immune, rabies
infections cease to spread, and eventually disappear.

In the areas where it has so far been introduced, the eradication programme
has been highly successful. The scientists organising the programme say
that if it continues for another few years, wild rabies could be eradicated
from western Europe. Even the draconian measures currently used to keep
rabid animals from entering Britain may no longer be necessary.

The choice now facing those responsible for eradicating the disease
is whether to continue with the use of the classic vaccine – whose active
component is weakened, live rabies virus – or to support the introduction
of a genetically engineered virus developed by the French company Transgene.

The engineered vaccine is currently being distributed in Belgium and
France in one of the largest releases of a genetically modified organism
yet carried out in Europe. But some scientists who work with the classic
vaccine distrust the new one. They think it may have hidden disadvantages
that have yet to surface.

Rabies is one of the oldest recognised diseases. Its victims invariably
die in agony, unless treated immediately with a series of immunisations
along the lines developed in the 19th century by the French scientist Louis
Pasteur.

Pasteur discovered that the virus that causes rabies can also induce
immunity to the infection. Weakened strains of the rabies virus are used
to vaccinate domestic animals and to treat infected humans. The virus must
be given alive, in order to multiply and expose the immune system to large
amounts of rabies antigen.

Since 1978, European scientists have been distributing live rabies vaccines
in several regions in an unprecedented effort to eradicate the disease among
wild animals. Rabies is a prime candidate for such an eradication strategy.
If a rabid animal does not bite another susceptible animal before it dies,
the virus will die with it. If the concentration of susceptible animals
is too low for most infections to be passed on, the chain of transmission
is broken, and rabies dies out in the region.

That is what happened in Europe early this century, when vaccination
became mandatory for domestic animals and wild packs of dogs were destroyed.
Rabies sprang up again on the Russian-Polish border in the 1930s. Jean Blancou,
of the French national rabies centre in Nancy, says the Second World War
helped the disease to spread by allowing packs of unvaccinated dogs to roam
loose, and by enlisting men who would otherwise have been hunting foxes.

Dogs and foxes contracted rabies from its ‘reservoir’ among wild animals
in Russia, and spread it among themselves to cause a high overall level
of infection. By the time dogs were brought back under control in the late
1940s, the infection was common enough among foxes to start spreading as
a major epizootic (as animal epidemics are called). Fox rabies has since
spread west through Europe at 30 to 40 kilometres per year. It reached West
Germany in 1950, Belgium in 1966, and crossed the Loire into western France
last year. But since 1945 only about 30 human cases have been reported in
Europe, says Blancou.

It was in the 1970s that European scientists first decided to develop
a vaccine for wild foxes, rather than attempt to control the spread of the
disease by keeping down the fox population.

The main difficulty with vaccinations has been in developing a virus-laden
bait for the foxes. The live virus dies after a few weeks at field temperatures,
so the bait must be eaten quickly for the vaccine to work. A preliminary
trial with virus injected into egg yolks failed because, unknown to the
virologists who planned the trial, foxes bury eggs and return to eat them
later, allowing time for the virus to die.

The first successful vaccine consisted of chicken heads stuffed with
packets of virus. This bait, distributed strategically in Alpine passes,
stopped rabies from crossing the Swiss Alps (New ÐÓ°ÉÔ­´´, 13 January 1983).
But the chicken heads were unpleasant to work with, and chicken baits are
hard to produce in the large numbers needed to cover wider expanses of country.

As an alternative, scientists working at the West German Federal Institute
of Virology in Tubingen developed a paste of fat, bonemeal and fishmeal
in which a plastic packet of virus could be embedded by machine. Foxes loved
it, and the baits were first distributed in flat country in West Germany
in 1983.

Since then, 5.2 million baits have been distributed. Over 70 per cent
of foxes in target areas take the baits, and are immunised against rabies.
This reduces the concentration of susceptible animals in a region sufficiently
to break the chain of transmission. Over 50,000 square kilometres of West
Germany were rendered rabies-free by 1987.

Significantly, no case of vaccine-induced rabies has been reported among
the 50,000 animals of various species trapped and examined in vaccinated
areas. This is important because of the theoretical possibility that the
live virus, though weakened, could revert to a stronger strain and end up
causing disease.

But the strain used in the vaccine has caused rabies in rodents in the
laboratory. The danger exists, at least in theory, of creating a new epizootic
of vaccine rabies among rodents.

No rodents that had eaten the vaccine in the field trials have shown
signs of rabies. But Paul-Pierre Pastoret, of the University of Liege in
Belgium, says that rodents that developed rabies from the vaccines might
have been missed in the survey because they were either dead or paralysed.

Pastoret is helping to test the new vaccine developed by Transgene,
the Strasbourg-based genetic engineering company, which avoids this risk
entirely. It consists, not of live rabies, but of live vaccinia virus.

Vaccinia is the pox virus used to immunise people against smallpox.
Transgene isolated a gene from the rabies virus that codes for a surface
antigen specific for rabies and inserted the gene into the DNA of vaccinia.
When given to animals, the vaccinia replicates and exposes the immune system
to the rabies protein, inducing immunity against rabies.

Transgene has sold the patent on its recombinant virus to the French
vaccine company Rhone-Merieux, which inserts plastic sachets of the virus
inside Tubingen baits. In 1987, according to Pastoret, the recombinant virus
was tested for safety on 6 square kilometres of a closed military base in
Belgium.

The following year, it was scattered over 435 square kilometres of infected
countryside in southern Belgium. This spring and autumn it will be spread
over 2200 square kilometres in the same region.

Some supporters of the classic vaccine are critical of the new one.
There has been much debate among virologists about the possible pathogenic
effects of vaccinia, despite the fact that it has been used safely as a
basis for several vaccines, including vaccinating patients with little immune
response, or people with AIDS. The main concern, according to these critics,
is that we know too little about the vaccinia virus to be certain that it
is not a human pathogen. They say that the recombinant vaccinia should at
least be tested for safety in primates before being distributed in the wild.

If there were evidence that it might be a human pathogen, then it would
be difficult to release the vaccinia in a genetically modified form under
the new European directives covering genetically modified organisms in commercial
use. The new regulations have yet to be incorporated into Belgian law.

France, the home of the companies producing the new vaccine, has been
slower to permit release into the environment. Last autumn, however, the
first recombinant vaccinia in fox bait was distributed in the centre of
the country, deliberately far from the border of West Germany, which is
sensitive about genetic engineering, says Blancou.

The advantage of vaccinia, says Pastoret, is that it is more stable
than weakened rabies virus. Classic vaccine is scattered in spring and autumn
to avoid the heat of summer. Vaccinia could permit more flexible timing,
as well as permitting rabies campaigns in hotter places than northern Europe,
such as the Indian subcontinent or North Africa, where rabies is a serious
problem.

Pastoret says that vaccinia has the advantage of over a century of handling
by doctors. And in areas in which the recombinant virus has been used to
vaccinate foxes, no animals have shown pox lesions, even in highly sensitive
tests for pox DNA.

The rabies gene is inserted into the region of the vaccinia DNA that
codes for an enzyme, thymidine kinase, which is essential to the pox virus’s
ability to grow in nerve cells, says Pastoret. The virus must grow in nerve
cells if pox disease is to develop.

But controversy stills exists. James Cox, of the virology laboratory
in Tubingen, says he cannot judge the usefulness of Transgene’s recombinant
virus until he sees data showing that it has induced immunity against rabies
in the wild.

Pastoret responds that the real proof of effectiveness is that the incidence
of rabies has dropped dramatically in areas treated with either vaccine.

It will be difficult to eradicate rabies in towns. With their constant
supply of tasty garbage, some have a much higher density of foxes than the
countryside. Vaccine pellets would have to be safe to use among high concentrations
of people and pets to vaccinate these foxes. The answer to the question
of which vaccine is safer – live rabies, or live pox – will be crucial in
any effort to eradicate rabies from towns.

Recombinant or classic, the commercial appeal of rabies vaccine is limited
in Europe, since it should wipe out the disease and so remove the market.
The big money will come from exports.

The European Community is already helping to pay for vaccination campaigns
in border regions of Czechoslovakia, Yugoslavia and East Germany, and a
large campaign has turned rabies back at the Finnish-Soviet border. The
East German state vaccine company is another potential manufacturer of classic
vaccine.

The recombinant vaccine will only compete with the classic one, says
Blancou, if the costs are comparable. Merieux, however, has not yet decided
what price to set for its recombinant vaccine.

One advantage beyond financial profit will be the possibility that a
genetically engineered rabies vaccine could counter popular fears about
genetic engineering. Rabies may be the one thing the public fears even more.

* * *

Do they really have to wait for six months?

THE QUESTION of rabies control is becoming politically sensitive in
the European Community, where border controls are due to end by 1992. French
rabies researcher Jean Blancou says British quarantine measures need not
be applied to other countries in the Community even now, because European
fox rabies spreads so poorly among any animals but foxes.

The only way to infect Britain with the current epizootic, says Blancou,
would be to release 50 rabid foxes deliberately inside the country. European
dogs and cats, which must now be quarantined for six months on arrival in
Britain, are unlikely to spread rabies, he says.

Officials of the European Commission in Brussels say that once the European
epizootic is under control, rabies controls will be needed only at Europe’s
external borders. Some officials question whether the British-style six-month
quarantine is needed. Spain is regularly infected by dogs crossing from
its African enclaves of Ceuta and Melilla, where the much more dan gerous
‘street’ strain of rabies is rampant.

The infection is readily contained by vaccinating animals in the surrounding
area, says Blancou – so dog rabies, he says, is unlikely to become a problem
in Europe.

French authorities have developed a technique for screening incoming
animals that requires not six months, but only a few weeks. The animal is
tested for rabies antibodies. If it has none, it is vaccinated and sent
on its way.

Any antibodies present could be due either to vaccination or to infection.
To find out which, the animal is given a rabies vaccine. This will significantly
increase the level of antibodies only if these are due to prior vaccination;
an infection stimulates the production of so many antibodies that the vaccine
will have little additional effect.

The success in detecting an incubating rabies infection by this means
is generally considered to be about the same as that achieved by six months’
quarantine.