Evolution des maladies des animaux sauvages

The Working Group of the Office International des Epizooties has just submitted its annual report on wildlife diseases reported throughout the world over the past 18 months.

Some of the major diseases to have emerged during this period include:

Diseases transmissible to humans

• Bovine tuberculosis, which affects many wildlife populations on several continents and remains a major source of concern. These concerns relate to both the potential effects this disease may have on certain wildlife populations, as well as the regulatory and veterinary public health concerns of having sylvatic reservoirs of infection in countries with active ongoing bovine tuberculosis eradication programmes.

Bovine tuberculosis is a particularly serious problem in the Kruger National Park in South Africa, where the geographical spread of the disease has been reported in buffaloes, with the incidental spread to other animal species living in the parks, including kudu, baboons, lions, cheetah and leopards.

In Uganda, the tuberculosis reported in the Queen Elizabeth National Park since the late sixties has just been confirmed in buffaloes in the Kadepo Valley National Park.

In Zambia, tuberculosis has been identified for several years in Red Lechwe on the Kafue flats. Transmission of the infection to herds of wildebeest was confirmed for the first time in 1998.

In Europe, tuberculosis was reported in Spain (fallow deer, red deer, wild boar and lynx), and in the United Kingdom (in badgers, roe deer, fallow deer and red deer).

In the United States of America, the outbreak of tuberculosis that had struck white-tailed deer in the state of Michigan continues to pose problems as it has been found that the disease has been transmitted to coyotes, red foxes, racoons, black bears and bobcat. Attempts are under way to combat the disease in cervids by reducing the density of their populations and by prohibiting the feeding of deer by humans.

In Hawaii, the search is on for a sylvatic reservoir of tuberculosis following detection of the disease in a cow; wild pigs seem to be the most likely source of the infection.

In Canada, bovine tuberculosis is endemic in a sub-population of bison, and a case has just been reported in an elk.

• Brucellosis is still endemic in several wild animal populations in Africa, the main species affected being buffalo, hippopotamus and waterbuck.

In Europe, the agent of the disease was identified in wild boar in France and Italy, as well as in brown hare in Austria, France, the Czech Republic and Switzerland. The recent spread of the infection amongst free-roaming domestic pigs seems to indicate that a wild reservoir is the source of the pathogenic agent. Brucellosis was also reported in chamois and red deer in the Alps.

In Canada, the pathogenic agent was isolated in American bison, reindeer and caribou, as well as in aquatic mammals, including beluga whale, narwhal and ringed seal.

In the United States of America, the disease was diagnosed in elk in the state of Idaho. In this region, elk have been fed by humans for several years, and this situation substantially increases the risk of transmission of the pathogenic agent.

• Epizootic rabies in foxes continues to recede in Western Europe following immunisation campaigns involving the distribution of bait containing vaccines against the disease. Other forms of rabies, in particular the infection in insect-eating bats, persists with no perceptible change. In early 1999, an Egyptian fruit-eating bat was found to be infected with a strain of rabies of African origin in the south of France. These bats (fruit bats) are not present in Europe, but they have been sold as pets for several years. The sick animal was imported into Belgium directly from Africa, in January 1999, then sold to a pet shop in Bordeaux (France) in March 1999. One hundred and twenty-two people had to be given preventive treatment against rabies. The sale of Egyptian fruit bats and other chiroptera, which is legal in the European Union, will certainly need to be reviewed very rapidly given the health risks associated with bats from tropical countries.

Rabies in terrestrial animals is enzootic in several parts of North America. Oral vaccination programmes against the disease are being carried out in four states in the United States of America and in one province in Canada.

• During the winter of 1998-1999, a major outbreak of anthrax extended over an area of 6,000 km² in South Africa, in the Kruger National Park and the neighbouring nature reserves. More than 160 cases, amongst 15 animal species, were confirmed, 68% of which affected kudu and buffalo. These two species are apparently more easily infected and play an important role in the spread of the disease, whilst the other thirteen species were only sporadically infected. A large number of lions were infected by consuming infected carcasses: they generally develop a sub-acute illness characterised by swelling of the head.

In Namibia, 76 cases of anthrax were reported in wild animals, most of them in the Etosha National Park where the disease in enzootic. Eleven wild species were affected, including elephant, zebra, wildebeest and springbok.

• Outbreaks of tularaemia were reported in Austria, Spain and Scandinavia during 1998. In Spain, the disease was observed in hare and in a rabbit, and several cases in humans were also reported. Apparently, the disease had been introduced into the country by imported hares.
• An outbreak of West Nile fever in humans was diagnosed in the district of New York, leading to the death of seven people and more than 50 cases of viral meningitis. Two laboratories have confirmed that the virus isolated in humans was practically identical to the one isolated in birds during the same period. The West Nile virus, transmitted by mosquitoes, had never before been identified on the American continent. The outbreak was identified in mid-August 1999 and the last case in humans was diagnosed on 16 September. The public health authorities asked the general public to take precautions to avoid mosquitoes and an intensive insect eradication programme was carried out. In October, fifteen horses were affected on Long Island, and at the same time, some birds, mainly crows, were found dead.

The West Nile virus belongs to a group of viruses transmitted by arthropods, hence their name "arthropod-borne" or "arbovirus". First discovered more than 60 years ago in Uganda, the West Nile virus was later identified in various African, Eurasian and Middle Eastern countries. In humans, the disease generally shows flu-like symptoms such as fever, headaches, muscular aches, sore throat and rash. Mortality in humans varies between 3 and 15% and is higher in elderly people.

The West Nile virus was isolated in more than 40 species of mosquitoes and certain ticks. In nature, the viral cycles include apparently healthy birds and mosquitoes, with birds considered to be the vertebrate hosts that act as a reservoir for the pathogenic agent. The problem currently facing the state of New York is different from previous episodes, because in this case the wild birds perish from the infection. American crows were the worst affected, but it also struck fish crows, blue jays, laughing gulls, American robins, rock doves, mallards, sandhill cranes, black-crowned night-herons, and several species of birds in captivity in the Bronx zoo.

Serious diseases affecting only animals

Several major diseases that can affect both wild and domestic animals have been reported to the Working Group:

• Classical swine fever which remains a very significant problem in European wild boar. In 1998, outbreaks were reported by Germany, France, Italy, the Czech Republic and Switzerland. The classical swine fever virus, which was isolated in Tessin (Switzerland), was identical to the one previously found in wild boar in the Varese region of Italy.

The virus can be transmitted from wild boar to domestic pigs both directly (where contact is possible) and indirectly (via contaminated feed). In Germany and Italy, the epidemiological data suggest that there is cross transmission of the virus between domestic pigs and wild boar. When domestic pigs and wild boar live well apart, human activity could be the cause of transmission of the disease from wild swine to domestic pigs, and vice versa.

The prevalence of this endemic situation seems to have increased in Europe over recent years.

• In the western United States of America, targeted surveys concerning hunting products have been under way for several years to study chronic wasting disease (a spongiform encephalopathy) in wild cervids living in an endemic zone located in north-eastern Colorado and south-eastern Wyoming. Given the possibility of transmission of this disease to other areas, surveillance efforts have recently been extended to populations of deer and elk living outside of the endemic zone. In total, microscopic examinations have been carried out on the brains of more than 3,500 such animals. All of the examinations proved to be negative, which confirms that chronic wasting disease is probably restricted to populations of deer and elk living in the above-mentioned endemic zone.

Surveillance was less intensive in the eastern United States of America because no abnormal deaths had been reported in this region. Since surveillance began in late 1997, laboratory examinations have not identified any cases of the disease.

• In 1998, thousands of gazelle were found dead in Mongolia. They had been infected by necrobacillosis. The agent infects the hoofs causing severe lameness. This episode was associated with the heavy rains in July and August 1998. In 1963-1964, a large number of gazelle had already died during an outbreak of foot and mouth disease and, in 1974, 140,000 had died of pasteurellosis.
• Cases of paratuberculosis (Johne’s disease) were reported in wallabies on Kangaroo Island in South Australia. However, to date there are no signs of transmission within the species and no contamination of domestic sheep has occurred. These are the first cases of paratuberculosis in wild animals in Australia.

Further information concerning wildlife health

• Since 1994, 56 bald eagles died in Arkansas (United States of America) of a nervous disease of unknown origin, known as avian vesicular myelopathy. In November 1996, American coots with the same nervous symptoms had been found on Lake DeGray. The infected coots and eagles that died had the same cerebral and medullar lesions, characterised by intramyelinic oedema. Lesions were also found in coots with no nervous symptoms. It is likely that the sea-eagles were infected from eating diseased coots, but it cannot be ruled out that the two species were exposed separately to the same source. The disease was also confirmed in mallards and ring-neck ducks, and suspected in buffleheads, an American widgeon duck and a northern shoveller on a residential lake in North Carolina. These observations, reported at many remote sites show that the disease is more widespread than initially believed, and that the Arkansas region is perhaps not the only area affected. Despite much laboratory and field research, the cause of the disease has still not yet been determined. An unknown neurotoxin, either natural or linked to human activity, is suspected.
• Numerous cases of crocodile pox have been reported on farms breeding these reptiles in Zimbabwe. Several outbreaks of polyarthritis of mycoplasmic origin have also been diagnosed on these farms, as well as cases of hepatitis caused by chlamydia. In the same country, out of 38 farms inspected, 12 were infected with trichinellosis.
• Six new cases of elephant floppy trunk syndrome were identified in pachyderms in the Matusadona National Park and two others at Malilangwe in Zimbabwe. Three other cases were reported in the Kruger National Park in South Africa. The cause of the disease remains unknown.
• In the winter of 1994, acute avian conjunctivitis, caused by a mycoplasma, was reported in house finches on the central section of the Atlantic coast (United States of America). Since then, other infected individuals have been found throughout the eastern United States. The adaptation of the pathogenic agent to this wild species could pose a problem for combating the disease in poultry. To assess the risks associated with this emerging problem, a survey was carried out between November 1997 and March 1999: more than 1,000 birds were captured simultaneously in the same zone, both on poultry farms and on sites with no poultry farming. Blood samples were taken for analysis, which showed that 19% of the birds caught on farms, and 11% of those caught at other sites, had been in contact with the pathogenic agent. The disease is currently limited to house finches, but the tufted titmouse could also be affected.