Emerging zoonoses

l8 World Heolth r 50lh Yeor, No. ), Jonuory-Februory I 997 Emerging zoonoses Irongois-Xovier tleslin, Klous Stiihri & P. Iormenly 'Mod cow" olert in Europe. The heods ond ofher offol of cottle sloughtered in the United Kingdom ore dyed blue to ensure thot fhey ore disposed of ond ore not usedTor humon or onimol food. Photo: Still Pictures,/N. Dickinson @. everal of the human pathogens which have emerged or re- emerged worldwide in recent years are agents originating from animals or from products of animal origin. The animal species and the categories of agents differ widely. For instance, wild animals (bats, rodents) as well as draught animals (horses) and food animals (poultry, cattle) have been implicated in the epidemiological cycles of certain diseases, while the agents responsi- ble for other new infections and diseases in human beings have in- cluded viruses, bacteria, especially enteric bacteria (salmonellae, Escherichia coli) aad parasites (Cryptosporidium) of animal origin. Outbreaks of otherwise endemic zoonoses such as rabies, brucellosis, leptospirosis, anthrax and arbovirus infections involving production animals (Venezuelan equine en- cephalitis, Congo-Crimean haemor- rhagic fever) have appeared in many developing countries in Africa, Asia and Latin America. The zoonotic nature of some other important human diseases - "potential zoonoses" - is suspected but not yet demonstrated. For exam- ple, the natural cycle of Ebola virus is not known and its reservoir (possi- bly an animal which plays an essen- tial role in maintaining the agent and its life cycle) remains to be identi- fied. On the other hand bovine spongiform encephalopathy (BSE), a fatal disease ofcattle, is suspected - although no scientific proof of a link yet exists - to be the origin of a new variant form of Creutzfeldt-Jakob disease (CJD) in human beings. These potential zoonoses have re- cently had a considerable impact on public health and on animal produc- tion worldwide in view of the need to minimize the potential risks for human beings. BSE ond CJD BSE first came to the attention of the scientific community in November 1986 with the appearance of a newly recognized form of neurological disease in cattle in the United Kingdom. Between November 1986 and 3l May 1996 about 160 000 cases were confirmed on 33 400 farms in the United Kingdom. Epidemiological studies suggested that the source was cattle feed pre- pared from the carcasses of rumi- nants such as sheep. Modifications of the physical conditions of the process used for preparing cattle feed introduced in 1981-82 may have created a risk factor by not eliminat- ing the disease agent from the feed. By May 1996, BSE had been reported from 10 other countries and areas. In France, Ireland, Portugal and Switzerland the disease occurred in native cattle and could be linked to importation of potentially infected cattle feed from the United Kingdom. In Canada, Denmark, the Falkland Islands, Germany, Italy and Oman, cases of BSE were identified only in cattle imported from the United Kingdom. BSE is one of several different forms of transmissible brain disease of animals. A number of similar severe, and fatal, neurological human diseases are due to nonconventional agents. These include kuru, a disease transmitted by ritual handling of bodies and brains of the dead and identified in Papua New Guinea in the 1950s, and the various forms of CJD, which is a rare disease with a worldwide distribution. When it was shown that these UlorldHeolth r 50thYeor, No. l, lonuory-tebruoryl997 t9 diseases (called transmissible spongiform encephalopathies) could be transmitted experimentally to a range of animals, WHO arranged four consultations between l99l and 1995 to review the possible public health implications, with special emphasis on BSE. The sudden announcement by the United Kingdom in March 1996 that 10 people had been identified with what appeared to be a variant ofCJD led to a fifth and then a sixth consulta- tion at WHO since - although there was no direct evidence of a link - the most likely hypothesis was that these cases might be related to exposure to BSE. These consultations issued recommendations designed to mini- mize risks for consumers in relation to food products of animal origin, and proposed a protocol for global surveillance of the new variant form of CJD. Ebolo ond Ebolo-reloted viruses ln 1976 the Ebola virus attracted worldwide attention with outbreaks at Nzara, Sudan, with 284 cases and at Yambuku, Zaire,with 318 cases. In both instances there were very high mortality rates (53Vo in Sudan andSSVo inZaire). The first patients may have been in contact with in- fected animals or their products (bats or rodents in Sudan, meat from monkeys or wild antelopes inZaire) but investigation of a possible animal reservoir remained inconclusive. Between 1977 and 1989 a few rela- tively small Ebola outbreaks were reported. Then in 1994 a Swiss ethologist became infected by a new Ebola variant after doing a post- mortem on a chimpanzee originating from the Tai forest in C6te d'Ivoire. The chimpanzee proved to be in- fected with Ebola and a number of deaths reported among these animals in the same area were associated with this agent too. In January 1995 a new Ebola epidemic was reported in Kikwit, Zare, with a total of 3 16 cases and 245 deaths. In January 1996, an outbreak occurred in Makokou, Gabon, with 37 cases and 21 deaths. In Gabon the investiga- tion showed that most of the patients had been in contact with a dead chimpanzee which they had butchered. Nothing definite is known about the reservoir of the Ebola virus, although a number of hypotheses have suggested that rodents or in- sects may be involved and even that the virus may be of plant origin. There is general agreement that the species of monkey and apes in which the virus strains have been isolated so far are only victims of the disease since, in view ofthe high mortality rates the infection causes, these groups of animals could not sustain themselves or the disease agent for very long. WHO has initi- ated a multidisciplinary study on mammals, birds and insects in the Tai forest of C6te d'Ivoire aimed at identifying the natural reservoir of the virus. The study involves scien- tists from Canada, France, Sweden, Switzerland, United Kingdom and USA. Identifying the reservoir is essential to understanding the mech- anisms for transmission in nature and helping to prevent future Ebola outbreaks. In both developing and developed countries, a number of zoonoses have emerged either as new pathological entities or as already known agents, appearing in areas or species where they had not been previously re- ported. The reasons for this increas- ing trend are complex but they include: I alteration of the environment, influencing the size and distribu- tion of certain animal species, and the vectors and transmitters of infectious agents that affect human beings; I increasing human populations, thus favouring an increased level of contact between humans and infected animals; I industrialization of animal pro- duction; I changes in food processing and consumer habits. As such problems are likely to con- tinue, the surveillance of animal diseases and zoonoses will need to be Plotforns ond footbridges ore buih in the conopy of lhe Toi Fotest in C6te d lvaire so thot onimols con be couohl ond checked for infecfion with the Ebololirus. Photo: WHO/D. Heymonn & P. Formenty. reinforced and maintained at na- tional and international levels. Further collaboration is essential between all professions involved in the development of food technology and control of the food industry in order to ensure that health risks in the food chain are minimized every- where. I Dr Froncois'Xovier Meslin ond Dr Kous Stohr ore Scienfists in the Division of Emerging ond other Communicob/e Diseoses Surveillonce ond Control, World Heolth Orgonizotion, l2l I Genevo 27, Swizelond. Dr P Formenl,y is Scienli/ic Coordinoror of the WHO Ebolo-Toi Forest Project in the C6te d'lvoire. His oddress is c,/o the WHO Represenlotive, 0l Boile postole 2494, Abidjon 0l , Cote d'lvoire.