Health

Emerging Zoonoses - A Global Threat

- Dr G.N. Gongal

ZOONOSES are disease the agents of which are transmitted between vertebrate animals and people. In recent years, there are many new human pathogens that have emerged worldwide originated from animals or from products of animal origin. Many animal species were implicated in the epidemiological cycles of these diseases. There is no single factor responsible for the emergence of these diseases but rather the interaction of multifactors. Several factors contribute to the emergence of new infectious diseases. Today, infectious diseases does not recognise regional and continental boundaries as a result of extensive network of international travel and trade link and mass movement of people from one place to another. The current pace of socio-economic development in developing countries indicates that it will be difficult for national government to satisfy adequately the demands for housing, safe drinking water, sanitation, sanitary waste disposal and basic health services.

The ecology resulting from these conditions will favour an increase in urban animal populations, especially dogs, cats, rodents and consequently an increase in the number of new zoonotic infections which were not reported in the past. Many emerging diseases are thought to be due to closer contact of man with their reservoirs in nature, with a successful jump of the infectious agent from animal to man across the species barrier. When people travel into the surrounding countryside and wildernesses, they can become exposed to novel infections. A classical example of this type of infection is Lyme disease. A mysterious disease of unknown origin was reported in those people in America who visited woodland habitats during early summer season. Detailed epidemiological and laboratory investigations showed that field rodents and deer are major reservoirs of the infectious agent and the infection is transmitted to human and animals when they are exposed to blood sucking ticks in forest areas. The causative agent was recognised in 1977. Today, the disease is reported in Europe, Australia and Russia also. Many of the agents responsible for new infections and diseases in human are viruses. New viral diseases are responsible for heavy loss of human life, spread very rapidly and are difficult to control once there is an outbreak of viral infection. Unfortunately, there is no specific treatment against viral diseases. There was a quick international response to localise Ebola Haemorrhagic Fever in outbreak area because it is the most highly fatal infectious disease ever known.

Some zoonotic disease problems are emerging as a result of production oriented livestock development activities and technological advancement. There has been unrestricted, improper and indiscriminated use of antibiotics, growth hormones and insecticides in disease control and animal production sector especially in developing countries may be responsible for new public health problems in the future. The possible future use of animal organs in human beings may lead, if precautions are not taken, to the emergence of new diseases called xenozoonoses. A number of Zoonoses have emerged either as new pathogenic entities or as already known agents, appearing in areas of species in which they had not been previously reported. It has been possible to identify new zoonotic infectious agents as we have made a significant progress in the field of molecular biology.

Some examples of emerging zoonotic infections are:

Hong Kong flu
There was a panic situation in Hong Kong in 1997 due to outbreak of highly fatal flu of avian origin in human population. There was a fear that the virus will cause a pandemic infection, much like those seen in 1977, 1968, 1957, and 1918 in which an estimated 20 million people died. The virus which causes flu symptoms that are typical of most other flu viruses was previously thought to affect only birds. This virus is quite different from all other influenza viruses in that it has never affected humans. Is the new flu that jumped from chickens into humans in Hong Kong a threat to the rest of the world ? The question is yet to be answered.

Enterohaemorrhagic E. coli infection in Japan
An outbreak of Enterohaemorrhagic E. coli infection among school children was reported in Sakai city in July 1996. Epidemiologic investigation has shown that E. coli 0157:H7 was responsible for fatal outcome of the infection. The reservoir of this pathogen appears to be mainly cattle. It is transmitted to man principally through consumption of contaminated foods, such as raw or undercooked ground meat products and raw milk. Examples of foods implicated in outbreaks of E. coli 0157:H7 include hamburgers, cheese, yoghurt, dried cured salami. Symptoms of the illness caused by E. coli 0157:H7 include abdominal crams and watery diarrhoea that can develop into bloody diarrhoea. The infection can result in life threatening complications among young children and the elderly patients. Preventive measures for E. coli 0157:H7 infection are similar to other food borne diseases.

Mad cow disease in the United Kingdom
On 20 March 1996, the UK announced the existence of a cluster of 10 persons identified with what appeared to be a variant of Creutzfeldt Jacob Disease (CJD). Full investigation of these cases led to the conclusion that exposure to mad cow disease was the most likely hypothesis. As a result, it was a hot topic for discussion in the international media in 1996 and it had a serious consequences in beef consumption and business activities around the world. It is interesting that most of the outbreaks were reported in native cattle in the UK and identified in some European countries where calves or cattle feed were imported from the UK. It had been reported from 10 countries and areas outside the UK. Epidemiologic studies suggested that the source of disease was cattle feed prepared from carcasses of dead animals. Mad cow disease, a fatal neurologic disease of cattle first discovered in the UK in 1986, is one of the transmissible spongiform encephalopathies caused by unconventional agents extremely resistant to heat and chemical treatments. The transmissible agent is considered as a protein like material called prion, the nature of which is not yet understood.

Hantavirus infection in Korea
Hantavirus are often heralded as new or at least as emerging pathogens, particularly in New World. In Europe, Hantavirus disease can hardly be called an emerging Zoonoses; it is rather a rediscovered disease. Each strain of Hantavirus evidently has its own rodent reservoir. It produces sub-clinical infections in rodents. Hantaviruses inducing Hantavirus pulmonaryo renal syndromes emerge through increased exposure to infected rodents and their excreta. Regardless of severity, almost all cases are characterised by sudden onset of high fever, chills, and pain in back, muscles, and abdomen. Hantavirus infection causing Haemorrhagic fever with renal syndrome has been reported from Myanmar and Sri-Lanka in South-East Asia region. As Zoonoses and animal diseases with the potential to affect human health likely will continue to emerge, Zoonoses surveillance will need to be reinforced and maintained at the national, regional and international levels. Top priority should be given to strengthen existing medical and veterinary laboratory service system in such a way that it will be able to diagnose emerging infection as early as possible. The emergence of new zoonotic infection in any part of the world may pose a serious threat to all countries if it is not recognised and controlled on time. The only way to combat this threat is to recognise these diseases early on the spot and take preventive and control measures rapidly. It is unfortunate that most of the developing countries have poor disease surveillance and service delivery system. The situation will not change in the near future. There is no doubt that these countries will continue to face out breaks due to emerging zoonotic diseases. Under such circumstances, more emphasis should be given to develop and early warning system and rapid response mechanism to combat emerging and re-emerging communicable diseases at national, regional and international levels. Last but not least, every effort should be made to advocate and mobilise international support for epidemic control specially in developing countries.