FAO Regional Office, 39, Phra Atit Road, Bangkok 10200, Thailand.
Disease in animals and humans is the end product of a complex process. The causative agent, the host and the environment are all important factors in this process. Many disciplines are necessarily involved in reaching an understanding of the disease process, and this understanding is essential if we are to not only increase the productivity of animals but also to protect human health and the environment.
The disease outcome and the frequency of occurrence are influenced by many variables called disease determinants. These usually include one or more specific agents, as well as factors associated with the host animals and their environment. The relative importance of all identifiable determinants and the interplay between them must be considered in attempting to prevent or control diseases.
It is important that policy advisors are aware of the consequences and implications of making decisions not based on sound advice on disease control as the economic consequences of a bad decision can be devastating and long-term.
The aetiological agents that affect animals may be genetic, physical, infectious, chemical or nutritional. Host determinants include: age, species, breed, sex, genetic characteristics and immunological state.
Environmental determinants can be broadly divided into those influenced easily by man, such as management and nutrition, and those not easily influenced by man, such as location, cultural practices, religion and socio-economic status.
The drive in Southeast Asia to satisfy the increasing demand for animal protein has resulted in many changes to common agricultural practices. The intensification of some of the animal production industries, combined with urbanisation, global changes and the increasing ease of travel/transport, has produced environments that may lead to an increase in impact of formerly uncommon diseases or even the emergence of new diseases. These changes challenge the normal control methods and indicate that new ways need to be found if these emerging diseases are to be controlled. Thought is now being given to the relocation of animal production systems away from where they have developed peri-urbanly to an area-wide integrated approach. These man-made relocations will significantly affect the disease determinants and can, if well understood, be used to significantly reduce the occurrence of disease in both animals and man.
"Emerging" infectious diseases can be defined as infections that have newly appeared in a population or have existed but are rapidly increasing in incidence or geographic range. Specific factors precipitating disease emergence can be identified in virtually all cases. They include ecological, environmental, or demographic factors that place animals in increased contact with a previously unfamiliar microbe or its natural host, provide potential pathogens with a novel host, or favour increased dissemination. These factors are increasing in prevalence in Southeast Asia because of the rapid development of livestock industries in the region. This increase, together with the ongoing evolution of viral and microbial variants and selection for drug resistance, suggests that infections will not only continue to emerge but also that the rate and effect will probably increase.
There is a conflict of interest between the scientist who looks for evidence that a disease agent exists in a country or region, where it is not expected to be, and the regulator who has to deal with the public health, or economic consequences, of a newly revealed situation. It does not seem to matter whether it represents a new but spreading disease or one being recognised for the first time in a stable situation. The national and international reaction to the discovery of many agents in countries where they have not previously been noted all too often does not take cognisance of the fact that the actual risk to humans and/or domestic/productive animals may not be changed. Fear and overreaction are sure to be the result when the risk is not quickly and correctly assessed, especially when this is compounded by uninformed and sensational media coverage.
Microorganisms and viruses are adapted to extremely diverse eco-niches. One of the most complex sets of adaptive characteristics concerns arthropod transmission of viruses. The arthropod-borne viruses are spectacular examples of emergence and re-emergence resulting from environmental manipulation or natural environmental change. Deforestation, amateur irrigation and the introduction of new species (usually livestock) give rise to many viral disease threats to humans and animals.
The revolution in international travel and trade over the past two decades has meant that disease agents can be transported quickly between susceptible populations. The corollary is also true in that susceptible populations can move into areas where they can become infected with agents to which they have had no previous exposure. For example, it has been estimated that approximately 1 million cattle per year will be moving from Australia (a continent free of many of the infectious agents present in Southeast Asia) to Southeast Asia by the year 2000.
Outbreaks of new diseases have been documented since antiquity, but these do not normally involve a genetically distinct "new" disease. More commonly, new diseases emerge because disease-causing viruses, bacteria, or parasites find a new pathway to previously unexposed populations.
However, the probability of interspecies transfer can be increased not only by increased contact between humans and an animal reservoir, but also by an increased opportunity for viral genetic reassortment or recombination within animal or insect hosts.
The complex relationships between the microbes that cause disease and the humans or animal hosts that contact the disease are constantly evolving as microbes mutate and either die or survive to reproduce. The widespread, even indiscriminate, use of broad-spectrum antibiotics has led to the evolution of bacteria that cannot be controlled by most common antibiotics. The common practice of adding antibiotics to the feed of animals intensively raised may well have led to the evolution of some of these bacteria.
Globalisation of food supplies is affecting outbreaks of food-borne illness, and this is an increasing problem in human health.
It is well known that conflicts within and between countries can lead to the breakdown in veterinary control measures, which can lead to the re-emergence of disease. Rinderpest control in Africa and South Asia is an often-quoted example of where civil disturbances have contributed to its re-emergence. The same could be said for foot-and-mouth disease control in this region. Severe constraints, including widespread strife, abject rural poverty, poor or absent infrastructure (personnel, facilities, roads, communications, law enforcement), limited logistical capacity, a long rainy season, frequent climate-associated natural disasters and inadequate funding have all contributed to the difficulties of controlling this disease.
While it is clear that there needs to be a correct balance, there are cases where livestock may create problems and difficulties. The intensive units, particularly of pigs and/or poultry, can create major environmental and health problems. In a global context, the movement of feed for livestock is itself causing massive shifts in resources which, in the long-term, create serious problems in both areas i.e. the provider and the recipient suffering from nutrient deficit and surplus respectively. The producer fails to replenish land with manure, etc, while the recipient area suffers pollution problems seriously affecting crop production as well as water quality, etc.
The opinions and views expressed in this paper are the author's and not necessarily those of FAO
Regional Workshop on Area-Wide Integration of Crop-Livestock Activities, 18-20 June, 1998, FAO Regional Office, Bangkok Thailand.
