Zoonoses: the deadly diseases that animals pass on to us

Diseases passed from animals to humans are on the rise.Why and what should we do to tackle them?

Zoonoses are diseases and infections that are naturally transmitted between vertebrate animals and man. More than 150 diseases are designated as zoonotic diseases according to the standard work published by the International Office of Epizootics (OIE), among which many are viral diseases. Cases of zoonotic viral diseases are increasing worldwide. Severe acute respiratory syndrome (SARS) and avian influenza (“bird flu”) are two of the most important recent examples of this phenomenon.

SARS, a rapidly spreading atypical pneumonia caused by a new coronavirus originating from animals, emerged in southern China late in 2002, though it was not publicized or identified as SARS until spring 2003. The disease spread quickly across Asia and North America, with 32 countries and areas eventually reporting cases. Almost 8,500 cases had been reported by late last year, with more than 900 deaths.

Since mid-December 2003, the rapid spread of bird flu has been reported in several areas in Asia. The chief outbreaks have been caused by the highly pathogenic H5N1 strain that had caused a severe disease with high mortality rate in humans before. In 1997 in Hong Kong, the H5N1 strain infected 18 people, six of whom died. In February 2003 it infected two people in Hong Kong. One died. The 2004 outbreak in humans, which is the largest recorded to date, had by late last year resulted in 44 officially reported cases, of which 32 were fatal.

The costs of zoonoses

The direct and indirect costs of a zoonotic disease are hard to quantify, since such disease can cause great social unease as well as direct economic losses.

Estimates of the global cost of SARS, for example, have varied from a conservative figure of $30 billion, from the World Health Organization (WHO), to $150 billion by some economists. And it is impossible to estimate the cost of panic in society caused by the early unawareness of the pathogen and SARS.

Outbreaks of bird flu can be devastating for the poultry industry and for farmers. An outbreak in Pennsylvania from 1983 to 1984 resulted in the destruction of more than 17 million birds at a cost of $65 million. During the first two months of the 2004 outbreak, more than 100 million birds died or were destroyed. The costs are especially devastating in Asia, as it raises about 40% of the world’s poultry population and accounts for a quarter of the world’s trade in poultry. Thailand, for example, exports up to $1 billion-worth of poultry each year.

But zoonotic diseases and their costs are not confined to Asia. Between November 1986 and September 2002, more than 180,000 cases of bovine spongiform encephalopathy (BSE, also known as mad cow disease) were confirmed in Britain. The costs of BSE, including the ban that many countries imposed on imports of British beef, and extensive interventions to support farmers and markets, had reached several billion dollars by the 1990s. The discovery of a single cow with BSE in the state of Washington in the United States in December 2003 resulted in more than 50 countries temporarily banning imports of American beef.

Cases of zoonotic diseases are increasing worldwide, especially in sub-Saharan Africa, south America and southeast Asia. With the ubiquity of global travel and world trade, emerging zoonotic diseases are a challenge not only in their region of origin, but across the world.

According to the World Health Organization (WHO), more than 30 new diseases have emerged since the mid-1970s, among them many zoonoses, such as the Ebola virus, which causes a contagious and incurable disease that makes victims bleed to death, the Nipah encephalitis virus that emerged in Malaysia five years ago and BSE. Although it is hard to be certain which diseases jumped from other animals to humans, there has been evidence that measles, smallpox, influenza and even HIV/AIDS might have started in other species.

Which animals?

Many species have been associated with zoonotic diseases. Wild animals, as well as domestic ones, have been implicated in the epidemiologic cycles of these diseases. Factors explaining the emergence of a zoonotic disease are usually complex, involving mechanisms at the molecular level. For example, the variation of antigen structures of viruses can enable them to avoid our immune system and cause new epidemics in a population. Many RNA viruses have high rates of mutation, that is, variation, and may have special mechanisms for recombination, making them extremely difficult to pin down. For example, influenza viruses undergo considerable variation in their antigen structure and can rearrange their genome by combining gene fragments from different viral isolates. A variety of bird flu viruses have been isolated from wild birds, particularly from waterfowl, which serve as gene pools for viral mutation.

What else is to blame?

Apart from genetic factors, social and ecological conditions such as population growth and movement, eating habits, the environment and agricultural models also play important roles in emerging diseases.

The major factor contributing to the appearance of new zoonoses in human populations is increased contact between humans and domestic animals and wildlife. This can be caused either by humans infringing into areas that were previously wilderness or by movements of wild animals into areas of human activity due to man-made or environmental disturbances.

An example of this is the outbreak of Nipah virus in peninsular Malaysia in 1999. Human activities such as slash-and-burn deforestation, intensified pig farming and agricultural encroachment on primary rainforest are believed to have played a central role in the emergence of the Nipah virus. All of these changes combined to alter bat foraging behaviour, bringing bats that carry the virus into close contact with pig farms. These bats infected the pig population which then acted as an amplifier host, eventually transmitting the virus to farmers. The result was 105 human deaths.

Deforestation is also behind the emergence of haemorrhagic fevers – those associated with severe blood loss, like Ebola and Lassa fever, a simian virus. With increased human presence in the rainforest, and exploiters pushing into deeper areas, man is encountering “new” micro-organisms with behaviour unlike those previously known. As the primary hosts of these pathogens are eliminated or reduced through forest disturbance and degradation, disease can break out among humans.

Too close for comfort?

SARS is also the result of close contact between humans and wild animals. Evidence has suggested that civet cats were the source of SARS. Although civets have been eaten in the south of China for some time, it was not until the past two decades that large numbers of civets have been raised for food. This has brought human handlers, such as traders and cooks, into close contact with civets.

Changes in farming practices – the emergence of high-density livestock rearing is the best example – also appear to promote zoonotic infection. As a result of so-called factory farming, animals are living closer to each other. This provides the conditions for viruses to accumulate mutations and spread rapidly, which may eventually result in an outbreak of an infectious disease. Frequent contact with domestic animals, poor animal sanitation and poor personal hygiene will then spread the disease among animals and humans.

Livestock rearing and feeding practices also determine the emergence of zoonotic disease. BSE is believed to have resulted from cattle, which are herbivores, being fed meat-and-bone meal protein recovered from bovine carcasses.

Today zoonoses are of practical interest because they tend to appear suddenly, be particularly virulent and spread rapidly. Take the H5N1 strain of highly pathogenic bird flu: since December 2003, eight Asian countries have confirmed outbreaks. Most of these countries were experiencing H5N1 outbreaks for the first time in their histories. In several countries, the disease was detected in poultry flocks in virtually every part of the country. The outbreaks are historically unprecedented in their geographical scope and international spread. From 1959 to 2003, 21 avian influenza outbreaks occurred worldwide, mainly in the Americas and Europe. The present situation is a threat to public health because, if the virus circulates long enough in humans and farm animals, there is an increased risk that it might evolve into a pandemic influenza strain that could cause disease worldwide.

What we must do

Emerging zoonotic diseases are a regional and global problem of growing importance, and the increase in occurrence is likely to continue. Infectious diseases do not stop at national frontiers. Therefore, we need very close global cooperation to address zoonoses.

It is laudable to see the WHO, the United Nations Food and Agriculture Organization and the OIE working together to set up procedures for international consultation to control and prevent zoonoses.

To conquer zoonoses, we need to bring together experts on animal health and public health. We need to use new tools and develop new mechanisms of surveillance and response. The integration and expansion of existing surveillance systems of international organizations as well as those of national governments will facilitate early detection of animal and public health events that may be linked.

As we have learnt from campaigns against SARS and AIDS, political awareness and support are extremely important for controlling emerging diseases. Although it is hard to predict what the next zoonosis will be and whether or when a pandemic is coming, we need to strengthen our animal and human health infrastructure to prepare for the worst.

The current bout of bird flu should particularly put us on our guard, as the history of influenza pandemics is a bleak one. The previous century experienced three flu pandemics (in 1918, 1957 and 1968): the pandemic of 1918 to 1919 killed between 20 million and 50 million people and is regarded as the most deadly pandemic in the history of humanity. Although the timing cannot be predicted, influenza pandemics have followed a recurring pattern. Therefore, there must be a special focus on current human and veterinary influenza surveillance, as well as a concerted attempt to develop vaccines and anti-virals against influenza.

There are other things we could do to prevent future zoonoses. A move away from factory farming would lower the risk of a zoonosis epidemic. But it is not clear that the world is ready to make that trade-off.

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