EMERGING AND RE-EMERGING DISEASES

by Dr Mike Townend MB ChB(Hons), Dip Trav Med, General Practitioner, Cockermouth, Cumbria

Some diseases appear as if from nowhere and rapidly achieve national or international significance; these are known as emerging diseases. Re-emerging diseases, after existing for many years or even reaching the verge of extinction, suddenly take on a new impetus and once again have a significant impact.

Emerging diseases (Box 1) are diseases that in the past two decades:

• have not existed previously; or
• have been newly recognised; or
• have become more widespread after a localised beginning.

Re-emerging diseases (Box1) are diseases that in the past two decades:

• have become more widespread, and
• clinically significant following a period of much lower activity.

Newly occurring emerging diseases

These are diseases for which there is no evidence that they existed before their recent description.

HIV/AIDS

HIV infection and AIDS is probably the best-known example in recent times of an emerging disease that has not existed previously. In 1981, the first cases of a new syndrome characterised by opportunistic infections in young male homosexuals, and later in other groups, were described.(1) Since then, the disease has become a global problem, with cases reported in all continents and most countries. The infection is thought to have originated in Africa, where it is a major health problem, spread principally by heterosexual intercourse and by vertical transmission from mother to child.(2) From Africa, it may have spread to Haiti(3) and subsequently to the USA, Europe and Asia.

Escherichia coli 0157

Recent high-profile cases in the UK of infection with Escherichia coli 0157 have drawn public attention to this new food-related health problem. Its emergence in the past two decades and its origin in cattle in the human food chain are well documented.(4) The strain probably arose as a mutation, by which it acquired a Shiga toxin,(5) and has become associated with sporadic outbreaks of diarrhoeal disease and the haemolytic-uraemic syndrome.

New-variant Creutzfeldt-Jakob disease

Another high-profile food-related disease in recent years has been new-variant Creutzfeldt-Jakob disease (nvCJD), the relationship of which to bovine spongiform encephalopathy (BSE) in cattle has been a source of much controversy and damage to the UK beef industry. BSE was first diagnosed in cattle in the UK in 1986 and the first cases of nvCJD were described in 1996.(6) The disease is the result of infection with a new prion agent.(7)

Toxic-shock syndrome

Toxic shock as a result of staphylococcal infection, often following surgery, has been recognised for many years. Within the past two decades, a new syndrome of toxic shock associated with the use of vaginal tampons has been described.(8) The new syndrome appears to be related not only to the use of tampons, but also to an absence of antibodies to toxin-producing strains of Staphylococcus aureus in young women.

Newly identified emerging diseases

Newly identified emerging diseases have patterns of symptoms that may have existed previously. However, these patterns have only recently been identified as distinct disease entities or as having a specific and newly identified cause.

Legionella infection

In 1976, an outbreak of pneumonia occurred in ex-servicemen attending an American Legion convention.(9) Atypical pneumonias can be caused by a variety of micro-organisms, but on this occasion the causative agent was a bacterium not previously identified, and subsequently named Legionella pneumophila. The organism has since been found to be widespread throughout the world.

Infection is acquired by inhalation of water aerosol derived from heating or air-conditioning systems, showers and spa pools. It is more likely to infect elderly or immunosuppressed individuals and can cause a severe multi-organ disease in addition to pneumonia.

Campylobacter infection

Campylobacter was initially isolated in 1909, and the first known human infection was reported in 1947. Within the past 10-20 years, the organism has emerged as a widespread gastrointestinal pathogen, possibly the most common food-borne bacterial pathogen.(10) It causes diarrhoea, often bloody, and is sometimes associated with reactive arthritis and Guillain-Barre syndrome.(11)

The principal source is meat, especially poultry meat, although untreated milk and even cheese have been identified as sources and it may also be acquired from dogs and cats.

Coccidia

Since the advent of HIV, the protozoa Cryptosporidium, Cyclospora and Isospora have become a common cause of opportunistic gastrointestinal infection.(12) They can also cause diarrhoeal illness in those who are not immunocompromised, as they are not removed from water supplies by chlorination or filtration. Sporadic outbreaks of infection occur in developed countries and these pathogens have also been identified as a cause of diarrhoea in travellers to developing countries.

Hepatitis C

As long as 50 years ago it was recognised that there was a causative agent for hepatitis that was different from hepatitis A and hepatitis B, but the hepatitis C virus was not identified until 1989.(13) It is transmitted in the same ways as hepatitis B but hepatitis C infection has a much higher rate of conversion to carrier status or chronic progressive liver disease and is more likely than hepatitis B to cause cirrhosis or liver cancer.

Viral haemorrhagic fevers

Lassa and Ebola fevers have achieved a high media profile. Yellow fever, which is not a newly emerging disease, is also a viral haemorrhagic disease.

Dengue, dealt with later in this article, may in some circumstances produce a haemorrhagic fever and several other such fevers exist. In more recent years, the causative viral agents of some of the African and other viral haemorrhagic fevers have been identified, including Lassa and Ebola.

In general, the viruses are transmitted to man from an intermediate host such as a rodent by bites from ticks or mosquitoes, or by aerosol inhalation directly from rodents.(14) Viral haemorrhagic fevers are unusual in travellers but have achieved great notoriety because of their high mortality and the possibility of human-to-human aerosol transmission, particularly after admission to hospital.

Lyme disease

Erythema migrans, the initial skin lesion in Lyme disease, was first described in 1909(15) and correctly identified as being caused by a tick bite. Lyme arthritis was not described until 1977, and in 1981 Burgdorfer identified the causative spirochaete, which is now identified by his name as Borrelia burgdorferi. The mode of transmission to humans by deer ticks is well established.

Hantavirus infection

Although hantavirus infections had previously been identified elsewhere, an apparently new hantavirus was identified as the cause of an apparently new syndrome in the USA in 1993;(16) the hantavirus pulmonary syndrome is characterised by fever, muscle pains, pneumonitis and a high mortality. It is transmitted by aerosol inhalation from small mammals.

Re-emerging diseases

These diseases are already well known but have undergone a resurgence in their incidence or prevalence, or have appeared in areas where they were not previously found.

Tuberculosis

Although tuberculosis has remained a problem in the developing world, it seemed likely that the disease would be eradicated from developed countries and possibly eventually from the developing nations. Resurgence of tuberculosis(17) is now well documented in both developing and developed countries. One factor in this resurgence is the emergence of drug-resistant strains of Mycobacterium tuberculosis, but another important factor is the association between HIV infection and tuberculosis. This is a particular problem in Africa(18) but occurs worldwide.

Diphtheria

In 1990, diphtheria re-emerged as a major health problem in the Russian Federation. By 1994, the problem had spread to involve all the new independent states of the former Soviet Union.(19)

The reasons for this resurgence include breakdown of immunisation and other public health programmes and of the healthcare infrastructure generally.

Malaria

Although malaria is widespread in tropical countries, experience in India illustrates how it can re-emerge even after effective control.(20) In the 1960s malaria was almost eradicated in the Indian subcontinent, but because of technical and financial problems the eradication programme was allowed to lapse.

Malaria is again a major health hazard in India. Another factor in the resurgence of malaria, now and in the future, is the possibility of climate change,(21) with the likelihood that alterations in temperature and rainfall will provide habitats for malaria-carrying mosquitoes in new areas. Human activity, such as cultivation, may also provide new habitats for mosquitoes.(22)

Dengue fever

Dengue fever was originally a disease found in Southeast Asia. Since the 1950s, it has spread to become the world's most common arborvirus (arthropod-borne virus) infection. In the past 15 years, it has become a problem in most urban centres in the tropics(23) and has emerged as a major problem in the Americas. Transmitted by mosquito bites, it is a severe, influenza-like illness that may be manifest as a viral haemorrhagic fever, particularly in previously exposed individuals.

Cholera

Cholera was not present in South America until 1991, when it suddenly became epidemic in Peru,(24) for reasons that are not altogether clear. Cholera is a water-borne disease and in Peru seems to have been transmitted among 'marginal-urban populations'. Eating unwashed fruits and vegetables was identified as a means of transmission in other areas of the country, and eating a raw marinated fish dish (ceviche) has been suggested as another source of infection.

How do diseases emerge or re-emerge?

A variety of mechanisms cause diseases to emerge or re-emerge and in some cases more than one plays a part.

Immigrants and their host countries

In areas of the country with a high proportion of ethnic minority residents, healthcare professionals should be aware of the changing patterns of disease in the countries of origin of newly arrived immigrants.

It is also important to recognise that ethnic minority travellers from the UK returning to their 'home' countries to visit friends and relatives are at increased risk of acquiring infections. Such travellers are less likely than others to consult a doctor or travel clinic before travelling(25) and once at their destination may perceive less risk than other travellers. In consequence, for example, they are more likely to acquire malaria than other travellers,(26) having lost their partial immunity within months of arrival in the UK and having a lower perception of the need for malaria prophylaxis.

Keeping up to date

How can we keep abreast of information about the emergence and re-emergence of diseases?

Printed databases from medical publications have a place in primary care, but they are unable to respond rapidly to new developments. Computer databases are better able to include new information quickly. An on-line database such as Travax can be updated much more rapidly than a database resident on the computer's hard disk, such as Traveller, which is updated monthly.

Those with Internet access can refer to websites relating to travel health, some of which are devoted entirely to outbreak information and emerging diseases. Signing up to the Promed website ensures regular e-mail information on new reportings of diseases.

Conclusion

Air travel is now easily available to large numbers of UK residents and its speed means that many travellers return home while still incubating diseases to which they have been exposed abroad. The primary care team needs to be aware of the emergence and re-emergence of diseases so as to advise patients on how to avoid them and to recognise them when they present in returned travellers.

Box 1. Important emerging and re-emerging diseases.
Emerging diseases
Newly occurring

• HIV/AIDS
• Escherichia coli 0157
• New-variant Creutzfeldt-Jakob disease
• Toxic-shock syndrome

Newly identified

• Legionella infection
• Campylobacter infection
• Coccidia
• Hepatitis C
• Viral haemorrhagic fevers
• Lyme disease
• Hantavirus infection

Re-emerging diseases

• Tuberculosis
• Diphtheria
• Malaria
• Dengue fever
• Cholera

References

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This paper first appeared in UPDATE.