Common infectious diseases in Hong Kong

T Y Wong 黃天祐, S T Lai 黎錫滔

HK Pract 2003;25:542-549

Summary

Emerging infectious diseases were prevalent in Hong Kong well before this year's outbreak of severe acute respiratory syndrome (SARS). Hong Kong is susceptible to infectious diseases due to a combination of geographical, historical, socio-economic and medical factors. There are currently 28 notifiable infectious diseases in Hong Kong. Certain diseases, such as influenza and sexually transmitted infections, are not notifiable but are important causes of morbidity and mortality on a community basis. Anti-microbial resistance, both community and hospital-acquired makes treatment and control difficult or impossible in some cases.

摘要

早在嚴重急性呼吸系統綜合症（SARS）爆發以前香港已經有傳染性疾病。因為地理、歷史、社會經濟和醫療等原因， 香港容易受傳染病影響。香港現有廿八種必須報告衛生當局的傳染病，但某些不需要向衛生當局呈報的傳染病， 如流感和性病，卻是社區內致病甚至致死重要原因。社區或醫院出現的抗藥性微生物感染， 都可能成為治療和控制的重大的挑戰，甚至難以治療。

Introduction

The severe acute respiratory syndrome (SARS) outbreak raised awareness of infectious diseases to our highest levels in the history of Hong Kong. Since the 1970s, there has been complacency both in the general public and in the medical community about infectious diseases, and a general impression that they no longer pose a major health threat to Hong Kong. As a result, the infectious disease specialty has been somewhat neglected in comparison to other specialties. In the last two decades, however, we have witnessed the emergence of new and old infectious diseases, including HIV/AIDS, tuberculosis, H5N1 influenza and dengue, to name a few.

Hong Kong is particularly vulnerable to the invasion of infectious diseases due to several geographical and historical factors. First, Hong Kong's close proximity to Mainland China and its dependence on the latter for food, water and livestock can result in direct importation of infectious disease via cross border movement of these goods. Secondly, cross border movement of people and international travel can bring infectious diseases into or out of Hong Kong. Thirdly, over-crowded living quarters and substandard hygienic conditions in some parts of Hong Kong encourage the spread of microorganisms. Infectious disease prevention awareness among the public leaves a great deal to be desired. Finally, the liberal and improper use of antibiotics both locally and in neighbouring regions is leading to the emergence of drug resistance, making treatment and control of infectious diseases more difficult.

Now is an opportune time to review the current infectious disease situation in Hong Kong so as to prioritise our strategies in this ongoing battle.

Notifiable infectious diseases

There are at present 28 statutory notifiable infectious diseases in Hong Kong that are reportable to the Department of Health. (List 1) SARS was the last added entity. Infectious disease surveillance is undertaken by the Disease Prevention and Control Division of the Department of Health (DH). A number of DH and private clinics act as sentinel surveillance points. Notifiable infectious diseases are admitted into the Infectious Disease Unit of Princess Margaret Hospital. Four diseases are quarantinable: these are cholera, plague, yellow fever and SARS. Non-notifiable infectious diseases are managed in other clinics and hospitals.

A comprehensive network of facilities consisting of hospitals, accident and emergency departments, general out-patient clinics, maternal and child health centres as well as clinics and hospitals operated by private medical practitioners provides a means for diagnosis and reporting of communicable diseases. This system of surveillance is further supplemented by facilities available in the various government hospitals for laboratory diagnosis, isolation and treatment of cases and contacts.

The top five notifiable diseases in terms of case number in 2002 were, in descending order: chickenpox (16786 cases), tuberculosis (6665 cases), food poisoning (2647 cases), viral hepatitis (431 cases) and bacillary dysentery (285 cases). There are no exact figures for non-notifiable infectious diseases, many of which are very important and occur commonly. Diseases in this category include (but are not limited to) influenza, otitis media, pneumonia and urinary tract infections.

In the year 2000, there were 310 deaths from notifiable infectious diseases in Hong Kong, the majority of which were from tuberculosis (96%). The rest were due to viral hepatitis, tetanus, chickenpox, malaria, meningococcal infection, rabies and typhoid fever. The incidence of underlying infectious diseases causing mortality is likely to differ from year to year, but this serves as an illustration of infectious diseases that may be associated with severe consequences.

In the ensuing discussion, we shall focus on selected important infectious diseases seen in Hong Kong, classified by their major routes of infection.

Food and water borne infections

Food poisoning, the most common notifiable infectious disease in this category, can be caused by a variety of microorganisms and their toxins. Biological toxins (e.g. Ciguatera poisoning) and chemical toxins (e.g. Insecticide poisoning) are also in this category. The top five causative microorganisms in Hong Kong are Vibrio parahaemolyticus, Staphylococcus aureus, Salmonella species, Norwalk-like virus and Clostridium perfringens.

Bacillary dysentery is an enterocolitis caused by the Shigella bacterium. The most common causative agent is Shigella sonnei. Transmission is from person-to-person, with an incubation period of 12 to 72 hours. Infections are associated with tissue invasion and enterotoxin production, but blood stream invasion is uncommon. Affected patients have abrupt onset of fever, headache, nausea, malaise, abdominal pain and watery stool that may contain mucus and blood. Symptoms usually last a few days to 2 weeks. Diagnosis is by isolation of bacteria from faeces or rectal swabs. Similar clinical features are found in infection caused by Vibrio parahaemolyticus,⁵ Plesiomonas shigelloides⁶ and Entamoeba histolytica (causing amoebic dysentery). Mild cases may be treated conservatively with rehydration. If antimicrobial treatment is indicated, fluoroquinolones are the drugs of choice.

Cholera is characterized in its severe form by sudden onset of profuse painless watery diarrhoea. In untreated cases rapid dehydration, circulatory collapse and renal impairment may occur. Mortality is infrequent in Hong Kong, but there have been reports of acute renal failure in the elderly and ill. Since Vibrio cholerae can be found in seawater, contaminated seafood, especially shellfish, is the culprit in many cases. Polluted seawater in fish tanks has been implicated in some restaurant outbreaks. Quick presumptive diagnosis is available by microscopy but culture is required to confirm the diagnosis. The mainstay of treatment is rehydration: the antibiotics of choice are the fluoroquinolones since tetracycline resistance is common.⁷ Cholera vaccine, having low protective efficacy (around 50%) and short protective duration, is not recommended. Vibrio vulnificus, also an halophilic organism, may cause gastroenteritis, bacteraemia or necrotising fasciitis and may be associated with mortality especially in cirrhotic patients.⁸

Typhoid and paratyphoid fever are causes of pyrexia of unknown origin especially in young adults. 10-20% of cases are imported. Some cases may be mistaken as surgical emergencies because of the associated acute abdominal pain. The gold standard of diagnosis is blood culture. The widal test is at best viewed as a surrogate marker for infection since about 20% are negative in culture-confirmed cases.⁹ However, previous antibiotics use may render blood cultures negative. Multi-drug resistant (MDR) strains are found, most commonly in cases from the Indian subcontinent. Fluoroquinolones are the drugs of choice but in recent year rising MICs have been noted, although the organism is still regarded as sensitive by sensitivity testing. The result may be a delayed response to antibiotics; third generation cephalosporins may be necessary in such circumstances.¹⁰

Enterovirus infections are caused by Polioviruses, Coxsackie viruses (groups A and B), echoviruses and enteroviruses (e.g. EV71). Transmission is mainly via the faecal-oral route; however the airborne droplet route can also apply. Infection is usually asymptomatic, although some patients may have low-grade fever. Systemic invasion is responsible for more serious illness in the form of a number of clinical syndromes including hand-foot-mouth disease and meningoencephalitis. Fatality due to enterovirus 71 infection has been reported in Hong Kong.¹¹

Hepatitis A is the most common form of acute viral hepatitis in Hong Kong. The prevalence of hepatitis A antibody has been found to be 12.5% and 26.8% in people aged 11-20 and 21-30, respectively.¹² Hence there is a large pool of non-immune individuals. Hepatitis A vaccine is highly immunogenic and safe and should be more widely promoted. The clinical course of acute hepatitis A may be more serious in patients who have underlying hepatitis B.

Acute Hepatitis E is not uncommon in Hong Kong. It accounts for around 40% of non-ABC hepatitis and has a similar clinical picture to hepatitis A. 11% of patients who have acute hepatitis A also have acute hepatitis E infection.¹³

Droplet/airborne infections

Influenza is not notifiable in Hong Kong. Its trend is monitored by sentinel surveillance in Department of Health and private clinics with regard to the number of influenza-like illness and samples isolated from nasopharyngeal aspirates and throat swabs. The peak season is spring time in Hong Kong. The major circulating influenza A serotypes include H3N2 (A/Sydney/5/97, Moscow/10/99, Panama/2007/99-like virus) and H1N1 (A/New Caledonia/20/99-like virus). Influenza B/Hong Kong/330/01-like virus has become the predominant B serotype since October 2001 and has replaced the B/Sichuan/379/99-like virus prevalent in the past few years. Increased morbidity and mortality occur in the elderly and patients who are immunocompromised. Complications include secondary bacterial pneumonia, myocarditis, encephalitis and Guillain-Barr syndrome.

Amantadine and rimantadine have been in use and are active against influenza A. Neuraminidase inhibitors like zanamivir and oseltamivir are active against influenza A and B. Influenza vaccines, if indicated, should be given annually in autumn in the northern hemisphere, just prior to the flu season.

The first documented outbreak of human respiratory disease caused by avian influenza A (H5N1) virus occurred in Hong Kong in 1997. A total of 18 proven human cases, many severe or fatal, were diagnosed in the same year. There were 2 cases in early 2003. The occurrence of novel influenza viruses has highlighted the gaps in our preparedness for dealing with another pandemic.¹⁴

Tuberculosis is the most commonly reported bacterial infectious disease in Hong Kong. In 2002, 6665 cases were notified. Tuberculosis can affect the lungs, gastrointestinal tract, genitourinary tract, central nervous system, skin, bones and joints, etc. The elderly and immunocompromised patients, including patients with acquired immunodeficiency syndrome (AIDS), are at high risk for reactivation of the infection.

Chickenpox has been a notifiable infectious disease in Hong Kong since February 1999. It was the most commonly reported notifiable infectious disease in Hong Kong in 2002 (16786 cases). The majority of infections occur in children 5-10 years of age; more than 90% of people above 10 years of age are seropositive. A significant number of adult infections occur in people brought up in the tropics (e.g. The Philippines) due to low rates of childhood infection. Complications are uncommon (less than 1%) and include scarlet fever, pneumonia, group A streptococcal infection and sepsis. Rarely, death occurs in immunocompromised persons. Herpes zoster is due to local reactivation of latent chickenpox virus in the dorsal root ganglia. Antiviral therapy is indicated for all cases of severe chickenpox or herpes zoster in the elderly or immunocompromised patients. Varicella zoster immunoglobulin given within 96 hours of exposure can help prevent or modify disease in contacts. Chickenpox vaccine is available and can achieve more than 90% seroconversion.

Measles, mumps and rubella are also vaccine-preventable diseases. Adult cases of measles still occur, due to waning antibody titres and failure to take the booster dose of vaccine. Measles can cause severe disease in adults, especially pregnant women and immuno-compromised patients. Immunoglobulin may be used within 6 days of exposure for susceptible contacts at high risk for complications.

As with measles, there is a problem with waning immunity to rubella. There has been a high notification rate of rubella in recent years, especially in young adults. In the outbreak of 1996-1997, there were 4 cases of rubella encephalitis and one death.¹⁵ A booster dose of vaccine at primary six is important, particularly for girls. There were 2 cases of congenital rubella of the foetus in pregnancy in 2000.

Mumps cases are continuously reported but there have been no epidemic outbreaks. Besides parotitis, the virus can cause orchitis, oophoritis, lymphocytic meningitis, pancreatitis, post-infectious encephalitis, thyroiditis and arthritis. Treatment is symptomatic.

Legionnaires' disease presents as severe community-acquired pneumonia. Mild cases are often missed. There were 4 cases in 2002. Contaminated water-cooling towers were the culprit in some cases in the past. Diagnosis has been based on serology, although a sensitive urinary antigen test now exists.

Meningococcal infections rose sharply from 3 cases in 1999 to 14 cases in 2000 and 10 cases in 2001. There were 6 cases in 2002. About half of the cases presented with signs and symptoms of meningitis and the other half with septicaemia.

SARS is caused by a novel Coronavirus. It is noted to be a triphasic disease, consisting of a viral replicative phase, an immunopathological phase and a lung damage phase. The optimal treatment has yet to be defined.

Vector-borne infections

The most frequent infectious diseases in this category include malaria, dengue, typhus and Japanese encephalitis. There have been no cases of yellow fever reported in Hong Kong.

Malaria vector mosquito density in Hong Kong is generally fairly low. There were indigenous cases of malaria before the mid 90s, the majority of which were from the areas along the border area, Sai Kung and Lantau Island. None were due to Plasmodium falciparum. Almost all cases of malaria were imported. Overall, around 80% were due to Plasmodium vivax and 17% due to . The main areas of acquisition included mainland China, the Indian subcontinent; South East Asia and Africa.¹⁶ There were 7 fatal cases out of a total of 965 cases of malaria from 1991 to 2000 inclusive. All the fatal cases were due to leading to cerebral malaria or multi-organ failure. The key to reduction of malaria mortality is early diagnosis and treatment. Personal protective measures against mosquitoes and chemoprophylaxis prior to travel to endemic areas are of vital importance in preventing malaria. Dengue fever has been notifiable in Hong Kong since 1994. Prior to 2002, all cases were imported and the number reported did not usually exceed 20 per year.

Dengue has attracted considerable public attention since the first local case was reported in Hong Kong in the year 2002 and 20 local cases were subsequently recorded during that year. Dengue is the most important mosquito-born viral disease globally. One of the major mosquito vectors, Aedes albopictus, can be found in Hong Kong. There have been no deaths due to dengue reported in Hong Kong. The majority of cases of dengue haemorrhagic fever (DHF), which carries with it a mortality of 10-30% if left untreated, occur in people infected with more than one serotype of the virus. In the absence of an effective antiviral or vaccine, personal protective measures against mosquitoes and vector control are the major means of prevention. It is imperative for us that vector control be heightened lest dengue establish itself as an endemic disease, as in neighbouring countries.

Japanese encephalitis is not a notifiable infectious disease in Hong Kong. The disease has appeared in Hong Kong only sporadically, confined mainly to rural areas in the New Territories. Only 6 cases have been reported from 1992 up till early November, 2003. The Government Virus Unit provides a serological test for Japanese encephalitis as part of its surveillance for this disease. Most infections are asymptomatic; however severe cases carry a mortality of 10-35% and neurological sequelae in 30 to 70% of survivors.

Typhus, an emerging infectious disease that does occur Hong Kong, is a systemic infection caused by transmission of rickettsiae from arthropods to humans. There are two groups of typhus fever and three main types: epidemic typhus (louse-borne), murine typhus (flea-borne) and scrub typhus (mite-borne). Epidemic typhus is caused by Rickettsia prowazekii and murine typhus is caused by R. mooseri. Faeces of lice or rat fleas are rubbed into the bite lesions. Scrub typhus is caused by Orientia tsutsugamushi. It is enzootic in wild rodents and is acquired by humans by bites transmitting larvae of the trombiculid mite. The triad of fever, headache and rash coupled with exposure history should alert the clinician to the diagnosis of typhus. Eschars are occasionally found on non-exposed areas of skin. This underscores the importance of a thorough physical examination in febrile patients. Typhus is treatable with the tetracycline group of antibiotics. Late treatment may lead to complications and death.

Blood borne infections

The overall carriage rate of hepatitis B in Hong Kong is around 10% with much lower prevalence in children since hepatitis B vaccine is now given to all newborns in Hong Kong.¹⁷ Acute hepatitis B has become largely a sexually transmitted disease.

Hepatitis C is found in 0.5% of blood donors in Hong Kong.

Antiviral therapies are available for chronic hepatitis B and C with variable success rates.

Sexually transmitted infections

The precise incidence of sexually transmitted infections (STI) in Hong Kong is unknown. The cases seen at the Social Hygiene Service of the Department of Health are estimated to represent only around 20% of the total cases.¹⁸ There is a general trend towards increasing incidence of STIs seen at social hygiene clinics since 1990. The most common STIs reported by the Social Hygiene Service in Hong Kong are, in descending order: non-gonococcal infections, gonorrhoea, genital warts, genital herpes and syphilis. This frequency order has been the same since 1990. STIs are known to cause a variety of complications including infertility. Treatment of sexually transmitted diseases reduces the likelihood that unprotected sex will result in HIV transmission.¹⁹

Up to 85% of HIV infection in Hong Kong is sexually acquired. As of 30 June 2003, there were 2116 HIV-infected cases and 641 AIDS cases in Hong Kong. Over the years, a significant proportion of HIV-positive cases have presented only after progression to full-blown AIDS. Overall, about a third of HIV infections are detected within three months of the corresponding AIDS diagnosis. Highly active anti-retroviral therapy has revolutionalised the prognosis of AIDS patients. Acute HIV infection is a mononucleosis-like syndrome which if recognized early, carries therapeutic and public health significance.²⁰

Zoonotic infections

The last local case of rabies occurred in 1988. There were 2 imported cases in 1998 and 2001. Leptospirosis is a notifiable occupational disease in Hong Kong. One case was notified in 2001.

Infection in immunocompromised hosts

Patients who have undergone transplantations, HIV infected persons, patients with certain medical conditions and patients who are on immunosuppressants are at higher risk of opportunistic infections. The spectrum of infections depends on which components of innate or acquired immunity are affected.

Antibiotics resistance

Worldwide, there is a growing trend of microbial resistance to many antibiotics due to their indiscriminate and extensive use. Hong Kong is no exception. MRSA resistance, for example, is found in nearly 60% of isolates in some hospitals in Hong Kong. It has typically been found in debilitated patients on antibiotics but community-acquired infections are increasingly reported. Prevalence is related to the antibiotics used and the degree of contact precautions especially hand washing. Penicillin resistant Streptococcus pneumoniae is another resistant organism of concern.

Conclusion

The prevention and treatment of notifiable infectious diseases in Hong Kong has reached international standards. Notifiable infectious diseases are limited in number. HIV/AIDS, infections in immunocompromised patients, nosocomial infections and antimicrobial resistance are emerging problems facing infectious disease physicians. The public health disease awareness of Hong Kong citizens needs to be intensified. This will help us better prepare for future disease outbreaks.

Key messages

1. Emerging infectious diseases can result from old infections re-emerging, new infections like SARS or infectious diseases which become difficult to treat due to drug resistance.
2. There are currently 28 notifiable infectious diseases in Hong Kong. On a community basis, non-notifiable diseases like influenza and sexually transmitted infections are of no less importance in terms of morbidity and mortality.
3. The major routes of transmission of infectious diseases include food/water borne, airborne/droplet, vector borne, blood borne, sexually transmitted and zoonotic transmission. Prevention, apart from specific measures, should be targeted at measures particular to individual routes of transmission.
4. Immunocompromised hosts, including those with AIDS and patients with underlying medical conditions, have increased susceptibility to infectious diseases.

Further reading

1. Appendix I & II, 3rd edition, Handbook of Dermatology & Venereology Vol 2 of 2 Venereology section. 2003.
2. Brown TJ, Yen-Moore A, Tyring SK. An overview of STD Part I.& II. J Am Acad Dermatol 1999;41:511-29;661-677.
3. Czelusta A, Yen-Moore A, Van der Straten M, et al. An overview of STD Part III. J Am Acad Dermatol 2000;43:409-432.
4. Centres for Disease Control and Prevention. 1998 Guideline for treatment of sexually transmitted diseases. MMWR 1998;47(No. RR-1) 1-116.