Emerging Infections of International Public Health Importance





Module 1:  The Paradigm of Emergence  
LECTURE 2 Readings

Emerging Infectious Diseases
Worldwide and in the Americas

David Brandling-Bennett,  MD

  1. To learn about which infectious diseases continue to be the major causes of illness and death worldwide and in the Americas
  2. To understand how infectious disease patterns are changing and what factors contribute to those changes
  3. To consider whether new and emerging infectious disease threats are a short-term and decreasing problem or will continue to present challenges for a foreseeable future


Overview of infectious diseases

This slide shows the leading infectious killers worldwide from 1998.

[Figure:   Leading infectious disease killers, millions of death, worldwide, all ages, 1998 estimate]

The graph is divided into age groups (over 5 years and under 5 years). We can see from this graph that of the top 6 diseases, one of these is AIDS, a new infection that emerged in the past 20 years. Infectious diseases account for 25% of deaths worldwide, a higher proportion in developing countries than developed countries. The primary deaths for children are from respiratory and diarrheal illnesses and measles. For adults, primary deaths due to infectious diseases are from AIDS and TB.

This graph breaks down the deaths in children under age 5 by the actual numbers of deaths for this year period.

[Figure:   550,000 deaths annually in children under 5]

Of the half million deaths occurring each year, 1/3 of them are due to 3 of the top 6 infectious agents. This slide shows the infant mortality and proportion of deaths in this age group in the Americas, by country.

[Figure:   Infant mortality and proportion of deaths from infectious diseases in children under 5 years of age for selected countries of the Americas, estimates for 1995-2000]

It is broken down into infant mortality on the left and proportion of deaths from infectious diseases in children under 5 on the right. You can see that the death rates vary widely based on the country, the lowest in Canada and the highest in Bolivia.

One of the agents of respiratory illness is Streptococcus pneumonia which can cause severe pneumonia, meningitis and sepsis. Surveillance for S. pneumonia has been ongoing. This graph shows the rates of penicillin resistance among the S. pneumonia isolates between 1993-1999 from the Americas.

[Figure:   Surveillance of invasive S. pneumoniae]

Both high level resistance (HR) and low level resistance (LR) have increased over the years.

Another major cause of illness for children and adults is tuberculosis. One third of the world’s population is infected with Mycobacterium tuberculosis. This map shows the estimated rates of TB by region of the Americas.

[Figure:   Estimated TB incidence rate in the region of the Americas, 1999]

Some of the highest rates are seen in Peru and Ecuador. An emerging problem with tuberculosis is the rapid rise of drug resistance.

[Figure:   Studies of anti-TB drug resistance in the Americas, 1994-2000]

This map shows where studies have been conducted into TB drug resistance in the Americas. Here, you see the results of these studies for primary TB drug resistance.

[Figure:   Primary anti-TB drug resistance in the region of the Americas, 1994-2000]

The highest rates of resistance are seen in the Dominican Republic, with over 40% primary resistance. Uruguay has some of the lowest rates, at 1-2% resistance.

Primary resistance is designated if the patient has not received drug therapy for TB in the past. Usually primary resistance is to only one of the TB medications such as isoniazid. Multi-drug resistance (MDR) is resistance to both isoniazid and rifampin, both potent first line agents for treatment of TB. The rise of MDR-TB worldwide has led to WHO supporting a DOTS strategy. This slide shows the MDR-TB prevalence for several countries.

[Figure:   MDR TB prevalence percent of MDR, 1998]

As you can see, Dominican Republic is the fifth in this list with 8% MDR-TB.

The cost of treating MDR-TB is significantly higher than drug sensitive TB and can cost upward of $100,000 per patient to treat. It can also carry a higher mortality rate especially among persons with HIV/AIDS.


Reintroduction of cholera into the Americas

Cholera is a disease that has been around for centuries. Epidemic cholera had been eradicated from the Americas since the 19th century. However, in January 1991, cholera cases were reported in Peru for the first time in the 20th century. The disease spread very quickly from Peru and then to neighboring countries. There were tens of thousands of cases in the early 1990’s. It has been suggested that this epidemic was part of the cholera pandemic that affected Indonesia in 1961. Reintroduction of cholera occurred from the port in Lima, Peru, then entered the food supply and water, and spread throughout South America. This slide shows graphically the number of cases of cholera reported to the WHO from 1990-1998.

[Figure:   Number of cases of cholera reported to WHO, by continent and by year, 1990-1998]

You can see that there were no cases of cholera reported from the Americas in 1990, but a very large number of cases in 1991 and 1992. The cases from the Americas surpassed both Asian and African cases during those years. We don’t know why the epidemiology of cholera has changed but it now has a more permanent presence in many parts of the world, including the Americas again.

These two maps show the incidence rate of cholera for 1999 and 2000 in the Americas.

[Figure:   Cholera incidence rate in the Americas, 1999]

[Figure:   Cholera incidence rate in the Americas, 2000]

The decreasing rate of cholera in South America was due to improved water sanitation. Both improved water systems and improved chlorination were utilized to help control the cholera epidemic.

An additional concern for cholera is the increase of drug resistance.

[Figure:   Countries with presence of resistant V. Cholerae, 1996-1997]

Treatment for cholera is primarily supportive with fluid and electrolyte rehydration and resuscitation. Some health care workers use antibiotics for this disease although it is unclear if it has any significant public health benefit; considering the resistance level there could be more harm by increasing drug resistance in a community.


HIV/AIDS in the Americas

Global estimates for HIV/AIDS worldwide range from 34 to 46 million persons infected.

[Figure:   Adults and children estimated to be living with HIV/AIDS as of end 2003]

There were an estimated 4-6 million new HIV infections in 2003 among adults and children. HIV/AIDS was responsible for 3 million deaths in 2003. The Caribbean has the second highest rates of HIV based on population, following behind some sub-Saharan African countries.

This study among pregnant women in selected Caribbean countries shows the prevalence of HIV.

[Figure:   Prevalence of HIV among pregnant women in selected Caribbean countries]

Haiti (not shown) has the highest burden of disease with 10-15% prevalence in their pregnant women. The Caribbean region does not have a broad economic base; much of their economy is based on tourism and export of agricultural products. HIV/AIDS affects the most productive age groups which impacts the economy even further. Treatment, while available, is expensive and complicated, but could help lessen the epidemic in these regions. Developing and continuing prevention programs along with treatment programs are essential to reduce the burden of this disease on their communities.


Vector-borne diseases in the Americas

We will briefly touch on four vector-borne diseases that affect the Americas: malaria, dengue, yellow fever and hantavirus.

The major vector-borne disease concern for the Americas is similar to Africa and Asia--the increase of drug resistant malaria. These two slides show the level of chloroquine resistance in Plasmodium falciparum from 1960 and 1999, respectively.

[Figure:   Chloroquine resistant Plasmodium falciparum, 1960]

[Figure:   Chloroquine resistant Plasmodium falciparum, 1999]

P. falciparum is the most lethal form of malaria. The spread of chloroquine resistance likely occurred after its appearance on the Kenyan coast. While Africa still carries the highest burden of resistance, this slide

[Figure:   Anti-malarial drug resistance]

shows an increasing level of both chloroquine and sulfadoxine-pyrimethamine resistance worldwide.

Dengue is a viral infection spread from the vector mosquito Aedes aegypti or Aedes albopictus. This slide shows the course of Aedes aegypti eradication efforts--you can see that in the 1930s there was extensive infestation of the dengue vector, and by 1970, the vector had almost been completely eradicated through public health measures of vector control.

[Figure:   Reinfestation with Aedes aegypti]

However, Aedes has made a comeback and has reinfested the Americas, at even greater concentration than in the 1930s. These mosquitoes are present in the tropics and subtropics.

Dengue cases have also been increasing from the 1980s-2001.

[Figure:   Dengue cases, 1980-2001]

This slide shows the number of dengue epidemics over two years in Central and South America.

[Figure:   Dengue epidemics, 2000-2001]

Dengue has four subtypes. The more dangerous form of dengue is dengue hemorrhagic fever (DHF), which can have death rates up to 10%. There has been a marked increase in DHF from 1980 to 2001.

[Figure:   Cases of DHF < 1980 to 2001]

DHF is more commonly seen when a person is infected with a second type of dengue. Most areas that are endemic for dengue (Asia and the Americas) also have all four subtypes present; this can increase the risk of DHF.

Yellow fever is present in two parts of the world currently, Sub-Saharan Africa and South America. This map shows the geographic distribution in South America for yellow fever.

[Figure:   Yellow fever geographic distribution, 1986-1999]

In the Americas, the disease is endemic in the rural setting--the main host is the monkey and the vector is Aedes aegypti (the same vector as dengue). There is no possibility of eradicating yellow fever because of this “jungle” transmission. Humans come in contact primarily when they enter an endemic region. There is concern that the number of urban cases could increase in the Americas, as it has in Africa. This graph shows the urban yellow fever cases from 1980-1998 in both Africa and the Americas.

[Figure:   Yellow fever cases, 1980-1998]

These yellow fever epidemics can be controlled with the use of yellow fever vaccination of the population at risk. It has been recommended to add yellow fever vaccine to the EPI vaccination schedule for children. Yellow fever vaccine is one of the more effective vaccines and has good longevity. To date, this recommendation has not been adopted by WHO. The vaccine is used by many countries as a control measure during large outbreaks.

Hantavirus pulmonary syndrome was first identified in the four corners region of southwestern United States in 1993. This map shows the hantavirus pulmonary syndrome cases during 1993-2000.

[Figure:   Hantavirus pulmonary syndrome cases, Americas, 1993-2000]

There have been cases of hantavirus pulmonary syndrome in South America. It is thought that this is unlikely to be from spread from the United States; it appears that HPS is an indigenous disease to this region that is either truly new or has recently been discovered.


Emergence of antimicrobial resistance

Another major factor for emergence of disease is the microbial adaptation to change. One organism, Salmonella, has been shown to have significant resistant patterns. Salmonella species cause a variety of intestinal and systemic disorders such as typhoid fever. They can commonly be found in the food supply and cause diarrhea. The majority is self limiting and requires no treatment. However, untreated S. typhi could lead to a carrier state or more serious illness. This map shows the countries with presence of drug resistant Salmonella.

[Figure:   Countries with presence of resistant Salmonella 1996-1997]

The resistance is to multiple antibacterials including sulfa, macrolides and chloramphenicol. These are the least expensive medications.

Newer antibiotics, such as the quinolones have been effective, but resistance has also been increasing. This graph shows the level of quinolone resistance in humans based on their use in animal husbandry and veterinary practices.

[Figure:   Resistance in Animals, Humans Quinolone-resistant Salmonella Typhimurium DT104, UK]

Based on this increase of resistance, the UK banned quinolones from being used in animal husbandry to prevent further resistance from occurring.


Bioterrorism concerns

Foot and mouth disease is a disease of cattle and other hooved animals. It is a vesicular disease that can cause high mortality among susceptible animals. It is a major concern in South America, especially in Argentina, Uruguay and Brazil, which are major exporters of beef. This world map shows the global distribution of Foot and Mouth disease.

[Figure:   Foot-and-mouth disease (FMD) outbreaks 1/00-2/01]

The potential to spread this disease by bioterrorist means is a concern as it could devastate an industry and food supply for a large numbers of people.


Infectious Disease Successes

Two serious childhood diseases that have been effectively eradicated or markedly diminished in number are polio and measles.

Polio eradication in the Americas started in 1988 and was completed by 1991. The Americas have been free of wild polio since 1991. This slide shows the efforts toward global polio eradication from 1988 until today.

[Figure:   Progress toward polio eradication, 1988-2003]

You can see that there are a few remaining countries in the world that still have endemic polio: Nigeria, India, Egypt, Pakistan and Afghanistan. Efforts continue to complete the eradication of this disease; it is hoped to be completed by the end of 2005. This would mark the second disease eradication of the 20th and 21st centuries (smallpox being the first).

The infrastructure and collaboration among countries was necessary to complete this eradication. Poliomyelitis laboratory network for PAHO (Pan-American Health Organization) exist throughout the Americas.

[Figure:   PAHO—Poliomyelitis laboratory network]

This network was used to help identify any suspect cases and determine if it was wild type polio or vaccine strain type.

Building on the success of polio eradication, PAHO decided to undertake measles eradication as well. The decision was made in 1994. Measles had been reduced at that time through increasing immunization coverage. In 1994, PAHO moved to a two-tiered strategy of routine immunization: the initial immunization occurs as part of the EPI vaccines at age 15 months, the second tier entails periodically giving an additional dose of vaccine to children under 15 years of age.

This slide shows the vaccine coverage and reported number of measles cases from 1980-2001.

[Figure:   Vaccination coverage and reported number of measles cases, region of the Americas, 1980-2001]

You can see that starting the two-tiered system led to even further decline of measles cases. Since 2002, there have not been any endemic, indigenous measles cases in the Americas. This graph shows the vaccine coverage and measles rates from 1990-2003.

[Figure:   Vaccination coverage and reported measles cases, region of the Americas, 1990-2003]

There are still a few cases that could have been imported from elsewhere in the world or due to partial immunity. Since measles presents as a rash, it could be confused for other diseases with similar presentation--an expanded laboratory network is necessary to do accurate testing on suspected cases

[Figure:   PAHO measles laboratory network]


The future of emerging infections

This map of the Americas shows the emerging infections of the past decade.

[Figure:   Emerging and re-emerging infectious disease threats]

What will we see in the next decade? The factors that lead to emerging infections--decay of public health, continued population growth and urbanization, international trade and travel, increase in microbial resistance and change in land use and others--will continue to exist.


What is our situation?

  • Humans may have developed their own diseases during the last 10,000 years
  • Recent studies of HIV and BSE show that humans are still creating new diseases
  • Human lifestyles and behavior will inevitably change over the next century
  • The appearance of new human diseases is highly likely


What can we conclude from this?

  • Infectious diseases remain a persistent challenge and threat worldwide and in the Americas
  • Along with old infectious diseases, new and emerging diseases pose new problems
  • Other new diseases are likely to emerge in the future
  • Control, prevention, and eradication of some infectious diseases is possible
  • Concerted and collaborative international action is required



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