—from Epidemiological Bulletin, Vol. 21 No. 4, December 2000—

Dengue in the Americas: The Epidemics of 2000

• Introduction
• Costa Rica
• El Salvador
• Guatemala
• Honduras
• Nicaragua
• Panama

Introduction
Prior to 1981, dengue and its more severe form, dengue hemorrhagic fever (DHF), were considered to be a public health problem of Asia and posed little or no threat to the Region of the Americas. This scenario changed suddenly as a result of the Cuban epidemic of 1981, the first major DHF epidemic in the Americas. For the following 10 years, sporadic cases of dengue and DHF were observed. Then in 1990, Venezuela experienced the second major epidemic in the Region. These events clearly marked dengue and DHF as an emerging disease in the Americas.

The epidemiologic pattern of the disease has evolved in the Americas in a similar way as it did in Asia several decades ago. Expanded distribution and increased infestations of Aedes aegypti occurred during and after World War II in Asia as it did in the Americas after the failure of the Ae. aegypti eradication initiative (1). In both of these regions, expanded distribution of the vector was followed by increased transmission of multiple dengue virus serotypes, resulting in hyperendemicity in many countries. In every country in Southeast Asia where epidemic DHF became a major public health problem, the disease first appeared sporadically for several years, ultimately culminating in major epidemics. Most of those countries subsequently developed a continuing cycle of dengue and DHF epidemics at three to five year intervals, with epidemics becoming progressively more severe. Some countries of Central America are experiencing similar continued cycles of epidemics.

During the summer of 2000, Costa Rica, El Salvador, Guatemala, and Nicaragua underwent outbreaks of dengue fever that also included cases of DHF and deaths. The deterioration of the control programs, associated with climatic conditions, prolonged and intensified the effects of the disease.

Costa Rica
Costa Rica was one of the last two countries in the Region to report cases of dengue. After about 30 years of being free of Ae. aegypti, Costa Rica became reinfested in 1988. Five years later, in 1993, the country reported 4,612 cases of dengue fever. In 1994, the number of cases rose to 13,929 and then decreased over the next two years, until 1997, when 14,267 cases were reported. The first manifestation of DHF occurred in 1995 in the Chorotega Region and in the following year, the first two deaths from DHF were registered. In 1997, an epidemic recorded 14,267 cases, the largest for Costa Rica in a single year. In the following three years, the numbers of reported cases decreased (2). The cycles of reported dengue cases can be appreciated in Figure 1.

Data presented in Figure 2 are numbers of dengue and DHF cases reported by epidemiological week of 2000. Throughout the first 17 weeks, there were few cases, but a sudden surge and drop followed in weeks 18 and 19, with 104 and 46 cases respectively. After this decline, a steady and sharp increase was observed starting on week 27 until week 37. Up to week 37, there were a total of 2,113 reported cases and an incidence rate of 52.52 cases per 100,000 population.

The serotype responsible for the first epidemic of 1993 was identified to be DEN-1, which was also associated with outbreaks in subsequent years. In 1994, all serotypes were detected in several regions of the country. Coincidentally, more case fatalities occurred the following year. From 1995 to 1998, DEN-1 and 3 were circulating simultaneously. For 1999 and 2000, the identity of the circulating serotypes was not reported.

El Salvador
There were no registered cases of dengue in El Salvador until 1980, when the first epidemic occurred with 2,060 cases. During the following 10 years, cycles of increasing and decreasing number of reported cases occurred. In 1993 and 1995, there were 9,015 and 9,658 reported cases, respectively, the highest numbers of cases ever registered in El Salvador at the time (2). The data presented in Figure 3 shows that the number of cases in 2000 surpassed the 1993 and 1995 data. This is the largest epidemic ever registered in El Salvadors history.

For 2000, the number of reported cases of dengue and DHF were low until weeks 29 through 36. During this time, a gradual increase was observed. By week 37, there were 1,438 cases, almost double the number of cases in week 36. The highest number of cases recorded was of 2,882 in week 38. Data presented in Figure 4 shows a sharp decrease by week 42 with 1,012 cases. Up to week 42, a total of 16,355 cases of dengue and DHF were reported, with 31 deaths of 336 confirmed cases of DHF. A case fatality rate of 9.22% for DHF and an incidence rate of 260.60 cases per 100,000 population for combined dengue and DHF were calculated.

As this epidemic progressed, there was a shift in the rates of cases per department. Initially, the department of Sonsonate had the highest rate, but by the end the department of San Salvador resulted in having the highest rate of 291 cases per 100,000 inhabitants, followed by Cabaas (231 per 100,000), La Libertad (179 per 100,000), Chalatenango (137 per 100,000) and Sonsonate (135 per 100,000).

Virus isolation studies show that serotypes DEN-1, 2 and 4 were co-circulating in the 1980s. Data presented in Table 2 shows serotype circulation per year from 1990 to 2000. Throughout most of these years, two serotypes or more were found to circulate simultaneously. Serotype 3 was isolated for the first time in 1991 and was detected later in 1995 and 1998. The only serotype isolated during the 2000 epidemic was DEN-2.

Guatemala
From 1948-1959 efforts were made to eradicate Ae. aegypti in compliance with the recommendations of the Pan American Health Organization. Guatemala was certified as free of Ae. Aegypti at the XI Meeting of the WHO Directors Council in 1959 (1).

However, by 1967, the Ae. aegypti larvae was discovered in the city of Escuintla. Five years later, in 1972, Ae. aegypti was found again in Escuintla and in Taxisco. With a combination of economic limitations, ecological factors, and rapid expansion, this species gradually re-infested other communities until the first dengue epidemic occurred in Escuintla in August of 1978 (1). Unfortunately, it took four months to control this epidemic. Subsequently, actions were taken to control and reduce the Ae. aegypti re-infestations but these efforts were not sufficient. In 1987, a new outbreak of dengue occurred and since then, the disease has spread to all areas infested by the vector. The appearance of dengue forced control schemes to change their approach, which had previously been oriented to avoid reintroduction of yellow fever, to a program based on source reduction and focal treatment with larvicides. Unfortunately, the program focused solely on infested areas, allowing the vector to spread and infest other free areas. In 1991, the re-evaluation of the vector control program made it necessary to reassume measures similar to those of a pre-control attack program.

In the early 1980s there were few cases of dengue reported each year. The first major outbreak occurred in 1987 with 2,318 cases. The serotype isolated from patients during this outbreak was DEN-1. In 1991, the number of cases of dengue and DHF peaked to 10,968. A sharp decrease in the number of reported cases was observed in 1992 with only 1,286 cases but in the following years the number of cases increased gradually (2). This trend can be seen in Figure 5.

Guatemalas dengue situation for 2000 is that 5,963 cases were reported up to epidemiological week 40, an 85% increase from the number of cases reported the previous year during the same period. During 2000, there have been 18 confirmed cases of DHF with six deaths, a case fatality rate of 33%. An incidence rate of 52.38 cases per 100,000 population has been observed to week 40. As can be seen in Figure 6, a steady increase in the number of cases starting in week 25 is observed with a peak of 519 cases in week 35. The most affected departments are Zacapa, Santa Rosa, Escuintla and El Progreso. During 2000, only the DEN-2 serotype has been isolated.

Prior to the 1990s, DEN-1 was the only serotype found circulating except in 1988 when serotypes 1, 2, and 4 were isolated. In Table 3, it can be appreciated that several serotypes have been found circulating in Guatemala simultaneously during seven of the past ten years.

Honduras
Like Guatemala, Honduras participated in the Dengue eradication programs of the 1950s. Ae. aegypti reinfestation occurred in 1968 in Honduras. By 1978, this country underwent its first major dengue epidemic of the DEN-1 serotype. Since then, dengue has become well established in most parts of the country. Throughout the 1980s, Honduras reported few cases, averaging 1,000 cases per year. As seen in Figure 7, the number of cases increased from 1,700 in 1990 to 27,560 cases in 1995 (2). Following this year, the number of reported cases has been consistently lower than 1995, but in comparison to the number of cases in the 1980s, it is about seven times greater.

During 2000, dengue activity in Honduras was not as prominent as it had been in previous years. Up to week 43 of 2000, the number of reported cases was lower than cases for the 1997-1999 period. A graph of the number of reported cases for 2000 by epidemiological week is presented in Figure 8. From week 22 to 40, the number of reported cases increased slightly from 90 cases to 370, respectively. However, suddenly, 1,065 cases were reported in week 41, a three-fold increase compared to the previous week. Following week 41 to 43, this upward trend continued. For 2000 up to week 43 there were 8,715 cases reported with an incidence rate of 134.39 cases per 100,000 population.

A national emergency was declared by the Honduran government due to the sudden increase in reported cases. According to the Ministry of Health of Honduras, almost all departments have reported a sharp increase in the cases after week 37. Departments with the highest number of cases are Tegucigalpa, Juticalpa, Comayagua with 2,369, 1,245 and 1,189 cases of dengue, respectively.

The serotype responsible for this rise in the number of cases was found to be DEN-2. Virus isolation studies show that serotype DEN-1 was responsible for the first outbreak of 1978. In the early 1980s, serotypes 2 and 4 were isolated on different years and were found co-circulating with DEN-1 by the end of the decade. Table 4 shows this trend until 1995, when serotype 3 was first detected. Since then, DEN-3 has been isolated and has circulated simultaneously with DEN 2 in 1998 and 1999.

Nicaragua
Nicaraguas first major dengue outbreak occurred in 1985 with 17,483 reported cases. This epidemic was associated with severe symptoms and a few fatal cases of DHF. During the following years, there were relatively low numbers of reported cases. By the end of 1994 and 1995, 20,469 and 19,260 cases were registered, respectively (2). Figure 9 shows an increase and decrease in the number of cases throughout this decade. Following the 1994 and 1995 epidemics, Nicaragua reported relatively low numbers of cases in 1996 and 1997. However, the cases continued to increase and decrease in the following years.

Figure 10 presents the number of reported cases of dengue and DHF for 2000 by epidemiological week. The number of cases increases steadily after week 21 and peaks at week 26 with 289 cases. Throughout the following weeks, the number of reported cases fluctuated but compared to week 26, a decrease in the number of cases can be observed. Up to week 42, 5,233 cases were reported with an incidence rate of 102.94 cases per 100,000 population.

Serotypes responsible for Nicaraguas first epidemic of 1985 were identified as DEN-1 and 2. Table 5 presents serotypes circulating in Nicaragua during the 1990s and 2000. For most of the years subsequent to 1993, two or three serotypes were found co-circulating. In addition, DEN-3 was first isolated in the 1994 and 1995 epidemics. For 2000, serological testing has isolated serotypes DEN-2 and 3.

Panama
Efforts towards eradication of the vector was undertaken in 1949 in Panama. In 1958, this country was certified as being free of Ae. aegypti. However, reinfestation occurred in Panama in 1985. Eight years later, in 1993, the first case of autochthonous dengue fever was confirmed since the last dengue epidemic of 1942. By the end of 1993, 14 cases of dengue fever were confirmed and DEN2 was isolated. Figure 11 depicts the rise in incidence of dengue cases in Panama from 1990-2000.

Although the largest outbreak occurred in 1995, with 3,083 cases, the number of cases did not decrease by much during the period of 1997 through 1999 (2). In comparison to most Central American countries, Panama has not experienced a major epidemic.

Panama is the only country that has detected autochthonous cases of dengue fever in the absence of a major epidemic. After the re-appearance of dengue in this country, appropriate control measures have been taken. This is primarily due to its active surveillance program, which has been maintained since September of 1988. Although Panama did not experience an epidemic during the summer of 2000 as did some of the countries in Central America, there was an increase in the number of cases during the first few weeks of 2000, followed by a rapid decrease until week 10. Few cases were reported in the following weeks. Up to week 36, there was a total of 220 cases, including one case of DHF. Panamas incidence rate for 2000 is 7.70 cases per 100,000 population. A graphical representation is found in Figure 12.

The circulation of the different serotypes from 1993 to 2000 in Panama is depicted in Table 6. Virus isolation studies in Panama show that DEN-1 serotype was detected for five years and has circulated with other serotypes. In 1994 and 1999, all four serotypes were isolated. For 2000, the identification of serotype has currently not been reported.

References:
(1) Halstead, S., Gomez-Dantes, H.: Dengue: A Worldwide Problem, a Common Strategy
(2) Gratz, N., Knudsen, B.: The Rise and Spread of Dengue, Dengue haemorrhagic Fever and Its Vectors: A Historical Review, World Health Organization, CTD/FIL(DEN) 96.7

Source: PAHO. Division of Disease Prevention and Control. Communicable Diseases Program (HCP/HCT)

Recent information on the same subject from the Epidemiological Bulletin include articles on the "Re-emergence of Dengue in the Americas" (Vol. 18, No. 2 - July 1997), and "Dengue and Dengue Hemorrhagic Fever, 1996" (Vol. 17, No. 4 - December 1996). The case definition of Dengue is also available (Vol. 20, No. 2 - June 2000).

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Epidemiological Bulletin , Vol. 21 No. 4, December 2000