Download Infectious Disease Morbidity in the US Region

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Infectious Disease Morbidity in the US Region Bordering Mexico, 1990–1998
Timothy J. Doyle1 and Ralph T. Bryan2,a
1
Division of Public Health Surveillance and Informatics, Epidemiology
Program Office, and 2Division of Quarantine, National Center
for Infectious Diseases, Centers for Disease Control and Prevention,
Atlanta, Georgia
The United States and Mexico share an international boundary ∼3000 km long. This border
separates 2 nations with great differences in health status. The objective of this study was to
assess morbidity due to infectious diseases in the US region bordering Mexico. The incidence
between 1990 and 1998 of 22 nationally notifiable infectious diseases was compared between
border and nonborder regions. Disease rates, reflected as rate ratios, were higher in the border
region for botulism, brucellosis, diphtheria, hepatitis A, measles, mumps, rabies, rubella,
salmonellosis, and shigellosis than in either of 2 nonborder comparison regions. These data
indicate that incidence rates for a variety of infectious diseases of public health importance
are significantly higher in the United States along the Mexican border than in nonborder
regions. These results suggest that an inadequate public health infrastructure may contribute
to excess morbidity due to infectious diseases in the border region.
The United States and Mexico share an international boundary ∼3000 km long between 4 US and 6 Mexican states. Perhaps
no other border in the world separates 2 nations with such
discrepancies in health status, entitlements, and utilization [1].
The border region is commonly defined as the area in the United
States and Mexico within 100 km of the border [2]. On the US
side, the border region is characterized by lower than average
socioeconomic status: 5 of the 14 poorest US counties are in
the Texas borderlands [3]. In Mexico, by contrast, the northern
border states are generally more prosperous relative to other
states, and the border region economy is heavily influenced by
commercial activity with the United States. In many respects,
US and Mexican border communities have more in common
with their sister communities across the international boundary
than with communities in their own countries outside the border region. Thus, in these 2 distinct countries, there is a region
of transition—i.e., the binational borderlands, which both separate and connect the United States and Mexico.
During the past century, the binational border region has
experienced steady population growth. Residents of border
communities commonly live on 1 side and work on the other
side of the border. Transborder human population movement
is enhanced by American manufacturing plants located on the
Mexican frontier. In one city alone, Ciudad Juarez, Chihuahua,
Received 11 May 2000; revised 13 July 2000; electronically published 27
September 2000.
Presented in part: International Conference on Emerging Infectious Diseases, Atlanta, March 1998.
a
Present affiliation: Office of the Associate Director for Minority Health,
Centers for Disease Control and Prevention, Albuquerque, New Mexico.
Reprints or correspondence: Mr. Timothy Doyle, CDC/EPO, Mail Stop
K-74, 4770 Buford Hwy., Atlanta, GA 30341 ([email protected]).
The Journal of Infectious Diseases 2000; 182:1503–10
q 2000 by the Infectious Diseases Society of America. All rights reserved.
0022-1899/2000/18205-0028$02.00
Mexico, 1200 companies have located plants, to take advantage
of lower production costs [4]. This concentration of manufacturing activity has attracted residents to the border region from
other parts of Mexico in search of employment opportunities.
Legal northbound crossings of the border exceed 200 million
annually, making this one of the world’s busiest international
boundaries (unpublished data, US Immigration and Naturalization Service).
Outside of urban areas, a number of US border residents
live in unincorporated settlements referred to as “colonias.” An
estimated 500,000 people live in 12500 colonias throughout the
US border region, with most settlements in Texas [5]. Residents
of colonias often lack access to indoor plumbing or adequate
sewage and waste disposal systems [6]. Groundwater on both
sides of the border has been shown to contain unhealthy concentrations of fecal coliform bacteria, thus posing a risk for
waterborne infectious diseases [7, 8].
The border states monitor the health of their residents through
standard, usually passive, public health surveillance methods.
Such methods, however, fail to treat the population living in the
border region as a single geographic or epidemiologic group, one
that shares many of the same health and demographic features
among states on both sides of the international boundary. The
unique health characteristics of the borderland population have
been qualitatively described, emphasizing the important contribution of infectious agents to morbidity and mortality [1]. In
addition, quantitative studies have been conducted, addressing
select diseases in specific border communities. Nevertheless, there
have been few quantitative analyses of disease incidence along
the border’s 3000-km length. Thus, the precise burden of infectious diseases in the region is unclear.
The creation of a truly binational infectious disease surveillance system is made difficult by substantive differences in case
definitions and laboratory disease confirmation methods be-
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Doyle and Bryan
Figure 1.
JID 2000;182 (November)
Border and nonborder comparison regions, continental United States
tween the United States and Mexico. Despite the close proximity and the many common characteristics of borderland communities, current disease reporting differences between the
countries require separate enumeration of disease incidence for
each country. We report here infectious disease morbidity, for
a 9-year period, for the US region that borders Mexico.
Methods
Case report data for 1990–1998 were obtained via the National
Notifiable Diseases Surveillance System (NNDSS) for 22 infectious
diseases reportable to public health authorities in the United States
[9]. The data included information on disease, the county and state
reporting the case, the year of illness, and the age and ethnicity of
the patient. Data on race were not consistently categorized throughout the study period and, therefore, were not analyzed extensively.
The continental United States was divided into 3 regions (figure
1): region 1, 48 counties within ∼100 km of the border with Mexico;
region 2, the remaining 312 counties in Arizona, California, New
Mexico, and Texas 1100 km of the border; and, region 3, 44 nonborder states and the District of Columbia. Annual and 9-year
mean, unadjusted, disease incidence rates were calculated for each
region after exclusion of case reports from border states with missing county information. After we excluded other cases with missing
age information, age-adjusted rates were computed by using the
indirect standardization method [10]. Population estimates for
1994, the midpoint of the study period, were used as population
denominators for all rate calculations (table 1) [11, 12]. Rate ratios
were then calculated, which compared mean rates between region
1 and regions 2 and 3. Similar analyses were performed after we
stratified for Hispanic ethnicity. During the 9-year study period,
Table 1.
regions.
Demographic characteristics of border and nonborder
Characteristic
a
Total population
a
Hispanic ethnicity, %
b
Age distribution, %
0–4 Years
5–17 Years
18–24 Years
25–44 Years
45–64 Years
164 Years
a
b
Data from [11].
Data from [12].
Region 1
Region 2
Region 3
9,871,639
36
45,572,965
26
203,067,933
5
9
20
10
32
17
12
9
19
10
33
18
11
7
18
10
32
20
13
JID 2000;182 (November)
US-Mexico Border Infectious Disease Morbidity
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Table 2. Total cases, average annual incidence, and rate ratios of incidence in border and nonborder regions for select nationally notifiable diseases, 1990–1998.
Rate per 100,000 population (no. of cases)
Disease
Botulism, foodborne
Brucellosis
Cholera
Diphtheria
Haemophilus influenzae,
c
invasive disease
Hansen’s disease
Hepatitis A
Hepatitis B
Legionellosis
Malaria
Measles
Meningococcal disease
Mumps
Pertussis
Plague
Rabies, human
d
Rubella
e
Salmonellosis
e
Shigellosis
Tetanus
Trichinosis
Typhoid fever
Region 1
Region 2
Region 3
Rate ratio
a
b
Regions 1:2
Regions 1:3
0.03
0.18
0.02
!0.01
(31)
(158)
(14)
(2)
!0.01 (27)
!0.01 (94)
0.09 (360)
0.03 (123)
!0.01 (5)
0.02 (398)
!0.01 (112)
!0.01 (13)
5.30
2.03
0.53
1.85
6.78
8.17
2.57
3.16
0.35
0.10
29.89
6.54
0.29
0.45
4.54
0.88
0.97
2.05
!0.01
!0.01
0.15
18.94
17.81
0.02
!0.01
0.16
(339)
(93)
(26,552)
(5811)
(255)
(403)
(4031)
(783)
(863)
(1817)
(4)
(3)
(105)
(5610)
(5274)
(21)
(2)
(145)
0.48
0.17
17.99
7.53
0.19
0.76
3.85
1.13
0.74
1.67
0.01
!0.01
0.09
17.49
14.22
0.02
!0.01
0.26
0.49
0.02
7.80
4.71
0.59
0.52
1.09
1.06
0.72
2.20
!0.01
!0.01
0.07
16.45
7.83
0.02
0.02
0.14
0.79
0.62
1.66
0.87
1.49
0.60
1.18
0.78
1.31
1.23
0.38
2.31
1.74
1.08
1.25
0.98
0.38
0.64
0.78
5.09
3.83
1.39
0.48
0.87
4.17
0.83
1.36
0.93
3.43
3.86
2.10
1.15
2.28
1.39
0.14
1.15
(1974)
(694)
(73,806)
(30,874)
(790)
(3097)
(15,792)
(4639)
(3049)
(6837)
(48)
(6)
(278)
(23,918)
(19,438)
(99)
(24)
(1054)
(8987)
(376)
(142,608)
(86,021)
(10,818)
(9519)
(19,903)
(19,441)
(13,069)
(40,200)
(24)
(16)
(1029)
(100,220)
(47,670)
(310)
(285)
(2586)
a
Ratio of disease incidence in region 1 divided by disease incidence in region 2.
Ratio of disease incidence in region 1 divided by disease incidence in region 3.
Eight-year average annual incidence, 1991–1998 (excluding 1990).
d
Seven-year average annual incidence, 1992–1998 (excluding 1990 and 1991, when 875 cases in border states
lacked information on county).
e
Three-year average annual incidence, 1996–1998 (excluding 1990–1995).
b
c
temporal trends were examined by comparing annual incidence
rates for each disease between the regions. The analyses were performed by using SAS statistical software (version 6.12 for Windows; SAS Institute, Cary, NC).
Invasive Haemophilus influenzae disease was reportable in !80%
of states during 1990, becoming nationally notifiable in 1991.
Therefore, incidence for this disease was calculated for the 8-year
period, 1991–1998. During 1990 and 1991, cases of rubella reported
from California lacked information on county. Therefore, incidence
of rubella in the 3 regions was calculated for the 7-year period,
1992–1998. Throughout the first 6 years of the study period, case
reports and information on county were lacking from some states
for cases of salmonellosis and shigellosis. Therefore, incidence rates
for salmonellosis and shigellosis were calculated over 3 years,
1996–1998.
Infectious disease morbidity in border and nonborder regions
was further assessed by pooling incidence data from 2 groups of
vaccine-preventable diseases. In both instances, pooling of such
diseases was done throughout the 9-year period, after we excluded
cases from border states with missing county information. Case
reports of measles, mumps, and rubella were pooled, to assess
incidence of diseases prevented by the conjugate measles-mumpsrubella (MMR) vaccine. Case reports of measles, mumps, rubella,
diphtheria, pertussis, and tetanus (MMR-DPT) were also pooled,
to assess the overall incidence of childhood vaccine-preventable
diseases. Finally, case report data were not obtained for tuberculosis, AIDS, or other sexually transmitted diseases, because re-
porting requirements differ for these diseases, and national data
on these conditions are maintained on separate information
systems.
Results
For the 9-year period, 1990–1998, we report the total cases
of disease, the unadjusted mean annual disease incidence by
region, and rate ratios between border and nonborder regions
(table 2). Incidence rates in the border region (region 1) were
greater than rates in nonborder counties of border states (region
2) for the following diseases: foodborne botulism, human rabies, brucellosis, diphtheria, rubella, hepatitis A, legionellosis,
mumps, shigellosis, pertussis, measles, and salmonellosis (figure
2). In comparison with national disease rates (region 3), incidence rates in the border region were elevated for brucellosis,
foodborne botulism, Hansen’s disease, measles, rabies, hepatitis
A, plague, diphtheria, cholera, shigellosis, rubella, hepatitis B,
tetanus, mumps, typhoid fever, and salmonellosis (figure 3). The
same patterns were observed for ethnic-specific and age-adjusted disease incidence rates compared between regions (data
not shown).
When temporal trends were examined, the incidence of hepatitis A was consistently higher in the border region each year
than in either of the 2 nonborder comparison regions. When
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Doyle and Bryan
Figure 2.
JID 2000;182 (November)
Rate ratios of disease incidence in regions 1 and 2, 1990–1998
compared only with rates in region 3, the incidence of brucellosis and hepatitis B was also consistently higher in the border
region during each year of the study period. During 1996–1998,
when complete data were available, the incidence of salmonellosis and shigellosis was consistently higher in the border
region than in either nonborder region. Further examination
of temporal trends revealed that 24 of the 31 cases of foodborne
botulism reported from the border region occurred in 1994.
This observation suggested that a single common source outbreak was responsible for the high 9-year mean incidence in
the border region. Indeed, most cases were part of an outbreak
linked to a restaurant in El Paso, Texas [13].
When the analyses were stratified by ethnicity, the regional
differences observed in the unstratified analyses generally persisted among both Hispanics and non-Hispanics. Although the
incidence of hepatitis A in the border region was over twice as
high among Hispanics than in non-Hispanics, non-Hispanics
in the border region continued to show a similar magnitude of
excess incidence of this disease, compared with non-Hispanics
in nonborder regions. Similarly, for brucellosis, among both
Hispanics and non-Hispanics, rates were higher in the border
region than in nonborder regions, even though the overall rates
were much higher among Hispanics.
When age-adjusted analyses were performed that stratified
by the age groups shown in table 1, no evidence of age confounding was observed. The population age distribution in the
border region is slightly younger than that in nonborder
regions. Although the mean age of hepatitis A cases in the
border region (21 years) was younger than that in nonborder
regions (23 and 28 years in regions 2 and region 3, respectively),
excess incidence of hepatitis A in the border region persisted
throughout all age groups. Similar results were observed for
measles and shigellosis, with the highest rates observed throughout all regions in children !5 years old; yet greater incidence
was observed in the border region within each age group. Overall, rate ratios derived from age-standardized incidence rates
differed little from those based on unadjusted rates.
Temporal trends for measles indicated a relative excess of
this disease in the border region in 1990, 1992, and 1998. During
1990, 13300 measles cases occurred in the border region, representing a significant portion (12%) of the 127,000 cases reported nationally. For 1991, 2848 measles cases were reported
throughout the 4 border states, yet 70% of these were reported
from California and lacked information on county that year,
making annual rate ratios difficult to compute. These cases are
most likely attributable to an epidemic that occurred in Los
Angeles [14]. Although reported measles cases declined during
1992 to 540 in the border region, cases there represented a
greater than 4- and 10-fold excess, respectively, when compared
with measles incidence in regions 2 and 3. By 1998, measles
incidence in the United States declined to record lows, yet the
14 cases reported in the border region that year represented a
9- and 5-fold excess, compared with incidences in regions 2 and
3, respectively.
The combined incidence of measles, mumps, and rubella
throughout the study period was ∼3-fold greater in the border
JID 2000;182 (November)
US-Mexico Border Infectious Disease Morbidity
Figure 3.
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Rate ratios of disease incidence in regions 1 and 3, 1990–1998
region than in region 3, even though we excluded all cases in
border states with missing county information and, thus, underestimated the true incidence in the border region. Taken
together over the study period, the 6 major childhood vaccinepreventable diseases (measles, mumps, rubella, diphtheria, pertussis, and tetanus) occurred in the border region at an incidence rate 85% greater than the incidence in region 3. The age
distribution of cases was similar among the 3 regions: measles
and pertussis most commonly affected preschool children,
mumps most commonly affected older children and adolescents, and rubella affected both young children and adults.
Discussion
When compared with US nonborder regions, we found significant excess morbidity due to certain infectious diseases in
the US region bordering Mexico. This excess morbidity was
noted in particular for botulism, brucellosis, diphtheria, hepatitis A, measles, mumps, rabies, rubella, salmonellosis, and shigellosis. Exceptions to this pattern included invasive H. influenzae disease, malaria, meningococcal disease, and trichinosis,
which occurred at lower rates in the border region than in the
2 nonborder regions.
In contrast to national rates outside the border states, we
observed a 13-fold excess in the incidence of hepatitis A in the
border region. Hepatitis A occurs worldwide and is often inversely correlated with socioeconomic status, occurring more
commonly in areas with poor sanitation conditions [15]. Al-
though the United States is generally considered an area of low
endemicity for hepatitis A, areas of intermediate and high endemicity exist, as evidenced by recurrent outbreaks of the disease on American Indian reservations [16].
In the United States, rates of hepatitis A are ∼2-fold higher
among Hispanics than among non-Hispanics [17]. Several recent studies of hepatitis A in the border region have helped to
further clarify the risk factors for infection. In a serosurvey of
antibody to hepatitis A virus among adult women in El Paso,
Texas, 75.8% of women had evidence of past infection [18].
Past infection was associated with increasing age and with Hispanic ethnicity. A similar serosurvey in a Texas border community found that 16.9% of primary school children were antibody positive for hepatitis A virus [19]. Past infection was
positively associated with characteristics of low socioeconomic
status (e.g., low maternal education, household crowding, and
inadequate water and sewage facilities). Underscoring these results, another study among children in south Texas reported
that the independent risk of hepatitis A infection was associated
with residence in a colonia [20]. To help reduce the burden of
this disease in the border region, routine use of the hepatitis A
vaccine is recommended among children in select communities
[21, 22].
Our data showed that shigellosis occurred in the border region at an incidence more than double that in region 3. There
was a consistent excess incidence of shigellosis in the border
region each year of the 1996–1998 period for which complete
data were available. Shigellosis is endemic in many parts of
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Doyle and Bryan
Mexico, and molecular studies of bacterial isolates among shigellosis patients in southern Arizona have suggested an association with travel to Mexico [23]. In addition, travel history
to Mexico is associated with resistance to trimethoprim-sulfamethoxazole among Shigella isolates in the United States [24].
Independent of the risks posed by travel to endemic regions in
Mexico, the risk of infection with enteric pathogens, such as
Shigella species, is undoubtedly increased by the inadequate
water and sanitation infrastructure in many colonias and other
border communities of the desert Southwest [6, 7]. Future research might appropriately focus on assessing the importance
of microbial resistance to antibiotics in the border region.
Although rare in the United States, brucellosis occurred in
the border region at a rate 8 times the national rate. The epidemiology of human brucellosis in the United States has
changed from an occupational disease associated with animal
contact to a foodborne disease associated with consumption of
unpasteurized dairy products that predominantly affects Hispanics [25–28]. Although our database did not include any information on bacterial species or behavioral risk factors associated with cases, 180% of brucellosis cases in the 4 border
states and 58% of cases occurring nationally were among
Hispanics.
Another rare zoonotic disease, human rabies, also occurred
at higher rates in the border region. In the Mexican states
bordering the United States, urban epizootics of canine rabies
continue to occur [29]. In such settings, children are often at
highest risk for exposure to rabies via dog bites. Viral studies
of human cases reported from US border states have implicated
urban canine rabies strains found primarily in the border region
[30]. These same urban canine strains have also been linked to
the recent coyote rabies epizootic in southern Texas that resulted in a massive effort to vaccinate coyotes with an experimental oral baiting program [31, 32]. Limited resources and
minimal public health infrastructure in some border communities may have hampered local efforts to maintain animal control and canine (dog) rabies vaccination programs.
We found that major childhood vaccine-preventable diseases
occurred in the border region at nearly twice the incidence in
nonborder states. This excess incidence in the border region
was driven largely by a 1 4-fold excess in measles incidence,
which accounted for most cases in the region for the 6 diseases
prevented by the MMR and DPT vaccine series. Nonetheless,
this observation suggests that the excess morbidity caused by
these readily preventable diseases may have resulted from an
inadequate system to ensure routine immunizations in the border region.
Between 1986 and 1994, Mexico was the leading source of
measles cases imported to the United States, accounting for
31% of all imported cases [33]. Five of the top 20 US counties
reporting imported measles were in the border region; 8 of the
top 20 were in border states. Imported measles cases may trigger
JID 2000;182 (November)
a local epidemic in a poorly vaccinated community because of
the high infectivity of the virus.
During the national measles resurgence witnessed from 1988
to 1991, cases occurred most commonly in unvaccinated preschool children [34, 35]. The national epidemic was characterized by large outbreaks in urban areas, such as San Diego, Los
Angeles, Dallas, Houston, Chicago, Milwaukee, and New
York. In 1990, 61% of measles cases reported nationally occurred in the states of California and Texas [36], and the border
region experienced outbreaks among preschool- and schoolaged children [37]. The large outbreak in Los Angeles continued
in 1991, encompassing Riverside County, California, in the border region [14]. Although cases reported nationally declined
dramatically during 1992, Hidalgo County in the Texas border
region reported a large outbreak that year [38].
Despite improvements in vaccination coverage resulting from
the measles resurgence, children living below the poverty line
continued to have lower measles, DTP, and oral polio vaccine
(OPV) coverage, when assessed immediately after the epidemic
period [38]. Three border counties (San Diego; Maricopa, AZ;
and El Paso, TX) included in the National Immunization Survey of 1994–1995 demonstrated coverage of 75%–81% for the
4:3:1 dose DTP:OPV:MMR vaccination series [39]. Although
measles incidence is currently at historic lows in the United
States [40], our analysis suggests that the border region continues to be overrepresented. Prevention of future resurgence
of vaccine-preventable diseases in the border region will require
ongoing surveillance of vaccination coverage in the region plus
innovative outreach methods in communities with inadequate
coverage [41]. Efforts to reduce the incidence of vaccine-preventable disease in the region may also benefit from the recent
change in Mexican vaccination policy that added MMR to the
routine schedule in place of a single-antigen measles vaccine
[42].
The NNDSS database used for this study is derived from a
passive surveillance system of disease reporting among states.
The NNDSS, therefore, likely underestimates the true burden
of disease, and reported incidence rates should be interpreted
with caution. Notifiable diseases are detected and reported with
varying levels of completeness across individual states because
of state-specific differences in surveillance methodology. Direct
state-to-state comparisons of disease incidence, therefore, are
potentially misleading. Rate ratios that compare multistate
regions of the country, such as those in this study, may be a
more accurate measure of relative morbidity, since the underestimation of disease inherent in NNDSS is less likely to differ
in magnitude between regions involving multiple states. Nevertheless, further operational research is needed to evaluate the
sensitivity of routine infectious disease surveillance in the border region. In addition, disease rates over many years are influenced by both endemic and epidemic patterns. Regional differences in focal endemicity for diseases such as plague and
JID 2000;182 (November)
US-Mexico Border Infectious Disease Morbidity
large multistate outbreaks of diseases such as salmonellosis
must be considered when interpreting these results.
This study was limited by missing county information for
some diseases during some years. Thus, rate ratios that compare
disease incidence in border counties to nonborder regions are
potentially biased for the diseases with a high proportion of
missing county information. Attempts were made to minimize
this bias by modifying the analysis period for diseases limited
by missing county information. Our study was further limited
by missing data from case reports on age, race, and ethnicity
of patients [43]. Therefore, morbidity was generally expressed
as crude incidence. Regional differences persisted, however, after stratifying by ethnicity and age group.
In conclusion, we found significant excess morbidity due to
infectious disease in the US region bordering Mexico, compared
with nonborder regions. In particular, this excess morbidity was
noted for diseases associated with poor sanitation, zoonotic
diseases, and vaccine-preventable diseases. Inadequate public
health infrastructure is likely an important underlying cause
for the excess morbidity observed. An improved water and
sanitation infrastructure is needed to address the excess burden
of enteric disease. Enhanced surveillance is recommended to
more accurately monitor enteric and zoonotic disease incidence
and vaccination coverage levels. Finally, a binational approach
to surveillance and prevention is needed to address public
health problems that defy international boundaries.
Acknowledgments
We thank Carol Knowles, Willie Anderson, and Man-huei Chang
for assistance in data management; Pani Ellinas and Douglas Thoroughman for helpful comments and suggestions; and Stephen Waterman, Samuel Groseclose, and Kate Glynn for critical review of the
manuscript.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
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