Infectious Diseases Associated with Environmental Exposures or Conditions

Infectious diseases are human illnesses caused by viruses, bacteria, parasites, fungi, and other microbes. They can be spread by direct contact with an infected person or animal, through ingestion of contaminated food or water, by insects like mosquitoes or ticks (disease vectors), or by contact with contaminated surroundings such as through touching animal droppings or breathing in contaminated air. Demographic and environmental factors such as population growth, increased urbanization, and alteration of habitats of disease-carrying insects and animals (e.g., irrigation, deforestation) may promote the spread of infectious diseases (CDC, 1998a). The three broad infectious disease categories included here are those whose appearance and spread may be influenced to some extent by environmental conditions and change. They include gastrointestinal (GI) diseases, vector-borne diseases, and legionellosis.

• Gastrointestinal diseases. Nine notifiable GI diseases caused by microorganisms are discussed below: cholera, cryptosporidiosis, shiga toxin-producing Escherichia coli (E. coli) (STEC), giardiasis, hepatitis A, Salmonella paratyphi infection (formerly reported as paratyphoid fever [in 2018] or considered salmonellosis [2017 and earlier]), Salmonella typhi infection (reported as typhoid fever prior to 2019), salmonellosis, and shigellosis. The major environmental source of GI illness is water or food that is contaminated with pathogenic microorganisms. The primary means of transmission for these diseases is through ingestion of contaminated food and water or through contact with and accidental ingestion of fecal matter (CDC, 2021a).

• Vector-borne diseases. Three vector-borne diseases are included: Lyme disease (transmission of Borrelia burgdorferi by ticks), spotted fever rickettsiosis (transmission of Rickettsia rickettsii by ticks), and West Nile virus (transmitted by mosquitoes). Certain ticks and mosquitoes (vectors) can carry bacteria and viruses that cause disease in humans. The vectors acquire bacteria or viruses when they bite an infected mammal or bird. Some studies indicate that the spread of vector-borne disease may be influenced by land use and/or other environmental changes (CDC, 2020a). Cases of spotted fever rickettsiosis are found across the U.S., though five states report more than 50 percent of cases (Arkansas, Missouri, North Carolina, Tennessee, and Virginia) (CDC, 2022a). Lyme disease cases were historically concentrated in the northeastern U.S., but have since spread to other areas of the U.S., such as California and Wisconsin (CDC, 2022c). Outbreaks of West Nile virus infections have varied in frequency and location in the U.S. each year (CDC, 2022d).

• Legionellosis. Legionellosis, or Legionnaires’ disease, is a serious and sometimes fatal form of pneumonia. It is caused by Legionella bacteria, which are found naturally in the environment and thrive in warm water and warm damp places. They are commonly found in lakes, rivers, creeks, hot springs, and other bodies of water. This bacterium has been associated with outbreaks in the U.S. linked to poorly maintained artificial water systems (e.g., industrial cooling systems, decorative fountains) and air ventilation systems. Infection results from inhalation of contaminated water sprays or mists (CDC, 2021b).

This indicator reflects the occurrence of these 13 notifiable diseases for varying periods between 1995 and 2019 as reported by health departments to the National Notifiable Diseases Surveillance System (NNDSS). A notifiable disease is one for which regular, frequent, and timely information regarding individual cases is considered necessary for the prevention and control of the disease. Data are collected by all 50 states, five territories, New York City, and the District of Columbia, based on a list of recommended nationally notifiable infectious diseases, and compiled nationally (CDC, 2021c). The temporal and geographic coverage of the data varies by disease. The number of states and territories reporting may also vary. For example, in 1995, when cryptosporidiosis was first nationally reported, only 27 states reported; by 2013, all 50 states, New York City, and the District of Columbia reported this disease (CDC, 1998b, 2015).

GI Diseases

Exhibit 1 presents the number of reported cases for each of the nine notifiable GI diseases from 1995 to 2019.

Cholera. In comparison to the other GI diseases, the number of newly identified cholera cases reported each year is low. From 1995 to 2019, just 259 laboratory-confirmed cases of cholera in total were reported to CDC, with the highest number of cases (40) reported in 2011.

Salmonella Typhi (reported as Typhoid Fever until 2019). The number of newly identified cases of Salmonella Typhi fluctuated from 1995 to 2019, ranging between a low of 321 cases in 2002 and a high of 467 cases in 2010.

Hepatitis A. Reported cases of hepatitis A declined from 1995 (31,582 cases) to 2011 (1,398 cases). The number of cases has continued to increase since 2014 (1,239 cases), with dramatic increases observed in 2018 (12,474 cases) and 2019 (18,846). According to the CDC (2020b), large Hepatitis A outbreaks with person-to-person transmission led to these dramatic increases.

Cryptosporidiosis. Reported cryptosporidiosis cases showed a noticeable increase from 1995 to 2007, fluctuated between 2008 and 2017, and increased since then hitting a peak in 2019 (13,975 cases). It remains unclear whether the greater number of case reports between 2005 and 2019 reflects changes in reporting patterns and diagnostic testing practices or a real change in infection transmission. For example, the increase in 2005 (and possibly 2007) was attributable primarily to outbreak-related case reporting. However, increased testing for Cryptosporidium following the introduction of nitazoxanide (2002-2004), the first licensed treatment for the disease, alongside changes to the national case definitions in 2011 and 2012, may have led to a possible increase in subsequent case reporting (CDC, 2008a, 2017b).

Shigellosis. The lowest number of shigellosis cases observed from 1995 to 2019 occurred in 2013 with 12,729 cases reported. This was followed by a variable trend with the highest number of cases (23,590) observed in 2015. 

Giardiasis. The number of cases observed for giardiasis remained relatively steady from 2002 to 2010, decreased continually to reach 14,485 cases in 2015, and has fluctuated since that time. 

Salmonellosis. Reported salmonellosis cases have shown a variable trend from 1995 to 2019. The highest number of salmonellosis cases during the reporting period occurred in recent years, with numbers remaining above 50,000 cases since 2010 and reaching a high of 60,999 cases in 2018. 

STEC. The number of reported cases of E. coli (presented in Exhibit 1 as shiga toxin-producing E. coli, or STEC) fluctuated from 1995 to 2009, and then increased almost every year up to 16,939 cases in 2019—the highest number of cases seen during the reporting period. Increases of reported cases seen in both 2018 and 2019 are associated with a rise in multistate STEC-related outbreaks (10 in 2018 and 16 in 2019), with the most considerable outbreak from contaminated romaine lettuce in 2018 (CDC, 2022b).

Salmonella Paratyphi. Prior to 2019, Salmonella Paratyphi cases were reported as paratyphoid fever (2018) or considered salmonellosis (2017 and earlier). Since being reported as a separate disease from salmonellosis, there were 133 and 155 cases in 2018 and 2019, respectively.

Vector-Borne Diseases

Exhibit 2 presents the number of reported cases for three vector-borne diseases.

Lyme disease. The most commonly reported vector-borne disease in the U.S. is Lyme disease. Surveillance for Lyme disease was initiated by the Centers for Disease Control and Prevention (CDC) in 1982 (CDC, 1993). It became nationally reportable with a standardized definition in 1991 (CDC, 2008b). The number of cases has fluctuated over time, but gradually increased from a low of 11,700 cases in 1995 to a high of 42,743 cases in 2017. Cases decreased to 34,945 in 2019.

Spotted fever rickettsiosis. Rocky Mountain spotted fever began as a nationally notifiable condition in the 1920s (CDC, 2022a). Starting in 2010, the category Rocky Mountain spotted fever changed to the category spotted fever rickettsiosis (including Rocky Mountain spotted fever). The number of new cases of Rocky Mountain spotted fever reported generally increased from 1995 to 2008, ranging between a low of 365 cases in 1998 and a high of 2,563 cases in 2008. The number of reported cases then fluctuated until reaching an all-time peak of 6,248 cases in 2017, followed by decreases in 2018 and 2019.

West Nile virus. Cases of West Nile virus were first documented in the U.S. in 1999, and the disease became nationally reportable in 2001 (CDC, 2010b). Cases have fluctuated from 2002 to 2019, with the lowest number of cases reported in 2009 (386 cases), and the largest number of cases reported (2,872) in 2012. 

Legionellosis

Exhibit 3 presents the number of reported cases of legionellosis within the U.S. population from 1995 to 2019. From 1995 to 2002, the number of new cases of legionellosis was relatively stable, ranging from a low of 1,108 cases in 1999 to 1,355 cases in 1998. However, a general increase in the number of new cases was reported since 2003 (2,232 cases), with the highest number reported in 2018 (9,933 cases).

• State and territory health departments report cases of notifiable diseases to CDC; reporting policies (and compliance with those policies) can vary by disease or reporting jurisdiction.
• Disease reporting likely underestimates the actual number of cases for a given time period because reporting nationally notifiable diseases to CDC is voluntary. Additionally, the completeness of reporting likely varies by disease. The degree of completeness of data reporting is influenced by many factors such as the diagnostic facilities available, the control measures in effect, public awareness of a specific disease, and the interests, resources, and priorities of state, territory, and local officials responsible for disease control and public health surveillance (CDC, 2017b).
• Factors such as changes in case definitions for public health surveillance, introduction of new diagnostic tests, or discovery of new disease entities can cause changes in disease reporting that are independent of the true incidence of disease (CDC, 2017b).
• Prior to 2001, shiga toxin-producing Escherichia coli (STEC) represents E. coli O157:H7, which was the only serotype that was nationally notifiable. From 2001 to 2005, STEC represents the sum of the three nationally notifiable shiga toxin-positive E. coli forms (O157:H7, non-O157, and not serogrouped). As of 2006, serogrouped E. coli cases are no longer nationally notifiable; STEC represents the notifiable form. These differences in reporting should be considered when making cross-year comparisons.
• Prior to 2005, only confirmed “neuroinvasive” cases of West Nile virus—the most severe form of the condition—were reported. Beginning in 2005, non-neuroinvasive domestic arboviral diseases for the six domestic arboviruses listed were added to the list of nationally notifiable diseases; these included West Nile fever, a non-neuroinvasive form of West Nile virus (CDC, 2015). In order to maintain reporting consistency, only neuroinvasive cases are presented for this indicator.
• Prior to 2018, Salmonella Paratyphi infection cases were considered in the case counts for salmonellosis (CDC, 2021c). This difference in reporting should be considered when making cross-year comparisons.

The data for this indicator were obtained from CDC annual reports that summarize data on nationally notifiable infectious diseases reported to CDC by state and territorial health agencies across the country (CDC, 1996, 1997, 1998b, 1999, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008a, 2009, 2010a, 2011, 2012, 2013, 2014, 2015, 2016, 2017a, 2017b, 2018, 2019, 2021c). Data are collected and compiled from reports sent by state and territorial health departments to the NNDSS, which is operated by CDC. The NNDSS is neither a single surveillance system nor a method of reporting. Certain NNDSS data are reported to CDC through separate surveillance information systems and different reporting mechanisms; however, these data are aggregated and compiled for publication purposes (CDC, 2017b).

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