Escherichia coli O157 outbreaks in the United States, 2003–2012

Infections with the Shiga toxin–producing bacterium Escherichia coli O157 can cause severe illness and death.

e.coli.O157.strawberryWe summarized reported outbreaks of E. coli O157 infections in the United States during 2003–2012, including demographic characteristics of patients and epidemiologic findings by transmission mode and food category.

We identified 390 outbreaks, which included 4,928 illnesses, 1,272 hospitalizations, and 33 deaths. Transmission was through food (255 outbreaks, 65%), person-to-person contact (39, 10%), indirect or direct contact with animals (39, 10%), and water (15, 4%); 42 (11%) had a different or unknown mode of transmission. Beef and leafy vegetables, combined, were the source of >25% of all reported E. coli outbreaks and of >40% of related illnesses.

Outbreaks attributed to foods generally consumed raw caused higher hospitalization rates than those attributed to foods generally consumed cooked (35% vs. 28%). Most (87%) waterborne E. coli outbreaks occurred in states bordering the Mississippi River. 

181 sick with Salmonella from chicks

The U.S Centers for Disease Control and Prevention reports that public health, veterinary, and agriculture officials in many states and the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (USDA-APHIS) are investigating four multistate outbreaks of human Salmonella infections linked to contact with live poultry.

OLYMPUS DIGITAL CAMERAAs of June 29, 2015, 181 people infected with the outbreak strains of Salmonella have been reported from 40 states.

33 ill people have been hospitalized. No deaths have been reported.

Epidemiologic, laboratory, and traceback findings have linked these four outbreaks of human Salmonella infections to contact with chicks, ducklings, and other live poultry from multiple hatcheries.

Eighty-two (86%) of the 95 ill people who were interviewed reported contact with live poultry in the week before their illness began.

CDC’s National Antimicrobial Resistance Monitoring System (NARMS) laboratory conducted antibiotic resistance testing on Salmonella isolates collected from seven ill people infected with one of the outbreak strains.

All seven isolates were susceptible to all antibiotics tested on the NARMS panel.

Antibiotic resistance testing continues on additional isolates collected from ill people infected with the outbreak strains.

Backyard flock owners should take steps to protect themselves and their families:

Always wash hands thoroughly with soap and water right after touching live poultry or anything in the area where the birds live and roam.

Do not let live poultry inside the house.


Almost 1800 sickened: It’s summer (up north), but beware the water

Outbreaks of illness associated with recreational water use result from exposure to chemicals or infectious pathogens in recreational water venues that are treated (e.g., pools and hot tubs or spas) or untreated (e.g., lakes and oceans).

caddyshackFor 2011–2012, the most recent years for which finalized data were available, public health officials from 32 states and Puerto Rico reported 90 recreational water–associated outbreaks to CDC’s Waterborne Disease and Outbreak Surveillance System (WBDOSS) via the National Outbreak Reporting System (NORS).

The 90 outbreaks resulted in at least 1,788 cases, 95 hospitalizations, and one death. Among 69 (77%) outbreaks associated with treated recreational water, 36 (52%) were caused by Cryptosporidium. Among 21 (23%) outbreaks associated with untreated recreational water, seven (33%) were caused by Escherichia coli (E. coli O157:H7 or E. coli O111). Guidance, such as the Model Aquatic Health Code (MAHC), for preventing and controlling recreational water–associated outbreaks can be optimized when informed by national outbreak and laboratory (e.g., molecular typing of Cryptosporidium) data.

A recreational water–associated outbreak is the occurrence of similar illnesses in two or more persons, epidemiologically linked by location and time of exposure to recreational water or recreational water–associated chemicals volatilized into the air surrounding the water. Public health officials in the 50 states, the District of Columbia, U.S. territories, and Freely Associated States* voluntarily report outbreaks of recreational water–associated illness to CDC. In 2010, waterborne outbreaks became nationally notifiable. This report summarizes data on recreational water–associated outbreaks electronically reported by October 30, 2014 to CDC’s WBDOSS ( for 2011 and 2012 via NORS.† Data requested for each outbreak include the number of cases,§ hospitalizations, and deaths; etiology; setting (e.g., hotel) and venue (e.g., hot tub or spa) where the exposure occurred; earliest illness onset date; and illness type. All outbreaks are classified according to the strength of data implicating recreational water as the outbreak vehicle (1). ¶Outbreak reports classified as Class I have the strongest supporting epidemiologic, clinical laboratory and environmental health data, and those classified as Class IV, the weakest. Classification does not assess adequacy or completeness of investigations.** Negative binomial regression (PROC GENMOD in SAS 9.3 [Cary, NC]) was used to assess trends in the number of outbreaks over time.

For the years 2011 and 2012, public health officials from 32 states and Puerto Rico reported 90 recreational water–associated outbreaks ( (Figure 1), which resulted in at least 1,788 cases, 95 (5%) hospitalizations, and one death. Etiology was confirmed for 73 (81%) outbreaks: 69 (77%) outbreaks were caused by infectious pathogens, including two outbreaks with multiple etiologies, and four (4%) by chemicals (Table). Among the outbreaks caused by infectious pathogens, 37 (54%) were caused by Cryptosporidium. On the basis of data reported to CDC, 37 (41%) of the 90 outbreak reports were categorized as class IV.

Outbreaks associated with treated recreational water accounted for 69 (77%) of the 90 outbreaks reported for 2011–2012, and resulted in at least 1,309 cases, 73 hospitalizations, and one reported death. The median number of cases reported for these outbreaks was seven (range: 2–144 cases). Hotels (e.g., hotel, motel, lodge, or inn) were the setting of 13 (19%) of the treated recreational water–associated outbreaks. Twelve (92%) of these 13 outbreaks started outside of June–August; ten (77%) were at least in part associated with a spa. Among the 69 outbreaks, 36 (52%) were caused by Cryptosporidium. The 69 outbreaks had a seasonal distribution, with 42 (61%) starting in June–August (Figure 1). Acute gastrointestinal illness was the disease manifestation in 34 (81%) of these summer outbreaks, with Cryptosporidium causing 32 (94%) of them. Since 1988, the year that the first U.S. treated recreational water–associated outbreak of cryptosporidiosis was detected (2,3) (Figure 2), the number of these outbreaks reported annually (range: 0–40 outbreaks) has significantly increased (negative binomial regression; p<0.001). Incidence of these cryptosporidiosis outbreaks has also, at least in part, driven the significant increase (negative binomial regression; p<0.001) in the overall number of recreational water–associated outbreaks reported annually (range: 6–84).

caddyshack.pool.poop-1For 2011–2012, 21 (23%) outbreaks were associated with untreated recreational water. These outbreaks resulted in at least 479 cases and 22 hospitalizations. The median number of cases reported for these outbreaks was 16 (range: 2–125). Twenty (95%) of these outbreaks were associated with fresh water; 18 (86%) began in June–August; and seven (33%) were caused by E. coli O157:H7 or O111. One outbreak associated with exposure to cyanobacterial toxins was reported.


Cryptosporidium continues to be the dominant etiology of recreational water–associated outbreaks. Half of all treated recreational water–associated outbreaks reported for 2011–2012 were caused by Cryptosporidium. Among treated recreational water–associated outbreaks of gastrointestinal illness that began in June–August, >90% were caused by Cryptosporidium, an extremely chlorine-tolerant parasite that can survive in water at CDC-recommended chlorine levels (1–3 mg/L) and pH (7.2–7.8) for >10 days (4). In contrast, among 14 untreated recreational water–associated outbreaks of gastrointestinal illness starting in June–August, 7% (one) were caused by Cryptosporidium. The decreased diversity of infectious etiologies causing treated recreational water–associated outbreaks is likely a consequence of the aquatic sector’s reliance on halogen disinfection (e.g., chlorine or bromine) and maintenance of proper pH, which are well documented to inactivate most infectious pathogens within minutes (5). Continued reporting of treated recreational water–associated outbreaks caused by chlorine-intolerant pathogens (e.g., E. coli O157:H7 and norovirus) highlights the need for continued vigilance in maintaining water quality (i.e., disinfectant level and pH), as has been recommended for decades (5).

In the United States, codes regulating public treated recreational water venues are independently written and enforced by individual state or local agencies; the consequent variation in the codes is a potential barrier to preventing and controlling outbreaks associated with these venues. In August 2014, CDC released the first edition of MAHC (, a comprehensive set of science-based and best-practice recommendations to reduce risk for illness and injury at public, treated recreational water venues. MAHC represents the culmination of a 7-year, multi-stakeholder effort and is an evolving resource that addresses emerging public health threats, such as treated recreational water-associated outbreaks of cryptosporidiosis, by incorporating the latest scientifically validated technologies that inactivate or remove infectious pathogens. For example, MAHC recommends additional water treatment (e.g., ultraviolet light or ozone) to inactivate Cryptosporidium oocysts at venues where WBDOSS data indicate there is increased risk for transmission. MAHC recommendations can be voluntarily adopted, in part or as a whole, by state and local jurisdictions.

The number of reported untreated recreational water–associated outbreaks confirmed or suspected to be caused by cyanobacterial toxins has decreased, from 11 (2009–2010) to one (2011–2012) (6). This decrease is likely the result of a decrease in outbreak reporting rather than a true decrease in incidence. CDC is currently developing a mechanism for reporting algal bloom–associated individual cases through NORS to better characterize their epidemiology.

The findings in this report are subject to at least two limitations. First, the outbreak counts presented are likely an underestimate of actual incidence. Many factors can present barriers to the detection, investigation, and reporting of outbreaks: 1) mild illness; 2) small outbreak size; 3) long incubation periods; 4) wide geographic dispersion of ill swimmers; 5) transient nature of contamination; 6) setting or venue of outbreak exposure (e.g., residential backyard pool); and 7) potential lack of communication between those who respond to outbreaks of chemical etiology (e.g., hazardous materials personnel) and those who usually report outbreaks (e.g., infectious disease epidemiologists). Second, because of variation in public health capacity and reporting requirements across jurisdictions, those reporting outbreaks most frequently might not be those in which outbreaks most frequently occur.

Increasingly, molecular typing tools are being employed to understand the epidemiology of waterborne disease and outbreaks. Most species and genotypes of Cryptosporidium are morphologically indistinguishable from one another, and only molecular methods can distinguish species and subtypes and thereby elucidate transmission pathways (7,8). Systematic national genotyping and subtyping of Cryptosporidium in clinical specimens and environmental samples through CryptoNet ( can identify circulating Cryptosporidium species and subtypes and help identify epidemiologic linkages between reported cases. Molecular typing could substantially help elucidate cryptosporidiosis epidemiology in the United States and inform development of future guidance to prevent recreational water–associated and other outbreaks of cryptosporidiosis (9,10).


State, territorial, local, and Freely Associated State waterborne disease coordinators, epidemiologists, and environmental health personnel; Lihua Xiao, Sarah A. Collier, Kathleen E. Fullerton, Division of Foodborne, Waterborne, and Environmental Diseases, National Center for Emerging and Zoonotic Infectious Diseases, CDC.

1Division of Foodborne, Waterborne, and Environmental Diseases, National Center for Emerging and Zoonotic Infectious Diseases, CDC; 2Environmental Protection Agency; 3Oak Ridge Institute for Science and Education, Oak Ridge, Tennessee.

Corresponding author: Michele C. Hlavsa,, 404-71

Outbreaks of Illness Associated with Recreational Water — United States, 2011–2012

Centers for Disease Control and Prevention Morbidity and Mortality Weekly Report

Michele C. Hlavsa, MPH; Virginia A. Roberts, MSPH; Amy M. Kahler, MS; Elizabeth D. Hilborn, DVM; Taryn R. Mecher, MPH; Michael J. Beach, PhD; Timothy J. Wade, PhD; Jonathan S. Yoder, MPH

Multistate outbreak of listeriosis linked to Blue Bell Creameries products

After that Jimmy John’s sandwich, how about some Blue Bell ice cream?


The U.S. Centers for Disease Control and Prevention has issued its final report, noting:

  • A total of 10 people with listeriosis related to this outbreak were reported from 4 states: Arizona (1), Kansas (5), Oklahoma (1), and Texas (3). All ill people were hospitalized. Three deaths were reported from Kansas (3).
  • On April 20, 2015, Blue Bell Creameries voluntarily recalledall of its products currently on the market made at all of its facilities, including ice cream, frozen yogurt, sherbet, and frozen snacks.
  • Consumers should not eat any recalled Blue Bell brand products, and institutions and retailers should not serve or sell them. This is especially important for people at higher risk for listeriosis. These products are frozen, so consumers, institutions, and retailers should check their freezers.
  • On May 7, 2015, the U.S. Food and Drug Administration released the findings from recent inspections at the Blue Bell production facilities in Brenham, Texas[PDF – 4 pages]Broken Arrow, Oklahoma[PDF – 11 pages]; and Sylacauga, Alabama[PDF – 5 pages].

US CDC says antibiotic resistance in foodborne germs is an ongoing threat

It’s nice that the scientists and PR-types at CDC, who for decades insisted that foodborne illness be publicly attributed to bacteria or viruses or parasites, out of scientific accuracy, are now referring to them as germs.

Family guy barfPeople really care about what is going to make them barf, not what it’s called.

Antibiotic resistance in foodborne germs, an ongoing public health threat, continued to show both positive and challenging trends in 2013, according to human illness data posted online today by the Centers for Disease Control and Prevention (CDC). Efforts are underway to curb the injudicious use of antibiotics, but each year, antibiotic-resistant infections from foodborne germs cause an estimated 440,000 illnesses in the United States.

The National Antimicrobial Resistance Monitoring System (NARMS) tracks changes in the antibiotic resistance of six types of common foodborne germs found in ill people, retail meats, and food animals. In 2013, NARMS tested more than 5,000 germs from sick people for antibiotic resistance and compared them with previous years’ data to assess changes in resistance patterns.

Among the findings in the new NARMS report on human illnesses:

The good news is that multidrug resistance (resistance to 3 or more classes of antibiotics) in Salmonella overall stayed steady, remaining at 10 percent of infections.

However, resistance in some types of Salmonella is increasing. For example, multidrug resistance in a common Salmonella serotype called I4,[5],12:i:- was 46 percent, more than double the rate from two years before. In the United States, resistance in this serotype to four drugs (ampicillin, streptomycin, sulfonamides, and tetracycline) rose from 18 percent in 2011 to 46 percent in 2013. Human illness with this serotype has been linked to animal exposure and consumption of pork or beef, including meats purchased from live animal markets.

NARMS also tests Campylobacter, another germ that is transmitted by food. One in four Campylobacter samples from sick people are still resistant to quinolones like ciprofloxacin.

Most Salmonella and Campylobacter infections cause diarrheal illness that resolves within a week without antibiotics. These germs can also cause infection of the bloodstream and other sites. In more serious infections and when germs are resistant, antibiotics may be ineffective, increasing the chance of a severe illness.

The 2013 NARMS Annual Human Isolates Report is now available at

Multistate outbreak of human Salmonella Muenchen infections linked to contact with pet crested geckos

We were eating dinner on the balcony – as you do in Brisbane – and our neighbor was showing his grandchild the geckos living in the utility box.

creasted-geckos-325There was no contact, but I still see a Salmonella factory.

According to the U.S Centers for Disease Control, as of May 13, 2015, a total of 20 persons infected with the outbreak strain of Salmonella Muenchen have been reported from 16 states since January 1, 2014.

Three (15%) of these ill persons have been hospitalized. No deaths have been reported.

Epidemiologic, laboratory, and traceback findings have linked this outbreak of human Salmonella infections to contact with pet crested geckos purchased from multiple pet stores in different states.

Ten (91%) of 11 ill persons interviewed reported contact with a crested gecko in the week before their illness began.

CDC’S National Antimicrobial Resistance Monitoring System (NARMS) laboratory conducted antibiotic resistance testing on Salmonella Muenchen isolates collected from two ill people infected with the outbreak strain.

Both isolates were susceptible to all antibiotics tested on the NARMS panel.

Antibiotic resistance testing on additional isolates collected from ill people infected with the outbreak strain is ongoing.

Norovirus from swimming in a lake

In July, 2014 a norovirus outbreak linked to a lake near Portland, Oregon sickened 70 people. Those who swam in the lake were 2.3 times more likely to develop vomiting or diarrhea than those who visited the park but didn’t go in the water.

kids.cottage.00More than half of those who got ill were children between 4–10 years old. Experts believe the outbreak began after a swimmer infected with norovirus had diarrhea or vomited in the water and other swimmers swallowed the contaminated water. To prevent other people from getting sick, park officials closed the lake to swimmers for 10 days.

“Children are prime targets for norovirus and other germs that can live in lakes and swimming pools because they’re so much more likely to get the water in their mouths,” said Michael Beach, Ph.D, CDC’s associate director for healthy water. “Keeping germs out of the water in the first place is key to keeping everyone healthy and helping to keep the places we swim open all summer.”

 Norovirus Outbreak Associated with a Natural Lake Used for Recreation — Oregon, 2014


Morbidity and Mortality Weekly Report

Amy Zlot, MPH, Maayan Simckes, MPH, Jennifer Vines, MD, Laura Reynolds, MPH, Amy Sullivan PhD, Magdalena Kendall Scott, MPH, J. Michael McLuckie, Dan Kromer, MPA, Vincent R. Hill, PhD, Jonathan S. Yoder, MPH, Michele C. Hlavsa, MPH


CDC: Reductions in foodborne STEC, some Salmonella, not so much the others in 2014

It’s becoming an annual rite: the U.S. Centers for Disease Control reports progress on some bugs, and no progress on others in the food supply. Batz could probably explain it better than I.

cdc.surveillance.2014But the data is important to focus policy efforts and make improvements.

In 2014, rates of infection from a serious form of E. coli and one of the more common Salmonella serotypes decreased compared with the baseline period of 2006-2008. Meanwhile, some other less common types of Salmonella increased. Campylobacter and Vibrio rose again in 2014, continuing the increase observed during the past few years. Today’s report summarizes the rates of infection per 100,000 population and tracks illness trends for key foodborne illnesses.

Infection with Shiga-toxin producing E. coli O157, which can sometimes lead to kidney failure, decreased 32 percent when compared with 2006-2008 and 19 percent when compared with the most recent three years. These infections are often linked to consumption of undercooked ground beef and raw leafy vegetables. Salmonella Typhimurium, which has been linked to poultry, beef, and other foods, was 27 percent lower than it was in 2006-2008, continuing a downward trend begun in the mid-1980s. Two other less common types of Salmonella, Javiana and Infantis, more than doubled for reasons that are unclear. Salmonella Javiana is concentrated in the southeastern United States, but has been spreading within the Southeast and to other areas of the country. However, when all Salmonella serotypes are combined, there was no change in 2014.

Campylobacter increased 13 percent and Vibrio increased 52 percent compared with 2006-2008. Yersinia has declined enough to meet the Healthy People 2020 goal.

cdc.surv.14The data are from FoodNet, CDC’s active surveillance system that tracks nine common foodborne pathogens in 10 states and monitors trends in foodborne illness in about 15 percent of the U.S. population. Today’s report compares the 2014 frequency of infection with the frequency in the baseline period 2006-2008 and in the three most recent years. Overall in 2014, FoodNet logged just over 19,000 infections, about 4,400 hospitalizations, and 71 deaths from the nine foodborne germs it tracks. Salmonella and Campylobacter were by far the most common– accounting for about 14,000 of the 19,000 infections reported.

“We’re cautiously optimistic that changes in food safety practice are having an impact in decreasing E. coli and we know that without all the food safety work to fight Salmonella that more people would be getting sick with Salmonella than we are seeing now,,” said Robert Tauxe, M.D., deputy director of CDC’s Division of Foodborne Waterborne and Environmental Diseases. “The increasing use of whole genome sequencing to track foodborne illness cases will also help; however, much more needs to be done to protect people from foodborne illness.” 

The recent decline in the incidence of Shiga toxin-producing E. coli (STEC) O157 follows several years of increasing scrutiny for beef products. Since 1994, the Food Safety and Inspection Service of the U.S. Department of Agriculture has taken STEC O157:H7 extremely seriously and made a number of changes in its regulatory oversight of the beef industry to protect public health.”We are encouraged by the reduction of STEC O157:H7 illnesses, which reflects our science-based approach to beef inspection, and we look forward to seeing further reductions in Salmonella and Campylobacter infections as our improved standards for poultry take effect later this year, ” said Al Almanza, Deputy Under Secretary for Food Safety at USDA. “Data sources like FoodNet allow us to be strategic in developing our food safety policies, and we will do everything within our power to keep reducing cases of foodborne illness from all meat and poultry products.”

Under the provisions of the FDA Food Safety Modernization Act, the U.S. Food and Drug Administration is planning to publish major new regulations in 2015. The regulations are geared toward ensuring produce safety, implementing preventive controls on processed foods, and improving the safety of imported foods.

“Prevention of illness is the fundamental goal of our new rules under the FDA Food Safety Modernization Act,” said Michael Taylor, deputy commissioner for Foods and Veterinary Medicine at FDA.  “We have worked with a wide range of stakeholders to devise rules that will be effective for food safety and practical for the many diverse elements of our food system. Once the rules are fully implemented, FoodNet will help us evaluate their impact.”

The FoodNet report also includes results of culture-independent diagnostic tests (a new method for diagnosing intestinal illnesses without needing to grow the bacteria) done in the many hospital laboratories in the FoodNet sites. In 2014, the results of more than 1,500 such tests were reported. More than two-thirds of the tests were for Campylobacter. Other tests performed were for STEC, Salmonella, Shigella and Vibrio. Some of the tests had a positive result. However, the infections were not confirmed by culture, and so CDC experts did not include them in the overall FoodNet results for 2014.

Public health has better things to do: Increased outbreaks associated with nonpasteurized milk

The number of US outbreaks caused by nonpasteurized milk increased from 30 during 2007-2009 to 51 during 2010-2012. Most outbreaks were caused by Campylobacter spp. (77%) and by nonpasteurized milk purchased from states in which nonpasteurized milk sale was legal (81%).

rw.milk.outbreaks.2Regulations to prevent distribution of nonpasteurized milk should be enforced.

Increased outbreaks associated with nonpasteurized milk, United States, 2007-2012.

Emerging Infectious Diseases, 2015 Jan;21(1):119-22. doi: 10.3201/eid2101.140447.

Mungai EA, Behravesh CB, Gould LH.

Beaver fever: Giardiasis surveillance, US 2011–2012

Problem/Condition: Giardiasis is a nationally notifiable gastrointestinal illness caused by the protozoan parasite Giardia intestinalis.

Reporting Period: 2011–2012.

beaver.feverDescription of System: Forty-four states, the District of Columbia, New York City, the Commonwealth of Puerto Rico, and Guam voluntarily reported cases of giardiasis to CDC through the National Notifiable Diseases Surveillance System (NNDSS).

Results: For 2011, a total of 16,868 giardiasis cases (98.8% confirmed and 1.2% nonconfirmed) were reported; for 2012, a total of 15,223 cases (98.8% confirmed and 1.3% nonconfirmed) were reported. In 2011 and 2012, 1.5% and 1.3% of cases, respectively, were associated with a detected outbreak. The incidence rates of all reported cases were 6.4 per 100,000 population in 2011 and 5.8 per 100,000 population in 2012. This represents a slight decline from the relatively steady rates observed during 2005–2010 (range: 7.1–7.9 cases per 100,000 population). In both 2011 and 2012, cases were most frequently reported in children aged 1–4 years, followed by those aged 5–9 years and adults aged 45–49 years. Incidence of giardiasis was highest in Northwest states. Peak onset of illness occurred annually during early summer through early fall.

Interpretation: For the first time since 2002, giardiasis rates appear to be decreasing. Possible reasons for the decrease in rates during 2011–2012 could include changes in transmission patterns, a recent change in surveillance case definition, increased uptake of strategies to reduce waterborne transmission, or a combination of these factors. Transmission of giardiasis occurs throughout the United States, with more frequent diagnosis or reporting occurring in northern states. Geographical differences might suggest actual regional differences in giardiasis transmission or variation in surveillance capacity across states. Six states did not report giardiasis cases in 2011–2012, representing the largest number of nonreporting states since giardiasis became nationally notifiable in 2002. Giardiasis is reported more frequently in young children, which might reflect increased contact with contaminated water or ill persons, or a lack of immunity.

Public Health Action: Educational efforts to decrease exposure to unsafe drinking and recreational water and prevent person-to-person transmission have the potential to reduce giardiasis transmission. The continual decrease in jurisdictions opting to report giardiasis cases could negatively impact the ability to interpret national surveillance data; thus, further investigation is needed to identify barriers to and facilitators of giardiasis case reporting. Existing state and local public health infrastructure supported through CDC (e.g., Epidemiology and Laboratory Capacity grants and CDC-sponsored Council of State and Territorial Epidemiologists Applied Epidemiology Fellows) could provide resources to enhance understanding of giardiasis epidemiology.