Giardiasis: A Return of Waterborne Disease?

Major outbreaks of waterborne disease have become so rare that most Americans have never experienced one and cannot even remember when or where the last one occurred. Moreover, thanks to penicillin and other antibiotics, the diseases that occasionally are transmitted through drinking water usually are not life-threatening.

This was not always the case. During and after the Industrial Revolution, urban water supplies in Europe and the United states were unsafe to drink. Cholera, typhoid, paratyphoid, dysentery, and enteritis, often transmitted through contaminated drinking water, were major causes of death. London's 1848-49 waterborne outbreak of cholera, to take a conspicuous example, killed more than 14,000 people.

Problems of drinking water contamination this period were exacerbated by poor sanitary conditions. Thousands of people had flocked to cities from areas in response to employment opportunities offered by newly constructed factories—London alone growing by almost 200,000 people between 1815 and 1820—and living conditions were deplorable. New housing could not keep up with the massive influx, many were homeless, and those fortunate enough to find housing were crowded into small living spaces. Potable drinking water for household use was unavailable, except to the wealthy, and indoor plumbing was nonexistent. Wastes were dumped in courtyards, alleys, and streets. Indeed, human excrement customarily was heaped in piles in the same courtyards where drinking water wells were located, leading frequently to their contamination by human fecal matter.

Moreover, disease transmission was poorly understood. The connection between drinking water and dreaded diseases was not even suspected prior to John Snow's classic study of London's 1848-49 cholera epidemic, which demonstrated the role of fecal pollution of drinking water. Even then, government was slow to act, and considerable effort was spent during the last half of the nineteenth century by doctors, lawyers, engineers, writers, and statesmen to awaken the social and sanitary consciousness of the people and their representatives in government. Thus began the period aptly described by historians as the "Great Sanitary Awakening."

Worry in the United States about waterborne diseases peaked during the early 1900s. Disinfection and other water treatment technologies, implemented by 1920, construction of modern sewage treatment plants between the world wars, and medical progress, especially the discovery of penicillin in 1919 and its practical use in 1941, all have contributed to a complacency in the United States concerning the safety of public drinking water supplies. By mid-century, safe water had become a birthright of nearly all Americans.

Now complacency has turned to concern. In addition to possible chemical contamination of drinking water, a new waterborne disease has emerged that was not considered a threat in the United States as recently as 1950, and many public water supplies are not equipped to protect the public against it. The disease is giardiasis, and it is caused by a protozoan called Giardia lamblia.

New problem or new awareness?

Giardia lamblia is the most commonly identified pathogen in outbreaks of waterborne disease in the United States today. Although seldom fatal, giardiasis has unpleasant and debilitating symptoms that mimic those of dysentery, bacterial food poisoning, and "travelers' diarrhea": abdominal cramps, explosive diarrhea, fatigue, weight loss, flatulence, belching, anorexia, nausea, and vomiting. In rare cases, hospitalization for dehydration may be necessary. Ingestion of as few as ten organisms by susceptible individuals may cause acute symptoms, which appear after an incubation period of six to twenty-eight days. Although the acute stage is thought to last only three to four days, untreated giardiasis often develops into a chronic infection characterized by recurrent periods of acute illness lasting several days. This stage may last for months, leading to malabsorption, increasing debility, and significant weight loss. Indeed, it is the intermittent nature of giardiasis, together with the patient's failure to develop a fever, that distinguish giardiasis from other gastroenteric illnesses.

Is giardiasis new to the United States? Or has it been around for a long time, undetected and perhaps masquerading as the better-known gastroenteritis, which has similar symptoms but no identified cause? The answer is unknown, in part because it is difficult to diagnose giardiasis, but mostly because the protozoan, prior to the mid-1950s, was not even suspected in outbreaks of waterborne disease, and therefore little or no effort was made to identify the organism.

In any event, the pathogenicity of Giardia was first demonstrated in the United States almost thirty-five years ago in a study using prison volunteers. A few years later, in 1955, it was the suspected cause of a massive epidemic (50,000 cases) of acute gastroenteritis in Portland, Oregon. Despite these reports, Giardia was regarded in the United States as a generally harmless organism until the mid-1960s, when it became evident that giardiasis was one of the most common ailments suffered by returning Peace Corps volunteers. It was also found among tourists and diplomatic personnel returning from overseas.

People vary considerably in susceptibility to giardiasis, and some infected with the parasite are asymptomatic. A national parasitology survey in 1977 revealed that nearly 5 percent of the U.S. population harbor the parasite, although most had no overt symptoms. And evidence suggests that resistance to acute symptoms can be acquired through repeated exposure. A survey of outbreaks of the disease in Colorado revealed a lower attack rate among the natives than among recent arrivals to the towns involved, where giardiasis is thought to be an endemic condition.

U.S. outbreaks

The first reported outbreak of giardiasis in the United States (not counting the suspected Portland episode) occurred in Aspen, Colorado, in 1965. Between 1965 and 1982, fifty-four outbreaks involving an estimated 20,000 illnesses were reported. Both numbers almost certainly are large underestimates: in only about half of all reported water-borne disease incidents is a cause ever established, and fully 44 percent of all reported outbreaks of giardiasis through 1982 occurred in Colorado—a pattern that suggests underreporting in the other states. Of the reported outbreaks between 1965 and 1982, 72 percent involved surface water supplies and 13 percent involved ground-water sources, with the remaining sources not reported. Moreover, most of the outbreaks involved either untreated drinking water (twenty-one) or deficiencies in the water distribution or treatment systems (twenty-six). In two cases where water treatment was provided, the treatment was limited to chlorination.

While most giardiasis incidents have involved small water supply systems, most of the reported illnesses through 1982 resulted from four large outbreaks:

• Rome, New York (1974), 4,800 cases
• Vail, Colorado (1978), 5,000 cases
• Lake Havasu, Arizona (1979), 2,000 cases
• Bradford, Pennsylvania (1979), 3,500 cases.

Later large-scale outbreaks occurred in 1983 in Reno, Nevada, and in late 1983 and early 1984 in Luzerne County, Pennsylvania, each involving more than 5,000 cases. The Luzerne episode will be explored in some detail below.

Transmission

Giardia lamblia, which inhabits the upper portion of the mammalian small intestine, passes through four stages in its life cycle: the trophozoite, the precyst, the cyst, and the metacystic trophozoite. The cyst is the infective stage by which the protozoa are transmitted from one mammal to another.

Giardia cysts are hardy. In water, they can survive for two months or more and are much more resistant to chlorine than other viruses or bacteria. Although boiling destroys the cysts, they are especially resistant in cold water, where free chlorine concentrations must be much higher than is desirable for finished water supplies to kill them within reasonable contact times.

Exposure to the cysts can come about through contact with infected animals and persons, as well as through consumption of contaminated water. Animal hosts include beavers, muskrats, coyotes, deer, cattle, raccoons, and dogs and cats (pet ownership is a risk factor for giardiasis). Contamination through food also has been observed. While direct transmission between adults is uncommon, the disease has been known to spread rapidly through some institutional populations, such as mental hospitals and, especially, day-care centers.

For most adults, the largest threat of exposure to Giardia comes from drinking contaminated water. Giardiasis has become a major concern of backpackers and hikers, especially in the mountainous West, but also in the East—Shenandoah National Park in Virginia posts signs warning hikers against drinking the Giardia contaminated water. Since the probability is high of a mountain stream supporting a carrier like beavers, most back-country travelers should avoid drinking water that has not been treated with chemicals or boiled.

The most serious public health problem associated with Giardia is its potential for contaminating drinking water. The protozoan's origin in U.S. water supplies is thought to be humans, although once a watershed has been contaminated, beavers and other animals can serve as significant long-term reservoirs of cysts. Giardia cysts in human and animal feces deposited in upland watersheds eventually find their way to watercourses, where they are transported to water-supply intakes. Once in the water distribution system, the cysts can be consumed by ans unless the cycle of contamination involving animals, water supplies, and people is broken by adequate water treatment.

Water treatment and risk

Because Giardia cysts are so resistant to chlorine, water supply systems that rely only on that chlorination are at substantial risk. Those that use only sand filtration also may be at risk, because cysts may percolate through them. The treatment technology required to prevent viable Giardia cysts from entering finished water supplies appears to consist of chemical pretreatment (coagulation) followed by filtration and chlorination. Diatomaceous earth filters also are effective, as are cartridge filters.

Many U.S. water supply systems—especially the smaller systems—do not have this combination of technology. In rural areas, some 3 million households get their drinking water from surface water supplies, of which only 3 percent have chemical pretreatment followed by filtration. Ironically, the systems most at risk tend to be those with otherwise high-quality raw water supplies, free of turbidity and color for which filtration had not been considered necessary.

EPA responsibility

Under the Safe Drinking Water Act of 1974, the U.S. Environmental Protection Agency (EPA) is required to establish drinking water regulations where there is an adverse effect on public health. Because giardiasis is a newly recognized threat, no federal regulation for controlling its incidence has yet been established; but one is currently under consideration at EPA. On October 11, 1985, the agency proposed a recommended maximum contaminant level of zero for Giardia in finished drinking water supplies. This is only a nonenforceable health goal, but it is the first step in establishing a regulation for Giardia in public water supplies.

Before EPA proceeds with a regulation, its Office of Drinking Water needs information on the costs and benefits of mandating specific water treatment technologies for water supply systems of particular sizes and risks of contamination. For this information, the agency asked Resources for the Future to develop methods for estimating the benefits of avoiding an outbreak of water-borne giardiasis and to apply them in a known outbreak of the disease.

RFF's research

The study involved the measurement of economic losses, which were grouped into four broad categories: losses caused by illness; by actions taken by individuals, businesses, and communities to avoid drinking contaminated water; to government agencies; and to the water supply utility. Other important losses, such as those associated with suffering and pain, were not part of EPA's charge and were not measured in the study.

The 1983-84 outbreak of giardiasis in Luzerne County, Pennsylvania, was selected for RFF's case study for three principal reasons. First, and perhaps most important, was the large size of the episode—an exposed population of 75,000, 370 confirmed cases of giardiasis, 6,000 estimated cases (based on an attack rate of 8 percent estimated by the Pennsylvania Department of Health). Second was the variety of economic losses involved, including effects on individuals, households, businesses, health-care facilities, schools, and local governments. Third was timing. The outbreak was recent (late 1983 and early 1984) and would be fresh in the minds of those affected. Indeed, an official warning to residents to boil their water still was in effect at the time the study began.

The Luzerne outbreak

During the late fall of 1983, giardiasis began to increase among the residents of a number of small Pennsylvania communities near Wilkes-Barre. The source of the disease was determined to be drinking water supplied from the Spring Brook Intake Reservoir in Lackawanna County. To contain the spread of the disease, on December 23, 1983,75,000 consumers in eighteen neighboring communities were advised by the water utility and the Pennsylvania Department of Environmental Resources to boil their drinking water. By late winter, 370 cases of giardiasis had been confirmed to the Pennsylvania Department of Health, making this outbreak one of the largest ever recorded in the United States, as measured by confirmed cases.

The affected area stretches along the east and west banks of the Susquehanna and Lackawanna rivers between Wilkes-Barre and Scranton in upper Luzerne and lower Lackawanna counties (shown shaded in figure 1). The 75,000 residents governed by the December 23 advisory to boil their water were located in the Spring Brook-Hillside Service Area, an integrated water distribution system with two sources of supply—Huntsville Reservoir, with filtration at the Hillside Filtration Plant, and Spring Brook Intake Reservoir, with no filtration prior to distribution.

Although the number of new confirmed cases dropped sharply after people were warned of the contaminated drinking water, the boil-water advisory continued for several months. It was lifted in March, April, and May for two-thirds of the residents, as new sources of uncontaminated water became available, but some 25,000 people remained on the boil-water advisory for almost nine months, until a new pipeline could be constructed from Nesbitt Storage reservoir around the contaminated Spring Brook Intake Reservoir.

Loss estimates

RFF developed and used four survey instruments to collect data for estimating losses. The first, an illness questionnaire, was sent to each of the 370 persons whose cases were confirmed. The second, an "averting-behavior" questionnaire, completed through a random telephone survey of fifty households, determined the defensive strategies people took to avoid the disease. A third questionnaire was mailed to each of the 250 restaurants and bars in the affected area; data on other businesses and institutions (hospitals, day-care centers, dentists, nursing homes, and schools) were collected through personal and telephone interviews. The fourth survey instrument was sent to selected government officials to determine the amount of time they devoted to the outbreak and to obtain data on related expenses. The public record of the Pennsylvania Public Utility Commission was used to obtain the cost of temporary measures taken by the water supply utility to provide a safe water supply.

Table 1 presents a summary of the estimated losses imposed by giardiasis. For both the high and medium estimates (a low estimate also was made but is not reported here), similar assumptions were used to estimate losses, with two exceptions:

• the value of time assigned to home-makers and retirees in providing alternative sources of water (the average wage rate in the area was used in the high estimate and the minimum wage rate in the medium estimate); and
• whether water was boiled and bottled water purchased in conjunction with other activities, and therefore whether the time lost avoiding the disease was attributable solely to avoidance (separate activity was assumed for the high estimate, and joint activity for the medium estimate).

It cannot be concluded from this or any other case study that all public water supplies should or should not use chemical pretreatment followed by filtration and chlorination. There are differences in the risk of contamination of watersheds, in the populations exposed, in the kinds of water system failures that have been associated with giardiasis, and in the per capita costs of water treatment, all of which affect the benefits and costs of water treatment in particular situations. For some water supply systems, the benefits of water treatment will exceed the costs. For others, particularly smaller systems with low risks of contamination, they will not. More information on the factors placing a water supply at risk for Giardia, on the benefits of avoiding waterborne giardiasis in particular situations, and on the per capita costs of treatment for different-size systems is needed to determine if the benefits of investment in a water treatment facility exceed the costs for a specific system.

Giardiasis is unpleasant and uncomfortable, and the actions necessary to avoid it are time consuming and bothersome. Still, it is not in the same class as cholera, typhoid, dysentery, and other diseases that were prevented by water treatment during the first half of this century. Nevertheless, American attitudes about public health and safety and aspirations for a better life have changed a great deal since 1900, and the benefits of protection from giardiasis may well be worth the costs expressed in terms of other opportunities forgone. This is what EPA needs to know and must determine before it issues a drinking water regulation for Giardia.