Appendices

Interpretation of an Epidemic Curve

An epidemic curve (epi curve) shows progression of illnesses in an outbreak over time. Epi curves depict when people became ill by day, week, or month. This information is often shown by the week people became ill. The horizontal axis (x-axis) is the week when a person became ill, also called the week of illness onset. The vertical axis (y-axis) is the number of persons that meet the case definition each week. During ongoing outbreak investigations, the epi curve is updated as new data becomes available. There are several important issues in understanding and interpreting epi curves during ongoing outbreak investigations.

Example Epi Curve

Example of an epidemic (epi) curve from an outbreak investigation of cryptosporidiosis in Milwaukee, 1993.

Example Epi Curve of an onset of watery diarrhea in the Greater Milwaukee Area

Figure 3. Reported Date of the Onset of Watery Diarrhea during the Period from March 1 through April 28, 1993, in 436 Cases of Infection Identified by a Random-Digit Telephone Survey of the Greater Milwaukee Area.

  • There is an inherent delay between the date that an illness starts and the date that the case is reported to public health authorities. This delay can be several weeks; someone who became ill last week is very unlikely to have his or her infection reported to public health authorities by now, and someone who became ill 3 weeks ago may have just had it reported.
  • Some background cases of illness are likely to occur that would have happened even without an outbreak. This makes it difficult to say exactly which case is the first in an outbreak. Epidemiologists typically focus on the first recognized cluster (group) of illnesses, rather than the very first case. Due to the inherent reporting delay described above, the cluster is sometimes not detected until several weeks after the persons initially became ill.
  • For some cases, the date of illness onset is not known because it takes time before someone from the health department can do an interview to ask for this information. Sometimes, this interview never occurs. If investigators know the date that a specimen from an ill person arrived in the laboratory for testing, they may estimate the date of illness onset as 2 or 3 days before the specimen collection or submission date.
  • It can be difficult to determine when cases start to decline because of the reporting delay. This information will become clearer as time passes.

It can be difficult to say when the outbreak is over because of the reporting delay. The delay means that the curve for the most recent 3 weeks always looks like the outbreak could be ending even during an active outbreak. The full shape of the curve is clear only after the outbreak is over.

Confirming Diagnosis

The tables below are intended to assist health departments with developing hypotheses about the possible cause of waterborne diseases and outbreaks during the outbreak investigation. If some of the clinical or exposure information is known, it might direct laboratory testing. Additionally, historical water exposure information might provide clues for developing investigation tools.

Infectious Disease – Bacterial
Etiology Incubation period Signs and symptoms Duration of illness Possible exposure categoriesi Notes on water exposure
Bacillus anthracis 2 days – weeks Nausea, vomiting, malaise, bloody diarrhea, abdominal pain Weeks Water
Food
Animal contact
Environment
Drinking, recreational: naturally-occurring spores, intentional contamination
Campylobacter jejuni 2 – 7 days Fever, cramps, vomiting, diarrhea (can be bloody) 2 – 10 days Water
Food
Animal contact
Environment
Drinking: human feces, animal (mammals, poultry, waterfowl) feces
Cronobacter spp. Unknown Diarrhea possible, sometimes bloody; sepsis; meningitis Variable; meningitis associated with brain abscesses and prolonged illness Water
Food
Environment
Drinking: powdered infant formula prepared with contaminated water
Enterotoxigenic Escherichia coli (ETEC) 12 – 72 hours Cramps, watery diarrhea, nausea and vomiting less common 3 – >7 days Water
Food
Animal contact
Person-to-person
Drinking, recreational: human feces, domestic sewage, animal feces
Shiga toxin-producing Escherichia coli (STEC) 1 – 8 days Abdominal pain, vomiting, watery diarrhea that becomes bloody. Usually no fever. 5 – 10 days Water
Food
Animal contact
Person-to-person
Drinking, recreational: human feces, domestic sewage, animal (cattle) feces
Francisella tularensis 1 – 10 days Tularemia can present as cutaneous ulcers, fever, chills, muscle aches, painful lymphadenopathy, headache, vomiting, abdominal pain, rash, general malaise. Symptoms may last for several days, remit, and then begin again. Learn moreexternal icon Up to >3 months Water
Animal contact
Environment
Drinking, recreational: animal (wild mammals) blood and tissue, arthropods
Legionella spp. 2 – 10 days for Legionellosis
24 – 72 hours for Pontiac fever
Legionnaire’s disease: Pneumonia, diarrhea, cough, vomiting, abdominal pain

Pontiac fever: milder self-limiting flu-like illness (fever, headache, and muscle aches)

LD: Variable

Pontiac fever: 2 – 5 days

Water
Environment
Drinking, recreational, other exposures to water: aerosolized mist from cooling towers, evaporative condensers, whirlpools, fountains, large plumbing systems, and respiratory therapy equipment- inadequate disinfection/biofilm removal, dead end systems, lack of maintenance
Leptospira spp. 4 – 19 days
Learn more
Fever, chills, headaches, diarrhea, vomiting, jaundice, kidney failure, stiff neck. Learn moreexternal icon Variable Water
Food
Animal contact
Environment
Drinking, recreational, other exposures to water: animal (rodents, cattle, pigs, dogs, horses, wild animals) urine and body fluids contamination water sources, cisterns exposure
Non-tuberculous Mycobacterium spp. Weeks – months Insidious onset of cough with purulent sputum, fever, weight loss, night sweats; lymphadenitis, skin, soft tissue, and skeletal infections; disseminated disease in severely immunocompromised hosts, such as AIDS patients. Learn moreexternal icon Variable Water
Food
Animal contact
Person-to-person
Environment
Drinking, recreational, other exposures to water: naturally-occurring (fresh/saltwater), biofilm-associated
Plesiomonas shigelloides 24 – 48 hours Diarrhea (usually watery), vomiting, fever, chills, and abdominal pain. Usually a self-limiting infection. Learn more pdf icon[PDF – 1 page]external icon 1 – 7 days Water
Food
Animal contact
Environment
Drinking: naturally-occurring (fresh/saltwater), potentially domestic sewage
Pseudomonas aeruginosa 8 hours – >5 days
Learn moreexternal icon
Generalized rash, pustules, ear infection, corneal infection, healthcare-associated (UTI, pneumonia, septicemia, etc.) Variable Water
Person-to-person
Environment
Drinking, recreational, other exposures to water: biofilm-associated, inadequate cleaning/chlorination- hot tub, pools, and water vessels
Salmonella spp. 6 – 72 hours
7 – 28 days for typhoid/
paratyphoid fevers
Fever, cramps, vomiting, diarrhea. S. Typhi and S. Paratyphi cause typhoid, characterized by fever, headache, constipation, myalgias, and abdominal pain 4 – 7 days Water
Food
Animal contact
Person-to-person
Environment
Drinking: human feces and urine, domestic sewage, animal (poultry, amphibians, reptiles, rodents) feces; farm runoff, meat/poultry processing plant wastes, back siphonage, inadequate disinfection
Shigella spp. 12 hours – 7 days Fever, cramps, vomiting, tenesmus, diarrhea that can be bloody and mucoid 4 – 7 days Water
Food
Person-to-person
Environment
Drinking, recreational: human feces, domestic sewage; back siphonage, inadequate disinfection
Vibrio cholerae (toxin) 1 – 5 days Profuse watery diarrhea (rice-water stool), vomiting, reduced skin turgor. Severe dehydration and death can occur within hours 3 – 7 days Water
Food
Environment
Drinking: human feces, domestic sewage, naturally-occurring (saltwater, warm weather)
Vibrio parahaemolyticus 2 – 48 hours Cramps, nausea, vomiting, watery diarrhea, wound/ear infections 2 – 5 days Water
Food
Environment
Recreational: naturally-occurring (brackish/saltwater, warm weather) + open wound
Vibrio vulnificus Around 16 hours Vomiting, abdominal pain, diarrhea, bacteremia, wound infections. More common in the immunocompromised and in those with chronic liver disease 2 – 8 days Water
Food
Environment
Recreational: naturally-occurring (brackish/saltwater, warm weather) + open wound + immunosuppression
Yersinia enterocolytica and Y. pseudo-tuberculosis 1 – 7 days Appendicitis-like symptoms (fever, abdominal pain, diarrhea, vomiting), typically in older children and young adults. Y. pseudotuberculosis may cause scarlitiniform rash 1 – 3 weeks Water
Food
Animal contact
Person-to-person
Environment
Drinking: animal (pigs, rodents) feces and urine

i Please refer to National Outbreak Reporting System (NORS) Guidance document for more information regarding exposure categories.

Viral
Etiology Incubation period Signs and symptoms Duration of illness Possible exposure categoriesi Notes on water exposure
Hepatitis A 15 – 50 days Diarrhea, jaundice, flu-like symptoms Variable
2 weeks- 3 months
Water
Food
Animal contact
Person-to-person
Drinking: human feces and urine, domestic sewage; back siphonage, inadequate disinfection
Norovirus 12 – 48 hours Nausea, vomiting, watery, large-volume diarrhea; fever rare 1 – 3 days Water
Food
Animal contact
Person-to-person
Drinking: human feces and vomit, domestic sewage
Rotavirus 1 – 3 days Low-grade fever, vomiting, severe watery diarrhea, dehydration, inappetence.  Temporary lactose intolerance may occur 4 – 8 days Water
Food
Animal contact
Person-to-person
Drinking, recreational: human feces, domestic sewage
Other viral agents (astroviruses, caliciviruses, adenoviruses, parvoviruses) 12 – 72 hours Headache, fever possible, nausea, vomiting, diarrhea, malaise, rash 2 – 9 days Water
Food
Animal contact
Person-to-person
Drinking, recreational: human feces, domestic sewage

i Please refer to National Outbreak Reporting System (NORS) Guidance document for more information regarding exposure categories.

Parasitic
Etiology Incubation period Signs and symptoms Duration of illness Possible exposure categoriesi Notes on water exposure
Acanthamoeba spp. Unknown (possibly weeks or months) Encephalitis: Fever, headache, mental status change, loss of coordination, other neurologic problems
Cutaneous: firm, reddish nodules, non-healing ulcers, or abscesses
Other symptoms depending on organ system affected.
Variable (weeks, months, encephalitis is usually fatal.) Water
Environment
Drinking: used for nasal or sinus rinsing
Acanthamoeba keratitis Unknown (possibly days or weeks) Eye pain, eye redness, blurred vision, sensitivity to light, sensation of something in the eye, excessive tearing Variable (weeks, months) Water
Environment
Drinking: when used for cleaning and storing contact lenses
Angiostrongylus cantonensis 1 week – >1 month Severe headache, neck stiffness, nausea, vomiting, paresthesias, seizures, other neurologic symptoms weeks to months Water
Food
Drinking: whole animal (snail/slug) and body fluids
Cryptosporidium 2 – 10 days Low-grade fever, cramps, nausea, vomiting, anorexia, prolonged diarrhea (usually watery) In those with Healthy immune systems, symptoms resolve within 2-3 weeks. Might remit and relapse over weeks to months Water
Food
Animal contact
Person-to-person
Environment
Drinking/recreational: human feces, domestic sewage, animal (ruminants, cats, rodents) feces, animal waste; inadequate disinfection/filtration
Cyclospora cayeanensis 1 – 11 days Cramps, nausea, vomiting, anorexia, weight loss, prolonged diarrhea (usually watery), fatigue May remit and relapse over weeks to months Water
Food
Drinking: human feces, domestic sewage
Entamoeba histolytica Few days to several months, typically 2 – 4 weeks Lower abdominal pain, diarrhea (can be bloody), fever, chills, liver abscess May last weeks to months Water
Food
Person-to-person
Environment
Drinking: human feces, domestic sewage; inadequate disinfection/filtration, back siphonage, water and sewer lines in same pits
Giardia 1 – 4 weeks Cramps, diarrhea (pale, greasy, malodorous stool), flatulence, bloating, vomiting, fatigue, fever Days to weeks Water
Food
Animal contact
Person-to-person
Environment
Drinking, recreational: human feces, domestic sewage, animal (beavers, ruminants, birds, dogs, cats, etc.) feces; inadequate disinfection/filtration
Naegleria fowleri 1 – 9 days Headache, fever, nausea, vomiting, neck stiffness, seizures, altered mental status, hallucinations, coma 1-18 days. Almost always fatal Water Recreational, drinking:
Water containing Naegleria fowleri enters the nose
Toxoplasma gondii
(enteric exposure)
4 – 28 days Generally asymptomatic. 20% may develop cervical lymphadenopathy and/or flu-like symptoms. Immunocompromised patients can get CNS disease, myocarditis, or pneumonitis Months Water
Food
Environment
Drinking: animal (cat) feces; inadequate filtration

i Please refer to National Outbreak Reporting System (NORS) Guidance document for more information regarding exposure categories.

Non-infectious etiologies
Etiology Incubation period Signs and symptoms Duration of illness Possible exposure categoriesi Notes on water exposure
Algal toxins Few hours – years May be in stages: vomiting, nausea, vomiting, fever, headache; then liver damage, multiple organ failure. Also, skin irritation, sore throat, muscle and joint pain, mouth ulcers, seizures Variable Water
Food
Environment
Drinking, recreational: naturally-occurring algal blooms (fresh/brackish/saltwater)
Antimony 5 minutes – 8 hours, depending on dose Vomiting, metallic taste Variable Water
Food
Environment
Drinking: natural deposits, industrial contamination (freshwater)
Arsenic 5 minutes – 8 hours, depending on dose Vomiting, cramps, diarrhea, cancer, skin changes, encephalopathy and peripheral neuropathy with high-dose exposure; lower-dose, chronic exposure may not produce GI symptoms Several days Water
Food
Environment
Drinking: natural deposits, agricultural/industrial contamination (freshwater); pesticides, back siphonage, indiscriminate disposal
Cadmium Several months Abdominal pain, salivation, nausea, vomiting, myalgia, kidney damage, lung damage, fragile bones Variable Water
Food
Environment
Drinking: natural deposits, agricultural/mining/industrial contamination (freshwater); galvanized pipe corrosion, volcanic eruptions, metal refinery discharge, waste battery and paint runoff
Copper Minutes – hours, depending on dose Nausea, vomiting with blue or green vomitus, liver or kidney damage Variable Water
Food
Environment
Drinking: natural deposits, agricultural/mining/industrial contamination (freshwater); surface water copper treatment or cooling from power plants, copper pipes and fittings, plumbing material corrosion, fungicides
Lead Weeks – months Irritability, anorexia, abdominal pain, high blood pressure, kidney problems, Developmental delays, cerebral edema, encephalopathy, seizures, death Weeks – months Water
Food
Environment
Drinking: plumbing/water pipe corrosion (freshwater)
Nitrite 1 – 2 hours
Variable, depending on dose and concentration
Headache, dizziness, loss of consciousness, nausea, vomiting, cyanosis (blue baby syndrome- bluish skin and brownish blood) Hours – days Water
Food
Environment
Drinking: natural-occurring, animal feces, agricultural contamination (freshwater); fertilizer
Pesticides (organophosphates or carbamates) Minutes – few hours Headache, nervousness, twitching, convulsions, miosis, cramps, nausea, vomiting, diarrhea, chest pain, cyanosis Weakness/
neuropathy may last weeks
Water
Food
Environment
Drinking: agricultural contamination; back siphonage, seepage following soil-foundation spraying
Sodium fluoride Minutes – months Salty/soapy taste, numbness in mouth, vomiting, diarrhea, dilated pupils, spasms, pallor, shock, collapse; severe hypocalcemia, hyperkalemia, bone disease Variable Water
Food
Environment
Drinking: natural deposits, water treatment plant malfunction

i Please refer to National Outbreak Reporting System (NORS) Guidance document for more information regarding exposure categories.