OUTBREAK INVESTIGATION – EXAMPLE



OUTBREAK INVESTIGATION – EXAMPLE

Outbreak of leptospirosis associated with swimming

Jackson, L.A. et al. Pediatr. Infect. Dis. 1993; 12:48-54

Leptospirosis

• Weil’s disease.

• Caused by Leptospira interrogans, about 240 serologic variants or serovars.

• Infection is common in rodents and other wild and domestic animals. Each serovar has a preferred animal host but each species may be host to one or more serovars.

• Infection usually results from exposure to water contaminated with urine of infected animals.

• The incubation period in humans is 2-20 days.

• Clinical manifestations are variable with varying degrees of severity, subclinical or inapparent infections are common.

• Two clinical types are recognized; icteric and anicteric. The more serious icteric or hepatonephritic type is much less frequent than the anicteric, which often resembles influenza (fever, myalgias, conjunctivitis, stiffness in the neck, nausea and vomiting).

• Infection is diagnosed by isolation of the agent from blood (early) or urine (later), and demonstration of a fourfold or greater rise in antibody titer between acute and convalescent serum samples.

On July 19, 1991, the Illinois Department of Public Health was notified that four adolescent boys were admitted to a hospital in Bureau county in the preceding week with a febrile illness associated with nausea, vomiting and myalgia. Two other boys with a similar illness who were not hospitalized were also identified.

The clinical syndrome and the fact that several of the boys were known to have recently been swimming in local ponds led to the consideration of the diagnosis of leptospirosis.

On July 30, 1991, an investigation was initiated to (1) detect additional cases, (2) determine the source of the outbreak, (3) identify risk factors for disease and (4) identify possible animal sources of the agent.

Steps in the outbreak investigation

1) Verify the diagnosis

The diagnosis was confirmed by serologic tests for all six of the boys.

2) Confirm the existence of an outbreak

From 1970 to 1990 a total of 25 cases of leptospirosis were reported from Illinois; none was reported from Bureau County.

3) Identify and count cases

METHODS

Establish case definition:

• Possible case: fever (>101.3 F) and at least three other symptoms compatible with leptospirosis (headache, chills, myalgia, nausea and/or vomiting and stiff neck) in a person between the ages of 5 and 60 years who did not have another proven etiology (Table 1).

• Confirmed case: the above criteria and a leptospiral microscopic agglutination (MA) titer of >1:100 (Table 2).

Acute and convalescent serum samples were obtained on all cases, and urine samples were obtained at least 48 hours after the last dose of antibiotics.

Case finding:

Hospital records from May 15 through August 1 were reviewed for the four area hospitals in Bureau and LaSalle counties. In addition, physicians in Bureau and LaSalle counties were contacted and asked to report patients seen in the last month with a syndrome similar to the case patients. Patients found by either method who met the definition of a possible case were contacted, interviewed and asked to submit a serum sample for confirmatory serologic testing.

The case definition incorporates elements of person, place and time, and uses clinical and laboratory criteria. Asympyomatic cases were excluded.

RESULTS

Only the 6 boys initially identified met the definition of a confirmed case. Four other individuals, detected by case finding methods met the definition of a possible case; however, all were seronegative.

Duration of symptoms ranged from 6-25 days with a mean of 13.8 days.

Cases had rising antibody titers to grippotyphosa (Table 2) and the agent was isolated from the urine of 2 cases. Persons infected with grippotyphosa may have titers to australis because of cross-agglutination reactions.

Note: When the agent is difficult to recover or detect, serology is usually used as a proxy for infection by the agent. When using antibody titers, the usual criteria is a four-fold rise in titer between acute (during illness) and convalescent (several weeks later) sera or a titer indicative of acute infection in a single serum sample (Table 2).

4) Orient data in terms of time, place and person

Person: The six cases were males, 12-16 years old (Table 3).

Place: Five of the cases were from Dalzell and one was from Oglesby.

Time: Onset of illness was July 7-18 (Fig. 2). Since the disease has been identified and the usual incubation period is known, the probable time period of exposure can be estimated from the epidemic curve (Fig. 3). Individuals with a single exposure can provide valuable information about contamination of the source. The epidemic curve is indicative of a common source outbreak, either continuous or intermittent.

5) Formulate and test hypotheses

METHODS

Because 5 of the 6 cases were residents of Dalzell, a retrospective cohort study of children in Dalzell was conducted to identify the source of the outbreak and risk factors for disease. The cohort was defined as all residents of the town enrolled in grades 7 through 11 (corresponding to the grades of the cases) in the 1991-92 school year. A questionnaire was administered to cohort members and one of their parents. The parent’s were asked questions about the child’s employment, recent illnesses, pets, visits to physicians and antibiotic use. The child was then asked about places where he/she had been swimming in June or July, swimming behaviors, animal contacts, outdoor activities and sports.

Preliminary analysis of the cohort study implicated two swimming areas, Steel Tunnel Pond (STP) and Black Bridge Pond (BBP) (Fig 1). At a second interview, children swimming in either location were shown a calendar and asked to mark down the dates of swimming in STP or BBP.

In addition, 43 of the 50 well cohort members had serum samples obtained for leptospiral serologic testing on August 12, 1991, 5 weeks after the onset of the outbreak (done to determine subclinical infections).

Epi-Info was used for data analysis. Results were analyzed by Mann-Whitney U test and Fisher’s exact test.

RESULTS

Questionnaires were administered to 55 of 58 (95%) cohort members (5 cases and 50 well cohort members).

Swimming in two areas, STP and BBP, in June or July, was associated with disease (Table 3). To define more accurately the time period during which cases may have acquired infection, the incubation period for leptospirosis and the dates of illness were used to determine a period of possible exposure to the agent. The incubation period ranges from 2-20 days. Because the onset of illness for cases was July 7-18, exposure resulting in illness for cases presumably occurred June 17-July 16. Swimming in STP but not in BBP during this period was associated with disease (Table 3). Hence, for all cases to be infected, the agent must have been in the pond on June 30 and July 12 at a minimum (Fig. 2).

There was no significant difference (P = 0.36, Mann-Whitney U test) in the number of days swimming between cases and cohort members in STP between June 17-July 16. Furthermore, no swimming behaviors, such as duration and frequency of swimming, swallowing or immersing the head, were associated with disease.

Six of the 43 cohort members that had blood drawn for serologic testing had a titer of 1:100 to one or more serovars. Two of the 6 had been swimming in STP June 17-July 16. Swimming in STP June 17-July 16 was not associated with a positive titer in cohort members (RR = 1.3, 95% CI = 0.3-6.1). None of the 6 with positive titers was ill in July.

6) Conduct animal/environmental studies to identify source of the agent

METHODS

The drainage creek emptying into STP ran through two pastures with cattle, Farms A and B (Fig. 1). On August 6, 1991, serum was obtained for leptospiral serologic testing from the cattle. No other livestock in the area with access to the drainage creek or Spring Creek were identified.

In an effort to obtain wild animals for testing which had access to the pond, live animal traps and glue boards were placed around STP for 4 days. A raccoon and an opossum were trapped. Serum was obtained for leptospiral serologic testing and urine was obtained for culture.

Water samples were collected between July 31 and August 7 for leptospiral culture from 6 ponds, creeks and rivers.

RESULTS (Table 4)

Four of the cattle had titers to grippotyphosa. No illnesses, stillbirths or abortions had occurred in the cattle. The raccoon had a titer to grippotyphosa and the agent was detected in its urine. The agent was isolated from water from STP.

7) Implement control and prevention measures

It was recommended that STP be closed to swimming. It was also recommended that individuals avoid swimming in other small, stagnant bodies of water.

COMMENTS

Only 5 of the 6 cases were considered to be part of the outbreak because only those cases were linked to the source of the outbreak, the STP. Hence, the one case in Oglesby can be considered to be “background”, i.e., the sporadic or endemic occurrence of the disease. If the agent had been recovered from all cases, molecular subtyping could have been used to complement the epidemiologic investigation. It may have revealed that the isolates from the cases in Dalzell represented a clone that was indistinguishable from the isolate from the raccoon but differed from the isolate from the case in Oglesby.

SUMMARY

Five boys from a small town in rural Illinois experienced the onset of an acute febrile illness confirmed as leptospirosis by serologic tests. A retrospective cohort study found that swimming in a small pond, STP, was associated with disease. The same leptospira species was isolated from urine cultures from two of the cases and from the water of STP. Both domestic and wild animals near the pond had antibodies to the same leptospira species.

Key points

THE INVESTIGATION

• Confirmed the existence of an outbreak.

• Organized data in terms of person, place and time.

• Identified high-risk groups.

• Formulated hypotheses.

• Conducted a retrospective cohort study to evaluate hypotheses.

• Found the source of the outbreak.

• Addressed the public health implications of the outbreak.

CMED/EPI 526

SPRING QUARTER 2007

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