VECTOR-BORNE DISEASES

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ISDH conducts an arbovirus surveillance program to serve as an early warning system for impending outbreaks of mosquito-borne viral disease. The principal arboviral diseases likely to be encountered in Indiana include St. Louis encephalitis, eastern equine encephalomyelitis (EEE), and western equine encephalomyelitis (WEE).

During the summer of 1996, above average rainfall in the Michiana (northern tier counties) area resulted in mosquito populations that exceeded the ten-year average as measured by light traps. Concurrently, two birds were found to be positive for antibodies to EEE. One bird was a juvenile brown thrasher and the other was an adult catbird. The presence of antibody in the juvenile bird gave definitive proof of transmission in the current season. Based upon these factors plus two reports of suspected cases of EEE in horses, an alert was issued for the Michiana area in which residents were advised to wear mosquito repellent when out of doors. No human cases were subsequently reported in this area.

Three California-group arboviruses cases were reported from Daviess, Wayne, and Howard Counties. The cases from the latter two counties may have been acquired during travel outside Indiana.

Cases = 7

The first documented case of human ehrlichiosis in Indiana was reported from Warrick County in June 1994. There were 4 more cases reported in 1994, 3 cases in 1995, and 7 cases in 1996. Six counties had one or more confirmed cases in 1996: Gibson (1), Jay (1), LaPorte (1), Spencer (1), Vanderburgh (1), and Warrick (2). All cases occurred between June and October. The 7 cases listed above were human monocytic ehrlichiosis (HME). There was an additional case of apparent human granulocytic ehrlichiosis (HGE) in Lake County. Unfortunately, the paired serum samples on which the diagnosis was based were run at different laboratories, leaving the interpretation of the results ambiguous. This would have been the first confirmed case of HGE in Indiana.

Ehrlichiosis is an acute febrile bacterial illness. Common, but nonspecific, clinical findings include fever, headache, myalgia, and nausea or vomiting. Thrombocytopenia and abnormal liver function test results, particularly increased hepatic transaminase levels, have also been noted. The spectrum of human infection ranges from asymptomatic to severe, sometimes fatal illness. The incubation period of the disease is not well established. There is no evidence of transmission from person-to-person.

Ehrlichiosis was first recognized in 1986 in the United States. The causative agent of human monocytic ehrlichiosis (HME) is Ehrlichia chaffeensis, a new member in the genus of Ehrlichia, family of Rickettsiaceae. E. chaffeensis is closely related to E. canis, the causative agent of canine ehrlichiosis. Patients with serologic evidence of a recent infection with E. chaffeensis have serologic cross-reaction with E. canis. Dogs have not been found to be reservoirs of human disease.

Epidemiologic data indicate that most cases of HME are found in the south central or southeastern United States. Studies confirmed the association of the disease with a history of tick exposure. Amblyomma americanum, the lone star tick, is a suspected vector. In addition to recent tick exposure, a hallmark of the disease is leukopenia.

Another, as yet unspeciated, Ehrlichia causes human granulocytic ehrlichiosis (HGE). This organism is closely related to both E. phagocytophila and E. equi. Ixodes scapularis, the deer tick, is the most important vector. HGE has so far been limited to the upper Midwest and the East Coast states. No human infections have been identified in Indiana to date. (See first paragraph.)

Accurate epidemiologic diagnosis requires laboratory confirmation by indirect fluorescent antibody tests. The ISDH Disease Control Laboratory facilitates diagnostic testing through CDC. Data suggest that prompt treatment with a tetracycline drug or with chloramphenicol markedly decreases the morbidity. The latter drug is recommended for pregnant women and children under 8 years of age.

Recognition of the existence of this disease provides an additional differential diagnosis besides Lyme disease and Rocky Mountain spotted fever for patients with febrile illness and a history of recent tick exposure. It also helps patients to access diagnostic testing services which are not widely available at this time. As a result of a correct diagnosis, patients can be treated properly and in a timely manner. Measures to prevent the disease include environmental control of ticks and avoidance of tick exposures.

Cases = 0

No cases of hantavirus pulmonary syndrome (HPS) were identified in Indiana in 1996. In 1994, an adult resident of Hendricks County died of adult respiratory distress syndrome (ARDS). Laboratory tests conducted by the Centers for Disease Control and Prevention (CDC) in Atlanta confirmed the infection in this person. No additional cases have been confirmed in the state since 1996.

A newly-identified strain of hantavirus (Sin Nombre virus) was determined to be the cause of an outbreak of ARDS in the southwest United States in 1993. This strain was also responsible for the HPS case in Indiana. As of April 1997, 158 cases have been documented in 26 states, including Indiana, since the recognition of this emergent virus.

Hantavirus is contracted by inhaling aerosolized droplets of feces, urine, or saliva from rodents, including the deer mouse, Peromyscus maniculatus, the predominant reservoir of infection identified in the outbreak in the southwestern U.S. Other possible modes of transmission include ingestion or contamination of open wounds with rodent droppings and urine. Person to person transmission has not been documented.

The death of the Indiana case led to an investigation of the rodent population in the area of the patient’s residence and work. Twenty-six rodents were trapped. Seven of these were Peromyscus maniculatus (deer mouse); seventeen Peromyscus leucopus (white footed mouse), one house mouse, and one meadow vole. Three of the Peromyscus maniculatus and the meadow vole were positive for hantavirus antibodies similar to those found in the southwestern United States.

The best way to prevent infection with hantavirus is to avoid contact with rodents or the places where they live. If a person can't avoid such contact, they can minimize the risk by following some simple measures: 1.) set traps when rodents are observed in the home; 2.) always wear gloves when handling rodents or their droppings; 3.) prior to cleaning, moisten and disinfect (with dilute bleach, for example) sites where rodents and their droppings have been observed; and 4.) rodent-proof homes as much as possible by sealing gaps where they might enter.

Cases = 16

In 1996, a total of 16 cases reported in Indiana met the CDC surveillance confirmed case definition (Lyme Disease-1). Seven cases were male and 9 were female. Their ages ranged from 7 to 64 years with a mean of 38.3 years.

The seasonal pattern of the disease is shown in Lyme Disease-2.

The number of reported cases has remained small and relatively stable over the past few years. Lyme Disease-3 summarizes the number of cases reported from 1989 to 1995.

Lyme disease is diagnosed clinically, using laboratory testing for confirmation. The current case definition of Lyme disease developed by the CDC for public health surveillance has been used since the summer of 1994. This definition requires the presence of either a physician-diagnosed erythema migrans (EM) or a positive serologic test for LD accompanied by the presence of specific bands on a Western blot of the patient’s serum.

Ixodes scapularis (which used to be classified as I. dammini) is the tick most commonly associated with LD in the Midwest. The tick distribution in Indiana revealed from the surveys were conducted and is illustrated in Appendix C. These data are helpful in assessing the possible transmission of LD to the public. The LD vector has been identified mainly in the northwest corner of Indiana but can potentially be found anywhere in the state.

Although a relatively small number of LD cases occur in Indiana, the ISDH Vector Control Section and the Communicable Disease Division are continuing aggressive surveillance and education programs. These programs emphasize "tick awareness" in order to prevent all tick-borne diseases.

Cases = 7

The disease occurs sporadically in Indiana. The seven cases were from six counties including Dubois (1), Franklin (1), LaPorte (1), Perry (1), Vanderburgh (2), and Warrick (1). All cases occurred between late April and early August. The secular trend of Rocky Mountain spotted fever (RMSF) is displayed below. Small fluctuations in the number of cases have occurred from year to year.

The disease is caused by Rickettsia rickettsii and is characterized by sudden onset with fever, malaise, deep muscle pain, severe headache, chills, and conjunctivitis. A rash may appear on the extremities and spread to much of the body. Transmission of the disease is through the bite of an infected tick. However, the association between tick populations and the occurrence of the disease is not well established. Therefore, education for tick awareness is given throughout the state.

Because no laboratory test is consistently positive during the first 2 weeks of illness, suspected patients should be treated empirically and serologic tests delayed until both acute and convalescent serum specimens are available. When paired serum specimens are not available, single specimens can be tested, but results may not be positive during the first 2 weeks of illness.

Since no vaccine exists for RMSF, the best preventive measure is avoidance of tick-infested areas. Persons who must enter these areas should wear protective clothing (long sleeves and legs, light-colored), use repellents such as DEET, and check for ticks afterwards.

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