Epidemiology

Epidemiology is the study of the spread, growth or development of disease among individuals in a given area, generally pertaining to infectious diseases.

This section will provide information and data on the epidemiology of certain vector-borne diseases (Lyme disease, Ehrlichiosis and Babesiosis) in the United States and more specifically, Dutchess County, NY.

Lyme Disease Surveillance Case Definition

Lyme disease was first described in the United States in the town of Old Lyme, Connecticut in 1975, but has now been reported in most parts of the United States.

In 1982, spirochetes were identified in the midgut of the adult deer tick, Ixodes dammini (referred herein by its original name, the black-legged tick, Ixodes scapularis) and given the name Borrelia burgdorferi. Finally, conclusive evidence that B. burgdorferi caused Lyme disease came in 1984 when spirochetes were cultured from the blood of patients with EM, from the rash lesion itself, and from the cerebrospinal fluid of a patient with meningoencephalitis and history of prior EM. The CDC began surveillance for Lyme disease in 1982 and the Council of State and Territorial Epidemiologists (CSTE) designated Lyme disease as a nationally notifiable disease in January 1991.

The surveillance criteria established by the CDC should not be used to diagnose, treat or withhold treatment for Lyme, but is merely the criteria that must be met in order to report the disease to a state health department or other agency. Treatment decisions should be based upon clinical findings, proximity to an endemic area or a known tick bite and should be verified by objective testing. However, many physicians continue to withhold treatment because a patient does not meet this criteria.

Lyme disease surveillance criteria includes:

Physician diagnosed EM rash greater than 5 centimeters
Recurrent joint effusion/arthritis (IgG Western Blot test is required to confirm joint effusion/arthritis)
Cranial Neuritis (Bell's Palsy)
Lymphocytic meningitis
Radiculoneurapathy
Encephalomyelitis
Acute 2 or 3 degree A-V nerve conduction defect

All criteria, with the exception of EM rash must be supported by a positive lab finding.

For information on Lyme disease visit our Ticks and Diseases section or search the Center for Disease Control, the National Institute of Health or PubMed.

Ehrlichiosis Surveillance Criteria

The emerging tick-borne zoonoses human monocytic ehrlichiosis (HME) and human granulocytic ehrlichiosis (HGE) are underreported in the United States. From 1986 through 1997, 1,223 cases (742 HME, 449 HGE, and 32 not ascribed to a specific ehrlichial agent) were reported by state health departments. HME was most commonly reported from southeastern and southcentral states, while HGE was most often reported from northeastern and upper midwestern states. The annual number of reported cases increased sharply, from 69 in 1994 to 364 in 1997, coincident with an increase in the number of states making these conditions notifiable. From 1986 through 1997, 827 probable and confirmed cases were diagnosed by serologic testing at the Centers for Disease Control and Prevention, although how many of these cases were also reported by states is not known.

Improved national surveillance would provide a better assessment of the public health importance of ehrlichiosis. A national ehrlichiosis surveillance program does not exist, so national incidence rates have not been determined because of wide variability in state surveillance activities.

The Council of State and Territorial Epidemiologists recommended that human ehrlichiosis be made nationally notifiable in 1998, but many states do not have a system for surveillance and do not test for ehrlichiosis in state diagnostic laboratories.As of August 1998, only 19 states considered ehrlichiosis notifiable, and fewer than one fourth of state health departments offered in-house diagnostic assays for HME or HGE. Average annual incidence rates, an important indicator of disease prevalence, could be calculated for only 21 states. These data underscore the need for better nationwide surveillance of ehrlichiosis

Ehrlichiosis surveillance criteria includes:

Confirmed Case

4 fold or greater in antibody titer in acute and convalescent sera
A positive PCR test
Morulae and an IFA greater than 1:64

Probable Case - A person with compatable symptoms and

A single IFA serologic greater than or equal to 1:64
Morulae present

Suspected Case - A person with compatable symptoms and

A single IFA serologic greater than or equal to 1:64

For information on Ehrlichiosis visit our Ticks and Diseases section or search the Center for Disease Control, the National Institute of Health or PubMed.

Babesiosis Surveillance Definition

Surveillance case definition is still under development by the CDC.

Sources: Dutchess County Dept. of Health, Center for Disease Control