Plant Science Day 2002 Short Talk
Dr. Louis A. Magnarelli
Department of Entomology
The Connecticut Agricultural Experiment Station
123 Huntington Street, P.O. Box 1106
New Haven, CT 06504
Phone: (203) 974-8466
Fax: (203) 974-8502
A research program on ticks, the infectious agents they carry, and associated diseases began at The Connecticut Agricultural Experiment Station in 1977. Dr. John F. Anderson and I conducted field and laboratory investigations to focus on the ecology of Rocky Mountain spotted fever. As the years passed, Lyme disease, ehrlichiosis, babesiosis, and tularemia were included for study. Babesiosis is caused by a protozoan organism, while the infectious agents for the remaining diseases are all bacteria. It was important to determine tick infection rates, the role of mice, birds, and deer as reservoirs of infection in nature, and geographic distributions of these diseases in Connecticut. In addition, there was a need to develop sensitive and specific antibody tests, which could be used to study the ecology of tick-borne infections, characterize antibody responses in mammals, and to improve laboratory diagnosis for humans, horses, dogs, and cattle.
Lyme disease is the most prevalent tick-associated disease in the United States. This disease is usually marked by expanding skin lesions at the sites of tick bites. Cardiac, neurologic, and arthritic manifestations may follow. In Connecticut, human infections occur in all 8 counties. Numbers of human cases reported to the Connecticut Department of Health during the period 1998 through 2001 ranged from 3,213 to 3,773. By contrast, numbers of human cases of ehrlichiosis, associated with a decline in platelets or white blood cells, and babesiosis, with reduced red blood cell counts (anemia), ranged from 42 to 118 and 40 to 56, respectively. Rocky Mountain spotted fever is a rare disease in Connecticut; annual numbers of reported human cases normally do not exceed 7. Although a reportable disease, the number of human cases of tularemia in Connecticut is unknown. Tularemia, also known as rabbit fever, is often characterized by high fever and skin lesions and can be contracted by the bites of ticks or deer flies or by direct skin contact with infected blood.
The blacklegged tick, Ixodes scapularis, is abundant in or near forests. One or more life stages may be active for several months. The immature stages, larvae and nymphs, are small and often unnoticed when attached to skin. We learned many years ago in our field studies that the immature ticks feed on over 50 species of birds as well as small, medium, and large-sized mammals. Male and female ticks prefer deer as hosts but will also ingest blood from dogs, raccoons, and people. The increase in populations of I. scapularis in Connecticut and other northeastern and upper midwestern states is strongly correlated with sharp rises in deer populations over the past four decades.
Blacklegged ticks transmit the agents of Lyme disease, granulocytic ehrlichiosis, and babesiosis when they ingest blood from a host. Results of our tick testing analyses over several years have revealed extensive variability in the numbers of ticks carrying the Lyme disease spirochete. The nymphs are most abundant during late May and June and appear to be most efficient at transmission. Prevalences of infected nymphs and adults in southeastern Connecticut are about 15% and 35%, respectively. Infection rates can even vary on a given street depending on the number of infected white-footed mice present at localized settings. The disease organism can be readily isolated from ticks or mice by introducing tissues into liquid culture media in the laboratory. In DNA analyses for the agent that causes human granulocytic ehrlichiosis, 50% of 118 adult I. scapularis ticks from Connecticut were infected. It is unknown what the infection rate is for the agent of babesiosis. Human cases of babesiosis are most prevalent in New London County, and the numbers of infected ticks there are probably higher than in the other seven counties. White-footed mice can be co-infected with the agents of babesiosis and Lyme disease. We isolated pathogens of both diseases from 25 (42%) of 59 mice captured in southeastern Connecticut. It is possible that some mice might harbor living disease organisms of all three diseases and may serve to infect larval and nymphal ticks that feed on them.
American dog ticks, Dermacentor variabilis, are common throughout Connecticut and can be abundant in grassy areas bordering woodlands during late April through June. These ticks feed on mammals and transmit bacteria that cause Rocky Mountain spotted fever (RMSF). We conducted intensive field studies in Newtown behind the houses where residents had RMSF. Antibodies were detected in mice, oppossums, and raccoons, but tick infection rates were surprisingly low (less than 1%). There is no convincing evidence that American dog ticks are important transmitters of the agents that cause Lyme disease, ehrlichiosis, and babesiosis, but they may carry the tularemia agent.
Lone star ticks (Amblyomma americanum) occur at scattered, widespread locations in Connecticut, and although not as abundant as blacklegged ticks or American dog ticks, their populations may be increasing. Deer and other mammals are important hosts. In southeastern United States, lone star ticks transmit a bacterium that causes human monocytic ehrlichiosis (HME). A program exists at The Connecticut Agricultural Experiment Station for citizens to submit ticks, via local health departments, for identification and analyses for Lyme disease spirochetes. Several lone star ticks were submitted, and DNA extracts were available for further analyses. Although the DNA of the Lyme disease organism was not detected, 8 (7.6%) of 106 lone star ticks from southwestern Connecticut were found to be carrying the agent for HME. This discovery established a more northern distribution of the infectious agent in the United States and showed that the disease organism co-exists in areas where the agent of HGE also occurs. Deer may be playing an important role in harboring both pathogens in nature.
Highly sensitive and specific antibody tests can be used to study the ecology of tick-borne infections and are required for accurate laboratory diagnosis of human and domesticated animal infections. Conventional tests for Lyme disease and HGE have had problems with false positive results because there are proteins found on the surfaces of the target disease organisms that are also present on common unrelated bacteria. Collaborative studies with scientists at Yale University and the University of Texas (Houston) were conducted to develop better antibody tests. Purified preparations of highly specific proteins were tested by state-of-the-art methods called enzyme-linked immunosorbent assays (ELISA). In Lyme disease testing, a VlsE antigen (surface lipoprotein) was analyzed with human serum samples. Tests for IgM antibodies produced during early weeks of infection revealed a sensitivity value of 40% and specificity rate of 98%. Sensitivity improved when an ELISA was conducted for IgG antibodies, which are present during later weeks of infection. The sensitivity and specificity values were 80% and 98%, respectively. For HGE evaluations, a protein 44 reagent was produced for use in an ELISA. In analyses of human sera, results were encouraging for the detection of IgM (sensitivity = 84%, specificity = 99%) and IgG antibodies (sensitivity = 91%; specificity = 100%). Similar studies are underway to improve antibody detection for white-footed mice, dogs, horses, and deer. Findings for mice and deer will help identify areas in Connecticut where the agents of Lyme disease and ehrlichiosis co-exist.
There is increased public concern regarding ticks, the infectious agents they transmit, and associated diseases. The extent of human infections, particularly babesiosis and tularemia, in Connecticut remains unclear. Moreover, tick-infection rates for these diseases are unknown. Future studies are needed to determine if babesiosis has a statewide distribution and if people and domesticated animals are being exposed to the agent of tularemia.