Medical Entomology News

Microbes to Control Ticks That Transmit Lyme Disease

Deer ticks continue to pose a serious public health concern, especially in midwestern and northeastern states, because they transmit the bacterium that causes Lyme disease. In 1996, the Centers for Disease Control in Atlanta, Georgia received more than 16,000 reports of the tick-borne disease. Although humans aren't the preferred host of the ticks, suburban sprawl has placed people in closer contact with white-tailed deer, the adult tick's natural host.

USDA scientists at the ARS Parasite Biology and Epidemiology Laboratory in Beltsville, MD are investigating the potential for biological control of this vector using nematodes and fungi.

Dolores Hill has studied the ability of nematodes, including nine strains of Steinernema and four strains of Heterorhabditis, to infect and kill ticks. Her studies show the nematodes are most effective against adult female ticks. When nymphs, larvae, flat (unfed) adults and engorged (fed) females were tested in petri dishes, only the engorged adult females were susceptible. The engorged females are most vulnerable to nematode attack because feeding expands natural body openings making it easier for nematodes to enter. This may be less important for Heterorhabditis species which have a short, sharp tooth that can be used to help penetrate the tick's body. Microscopic observations revealed that unfed ticks bat away the nematodes that are trying to get into them. But once infected, the ticks die within 24 hours.

Field experiments are being planned that will help determine where, when and how best to apply the nematodes, and what concentrations are most effective under outdoor conditions. Hill also hopes to identify cold-hardy strains that can withstand the fluctuating temperatures characteristic of the late fall and early spring months when female ticks are abundant.

Patricia Allen and Gary Samuels are examining fungi from naturally-infected ticks, and have tentatively identified one of the most promising fungi, which killed 70% of unfed ticks in one experiment, as Gliocladium roseum. The fungus Metarhizium anisopliae can kill up to 100% of the ticks. The ultimate aim of this research is to develop a practical method of applying pathogenic fungi to the Lyme disease tick, possibly by formulating fungal spores to be sprayed onto leaf litter, shrubs, or other vegetation where ticks wait for a passing host.

Source:

Suszkiw, J. 1998. Tackling ticks that spread Lyme disease. Agricultural Research/March 1998 pp. 22-24.


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