Tick-Borne Diseases In North America
Special thanks to Tom Bush of GoodGear
Ticks are the number one vector of infectious disease in the United States and run a close second to mosquitoes worldwide. Tick-borne illnesses constitute an important health problem that has increased over the past 20 years as humans increasingly live and play in rural areas. Tick-borne disease can result from infection with bacteria, viruses, parasites and from toxins or venom from the tick itself.
Ticks belong to the class Arachnida, a group of arthropods including spiders and scorpions and are closely related to mites. Two of three families of ticks can transmit disease to humans. Ixodidae, or hard ticks, and Argasidae or soft ticks. Ixodids are referred to as hard ticks because of a thick dorsal shield. The head of an Ixodes tick is anterior and can be seen from above. During feeding, hard ticks remain attached to the host for hours or days at a time. All of the major tick-borne diseases in North America are transmitted by Ixodes ticks except for relapsing fever.
Argasidae ticks (soft body) are identified by their leathery skin and downwardly directed mouth parts. Argasidae ticks live a long time and can survive years between meals. Blood meals last less than 30 minutes.
Ticks cause human disease by transmitting microorganisms or by direct effect of toxins or venoms. Local reactions range from formation of a small itchy nodule to extensive ulceration that can remain as long a 6 weeks after tick removal.
Tick-borne paralysis involves weakness of the lower extremities and is reported worldwide, with most cases in North America and Australia. Both Ixodid and Argasidae species are reported to cause tick paralysis. In the US, the Pacific Northwest and Rocky Mountain states account for most cases although it has been documented in the southeast. Children are affected more than adults and girls twice as often as boys likely due to hair length. Men are affected more often in adults probably from occupational and recreational exposure.
Lower extremity weakness develops 5-6 days after exposure beginning with restlessness, irritability and tingling or numbness of the hands and feet. After 24-48 hours an ascending, bilateral paralysis develops with loss of reflexes. Severe weakness follows with eventual respiratory paralysis and possible death.
Resolution of symptoms after removal of the tick helps make the diagnosis. Recovery occurs over hours with complete resolution of symptoms within days. Once tick-borne paralysis is suspected it is a simple matter of finding and removing the tick.
Ticks as Vectors
Ticks act as reservoirs or amplifiers for microorganisms. In the amplifier system, the reservoir is a vertebrate such as a mouse or deer and a tick transmits or amplifies the disease. The organism responsible for Lyme disease has many hosts and relies on the tick for transmission. Humans are incidental or dead end hosts for the microorganism.
In the reservoir system the microorganism is passed from one generation to the next. The pathogen depends only on the tick for survival. The organism causing Rocky Mountain Spotted Fever needs only the tick population to sustain the life cycle.
It is not clear which ticks can transmit which diseases. The deer tick (Ixodes scapularis) has long been known to transmit Lyme disease and the dog tick (Dermacentor variabilis) has been associated with Rocky Mountain Spotted Fever. The dog tick has also been shown to harbor the organism responsible for Lyme disease. Theoretically, one tick could transmit as many as three diseases. More work needs to be done to determine what diseases each tick can transmit.
Studies have shown that the longer a tick stays on the more likely it is to transmit disease. If a deer tick is attached for less than 36 hours the risk of transmitting Lyme disease is pretty low. Other diseases transmitted by different ticks can occur more quickly. The efficiency by which ticks transmit disease needs further study. Common sense suggests that the sooner you remove a tick the better. The incidence of disease carrying ticks varies across the country and is probably due to availability of suitable hosts.
© Article copyright Tom Bush and Goodgear. All rights reserved.
Details mentioned in this article were accurate at the time of publication