Ticks and Rocky Mountain Spotted Fever What are ticks? Ticks are not insects like fleas, flies, and lice, but are arachnids like mites and spiders. They are classified into two families based on their structure. The family Argasidea contains the arg asid ticks, which are soft-shelled. Their body lacks a hard shell which is the protective outer covering found on some ticks. The other tick family is named Ixodidae and these ticks possess the hard outer covering, and therefore, are termed hard-shelled ticks. The two hard shelled ticks, Dermacentor andersoni and Dermacentor variabilis are responsible for spreading the organism that causes Rocky Mountain Spotted Fever.
All ticks have three pairs of legs during the immature stage and four pairs as an adult. Ticks possess a sensory apparatus called Haller’s organ that senses odor, heat and humidity. They use this organ to locate their food source. A tick’s diet consists of only blood and ticks require a blood meal to progress to each successive stage in their life. What is the life cycle of ticks? Most ticks are three host ticks.
This means that during their development which takes two years, they feed on three different hosts. All ticks have four stages to their life cycle: egg, larvae, nymph, and adult. An example of a life cycle would be to look at the deer tick. Adult female deer ticks lay eggs on the ground in spring.
Later in the summer the eggs hatch into larvae (seed ticks). The larvae find an animal (the first host, which is usually a bird or rodent), live off it’s blood for several days, then detach and fall back onto the ground. For deer ticks, this most commonly occurs in the month of August. In the ground the larvae now molt into the next stage called nymphs.
These nymphs remain inactive during the winter months and in spring become active. The nymph now finds an animal (the second host- a rodent, pet, or human) and feeds again. It then detaches and falls back to the ground. Here it molts and changes into an adult. Throughout the fall, both adult male and female ticks now find another animal (the third host- a rodent, deer, pet, or human) and feed on blood and mate. Once well fed, both males and females fall back to the ground.
The male now dies and the female lives through the winter and lays eggs in the spring completing the cycle. What is Rocky Mountain Spotted Fever? RMSF is a disease transmitted by ticks and is most prevalent in the east coast, midwest, and plains regions. RMSF affects man, dogs, and other small mammals. It has been demonstrated that cats may also become infected, but the disease in cats is minimal.
Various rodents, raccoons and fox may carry the organism, Rickettsia rickettsii, that causes RMSF. The role of ticks in the transmission of the disease was first described in the early 1900’s by Howard Ricketts, for whom the organism was named. How is Rocky Mountain Spotted Fever transmitted? R. rickettsii is transmitted from animal to animal through the bite of the American dog tick (Dermacentor variabilis) and the Rocky Mountain Spotted Fever tick (Dermacentor andersoni). The seasonal activity of the ticks contributes to the seasonal aspect of RMSF which runs from March to October. Any of the various life stages could be infected with R.
rickettsii and transmit RMSF. The tick must be attached to a host for a minimum of 5-20 hours for transmission of R. rickettsii to occur. Transmission can also occur through blood transfusions.
What are the symptoms of Rocky Mountain spotted fever? There are two stages of RMSF: Subclinical and acute. In the subclinical stage, dogs are infected, but do not show outward signs of the disease though they may have laboratory test abnormalities. These dogs recover quickly. In the acute stage, dogs have a loss of appetite, fever, pain in the muscles and joints, swollen lymph nodes, and edema in the face and legs. Some animals develop pneumonia or heart arrhythmias, which can lead to sudden death.
Most dogs have neurological signs which can include dizziness, depression, stupor, and seizures. These signs appear 2 to 14 days after the tick bite. The dogs may have anemia, a low number of platelets, ulcerations of the mucous membranes and extremities. How is Rocky Mountain spotted fever diagnosed? Blood tests are available to test for the dog’s antibodies to R. rickettsii. A test is also available which detects antigens of the organism by doing a biopsy of an affected lesion.
This test can show a positive as early as 3-4 days after exposure. A tentative diagnosis of RMSF is supported by appropriate historical, physical, and laboratory findings in endemic areas during the months of March to October as well as recent exposure to ticks. How is Rocky Mountain spotted fever treated? The antibiotics tetracycline, doxycycline, and enr ofloxacin are used. Treatment is usually for 10 to 14 days.
Dogs that develop severe disease must be treated for shock or severe nervous system symptoms. If treated within the first several day, most dogs will recover completely with some dogs showing improvement within hours of starting antibiotics. Purebred dogs seem more prone to develop clinical illness than mixed breed dogs with German Shepherds having a higher prevalence than other breeds. How is Rocky Mountain spotted fever prevented? There is no vaccine for RMSF so tick control is the main way to prevent RMSF as well as avoiding tick infested areas. Can people get Rocky Mountain spotted fever? People can get RMSF through a tick bite or the contents of a tick that comes in contact with a person’s abraded skin or conjunctiva during removal of an engorged tick from pets. People do not get infected directly from a dog.
The common symptoms in people include a rash, fever, chills, headache, and muscle aches. Other less common symptoms include nausea, abdominal pain, and swollen lymph nodes. Approximately 400-1200 cases of RMSF are reported in people in the United States each year.