Among the various families of ticks, only hard ticks (family Ixodidae) and soft ticks (family Argasidae) can transmit diseases. Hard ticks are responsible for the transmission of “Tick Borne Rickettsial Diseases (TBRD)”.
Ticks attach themselves to the skin of the host by means of “hypostome”. Ixodes ticks secrete certain substances into the area of the tick bite which mask the itchiness associated with the bite and help the tick to obtain a blood meal without coming into notice. Outdoor activities including occupational activities or recreational pursuits, during April--September, particularly in areas with dense woods and brushy areas can increase the risk for tick bites.
Ticks can transmit a number of pathogens, like bacteria, spirochetes, rickettsiae, protozoa, viruses, nematodes, and many types of toxins.These pathogens complete their life cycles partly in mammals like deer, and partly in hard-bodied ticks.
Commonly found Tickborne rickettsial diseases
The commonly found Tickborne rickettsial diseases, their description, and their causative agents are detailed below:
Rocky Mountain Spotted Fever (RMSF):
RMSF is a tick borne infection caused by Rickettsia rickettsii .The species that transmit R. rickettsii (responsible for RMSF) include the American dog tick (Dermacentor variabilis) in the eastern part of US and the Rocky Mountain wood tick (Dermacentor andersoni) in the western parts. Rickettsiae enter the human skin after an infected tick feeds for more than 6 hours. The tick bite is painless and is normally not noticed. They spread through the bloodstream and infect the vascular endothelial cells. The resulting vasculitis causes a maculopapular or petechial rash in the majority of patients. The rashes typically begin as small, pink macules on the extremities which blanch on applying pressure, evolve to maculopapules and may turn into petechiae over the next few days of illness. The classic centripetal spread of rash is typical of RMSF. Vasculitis leads to swelling, low blood pressure and loss of proteins and when it involves important organs like the brain or lungs, it can be fatal.
Human Monocytotropic (or monocytic) Ehrlichiosis (HME):
Lone star tick A. americanum is responsible for the transmission of Ehrlichia chaffeensis, the causative agent of HME. The white-tailed deer is the main carrier of lone star ticks. E. chaffeensis most frequently infects monocytes, a type of white blood cell and multiplies to form small colonies called morulae. Rash is observed in only one third of all patients of HME although its frequency increases to 66% in affected children.
Human Granulocytotropic (or granulocytic) Anaplasmosis (HGA):
The blacklegged tick (Ixodes scapularis) and the western blacklegged tick (Ixodes pacificus) are the principal vectors of Anaplasma phagocytophilum which causes HGA. Anaplasmosis is characterized by infection of granulocytes. Rash is rare in HGA infection.
Ehrlichia ewingii Infection:
Amblyomma americanum is the carrier of the ehrlichial pathogen, E. ewingii. Ehrlichioses is characterized by infection of leukocytes, with a special predilection for granulocytes.
Signs, Symptoms and Diagnosis of TBRD
The disease causing organisms multiply inside the body of the patient for a period of about 5 to10 days after a tick bite, without causing any symptoms. Most of the patients of tickborne rickettsial diseases visit a physician during the first 2 to 4 days of illness after the incubation period. The early presentation of the various tickborne rickettsial diseases resembles closely. Initially the patients present with sudden fever associated with chills, and headache, along with muscular pain and general lethargy. Certain patients may show hypersensitivity to light. Severe headache is a uniform feature in almost all cases. Complain of nausea, vomiting, and loss of appetite in the early stage of disease, especially with RMSF and HME in children, is also noticed frequently. Some patients may suffer from diarrhea. Other commonly observed symptoms in TBRD are swelling of conjunctiva, severe abdominal pain, and a confused state of mind. Pain in calf muscles and swelling around both eyes accompanied by swelling of hands and feet are some of the rare findings in TBRD. Failing kidneys, low blood pressure, continuous fever, inflammation of heart and brain, shortness of breath due to the involvement of lungs, and multisystemic failure are the major complications of untreated tickborne rickettsial diseases.
Decisions regarding the treatment depend on the initial clinical presentation of the patient and the history of the disease. This is due to the fact that the laboratory tests for detecting tickborne rickettsial diseases become positive only later on in the course of the disease. A low platelet count, low serum sodium levels, abnormal white blood cell counts, and increased levels of liver enzymes are suggestive of TBRD. A specific test for TBRD involves examining paired serum samples obtained during the course of the disease and after two to three weeks. An increase in serum antibody titer, shown by IFA or ELISA confirms the illness to be TRBD.
Treatment of Tickborne rickettsial diseases
Pathogens causing tickborne rickettsial diseases respond very well to antibiotics of tetracycline group. Doxycycline is the drug of choice in most of the cases. The fever usually settles down within one to two days if treatment with doxycycline is initiated early on in the course of the disease. Chloramphenicol can be used as a substitute to doxycycline to treat RMSF, but it might have certain adverse effects. The medication for TBRD should be continued for a minimum of 5 to 10 days and for at least 3 days after the fever has come down and there is substantial improvement in the condition of the patient. Patients not responding to oral medication or having multiple organ system involvement should be hospitalized.
Prevention of Tickborne rickettsial diseases
Avoiding venturing into areas endemic to ticks, wearing protective clothing, and applying insect repellents which contain DEET or permethrin are various steps that one can take to avoid tickborne rickettsial diseases. Ticks, if detected, should be promptly removed with the help of tweezers and the hands should be washed with an antiseptic solution.