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Understand how zoonotic diseases are transmitted, specifically tick-borne diseases. Recognize that although zoonotic diseases are typically endemic in geographic distribution, they require a high degree of clinical suspicion for early diagnosis through careful history taking and a thorough physical exam. Understand the clinical manifestations of tick-borne diseases ranging from no symptoms to tick paralysis and eventually, respiratory failure. The course discusses practical interventions in the prevention of tick-borne diseases and provides clinicians with actionable steps they can immediately start applying in their clinical practices today.
After completing this continuing education course, the participant will be able to meet the following objectives:
Ticks are worldwide parasites that infect both animals and humans. They are hematophagous parasites meaning they feed on blood products and cause disease by transmitting toxins to the host organism. Ticks are some of the most common vectors of human diseases and come second only to mosquitoes in transmitting human diseases.1
They are typically encountered or acquired when the host travels to an endemic area, usually during the summer months. However, recently, there are increasing reports of tick-borne disease in urban areas, even in patients who have no reports of recent travel. Needless to say, obtaining a social history is imperative in raising clinical suspicion for tick-borne diseases.1
Tick-borne diseases are considered to be a zoonotic disease. Zoonotic diseases are defined as diseases that affect both animals and humans. According to the Center for Disease Control (CDC), tracking and reporting the incidence and prevalence of zoonotic diseases is necessary since tens of thousands of Americans are infected every year.2
According to the CDC, some tick-borne diseases such as tularemia can be used for biological warfare. As such, it is incredibly important to educate both clinical providers and the public how to prevent the spread of tick-borne diseases.2
Zoonotic diseases can be mild or severe in their presentation. In addition, they can be transmitted by people or animals who appear healthy. According to the CDC National Center for Emerging and Zoonotic Infectious Diseases, 60% of all known infectious diseases in people are spread from animal vectors, while 75% of every new or emerging infectious disease is also spread from animals.
The disease is spread by being exposed to contaminated surfaces, objects, or areas where animals live and roam. Examples of indirect contact include exposure to contaminated soil from infected animal urine and pet food and water dishes.2
Direct contact implies being exposed to animal secretions such as feces, urine, saliva, mucus, or blood. This can occur while petting animals at a zoo, playing with pets at home or exposure to an infected animal on a hike.2
Common vectors include mosquitoes and ticks. Examples include being bitten by an insect, a tick, or a flea.
Humans can get sick from eating contaminated food. Examples include contaminated raw fruits and vegetables, undercooked meat, undercooked eggs, or unpasteurized milk.
Most humans, even those who appear healthy can be infected with zoonotic diseases. However, some patient groups are at increased risk of experiencing severe disease courses that potentially lead to death. These include children under the age of 5, patients who are immunosuppressed and elderly patients (those older than age 65).2
Hand washing is extremely important and effective in preventing the spread of germs and diseases from one vector to another. This is incredibly important when exposed to an infected animal or animal bodily fluids. Teach people exposed to animals to wash their hands even though they were not touching the animals. Ideally, handwashing should be done with clean running water and soap. If clean running water is not available, soap and any available water should be used, otherwise, consider using alcohol-based sanitizers, which contain at least 60% alcohol. However, note that alcohol-based sanitizers do not kill all germs, so hand washing should be performed as soon as soap and water become available.2
Prevent animal bites and bites from other vectors such as ticks or insects, including mosquitoes and fleas. This may mean dressing in the appropriate clothing while going on hikes or while caring for animals. To that extent appropriate, protective apparel such as gloves and boots may be required necessary when interacting with or caring for animals. This is very important for people working in animal shelters or zoos.
There are certain occupations that are at increased risk of tick-borne diseases. These include roofers, construction workers, laborers, mechanics, farmers, field workers, park workers, or landscapers. Typically, ticks are more active in the summer months.3
Suspicion for tick-borne diseases is based on two main factors; presenting symptoms and a history of possible exposure to ticks.
Common interventions that could help prevent tick-borne diseases include wearing long sleeve clothing, long pants tucked in boots or socks, wearing light-colored clothing, and wearing insect repellant spray.3
Any patient who experiences exposure to ticks or a tick bite should take a shower as soon as possible using soap and water.
Zoonotic diseases can be caused by fungi, bacteria, viruses, and parasites.
Lyme disease is a bacterial disease caused by the pathogen Borrelia burgdorferi. The vector for Lyme disease is Ixodes scapularis, also known as the black-legged tick or the deer tick. The average life span of the deer tick is 2 years. In the United States, the deer tick is mostly found in the Northeastern US. Other vectors include Ixodes pacificus also known as the western black-legged tick found in the upper midwestern US. Lastly, Ixodes ricinus also known as the castor bean tick which is mostly found in Europe and the Pacific coast of the US.1 Lyme disease will be discussed thoroughly later during the course.
Tularemia is a bacterial disease caused by Francisella tularensis. The arthropod vector for Tularemia is Ixodes scapularis, which is fully described above.
Rocky Mountain Spotted fever is caused by Rickettsia rickettsii. The vector is Dermacentor andersoni, also known as the Rocky Mountain Wood Tick or the hard tick. The geographic distribution is predominantly the southeastern US.1
Q fever is a rickettsial disease caused by Coxiella burnetii. The arthropod vector is Dermacentor andersoni, and the disease distribution is worldwide.1
An example of parasitic tick-borne disease is Babesiosis, which is caused by Babesia microti. The arthropod vector is Ixodes scapularis (deer tick), and the geographic distribution is the coastal New England.1
The Colorado tick fever is a viral disease caused by the Orbivirus. The arthropod vector is the Dermacentor andersoni (Rocky Mountain wood tick). The geographic prevalence of the mountain areas of the western US and Canada.1
Ticks are arthropods which are different from insects. The best way to distinguish between insects and arthropods is by counting the number of legs. Arthropods have eight legs while insects have six legs. Arthropods mainly have a head and an abdomen, while insects have a three-part body. Note that color is not a reliable way to identify ticks since they can change color with the seasons.
There are four stages in the life cycle of a tick, which include the egg, the larva, nymph, and adult. An important part of understanding how vectors transmit diseases is understanding how they feed. Some vectors feed directly on blood from the capillaries, also known as solenophagic feeders such as mosquitoes. While others feed on blood, tissue, and extracellular fluid, these are known as telmophagic feeders. Note that telmophagic feeders do not discriminate on the type of tissue that they feed on. Ticks are telmophagic feeders.
Once ticks bite into their hosts, they are attached by a salivary secretion, which is cement-like as well as a structural part of their sucking structure called a barbed hypostome. Note that ticks can remain attached to their hosts for up to 2 weeks.
During a tick bite, the physical destruction of the tissues by the biting apparatus, as well as the inflammatory response to the saliva secreted. This leads to local swelling, bleeding, increased vascularity, and focal skin thickening. Eventually, the enzymes produced in the saliva breakdown the tissues turning them into a liquid that is absorbed by the tick. Specifically, the deer tick releases a carboxypeptidase, which prevents inflammatory mediators from being released which would cause hemostasis if released.
After a tick bite, there is a characteristic small reddish spot. This is different from the rash that occurs with disease progression.
Secretions from a tick will not only produce a local skin reaction, but it also transmits pathogens and cause systemic reactions such as a febrile illness or paralysis. Soft ticks can attach for about 1 hour while hard ticks can feed for up to 2 weeks.
In the absence of a pathogen being transmitted, tick induced fever can occur, and it is associated with nausea, malaise, and headaches. It typically resolves in 36 hours or less after the tick has been removed.4
It is an ascending flaccid paralysis that happens secondary to a toxin transmission via tick saliva. It causes decreased nerve conduction, which eventually causes the neuromuscular block.4
Tick paralysis can be extremely frightening for the patient. The weakness begins in the lower extremities and is symmetric. It typically begins less than 6 days after the tick has attached. The paralysis can ascend and eventually lead to cranial nerve paralysis as well as complete paralysis of the upper and lower extremities. On physical exam, the sensory examination is normal, but the motor reflexes are either absent or decreased. Once the tick is removed, the patient’s condition improves in a matter of hours. Failure to remove the tick can lead to death by respiratory paralysis.4
The most critical part of clinical management is tick removal because it is directly linked to the improvement of symptoms. Needless to say, a thorough physical exam and history taking process are critical to raising clinical suspicion.
When ticks are being removed, it should be done with forceps applied as close as possible to the point of attachment. Patients who were bitten in an area endemic for Lyme disease should receive prophylactic treatment with a single dose of doxycycline.
Lyme disease is the most common disease transmitted by a vector in the US. It was first identified in 1975 when two parents noticed an abnormal amount of juvenile rheumatoid arthritis diagnosis in their community in Connecticut. They decided to inform physicians at Yale University and also alerted the department of health. The disease was eventually named after their community, Lyme disease from Old Lyme, Connecticut.5
Cases of Lyme disease have been reported worldwide except for the continent of Antarctica. However, most patients diagnosed with Lyme disease do not remember being bitten by a tick. Most cases of early Lyme disease occur between the months of May and August. Later manifestations of Lyme disease can occur throughout the year.5
There is some heterogeneity in the clinical course and clinical presentation of Lyme disease. It is unclear why this heterogeneity occurs, but some patients with chronic manifestations of Lyme disease are reported to have an increased frequency of human leukocyte antigens.
Clinical manifestations of Lyme disease are divided into 3 stages; early localized, early disseminated, and late disease. Early Lyme disease is characterized by a rash and other constitutional symptoms suggestive of a viral syndrome. The rash associated with Lyme disease is also called erythema migrans and is often described as a “bull’s eye pattern.” Early disseminated is characterized by cardiac, neuro or joint symptoms. While the late disease is characterized by chronic arthritis. Note that patients may be asymptomatic at any given time point in the disease course.1
Currently, there are no vaccinations for Lyme disease available in the US. Prophylactic treatment is currently recommended in patients who have been exposed. Prior to administering doxycycline, certain criteria must be met1:
Typically doxycycline is administered as a single dose.
You are a school nurse chaperoning a group of students on a field trip to a local park. About 30 mins after arriving at the park, the class spots a deer in the bushes, which promptly dashes away after only a few seconds. The kids play in the park and surrounding bushes for a few hours, and then the class returns back to school without incident. The next day one of your student’s mother calls to report that her son has a fever of 101 and inquires if anything unusual happened at school the day before. She also wants to know if she should keep giving Tylenol and watch the patient at home.
You should immediately instruct the parent to contact their pediatrician, who will further direct them to either present in a clinic or present to a local emergency room. In addition, you should inform the mother of the potential exposure to a deer’s secretions or excretions since the boy played in the bushes and may have been exposed either through direct or indirect contact. This information should be relayed to the medical staff. In addition, ask the mom to report pertinent information such as the presence or the absence of a rash. Ultimately, the patient was seen by their pediatrician, who elected to treat the patient prophylactically.
Ticks are incredibly tenacious vectors that are efficient at transmitting diseases to their hosts. Note that ticks may remain infectious for multiple generations without having to be re-infected by the host. Patient education should be focused on disease prevention by mitigating patient exposure. Finally, some tick-borne diseases can be used as biologic weapons especially in the era of antibiotic resistance.
Given that tick-borne diseases are varied in both the type of pathogens as well as the vector, it is essential that clinicians not only understand the clinical presentation and clinical course of tick-borne diseases, but they must also maintain high clinical suspicion for these diseases in order to provide timely diagnosis and treatment, which could potentially save the patient’s life.
