It's a warm January afternoon 50 miles outside of Boston, Massachusetts, and Dr. Sam Telford,
Professor of Infectious Disease and Global Health at Tufts University, is flagging for ticks.
Flagging involves sweeping a cotton or flannel cloth over areas where ticks may be present.
And today we're in luck... Ah, we've got one.
Ticks are common parasites that reside in many parts of the world,
and may transmit viral, bacterial, or protozoal infections. In this video,
we'll introduce characteristics of ticks, and a clinical overview of tickborne infections.
Ticks are most active in warmer months and are commonly found in tall grass, brush,
and wooded areas where they await contact with passing animals, including pets and humans.
Ticks are remarkable animals. They are arthropods more related to horseshoe
crabs and spiders than they are to insects. They are obligate blood feeders. They're parasitic,
so they require blood in order to grow and develop. Size of ticks can vary greatly,
and not just by species. Adult ticks are 4 to 8 times larger than nymphs. Because nymphs often
go unnoticed, they play a major role in disease transmission. The vast majority of the
tickborne infections are not inherited by the tick. So, an infected female is not going to
pass infection on to its progeny. The larvae need a blood meal in order to grow to a nymph,
and nymphs need a blood meal in order to grow to an adult. And during those blood meals,
if they feed on an animal that is infected, they will pick up the infection. We believe a
lot of these tickborne infections are what we call peridomestic infections. That is, things
that you acquire around your home through the activities that you engage in around your home.
Once attached, a tick uses its hypostome, specialized mouth parts, to pierce the host's
skin to feed upon their blood. In most cases, when an infected tick feeds, pathogens aren't
immediately transmitted to the host being bitten. Reactivation of the pathogen in the tick's gut is
first required. The surrounding temperature change during the tick's feeding induces the expression
of genes and proteins in the pathogen required for disease transmission. These reactivated pathogens
then migrate from the gut to the salivary glands and then to the host. There are exceptions, though.
Pathogens that reside in the salivary glands, like Powassan virus, can be immediately transmitted to
the host. The time required for reactivation varies greatly, from as little as 15 minutes
to as long as 72 hours. So, to minimize the risk of transmission, it's important for individuals
to remove an attached tick quickly once it's been found and before reactivation
enables transmission. The mechanism by which these microbial agents are transmitted by
the tick is through the saliva. The tick injects saliva as it feeds because it has antihemostatic,
immunosuppressive, and antiinflammatory agents within it. And the microbes use that to their
advantage because it prepares the site and knocks out host defenses at the site. So,
it's the saliva delivery of these pathogens that makes tickborne infections so effective.
In the contiguous United States, there are several human-biting tick species that are vectors for
tickborne diseases. Some of the most frequently reported tickborne diseases in the U.S. include
Lyme Disease, babesiosis, and anaplasmosis, all transmitted, mainly, by black-legged ticks, also
known as deer ticks; Rocky Mountain spotted fever, transmitted by the American and brown dog ticks
and Rocky Mountain wood tick; ehrlichiosis, transmitted mainly by the lone star tick;
and tularemia, transmitted by several species of ticks. Other diseases include Powassan virus
disease and hard tick relapsing fever, both transmitted by black-legged ticks,
also known as deer ticks; Heartland and Bourbon virus diseases, both transmitted by the lone star
tick. The lone star tick can also cause alpha-gal allergy. Rickettsiosis caused
by Rickettsia parkeri, closely related to the causative agent of Rocky Mountain spotted fever,
and is transmitted by the Gulf Coast tick. The woodchuck tick, also known as the groundhog tick,
can also transmit Powassan virus disease. In the western United States, Colorado tick fever is transmitted
by the Rocky Mountain wood tick, and tickborne relapsing fever is associated with soft ticks.
All ticks can become infected and potentially transmit more than one infectious agent,
but sequential infection is more likely. Most ticks have foci, central areas of localization,
but, as described by Dr. Robert Smith, Infectious Disease Physician at Maine Medical Center, these
distributions have been changing. We have seen real changes in the last 10 to 20 years in
the range of certain ticks in the United States. In particular, the lone star tick has moved
northward into middle Atlantic states and southern New England. The deer tick has moved northward,
but also is increasingly reported from states where we didn't usually find deer ticks. Presence
of these vectors is of particular significance for physicians who are on the front line and
see patients coming in with fever or other unexplained symptoms. In certain areas we've seen
real changes in the presence of certain tickborne diseases in northern New England, for example.
We've had Lyme disease present for a couple of decades, but we've seen a marked increase in some
of the other diseases the deer tick carries, such as anaplasmosis and babesiosis. Nationally, if you
look at the reported numbers of Lyme disease cases and babesia cases and anaplasmosis cases and other
diseases such as Rocky Mountain spotted fever, it's generally been pretty stable. But there have
been changes in particular regions in terms of the frequency of these diseases.
A thorough physical examination and medical history is very important when evaluating
patients with fever and nonspecific symptoms, since most tickborne infections present with
nonspecific fever, muscle aches, headaches, fatigue, malaise. Where somebody's been
and what they've been doing is very important information in figuring out the probabilities
that they might have one of these infections and developing your differential diagnosis.
Time of symptom onset, progression and nature of symptoms, and potential disease exposures
other than tick bites may be relevant. The exam should focus on vital signs, inspection of skin
and mucosal surfaces, palpation for enlarged lymph nodes, and auscultation of the heart and lungs.
Particular attention to the abdominal examination may be helpful when nausea or abdominal pain are
present. Besides a medical history, there are other dermatological signs to
consider if tickborne diseases are suspected, and familiarity with the spectrum of variation
is important. More than 80% of patients with early Lyme disease have the presence of erythema
migrans on careful initial examination, but erythema migrans can vary in appearance, size, and
location from the typical textbook bull's-eye or target rash. Early on, most are a homogeneous red
circle or band, and can easily be missed. Ten percent or more of these rashes may also appear as multiple
lesions. Patients bitten by lone star ticks will occasionally develop a similar appearing
circular rash caused by southern tick—associated rash illness, or STARI, and requires further
diagnostic testing to exclude Lyme disease as the cause. While half of patients with Lyme
disease will test positive on an antibody-based laboratory test at the onset of erythema migrans,
most will test positive by about 4 to 6 weeks. The rash of Rocky Mountain spotted fever is present in
80% of cases after 2 to 5 days of illness but may be more difficult to appreciate on darker skin. It
often starts as small pink macules on the ankles and wrists and may involve the palms and soles,
with petechial or purpuric components present in half of the cases. Also, tick-bite sensitized
individuals may present with an immediate-type hypersensitivity reaction, and the site may remain
irritated and itchy for weeks. If somebody comes in and they've had a tick, that's what we
call engorged or that has been attached for more than 24 hours or so, there is a risk
that tickborne illness may be transmitted. And if it's particularly engorged, and it's a deer tick,
treatment with a prophylactic dose of doxycycline may be indicated. Unfortunately, that doesn't
prevent the other tickborne diseases that might be carried. So, in most instances, other than that,
it's watchful waiting and if somebody develops symptoms over the next week, that information
that they've had exposure to a particular species of tick will be very helpful in guiding treatment.
While rare, symptoms inconsistent with just one tickborne infection may suggest a coinfecting agent.
With multiple tickborne infections, you could be misled because of the particular
combination of symptoms. So, somebody coming in with a rash that might be due to Lyme disease,
but it's not typical who also has significant gastrointestinal symptoms,
which we don't see with Lyme disease, would be a confusing mix. And that could be due to
the presence of a coinfection. In patients at risk of serious illness, or those
who are moderately or seriously ill on initial evaluation, baseline laboratory testing will
likely be pivotal. If a clinician is thinking of a tickborne infection, particularly
if they're thinking of infections other than Lyme disease, a complete blood count is very helpful.
CBCs with leukopenia and thrombocytopenia can be indicative of a tickborne infection,
and including a hepatic transaminase test as part of a metabolic panel can be helpful as well.
If a patient presents to a clinician with a tick they've discovered, they may ask to
have the tick tested, but a negative test of the tick doesn't necessarily mean that there is
no risk of infection since there may be other unnoticed tick bites. A positive test of a tick
may also be misleading. The Centers for Disease Control do not recommend and the
IDSA guidelines as well do not recommend testing for the particular pathogens in a tick for medical
diagnostic purposes. And the reason for that is that the actual rate of transmission, for example,
of Lyme disease from an infected tick to somebody who's bitten by that tick is quite low. So,
20 to 40% of the ticks might be infected, but the actual infection rate in the patient
is 2%. And so the presence of the pathogen in the tick does not imply that the person
has been infected or will necessarily become ill from it. With the changing distribution
of ticks and the diseases they carry, it's important for physicians to be aware of the
most common endemic tickborne infections in their region, as well as their seasonality,
in order to properly diagnose the presence of these diseases in their patients. Physicians
can stay informed by checking the CDC or their local departments of public health.