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Regarding the problems caused by ticks, Lyme disease attracts a lot of attention. However, various tick species carry and transmit a collection of other pathogens, some of which cause severe or life-threatening conditions.
In fact, the number of cases of tick-borne diseases is increasing in the United States. The range where various species of ticks live in North America could be expanded due to climate change. Researchers continue to discover new pathogens that live in ticks. And new species of invasive ticks continue to appear.
During my career as a public health entomologist, I was amazed by the ability of ticks to bounce back in all the ways that people try to control them, including with pesticides. Ticks excel at finding new ecological niches for survival. People and ticks often cross, exposing us to their bites and the diseases they carry.
Here are some of the less known but growing tick threats.
Ticks can transmit bacterial diseases
Some very small species of bacteria that can cause human diseases, such as rickettsia, ehrlichia and anaplasma, live in ticks. Ticks ingest these bacteria when they drink blood from animals. Then, when the ticks take a blood meal, they transmit the bacteria to the next animal or person they feed on.
The most well-known of these bacterial diseases is probably Rocky Mountain Spotted Fever, the most frequently reported rickettsial disease in the United States, with about 6,000 cases a year.
The number of diagnoses seems to be increasing throughout the country, especially among Native Americans, probably because of exposure, on reserves, to free-ranging dogs that can carry ticks.
People with Rocky Mountain fever usually have three factors: fever, rash, and a history of tick bite. They can also report severe headaches, chills, muscle aches and gastrointestinal symptoms such as abdominal pain and diarrhea.
A rash is usually present after a few days, but not always. Mental confusion, coma and death can occur in severe cases. Untreated, the mortality rate is about 20%; and even with treatment, 4% of infected people die.
Not all tick species transmit the rickettsia bacteria effectively. Even within vector species, often only 1% to 5% of ticks in an area are infected. So getting stung by a tick that transmits the rickettsial bacteria to you is like getting stuck with a needle in a haystack.
The main vectors are the American tick in the eastern United States and the Rocky Mountain tick in the west. The brown dog tick has also recently been shown to be a vector.
In most tick-borne diseases, ticks must feed for a period of time before any pathogens they carry are transmitted to the animal whose blood is eating them.
Macular fever transmission in the Rocky Mountains usually takes between one and three hours. Therefore, ticks attached must be removed quickly. Doctors usually prescribe doxycycline, an antibiotic, to treat Rocky Mountain Spotted Fever, which works pretty well if the disease is detected early.
Ehrlichiosis is another bacterial disease transmitted from ticks to humans. In the United States, it is most often caused by Ehrlichia chaffeensis Bacteria, mediated by single ticks that are common in the East of the American bacterium Ehrlichia, infects a type of blood cell called leukocytes. Human monocytic ehrlichiosis occurs mainly in the south and south-central United States; 1,642 cases were reported to the CDC in 2017.
Patients with ehrlichiosis usually have fever, headaches, muscle aches and a gradual decrease in the number of their white blood cells. Unlike Rocky Mountain Spotted Fever, people have a rash only about 20-40% of the time. Doctors usually treat ehrlichiosis with doxycycline.
Another bacterial disease transmitted by ticks to worry about is human granulocytic anaplasmosis. Human granulocytic anasplasmosis, Anaplasma phagocytophilum Bacteria infect a type of white blood cell called granulocytes.
It occurs mainly in the Upper Midwest and Northeastern United States, and the incidence increases, with 5,762 cases of human granulocytic anaplasmosis reported to the CDC in 2017.
Symptoms include fever, headache, muscle aches, and a gradual decline in the number of white blood cells. That's the deer tick Ixodes scapularis – famous also responsible for Lyme disease – which transmits Anaplasma bacteria to humans. There is an unfortunate risk that a bite from a deer tick can infect you with both diseases. Again, the recommended treatment is doxycycline.
Ticks can also carry viruses
People usually think of mosquitoes when they think of insect-borne viruses – dengue, Zika or West Nile are a big hit in the news. But ticks can also transmit viruses.
Scientists have historically grouped tick-borne viral diseases into two categories. One is diseases similar to dengue fever. The main tick-borne dengue viral disease in the United States is Colorado tick fever, which occurs in mountainous areas in the west.
The other group of tick-borne diseases resembles encephalitis transmitted by mosquitoes. Most of these diseases, characterized by inflammation of the brain, are not found in American encephalitis in Powassan, which is prevalent in the northeastern United States and adjacent areas of Canada.
Powassan is a relatively rare but serious human disease characterized by sudden fever with temperatures as high as 104 degrees Fahrenheit and convulsions. The brain inflammation is usually severe, with vomiting, respiratory distress and prolonged fever.
Less than 100 cases of Powassan have been reported in North America, about half of which are fatal. Its incidence seems to increase. In 2017, 34 cases of Powassan were reported. The prisoner of war is kept in a natural cycle when ticks – mainly Ixodes Cookei – infect the animals with the virus through bites.
Then, these infected animals can constitute what scientists call reservoirs of diseases, infecting new ticks when they feed on their blood.
In the last decade, researchers have discovered new tick-borne viruses in the United States. To date, approximately 30 cases of Heartland virus have been identified. It is associated with the only starry tick and has been recognized in Missouri, Oklahoma, Kentucky and Tennessee.
A few cases of a new thogotovirus called Bourbon virus have been identified in the Midwest and Southern United States. The unique star tick could also be the vector of the Bourbon virus.
A food allergy triggered by a tick bite
Perhaps the strangest threat from ticks is the "allergy to red meat" that scientists have recently attributed to tick bites. People may become allergic to meat consumption when a tick's saliva passes on the galactose-α-1,3-galactose carbohydrate that she had previously taken in a blood meal from an animal.
In case of allergies, the person may be sensitized to the alpha-gal molecule present in animal blood and in other tissues.
Then, days or weeks later, he / she may develop hives, swollen skin and lips, or life-threatening anaphylaxis three to six hours after consuming red meat.
Meats containing alpha-gal include beef, pork, lamb, squirrels, rabbits, horses, goats, deer, kangaroos, seals and whales. People sensitized to alpha-gal can still eat chicken, turkey and fish.
Overall, people should be aware of tick-borne diseases present in their area and use personal protection techniques outdoors in tick-infested areas.
Remember that ticks often come into close contact with people through dogs or cats. It's a good idea to check for ticks yourself after being outside in tick-infested areas. Reducing the number of tick bites and the amount of time ticks can go a long way toward protecting you from tick-borne diseases.
Jerome Goddard, Extension Professor of Biochemistry, Molecular Biology, Entomology and Plant Pathology, Mississippi State University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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