Forget Lyme, ticks can do a lot worse …
When it comes to health problems caused by ticks, Lyme disease gets all the attention. But various tick species carry and transmit a multitude of other pathogens, some of which cause serious or life-threatening diseases.
In fact, the number of cases of tick-borne diseases is increasing in the United States and Canada. The range of various tick species in North America may be expanding as a result of climate change. The Public Health Agency of Canada has just issued an opinion on increasing the risk of tick-borne diseases in the context of environmental change. “The temperature increase in Canada is creating a more favorable environment for ticks and extending their season of activity. “
Researchers are constantly discovering 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 adaptability of ticks. They recover from the many attacks we make on them, including pesticides. Ticks are good at finding new ecological niches to survive. Humans and ticks therefore cross frequently, exposing us to their bites and the diseases they carry.
Here are some of the lesser-known, but growing, threats that ticks represent.
Some very small species of bacteria, which can cause diseases such as rickettsia, ehrlichia and anaplasma, live in ticks. These ingest these bacteria when they absorb the blood of animals. Ticks then transmit the bacteria to the animal or person they are feeding on.
The best-known of these bacterial diseases is undoubtedly the Rocky Mountain Spotted Fever, the most frequently reported rickettsial disease in the United States, with about 6,000 cases each year. The number of diagnoses appears to be increasing nationally, particularly among natives, likely due to exposure to free-ranging dogs that may carry ticks.
When people get Rocky Mountain spotted fever, they usually show up in the clinic with three symptoms: fever, rash, and a history of tick bites. They can also report severe headaches, chills and muscle aches, as well as 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. In the absence of treatment, the mortality rate is about 20 percent; and even with treatment, 4 percent of infected people die.
Not all tick species are transmitters of the Rickettsia bacteria. Even within vector species, only 1 to 5 percent of ticks in an area are infected. Being bitten by a tick that transmits the rickettsial bacteria is like being stung by a needle in a haystack. The main carriers are the American dog tick in the eastern United States and the Rocky Mountain tick in the west. The dog’s brown tick has also recently been identified as a vector.
In most tick-borne diseases, ticks must feed for a period of time before the pathogens they carry are transmitted to the animal from which they absorb blood. In the case of Rocky Mountain spotted fever, it usually takes one to three hours for transmission to occur, so ticks that cling to the skin must be removed quickly. Doctors usually prescribe the doxycycline antibiotic to treat Rocky Mountain spotted fever, which works very 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 the bacterium Ehrlichia chaffeensis , carried by solitary ticks, widespread in the eastern United States. This bacterium 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 Centers for Disease Control and Prevention (CDC) in 2017.
Patients with ehrlichiosis usually have fever, headache, muscle aches, and a progressive decrease in white blood cells. Unlike Rocky Mountain spotted fever, people with skin rashes only develop about 20 to 40 percent of the time. Doctors usually treat ehrlichiosis with doxycycline.
Another tick-borne bacterial disease that needs to be addressed is human granulocytic anaplasmosis. Anaplasma phagocytophilum bacteria infect a type of white blood cell called granulocytes. It occurs mainly in the northwestern and northeastern United States, and its incidence is increasing, with 5762 cases of human anaplastic granulocytic anaplasmosis reported to the CDC in 2017.
Symptoms include fever, headache, muscle aches, and a gradual decrease in white blood cells. It is the tick of the deer Ixodes scapularis – also responsible for Lyme disease – that transmits the Anaplasma bacteria to humans. There is a risk that a stag tick bite can infect you doubly. Again, the recommended treatment is doxycycline.
Also carrying viruses
People usually think of mosquitoes when it comes to insect-borne viruses – dengue, Zika or West Nile virus are in the news. But ticks can also transmit viruses.
Historically, scientists have grouped tick-borne viral diseases into two categories. One is a disease 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 is like encephalitis transmitted by mosquitoes. Most of these diseases, characterized by brain inflammation, are not found in North America, with the exception of Powassan encephalitis, found in the northeastern United States and Canada.
Powassan is a relatively rare but serious human disease characterized by the sudden onset of fever reaching 104 degrees Fahrenheit (40 degrees Celcius), accompanied by convulsions. 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 have been fatal. Its incidence appears to be increasing: 34 cases of Powassan have been reported in 2017. The spread occurs when ticks – mainly Ixodes cookei – infect animals with the virus through their bites. These infected animals can then serve as what scientists call a reservoir species, infecting new ticks when they feed on their blood.
Over the past decade, researchers have discovered other new tick-borne viruses in the United States. About 30 cases of Heartland virus have been identified to date. It is associated with the solitary star tick and has been identified in Missouri, Oklahoma, Kentucky and Tennessee.
Some cases of a new Thogotovirus called Bourbon virus have been identified in the Midwest and Southern United States. The tick of the isolated star may also be the vector of the Bourbon virus.
Food allergy triggered by a sting
Perhaps the most bizarre threat of ticks is the allergy to red meat, which scientists have recently traced back to tick bites. People may become allergic to meat when a tick’s saliva passes on galactose-α-1,3-galactose, a carbohydrate that has already been extracted from an animal’s blood. If it is allergic, the person may be sensitized to this alpha-gal molecule found in animal blood and other tissues.
A few days or weeks later, this person may develop hives, inflammation of the skin and lips or even life-threatening anaphylaxis three to six hours after eating red meat. Meats containing alpha-gal include beef, pork, lamb, squirrel, rabbit, horse, goat, deer, kangaroo, seal and whale. People who become sensitive to alpha-gal can still eat chicken, turkey and fish.
Overall, people should know what tick-borne diseases are present in their area and use personal protection techniques when outdoors in tick-infested areas.
Remember that ticks often come into contact with people through dogs or pet cats. It is a good idea to check for ticks after being outside in tick-infested areas. Reducing the number of tick bites and the amount of time they remain clinging to your skin can go a long way toward protecting you from the diseases they transmit.
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This text first appeared on the French-Canadian website of The Conversation. Reproduced with permission.
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