Doctors are seeing an increase this summer in a tick-borne illness that many people don't know about.

Called babesiosis, the illness is carried by the same deer ticks that carry Lyme disease (some ticks carry both). But while most people are familiar with Lyme disease, they may not think about babesiosis -- or get tested for it when they go for a Lyme test. That could mean unnecessary discomfort and, in rare cases, serious illness.

But in Rhode Island, ticks that carry the disease have been found only in South County.

Most infected people don't get ill, and their immune systems clear out the parasites that cause babesiosis. Those who get sick often feel as if they have the flu in the off-season -- fever, chills, sweating and muscle aches. People whose immune systems are weak will get these symptoms more severely, and can become seriously ill with anemia. But drug treatments are effective.

In 2004, there were 48 reported cases of babesiosis in Rhode Island, and in 2005, there were 43. So far this year, the Health Department has already received reports of 46 cases of babesiosis. Four Rhode Islanders have died from babesiosis in recent years, the most recent last year. (To put these numbers in perspective, consider that an average of 477 Lyme disease cases and 108 AIDS cases are reported each year.)

A person will experience symptoms of babesiosis three to four weeks after being bitten by an infected tick. With ticks still plentiful today, it looks as though the state will be heading for a record year for babesiosis.

The main reason is the increase in the number of ticks because of the wet weather, said Thomas N. Mather, the entomologist who is director of the University of Rhode Island's Center for Vector-Borne Disease.

"It's a relatively rare infection, but it's increasing for sure," Mather said. "People are still very confused about this. A lot of people think it's just another form of Lyme disease."

But the antibiotic treatment for Lyme disease won't touch the babesia, the blood parasites that cause babesiosis. And if you get both babesiosis and Lyme disease, your symptoms of both will be more severe. The antibiotic and antiparasitic medications used to treat babesia can eliminate the bugs from the bloodstream.

Mather, who has been studying the deer-tick population in Rhode Island for many years, has recently identified the areas where deer ticks are infected with babesia -- primarily the southern half of the state. (In contrast, ticks throughout the state are infected with Lyme disease, except for those in the far northeastern corner, chiefly Cumberland and part of Lincoln.)

The determining factor is the concentration of ticks: babesia-infected ticks live only in areas where there are a lot of ticks. Researchers consider those high concentration areas to be those where they can drag a cloth through bushes and collect 20 or more ticks per hour.

About 12 percent of ticks in the nymphal stage -- the tiny ones that are out in the spring and summer -- carry babesia, while 20 percent carry Lyme disease, Mather says. Some carry both.

The illness is caused by blood parasites called babesia, protozoans that are similar to those that cause malaria. They attack red blood cells. Babesia enter people's bloodstreams in one of two ways: the bite of a deer tick, which must stay attached for 48 hours before it can transmit the parasite; or through blood transfusions (there is no licensed screening test for babesia).

Most infected people have no symptoms, or only mild ones. People who have had their spleens removed, who are very old, or who have an illness that suppresses the immune system can become very ill.

"The tricky part is thinking about it and recognizing it," said Dr. Christopher C. Brown, an infectious disease specialist with an office in South Kingstown. "It's not hard to treat. People tend to respond well to treatment."

Dr. Jennifer Adelson Mitty, an infectious disease specialist who runs the Lyme disease clinic at Rhode Island Hospital, said that this year she is seeing more of all three diseases transmitted by deer ticks: Lyme disease, babesiosis and ehrlichiosis, a bacterial infection that attacks white blood cells. (Ehrlichiosis, also called anaplasmosis, is slightly more common but usually less serious than babesiosis, with an average of 51 cases a year in Rhode Island.)

The antibiotics that treat Lyme disease will also wipe out ehrlichiosis. But not babesiosis, Mitty said.

Asked which of the three is more worrisome, Mitty says it depends on the patient. "In an immunocompromised patient, I'd worry more about babesiosis," she said.

But anyone who gets sick with a fever in the summer should tell his or her doctor, she said.

Dr. Staci A. Fischer, an infectious disease specialist at Rhode Island Hospital, said that in this region, babesiosis is one of the most common diseases that a person can get through blood transfusions. That's because blood donors can temporarily carry the parasite in their blood without knowing it. Blood recipients are likely to have weakened immune systems that will allow the parasite to multiply and make the person ill.

Although a blood test can diagnose babesiosis in a sick person, a good screening test that would detect it in a healthy person's blood has not yet been developed, said Dr. Carolyn Te Young, medical director of the Rhode Island Blood Center.

But in the end, people have considerable control over whether they get any of the tick-borne illnesses. Mather notes that a tick has to be on your body for at least 24 hours before it can transmit illness, so checking yourself daily for the poppy-seed-sized bugs can dramatically reduce your risk of infection. You can also reduce the number of ticks in your yard, and keep them off you by spraying your clothes with permethrin and your skin with DEET.

"People don't have to be scared about tick-borne illness, if they're aware and take precautions," said Brown, the South Kingstown doctor. "People don't have to sit there worrying about it. They can do a lot to take care of themselves."

ffreyer@projo.com / (401) 277-7397

Babesiosis: Key facts

Cause: Babesia, malaria-like parasites that attack the red blood cells.

Transmission: Deer-tick bites or blood transfusions. Deer tick must remain on the skin for 48 hours to transmit infection.

Symptoms: Fever, chills, sweating, muscle aches, anemia. Most people have no symptoms, however, and their immune systems eliminate the parasites.

Who's in danger: People without spleens, elderly people, and people whose immune systems are weakened can get sick from babesia, sometimes severely.

Treatment: Clindamycin plus quinine, or atovaquone plus azithromycin. These drugs can eliminate the parasites.

Geographic distribution: Most common in the Northeast and Midwest.

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Keeping ticks at bay

Check yourself thoroughly for ticks every day. The size of a poppy seed, they tend to crawl to constricted places such as waist bands, the back of the knee, or armpits. If you find a tick, remove it with pointed tweezers.

Apply a repellent containing permethrin on your clothing and one containing DEET on your skin when you go into tick-infested areas. DEET will keep a tick from biting, but not from crawling to a spot where there's no DEET. Wearing long pants tucked into socks and sprayed with permethrin is a good way to keep ticks off you.

Keep the edge of your yard clear of leaf litter.

Hire a trained professional to apply tick treatment to the perimeter of your yard.

For more information, go to tickencounter.org.