Thursday, June 22, 2017

Mosquito Season

Written by  Dr. Eva Hersh, MD

Dear Dr. Eva,

The mosquito season is already exploding in my back yard. After a couple of hours of gardening, my wrists and ankles have so many bites they are swollen. I put on mosquito repellent, but is there anything else I can do? I’m thinking of hiring a mosquito spraying service, but I wonder if the spray is dangerous and also won’t it interfere with the ecosystem?


Dear Bitten,

The most important thing you can do to decrease the number of mosquitoes in an area (like your yard) is to get rid of all standing water. Mosquitoes can breed in large numbers in less than an ounce of water, even in the small amount of water that collects in a bottle cap.

Water collects in saucers under planters, on tarps or plastic bags, in cups and bottles, rainwater collection systems, bird baths, children’s pools, tires, and many other things. Drain the water off and put the object where it won’t collect water next time it rains. Hydroponic planters, like Earthbox and Growbox, can grow huge numbers of mosquito larvae in their water tanks. This can be prevented by placing aluminum foil caps on top of the water pipe. If you want to keep water in a bird bath, change the water and wash out the bath basin once a week.

That should help, but if we have a bad mosquito season or you live near areas with standing water you cannot drain, and bug repellent does not do enough, then a spraying service is safe and reasonable. The drawback is the cost, about $90 a month through the mosquito season. The main pesticide these services use is pyrethrin, a derivative of chrysanthemum leaves. Pyrethrin is also used to treat children for scabies and head lice. It is safe for people and animals.

As far as the ecosystem, the mosquitoes will always come out ahead. They reproduce in the hundreds of millions. In areas with dangerous mosquito-borne diseases like malaria and yellow fever, heavy spraying year after year has not reduced the numbers of new mosquitoes that hatch each year.

Dear Dr. Eva,

Could you please give an update on diseases carried by mosquitoes? Is West Nile virus still a problem? What about Zika? Does Zika affect people other than pregnant women? Is it possible that other mosquito – carried diseases could come to our area due to climate changes?


Dear Concerned,

There are many mosquito-transmitted illnesses. Most are caused by viruses (Zika, West Nile, Dengue fever, Chikungunya fever), and some by parasites (malaria, heartworm in dogs). There are no vaccines available for most of these infections.

West Nile virus infections are still occurring in Maryland. West Nile is rarely diagnosed because the symptoms are the same as many other viruses: rash, fever, headache, conjunctivitis (“pink eye”), joint and muscle pain. Rarely, West Nile causes life-threatening brain inflammation and swelling (meningitis and encephalitis). Originally discovered in Uganda in 1937, West Nile was first found in the U.S. in 1999 and has now been found in 41 states and the District of Columbia.

Yellow fever was a serious problem in Maryland in the 1800s. Wealthy people left their downtown homes in the summer to avoid yellow fever by staying in higher, drier areas. This is no longer necessary because mosquito-infested swamps were drained, and regular spraying continues to kill mosquitoes and their larva (the immature, water-living life stage.)

Some mosquito-borne diseases can only be carried by one certain species of mosquito. This is the case with Anopheles, the mosquito that carries the malaria parasite. The Anopheles mosquito, and malaria, are widespread in tropical and subtropical regions especially in Asia, Africa, and South and Central America.

There are many examples of changes in the distribution of mosquito-borne illnesses. For example, from its discovery in 1947 until 2007, the Zika virus was found only in Africa. In 2007, Zika was found in Micronesia (South Pacific islands). By 2015, Zika virus had crossed the Pacific and arrived in Brazil. Zika virus transmission has now been found in 40 countries, including a few cases in Florida. Zika virus can be sexually transmitted as well as transmitted by mosquitoes. 80% of people infected with Zika virus don’t feel sick at all. The 20% who do feel ill have the same general viral symptoms described above for West Nile virus. The real danger of Zika is the risk to fetuses. When women are infected with Zika during pregnancy, there is a high risk of microcephaly in the infant. Microcephaly is a severe developmental defect with underdevelopment or total lack of development of the brain. Other birth defects are also common after maternal Zika infection.

Scientists who study climate change expect that, since mosquitoes reproduce and survive better in warmer temperatures, mosquito-borne diseases are likely to increase and spread north as temperatures warm. Malaria, which is by far the most deadly human mosquito-borne disease, has already been found at higher altitudes where temperatures are rising.

Eva Hersh is a Baltimore family physician. Send your comments and questions to her by email at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .


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