NEWS RELEASE: (Strasburg, Va.) – The North Fork of the Shenandoah River in Shenandoah County is experiencing a benthic harmful algae bloom (HAB) caused by cyanobacteria. Out of an abundance of caution, the public is advised to avoid contact with blue green algal mats and scum in the river from Bethel Road to Strasburg. Some harmful algae, called cyanobacteria, can cause skin rash and gastrointestinal illnesses, such as upset stomach, nausea, vomiting and diarrhea. Cyanotoxins, including those identified at this location can be fatal to dogs and other animals when ingested. The cyanobacteria in this portion of the river creates algal mats. The cyanobacteria are producing toxins which are contained within the mat material only – avoid contact with mats by people and animals. The area of the river where algal mats should be avoided can be seen on an interactive map at

On July 13, 2021 and July 19, 2021, the Department of Environmental Quality collected samples at two sites on the river – at Bethel Road and Strasburg, near S. Holliday St. (Rt. 648). The sample results and observations made during sampling indicated swimming advisories were necessary due to wide-spread potentially toxic cyanobacteria mats observed and the identification of toxins detected within the cyanobacteria mats which may pose a human health risk should contact or consumption occur. An initial advisory was issued by the Lord Fairfax Health District July 16, 2021, in an abundance of caution to alert the public to the potential cyanobacteria bloom being investigated. That initial advisory is being expanded to include locations along  the river from Bethel Road to Strasburg.

People and pets should avoid contact with the mat material and ensure pets and livestock avoid consuming this material to avoid poisoning.Accidental ingestion by dogs and other animals often includes the animals licking their fur following swimming.  Cyanotoxins, including those identified at this location can be fatal to dogs and other animals, with dog deaths occurring suddenly following exposure.  People should also avoid allowing their pets to swim in areas where mat material is observed. Humans should never consume water or material from a natural waterbody because this water is not treated water and is not suitable for consumption. People are advised to avoid swimming, wading, and stand-up-paddle-boarding as well as other activities that pose a risk of coming in contact with mat material. Boating such as kayaking and other forms of activity, where contact with the cyanobacteria mat material are low, are activities which may continue. There were no potentially toxic cyanobacteria cells or cyanobacteria toxins detected within the water column itself at either site noted above.  Cyanobacteria and cyanobacteria toxins were only observed within the cyanobacteria mat material.  The Virginia Department of Environmental Quality (DEQ) field staff that investigated the waterway did not report the presence of the cyanobacteria mats across the extent of the river segment, and that the most wide-spread mats were at Bethel Road and a downstream location near Strasburg.  DEQ staff did not observe cyanobacteria mats elsewhere in the river, including the Deer Rapids location. Mats may however move in waterways.

Follow-up monitoring and sampling for this area is planned (weather permitting) for early next week.

The section of the North Forth of the Shenandoah River currently under advisory:

North Fork of Shenandoah River

  • From Bethel Road (near the intersection of Rt. 644) and at Strasburg, near S Holliday Street (Rt. 648) near Strasburg, VA. This river segment is approximately 5 miles in length.

The river serves as the raw water source for the Town of Strasburg, which provides public drinking water for a population of approximately 6,495. Since the bloom was discovered, the Town of Strasburg has adjusted the treatment protocol and increased monitoring. VDH has observed no evidence of impacts to drinking water at this time.

The North Fork of the Shenandoah River is a popular local recreation area for boating, swimming and fishing.  Advisory signs will be posted along the River shoreline at public access points within the advisory areas in both English and Spanish. Please observe the advisory signs in these posted areas.

Algae blooms can occur when warm water and nutrients combine to make conditions favorable for algae growth. Most algae species are harmless, however, some species may produce irritating compounds or toxins. Avoid discolored water, scums or mat material that are green or bluish-green because they are more likely to contain toxins.

To prevent illness, people should:

  • Avoid contact with any area of the lake where water is green or an advisory sign is posted,


  • Do not allow children or pets to drink from natural bodies of water or consume material in the water or along the shoreline.
  • Keep children and pets out of the areas experiencing a harmful algae bloom and quickly wash them off with plenty of fresh, clean water after coming into contact with algae scum or bloom water.
  • If you or your animals experience symptoms after swimming in or near an algal bloom, seek medical/veterinarian care. You may also contact your local poison control center.
  • Additional resources to pet owners and veterinarians are available from the CDC through this link:
  • To ensure fish fillets are safe to eat, properly clean fish by removing skin and discarding all internal organs, and cooking fish to the proper temperature.
  • To view the advisory area, view the HAB Map online
  • To report a report an algae bloom or fish kill, please use the online HAB report form:
  • If you suspect you or your animal experienced health-related effects following exposure to a bloom, contact the Virginia Harmful Algal Bloom Hotline at 1-888-238-6154.

The Virginia Harmful Algal Bloom Task Force, which includes the Virginia Department of Health, Virginia Department of Environmental Quality, and the Old Dominion Phytoplankton lab, will continue to monitor the cyanobacteria bloom in the River. In general, advisories will be lifted following two consecutive test results with acceptable levels for algal cell counts and/or toxin concentration. An advisory may be lifted or maintained at the discretion of the health department. For example, after one test an advisory may be lifted if results are within safe levels for swimming if other information indicates exposure or human health risk is low.

A status report has been created to reflect the HAB advisory status of North Fork of Shenandoah River which may be found at “North Fork of Shenandoah River Status Report (7.23.21)”.