Blooms of blue–green algae grow rapidly and appear like a suspension of greenish paint or curdled greenish milk on downwind water surfaces. Ranging in colour from pale green to blue–green, dark green to brown, blue–green algal blooms often have a foul sewerage-type odour.
Blooms form a scum on shorelines when concentrated by winds or currents and can also form a suspension in the water.
Harmful toxins produced by some species can kill large numbers of stock quickly with most deaths occurring during late summer and early autumn.
Toxicity can change rapidly and can increase as a bloom ages or starts to die. Some toxins can persist for more than three months before being degraded by sunlight and microbial activity.
As little as one cup or many litres of toxic water can cause death. Signs of poisoning vary among livestock and also depend on which algae are present.
Cattle, sheep and goats are susceptible and usually develop muscle tremors and start staggering within 30 minutes of drinking toxic water. Signs can include breathing difficulties, muscular weakness and paralysis of skeletal or respiratory muscles.
Affected stock lie down and die with convulsions within 24 hours. The few animals that survive the first 24 hours develop liver damage, jaundice and photosensitisation and could die 1–2 weeks later or become chronically ill from liver damage.
Medicinal activated charcoal or bentonite are the only treatments available and need to be administered by a veterinarian before signs develop. The treatment is expensive and is an option only for very valuable stock.
Sometimes the algal scum can be found on the forelimbs, lips and muzzle. Most commonly, stock are simply found dead near affected water.
Sheep are more likely to be affected than cattle as they tend to drink from dam edges whereas cattle often wade out into the dam beyond the toxic area. Dogs that lick their coats after swimming in contaminated water can also be poisoned.
If algal poisoning is suspected, prevent access by stock and arrange access to a different water supply. If stock must be moved to another paddock, do it at a very easy pace.
Once the stock has been moved, seek veterinary advice to confirm the diagnosis and also send a sample of algae for identification.
Algae submitted for identification should be fresh and healthy, so ideally, should reach the laboratory within 24 hours of being collected. Collect 100mL or more of water containing the most concentrated (strongest colour) algae in the dam, refrigerate the sample and courier to:
Duty Pathologist, Animal Health Laboratory
Department of Agriculture and Food WA
South Perth WA 6151
Check water sources daily for blooms and ensure stock and people are isolated from contaminated water if blue–green algae is suspected and have water samples tested. Water toxicity can vary from hour-to-hour and day-to-day and water can be toxic to stock even after a bloom has disappeared.
Do not allow stock access to affected water for at least 10 days after a bloom has disappeared. Deep dams are better than shallow dams as they reduce algal use of the sun’s energy.
Treating algal blooms
Ferric alum can be used to treat algal blooms but is best used for prevention. It restricts algal growth by removing phosphorus from the water. Apply 50g of ferric alum per 1000L of water by placing it in a porous bag attached to a float.
Algicides or herbicides are not recommended for algal control as they can make water more toxic. Decaying algal cells release even higher levels of toxins which can persist for many months.
Water tanks that become contaminated with algae may be controlled by covering the tank to exclude light and reduce the temperature.
Nutrient levels in farm dams may be reduced by screening systems that reduce fouling of dam water by stock excreta and plant debris, by restricting the debris from entering the dam.
Algae can be killed with several chemicals, including Simazine, calcium hypochlorite and Cupricide. For best results, treatment should be done when algal development is first seen.
Note that some chemicals may be toxic to plants or livestock and aquatic animals, such as fish and crustaceans.
It is important to read the product label carefully and follow the manufacturer’s instructions. Chemical treatment of toxic algae can cause a sudden release of its toxin, which can persist for several days.
Do not use water containing toxic algae, and which has been treated with Simazine, for irrigation or watering livestock for two weeks to ensure the residual effects of both the algal toxin and simazine have dissipated.
Straw breaks up algae
Another preventative measure is spreading 100g of barley straw over 1000L of water (apply by mooring it in plastic mesh bags) — this breaks up algal rafts. It can take one month to start working but lasts for up to six months.
Farm dams should be monitored regularly for outbreaks of blue-green algae and treatment sought quickly if an outbreak does occur.