In this article we will discuss about the determination of LC50 and LD50 of toxicants.
Determination of LC50:
Different procedures are adopted depending on the size of the organisms subjected to a toxicant.
Mosquito Larvae (Test Animal):
Four 1000 ml glass beakers are half-filled with tap water/tank water. A measured amount of toxicant-liquid/solid—is thoroughly mixed with water in three beakers—1, 2, 3 to prepare an aquatic medium of toxicant of three known concentrations.
The concentration of the toxicant in the three beakers should be in a gradually increasing order from a lower to higher level. The fourth beaker containing only water is used as a control. Properly label the beakers with necessary information.
A batch of 10 mosquito larvae of same age and size are released in each beaker.
Observe the behaviour—movement, irritability, etc. of the larvae both in test and control beakers and record those with reference to time of exposure.
Continue observation, count the dead larvae in each beaker and record the time of death till the mortality reaches 50%.
The period of 50% mortality is dependent on the concentration of the toxicant.
A dose-response curve for each dose may be prepared by plotting the period of exposure on x-axis (abscissa) and the number of dead larvae on the y-axis (ordinate).
A dose — 50% mortality time curve may be prepared by plotting dose (concentration of toxicant) on x-axis and time for 50% mortality on y-axis.
Major Carp Fingerlings (Test Animal):
Four aquaria measuring 75 x 30 x 50 cm each are filled with tank water, preferably from a fish fingerling culture pond up to a height of 45 cm. A measured quantity of toxicant—liquid/ solid is thoroughly mixed with water in three aquaria — 1, 2, 3 to prepare an aquatic medium of toxicant of known concentration.
The concentration of the toxicant in the aquaria should be in a gradually increasing order from a lower to higher level. The fourth aquarium containing only water is used as a control. Properly label the aquaria with necessary information. All the aquaria are covered with mosquito net to prevent jumping out of fingerlings.
A batch of 10 healthy fingerlings — size 75 mm — are released in each aquarium. Regular food supply and aeration of aquarium water is maintained.
Observe the behaviour—movements irritability, loss of balance, etc. of the fingerlings — both in test and control aquaria and record these with reference to the time of exposure.
Continue observation, count the dead fingerlings in each aquarium and record the time of death till the mortality reaches 50%.
The period of 50% mortality is dependent on the concentration of the toxicant.
A dose-response curve for each dose may be prepared by plotting the period of exposure on x-axis and number of dead fingerlings on y-axis.
[Dose-50% mortality time curve (p. 496)]
Determination of LD50:
Albino Mice (Test Animal):
Four batches of healthy albino mice, each consists of six individuals of almost same age group and weight are taken. The toxicant may be used in solid/liquid state. The doses are prepared by mixing the toxicant with food at the ratio mg (toxicant)/kg (weight of mice) — 1 mg/kg, 2 mg/kg, 3 mg/kg.
The total amount of food for each concentration of toxicant should be such that it can be consumed by 6 mice of a batch in one meal. Six baits are prepared for each concentration. The total number of baits will be 3 x 6 = 18. Six baits are prepared without toxicant for the control batch.
The animals are not given food for about 12 hours prior to commencement of the experiment. The control group is treated in the same way.
The fasting animals are given the bait—one for each mice and water. The control batch is given bait without toxicant and water.
Observe the behaviour—movement, irritability, lack of activity, balance, etc. in both the test and control mices and record those with reference to the period of exposure.
Continue observations, count the dead mices in each batch and record the time of death, till the mortality reaches 50%.
The period of 50% mortality is dependent on concentration of the toxicant.
A dose-response curve for each dose may be prepared by plotting the period of exposure on x-axis and the number of dead mices in y-axis.
[Dose-50% mortality time curve]