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Toxicity Testing : An Overview of Clinical Signs of Toxicity

Nearly 15 million animals are used across the world each year for testing medications and newly developed drugs. The National Centre for Laboratory Animal Sciences in Hyderabad supplies nearly 50,000 animals to various laboratories in India.

Why are so many animals sacrificed at the hands of scientific researchers? What do these researchers look for during animal testing of drugs and chemicals?

These experimental animals are used to test the safety and efficacy of newly developed drugs or enhance the potential of those previously known. Labelled as toxicity testing, the toxic effects of a drug noticed in animal species will predict its adverse effects in humans. Thus, it is an important step in the drug development process. Without animal testing, no drug can be approved for a clinical trial on human beings.

Thus, researchers look for the signs of toxicity produced by any drug on these experimental animals. This helps them to approve or disapprove a particular drug for human use.

But, what are these signs of toxicity? How to know that they have been produced by the investigational drug?

Here, we have compiled a comprehensive list of clinical signs of toxicity that indicate a harmful effect of the investigational drug on animals.

1.Respiratory System

  • Blockage of nostrils
  • Changes in skin colour
  • Changes in rate and depth of breathing
  • Abdominal breathing, gasping, apnoea, cyanosis
  • Pulmonary oedema
  • Increased mucus production in the respiratory passages

2.Locomotor System

  • Increase/decrease in spontaneous motor activity
  • Somnolence or unusually sleepy
  • Loss of the righting and other reflexes
  • Loss of sensation of pain
  • Unusual gait
  • Tremors and fasiculations
  • Muscle cramps
  • Twitching of muscles

3.Ocular System

  • Abnormal watering of eyes
  • Abnormal constriction or dilation of the pupil
  • Drooping of eye-lids
  • Inflammation of the iris or conjunctiva
  • Bulging of eyes
  • Opacity of cornea

4.Cardio-vascular System

  • Abnormally slow or fast pulse
  • Abnormal heart rate or rhythm
  • High or low blood pressure
  • Heart attack
  • Abnormal constriction or dilation of coronary arteries
  • Difficulty in breathing

5.Digestive System

  • Involuntary evacuation of faeces
  • Involuntary urination
  • Blood discharge in stool or urine
  • Vomiting
  • Loose stools or increased frequency of stools

6.Nervous System

  • Giddiness
  • Apathy or indifference to its surroundings
  • Drowsiness
  • Convulsions or seizures of any type
  • Lack of muscle coordination leading to the loss of voluntary movements
  • Confusion
  • Coma

7.Other signs

  • Increased salivation or drooping of saliva
  • Piloerection or involuntary erection of the hair of the skin
  • Increase or decrease in muscle tone
  • Excessive dryness of the mouth

Besides these signs, decreased body weight and death of the experimental animal are also considered as the signs of toxicity. Plus, the researchers also examine the cells and tissues of the animals under a microscope to determine any histopathological changes in addition to the clinical signs.

The results of this toxicity testing are compiled to calculate the benefit: risk ratio of the investigational drug, a leading point to proceed or deter from further clinical trials.

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