Blood sample identification is an essential part of a forensic investigation particularly for serious and violent crimes. Blood from animals and humans may appear similar to the naked eye but their biochemical aspect differs. Analyzing whether or not evidentiary blood stains are human-sourced is crucial in forensic investigations.
What is Forensic Serology?
Forensic Serology is the study and examination of body fluids such as semen, blood specimens, sweat, urine, saliva etc. It is used in primarily for investigation purposes in criminal and medico-legal cases. Blood is the most commonly recovered and an important piece of evidence in forensic investigations.
How to differentiate between human and animal blood?
There are various lab methods every forensic student should know to differentiate between human and animal blood. These are:
1. Microscopic Analysis
Forensic scientists can identify whether a sample is from a human or an animal by looking at the structure and form of red blood cells.
A respiratory pigment is present in the majority of animal blood. The respiratory pigment hemoglobin which binds four oxygen molecules is composed of four polypeptide chains in human blood. Human hemoglobin is attached to iron. Transporting oxygenated red blood cells to different body systems and organs is its primary function. Without the binding of haemoglobin to oxygen, it will difficult for end organs to receive oxygenated blood from the lungs. So in essence, respiratory pigment is the engine of the vehicle (blood) for effective transportation of nutrients and substances around the animal or human body.
In animals, the case is different because they have not just one but 4 respiratory pigments namely - Haemoglobin (present in mammals), Haemerythrin (found in marine invertebrate animals like the annelid worm), Haemocyanin (they contain copper and found in arthropods and molluscs) and Chlorocruorin (This is violet in colour and mostly found in Marine polychaete worms).
Therefore you would’nt anticipate seeing the typical red-colored hemoglobin found in human blood when examining animal blood animals do have hemoglobin but it is different. This is an important point to note: hemoglobin is what gives blood its color. Since humans have a different type of hemoglobin than animals our blood is primarily red. Essentially this can also serve as a differentiator. Another differentiator between human and animal blood is the shape of RBCs. Human red blood cells have a round biconcave shape whereas many animals have oval shaped RBCs.
White blood cells (WBCs) are also an important differentiating factor. Human blood contains only five different types of white blood cells but the blood of animals contains a variety of types. Haemocytes are the cellular component of animals in addition to the various white blood cells in human. These cells play a protective role in immunity where they fight against foreign pathogens that invade the body system.
These distinctive features are clearly visible under the microscope.
2. Teichmann and Takayama test
Bloodstains at crime scenes are a major source of DNA and crucial pieces of evidence. The two most popular microcrystal confirmatory assays for blood are the Teichmann and Takayama tests.
Both the Teichmann test and the Takayama test are qualitative techniques used in forensic science to verify whether blood is present in a sample and to support the distinction between human and animal blood. The foundation of these tests is the identification of distinctive crystals that form when hemoglobin is present.
3. Pattern Analysis of Bloostains
Bloodstains pattern analysis can identify whether the blood is human or animal. Animal and human blood behaves differently in different situations. For example the viscosity and volume of human blood can result in bloodstains that differ from those produced by animals. Based on the features of the stains bloodstain pattern analysis can assist forensic specialists in identifying whether the blood is from an animal or a human. Higher blood pressure or the sheer volume of blood in larger animals like cows, pigs or horses may result in larger droplets and more dispersed patterns. On the other hand the blood of smaller animals such as cats or dogs may produce more localized and smaller patterns. Blood patterns are also influenced by animal behavior: an animal that is escaping or that has been hurt may produce different stains than a person who has been stabbed or involved in an altercation. Particularly when used in conjunction with other techniques bloodstain pattern analysis helps forensic specialists make educated guesses about whether the blood is human or animal and offers valuable insight into the dynamics of a crime scene.
4. Electrophoresis of Haemoglobin
Hemoglobin proteins are separated using electrical currents in this method. Hemoglobin patterns vary by species which can be used to identify whether the blood is human or animal.
5. Marker enzymes
Blood contains specific enzymes that can be used to differentiate between species. Experts in forensics can determine the bloods species of origin by examining these indicators.
6. Immunological Testing
There are various proteins that are unique to certain species and these are detected by specific antibodies. Based on these distinctive proteins, immunoassays such as ELISA can distinguish between human and animal blood.
7. Precipitin Test
Precipitin test is the most reliable test to distinguish between human and animal blood. This test detects human serum proteins using specific antibodies.
8. Raman Spectroscopy
This is a modern non invasive test that uses laser beam for analyzing the molecular composition of the dry bloodstain to determine whether it is human or animal blood.
Conclusion
To conclude it is crucial for forensic professionals to be able to distinguish between human and animal blood. This will be useful in the investigation of criminal cases, hit-and-run cases and other incidents.