How Is Rabies Diagnosed in Animals?

Significance

• Animals infected with the Rhabdoviridae virus that causes rabies secrete the virus in saliva and transfer it to other animals through bite wounds. Only mammals contract rabies, with most cases appearing in skunks, bats, foxes and coyotes. The American Veterinary Medical Association states that, "In recent years, cats have become the most common domestic animals infected with rabies...because many cat owners do not vaccinate their cats before the cats are exposed to rabid wildlife outdoors." Rabies also occurs in cattle, horses, goats, swine, sheep, ferrets and dogs in smaller numbers.

Harvesting Procedure

• The test for rabies in animals requires harvesting tissues from at least two locations in the brain, "preferably the brain stem and cerebellum" says the CDC, requiring that the animal be euthanized. Labs examine the infected animal's brain because the Rhabdoviridae virus that causes the disease travels from the bite wound to the central nervous system to cause symptoms including ataxia (muscle incoordination), aggression, paralysis of the jaw and throat, and drooling and foaming of saliva, according to The Animal Health Channel.

  Veterinarians either remove the samples in the clinic or ship the entire head to a state public health laboratory for diagnosis. A veterinary diagnostic laboratory can also test for rabies depending on individual state regulations and the type of test required.

Types of Tests

• Typically, a laboratory conducts a direct fluorescent antibody test (dFA test) on the sample brain tissue. A fluorescent rabies antibody binds to a specific protein and antigen of the rabies virus. The body produces this specific antigen as an immune response to the rabies virus. Because the virus grows within the brain cells, the dFA test shows bright green dots and particles within the individual brain cells when they are viewed microscopically.

  According to the CDC, using an electron microscope allows lab technicians to view the individual viral cells in detail. Described as bullet-shaped particles, the stained rhabdovirus resides in the upper portions of brain and spinal cord cells where it destroys their functioning. The electron microscope works so well that scientists can see RNA particles of the virus in the nucleus of its cells.

  Occasionally, scientists will stain suspect brain tissue and view it under a microscope looking for evidence of a strain of encephalomyelitis (brain and spinal cord inflammation) that shows the rabies virus infiltrating single nerve cells and gathering around white blood cells in blood vessels of the circulatory system. This histopathology (disease-causing tissue changes) also includes the discovery of Negri bodies---round cellular particles found specifically in the nerve cell of animals dead of rabies.

  Immunohistochemistry methods of diagnosing rabies test for the disease in formalin-fixed (not fresh) brain tissue by staining the tissue and testing for specific antibodies using ultra-sensitive enzymes.

  When the tissue sample contains a smaller than normal amount of rabies virus, scientists amplify the virus in cell cultures, allowing it to grow in the cells of mice and baby hamsters. They also replicate the virus' RNA in its own DNA molecule creating a chain reaction that confirms rabies in saliva and skin biopsies.

History

• The CDC states that by the end of the 19th century, Louis Pasteur's experiments with rabies vaccines led other scientists to discover and note the pathology of the rabies virus. The discovery of inclusion bodies (parts of the rabies virus found inside the nerve, brain, tongue and salivary gland cells of rabid animals) in 1903 by Dr. Adelchi Negri gave scientists and veterinarians another way of diagnosing rabies by viewing these Negri bodies under the microscope. This was the most common form of diagnosis prior to the development of the dFA antigen test in the 20th century.

Regulations

• The Animal and Plant Health Inspection Service, part of the U. S. Department of Agriculture, regulates all rabies test kits (typically the dFA antigen test) in U.S. laboratories, keeping them up-to-date and making sure they include the same materials. Labs are directed to report all positive tests to the USDA's Center for Veterinary Biologics.