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Virus Hunters Find Cattle Disease

December 2, 2014

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UC Davis researchers involved in this study on astroviruses in cattle include members of Patty Pesavento's lab and CAHFS: Santiago Diab, Sabrina McGraw, Bradd Barr, Ryan Traslavina, Robert Higgins, Pat Blanchard, Guillermo Rimoldi and Patricia Pesavento. This article originally appeared in The Western Producer and was written by Jamie Rothenburger.

Scientists at the University of California Davis have discovered a new virus associated with brain inflammation in cattle. They had considered the usual causes when a yearling steer developed clinical signs consistent with brain and spinal cord inflammation, including rabies virus, herpes viruses, histophilus somni, listeria monocytogenes, which is the causative agent of listeriosis, and sporadic bacterial causes such as salmonella. BSE, the parasite neospora caninum and non-infectious causes such as lead poisoning and thiamine deficiency, sometimes referred to as polio, were also considered.

Since none of the known causes were found, the scientists then went hunting for novel viruses. They used a new technique called metagenomics to search for virus genes and discovered a new virus, which they called BoAstV-NeuroS1. Genetic analysis grouped it in the astrovirus family. It is distantly related to other astroviruses that cause diarrhea in infant children and a shaking disease in farmed mink.

The scientists’ next step was to look back at cases of neurological disease in cattle in which an exact cause was not found. Three of the 32 cattle that were tested had this new virus in their brain tissue.

Researchers in Switzerland independently found the same virus in cattle with neurological disease, further supporting the discovery. More research is needed to understand how the new virus is transmitted, what proportion of cattle is exposed, what other countries have infected cattle, how the virus causes brain inflammation and if a vaccine can be developed.

Diagnosing the cause of neurological disease in cattle can be challenging. Clinical signs can include behaviour changes such as loss of appetite, teeth grinding, restlessness and increased excitability. More dramatic signs include wobbly gait, loss of balance, blindness, paralysis, head-pressing, inability to stand, seizures, coma and death.

It is important to determine the exact cause when brain inflammation is found during an autopsy. We don’t want to miss a case of rabies or BSE. For example, a public health investigation is needed if a heifer is diagnosed with rabies to determine if people were exposed and require treatment.

The new virus discovery is important because it gives us one more option for which to test. We look at microscopic pieces of brain tissue and can usually confirm or rule out causes such as neospora and bacteria. However, changes to brain tissue all appear pretty much the same when it comes to viruses. It takes additional testing to find out which virus is the cause, but generally we have to know what to look for. 

This is where the virus hunting technology shines. Metagenomics recently became affordable and fast enough to apply to cases such as the steer in the California study. The technology was originally developed and perfected through the Human Genome Project, which provided the funding to automate these techniques and make them cheaper. As a result, tissues from clinical cases can now be screened for unknown viruses.

An explosion of new viruses has been discovered in recent years. Scientists previously had to grow viruses in cultures, which left a vast amount of virus types undiscovered because they cannot be propagated in this way. Other molecular techniques be-came available in the last 30 years that didn’t require virus culture, but the investigators still had to have a rough idea what virus type they were looking for. These virus hunters can now “shoot in the dark” and come up with answers.

The astrovirus discovery is just the tip of the iceberg. Metagenomics will quickly open up the world to many bacteria and viruses that were previously unknown. It is an exciting time for infectious disease discovery as science continues to document, explain and understand the natural world around us. As well, veterinary pathologists now have one more virus to test for in cattle with brain inflammation.

 Dr. Jamie Rothenburger is a veterinarian who practices pathology and a PhD student at the Ontario Veterinary College.