Steve Wikse, a retired professor of large-animal clinical sciences, College of Veterinary Medicine at Texas A&M University, answers this month's beef cattle health question.
Q -- Recently I was talking to a friend about rabies in animals. He said rabid cows just lie down and die. I thought rabid cows would act like any other animal with rabies -- wildly running around and attacking anything in their way. Which of us is right?
A -- You are both correct. Rabid cows can show symptoms of either the furious or the paralytic form of rabies. Your question is timely, because rabies is most common in Texas in spring and fall, although cases are reported every month. It's important because Brazos County is usually one of the top counties in Texas in the number of rabies cases reported annually.
Rabies is caused by a virus that circulates in wildlife such as skunks -- the most common reservoir -- plus bats, foxes, raccoons and coyotes. In 2008, rabies was reported in 31 skunks and two bats in Brazos County.
The rabies virus is transmitted from wildlife to domestic animals through the bite of an infected animal. An incubation period between the bite and the onset of clinical signs is usually three weeks in cattle, but can last as long as six months. Incubation periods longer than one year have been reported in humans. There is no effective treatment for rabies in animals or man once the clinical stage is reached at the end of the incubation period. It is always fatal.
Rabies: Know the clinical signs
There is much variation in symptoms of rabid cattle, depending on whether they have the paralytic or furious form of rabies. Cows with paralytic rabies first have a rear weakness with knuckling of fetlocks and swaying of hindquarters. Their tails may be flaccid or pulled to one side and they sometimes strain to pass manure. Bulls can have paralysis of the penis. Rabid cattle may drool. Many cows have yawning movements of their mouths, as if they are trying to bellow. In three to seven days, affected cows lie down, are unable to rise, enter a coma and die within two days.
Cows with the furious form of rabies act alert and aggressive. They are attracted to noise. They may mount uncoordinated attacks accompanied by loud bellowing. Bulls with the furious form of rabies often show sexual excitement. Clinical signs of the furious form of rabies eventually progress to paralysis, coma and death.
Rabies is a not a common disease in cattle. Out of 405 reports of animal rabies in Central Texas in 2008, only two were in cattle. The clinical signs of rabies can look like many other cattle diseases that are much more common. These include polioencephalomalcia (polio), lead poisoning, thromboembolic meningoencephalitis, salt toxicity, tetanus, back injuries and hypomagnesemia (grass tetany). There is a great danger of human exposure to the rabies virus from treating rabid cattle that have a look-alike disease. A rabid cow can fool you. Several years ago one fooled me.
My students and I were called in January to a ranch near Snook to treat a downer cow. The manager said the cow was weak in the hindquarters for a day before going down. She was a crossbred cow in good condition with a one-month-old calf. The cow was lying on her sternum and appeared alert. Her temperature was normal and she was moderately dehydrated. She did not have excessive salivation. We could find no evidence of back or leg injuries. Well, rabies was on our list of possible diseases, but we thought most likely the cow had hypomagnesemia. We collected blood for laboratory work and administered intravenous magnesium and calcium. We also pumped 10 gallons of water through a stomach tube into the cow's rumen.
The next day we knew our diagnosis of hypomagnesemia was not correct. The cow was still down and the laboratory reported her serum concentrations of magnesium and calcium to be normal. Rabies jumped to the top of our diagnosis list. Following three days of deterioration in the cow's condition in spite of daily treatment, I administered a lethal injection, autopsied her and submitted samples to the laboratory for rabies diagnosis. They were positive. Yup, three veterinary students, the ranch manager and I received post-exposure prophylaxis (PEP) for rabies. We were exposed to the cow's saliva when we passed a stomach tube to give her water. By the way, the cow never had excessive salivation, a clinical sign that many textbooks list as present in every case of rabies.
Bottom line
Cows with rabies can have tremendously variable symptoms. Body temperature is usually normal. Some affected animals eat well; others don't. As expected, rabid cows can show symptoms of brain infection, but they may not. Instead they may act lame or act weak, as if they've had a back injury, or even appear to have a metabolic disease such as grass tetany. Do not hesitate to have your veterinarian examine a cow that has any combination of clinical signs that make you suspicious of rabies.
Ask the Vet
Steve Wikse, a retired professor of large-animal clinical sciences, College of Veterinary Medicine at Texas A&M University, answers this month's beef cattle health question.
Q -- Recently I was talking to a friend about rabies in animals. He said rabid cows just lie down and die. I thought rabid cows would act like any other animal with rabies -- wildly running around and attacking anything in their way. Which of us is right?
A -- You are both correct. Rabid cows can show symptoms of either the furious or the paralytic form of rabies. Your question is timely, because rabies is most common in Texas in spring and fall, although cases are reported every month. It's important because Brazos County is usually one of the top counties in Texas in the number of rabies cases reported annually.
Rabies is caused by a virus that circulates in wildlife such as skunks -- the most common reservoir -- plus bats, foxes, raccoons and coyotes. In 2008, rabies was reported in 31 skunks and two bats in Brazos County.
The rabies virus is transmitted from wildlife to domestic animals through the bite of an infected animal. An incubation period between the bite and the onset of clinical signs is usually three weeks in cattle, but can last as long as six months. Incubation periods longer than one year have been reported in humans. There is no effective treatment for rabies in animals or man once the clinical stage is reached at the end of the incubation period. It is always fatal.
Rabies: Know the clinical signs
There is much variation in symptoms of rabid cattle, depending on whether they have the paralytic or furious form of rabies. Cows with paralytic rabies first have a rear weakness with knuckling of fetlocks and swaying of hindquarters. Their tails may be flaccid or pulled to one side and they sometimes strain to pass manure. Bulls can have paralysis of the penis. Rabid cattle may drool. Many cows have yawning movements of their mouths, as if they are trying to bellow. In three to seven days, affected cows lie down, are unable to rise, enter a coma and die within two days.
Cows with the furious form of rabies act alert and aggressive. They are attracted to noise. They may mount uncoordinated attacks accompanied by loud bellowing. Bulls with the furious form of rabies often show sexual excitement. Clinical signs of the furious form of rabies eventually progress to paralysis, coma and death.
Rabies is a not a common disease in cattle. Out of 405 reports of animal rabies in Central Texas in 2008, only two were in cattle. The clinical signs of rabies can look like many other cattle diseases that are much more common. These include polioencephalomalcia (polio), lead poisoning, thromboembolic meningoencephalitis, salt toxicity, tetanus, back injuries and hypomagnesemia (grass tetany). There is a great danger of human exposure to the rabies virus from treating rabid cattle that have a look-alike disease. A rabid cow can fool you. Several years ago one fooled me.
My students and I were called in January to a ranch near Snook to treat a downer cow. The manager said the cow was weak in the hindquarters for a day before going down. She was a crossbred cow in good condition with a one-month-old calf. The cow was lying on her sternum and appeared alert. Her temperature was normal and she was moderately dehydrated. She did not have excessive salivation. We could find no evidence of back or leg injuries. Well, rabies was on our list of possible diseases, but we thought most likely the cow had hypomagnesemia. We collected blood for laboratory work and administered intravenous magnesium and calcium. We also pumped 10 gallons of water through a stomach tube into the cow's rumen.
The next day we knew our diagnosis of hypomagnesemia was not correct. The cow was still down and the laboratory reported her serum concentrations of magnesium and calcium to be normal. Rabies jumped to the top of our diagnosis list. Following three days of deterioration in the cow's condition in spite of daily treatment, I administered a lethal injection, autopsied her and submitted samples to the laboratory for rabies diagnosis. They were positive. Yup, three veterinary students, the ranch manager and I received post-exposure prophylaxis (PEP) for rabies. We were exposed to the cow's saliva when we passed a stomach tube to give her water. By the way, the cow never had excessive salivation, a clinical sign that many textbooks list as present in every case of rabies.
Bottom line
Cows with rabies can have tremendously variable symptoms. Body temperature is usually normal. Some affected animals eat well; others don't. As expected, rabid cows can show symptoms of brain infection, but they may not. Instead they may act lame or act weak, as if they've had a back injury, or even appear to have a metabolic disease such as grass tetany. Do not hesitate to have your veterinarian examine a cow that has any combination of clinical signs that make you suspicious of rabies.