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Last Updated: Jul 23rd, 2009 - 18:22:26 |
From the Desk of:
Dr. Kendra B. Ewalt, DVM
Central Kentucky Veterinary Services
(A full service camelid veterinary hospital)
400 Bethlehem Rd.
Paris, KY 40361
859-987-6721
I have received numerous phone calls in the last couple of weeks regarding BVD in alpacas. Questions range in scope: what is it?, where does it come from?, why does it cause different disease syndromes?, what is a “PI”?, how do we treat &/or prevent it?, what are the implications for showing, breeding ,travel, etc.? how do we test for it?, should we start testing?, just to name a few.
Many people I have spoken with have said to me that they are very confused about this disease and are having a difficult time understanding what they are hearing or reading. I decided to write a discussion article in order to help alpaca and llama owners better understand the virus and disease processes, as well as available testing procedures and recommendations for owners.
This can be a very confusing disease-causing virus, so I’ll try to keep things fairly simple. I will describe the diseases as they occur in cattle, as this is the better-known model. We can then take the information known about the disease in cattle and use it to better understand possible disease processes in camelids.
Finally, I will relay my recommendations for testing and prevention of the virus/disease. These recommendations come from studying and researching the current published papers, as well as discussions with virologists and veterinarians. These represent my opinion based on my own experiences and my studies. My recommendations are not meant to be substituted for consultation with your veterinarian. You should work with your veterinarian to tailor a BVDV control program specific for your farm and situation.
BVD (Bovine Viral Diarrhea)
An Emerging Issue in Camelids
Bovine viral diarrhea has long been an important cause of illness and death worldwide in cattle. Until recently it was thought to be an uncommon cause of illness in camelids.
Bovine viral diarrhea virus (BVDV) can cause different diseases of varying degrees of severity in cattle. There are two types of BVDV: Type I and Type II. These can cause similar disease syndromes in cattle, but Type II BVD is usually a more severe disease (although Type I can also cause severe disease and death). Also within these two types of BVDV exists another variant. Each type can be classified as cytopathic or non-cytopathic. Both Type I and Type II also have many different strains. Simply stated, there are many, many different strains of the BVD virus.
Infection with a BVDV can lead to one of many disease syndromes, mostly dependent on the age of the animal when it becomes infected. These can be grouped into the following categories: (1) Fetal infections occurring in-utero, (2) Acute infections occurring after birth through adulthood, and (3) Mucosal Disease. Within each of these broad categories, multiple outcomes exist and are listed in the following table.
Table 1.
Fetal Infection Outcomes:
1. Early embryonic death
2. Abortion
2. Stillbirths
3. Congenital Birth Defects
4. Persistently Infected Animal
5. Normal Animal Born with Antibodies to BVDV
Acute Infection Outcomes:
1. Subclinical—no sign of disease
2. Severe acute BVD infection
3. BVD Infection with or without secondary disease complications (often pneumonia in cattle)
Mucosal Disease 1. Persistently infected animal superinfected with cytopathic strain (causes severe disease and death)
Fetal Infections: A cow that becomes infected while she is pregnant can pass the infection on to her unborn fetus. During the first 120 days a calf is developing, its immune system is incomplete and not able to mount an immune response against the invading virus. When a fetus is infected during that time frame, one of two things may occur. The fetus will either succumb to disease and die (usually the case when infection occurs before 60 days of pregnancy), resulting in early embryonic death loss or abortion; or the fetus will survive and be born as a persistently infected (PI) animal (occurs between 60 and 120 days of pregnancy).
In the situation of a persistently infected animal, the fetus becomes infected with the virus, but since its immune system is still incompetent, its body does not recognize the virus as a “foreign” entity and does not produce antibodies to fight off the virus infection. In other words, these animal’s immune systems think that this virus is a normal part of the body, they are “immune tolerant” of the disease. These “PI” animals are born with a large population of circulating virus that continues to replicate (breed/increase in number). PI animals shed a large number of virus particles in all secretions (nose, mouth, and eye discharges, feces, and semen) daily for the rest of their lives. PI animals may be born looking completely normal; or they may be weak at birth, grow poorly, be susceptible to common diseases (because BVDV causes suppression of the immune system, leaving the animal incapable of fighting off many of the common illnesses), and die before they are weaned. Most PI calves live less than one year and usually die before weaning; but some PI calves have grown normally and reached breeding age. They subsequently produce more PI calves since the virus can be transmitted in-utero (presenting a scenario where the virus is passed from generation to generation).
Fetuses that survive infection after 120 days of pregnancy may be born normal or with birth defects. In cattle, the immune system is developed enough to fight off the virus at about 180 days. Those fetuses that successfully fight off the viral infection will be born with antibodies to the virus. They may be completely normal at birth or they may have one or multiple birth defects (eye defects/blindness, skeletal abnormalities, brain defects). Birth defects are dependent upon the stage of development the fetus is in when it becomes infected with the virus. Birth defects are common if an infection with BVDV occurs from 100 to 150 days of pregnancy. Still some fetuses will not survive an infection that occurs after 120 days of pregnancy and will either result in abortion or a stillbirth.
Acute Infections: Many animals that become infected with BVDV never show signs of disease, this is termed a subclinical infection, but are contagious to other animals during infection. BVDV also causes immunosuppression in animals, or lowers their resistance to other viral or bacterial infections. These animals will become sick due to secondary infections, and signs will be relative to the organ system(s) involved. This is very common in the “shipping fever” complex well known in cattle. The virus attacks the animal, the immune system is suppressed, allowing a sometimes-overwhelming bacterial infection of the lungs.
Acute infection can also cause primary disease. The “Classic” acute form of BVD is characterized by one or more of the following signs: a moderately high fever (104 to106 F), depression, discharge from the eyes and nose, ulcerated areas of the nose and mouth (and throughout the intestinal tract), and diarrhea with or without mucus and blood. Although the name of the virus is viral diarrhea, oftentimes, animals do not have diarrhea. Recovery or death is dependent upon immune status of the animal, strain of virus causing the infection, age of the animal, and other factors.
A severe acute form of BVD infection exists. It is usually (but not always) associated with the Type II BVD. An animal infected with this form will display extremely high fevers (107-110 F), occasionally have diarrhea, have pneumonia, loss of appetite and usually succumb to death within 48 hours of disease onset.
Mucosal Disease: Persistently infected cattle that become infected with a different strain of BVD suffer from mucosal disease. It is characterized by profuse watery diarrhea, more severe signs of BVD, a lack of appetite, occasional lameness, and secondary infections. Most of these animals die within 3 to 10 days. Occasionally, a chronic condition will exist.
How does timing of infection relate to alpacas and llamas? Since the main concern is preventing persistently infected animals, we need to be cautious about exposure of early pregnant females to possible viral infection. Cattle have a gestation period of approximately 9 months, whereas camelids have a period of approximately 11 to 11 ½ months. The condition of a persistently infected calf occurs up to about 120 days, which is into the beginning of the second trimester. One month into the second trimester in camelids would correspond to approximately 4 to 5 months of gestation. Since it is not currently known the actual window of time that an infection of the camelid dam will produce a PI cria, the first 5 months of gestation should be considered to be critical. Keep in mind though, that infection of the dam at any time during pregnancy can lead to other disease considerations in the cria.
Transmission of BVDV: BVDV is contagious. The virus generally gains entry via the nose or mouth. Transmission occurs through close contact with an animal acutely infected or persistently infected. Acutely infected animals only shed the virus for a short period of time, about 2 weeks. Persistently affected animals shed the virus for their lifetime in all bodily secretions (feces, nasal, ocular, etc.). PI animals will also produce and shed a much greater amount of virus since they produce no antibodies to fight viral replication. Also keep in mind that males will secrete and shed virus in their semen (whether acutely or persistently infected), which could then be transmitted during breeding.
The virus is not very stable outside of the host animal. It is very susceptible to detergents, light, temperature changes, and other environmental conditions. That said, though, it can still by transmitted by inanimate objects until one of the above conditions causes it to be destroyed. Inanimate objects include a variety of everyday implements that come into contact with infected secretions: gloves, needles & medicating supplies, water/feed troughs, our clothing, hands, shoes; barn cleaning tools, and numerous others.
Treatment: There is no effective treatment for this disease. Most often the disease is self-limiting, meaning it runs its course and the animal makes a full recovery. Supportive care should be provided to very sick animals and may include fluids, antibiotics to prevent secondary infections, and highly palatable feeds.
Control and Prevention: BVD can be an emotionally and economically devastating disease. Control of its spread and prevention is of utmost importance. The first step in this process is to recognize that this can be and may already be a widespread problem in the camelid industry. Camelids are and have been “long distance travelers”—sometimes traveling across country for shows, sales, and breeding. It would not take very many PI crias traveling to expose a very large population of animals.
Persistently infected crias need to be identified and eliminated from the herd. Exposure status of a herd needs to be known. Further exposure of the herd to the virus needs to be prevented by limiting the chance of introduction of the virus into a “clean” herd or by limiting the spread within a herd.
Testing: There are a variety of testing procedures available . These will only be discussed very briefly. Tests either “look for” the actual virus or antibodies to the virus. When the virus is identified, the animal either has an acute infection or is persistently infected. When the antibodies are identified, the animal has been exposed and has recovered from the virus. These animals are not contagious and may be resistant to future infections. (If they will be “immune for life”, as occurs with some diseases, has yet to be determined in camelids. This would be a situation like chicken pox in humans—it’s extremely rare to have the disease more than once in a lifetime.)
Testing procedures also vary in degrees of sensitivity and specificity. Simply stated, some tests may produce more false negatives than others. Testing procedures also will vary in pricing (the more sensitive tests are usually more costly) and will vary in the length of time for results to be available. Testing for antibodies is accomplished through a Serum Neutralization (SN) Test, which is relatively inexpensive. Any positive means that your animal and possibly the herd have been exposed to the virus. Testing for the actual virus can be accomplished through Virus Isolation (VI), Polymerase Chain Reaction (PCR), Immunohistochemistry (IHC), Antigen-Capture ELISA. The VI has been the gold standard for detecting BVDV. It requires either a tissue sample or blood sample. It is costly and usually takes several weeks before results are available. The PCR is commonly used and is the testing procedure being recommended for detecting PI’s in camelids. It is very sensitive, there is no interference in young animals due to antibodies from the dam. It produces a rapid result (depending upon the lab situation) and is higher in cost than other tests. The IHC is commonly used in cattle to detect PI’s, it requires a skin sample, is relatively inexpensive, and early tests are showing this may not be as reliable in camelids. The ELISA test on either blood or skin is inexpensive, but may be less accurate.
Testing procedures will vary from lab to lab. Samples must be collected, handled, and shipped properly for accurate results. The length of time for results will vary, as will the cost of testing.
Vaccination: There is a vaccine available for cattle. Most are combination respiratory vaccines that contain more than just the BVD virus component, and not all are equal in effectiveness for the various types and strains of BVDV. That said, I do not believe that vaccination is the answer at this time. The safety and efficacy of the cattle BVDV vaccines have not been tested in camelids. Also, animals that have been vaccinated will have positive titers (meaning that they produce and have antibodies). It is difficult to distinguish vaccine titers from titers due to actual infection/exposure. This hampers the process of identifying exposure within a herd. It also makes testing more difficult and more costly.
Herd Recommendations: The following are my general recommendations. Each farm situation is different. Testing, isolation/quarantine, and culling procedures will differ. A general health program should be tailored to each farm. Work with your veterinarian to determine the best program for your situation. Sanitation and biosecurity measures will be of utmost importance.
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