|
Bovine viral diarrhea virus (BVD virus or BVDV), a serious problem in cattle, has nowbeen proven to cause illness, abortions, and most important of all, the persistentlyinfected (PI) state in alpacas. The virus’s ability to produce persistently infected cria, the main way this disease would be spread, and its ability to cause abortions are extremely important for the alpaca industry.At this point, the vast majority of knowledge about BVDV and the disease that itcauses, bovine viral diarrhea (BVD), is about its effects in cattle, where it is considered one of the most significant viral infections, causing major economic losses. Despite the name, many animals with BVD do not have diarrhea. Other manifestations of the virus include subclinical infections, immunosupression, abortions,congenital defects, persistent infection, and mucosal disease. The majority of cows infected are either subclinically ill (do not appear unwell) or only mildly unwell with low grade fever and diarrhea. Because BVDV depresses the immune system, some animals willbecome ill with other infections, usually pneumonia; others will have a classic case of BVD with fever, discharge from the nose and eyes, erosions of the muzzle and in the mouth, and severe diarrhea; others may have severe hemorrhagic (bloody) diarrhea and die. Severity of illness is influenced by the age of the animal and its immunological and physiological status, and the particular strain of the virus involved. The most important aspect of BVDV is its effect on the developing fetus. BVDV can cause abortions at any stage of gestation – from early embryo loss up to stillbirths at term. Even a subclinically infected cow can abort, and abortions may occur up to several months after exposure to the virus. A unique feature is that if the cow is exposed to the virus at a critical phase of her gestation (approximately 40 - 120 days) and does not abort, she will produce a persistently infected (PI) calf. Because the developing fetus is not immune competent at that stage of development it becomes immunotolerant to the virus(does not recognize the virus as foreign); it is unable to make an immune response to rid itself of the virus, and once born, is a permanent carrier and sheds huge concentrations of the virus in every secretion – tears, nasal discharge, saliva, urine, and feces. PI calves are the major source of the spread of BVD – they shed several billion viral particles a day– about a thousand times more than what is shed by an acutely infected non-PI animal.The only way to be PI is to be born PI. Some PI calves appear completely normal, butmost are poor-doers – poor weight gain, weak, and susceptible to other diseases such as pneumonia; many PI calves die before they are a year old. Infection of the pregnant cow with BVD later in pregnancy can result in calves that are not PI but that have congenital defects such as cerebellar hypoplasia (underdevelopment of the part of the brain that controls balance and co-ordination), cataracts, blindness, hypotrichosis (sparse hairgrowth) or general growth retardation. Acute infection with BVDV occurs through the nose or mouth by contact with secretions from an infected animal (usually a PI animal) – saliva, nasal discharge, tears,urine or feces – either directly from the infected animal itself or from items that havebeen contaminated with those secretions, such as water troughs. An acutely infected butnon-PI animal sheds virus for a relatively short length of time (4 – 10 days, possibly up to2 weeks) in comparison with a PI animal which sheds virus for its entire life. Incubation period is 5 to 7 days. The virus cannot survive long in the environment – a maximum of 2 weeks. Because it is a virus, antibiotics are of no use in treating it. There are hundreds of different strains of the virus, which can also be categorized undertype 1 (BVDV1) or type 2 (BVDV2) and by the terms cytopathic or non-cytopathic. BVDV2 tends to produce a more severe clinical illness than BVDV1. PI animals alwayshave a non-cytopathic strain of BVDV. An entity called mucosal disease occurs only inPI animals – they become superinfected with an antigenically similar cytopathic strain of the virus (usually from a mutation in their own non-cytopathic strain, or from a modified live vaccine); this leads to severe diarrhea and inevitably death. Vaccines for cattle are available for BVD; however they do not confer 100% immunity. Testing for BVDV Testing for BVDV is complicated, with different tests being used in different situations. Having antibodies (blood test) to BVDV shows that the animal was exposed to the virus(from a clinical or subclinical infection, or from immunization), but it is unknown how long antibodies are detectable for after exposure. PI animals would not have antibodies (unless they were tested as newborns soon after ingesting their mother’s antibody-containing colostrum). To detect the PI state the animal has to be tested for the virus itself, and there must be two positive tests three weeks apart to prove the PI state, in case the first positive test was an acute infection and not from being PI. Virus isolation is the ‘gold standard’ test for detecting BVDV – it can be done on blood (live animal) or tissue (dead animal). The ELISA blood test for the virus is used to detect PI animals over the age of 3 months (the maternal antibodies from colostrum interfere with the test beforethat age, and also sometimes interfere with virus isolation.) The ELISA test on skin biopsies or ear notches can be used to detect PI calves under the age of three months. The PCR test (blood) is not affected by maternal antibody and is also used to detect PI animals under the age of three months. Immunohistochemistry is used on formalin fixedtissue from autopsies. Research has shown that alpacas can be infected with the virus and develop clinical signs. There have also been reports of suspected persistent infections in crias. In cattle, persistent infected calves are the primary source of spreading the infection to other animals. It is not known if persistently infected crias are the primary source of herd infection in camelids, but it is suspected. Alpacas are sent all over North America and lapses in biosecurity could permit a persistent infected cria to infect other animals and herds. The most efficient method of BVDV transmission in camelids is not known. Transmission in cattle has been primarily by ingestion or inhalation of the virus. The virus can be found in all body fluids (respiratory and oral secretions, urine, milk, and semen) and feces. Transplacental (cow to fetus) transmission also occurs. Transmission is assumed to be similar in other susceptible species including alpacas and llamas. Currently there is no BVDV vaccine licensed for use in camelids. There are several vaccines available for use in cattle. The vaccines do not prevent infection but reduce the clinical disease effects. At this time, it is not recommended to vaccinate camelids until more is understood about the virus. Unwarranted vaccination can interfere with diagnostic testing and identifying truly infected animals. BVDV infections cannot be prevented but they can be reduced. Maintaining a closed herd, implementing strict biosecurity protocols for all incoming animals (recommended not just for reducing BVDV infections), and periodic screening of open herds can reduce the occurrence. At this time, a reasonable recommendation is that all aborted or stillborn fetuses, allunusually low birth weight and ‘poor doing’ cria and all unexplained deaths be tested forBVD virus and/or antibodies, depending on the case. If BVDV is found then further testing should be done to determine how the virus entered the herd - specifically is there still a PI animal present, or has it gone back to another farm. Any cria subsequently born to females who were pregnant when BVD was active in the herd should be tested to see if they are PI. Testing specifically for BVDV should become routine in abortions, stillbirths, and unexplained deaths in alpacas. 1. Herd Screening Submit whole blood samples (purple top tube, (PTT) (whole blood in EDTA), and red top tube, (RTT) (serum) individually marked to WADDL. WADDL will test PTT sample for BVDV by PCR. Up to 10 samples can be pooled and tested, which may reduce testing costs. Individual samples from positive pools would be retested to identify individual positive animals within the pool. - Negative results (BVDV not detected) = no BVD infection
- Positive results (BVDV detected) = persistent OR transient BVD infection suspected
- Definitive diagnosis of persistent infection requires submission of another blood sample (PTT) from an individual positive sample to be collected in 3-4 weeks and re-tested.
Serum samples (RTT) can be used to check for BVDV antibody to determine prior exposure through serologic testing.If unable to test entire herd, test all juveniles less than 2 years old and breeding males and females. Again diagnosis of BVDV persistent infection would require 2 blood samples collected 3-4 weeks apart. 2. New Arrivals to a Herd Quarantine for minimum of 30 days. Quarantining is not only important to allow screening for BVDV but also for other diseases. Herd biosecurity is important to protect your herd from diseases new animals may bring with them. For animals that will be remaining on your property a minimum 30-day quarantine is recommended before introducing new animals to your herd. - Negative results (BVDV not detected) = no BVD infection
- Positive results (BVDV detected) = persistent OR transient BVD infection suspected
- Remain in quarantine until retested in 3-4 weeks
Negative re-test = most likely a transient infection
Positive re-test = persistent infection suspected
A negative-tested dam can be returned to the herd, but recommend quarantining just before delivery until newborn cria is tested with PCR and identified as BVD infected or not. 3. Other Recommended Tests Necropsy and submit fixed and fresh tissues to test all aborted and stillborn crias and crias or adults with unexplained deaths. Submit whole blood (PTT) and serum (RTT) from the respective dam as well.
In addition to performing an abortion screen it is a good opportunity to evaluate the herd’s trace mineral status. MAJOR POINTS - BVD virus, a major problem in cattle, has now been shown to also cause illness,abortions, and, most important of all, the persistently infected state in alpacas. If the alpaca is exposed to BVDV during early pregnancy she can produce a persistently infected (PI) cria who sheds huge amounts of virus its whole life and is the major source of the spread of BVD. The only way to be PI is to be born PI.
- Because a PI cria may not show any signs of illness for several months or longer,the potential for BVD to spread between herds is significant because of the practice of females with cria at side going to other farms for breeding.
- It is unknown yet how common BVD in alpacas is – this has the potential to have a profound impact on the alpaca industry.
- All aborted or stillborn fetuses and unusually low birth weight or poor doing cria should be tested specifically for BVDV, as there are usually no pathological changes to suggest BVDV. Your veterinarian should consult with a veterinary virologist or lab that does BVDV testing about the appropriate tests to be ordered.
|
