Bovine Viral Diarrhea Virus (BVDV)
This article by Dr. Kim Gardner-Graff is reprinted from the the Alpaca Breeders of the Rockies 1/06 Newsletter.
There have been cases of alpaca
crias born persistently infected with Bovine Viral Diarrhea Virus that originated
from other farms. Some of these cases became infected with BVD during the early stages of
gestation, when their dams were exposed to a suspected BVD persistently infected cria for
approximately 10 days. The Alpaca Research Foundation census on their website lists 9 defined PI cases from several regions and Canada as of Dec.1, 2005, not including the recently diagnosed cases mentioned above. It’s truly an entire North American alpaca issue.
Prior to June of 2004 year when articles appeared about cases of BVDV in alpacas confirmed in Canada,
BVDV was not thought by large animal veterinarians, to be a major source of concern in alpacas.
As the fall has progressed, veterinarians are now testing for the disease it appears to be much more prevalent than we thought. The following list contains frequently asked questions regarding BVDV that I have been asked as my clients become more aware of this disease.
What is BVDV?
Bovine Viral Diarrhea Virus is a disease of cattle that has been around for many years affecting cattle all over the world. In cattle it is know to cause infertility, still borns, abortion, congenital defects, diarrhea, ulcerated oral lesions and the production of persistently infected calves if the adult cow contracts the virus during a certain period in her pregnancy. These calves serve as a constant source of virus that continually infects the herd. BVDV has also been isolated in deer, elk, goats, swine and other species.
How does it affect my alpacas?
BVDV in alpacas has been linked to abortions, still births, possibly early embryonic death (under 30 days of gestation), infertility, and the production of persistently infected poor doing crias.
What does persistently infected (PI) mean?
Persistently Infected means a cria (or calf) that was exposed to the virus in utero (as a fetus) during the first 18-120* days of pregnancy. The virus infects the developing cria and its immune system never recognizes it as foreign. Therefore, the cria is born “tolerant” to the virus and never mounts an immune response against it (such as forming antibodies). The virus thrives in that cria and continues to be shed by that animal for the rest of its life resulting in the exposure to all the animals in the herd. The only way to produce a PI cria is by exposure of the developing fetus in utero. (Exact timing may be more or less – this is extrapolated from research on cattle.)
What does a Persistently Infected cria look like?
Most PI crias appear to be poor doing animals and rarely live to 2 years of age. Yet, in cattle there are documented cases of “normal” appearing calves that test positive as a PI, survive long enough to become pregnant, and give birth to a persistently infected calf. Therefore, “normal appearing” doesn’t rule out PI status completely.
To date most documented PI crias have been born at low birth weights (9-15) pounds. Many are premature by a couple of weeks and may do well for the first month if they received adequate colostrum or a plasma transfusion. As they use up that maternal antibody they tend to develop chronic illness [nasal and eye discharges being the most common in my practice]. PI’s tend to have poor weight gain and many have an abnormal hair coat (long and silky in the huacayas, similar to suri fiber but doesn’t lock).
How do alpacas get BVD?
Alpacas contract BVD primarily from the persistently infected crias that are shedding the virus in huge amounts into the environment in their body fluids. The virus is then ingested or inhaled by another animal.
"Normal" animals that contract the virus shed it in small numbers while fighting it and can potentially could serve as a source of infection as well as virus carried in on peoples shoes, hands, clothing or equipment (rakes, shovels, buckets, etc). But the main source of infection appears to be contact with a PI cria.
Is diarrhea a sign my herd has BVDV?
No. Most of the healthy adults that appear to be recently infected with BVDV have not shown any signs of diarrhea, nor does diarrhea appear to be a consistent sign in persistently infected crias.
What are the symptoms in adults?
Healthy adult animals do not appear to show any external clinical signs of infection with the BVD virus. There have not been any consistent reports of diarrhea, temperature elevations or loss of appetite. Healthy adults appear to contract the virus and clear it in about 2 weeks without any outward ill effects.
What are the clinical signs in young animals?
Currently we haven’t correlated any specific clinical signs of a recent, active BVDV infection in “normal” (non PI) crias, but there is much to be studied about this disease in alpacas.
How do I know if my herd has been exposed?
The best way to determine if your herd has been exposed is to have blood drawn for a serology test on your breeding females. Serology looks for antibodies in the female’s blood stream that would indicate that she was exposed to the virus and fought it off. Many farms will probably test negative for the disease. Farms with a history of poor doing crias, abortions, infertility, stillborns or animals that travel or have many new animals coming on to their farm are more likely to have been exposed
What do I do if my females test positive?
On farms with animals testing positive on serology all crias under 2 years of age as well as the new crias as they are born should be tested by PCR (Polymerase Chain Reaction) that tests for a portion of the actual virus. Even females that test negative on serology should have their crias tested since there is a small chance that those females themselves are PI animals and potentially could give birth to a PI cria.
PI animals do not fight the virus so they do not form antibodies therefore serology would be negative. Unless they have maternal antibody for BVD from the colostrum (in newborns) they will be negative on serology but positive for the actual virus on PCR.
Crias testing positive on PCR should be isolated and retested two to three weeks later to positively and scientifically determine it to be a PI. There are two types of animals that will test positive on a PCR test: (1) the PI crias and (2) “normal” animals recently infected with the virus. PI crias never clear the virus so they will continue to test positive three weeks later. The “normal” crias, with a normal immune system, will fight the virus off, form antibodies and test negative on a second PCR test three weeks later (and positive on serology).
If my female test positive does that mean she is going to have a PI cria?
If a female tests positive that only indicates exposure. It does not indicate active virus in her, it does NOT guarantee that she is carrying a PI cria. Development of a PI cria takes place if the female is exposed to the virus during the first 18-120 day period of her pregnancy. There is chance she was exposed prior to getting pregnant or at some other point in the pregnancy. Therefore, view it as a flag of caution. As that female is nearing her due date she should be isolated from early pregnant females and blood should be drawn from the cria for a PCR test as soon as it is born and before it nurses. If the cria’s PCR test is positive that dam and cria should remain isolated until a second PCR test is run on the cria 2-3 weeks later. If that test too is positive then the cria most likely is a PI cria and euthanasia to prevent further infection of the herd should be considered. If the cria is PCR test negative – that would indicate there is no virus in the cria, it is not a PI cria and is safe to mix with the herd.
If my female gives birth to a PI cria is she likely to do it again?
No, it appears that in most cases once a female has contracted the virus and cleared it she will be immune to the disease and should not be likely to produce another PI cria unless faced with overwhelming re-exposure.
How long do does it take to get the test back?
Both serology and PCR testing take 7-10 days to get the results.
Is there a vaccine? And should I use it?
Yes, there are a number of vaccines on the market for cattle. Currently the virologists are recommending that we not use them since they will confuse testing and have not proven to prevent the development of PI’s in cattle.
How do I protect my herd?
My suggestion is to determine the status of your herd, and do what you can to decrease exposure of your pregnant females to new young crias from outside farms.
- All alpacas returning from shows or outside breedings should be isolated for two weeks from the rest of your herd. This is recommended as a general herd health practice.
- New additions to the herd should be tested prior to arrival by serology for exposure and if positive their current cria should be tested by PCR and only allowed on the farm if PCR negative and healthy appearing.
- All poor doing or ill thrift crias or young alpacas should be tested by PCR.
Stillborn and aborted fetuses should be necropsied and samples sent to labs for testing for BVDV and other infectious causes of abortion.
- Females that are early in their pregnancy that are being transported by commercial haulers ideally should be shipped on trailers that are not carrying crias or on a trailer where every cria on the trailer is PCR negative.
- Pregnant females and crias under six (6) months should not go to shows.
Our best defense against this disease is going to be education, testing and removing the sources of exposure, i.e. the PI crias. Hopefully we will find after further testing that this disease is not wide spread and with the proper application of testing and culling we can get it under control before it affects the industry detrimentally. Dr. Bedenice at Tufts University recommends PCR testing for animals less than 4 yrs of age.
Where can I learn more?
Great Websites for more information:
The Alpaca Research Foundation
Cornell University College of Veterinary Medicine
This page was last updated on June 9, 2009.