Introduction

BVD accounted for up to 2.1 billion dollars in losses to the cattle industry in 2004. The seriousness of BVD is alarming. In fact, BVD has been termed the "AIDS" virus of cattle due to the severe suppression of the immune system which allows other disease organisms to invade the animal. Sadly, much like the AIDS virus, BVD is similarly difficult to detect within its community. Persistently infected (PI) animals are the primary source of the spread of BVD infections. Veterinarians and producers have worked for years to try to eliminate this disease, mostly through vaccination. It is now known that vaccination alone will not get rid of this disease, but with the increased knowledge of PI animals and how it affects herds, producers and veterinarians can work together to formulate workable plans to lessen the severity of the disease and work towards its eradicaton.

What is BVD?

Bovine Viral Diarrhea is a disease of cattle caused by the Bovine Viral Diarrhea Virus (BDVD). The virus is widespread and most herds are at risk for infection if exposed to the virus. Today it is primarily looked at as a cause of reproductive problems. However, it can be devastating to overall cattle health when introduced to a herd with little immune protection. BVD is currently one of the most costly diseases of cattle. Cost estimates at a herd level in herds with BVD range from $24 to $200 per cow per year. The range in both clinical signs and cost estimates is due to varying levels of herd immunity, the virulence of the infecting virus strain, and the pregnancy status of cows at the time of the initial infection. BVD also can have profound effects on performance, health, and profitability of feedlot cattle as well. The cost of persistently infected (PI) animals in the feedlot can vary as much as it does in the cow herd for the same reasons – herd immunity levels, the type of virus strain, and the number of PI animals in a group.

What is the impact of BVD in feedlots?

While numerous studies have looked into BVD, the consensus is that previous research has provided the industry with only partial insight into this complex disease. To get a better handle on the disease, feeders need to know more about its prevalence and cost impacts — especially in regard to BVD persistent infection (PI).

Persistently infected (PI) cattle are animals that had the virus transferred to them in utero from the dam before their immune system was fully developed. PI cattle may or may not appear sick, but will shed BVDV from body secretions throughout their life. PI cattle are considered the primary reservoir for BVDV in both cow herds and feedlots. A current estimate is that about 10% of beef cow herds have at least one PI animal and about 0.25 to <1% of calves born are PI. The main concern with PI cattle is two-fold: their ability to spread the virus (a single PI calf will shed up to 10,000,000 virus particles every day of its life), and the economic losses due to reduced gains and increased need for treatment and even death. PI cattle have a weakened immune system, which makes them more susceptible to other diseases.

What is the rate of incidence of PI cattle in feedlots and what are the costs associated with having a PI animal in the feedyard? What degree of treatment and prevention measures do these costs warrant?

Research conducted by Guy Loneragan of West Texas A&M University (WTAMU) found feedyard PI prevalence to be about 0.17% (1.7 PIs per 1,000 head). In his 6,000 head study, he found pens that included a PI animal had 35% higher morbidity than pens with no apparent exposure to PI animals. He also found that the non-PI pens placed adjacent to the pens with a PI animal in them also had 35% higher morbidity. The landmark WTAMU study also showed 3% of railers and deads are BVD PIs.

This prompted Paul Brown and Ron Shortridge, partners in Cattle Empire Feedyards, LLC to conduct their own research to answer some of their questions about BVD PI’s. In October 2003, they teamed up with Boehringer Ingelheim Vetmedica, Inc.,

providing 2,284 head of “high-risk” calves. The 572-lb. calves came from the Southeast via five different order buyers.

In this trial, five of 24 (21%) of the test pens, had at least one PI animal. The PI and non-PI pens were randomly placed throughout the feedyard, with the PI status of adjacent pens unknown. Health and performance parameters were tracked through the finishing period. Feedyard workers weren't told if pens were PI positive or PI negative. The results of this trial are displayed in the tables below.

Table 1. Performance of persistently infected (PI) and non-PI (NPI) calves

	Head count	Weight in (lbs.)	Weight out (lbs.)	Weight gain (lbs)	Days on feed	ADG	Consump. dry (lbs)	F/G (deads in)	Cost of gain (deads in)
NPI	1,731	571	1,051	405	180	2.25	14.02	6.26	.691
PI	553	574	1,040	357	177	2.00	13.75	6.94	.767
p-value		0.76	0.70	0.04	0.58	0.07	0.53	0.02	0.05

Table 2. Health of PI and NPI calves

	Head count	% Morbidity	First relapse rate	Second relapse rate	Avg no. of treatments	Med $/hd	Med $/ hd treated	% Mortality	% Railers
NPI	1,731	49.52	46.04	55.44	1.72	25.40	52.80	6.96	6.34
PI	553	42.31	43.74	54.84	1.68	23.05	55.41	10.37	6.39
p-value		0.22	0.71	0.93	0.76	0.43	0.24	0.14	0.97

Total difference in the COG between PI and non-PI (NPI) pens was $7.60/cwt. gained. Therefore, cattle that did not have a PI animal in their pen had a $30.78/head cost advantage (405 lbs. gained, $.076/lb. advantage) over cattle with a PI animal in their pen.

The PI animal causes a distinct increase in mortality in the PI pen — with the majority of the mortality increase occurring in the first 30 days on feed, he says. Based on these data, it's apparent there's a substantial benefit to feeding cattle that are PI free.

How does the BVD virus spread?

Most animals become exposed through contact with other recently infected or persistently infected animals that are shedding the virus. It is also possible for cattle to become infected via contact with contaminated water buckets, calf feeders, feed bunks, IV equipment, nose leads, and cattle trucks.

At what age are cattle susceptible to BVDV infections?

Cattle of all ages are susceptible to acute infection. However, since the colostral antibodies are effective in preventing infection in young animals, the disease is seldom seen before 3 months of age when adequate colostrums feeding and management is practiced. Persistent infection (PI animals) only develops in utero, and then only if the dam is exposed to BVDV at less than 125 of pregnancy. An animal cannot become persistently infected after it is born.

How can PI cattle be identified?

PI cattle cannot be identified visually. While many PI animals are unthrifty, being thin and having rough haircoats, reports have indicated up to 50% will appear normal and may enter the breeding herd or feedlot in excellent condition. BDVD infection is diagnosed on the basis of the clinical signs plus confirmation through necropsy findings and laboratory tests of blood samples. There are several ways to test for BVD and for PI animals, including virus isolation, immunohistochemsitry, PCR testing, serology, and ear notch testing. If you suspect BVD in your cattle, it is best to contact your veterinarian to assist you in testing, identifying infected cattle, and developing a treatment and control plan. One of the more recent and easier methods of testing for PI animals is the ear notch sample. Following are steps for accurately collecting the samples. Once the samples have been collected, they should be sent to your state’s veterinary lab for analysis.

How to Collect Ear Notch Samples for Immunohistochemistry

1. Sample can be collected from any age animal.

2. Using hog ear notch pliers, take sample from the margin of the ear.

3. Recommend notching ears in the same place to use as permanent indicator that animal has been tested.

4. Ensure that animal has been properly identified.

5. Place sample in glass or plastic tube with water-tight lid.

6. Add 10% formalin to tube equal to 10X the size or volume of the sample.

7. Send tissue sample to laboratory within 14 days to be processed.