We expanded two years ago and added 500 cows."
I think I hear that statement, or one similar to it, at least once a month while trouble-shooting on dairy farms. My standard
reply is, "Did you do any disease screening on them?"
That question usually gets a bit of a blank look, then they either state that no testing was done, or that the purchased animals
were tested only for TB and brucellosis.
While I have heard this answer many times, it still leaves me perplexed. With all that we know about bovine viral diarrhea
(BVD), Johnes Disease and Mycoplasma, why do most producers purchase new animals as if none of these diseases exists? Where
are the veterinarians? Are they never consulted or are they as complacent as the producers?
In my opinion, most producers do not screen new animals for disease because they are uncertain regarding what tests to request.
They realize that there are risks associated with bringing in new animals, but without specific information regarding how
to minimize that risk, they simply proceed. In most cases, there are no immediate signs of illness after the new animals arrive,
and the producer presumes that all is well. There are at least two scenarios under which this assumption would be wrong.
BVD most often is brought into a herd in the form of a non-infected animal, often a bred heifer carrying a fetus that was
infected early in the heifer's pregnancy. That fetus, if it survives to birth, will be born infected and will remain as a
persistently infected (PI) animal its entire life. It will shed virus continually, and if it survives to breeding age, it
will begin infecting herd-mates in early pregnancy, thus creating more PI fetuses and perpetuating the disease.
In time, some of these PI animals will reach the lactating herd, setting up early embryonic deaths and reduced immunity. All
of this will occur years after the original animal carrying the original PI fetus was purchased. So the absence of disease
signs following a new animal's arrival does not rule out the introduction of BVD. Breeding problems and overall health issues
can be the ultimate result and not apparent until years later.
Producers can protect themselves against BVD by testing every animal they purchase, but more importantly, testing the calf
born of any pregnant animal they buy. Testing the calves will identify any PI animals before they are raised and allowed to
commingle with pregnant animals. Newborn PI calves are accurately identified by testing a small sample of skin for the presence
Johne's disease also can have a long lag period from when the disease is brought onto a farm until symptoms appear. A group
of bred heifers can contain several infected with Johne's, but they will not show symptoms until they become older and the
disease progresses. Testing individual animals, especially heifers, will not accurately identify those that are infected.
The best defense against Johne's is to know the purchased animals' herd of origin. When this is possible, testing approximately
30 mature cows from that herd will give you an idea of the disease's prevalence in that herd. If all 30 are negative, the
herd is truly free of Johne's or the prevalence is low. If 10 or more are positive, then the prevalence is high. The prevalence
of the disease in the purchased cows will probably mimic the prevalence in the herd.
Dairy producers who buy cows should maintain strict separation of young calves from adult animals. Manure-handling equipment
should never be used to transport feed. Feed refusals from adult cows should never be fed to calves and heifers less than
a year old. Fecal testing for the Johne's organism can be employed to help identify carrier animals, and this information
can be used in making culling regarding colostrum usage and culling.
Mycoplasma can appear as a respiratory syndrome, mastitis or both after new cows are brought into a herd. Swollen joints
and lameness also might be present. These symptoms usually will become apparent within a few days to a few weeks. Other forms
of contagious mastitis also are a risk when cows are purchased. The best prevention in this case consists of milking new cows
last for at least two weeks as well as running CMTs (California Mastitis Tests) followed by cultures on positive cows.