-----Biosecurity Considerations for the Expanding Dairy
River Road Veterinary Clinic
P O Box 309
Norwich, Vermont 05055
802-649-3877
BIOSECURITY CONSIDERATIONS FOR THE EXPANDING DAIRY
R. Page Dinsmore DVM Diplomate ABVP (Food Animal) College of Veterinary Medicine & Biomedical Sciences Colorado State University, Fort Collins, CO
By definition, an expanding dairy must introduce new animals to the existing herd. "Biosecurity" implies that the introduction of infectious disease is minimized or prevented whenever possible, from the new animals to the existing herd and vice versa. Ideally, introduced animals should be screened for the disease agents of interest and/or should originate from herds free of those agents; the introduced animals and the existing herd should be vaccinated for endemic diseases whose detection and eradication is impossible; and the introduced animals should be quarantined for 2-3 weeks to allow incubating endemic disease to run its course before the rest of the herd is exposed. Dairy herds are notoriously bad at biosecurity. The most recent NAHMS survey (Dairy '96) revealed that fewer than 10% of dairies quarantined introduced lactating cows an average of 10 days. Veterinarians must take the initiative and convince their clients of the importance of biosecurity.
There are at least 5 major infectious diseases that dairy producers should eradicate from their own herds. Contagious mastitis causes reduced milk production and increased somatic cell counts. Bovine Virus Diarrhea (BVD) causes clinical disease and abortion. Johne's Disease causes chronic diarrhea and weight loss and spreads insidiously. Digital dermatitis can cause lameness and reduced milk production in affected cows and will become endemic on a dairy after its introduction. Venereal diseases such as Campylobacter and trichomoniasis can be introduced through the purchase of an infected carrier bull; early embryonic death, pyometra, and infertility can result. Dairy producers should make all efforts to prevent the introduction of these disease agents to their herds by testing all introduced animals and/or becoming familiar with the disease status of the herds of origin.
CONTAGIOUS MASTITIS
The contagious mastitis pathogens Streptococcus agalactiae
and Mycoplasma sp. are commonly introduced to previously
uninfected herds through the purchase of infected cows. The purchase
of heifers ready to calve will reduce, but not eliminate, the risk.
The udder of a virgin heifer can become infected with S. agalactiae
or Mycoplasma if milk containing the organism is fed to baby
calves that are housed in groups. Calves in groups will often suckle
each other's teats and transfer the organism from the mouth to the teat
end and mammary gland. Mycoplasma sp. are common respiratory
pathogens as well, and the risk of Mycoplasma pneumonia is greatly
increased by the feeding of milk containing Mycoplasma organisms.
Severe pneumonia may lead to Mycoplasma septicemia, allowing
the organism to localize in the joints and the udder, where the infection
may remain subclinical until calving.
Once these two contagious pathogens are introduced, they will spread steadily throughout the herd. Bulk tank somatic cell counts will climb reflecting the increase in the prevalence of subclinical intramammary infections. Mycoplasma will also cause clinical mastitis that does not respond to routine treatments. Economic loss results from the loss of bonus payments for milk quality, reduced milk production, and eventual loss of Grade A permit due to illegal somatic cell counts. Control of these pathogens is expensive in the short run due to the costs of culturing milk samples to identify infected cows, treatment of cows infected with S.agalactiae, and culling of Mycoplasma cows.
To prevent the introduction of contagious mastitis pathogens to a herd, all purchased replacements (heifers and cows) should be sampled and cultured before being allowed to mingle with the milking herd. Samples can be taken at calving, and the purchased animals kept in a separate group and milked last. In addition, any mastitis control program begins with regular screening of bulk tank milk for contagious pathogens. The goal is to detect the appearance of a pathogen before it has spread widely throughout the herd.
BOVINE VIRUS DIARRHEA
Bovine Virus Diarrhea (BVD) continues to be responsible for significant
economic loss in dairy herds. In recent years, outbreaks of severe mucosal
disease due to BVD have caused severe losses in dairy herds in Pennsylvania
and elsewhere. These outbreaks have been blamed on the introduction
of a new type of BVD virus to an improperly vaccinated herd. Abortions
and stillbirths due to BVD have continued to plague dairy herds for
many years, even in herds with excellent vaccination programs.
There are two distinct situations in which BVD can be introduced to a herd. Purchased animals, especially those obtained at sale barns, may contract the disease during the shipping and sale process after contacting other infected animals. Recently infected adult animals may not show clinical signs of BVD infection, but will be shedding the virus from respiratory secretions. Alternatively, some fetuses that were infected with BVD virus in utero in the first trimester of gestation are carried to term and become persistent carriers of the virus. Persistently infected (PI) cattle often do poorly with reduced resistance to infections, but some will thrive and become productive adults. They may break with mucosal disease when infected with another type of BVD virus, but more commonly they remain in the herd and continue to shed virus. In either scenario, BVD infection can then spread throughout the herd to which the recently or persistently infected animal is introduced. The clinical manifestation of the new infection in the herd will depend on the herd's vaccination status and the type of BVD virus. An insidious aspect to the introduction of BVD to a herd may not become apparent for years: Pregnant cattle in their first trimester that are either recently exposed to, or persistently infected with, BVD virus are likely either to abort or to give birth to persistently infected calves. As described above, the PI calves may become productive adults and will continue to shed BVD virus.
To prevent the introduction of animals recently infected with BVD virus, purchase of replacement animals from sale barns should be avoided, and all recently obtained cattle should be kept isolated for 2 weeks prior to introduction to the herd. This quarantine period may be adequate to allow the acute infection to run its course and the virus to be eliminated. In the past, the identification of PI animals involved virus isolation from the buffy coat of blood samples. The process is expensive and time consuming. Great progress in BVD diagnosis has been made recently with the development of ELISA testing methods for BVD virus. The test uses serum from clot tubes, takes less than a week to perform, is much less expensive, and its sensitivity for the detection of Pl animals appears to be very high. It is recommended that all newly purchased animals be tested for PI status with this new test. Furthermore, one must also consider the fetuses carried by introduced animals that may have been exposed to BVD virus in their first trimester of gestation. Calves born to introduced animals should also be tested for PI status at 6 months of age or greater, to prevent maternal antibody from binding free virus in the serum and leading to false negative tests.
JOHNE'S DISEASE
(Mycobacterfum avium subsp. paratuberculosis)
Johne's Disease (JD) is an insidious enteric infection that is responsible
for considerable economic loss across the US. Animals are usually infected
at or shortly after birth, and harbor the infection for years before
breaking with clinical disease. Diagnosis is difficult since the organism
grows very slowly, and serologic tests are plagued by false negative
results. The recent NAHMS Dairy '96 Study report found that more than
20% of operations had 10% or more animals test positive for antibodies
to Mycobacterium paratuberculosis, with little regional difference.
The NAHMS Study estimated that the cost of the disease in herds where
at least 10% of cull cows showed clinical signs of JD exceeded $200
per cow per year.
There is a significant risk that purchased replacement heifers may be subclinically infected with JD. Ideally, all purchased animals should be tested for JD before calving. No single test will detect all subclinically infected animals, and all are more likely to give false negative results in younger animals. However, two tests that have similar false negative rates (50%), and extremely low false positive rates (0-1 %), are fecal culture and an ELISA test developed by Idexx Laboratories. Fecal cultures require 8-12 weeks and cost between $7 and 25; the ELISA results can be obtained in 1 week and the cost is $4-8. The two tests are clearly different and will not necessarily detect the same positive animals. Neither test is very helpful when testing introduced heifers because of the high false negative rate In young subclinically infected animals. A better strategy would be to purchase animals from test negative herds. Unfortunately, very few herds are aware of their JD status. Efforts are underway on an industry-wide basis to standardize the voluntary guidelines used by state animal-health agenciesto control and certify the JD status of cattle herds.
DIGITAL DERMATiTis (Hairy Warts)
This infectious cause of lameness has received a great deal of attention
lately because of its appearance in epidemic proportions in many dairies
across the country. It is believed to be caused by a spirochete of unknown
origin. There are many instances in which the disease has first appeared
in a dairy coincident with the purchase of replacement cattle. Cattle
affected with digital dermatitis are often lame, but a subclinical carrier
state exists. Purchased replacements should be examined carefully for
interdigital lesions consistent with the disease, and as a further precaution
the interdigital space should be sprayed with a tetracycline solubon
to eliminate a subclinical infection.
CAMPYLOBACTER/TRICHOMONIASIS
These organisms are the cause of early embryonic death and pyometra
in infected cows. Transmission is exclusively by the venereal route
and can be introduced to the herd by an infected bull. Most dairies
will purchase young bulls (<3 years of age) that are much less likely
to carry these infections than older bulls, but a breeding soundness
exam that includes a culture for Campylobacter and Trichmoniasis should
be required before the purchase and introduction of breeding bulls.
Selected References
1 . Biosecurity practices of US dairy herds. Dairy '96,
Fort Collins, CO: National Animal Health Monitoring System, USDA, APHIS,
Veterinary Services, 1996.
2. Johne's Disease in US dairy herds. Dairy '96, Fort Collins, CO: National
Animal Health Monitoring 'System, USDA, APHIS, Veterinary Services,
1997.
3. Report of the USAHA Committee on Johne's Disease. Proc 102nd Ann
Mtg U. S. Animal Health Association, 1998.