Heifer exporters should limit disease incidence
Heifer exporters should limit disease incidence
by Michaela Kristula, D.V.M.
The author is the section chief of field service at the University of Pennsylvania’s School of Veterinary Medicine.
World demand for U.S. dairy cows and heifers has surged. Exports of bred dairy heifers rose from 8,385 in 2007 to 73,639 in 2011, with Turkey accounting for more than half of the exports. Since 2011, exports to Russia have also been on the rise. Holstein dairy heifers from the West, Midwest and Mid-Atlantic states account for the majority of stock. Because of its bluetongue-free status and location relative to ports in Delaware and Maine, Pennsylvania is a popular state to quarantine cattle prior to export.
If you are interested in marketing your heifers for export, apply practices to lower the prevalence of diseases for which testing is required prior to export. In addition to negative tests for brucellosis and tuberculosis, heifers must test negative for bovine leukosis virus (BLV), bluetongue virus (BTV) and Johne’s disease prior to export to both Turkey and Russia. Turkey also requires that heifers test negative for bovine viral diarrhea virus-persistently infected (BVD-PI).
Since 2010, the Field Service veterinary team at the University of Pennsylvania has been involved with the export process, handling 7,339 pregnant Holstein heifers for overseas export to Turkey at USDA-approved isolation facilities in Pennsylvania. Heifers from endemic bluetongue states must be isolated for 21 days in a bluetongue-free state such as Pennsylvania prior to export to Turkey.
The Field Service veterinarians and students process the heifers according to Turkish import requirements. The heifers are bled for serology testing for certain diseases, tested for tuberculosis (TB), vaccinated for all the respiratory diseases, palpated for pregnancy and dewormed. Fourteen days prior to shipping, the heifers are given oxytetracycline and vaccinated for pinkeye. Any warts and extra teats are removed, and pinkeye and ringworm are treated. All heifers are given RFID (radio frequency identification) tags enabling them to be traced back to the herd of origin in the United States.
BLV is biggest battle
Because of the high prevalence of leukosis in U.S. cattle, all heifers are routinely screened prior to purchase and entry in quarantine. One study reported that 17 to 24 percent of dairy heifers tested prior to export to Russia were positive for leukosis. In our quarantine, 5.7 percent of prescreened negative heifers tested positive for leukosis during the quarantine period.
The leukosis virus produces fatal tumors in a small percentage of positive cattle. The organs most frequently affected are the lymph nodes, heart, abomasum, uterus, kidney, spinal cord and eyes. The highest incidence occurs in adult cattle, and once animals acquire BLV they are infected for life.
Leukosis is most commonly spread through the horizontal transmission of blood from an infected animal with BLV to an uninfected animal. The risk of in utero infections is generally low, but an infected cow can transmit BLV to her calf through colostrum or milk. Herd level control measures to lessen the spread of blood between infected cattle include using bloodless dehorning methods, sanitizing ear taggers between heifers, and using individual needles and palpation sleeves.
North lacks bluetongue
Heifers from endemic BTV states are routinely prescreened for BTV prior to entry in quarantine. In our quarantine, 4.9 percent of heifers tested positive for BTV. Bluetongue virus causes vasculitis and is transmitted by Culicoides spp. (biting midges) to both domestic and wild ruminants. The disease is not contagious and is only spread by competent vectors of Culicoides spp. In the U.S., cattle typically have unapparent infections, but the disease causes severe clinical signs in sheep resulting in facial and pulmonary edema, oral ulcers, lameness and pneumonia.
Northern states from Maine west to Montana, and those states extending as far south to Maryland and Pennsylvania, are considered BTV free because they are free of the BTV vector species. Historically, live cattle have not been exported from the U.S. to the EU primarily because of BTV restrictions on U.S. cattle, but there is general agreement that live cattle can be exported from BTV-infected countries by following recognized testing and quarantine procedures. Presumably, application of fly repellents to both the premises and animals would limit the prevalence of bluetongue in BTV-endemic areas. But research is lacking on the efficacy of any repellents to control the vectors.
Johne’s and BVD lurking
Johne’s disease is a chronic granulomatous gastrointestinal disease of cattle caused by the bacteria Mycobacterium avium subsp paratuberculosis (MAP). In our quarantine, 0.65 percent of the heifers tested positive for MAP. Calves are infected in utero or when they ingest cow manure with MAP and typically will not turn positive on tests until many years after infection.
The disease can be controlled by eliminating the contact of heifers with adult manure and by separating calves from cows after calving. In high-prevalence herds, additional helpful practices include feeding calves colostrum from only MAP-negative cows and removing high shedders from the herd.
In our quarantine, 0.68 percent of the heifers tested positive for bovine viral diarrhea-persistently infected (BVD-PI). Infection with bovine viral diarrhea (BVD) causes primarily reproductive problems and worse animal health.
BVD can cause either transient or persistent infections (PI). A PI animal is created when a fetus is exposed to a noncytopathic BVD virus, while in utero prior to the development of a competent immune system generally described as less than 125 days of gestation. The PI cattle continually shed large amounts of BVD virus during their lifetime.
BVD is generally introduced in a herd through purchase of PI cattle or PI-negative cows pregnant with PI fetuses. Control for BVD is centered on annual herd vaccination and eliminating BVD-PI cattle because they are viremic and continually shedding BVD, making more BVD-PI calves in utero.
Bulk tank milk samples can be used to screen the adult milking cows for BVD. During the testing period, all calves from pregnant females should be tested for BVD- PI, all BVD-PI animals should be slaughtered and any incoming cattle should be tested.
Dairy heifers bound for export also need to be properly vaccinated as heifer calves for all the respiratory diseases, BVD and leptospirosis, and receive a booster vaccine prior to entering quarantine.