If you don't know your animals' BLV status - or if you do - I'll strongly recommend a fresh, new needle for every animal - or at least for every test-negative animal in the herd. It's what the experts recommend, what I recommended to my own clients when I was in active veterinary practice. It's a pain in the butt to do, but disposable needles are pretty cheap - but I still didn't do it with my own herd, and now I'm paying the price.
I knew I had BLV(Bovine Leukosis Virus) present in my herd - I'd lost a 10+ yr old cow every couple of years for the past 5-6 yrs to lymphosarcoma - malignant tumors of the lymphatic system, which occurs in about 5% of cows infected with BLV. Lost two this past winter, and it bothered my farm manager(wife) enough that we finally bit the bullet and tested the entire herd, including every heifer over 6 months of age.
Out of 45 cows, 40 were BLV positive; fortunately, out of the 10 heifers we retained for this spring's breeding, only 1 was infected.
So...from now on, all negative animals get a fresh new needle every time. And a fresh new OB sleeve if we're palpating or inseminating. Just because they were negative 6 months ago, I can't guarantee they're still 'clean', so I'll use a new needle for each 'clean' animal - I won't take the chance of using the same one on 3 or 4 'clean' animals, running the risk that one or more of them may now be infected.
For multi-dose vials of vaccine, I stick one fresh new needle in the top and withdraw through that needle, and that needle only - no going in and out with a needle that's already been stuck in a cow - I use the same syringe on 'clean' or 'dirty' animals, but not the same needle.
Ear taggers, tattoo sets, etc. need to be disinfected between animals, as any transfer of blood can result in transmission of BLV to the next animal.
The BLV-positive animals? Well, the damage is done - they're infected for the rest of their lives, so I don't bother using a fresh new needle on them - I do re-use the once-used needles from the 'clean' animals to vaccinate 'dirty' cows.
All the BLV-infected cows are on a 'fast-track' to be culled. Any infected cow is gonna leave once she reaches 10 yrs, regardless of her production record. I anticipate that almost all of the current BLV+ cows will be gone within 5 yrs.
Even with these safeguards(clean sterile single use needles & OB sleeves on 'clean' cows, not contaminating vaccines, etc.), there's still a risk of horizontal transmission - animal to animal - and vertical transmission from dam to offspring, so I'll have to re-test 'clean' cows on a yearly basis to keep up with whether they've actually remained free or have become infected. All retained heifers will be tested somewhere between weaning and 10 months of age. Depending on infection rate in heifers, I may start culling on BLV status as heavily as I do on appearance and genetic potential.
So...please save yourself - and your cowherd - the possibility of spreading this disease by using clean sterile needles.
BLV infection is pretty widespread in both beef and dairy herds - you may or may not know whether you have it in your herd, but if you've ever lost a cow to lymphosarcoma, chances are your herd may have a fairly high infection rate, like my own.