The risks associated with intranasal strangles vaccination have discouraged its use by some veterinarians. These risks include abscess formation at the site of concurrent injections, mild clinical disease, and immune-mediated complications such as purpura or thrombocytopenia. Even minimal vaccine contamination of the needle or skin at the site of IM injections after Pinnacle I.N. administration has led to marked abscess formation at the injection site. Veterinarians quickly learned to administer all IM injections first to all horses on the premises before even mixing up the intranasal vaccine; this eliminates the risk of contamination. Unfortunately, even with this precaution, a very small number of horses still developed an abscess at the site of an IM vaccination, and the attenuated Streptococcus equi var. equi was isolated [h
]. Presumably, in these animals, the vaccine strain was able to overcome natural pharyngeal barriers and create a transient bacteremia, which led to the colonization at inflamed muscle sites. After experiencing this adverse reaction, a veterinarian may choose to avoid administering any other vaccines on the same visit as the intranasal vaccine or may resort to using the IM products for protection against strangles.
Horses vaccinated with the intranasal strangles vaccine
may very occasionally develop mild signs of strangles including depression, lymphadenopathy, and fever. Rarely, mandibular lymph nodes may progress to rupturing. The vaccine strain can also purportedly be transmitted to other horses in the herd, causing similar signs, but this has not been verified by polymerase chain-reaction (PCR) testing of the isolate. This possibility can be of particular concern in boarding stables where multiple owners can independently choose the vaccines that they wish their horse to receive. If a veterinarian chooses to use the intranasal vaccine at the beginning of a strangles outbreak, clients should be advised of this risk. The veterinarian must also be cautious not to inadvertently spread the wild type S. equi during administration of the vaccine to multiple horses [14
]. Clinical experience suggests that early use of the intranasal vaccine in minimally exposed horses may be effective in reducing outbreak severity. However, the vaccine should be avoided in horses that have developed early signs of the disease, such as depression or fever. The recent ACVIM consensus statement takes a more conservative approach and recommends that the use of the intranasal vaccine during an outbreak should be avoided, except in horses with no known contact with infected or exposed animals. However, it also states that no published data show that use in the face of exposure is detrimental [14
--2005 AAEP Proceedings