Ask a Vet: Let’s Talk About the Latest on Injection-Site Sarcomas in Cats


Many people are surprised to learn that the veterinarian’s oath does not contain the phrase “do no harm.” Nonetheless, this does not stop the vets I know from avidly attempting to follow the mantra.

For thoughtful veterinarians, the subject of injection site sarcomas is painful. Injection site sarcomas (also known as IJS) were formerly known as vaccine-associated sarcomas, but it turns out that injections other than vaccines also can cause them. They are high-grade, life-threatening cancers that occur at sites of injections — in other words, caused by the actions of veterinarians. They represent the exact opposite of most vets’ goals. I despise injection site sarcomas, and I devour any material that I can find that might lead to insight into these malignancies.

Although I have written about IJS in the past, the subject is ever timely. An article By Dr. Richard Ford recently was published in Today’s Veterinary Practice. The article rounds up the most recent knowledge on the development, treatment, and avoidance of these horrible cancers. Every cat owner should be familiar with the material. Here’s a quick summary:

The discovery of injection site sarcomas

The revelation that vaccines were causing cancers on a large scale occurred in the late 1980s. Pennsylvania enacted mandatory rabies vaccination requirements for cats in 1987; researchers at the University of Pennsylvania noted a 61 percent increase in IJS in the years just after it came into effect. The world of veterinary medicine has been trying to come to grips with the situation ever since.

Causes and prevalence of injection site sarcomas

Despite decades of research, the cause is not fully understood. However, two key factors appear to play a role in the development of the cancers. One factor appears to be hereditary — some cats are predisposed to the cancers. Some progress is being made on the front of understanding which genes are involved, but at this time no test for susceptibility exists.

The other major factor in the development of the tumors is inflammation. Inflammation is a fancy way of saying irritation, a complex process involving the immune system. Inflammation is necessary for vaccines to be effective — the immune system needs to be stimulated in order to trigger immunity. However, IJS appear to occur as a result of inflammation run awry.

Adjuvant is a component of vaccines that increases inflammation. This subsequently increases vaccination efficacy. Some experts have theorized that the increased inflammation caused by adjuvant contributes significantly to the development of IJS. Although this notion is logical, it has not been conclusively proved, and it is still controversial. It is a fact that adjuvant need not be present for IJS to develop; therefore, it is certain that adjuvant alone is not the cause of the cancers.

The prevalence of IJS is not known. Estimates range from 1 in 10,000 cats to 1 in 1000 cats.

Treatment of injection site sarcomas

Injection site sarcomas typically develop as firm masses at the site of vaccination. Like all cancers, they are best treated if caught early when they are small. It is important for owners to know precisely where on their cat vaccines and other injections have been administered. Have the vet show you where the injections were administered and monitor those areas. Most IJS develop between three months and four years after injections. Any lump that develops at the site of an injection should be evaluated by a veterinarian. Be aware that most injection-associated lumps are not sarcomas, so don’t panic if you find a lump on your cat.

Standard sites for vaccination have been recommended for over two decades. In particular, it has been recommended that leukemia vaccines be administered near the left ankle and that rabies vaccines be administered near the right ankle. The reason for this is morbid but straightforward: these two vaccines are the most likely to cause IJS. When they are administered between the shoulders or closer to the hip, they are exceptionally difficult to remove. When they are administered near the ankle, many cats can be cured of IJS through amputation of the limb. This solution is very far from perfect, but unfortunately at this time — despite decades of research — I am very sorry to say it is the best one available.

Prevention of injection site sarcomas

One thing is certain: less antigenic stimulation (in other words, fewer vaccines) means less risk for the cancers. However, this must be balanced against the very real risk of diseases such as leukemia and panleukopenia, which can be prevented through vaccination. And cat owners and veterinarians have no choice but to obey the law of their jurisdictions regarding rabies vaccinations.

Some degree of vaccination is absolutely beneficial for cats. Over-vaccination is not. The line between the two can be impossible to determine. However, cat owners should consider avoiding vaccinations that aren’t necessary for their cat, based upon lifestyle. For instance, indoor cats do not require leukemia vaccines. Older cats aren’t likely to benefit from panleukopenia vaccines. Vaccines for coronavirus (FIP) and Bordetella are of limited benefit for most cats.

Non-adjuvanted vaccines for rabies and leukemia exist but their benefit is not clear. They are labeled to be given every year, whereas most adjuvanted vaccines are given every three years. I have been watching this matter carefully for years, and I have yet to see any evidence that the non-adjuvanted vaccines are three times less likely to cause sarcomas. This is the threshold they must cross in order to be truly beneficial.

Advice for cat owners

I have been writing about IJS for many years, and my advice today is the same as it was in 2005. Find a good vet — one who will sit down and talk with you about the subject of IJS. Sit down with him or her and discuss your cat’s history and lifestyle. Make sure that the vet is staying up-to-date on IJS and vaccination recommendations. Work out a protocol together that you both believe will be in the best interests of your cat. Cats with a familial history of IJS should receive special consideration.

After your cat has received an injection, monitor the site for development of a lump. Have a vet check any lump that develops.

Finally, if your cat develops an IJS, make sure that it gets reported. The world of veterinary medicine is struggling to tackle this intractably fractious problem. Any bit of data may help us to understand, and eventually eliminate, this horrible syndrome.

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Got a question for Dr. Barchas? Ask our vet in the comments below and you might be featured in an upcoming column. (Note that if you have an emergency situation, please see your own vet immediately!)

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