You don't have to spend long on social media to know that cats and kittens seem to dominate the internet. They seem to dominate our homes too - apparently one in three households in the USA have a pet cat. Research shows that being able to look after a pet makes us happier and also teaches our children compassion and gentleness. So it's understandable that there are a lot of cat lovers out there and if you are one and are reading this, don't panic - cat scratch disease is pretty rare. But it's worth knowing about, as it's also preventable.
What is cat scratch disease?
It's an infection caused by a germ that gets into your body from an infected cat - or more likely kitten - when it scratches or bites you. It's usually fairly mild and eventually goes away without any treatment, although it can take several months.
What causes cat scratch disease?
Cat scratch disease is caused by a germ (bacterium) called Bartonella henselae (B. henselae). It used to be called Rochalimaea henselae.
Most cats in the world are infected by B. henselae at some point in their lives, usually when they are kittens less than a year old. It doesn't cause them any problems at all, except very rarely. They catch it from fleas that have picked it up from another cat. They can also catch it directly from an infected cat if they have been fighting.
Most people who get cat scratch disease have caught it from a cat and can remember a scratch, bite or a lick from a cat in the previous week to two months. Very occasionally the affected person doesn't remember any contact with a cat. It is possible they have caught it from an infected cat flea. Some people think it can be caught from other animals such as dogs and rabbits, but this hasn't been proven.
Who gets cat scratch disease?
Cat scratch disease is quite rare. In a study from the Centers for Disease Control and Prevention (CDC) in the USA, it was estimated that one person out of every 20,000 to 25,000 seeks medical advice for cat scratch disease every year. It was more frequent in people who lived in the Southern USA, where cat fleas are more common as it is warmer.
Anyone can get it, but it particularly affects children aged 5-9. This may be because they are more likely to play with a cat or kitten in a way that encourages it to scratch them.
It seems that it is more likely to follow a scratch from a kitten than from a cat. This is because, although most cats are likely to pick up the germ that causes it at some point, they are most likely to be infected with it when they are kittens. Their immune system then gets rid of it, so they no longer have it when they are older. Kittens are also more likely to scratch than older cats, as they learn how to catch prey.
What are the symptoms of cat scratch disease?
Most people who get cat scratch disease start by getting little spots where their skin has been in contact with a cat's saliva - so a scratch or a bite nasty enough to have broken the skin is the usual cause. If a cat has licked an open wound it can infect you that way too.
- The spots start as just small red marks.
- Then they become fluid-filled, like little blisters.
- Finally they crust over and scab.
- Sometimes they can be yellow, pus-filled spots.
- It is common to think that they are insect bites.
- They appear sometime between 3 and 12 days after you have been scratched or bitten (or licked if you have a cut or graze) by a kitten or a cat.
The next symptom to develop is a swollen gland (lymph node) or glands.
- Lymph nodes are part of the body's immune system which defends the body against infection.
- The lymph node that is nearest the place where the spots developed is the one that swells, although others may swell too.
- The swelling usually starts about two weeks after the scratch.
- The most common lymph nodes to swell are the ones in the armpit (axilla) and in the head and neck.
- The swollen lymph node or nodes can get very large - bigger than a tennis ball.
- Any affected lymph node is painful, hot, red and tender.
- Sometimes the lymph node can fester and discharge pus (suppurate) - this affects 2 or 3 people out of every 10 who get cat scratch disease.
What other symptoms can you get with cat scratch disease?
About half the people who get cat scratch disease only get swollen lymph nodes. The other half can feel generally unwell, with symptoms such as:
- Malaise and exhaustion.
- Going off food.
About 1 in 10 get a slight temperature (fever), painful joints (arthralgia) and muscle pain (myalgia).
Rarely it can cause:
- Sudden loss of sight.
- Tummy (abdominal) pain.
- Pain in a bone.
- Epileptic fits.
- Severe headache and being sick (vomiting).
What are the symptoms if my immune system isn't working properly?
Having an immune system which isn't working properly is called immunodeficiency. If you have HIV or AIDS or are on treatment that suppresses your immune system, such as chemotherapy, the symptoms can be worse and you are more likely to feel ill. It can be very serious and even life-threatening if not treated .
Other symptoms you may develop are:
- A rash: raised red or purple spots, which can bleed if knocked.
- High fever.
- Feeling sick (nausea) and being sick (vomiting).
- Becoming short of breath.
- Bringing up blood.
How is cat scratch disease diagnosed?
Your doctor is likely to suspect cat scratch disease from what you tell them and what they find when they examine you or your child. It's really important that you tell your doctor if you have a cat or a kitten, especially if they have recently scratched you or your child.
If your doctor suspects cat scratch disease, you will also need to have blood tests done to confirm the diagnosis. The blood tests will look for signs of inflammation in your body and for signs that your body is trying to get rid of B. henselae, the germ (bacteria) that causes cat scratch disease.
What other tests will I need?
If the only symptoms you have are a swollen gland (or glands) and a history or being scratched by a kitten or a cat and you have had the typical spots of cat scratch disease, you may not need any other tests other than the blood tests described above.
If you have other symptoms or if you have no recollection of being in contact with a cat or a kitten, you might need other tests. There are lots of other illnesses that have very similar symptoms to cat scratch disease, particularly cancer. It is therefore extremely important to get the correct diagnosis.
Other tests you might need could include:
- Lymph node biopsy - a small amount of tissue is removed from a gland (lymph node). It is then looked at under a microscope to make sure there aren't any cancer cells in it and it is also tested for germs. See separate leaflet called Biopsy for more information.
- Computerised tomography (CT) scan.
- Ultrasound scan.
- Magnetic resonance imaging (MRI) scan.
- Lumbar puncture.
- Electroencephalogram (EEG).
What are the possible complications of cat scratch disease?
Complications can occur if the infection gets into the eye or if it spreads to other parts of the body. Some of the complications include:
- Parinaud's oculoglandular syndrome - occurs when the germ gets into the eye and spreads to the gland (lymph node) in front of the ear.
- Neuroretinitis - occurs when the nerve that takes messages to and from the eye to the brain (the optic nerve) is damaged by the infection.
- Osteomyelitis - when the infection spreads to the bone. This is rare.
- Pseudo-malignancy - occurs when the symptoms and changes in the body make the infection look like there is a cancer present.
Some complications can be potentially life-threatening:
- Bacillary angiomatosis - is a rare complication in which blood vessels grow out of control in the skin and in organs inside the body. It mostly affects people who have AIDS or HIV.
- Hepatitis and splenitis (bacillary peliosis) - occurs when the germ infects the liver and the spleen.
- Infective endocarditis - can occur if infection spreads to the heart.
- Encephalitis and/or meningitis - inflammation of the brain (encephalitis) and/or the covering of the brain (meningitis) - is rare.
What is the treatment for cat scratch disease?
Most people do not need any treatment - the body is usually good at getting rid of the germ (bacterium) that causes cat scratch disease. Some people get cat scratch disease and get better again without even knowing that they had it. However, for most people it can take time for the body to get rid of the infection. In the meantime you might need to take simple painkillers, such as paracetamol, if your swollen gland (lymph node) is really painful.
About 14 out of every 100 people who get cat scratch disease develop complications with spread of the infection to the liver, spleen, eye or nervous system. This is more likely in children younger than 5 years of age and in people whose immune system is weakened, such as by AIDS. Treatment with an antibiotic, or a combination of antibiotics, is then likely to help.
What is the outlook if you have cat scratch disease?
Most people make a complete recovery in 2-5 months. You should be seen again to make sure that the affected lymph gland has gone back to normal.
The outlook (prognosis) is different if you have developed one of the rare complications. The outlook will then depend on the particular complication and it will be different for different people, according to your age and any other illnesses you have.
How can cat scratch disease be prevented?
Cat scratch disease is preventable. It is rare and most people who get it have a mild illness from which they recover completely. However, not everyone does, so why take the risk? There are some simple things that you can do to reduce the chances of you or a member of your family catching it, such as:
- Wash cat bites and scratches well with soap and clean, running water.
- Don't allow cats or kittens to lick your wounds.
- Make sure you have good flea control for your cats, particularly in households with children.
- Wash your hands after contact with cats and teach your children to do the same. This is so as to remove cat flea poo that could be infected with B. henselae. Otherwise the germ could get into little breaks in the skin.
Further reading and references
Klotz SA, Ianas V, Elliott SP; Cat-scratch Disease. Am Fam Physician. 2011 Jan 1583(2):152-5.
Nelson CA, Saha S, Mead PS; Cat-Scratch Disease in the United States, 2005-2013. Emerg Infect Dis. 2016 Oct22(10):1741-6. doi: 10.3201/eid2210.160115.
Mazur-Melewska K, Mania A, Kemnitz P, et al; Cat-scratch disease: a wide spectrum of clinical pictures. Postepy Dermatol Alergol. 2015 Jun32(3):216-20. doi: 10.5114/pdia.2014.44014. Epub 2015 Jun 15.