Cat scratch fever isn’t just a song by Ted Nugent. Being scratched by a cat is just one of the hazards of having one, but seemingly minor nicks can lead to nasty infections. It’s important to take good care of any cat-induced injuries.
Not only are cats’ claws sharp, but they can carry bacteria [1-5]. So, step one after getting scratched by a cat is to clean the wound [1-5]. Flush the wound with running water [2, 3] and then wash it with mild soap [1-5]. If it’s a serious enough scratch that it’s still bleeding after that, apply pressure with a clean cloth or bandage to stop the bleeding [2, 3]. Then apply a topical antibiotic (like Neosporin) and cover with a sterile bandage [2, 3].
For most cat scratches, that’s all the attention the injury needs. If it doesn’t break the skin, all you have to do is clean any bacteria off, and you’re done. But sometimes, a scratch can get infected and even make you quite sick, so keep an eye on the wound and be vigilant for any symptoms of the infections below [1-5].
Cat Scratch Fever
Cat scratch fever, also called cat scratch disease or CSD, is an infection caused by the bacterium Bartonella henselae [1-3, 5]. Cats become infected with B. henselae from fleas or by fighting with other infected cats [1-5]. Up to 30% of cats carry this bacterium in their blood [1]! But cats rarely become sick from it [1, 3, 5]. When an infected cat scratches or bites a human and breaks the skin, or licks a human’s open wound, then the B. henselae bacteria get into that person’s bloodstream, and that’s what causes CSD [1-5].
Symptoms typically start between 3 days and 2 weeks after the cat scratch or bite [1, 3, 5]. The injury may become red, swollen, warm, and painful; develop lesions; and exhibit pus [1, 3-5]. Later, lymph nodes near the injury may swell and get sore [1, 3-5]. Other symptoms that may develop include fever, headache, loss of appetite, fatigue, and joint pain, somewhat like the flu [1, 3-5].
Usually, CSD is not serious and will resolve on its own or with antibiotics [1-5]. However, in rare cases it can cause serious problems with the brain, eyes, heart, or other organs and require intensive treatment [1-5]. Children from ages 5-14 and people with weak immune systems are most likely to have these complications [1, 4, 5]. If you notice any signs or symptoms of CSD after being scratched by a cat, see a medical professional right away, just to be safe [1-5].
Other Infections You Can Get from Being Scratched by a Cat
Other pathogens can be present on a cats claws and be introduced under your skin by a scratch. One example is Staphylococcus aureus, a common bacterium on the skin of humans and other animals that can cause several types of infections when introduced inside the body [2]. Another culprit is Campylobacter [2]. Campylobacter is typically spread through cat feces, but cats bury their poop with their paws, so the bacteria can be on their claws, too [2]. A Campylobacter infection can cause fever, stomach cramps, nausea, and diarrhea [2]. Pasturella bacteria are spread by cat bites and scratches, too [2]. The bacteria attack the joints and tendons, causing pain and swelling, and symptoms can appear within a few hours [2].
Quick Tips to Prevent Scratches
Anything can be a toy, and toys will be destroyed – Image by Birgit from Pixabay
If you have cats, you’re probably going to get scratched sooner or later. There are a few things you can do to make it less likely, however. First of all, DO NOT play with your cat using your hands [3]! Only use toys, preferably ones that keep your hands far away from the action. If you use your hand like a toy, your cat will treat it like a toy. And you’ve seen how they treat their toys.
Keeping your cats nails trimmed can help prevent scratches as well, especially accidental ones [3]. Many cats don’t care for this, but they can usually be accustomed to it over time. You can have a groomer do it for you, but keep in mind that your cat will associate the groomer with the claw trimming. They may learn to resist their grooming appointments like they do their vet visits. There are advantages to doing it yourself at home and giving them their favorite treat when you’re done.
Sources
Centers for Disease Control and Prevention, National Center for Emerging and Zoonotic Infectious Diseases. (2023, June 13). Cat scratch disease (Bartonella henselae infection). https://www.cdc.gov/healthypets/diseases/cat-scratch.html
Shoutout to Dad for suggesting calicivirus as a topic.
Upper respiratory infections are common in cats. Often called cat flu, these infections are a lot like the colds and flus that humans get in the winter. Cat flu is highly contagious, but not very serious for most healthy adult cats. It can be dangerous for young kittens, seniors, and cats with chronic illnesses or weak immune systems. Like human colds, cat flu can be caused by more than one virus, but one of the most common culprits is feline calicivirus.
What is Feline Calicivirus?
A calicivirus showing the characteristic chalice shapes on the surface – Photo by Marian C. Horzinek [2]
Feline calicivirus (FCV) is part of the Caliciviridae family of viruses [1, 2]. There are viruses from this family that afflict a wide range of vertebrate animals, including humans [1, 2]. Norovirus is a member of the Caliciviridae, as are some diseases of rabbits [1, 2]. The name calicivirus comes from the Greek work “calyx,” meaning “cup” or “goblet” [2]. This refers to the cup-shaped depressions on the surface of the hexagonal or star-shaped virus particles [2].
Feline calicivirus is specific to cats, both housecats and wild species [1, 2]. It is a single-stranded mRNA virus, a type which is particularly prone to mutation [2]. This has led to there being many different strains of FCV around the world.
How Are Cats Infected with Calicivirus?
FCV spreads most often in environments where a large number of cats live in close proximity to each other, such as boarding kennels, pet stores, shelters, breeding facilities, and dense feral colonies [1-5]. The risk begins to increase dramatically when as few as four cats are in close proximity [2]. In groups of fewer than four healthy cats, the infection rate is 2.5%, but groups of four or more healthy cats have an infection rate of 32% [2]! In colonies and shelters, the rate can range from 50-90% [2].
The virus is spread through direct contact with body fluids from an infected cat, primarily saliva and eye and nose secretions [1-5]. Blood, urine, and feces can also spread the virus, as can aerosolized droplets from sneezes [1-5]. This means sharing things like food bowls, water bowls, litterboxes, and bedding can spread FCV [1-3]. So can just sharing an environment, or human caretakers can accidentally spread the virus around by getting it on their hands or clothes [1-3]. Most infected cats shed virus for 2-3 weeks, but some become carriers and continue to spread FCV for months or years [1, 2].
Symptoms of Feline Calicivirus
The typical presentation of FCV is an upper respiratory infection [1-5]. The virus attacks the inside of the mouth and the lungs [1, 2]. The symptoms include the usual upper respiratory signs like sneezing, stuffy/runny nose, runny eyes, and sleepiness [1-3]. However, FCV may also cause ulcers inside the mouth, which may in turn lead to drooling and refusal to eat due to oral pain [1-5]. Occasionally vulnerable cats, usually young kittens, will develop pneumonia [2, 3]. It is rarely fatal, however, and most cats will recover in a week or so, although severe cases may take up to six weeks [1].
Some cats who become infected with feline calicivirus go on to develop chronic gingivitis [1-5]. Researches do not yet know why this happens, or why it only happens to certain cats [2]. If your cat has recently recovered from cat flu and is still having problems with their mouth, this is probably something to talk to your veterinarian about.
Limping Syndrome
FCV can sometimes attack the joints and result in a strange–and upsetting–manifestation known as limping syndrome [1-5]. It mostly occurs in kittens [3, 4], and it can look very scary. Affected kittens develop a fever as well as pain, stiffness, and lameness in their joints, but which limb(s) are affected may change over time [2-4]. Sometimes limping syndrome occurs as a side effect of an FCV vaccination, not from an infection [2, 3, 5]. Although it is painful, limping syndrome only lasts 2-3 days and usually clears up on its own [2-4]. However, during that time, your veterinarian may be able to prescribe anti-inflammatory and/or pain medication to make your kitten more comfortable [2, 3]. Kittens that are severely affected may be reluctant to move at all [3] and need your help getting to their food, water, and litterbox until they’re feeling better.
Advice from Kitten Lady Hannah Shaw on taking care of kittens with limping syndrome
Virulent Systemic Feline Calicivirus (VS-FCV)
In rare instances, the FCV virus mutates into a much, much more dangerous form called virulent systemic feline calicivirus (VS-FCV) [2, 3]. VS-FCV is able to infect multiple organ systems and the lining of blood vessels [2, 3]. It causes high fever, inflammation throughout the body, swelling of the head and limbs, sores on the face and feet, jaundice, and multiorgan failure [1-3]. About 60% of cats infected with VS-FCV die [1-3]. Fortunately, outbreaks are very rare [1-3] and have only been observed in some countries [3]. Researches have not found any genetic traits that distinguish VS-FCV strains from regular FCV strains, however, making it difficult to specifically vaccinate against VS-FCV until there’s a breakthrough [2].
Treatment and Prevention
FCV usually goes away on its own in healthy cats [1-4]. There are currently no antiviral drugs that are proven to be effective against FCV, although a couple have shown promise in preliminary research [2]. As such, treatment focuses on relieving symptoms and supporting your cat’s body while it fights off the virus [1-4]. Your veterinarian might decide to prescribe medication like a non-steroidal anti-inflammatory drug or anticongestant [1, 2, 4]. Keeping the eyes and nose clean is important–nebulizers and saline eye or nose drops can help [1, 2, 4]. Some cats may need intravenous fluids [1, 2, 4]. Since many cats lose their appetite or simply find eating difficult, you may have to try different foods or ways of presenting their food to make sure they eat [1, 2, 4]. In severe cases, they may require a feeding tube [1, 2, 4].
The bad news is that FCV is extremely widespread. The good news is that there is a vaccine for it, which the American Association of Feline Practitioners [5] and European Advisory Board on Cat Diseases [2] both consider a core vaccine that all cats need to have. FCV is usually included in a combination vaccine with other common viruses [2].
Because there are so many different strains of FCV, the vaccine doesn’t always prevent a cat from becoming infected, but it is effective at preventing them from getting sick [2, 5]. Kittens lose the protection of the immune cells they get from their mothers somewhere between 6 and 14 weeks of age, so the American Association of Feline Practitioners recommends vaccinating kittens against FCV at 6 weeks, or as early as is allowed in your country [5]. Kittens should receive a primary vaccination series of 2-3 shots, and adult cats should receive a booster every 1-3 years, depending upon their risk level [2, 3, 5].
Hofmann-Lehmann, R., Hosie, M. J., Hartmann, K., Egberink, H., et al. (2022). Calicivirus infection in cats. Viruses, 14(5), 937. https://doi.org/10.3390%2Fv14050937
Feline Infectious Peritonitis (FIP) is a severe multi-organ-system viral disease that affects domestic cats as well as other felines [2, 5]. Until very recently, a diagnosis of FIP was a death sentence for a cat. It is still often fatal. However, knowledge is power. Knowing when to go to the vet and what your options are can save a life.
What Causes FIP?
FIP starts with an intestinal virus called feline enteric coronavirus (FeCV or sometimes FCoV). You can’t catch this coronavirus; only felines can [1, 4, 5, 9]. FeCV is very common among cats, especially in places where many cats live close together like shelters and catteries [1, 2, 5, 8]. Most cats infected with FeCV experience no symptoms, mild diarrhea, or mild respiratory illness before defeating the infection on their own [1, 4, 5, 8, 9]. In the meantime, they can easily spread the infection through their fecal matter or, less often, saliva [1, 2, 4-6]. Cats usually become infected by getting the virus in their mouths [1, 5, 6]. Shared litter boxes are a typical culprit [1, 2, 4-6]. An uninfected cat needs only to step in the litterbox after an infected cat has used it, and then when they clean their paws later, they become infected themself.
But what does this benign virus have to do with FIP? It isn’t well understood, but in about 10% of cats a FeCV infection turns into FIP [1, 2]. This is believed to occur as a combination of two phenomena. First, FeCV undergoes a spontaneous mutation that enables it to infect macrophages, a type of white blood cell [1, 2, 5, 8, 9]. This is the feline infectious peritonitis virus (FIPV). Second, the immune systems of certain cats overreact to the mutated virus, causing out-of-control inflammation and damage [1, 4, 6]. Incidentally, it is an overblown immune reaction to the coronavirus of the moment, COVID-19, that makes it so dangerous for a proportion of the population.
In the bodies of cats where both of these things happen, FIP develops. Some cats are more prone to FIP than others. Although the specific genetic risk factors remain unclear, specific breeds are at greater risk: Abyssinians, Bengals, Birmans, Himalayans, Ragdolls, Cornish Rexes, and Devon Rexes [6, 8]. Younger cats and most especially kittens make up the majority of FIP cases [1, 4, 6]. Their weaker immune systems are less able to fight off a FeCV infection, giving it more time to mutate [5]. Unneutered cats also seem to be at greater risk, for unknown reasons [8].
Symptoms of FIP – Image from Cornell University eCommons
Symptoms and Subtypes
The most common and earliest symptoms of FIP are, frustratingly, not very specific. They may include fever, change in appetite, weight loss, and depression [1, 5, 6]. As FIP advances, it develops into one of two general forms: wet/effusive or dry/non-effusive. In the wet form, fluid builds up in the abdominal and chest cavities and may cause breathing difficulty [1, 4-6, 8]. The dry form causes neurological and eye symptoms such as seizures and uveitis (inflammation of the outer layers of the eye) in addition to the non-specific symptoms [1, 4-6, 8].
Diagnostic Map for FIP (Hartmann, 2005)
Diagnosis
Diagnosing FIP is difficult and complicated. The symptoms can all be signs of other diseases. Once it is suspected, there is no definitive, foolproof test for FIP. At right is a diagnostic scheme for FIP from one scientific article as an example of the mental gymnastics veterinarians have to do when deciding how to pursue FIP testing.
The options include blood and fluid tests, genetic tests for FIPV, and the veterinarian’s observations during an exam [1, 2, 4-6]. Many of the tests are complicated by the fact that so many cats have been infected by FeCV [1, 2, 5]. A cat’s veterinarian has to determine the best course of testing based on their individual case.
Treatment
Until just a couple years ago, nothing could be done for cats with FIP except supportive care to ease their suffering and extend their life a short time. For many cats, that is still the case, for reasons I will get to shortly, or simply because their disease is too severe. Supportive care consists of subcutaneous or intravenous fluids, draining fluid from body cavities if applicable, steroids to control inflammation, and immunosuppressive medication [1, 2, 4-6].
GS-441524
GS-441524 is an antiviral drug that prevents viruses from synthesizing RNA, a molecule which many viruses, including FeCV/FIPV, use instead of DNA [3, 9]. It was created by the pharmaceutical company Gilead, which specializes in antivirals [9]. Gilead first tested GS-441524 against Ebola but was ineffective [9]. However, FIP researcher Niels Pedersen knew someone at Gilead and asked if they could send him anything to test [9]. His friend at Gilead obliged, and Pedersen found two promising candidates that killed FIP in cultured cat cells [9]. He moved on with testing one of them, GS-441524, in cats with FIP [9].
The results were incredible. In a laboratory study, ten of ten infected cats recovered [9]. In a field trial with 31 pet cats with non-neurological FIP, 25 recovered, still an unheard-of success rate for this otherwise inevitably fatal disease [7, 9]. A more recent pilot study suggests that GS-441524 is effective in neurological and ocular FIP, too [2]. Side effects were not too bad. Cats experienced pain and skin reactions at the injection site, sometimes developing sores or scars [7]. This may be because the formula is very acidic [7].
Gilead, however, has declined to license GS-441524 for animal use [9]. GS-441524 is almost identical, molecularly, to another one of their drugs, remdesivir, which they are trying to get licensed for human use, and they believe that licensing GS-441524 for FIP would harm their chances with remdesivir [9]. Cat people and Chinese entrepreneurs have gotten creative, however.
GS-441524 is not a difficult molecule to synthesize, so numerous outfits in China have chosen to manufacture and sell it, filling the gap Gilead left open [9]. Because GS-441524 is not FDA-approved, veterinarians cannot prescribe it, and it’s illegal for cat parents to purchase and administer it [1, 9]. That has not stopped desperate people who want to save their cats. Because it is a black market, there is a risk that the product will not be as advertised [1, 9]. And it is incredibly expensive: it costs 7,000-12,000 USD for the 12-week course of medication [9]. When people can manage to acquire it, however, GS-441524 seems to be as good as the research suggested [1, 9].
If your cat has been diagnosed with FIP, and you would like help accessing GS-441524 treatment, contact FIP Warriors.
Watch “Kitten Lady” Hannah Shaw’s journey through FIP with her cat Coco
Preventing FIP
Because FeCV is so common and easily spread, it’s hard to prevent FIP entirely. There are some things you can do, however. Keep litter boxes clean and far away from food and water dishes [1, 6]. Cats are more vulnerable to developing FIP if they have another disease like FeLV or FIV, so get your cats vaccinated against all those easily preventable diseases [1, 6]. There is one FDA-approved vaccine against FeCV, but it does not seem to be all that effective [1, 5, 6]. You can always ask your veterinarian about the FeCV/FIP vaccine if you’re concerned, however. Keep in mind that cats often get FeCV infections at animal shelters and catteries, so you may want to isolate any new cats for a couple weeks [1, 5].
Works Cited
Cornell Feline Health Center. (2020). Feline infectious peritonitis. Cornell University College of Veterinary Medicine. Link
Diaz, J. V. & Poma, R. (2009). Diagnosis and clinical signs of feline infectious peritonitis in the central nervous system. The Canadian Veterinary Journal, 50(1), 1091-1093. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2748294/
Dickinson, P. J., Bannasch, M., Thomasy, S. M., Murthy, V. D., et al. (2020). Antiviral treatment using the adenosine nucleoside analogue GS-441524 in cats with clinically diagnosed neurological feline infectious peritonitis. Journal of Veterinary Internal Medicine, 34(4), 1587-1593. https://doi.org/10.1111/jvim.15780
Pedersen, N. C., Perron, M., Bannasch, M., Montgomery, E., et al. (2019). Efficacy and safety of the nucleoside analog GS-441524 for treatment of cats with naturally occurring feline infectious peritonitis. Journal of Feline Medicine and Surgery, 21(4), 271-281. https://doi.org/10.1177/1098612X19825701
Pesteanu-Somogyi, L. D., Radzai, C., & Pressler, B. M. (2006). Prevalence of feline infectious peritonitis in specific cat breeds. Journal of Feline Medicine and Surgery, 8(1), 1-5. https://doi.org/10.1016/j.jfms.2005.04.003
Thyroid disease is fairly common in cats, especially as they get on in years. The most frequent type of thyroid disease in older cats is hyperthyroidism [1, 4]. My parents’ elderly cat, Rosie, has it. So did my roommate’s cat Coco. And so do my best friend’s two ancient gray ladies. Even though hyperthyroidism is a common illness, I will attest that it can still be mystifying at times. It can impact a cat’s entire body, and it isn’t completely understood even by scientists and veterinarians. It is diagnosable and treatable, however. Sometimes, hyperthyroidism can even be cured.
The thyroid gland – Image from Cornell Feline Health Center
What is Hyperthyroidism?
The thyroid gland is a small organ found in all vertebrates which produces the thyroxine (T4) and triiodothyronine (T3) hormones. In mammals, the thyroid is found in the throat and has two lobes. Thyroid hormones regulate a wide variety of bodily functions. When the thyroid produces too much T3 and T4, it’s called hyperthyroidism. Hyperthyroidism increases the body’s metabolism which can cause problems with many different organs and systems.
Cats typically develop hyperthyroidism when middle-aged or older [1, 2, 7, 8, 10]. Up to 10% of cats age 10 and older may have hyperthyroidism [1, 3, 8]. It’s uncommon in cats younger than that, however [1, 2, 7, 8]. Untreated hyperthyroidism is fatal, but today the prognosis for a cat with this disease is usually optimistic.
How is Hyperthyroidism Diagnosed?
If your veterinarian suspects hyperthyroidism, they will probably do a physical examination of your cat’s neck to see if they can feel that the thyroid is enlarged [1, 2, 4, 7]. The presence or absence of a suspicious lump isn’t enough for a diagnosis, however. Most of the time, the final diagnosis can be made with a total thyroxine, or TT4, blood serum test [10]. The vast majority of hyperthyroid cats will have very high T4 levels with a TT4 test, making the diagnosis clear right away [1, 2, 7, 10].
However, cats with hypothyroidism occasionally have T4 serum levels that fall within normal limits [1, 2, 7, 10]. T4 levels fluctuate, so your veterinarian may decide to try the test again in a few weeks [1, 7]. Or they might try other thyroid hormone blood tests. A thyroid scan is another option but must be performed at a specialty veterinary facility [7, 10].
What are the Symptoms?
Because thyroid hormones affect so many parts of the body, the symptoms of hyperthyroidism are varied and sometimes nonspecific. They include:
Characteristic appearance of hyperthyroid cat – Image from Cornell Feline Health Center
Weight loss/muscle wasting
Increased appetite
Increased thirst and urination
Vomiting
Diarrhea
Hyperactivity
Unkempt fur
Aggression or irritability
Fast heartrate
Increased vocalization, especially at night
Weakness
Heat intolerance
Some cats demonstrate “apathetic hyperthyroidism,” where they are lethargic, anorexic, and depressed/dull instead of the more usual symptoms [1]. Hyperthyroidism is progressive, so symptoms may be very mild at first [2]. It’s also not uncommon for hyperthyroid cats to also have other diseases, especially since they tend to be older. Many of these symptoms can be symptoms of something else. The safest bet is to consult your veterinarian anytime your cat seems different or unwell.
Secondary Illnesses
An overactive thyroid can sometimes cause other organs to become diseased. The heart is particularly at risk. There is a form of heart disease known as thyrotoxic cardiomyopathy which is caused by the heart muscle working overtime to meet the demands of a metabolism on overdrive [10]. The heart muscle thickens as a result of pumping so hard, but then that makes it difficult for the heart to work properly [2, 4, 10]. This can lead to heart failure. Fortunately, successful treatment of the hyperthyroidism can improve and even reverse the cardiomyopathy [2, 7, 10].
Hyperthyroid cats frequently experience high blood pressure as a cardiac complication, too [2, 4, 10]. About 25% of the time, in fact [10]. In severe cases, the high blood pressure can damage their organs [2, 4, 10]. It can even lead to retinal detachment that renders them permanently blind [4, 10]. High blood pressure is no joke and needs to be managed alongside hyperthyroidism until the thyroid hormones are under control [2]. If the hyperthyroidism is resolved, the blood pressure usually takes care of itself [2, 10].
What Causes Hyperthyroidism?
Hyperthyroidism usually begins when the thyroid gland becomes enlarged [2, 10]. In cats, this is almost always the result of a benign tumor called an adenoma [2, 4]. In less than 2% of cases, the growth is cancerous [1, 10]. What causes the abnormal thyroid tissue growth to begin with, however, remains uncertain. Researchers have found evidence for both environmental and biological factors that may play a role.
Environment
There is a demonstrated association between diet and the chances of a cat developing hyperthyroidism. Too much iodine in the diet may make a cat susceptible [10]. Canned cat food is also associated with higher rates of hyperthyroidism [1, 5]. The results of a 1999 study suggest cats fed primarily canned food have a rate of hyperthyroidism 2 to 3 times higher than other cats [5]. Researchers still don’t know why canned food would lead to an overactive thyroid. Correlation is not causation, so there may be more going on here that we don’t understand yet.
Using cat litter has also been found to be a potential risk factor, with the same study showing triple the rate of hyperthyroidism in cats that use litter [5]. Again, although there is a potential link, there is as yet no explanation. From looking through a lot of research papers to write this article, I know that veterinary scientists are still hard at work trying to figure out feline hyperthyroidism. The disease just doesn’t want to give up its secrets easily.
Certain household chemicals are associated with higher rates of feline hyperthyroidism, too. Polybrominated diphenyl esters (PBDE), which are used as flame retardants, have been found in high levels in the blood serum of hyperthyroid cats [1]. According to the Environmental Protection Agency factsheet for PBDE, rodent studies have shown thyroid toxicity from PBDE exposure. Per- and polyfluoroalkyl substances (PFAS) are a versatile group of artificial chemicals with a wide variety of applications in industry and the home. Wang et al. found that PFAS levels, particularly perfluorooctanoic acid, were higher in hyperthyroid cats than the cats in the control group [9]. The Environmental Protection Agency lists tumor growth and thyroid hormone disruption among the effects of PFAS in animals.
Siamese have a reduced rate of hyperthyroidism – Image by rihaij from Pixabay
Biology
Hyperthyroid cats may have defects in the way their cells communicate that lead to the adenomas and other tissue abnormalities that overproduce thyroid hormones. Cats with thyroid nodules sometimes have low levels of the G protein in their thyroids [1]. A lack of G protein can decrease the inhibition of cell growth [1]. Some cats with thyroid nodules or adenomas have a cancer-causing mutation, known as an oncogene, which could cause the excessive cell growth [1].
Certain breeds of cat are less likely to develop the disease: Siamese, Burmese, Tonkinese, Himalayan, Persian, Abyssinian, and British Shorthair [3, 5, 9, 10]. At one time, only the Siamese, Burmese, and Persian/Himalayan were known to have these decreased risk, so the benefit was thought to be associated with the color-point pattern or lighter coat colors [3]. This hypothesis ultimately didn’t hold up [3], but perhaps there is something special these breeds have in common which will one day come to light. The same study which discovered the three new resistant breeds also found that long-haired, nonpedigree cats have higher rates of hyperthyroidism [3]. This despite the extremely long-haired Persian and Himalayan’s decreased rate.
What Treatments are Available?
Radioactive Iodine
It might sound a little scary, but radioactive iodine is actually the gold standard for treating feline hyperthyroidism. This treatment can only be done in specialized veterinary clinics that are authorized to work with radioactive materials, so it may not be accessible or affordable for everyone [2, 7, 10]. It’s a great option if you can take it, though, because for about 95% of cats it cures their hyperthyroidism–even those few with cancerous thyroid tumors [1, 2].
Radioactive iodine therapy relies on the fact that the thyroid takes up iodine to synthesize thyroid hormones. The cat is injected with the radioactive version of iodine, I-131 [1, 2, 7, 10]. No anesthesia is required [2, 7, 10]. The cat’s thyroid absorbs the I-131, just like it would normal iodine, but the abnormal thyroid tissue that’s churning out T3 and T4 like crazy is hogging the iodine. So the radioactive molecules get sucked into the abnormal tissue and kill it, leaving alone the cat’s healthy tissues, including any healthy thyroid tissue [1, 2, 7, 10].
Although the procedure is quite safe for the cat, it can be dangerous for the people around the cat which is now shedding radioactive particles [2]. Cats have to be hospitalized for a week or two after radioactive iodine therapy so that their radiation levels can get low enough for release [1, 2, 7, 10]. Even then, special care has to be taken at home for a two weeks after [1].
Radioactive iodine therapy may be a good option for cats that cannot undergo anesthesia. It also works well for cats with ectopic thyroid [7]. Ectopic thyroid is thyroid tissue somewhere other than the usual location of the thyroid gland such as under the tongue or in the chest [10]. It can be in locations that would be difficult to remove surgically. Radioactive iodine therapy doesn’t usually have complications, but for the rare cat it works too well and they become hypothyroid instead [2]. The solution to this is medication with thyroid hormones.
Medication
Hyperthyroidism cannot be cured with medication, but it can be managed with it. The medicine is called methimazole [1, 4, 7, 10]. Its brand names are Felimazole and Tapazole [4, 7, 10]. Methimazole is available as an oral tablet or a gel applied to the skin [1, 2]. It works by decreasing the production and release of hormones from the thyroid [2, 4]. It’s usually given twice a day [1, 4].
Cats may be given methimazole as a long-term method of managing their disease or to stabilize them before trying a more permanent solution [1, 4, 7]. Like all drugs, methimazole can have side effects. According to the Food and Drug Administration, the most common side effects of methimazole in cats are changes in appetite, lethargy, vomiting, diarrhea, skin lesions, itchiness, unusual vocalizations, weight loss, fur abnormalities, weakness, and agitation [4]. However, a small percentage of cats develop severe side effects with methimazole [1, 4]. Namely, liver disease, anemia, low blood platelets, and low white blood cell counts [1, 4, 7, 10]. Always report medication side effects, even seemingly small ones, to your veterinarian.
When Dr. Jean Holzworth first recognized feline hyperthyroidism in 1978, the only way to treat it was to remove the offending thyroid gland [1]. Although there are now other options, for some cats this is still the chosen course of treatment. For most cats, surgery cures their hyperthyroidism [2]. It can also be done at a regular veterinary surgery rather than the special facility required for radioactive iodine therapy, making it accessible to more people. Surgery requires a cat to go under anesthesia, however, and for older cats who often have other health problems, that can be a risky proposition [2, 10].
Surgery also carries a chance of complications. Complications aren’t likely, but they do happen. Most cats still have enough thyroid cells left after the surgery to keep up their T3 and T4 hormone levels, but some become hypothyroid and require medication [7]. Another possible complication is damage to the parathyroid glands, which are near or within the thyroid are necessary for maintaining normal calcium levels in the blood [2, 7]. If the parathyroid glands become damaged, the cat may develop low blood calcium and require supplemental calcium or vitamin D [1, 7]. Other complications include damage to nearby nerves or the larynx [1, 7].
Due to the risks associated with surgery and the availability of alternatives, it isn’t a very common choice anymore [2]. That being said, it might still be the right option for you and your cat. That’s something you would have to figure out with your veterinarian.
Diet
The research is still ongoing for diet as a means of controlling hyperthyroidism in cats [2]. For cats with health concerns that make other treatment options difficult or impossible, however, it can be the best–or only–choice despite being somewhat controversial. The principle is that limiting the amount of iodine in the cat’s diet to a small, controlled amount restricts the amount of hormones the thyroid can make. Cats fed a prescription iodine-restricted diet and nothing else should return to normal thyroid function [1, 7, 10]. Nothing else really means NOTHING else. No treats, don’t even put the prescription food in a bowl that has been used for other food or untreated water [1, 10]. The iodine-restricted diet contains only 0.2 parts per million of iodine [10]. That is a really, really small amount. It would not take much to throw off the balance.
Crossley, V. J., Debnath, A., Chang, Y. M., Fowkes, R. C., et al. (2017). Breed, coat color, and hair length as risk factors for hyperthyroidism in cats. Journal of Veterinary Internal Medicine, 31(4), 1028-1034. https://doi.org/10.1111/jvim.14737
Kass, P. H., Peterson, M. E., Levy, J., James, K., et al. (1999). Evaluation of environmental, nutritional, and host factors in cats with hyperthyroidism. Journal of Veterinary Internal Medicine, 13(4), 323-329. https://doi.org/10.1111/j.1939-1676.1999.tb02189.x
Peterson, M. E., Castellano, C. A., and Rishniw, M. (2016). Evaluation of body weight, body condition, and muscle condition in cats with hyperthyroidism. Journal of Veterinary Internal Medicine, 30(6), 1780-1789. https://doi.org/10.1111/jvim.14591
Stephens, M. J., O’Neill, D. G., Church, D. B., McGreevy, P. D., et al. (2014). Feline hyperthyroidism reported in primary-care veterinary practices in England: Prevalence, associated factors and spatial distribution. Veterinary Record, 175(18), 458. https://doi.org/10.1136/vr.102431
Wang, M., Guo, W., Gardner, S., Petreas, M., et al. (2018). Per- and polyfluoroalkyl substances in Northern California cats: Temporal comparison and a possible link to cat hyperthyroidism. Environmental Toxicology and Chemistry, 37(10), 2523-2529. https://doi.org/10.1002/etc.4239
To quote Cats the musical, “A cat is not a dog.” This is a fact that is both obvious and often overlooked. Sometimes, cats and dogs are subject to the same illnesses, but that does not mean that they will have the same symptoms or can receive the same treatments. Heartworm disease is a prime example.
Heartworm lifecycle – American Heartworm Society
Heartworm is the common name for a blood-borne parasite, Dirofilaria immitis, which normally resides in the heart or attached large blood vessels of an infected animal (1). Mature female worms are 6-14 inches (15-36 centimetres) long and 1/8 inch (3 millimetres) wide, with mature males about half that size (1). They can infect dogs, cats, ferrets, and even, rarely, humans (2). Wild hosts include wolves, coyotes, foxes, and sea lions (2). The heartworm lifecycle begins when a mosquito bites an infected animal. The mosquito sucks up larvae circulating in the bloodstream, called microfilaria (2). The larvae develop into an infectious stage in the mosquito and then enter a new host animal when the mosquito bites it (2). The larvae migrate to the right side of the new host’s heart, where they will mature into adult worms over the next six months.
Heartworms can only be transmitted by mosquitos, not between individual animals, although having a significant population of infected hosts increases the likelihood of infection. Heartworms can live for 5-7 years in dogs and 2-4 years in cats (3), so the infection burden in an animal population can increase quickly over several mosquito seasons, and individuals can be infected multiple times. Heartworm infection is possible throughout the United States and in many other countries throughout the world. Thirty species of mosquito can carry the larvae (1). Infected wildlife or mosquitoes can travel great distances, and infected pets may be moved or adopted to new areas (1), so it is safest to assume that if there are mosquitoes where you live, there is a risk of heartworm.
Preserved heartworms (I know, gross) – Image by Mirko Sajkov from Pixabay
Cats are not a very good host for heartworms. Canines are more hospitable to the parasites. Cats are infected 5-20% as often as dogs in the United State, depending upon the location (4), and the worms often fail to mature inside their bodies. Cats also are unlikely to have circulating microfilaria, and about a third of infections in cats have mature worms of the same sex and therefore produce no larvae at all (5). However, just because cats are an imperfect host does not mean that they cannot suffer severe or sometimes fatal symptoms.
Because cats are so much smaller than most dogs, they don’t have space for nearly as many of the parasites. They usually have only 1-3 mature worms, if they have any adults at all (2). Even a few worms is still considered a heavy infection, however, due to a cat’s size (3). And even a cat with only immature worms can still have significant disease. Although the term “heartworm disease” would suggest that the symptoms primarily affect the heart, in cats heartworms tend to cause respiratory problems. Heartworms, once mature, often take up residence in the pulmonary arteries that take blood from the heart to the lungs (6). There, worms can block the arteries, and dead and dying worms cause inflammation that compounds the problem (6).
Cats are subject to a complication known as heartworm-associated respiratory disease (HARD). The symptoms of HARD are rapid and labored breathing, coughing, and gagging. Unfortunately, these symptoms are also indicators of asthma or bronchitis, making it difficult to distinguish a heartworm infection from other forms of respiratory distress. Other symptoms of heartworm infection are also non-specific: vomiting, loss of appetite, and weight loss. In more severe cases, cats may have seizures, faint, struggle to walk, or accumulate fluid in their abdomens. Sometimes, the first sign that something is wrong is sudden death.
If heartworm is suspected in a cat, it is much harder to diagnose an infection than in a dog. It is typical to have to use multiple tests, often more than once (7). This is because the unusual nature of heartworm infection in cats makes many tests less reliable, including the antigen blood test which is the gold standard for dogs (7). The antigen test detects mature female worms, but will not detect an infection with only male or only immature worms (5). Because both of those types of infection are likely in cats, there is a high rate of false negative results. Still, 50-70% of cats are estimated to have at least one female worm, so the test is worth a try (7).
The antibody test is another blood test which detects the cat’s immune response to the parasite. While antigen tests can only detect an infection after 6-8 months, once the parasites are mature, the antibody test is effective as early as two months after infection (7). The antibody test can be highly sensitive, but it’s reliability decreases over time and may be affected by the use of certain medications to treat the infection (7). It can also still be positive after a cat has fought off an infection but the antibodies are still in their blood (7). As the antigen and antibody tests are both useful but have major pitfalls for cats, they are best used together.
If the results of the antigen and antibody blood tests are inconclusive, there are other options. The blood can be tested for circulating microfilaria, but less than 20% of cats will have microfilaria, and then only for 1-4 weeks, so the chances of finding them are low (1). X-rays and ultrasounds of the heart and its vessels can look for changes in the shape of the heart and pulmonary arteries (1). Sometimes an ultrasound will even find the mature worms themselves, although this is uncommon since cats have so few worms (1).
When a heartworm infection can be diagnosed, treatment is its own challenge. The medication used to treat heartworm in dogs, melarsomine dihydrochloride, has been shown to be toxic to cats (8, 9). Cats that appear to be doing well or have no symptoms at all may be allowed to continue fighting the infection on their own while being continually monitored and routinely re-tested (7). A course of steroids is a common treatment for cats that need more support.
If a cat is in need of acute care, a veterinarian will step in with treatments for shock and symptoms, such as oxygen, intravenous fluids and steroids, or bronchodilator drugs to open up their airways (7). The goal is to stabilize the cat until they can eliminate the infection on their own. As measures of last resort when an otherwise stable cat seems to have trouble doing this, the drug ivermectin can be given to kill adult worms, or worms may even be surgically removed (7).
Take care of your cat’s heart <3 – Image by Melinda Rózsa from Pixabay
Obviously, heartworm disease in cats is a complicated and difficult matter. The best treatment is prevention. Heartworm preventatives are available as monthly oral or topical medications, or an injection given every six months by your veterinarian (2). In the United States, heartworm medication is available by prescription only, but on the bright side, most heartworm medications also prevent intestinal parasites, and some are effective against external parasites like fleas and ticks (2). The American Heartworm Society recommends using year-round prevention even in places with cold winters, just in case (2). Indoor cats should be treated as well. Mosquitoes have a knack for making their way inside. About a third of cats with heartworm infections are indoor-only (1). You can’t underestimate the diabolical sneakiness of mosquitoes.
For more information about heartworms, visit the American Heartworm Society at https://www.heartwormsociety.org/ or any of the resources below.
Cornell Feline Health Center. (n.d.). Heartworm in cats. Cornell University College of Veterinary Medicine. link here
Jones, S., et al. (2014, October). Current feline guidelines for the prevention, diagnosis, and management of heartworm (Dirofilaria immitis) infection in cats. American Heartworm Society. PDF
McLeroy, L.W. (1998). Evaluation of melarsomine dihydrochloride for adulticidal activity against Dirofilaria immitis in cats with intravenously transplanted adult heartworms. MS Thesis, 95. University of Georgia, Athens, GA.
Goodman, D.A. (1996). Evaluation of a single dose of melarsomine dihydrochloride for adulticidal activity against Dirofilaria immitis in cats. MS Thesis, 95. University of Georgia, Athens, GA.
