VetInfo Digest April 2004
Table of Contents:
When not to do certain tests
Tick Borne Disease Information Updates
(continued from last month's issue)
New server, new mail lists, new anti-spam software
This month's note:
One of the greatest strengths a veterinarian can have is a clear understanding of what he or she is not capable of doing well. It is almost certainly better to have an ultrasound examination done by someone who performs several of these examinations a day than by a practitioner who does one a week or one a month. If you were going to have knee surgery would you pick a general surgeon or an orthopedic surgeon? Do you need an expert to look at your pet's X-rays?
When your pet needs a diagnostic, medical or surgical procedure that seems to fall outside the normal bounds of care that might be expected at a general practice, consider asking for referral to a specialist. In some cases, such as review of X-rays, it is possible for your veterinarian to do the initial work of taking the X-rays but to send the films to a board certified veterinary radiologist for review. To some extent this works for ultrasound examinations and review of blood smears or cytology specimens by sending digital images to appropriate specialists. This use of "telemedicine" can be very helpful, but is still a little limited by the quality of the data that can be transmitted.
If you believe your pet might benefit from a specialist's care, don't hesitate to ask your vet for a referral or to ask if review of X-rays, lab work or ultrasound examination by board certified specialist can be arranged. In many instances this is better for your pet and for your veterinarian, too.
Testing for Disease -- When it is not appropriate?
As our knowledge of disease processes has increased, more and more tests have been devised to help detect and quantify different disease processes. In some instances, the tests have made a huge difference in the quality of life for pets. In other instances, the tests themselves have introduced new risks that have to be accounted for when making the decision to test or not test. In addition, there are a number of tests used in veterinary medicine which only work under certain conditions, making it necessary to check to be sure that the testing procedure is appropriate before pursuing testing. With this in mind, it seems like a good idea to review some of the most common tests and to give some guidelines for when they are not appropriate.
Heartworm Testing in Dogs
It is not possible to detect heartworms that are less than 5 to 6 months of age, so heartworm testing is not appropriate in dogs when:
- The dog is less than 5 to 6 months old.
- Potential exposure to heartworms occurred less than 5 months previously, such as when a monthly heartworm preventative dosage is missed or after a trip to an endemic area where heartworm prevention was not administered such as a family trip to Florida that included the pet.
- There is no reason I am aware of to test for heartworm disease more than once a year when pets are on heartworm prevention medications.
Heartworm Testing in Cats
Most of the available in-clinic tests for heartworms in cats are tests for antibody against heartworms instead of direct tests for heartworms. This is an important difference because antibodies may be formed when a cat is bitten by a mosquito carrying heartworms but may not actually develop heartworm disease as a result of the bite. There are only limited time constrictions on heartworm antibody testing in cats. Antibodies are present from about 2 to 3 months after exposure to heartworms to a year or more after exposure, even if heartworms never develop in the cat. So this particular type of test indicates that heartworms may be possible but does not confirm their presence.
There are antigen tests for heartworms in cats but they have a low sensitivity. Approximately 50% of cats who are actually infected with heartworms will test negative on these tests because of the low numbers of heartworms that tend to be present in cats. It is still a good idea to do an antigen test if a cat is positive on an antibody test to try to determine if infection is actually present.
There are no reasons not to do fecal examinations, including fecal smears, direct fecal examination in saline or fecal floatations repeatedly when diarrhea, vomiting or weight loss are present. Lots of people ask why this test is done so frequently when it is negative several times in a row but almost all gastroenterology specialists agree that multiple fecal exams are necessary in order to rule out intestinal parasites and other intestinal disorders. For this particular test it is hard to say it is ever inappropriate.
It is important to point out that the type of testing done does make a difference. Fecal floatation is much better at detecting worm eggs than direct exam of feces mixed with saline. Fecal floatation usually requires at least a teaspoonful of feces. Stained fecal smears or rectal swabs are better for detecting bacterial infections. Examination of a fecal smear in saline is sometimes the best way to find Giardia and protozoan infections, although soon there will be an available ELISA test for Giardia which should help in detection of this particular organism. Try to ensure that the correct sampling has occurred when diagnosis is difficult.
Feline Leukemia Virus Testing
I know that this may be controversial, but it is questionable whether the currently available feline leukemia virus (FeLV) tests should be used as screening tests for this disease. In an apparently healthy cat a positive test for FeLV will be a false positive test approximately 50% of the time (this is explained in the November 2000 issue of the VetInfo Digest: http://www.vetinfo.com/subscriber/1100digest.html). Remember that this information applies to cats that are apparently healthy. A cat with visible clinical signs of FeLV and a positive test result is much more likely to actually have the disease. Conversely, a negative test will be incorrect less frequently, probably less than 1% of the time, but the true false negative rate is unknown due to sequestering of the virus in some cats.
Feline infectious peritonitis (FIP) testing in cats
It is critically important to understand that any test referred to as an FIP test in cats is actually a test for exposure to feline coronavirus and nothing more. There is no test currently available that distinguishes well between these two viruses other than biopsy of diseased organs or necropsy (post mortem) examination. For this reason, routine testing for FIP as a screening procedure for cats who appear healthy is generally not a good idea. More cats have probably been euthanized inappropriately due to "positive" FIP tests than have actually died from the disease (although this is hard to document with certainty). Cats with clinical signs of FIP and high feline coronavirus titers are more likely to actually have FIP but even this is not certain.
Lyme Disease Testing
Screening tests for Lyme disease are also very likely to cause confusion. Most dogs who have positive titers to Lyme disease do not have Lyme disease and will never develop Lyme disease. They have been exposed to the bacteria, Borellia burgdorferii that causes Lyme disease but that doesn't mean they will develop clinical disease. Therefore, it is questionable whether there is any value in routinely screening for Lyme disease in an individual patient. It can be useful in establishing whether the disease is present in a locality, though.
At the present time I think that most of the infectious disease researchers question whether Lyme disease occurs in cats, even though some do have positive titers to the bacteria. If this thinking continues to hold true then testing cats for Lyme disease is unnecessary.
Kidney biopsy is a relatively high risk biopsy procedure and it should probably only be used in situations in which it is reasonable to expect that accurate diagnosis of the particular type of kidney disease affecting a dog or cat will result in a better ability to manage the kidney disease. Ask your vet if this is the case before authorizing a kidney biopsy.
- Kidney biopsy should not be done in a patient with a bleeding disorder.
- Kidney biopsy should not be done if large cysts or detectable abscesses are present in the kidney.
- Biopsy of the kidneys should not be attempted in a patient who may have a urinary tract obstruction.
In a patient with no known tendency towards bleeding it may be reasonable to do a fine needle aspirate of the liver without testing for blood clotting disorders but any other biopsy technique requires some confirmation of blood clotting ability prior to the biopsy, even in patients not suspected of having blood clotting disorders. Fine needle aspirate provides very small samples of the liver and destroys the normal architecture of the tissue so its usefulness is limited to disorders in which this may not matter, such as hepatic lipidosis in cats.
- Liver biopsy should not be attempted in a patient with a blood clotting disorder, regardless of the method of biopsy.
Aspiration from tumors
Sometimes I get questions from people wondering if it is necessary for their vet to do a fine needle aspirate on every lump their dog has. A fine needle aspirate consists of suctioning a small sample from a tumor using a syringe and hypodermic needle and examining it under the microscope to try to identify the tumor type. From a practical standpoint, it often seems unnecessary to me to aspirate every single tumor a dog develops after we have identified tumor types that are occurring, such as lipomas or inclusion (sebaceous) cysts and new ones appear. On the other hand, many veterinary pathologists believe that each individual tumor should be examined independently on the small chance that a tumor that resembles one that is already present turns out to be something else. This is another situation in which it is pretty hard to argue against taking the cautious approach except for the cost, discomfort to the patient and time involved. Even though I tend to take a middle of the road approach and aspirate anything I think is suspicious but not every single lipoma a dog or cat has, I am not able to say that people who approach this issue by just aspirating every single tumor are wrong.
Biopsy of tumors
Tumors should not be biopsied except in a manner in which the entire biopsy tract will be removed when (or if) the tumor is subsequently removed. This is especially important when dealing with very highly aggressive tumors such as vaccine associated sarcomas in cats. In a small number of instances in which a biopsy punch passes through normal tissue that is not subsequently removed, there is "seeding" of tumor cells into the normal tissue that later causes tumors. This is not an admonition not to do biopsies prior to removing tumors. It is often critically important to know what type of tumor is present so that the best possible surgical plan can be developed prior to attempting surgery. It is just important to do biopsies carefully. Make sure that the biopsy tract will be removed later if a malignant tumor is diagnosed.
Presurgical and pretreatment blood panels
I have had many arguments with other veterinarians over the routine use of presurgical blood panels. I don't have any objection to drawing blood prior to surgery to look for preexisting problems that might alter the surgical or anesthetic plan. I object to the implication that these tests make surgery or an anesthetic event safer. I think this is simply not true, with the possible exception of truly optional surgical procedures that can be indefinitely postponed without causing the patient ill effects.
This is the scenario that I find objectionable. The client is asked about presurgical or pretreatment lab work with some phrase like "We will ensure that you pet can safely be anesthetized or safely treated for heartworms." Blood is drawn. A problem such as an elevated liver enzyme is found on the lab work. The patient still needs surgery or still has heartworms but the veterinarian is now in a bind. If he or she goes ahead with the surgery or treatment immediately then it looks bad because the blood work seems to preclude the treatment - even though it really doesn't. So the vet postpones, or even refuses to perform, a necessary surgery or treatment. In many instances this means that the patient will suffer unnecessarily simply due to the results of the lab work.
Alternately, if a client assumes that drawing presurgical or pretreatment lab work will somehow guarantee that the patient can be safely anesthetized or will survive a difficult surgery or treatment, then an improper understanding of the value of the test has been reached by the client. The lab work does not and can not guarantee that complications won't occur. So in this case, the test is fine but the interpretation of the test results is a critical issue. Presurgical lab work is valuable as a planning tool and worth paying for as long as you understand that it is not a guarantee of a successful outcome nor should findings necessarily preclude a necessary surgical or medical procedure if appropriate compensation for the findings is possible.
Hypothyroidism, or low thyroid levels, is common in dogs. It is is very rare in dogs less than two years of age and when present in juvenile dogs almost always results in very poor growth or dwarfism. In female dogs less then two or three years of age low thyroid levels may be the cause of failure to come into estrus. Except for these two conditions it is questionable whether thyroid testing is necessary in dogs less than two years of age, although some vets do feel it is possible to predict autoimmune thyroiditis in the future based on the presence of anti-thyroid antibodies in young dogs. This is still controversial.
When testing dogs for hypothyroidism it is inappropriate to rely on a single total thyroxine (TT4) test to make the diagnosis. It is likely that hypothyroidism is the most frequently misdiagnosed hormonal disease in dogs, partially due to the use of inappropriate tests. The most accurate test for hypothyroidism is the free thyroxine test done by equilibrium dialysis (fT4ed). When this test is combined with a TT4 test it is accurate more than 90% of the time.
Low thyroid levels can occur in almost any highly stressful situation, when dogs are ill with other diseases and when they are on prednisone or other corticosteroids. Long term supplementation with thyroxine should not be started based on the results of thyroid tests done when any of these conditions is present. It is thought that greyhounds have naturally low total thyroxine levels and it is important to remember this when testing for hypothyroidism in this breed.
Hyperthyroidism, or too much production of thyroid hormone, occurs most commonly in older cats. It is rare in cats less than five years of age. In the past I have been against routine testing of older cats for this condition. I have preferred to wait for clinical signs to appear prior to testing for the disease. As time goes on I am becoming less resistant to routine testing but we still do not test all older cats on a yearly basis, as is recommended by some feline specialists. It is appropriate to use total thyroxine (TT4) testing for the detection of hyperthyroidism as a screening test.
When testing for hyperthyroidism it is important to remember again that illness, chronic stress and the use of corticosteroids can lower thyroid levels. A cat with clinical signs of hyperthyroidism and "normal" T4 values who has one of these conditions may need to be tested at a later time when other health issues have been resolved or may need specialized testing, such as fT4ed or T3 suppression testing. Very old cats may have hyperthyroidism even though their T4 levels fall within the normal range, since T4 levels decline with age. This is another reason to consider more specific testing procedures.
Vector Borne Disease (continued from last month)
A vector borne disease is an infectious disease that is carried from one victim to another by insects like ticks, fleas and mosquitos. These diseases are generally not directly contagious from one disease host to another, although there are diseases that are predominantly transmitted through vectors that occasionally can be transmitted directly when circumstances are just right, such as through a bite wound.
The ultimate vectors for disease appear to be ticks. There are a number of diseases that rely on ticks to move from one animal to another. In recent years more accurate tests for tick borne diseases have become available and it is becoming obvious that animals who have one tick borne disease may have two or even three tick borne diseases at the same time. This probably explains some of the treatment failures associated with tick borne diseases in the past. If only one disease responds to the medication given and two diseases are present, there is likely to be failure to successfully treat the patient. If your pet has tick borne disease and is not responding appropriately to treatment it is critical to keep the problem of multiple infections in a single host in mind.
Some tick borne diseases can be found in many parts of the world and others have very limited geographical distribution. Lyme disease, which was discussed in last month's VetInfo Digest, is an example of a tick borne disease that is most prevalent in a fairly limited geographical area. The Ehrlichia family of parasites is found in many areas of the world.
Ehrlichia canis is probably the most widely recognized tick borne disease other than Lyme disease. It is found in many areas of the world and is primarily spread by the brown dog tick. Infection rates are higher in warmer areas, so this disease tends to have a Southern distribution in the U.S. The first signs of infection with E. canis typically occur within 1 to 3 weeks after the organism enters the body. Low grade fever, decreases in platelet counts, weight loss, lymph node enlargement, lessening or lack of appetite and weight loss are the typical clinical signs. This phase of the clinical signs is often missed by pet owners and does not often cause severe enough illness to cause death. The majority of dogs manage to kill the Ehrlichia organisms during this acute phase of the infection and are no longer infected.
Dogs whose immune systems can not totally eliminate the E. canis infection become chronically infected. This group of dogs has a much higher risk of dying from the infection or problems the infection causes, over time. During chronic infections there is often development of a bleeding disorder through signs such as nose bleeds, blood in the stool, bruising of the skin, hemorrhage in an eye or blood in the saliva. Anemia without obvious bleeding occurs in some dogs, leading to very pale gums or eye tissue. Swollen lymph nodes are common in affected dogs. Some dogs develop neurologic signs such as difficulty walking or a head tilt. Kidney failure may occur due to glomerular damage in the kidneys. White blood cell counts may be so depressed that secondary infections occur easily and these may be the reason that the dog is first seen by a veterinarian. Ehrlichiosis is one of the causes of immune mediated hemolytic anemia (IMHA) and this must be kept in mind in areas in which the infection is likely.
Testing for ehrlichiosis is usually done by blood test. A serum sample is tested for the presence of antibodies against E. canis. This means that a positive test can only be used to indicate exposure to the disease. Dogs with positive titers may have cleared the infection themselves, may be in the process of clearing the infection or may be chronically infected. If a titer is measured prior to treatment and then compared with a second titer done two to three weeks later by the same laboratory, a rise in titer is indicative of an active infection being present. Paired titer samples are nice to have but most dogs are going to be better after two to three weeks of treatment and so it not uncommon for veterinarians and veterinary clients to presume treatment was successful based on the response to the treatment.
New DNA tests, polymerase chain reaction (PCR) tests, can indicate the presence of the E. canis organism directly. This is a big advantage when trying to decide if there really is an infection but due to the nature of E. canis infections it is possible to get a negative test in a dog that is really infected. Lab errors are a constant risk when doing PCR testing, as contamination of samples with even one organism can lead to a positive test result. For this reason, it is usually advised to combine PCR testing with serologic testing to get the most information possible when testing for ehrlichiosis.
Sometimes it is possible to see a cluster of Ehrlichia organisms in a white blood cell on a blood smear. If clinical signs of ehrlichiosis are present, this is sufficient for a presumptive diagnosis of infection. These inclusions are referred to as a morula if one is seen or morulae if more than one is seen.
Ehrlichisosis is probably missed when it is present in a fairly large percentage of dogs. The most common reason for this is simply not looking for it. The signs of the disease are fairly vague and are not specific for ehrlichiosis. The tests only give information on this specific disease, are somewhat expensive and take several days to get results from, making it difficult to recommend testing for every patient who has some sign of this disease. There is a new test kit for E. canis infection that can be run in any veterinary practice and this test appears to work well as an indication of antibodies to E. canis but does not provide enough information to differentiate between past exposure that was successfully handled by the dog's immune system and current infection. Still, the availability of this test kit may make it easier to screen for ehrlichiosis and decrease the potential for missing the disease early on.
Treatment for ehrlichiosis is usually accomplished with doxycyline (usually generic) or imidocarb dipropionate (Imizole Rx). Doxycycline is given in capsule form most of the time and can be administered at home. Imidocarb is an injectable medication given initially and then repeated in two weeks. The injection can cause a lot of salivation, nasal exudates, diarrhea or breathing difficulty so it is often given after an atropine injection, which cuts down on the side effects of the imidocarb.
Figuring out if treatment has been successful is quite difficult in dogs. If the infection has been cured the titer to E. canis should decrease over the course of 6 to 9 months but this isn't completely consistent. If clinical signs of illness persist it is important to keep in mind the possibility of a second vector borne illness and to consider changing the medication being used to treat the ehrlichiosis. Fortunately, treatment for ehrlichiosis is successful more often than not.
There is no vaccination to prevent this disease, so prevention efforts have to be directed towards avoidance of tick bites. Fipronil (Frontline, Frontline Topspot tm), permethrin (many products) and amitraz (Preventic Collars tm) are the most consistently successful tick control products. In severely endemic areas it might even be reasonable to administer doxycycline periodically to try to eliminate infections before signs appear but this is a questionable approach to prevention given what is known about antibiotic resistance at this time.
Cats appear to be able to be infected with E. canis but reported infections are uncommon and it is hard to be certain that E. canis is the infecting organism, although that is the best guess in some infections, currently. Affected cats have similar clinical signs based on the case reports so far. Serologic testing doesn't seem to be reliable in cats so PCR testing is the mainstay of diagnosis in this species. Doxycycline is usually recommended for treatment of cats but care must be taken to ensure the capsules or tablets are passed through the esophagus and into the stomach, usually be giving water with a syringe after administration of the doxycycline. There have been a number of case reports of esophageal damage attributable to doxycyline capsules hanging up in the esophagus and causing scarring or even complete strictures. Having a compounding pharmacy make the doxycyline into a liquid form can be helpful in preventing this problem. Immune mediated hemolytic anemia, which can easily be confused with ehrlichiosis, is also uncommon in cats, so it makes sense to consider both of these problems when signs of IMHA occur in cats.
Ehrlichia ewingii is another Ehrlichia species that infects dogs. It is carried predominantly by the brown dog tick and is most common in the Southern states. At this time it does not appear to infect cats. Affected dogs usually have acute severe lameness with discernible joint pain. There may be swelling of the joints in some dogs. Fever is usually present. Lymph node swelling is not a consistent sign. More rarely, neurologic signs have been reported, such as inability to balance. Bleeding disorders are not as likely as they are with E. canis but have been reported. Due to the clinical signs it is likely that at least some cases of E. ewingii infection are mistaken for Lyme disease. Treatment with doxycycline is usually effective and recovery is usually swift. This disease is most common in the spring and summer since the brown dog tick is most active during these times.
Ehrlichia caffeensis is a disease that most commonly affects people. It causes fevers, headaches and muscle pain. It can be fatal if untreated. It is carried by the brown dog tick (Rhipcephalus sanguineus) and the American dog tick (Dermacentor variabilis) This organism can infect dogs, although it is thought that most will not show clinical signs, or will have only mild signs of illness such as lethargy. There have been a very small number of more serious cases of disease in dogs. Clinical signs included nose bleeds, lymph node enlargement, lethargy, vomiting and eye inflammation (uveitis). Unfortunately, E. chaffeensis antibody cross reacts with E. canis antibody on serologic tests, making it difficult to be certain how frequently this infection occurs. Keeping ticks off of your dogs so that they do not serve as a reservoir for this infection in people seems like a reasonable preventative health care measure to me.
Another species of organism carried by ticks that infect people, cats and dogs is Anaplasma phagocytophilia (was Ehrlichia equi). This organism is carried by Ixodes ticks, the same ones that carry Lyme disease. In dogs infection with this organism may not cause clinical signs or may cause lethargy, fever, decreased platelet numbers and sometimes lameness. In cats the disease is also thought to be inapparent in most cases but may cause mild disease with the same signs seen in dogs. Perhaps the major worry with this disease is that it is thought to be a fairly frequent coinfection with Lyme disease and may be a factor in some cases of Lyme disease that appear to be non-responsive to treatment.
Anaplasma platys (used to be Ehrlichia platys) causes a cyclic thrombocytopenia, or periodic decreases in platelet numbers. This usually does not cause bleeding disorders but A. platys infection sometimes occurs concurrently with E. canis infection and the combination may lead to severe clinical signs in some dogs. This disorder can cause severe platelet deficiencies in some dogs (it has not been documented in cats). It would definitely be worth considering testing for A. platys if a dog had recurrent episodes of low platelet counts that can't be explained by the presence of another disease. This disease seems to have worldwide distribution, as it has been seen in the U.S, Europe, Asia and Africa. A paper with case reports from Israel reported more severe clinical signs, including lameness, fever, lymph node enlargement and lethargy in association with the cyclic platelet count abnormalities. (Harrus S, 1997)
Neorickettsia risticii (was Ehrlichia risticii) is most commonly recognized as the cause of Potomac horse fever, but it can infect dogs and cats, as well as horses. In dogs and cats it causes lethargy, vomiting, joint pain and bleeding disorders. This disease is not thought to be carried by ticks. It's life cycle is more complex than the Ehrlichia organisms and involves development in small snail species. It is not clear whether the snails have to be ingested or whether there can be other vectors such as aquatic insects. This life cycle is probably the main reason that the infection was noticed first in horses and only later discovered in dogs and cats, since horses are more likely to be eating vegetation that might harbor the snails or insects.
Salmon poisoning is a disease in the Northwest United States that is caused by Neorickettsia heminthoeca. This organism has a life cycle that involves a fluke worm (Nanophyetus salminocola), snails, fish and then birds or mammals. It causes a sudden severe diarrhea in dogs who have eaten raw fish or salamanders that are infected with fluke worms that are infected with Neorickettsia. This causes a severe diarrhea that closely resembles parvoviral diarrhea except that it is responsive to treatment with doxycycline or other tetracycline antibiotics, along with medications to kill the flukes. If not treated this disease is often fatal but it responds very well to treatment in most cases.
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