Heartworm Associated Respiratory Distress Syndrome (HARD) and HWD in Feline Patients (3 Pages)
This disease is very frustrating for the practitioner and for the feline patient. HW behaves differently in the cat than in the dog and therefore clinical presentation and treatment options are vastly different and represent notable challenges. The current testing options are limited at best. The results between tests are highly variable and somewhat unreliable due to poor test performance in some cases and general limitations due to heartworm behavior in all testing options.
It is a known fact that HW is transmitted to the feline patient when a mosquito that has fed on an infected dog feeds on the cat. It should therefore be intuitive that indoor cats should be at less risk. However, in some studies as many as 1/3 of the cats definitively diagnosed with HW based on necropsy were indoor cats. In endemic areas, all cats are at risk and should be on prevention medication. FELV, FIV infections do not appear to increase risk.
TOP 5 KEY DIFFERENCES BETWEEN FELINE AND CANINE HEARTWORM DISEASE:
In cats a much lower % of larvae survive to adulthood. The adult worms are smaller and cats have less worm burdens overall. There are higher rates of single worm or all male infections. The chances of seeing microfilaria is < 20% modified Knott or Millipore filter), and the worm survives only 2 to 3 years in the cat when compared with 5 to 7 years in the dog.
Cats are more likely to have HW larvae migrating to locations outside heart and lungs. The cats can remain asymptomatic or signs may be associated with local infiltration. An example of this is neurologic signs when larvae are migrating in the brain.
The organ most affected in cats is the lung and they can develop clinical signs during larval stages when the HW is not yet present in the heart.
Clinical signs are most commonly associated with respiratory, digestive, neurologic systems as well as sudden death. Dyspnea 45%, vomiting 34%, chronic vomiting 38%, coughing 29%, sudden death 19%, neurologic signs 14%.
The clinical signs often present in two different phases. The first phase is seen during the 2-5 months while larvae are migrating and molting (late fall and early winter) and then the 2nd phase is when adult worms die and emboli in the pulmonary arteries.
Most visible lung changes occur after 7 months post infection and EVEN transient infections can leave cats with long-term lung pathology.
Pulmonary artery blunting, increased tortuosity with flattening of the diaphragm is seen less often in cats than dogs. RVE, pleural effusion, chylothorax and gallop rhythms are uncommon and should lead you to consider alternative diagnosis first.
RECOMMENDED DIAGNOSTIC WORK UP:
Combination of Ag, Ab, and microfilaria tests. POS microfilaria only 100% reliable result.
EKG is cases where suspicion is present and other results are negative
Echocardiogram: Should be done to differentiate primary cardiac disease causing respiratory signs.
TESTING: WHAT DO THE RESULTS MEAN?
The current state of testing options is almost as frustrating as the lack of effective treatment. Serological tests vary significantly in terms of performance. Neither AG nor AB test correctly identified all HW positive cats, so you cannot eliminate the possibility of infection based on Negative test results.
Brief Description Interpretation Limitations Antibody Test
Antibodies confirm infection with heartworm larvae,
exposure, infection < 60 days old, or
but do not confirm disease causality. May remain
positive for an unpredictable time after infection is
cleared. Means infection has occurred, not
mean current infection with larval adults, previous infection with
Antibody persistence, false positive, or
may have aborted infection prior to maturation into adult (ex: those cats on prevention)
Low worm counts and male-only worm infections
adult female repro tract
no HW, infections < 7 months old, all
unpredictable amount of time after infection
Confirms presence of HW, recently cleared infection or false positive result.
Radiographic signs subjective and affected by
clinical interpretation. Most radiographic changes
occur >7 months after infection. Cats can develop
clinical signs before HW reaches the heart. Severity
of clinical signs does not correlate with worm
burden. Transient infections can leave cats with long-term lung pathology.
Echocardiography Detects echogenic walls of the No worms
Ultrasonographer experience with heartworm
detection appears to influence accuracy rate.
arterial tree, if within the visual window of the ultrasound.
THE ROLE OF WOLBACHIA:
Research shows there is a bacterium called Wolbachia, which colonizes each HW. It has substantial inflammatory effects when the HW’s are dying or molting. In humans, doxycycline is used prior to treatment to sterilize the HW and lead to fewer anti-inflammatory effects. The treatment of HW in cats with an adulticide or manual removal is associated with high morbidity rates and is generally contraindicated. Elimination of the Wolbachia organisms may help the cat co-exist more comfortably with its HW when they are dying or molting.
Respiratory signs can often be managed with anti-inflammatory medications, bronchodilators and antibiotics. Most cats outlive their HW’s but clients must be warned of the consequences, such as thromboembolism, sudden death or neurologic signs.
TREATMENT/PREVENTION Clinical Signs: Chronic or Cyclical
HW prevention monthly Prednisone 1-2mg/kg PO q48-72 hours +/- Doxycycline 5mg/kg PO
Acute Respiratory Distress
Oxygen Dex 1mg/kg IV/IM or Prednisilone sodium succinate 50-100mg/cat Aminophylline 6.6 mg/kg IM q12 or Theophylline ER 10mg/kg P.O. or Terbutaline 0.01 mg/kg S/C +/-Doxycycline 5mg/kg PO HW prevention Monthly
Asymptomatic Ag +ve Asymptomatic Ab +ve Asymptomatic Ag and Ab negative REFRENCES:
1. The role of Wolbachia in Feline HWD, 17th ECVIM-Congress 207, Julie K. Levy DVM, College of Veterinary Medicine ,
2. HW disease without the HW, AAFP Spring 2008 Meeting, Julie Levy DVM, College of Veterinary Medicine, University of
3. Feline Heartworm Disease- the HARD part, ACVIM 2008, Danielle Russ, LVT, BS, BA, AS, Virginia Beach, VA 4. American HW Society Website 2007: guidelines for the Diagnosis/Prevention and Management of HW (Dirofilaria Immitis)
infection in cats. Available at http:/www.heartwormsociety.org/
Judy McFarlen DVM Mobile Sonographer and Medical Consultant Eric C. Lindquist DMV (Italy), DABVP (K9/Feline medicine) Founder of SonoPath.com
Contact Information - Dr. Marty Henderson – SonoVet, Inc.
301 Main Plaza, Ste. 332, New Braunfels, TX 78130
Phone/Fax: 800-792-5106 | Web: http://SonoVet.us | Email: Info@SonoVet.us SonoVet, Inc. is independently owned and established by Dr. Marty Henderson.
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