Today's Veterinary Practice provides comprehensive information to keep every small animal practitioner up to date on companion animal medicine and surgery as well as practice building and management.
Issue link: http://todaysveterinarypractice.epubxp.com/i/38074
| DIAGNOSIS OF CANINE HEARTWORM INFECTION When to Test Periodic testing, usually yearly, is rec- ommended for all dogs, and it is advisable to retest whenever changing chemoprophylaxis agents or starting prophylaxis after a period of non- compliance. When changing heart- worm preventive medication, addi- tional testing 4 months after switch- in}Ê «Ào`ÕVÌÃÊ iÃÊ ÀiµÕiÀi`Ê ÌoÊ iÛ>lÕ>ÌiÊ the efficacy of the original product. This process is especially important in light of recent suspicions of grow- ing CHW resistance to macrocyclic lactones in some regions of the U.S. (see Prophylaxis Update). Microfilaria Detection Microfilaria may be visualized in whole blood using a direct smear ÌiVhniµÕiÊ oÀÊ vÀomÊ Lloo`Ê VonVin- trated using a modified Knott’s test or membrane filtration. Compared to other methods of heartworm detec- tion, blood smears and concentration ÌiVhniµÕiÃÊ >ÀiÊ Àil>ÌiÛilÞÊ inÃinÃiÌiÛiÆÊ therefore, a test should not be con- Ãi`iÀi`Êni}>ÌiÛiÊÕnÌilÊ>ÌÊli>ÃÌÊ£Êm ÊovÊ blood has been examined.4 However, the sensitivity of the modified Knott’s test is greater than that of the direct smear method when samples contain Courtesy of the American Heartworm Society ©2007 PROPHYLAXIS UPDATE Chemoprophylaxis is the primary weapon available to prevent heartworm infections in individual animals and controls spread of this parasite by reducing infections in the primary reservoir population. Macrocyclic lactones (ivermectin, moxydectin, selamectin, and milbemycin oxime) are the most commonly used heartworm preventives. The American Heartworm Society’s current guidelines4 for control of CHW infections in dogs include: r #FHJO $)8 QSPQIZMBYJT JO QVQQJFT BT FBSMZ BT QPTTJCMF VTVBMMZ CFGPSF 8 weeks of age. r :FBS SPVOE DIFNPQSPQIZMBYJT FWFO JO BSFBT XIFSF DPME XFBUIFS interrupts the CHW life cycle) is highly recommended to increase compliance. r "MUIPVHI DPMMJFT BOE TPNF PUIFS EPHT BSF TFOTJUJWF UP NBDSPDZDMJD lactones, commercially available products for dogs used at the recommended doses are considered to be safe in all breeds. r 3FQPSUT PG MBDL PG FGGJDBDZ PG NBDSPDZDMJD MBDUPOFT JO TPNF BSFBT PG the U.S. are being investigated; currently, there is no strong evidence of widespread emergence of resistant strains of CHW. Lack of treatment and lack of compliance are of much greater concern. The issue of apparent lack of efficacy is yet to be resolved. A recent re- port detailed genetic changes in some populations of heartworms that may confer greater resistance to chemoprophylactic agents.5 Whether or not these changes translate to the emergence of macrocyclic lactone-resistant strains of heartworms in endemic populations remains to be seen. 32 Today’s Veterinary Practice July/August 2011 <ÊxäÊmiVÀovil>Ài>Ê«iÀÊ£Êm ÊovÊÜholiÊ blood6 and the test provides enhanced ability to distinguish D immitis from other canine microfilaria. Microfilaria tests are now viewed as supplemental assays, secondary to the more sensitive heartworm antigen tests. Microfilaria detection can be used to: UÊ6>li`>ÌiÊ>Ê«oÃiÌiÛiÊ>nÌi}inÊÌiÃÌÊ UÊ iÌiÀminiÊivÊÌhiÊ`o}Ê iÃÊ>Ê«oÌinÌi>lÊ reservoir for infection UÊ `inÌivÞÊ`o}ÃÊ>ÌÊÀiÃkÊvoÀÊÃiÛiÀiÊÀi>V- tion to treatment with microfilari- cides. imiÌ>ÌionÃÊ ovÊ miVÀovil>Ài>Ê ÌiÃÌÃÊ include: UÊ/hiÊ v>VÌÊ Ìh>ÌÊ >LoÕÌÊ Óä¯Ê ovÊ hi>ÀÌ- worm-infected dogs are not micro- filaremic (occult infection), usually because only 1 sex of worm is pres- ent or the infection is prepatent.4 UÊ/hiÊ ÕÃiÊ ovÊ m>VÀoli`iÊ «ÀiÛinÌiÛiÃÊ may result in clearance of microfi- laria from the blood of treated dogs after several months of use.2,4 LIFE CYCLE OF HEARTWORMS 1. The life cycle of a patent CHW infection starts with introduction of L3 larvae from an infected mosquito into the canid host. 2. In the dog, the larvae mature into L4 larvae and then into adult worms that localize in the pulmonary arteries and occasionally in the right cardiac ventricle. 3. Following a prepatent period of 6 to 7 months, microfilariae are produced via sexual reproduction and are released into the dog’s bloodstream, where they may remain viable for months to years. 4. Mosquitoes feeding on the infected host ingest the microfilaria, which then migrate to the Malpighian tubules where they mature into first- stage larvae. 5. Ultimately, the larvae mature to the L3 stage in the salivary glands of the mosquito and become infective, thus completing the heartworm life cycle.