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Compendium April 2010 (Vol 32, No 4)

The Emerging Role of Wolbachia Species in Heartworm Disease

by Kristen A. Frank, DVM, R. Dennis Heald, DVM, DACVIM

    CETEST This course is approved for 3.0 CE credits

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    Heartworm disease was first recognized in dogs more than 100 years ago and is still prevalent among dogs and found in cats worldwide. The complications of heartworm disease can be devastating, and treatment carries risks. Wolbachia spp are gram-negative bacteria that infect filarial nematodes, including Dirofilaria immitis, and elicit an inflammatory response in cats and dogs. Antimicrobial therapy directed against these bacteria has resulted in decreased microfilarial loads, inhibition of the development of larval worms, female worm infertility, and reduced numbers of Wolbachia organisms. Antimicrobial therapy against Wolbachia spp may be useful in treating heartworm disease in cats and dogs, but further research is needed.

    Canine and feline heartworm infections remain a worldwide clinical problem. Heartworm infection was first described in dogs more than 100 years ago and was recognized in cats in the 1920s. Heartworms (Dirofilaria immitis) primarily infect members of the family Canidae, including domestic dogs, wolves, foxes, and coyotes; however, domestic and nondomestic cats, ferrets, muskrats, sea lions, coatimundi, and humans may also be infected.1 D. immitis is a filarial nematode in the Onchocercidae family that has a worldwide distribution but is most commonly found in mild and warm climates. Cases of heartworm infection have been identified in all 50 US states. In the United States, the highest prevalence of infection is mainly in the Southeast and Mississippi River Valley, and in some endemic areas, the infection rate approaches 45% in canids.1 Cats are unnatural hosts of D. immitis, but overall infection rates between 5% and 10% of the rate in dogs in any given area have been reported.2 Mosquitoes are responsible for transmitting infection; as many as 60 different species are potential vectors. The geographic distribution of infection is directly related to that of the susceptible mosquitoes.3

    Clinical Signs of Heartworm Disease

    Susceptible hosts are infected when feeding mosquitoes transmit L3-stage D. immitis larvae. Under ideal conditions,1 the larvae mature into adults in 6 to 7 months in dogs and 7 to 8 months in cats.4 While some dogs with heartworm infection are asymptomatic, most affected dogs develop clinical signs due to their immune response to D. immitis; resulting diseases include glomerulonephritis and allergic pneumonitis. Dogs with a high worm burden can also develop clinical disease secondary to the physical presence of the worms, such as congestive heart failure, caval syndrome, pulmonary hypertension, and pulmonary thromboembolism. Cats, too, can be asymptomatic when infected with heartworms. However, they can develop serious illness when infected with even a few heartworms. When clinical signs do occur in cats, they develop when the juvenile heartworms arrive in the pulmonary vasculature and when the adult heartworms die.5 Clinical signs in cats with heartworm infection are respiratory in origin and include dyspnea and cough due to an intense inflammatory response by the pulmonary arterial system to adult and juvenile heartworms.4,6 The clinical signs in these cats can be mistakenly attributed to asthma or allergic bronchitis, but they are part of a syndrome now known as heartworm-associated respiratory disease (HARD).5

    Wolbachia Species and Dirofilaria Immitis

    In the 1970s, intracellular bacteria were observed in various filarial nematodes, including Onchocerca volvulus, D. immitis, Litomosoides sigmodontis, and Brugia malayi, by electron microscopy.7 DNA sequence data have since identified these bacteria as belonging to the genus Wolbachia, gram-negative bacterial endosymbionts belonging to the Rickettsiaceae family. In filarial nematodes that harbor Wolbachia spp, the prevalence of infection appears to be 100%, suggesting an obligatory endosymbiosis.7 Symbiosis refers to a constant and intimate relationship between two dissimilar species, and an endosymbiont is any organism that lives within the body or cells of another organism.8 Many instances of endosymbiosis are obligate; that is, neither the endosymbiont nor the host can survive without the other. Immunochemical, immunogold, and electron microscope studies have shown that Wolbachia spp usually occur as homogenous clusters of bacteria, principally in the lateral chords of the adult worms as well as in the microfilariae and all larval stages of D. immitis.9 In female worms, they are also found in the reproductive tract, including the oocytes, morulae, and microfilariae.9 The recognition of intracellular bacteria in D. immitis has raised concerns that these bacteria may contribute to the pathogenesis of heartworm disease and other filarial infections. Hosts are continuously exposed to Wolbachia spp released from the uterus of adult female worms; they can also be exposed when adult worms or larvae die due to natural causes, host immune defenses, or chemotherapy.10

    How Wolbachia Organisms may Contribute to Heartworm Disease

    Data exist to support the potential role of Wolbachia spp in the development of the inflammatory reaction associated with heartworm disease in dogs and cats. In one study,11 cats developed an IgG immune response against Wolbachia surface protein (WSP) after infection with D. immitis. Additional studies have confirmed the IgG response to Wolbachia organisms in cats and that antibodies against Wolbachia spp increase after larvicidal therapy.12 In dogs, it has been shown that WSP activates canine polymorphonuclear neutrophils by inducing chemokinetic activity and IL-8 production.13 Human studies have shown that in river blindness (O. volvulus infection), Wolbachia organisms are direct and indirect sources of signals accounting for neutrophil accumulation around adult filariae.14 In one veterinary study, all dogs infected with D. immitis had circulating antibodies against WSP; these antibodies were detected independent of clinical signs of heartworm disease.15 In the same study, immunohistochemical staining for WSP in tissue from patients that had died from heartworm disease showed positive staining in lung, liver, and kidney samples. The Wolbachia organisms were localized within renal tubules and glomeruli and hepatic monocytes; they were found as extracellular bacteria in the lungs.15

    Wolbachia spp have been shown to contain lipopolysaccharides (LPS) that can induce monocyte activation in vitro.16 LPS, which are released by gram-negative bacteria, are the major mediators of the inflammatory response in heartworm disease.16 It was initially believed that the death of adult filarial nematodes was the primary cause of generation of an anti-WSP response because this event increases the levels of Wolbachia DNA in the bloodstream of people.17 However, exposure to the L3 larval stage of D. immitis initiated the strongest immune response in human and mouse studies,18 indicating that the role of Wolbachia spp in the inflammatory response in heartworm disease may be most significant in the early stages of infection.

    It has also been shown that Wolbachia spp induce a helper T 1 (Th1) lymphocyte response during filarial infection, whereas nematode antigens may be responsible for the helper T 2 (Th2) lymphocyte response seen in heartworm infections.19,20 Th1 lymphocytes secrete cytokines that support cell-mediated immune responses to viral and bacterial infections, characterized by enhanced cytotoxicity, recruitment of inflammatory cells to the infected site, and production of selected classes of antibodies.21 Th2 lymphocytes, which produce cytokines that generally stimulate antibody production, are activated during parasitic infections and allergic reactions; these responses are characterized by the production of all classes of antibodies and the stimulation of eosinophils and mast cells.21

    The presence of Wolbachia bacteria results in an inflammatory response in humans, dogs, and cats. In one study, 65.6% of heartworm-positive dogs were found to have Wolbachia DNA in their blood.21 Because of the high prevalence of Wolbachia spp in heartworm-positive dogs in this report and the evidence supporting the role of Wolbachia spp in inflammatory responses seen in heartworm disease, the potential for including chemotherapy to eliminate Wolbachia spp in the heartworm treatment protocol is being studied.

    Current and Potential Therapies

    The current therapy for canine heartworm disease is melarsomine dihydrochloride, an organoarsenic with an unknown mechanism of action that is effective in killing mature and immature adult D. immitis worms. Although the safety of melarsomine is better than that of thiacetarsamide (the organoarsenic previously used to treat heartworm disease in dogs), adverse effects of therapy still occur. Adverse effects associated with melarsomine administration include irritation and pain at the injection site, swelling, tenderness, and reluctance to move.22 Sterile granulomas or abscesses may also form at the injection site.23 Additionally, neurologic complications following melarsomine administration have been reported, consisting of marked paraparesis or paralysis of the hindlimbs.24 Pulmonary thromboembolism is a potential adverse effect of treating heartworm disease in dogs.

    Melarsomine therapy is currently not recommended in cats due to previous studies that indicated melarsomine was toxic to cats even at low doses.5 Symptomatic therapy for heartworm disease continues to be recommended for affected cats; therapies including corticosteroids, balanced electrolyte solutions, bronchodilators, and oxygen have been used.25

    Because Wolbachia spp are members of the Rickettsiaceae family, research has investigated the effect of antibiotics commonly used to treat rickettsial infections on these bacteria.26–29 Wolbachia spp are susceptible to tetracycline, doxycycline, rifampin, and azithromycin but are not affected by chloramphenicol, erythromycin, or ciprofloxacin.26–29 Use of effective antibiotics for Wolbachia spp in animals and humans with filarial nematode infections has resulted in decreased microfilarial loads, inhibition of the development of larval worms, and female worm infertility.28,30,31 Direct adulticidal effects on the filarial nematodes with antibiotic chemotherapy were not demonstrated, and there are conflicting results on whether antibiotic therapy completely eliminates Wolbachia DNA.31,32 However, this may be a result of the various antibiotic protocols used in these studies. A protocol has not been established for the treatment of Wolbachia infection, but effects on the fertility of filarial nematodes and a reduction in the number of Wolbachia bacteria present have been reported in dogs and humans treated with doxycycline and tetracycline at doses recommended for rickettsial infections.27,31


    Despite efforts by the veterinary community to increase pet owners' awareness of heartworm disease in dogs and cats and to emphasize the importance of heartworm preventives, heartworm disease continues to be a major problem in veterinary patients. The complications of heartworm disease, such as congestive heart failure, glomerulonephritis, allergic pneumonitis, eosinophilic granulomatosis, pulmonary thromboembolization, and HARD (in cats), can be life threatening. The treatment available for heartworm disease is not without risk in canine patients, and only supportive therapy is recommended in cats. Research has proven that the Wolbachia bacteria present in D. immitis cause an inflammatory response in both cats and dogs. Additionally, these bacteria have been found in the lungs and kidneys of heartworm-infected dogs. The emerging role of Wolbachia spp in the pathogenesis of heartworm disease offers the potential for novel therapies for this disease that may reduce the complications of heartworm infections as well as the adverse effects associated with treatment. In cats, antibiotic therapy has the potential to reduce clinical signs and the risk of acute death associated with heartworm disease. Furthermore, the ability to detect Wolbachia antigens may lead to the development of new testing methods that may enhance the diagnostic sensitivity of heartworm testing in cats and dogs with a low worm burden.

    Although the role of Wolbachia spp in inflammation has been proven and chemotherapy with tetracycline, doxycycline, rifampin, or azithromycin reduces worm viability, to our knowledge, no studies have yet been performed establishing a protocol for antibiotic use in dogs and cats to eliminate, reduce, or sterilize Wolbachia bacteria. One study of dogs that had been experimentally infected with D. immitis found that dogs treated with doxycycline in combination with melarsomine and ivermectin had less pulmonary pathology compared with dogs treated with melarsomine alone.33 Additionally, the combination of ivermectin and doxycycline has been shown to be adulticidal.34 Antimicrobial therapy for Wolbachia infection may be useful in treating heartworm disease in cats and dogs by affecting D. immitis development and reproduction and by decreasing the inflammatory responses associated with infection and treatment. At our practice, a 3-week course of doxycycline at 10 mg/kg/day is administered before adulticidal therapy is initiated. Prospective studies are needed to evaluate whether adding antibiotics to standard heartworm therapy in naturally infected dogs and cats improves clinical outcome and survival. The potential for adverse effects of therapy, including antibiotic resistance and gastrointestinal signs, must be considered when deciding whether to use antibiotics in dogs and cats with heartworm disease.

    Downloadable PDF

    1. Atkins CE. Canine heartworm disease. In: Ettinger SJ, Feldman EC, eds. Textbook of Veterinary Internal Medicine. 6th ed. St. Louis: Elsevier Saunders; 2005:1118-1136.

    2. Litster AL, Atwell RB. Feline heartworm disease: a clinical review. J Feline Med Surg 2008;10(2):137-144.

    3. Calvert CA, Rawlings CA, McCall JW. Canine heartworm disease. In: Fox PR, Sisson D, Moise NS, eds. Textbook of Canine and Feline Cardiology. Philadelphia: WB Saunders; 1999:702-726.

    4. Atkins CE. Feline heartworm disease. In: Ettinger SJ, Feldman EC, eds. Textbook of Veterinary Internal Medicine. St. Louis: Elsevier Saunders; 2005:1137-1144.

    5. McCall JW, Genchi C, Kramer LH, et al. Heartworm disease in animals and humans. Adv Parasitol 2008;66:193-285.

    6. Atkins CE, DeFrancesco TC, Coats JR, et al. Heartworm infections in cats: 50 cases (1985-1997). JAVMA 2000;217(3):355-358.

    7. Bandi C, Trees AJ, Brattig NW. Wolbachia spp. in filarial nematodes: evolutionary aspects and implications for the pathogenesis and treatment of filarial diseases. Vet Parasitol 2001;98:215-238.

    8. Daida JM, Grasso CS, Stanhope SA, Ross SJ. Symbionticism and complex adaptive systems I: implications of having symbiosis occur in nature. In: Fogel LJ, Angeline PJ, Bäck T, eds. Evolutionary Programming V: Proceedings of the Fifth Annual Conference on Evolutionary Programming. Cambridge, MA: MIT Press; 1996:177-186.

    9. Kramer LH, Passeri B, Corona S, et al. Immunohistochemical/immunogold detection and distribution of the endosymbiont Wolbachia of Dirofilaria immitis and Brugia pahangi using a polyclonal antiserum raised against WSP (Wolbachia surface protein). Parasitol Res 2003;89:381-386.

    10. Kozek WJ. What is new in the Wolbachia spp./Dirofilaria interaction? Vet Parasitol 2005;133:127-132.

    11. Bazzocchi C, Ceciliani F, McCall JW, et al. Antigenic role of the endosymbionts of filarial nematodes: IgG response against the Wolbachia spp. surface protein in cats infected with Dirofilaria immitis. Proc Biol Sci 2000;267:2511-2516.

    12. Morchon R, Ferreira AC, Martin-Pacho JR, et al. Specific IgG antibody response against antigens of Dirofilaria immitis and its Wolbachia spp. endosymbiont bacterium in cats with natural and experimental infections. Vet Parasitol 2004;125:313-321.

    13. Bazzocchi C, Genchi C, Paltrinieri S, et al. Immunological role of the endosymbionts of Dirofilaria immitis: the Wolbachia spp. surface protein activates canine neutrophils with production of IL-8. Vet Parasitol 2003;117:73-83.

    14. Brattig NW, Buttner DW, Hoerauf A. Neutrophil accumulation around Onchocerca worms and chemotaxis of neutrophils are dependent on Wolbachia endobacteria. Microbes Infect 2001;3:439-446.

    15. Kramer LH, Tamarozzi F, Morchon R, et al. Immune response to and tissue localization of the Wolbachia spp. surface protein (WSP) in dogs with natural heartworm (Dirofilaria immitis) infection. Vet Immunol Immunopathol 2005;106:303-308.

    16. Taylor MJ, Cross HF, Ford L, et al. Wolbachia bacteria in filarial immunity and disease. Parasite Immunol 2001;23(7):401-409.

    17. Keiser PB, Reynolds SM, Awadzi K, et al. Bacterial endosymbionts of Onchocerca volvulus in the pathogenesis of posttreatment reactions. J Infect Dis 2002;185(6):805-811.

    18. Lamb TJ, Le Goff L, Kurniawan A, et al. Most of the response elicited against Wolbachia surface protein in filarial nematode infection is due to the infective larval stage. J Infect Dis 2004;189:120-127.

    19. Marcos-Atxutegi C, Kramer LH, Fernandez I, et al. Th1 response in BALB/c mice immunized with Dirofilaria immitis soluble antigens: a possible role for Wolbachia? Vet Parasitol 2003;112:117-130.

    20. Marcos-Atxutegi C, Gabrielli S, Kramer LH, et al. Antibody response against Dirofilaria immitis and the Wolbachia endosymbiont in naturally infected canine and human hosts [abstract]. Proceedings of the IX European Multicolloquium of Parasitology, Valencia (Spain). Bologna, Italy: Medimond; 2004:297-302.

    21. Lappin MR, Hawley JR, Levy JK, et al. Wolbachia spp. DNA in blood of dogs with Dirofilaria immitis infection [abstract]. Am Coll Vet Med Proc 2006:732.

    22. Plumb DC. Plumb's Veterinary Drug Handbook. 5th ed. Ames, IA: Blackwell; 2005:487-489.

    23. Rawlings CA, McCall JW. Current uses and hazards of melarsomine. Kirk's Current Veterinary Therapy XIII Small Animal Practice. Philadelphia: WB Saunders; 2000:787-790.

    24. Hettlich, BF, Ryan K, Bergman RL, et al. Neurologic complications after melarsomine dihydrochloride treatment for Dirofilaria immitis in three dogs. JAVMA 2003;223(10):1456-1461.

    25. Dillon R. Clinical significance of feline heartworm disease. Vet Clin North Am Small Anim Pract 1998;28:1547-1565.

    26. Hoerauf A, Mand S, Adjei O, et al. Depletion of Wolbachia endobacteria in Onchocerca volvulus by doxycycline and microfilaria after ivermectin treatment. Lancet 2001;357:1415-1416.

    27. Hoerauf A, Volkmann L, Hamelmann C, et al. Endosymbiotic bacteria in worms as targets for a novel chemotherapy in filariasis. Lancet 2000;355:1242-1243.

    28. Hoerauf A, Volkmann L, Neissen-Paehle K, et al. Targeting of Wolbachia endobacteria in Litomosoides sigmodontis: comparison of tetracyclines with chloramphenicol, macrolides and ciprofloxacin. Trop Med Int Health 2000;5:275-279.

    29. Townson S, Hutton D, Siemienska J, et al. Antibiotics and Wolbachia in filarial nematodes: antifilarial activity of rifampicin, oxytetracycline, and chloramphenicol against Onchocerca gutturosa, Onchocerca lienalis and Brugia pahangi. Ann Trop Med Parasitol 2000;94(8):801-816.

    30. Bandi C, McCall JW, Genchi C, et al. Effects of tetracycline on the filarial worms Brugia pahangi and Dirofilaria immitis and their bacterial endosymbionts Wolbachia. Int J Parasitol 1999;29:357-364.

    31. Smith HL, Rajan TV. Tetracycline Inhibits development of the infective-stage larvae of filarial nematodes in vitro. Exp Parasitol 2000;95:265-270.

    32. Rajan TV. Relationship of anti-microbial activity of tetracyclines to their ability to block the L3 to L4 molt of the human filarial parasite Brugia malayi. Am J Trop Med Hyg 2004;71:24-28.

    33. Kramer LH, Grandi G, Leoni M, et al. Wolbachia and its influence on the pathology and immunology of Dirofilaria immitis infection. Vet Parasitol 2008;158:191-195.

    34. Bazzocchi C, Mortasino M, Grandi G, et al. Combined ivermectin and doxycycline treatment has microfilaricidal and adulticidal activity against Dirofilaria immitis in experimentally infected dogs. Int J Parasitol 2008;38:1401-1410.

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