Heartworm Infection in Bears (with a note on Lagomorphs)

• This parasite is found primarily in dogs, occasionally in cats and humans. (B46)
• Adult worms are long (20 - 30 cm) and slender; the tail of males has a loose spiral. (B46)
• They are found in the right ventricle, pulmonary artery and posterior vena cava. (B46)
• Microfilariae are found in the blood: 307 - 332 um long, 6.8 um wide, not ensheathed, with a tapered anterior and blunt posterior. (B46)
• Females release microfilariae into the bloodstream. Mosquitoes ingest these while feeding. Larvae develop in the mosquito to the L3 stage, which takes about two weeks. These larvae are found in the mosquito's mouthparts and are injected into the new host when the mosquito takes further blood meals. (B46)
• Several species of mosquitoes can act as the intermediate hosts. (B504)
• In dogs, L3 migrate to subcutaneous or subserosal tissues where they remain through two moults; young adults then pass via the venous circulation to the heart and adjacent blood vessels. (B46)
• Prepatent period: minimum six months (B46); six to seven months. (B504) Patent period: more than five hears has been recorded. (B46)
• Dead worms are carried in the bloodstream to the lungs; here they occlude branches of the pulmonary artery and produce infarcts. (B504)
• Microfilariae also can obstruct capillaries. B504)

Note: Distinguished from the connective tissue worm of bears, Dirofilaria ursi by having fewer preanal papillae per row (4-5, average 4, versus 4-9, average 7), lack of longitudinal cuticular ridges, and a more anterior placement of the vulva. (J11.61.w1)

• Dirofilaria immitis (Nematoda - Roundworms (Phylum-Division))

• Clinical signs are not seen with light infections. (B46)
  • Note: As few as five adult worms can be fatal in ferrets Mustela putorius fero (Mustela putorius - Poleca). (B504)
• With heavy infections, listlessness, gradual loss of condition, exercise intolerance, soft cough, haemoptysis (coughing up blood). Later dyspnoea, sometimes oedema and ascites. (B46)
• Episodes of acute respiratory distress with coughing of blood and worms following pulmonary embolism. (B504)
• With acute vena caval syndrome (due to a mass of worms lodged in the posterior vena cava), haemolysis, haemoglobinuria, jaundice, bilirubinuria, anorexia, collapse and death due to acute hepatic insufficiency. (B46, B504)

In Bears

• No clinical disease has been reported in bears in association with heartworm infection. (B336.51.w51)
• In a wild male hunter-killed Ursus americanus - American black bear in North Carolina, five adult Dirofilaria immitis, a 13 cm long male and four 22 - 28 cm long females, were found in the right ventricle and pulmonary artery. Heartworms also have been observed in other black bear individuals from North Carolina. (J11.61.w1)
• During a survey for parasites in Ursus americanus - American black bear in the southeastern USA, Dirofilaria immitis were found in the right ventricle/pulmonary artery of 8% of 52 individuals (four of the 13 bears from the coastal plain of North Carolina), with a mean of three worms per infected bear (range one to six). All the infected bears were at least 2.5 years of age. (J1.14.w10)
• "Polar bears [Ursus maritimus - Polar bears] are susceptible to heartworm disease but reports are rare." (D315.3.w3)

In Lagomorphs

• Rabbits have been reported as an aberrant host for this heartworm, with a granulomatous nodule associated with an immature worm found in the lung of a Oryctolagus cuniculus domesticus - Domestic European rabbit. (B614.11.w11, J11.68.w1)
  • Histologically, the nodule was found to be an organized thrombus around a degenerated worm, identified morphologically as Dirofilaria immitis, in a pulmonary artery. (J11.68.w1)

In Bears

• Despite the presence of sexually mature individuals of both sexes of the worm, and many infertile ova in the uteri of two females, microfilariae were not found in any of the females. It was suggested that the worms were not reproducing and that Ursus americanus - American black bear is not a suitable host to allow the worm to complete its life cycle. (J11.61.w1)
  • Note: while Ursus americanus is generally considered an unsuitable host for Dirofilaria immitis, with females not producing microfilaria, one adult female worm, found during a survey of black bears from six states in the southeastern USA, July 1973-November 1976, was found to contain microfilariae. (J1.14.w10)

Diagnosis

• Diagnosis of infection is normally based on the combination of clinical signs and the detection of microfilariae in the blood. (B46)
  • Microfilariae are more easily detected by lysis of cells, concentration, filtration and staining with methylene blue. In the absence of a specific kit, mix one part blood with nine parts formalin, centrifuge, mix the sediment with blue stain and examine as a smear under the microscope. (B46)
• Radiography of the thorax may reveal thickening/enlargement of the pulmonary artery, a tortuous course of the pulmonary artery, right ventricular hypertrophy and a perivascular parenchymal pattern (caudal lobar artery distribution). (B46, B208.12.w12)
• Ultrasonography (M-mode echocardiography) can reveal evidence of adult worms in the right atrium with movement into the right ventricly; this is considered pathognomonic for the disease. (B208.12.w12)
• An ELISA for Dirofilaria immitis antigens can be used to detect silent infections. (B208.12.w12)
• In Ursus maritimus - Polar bears it has been recommended that if the bears are in a heartworm endemic area and are exposed to mosquitoes, ELISA antigen testing should be carried out annually. If this is positive, further diagnostic workup (e.g. radiography) should be carried out to determine severity of infection and appropriate treatment. (D315.3.w3)

Treatment (not required in bears)

• To kill adult worms:
  • Thiacetarsamide, 2.2 mg/kg, twice daily for two days (three days sometimes recommended). Adult worms dies over days to weeks, are carried to the lungs and temporarily obstruct the circulation, before being removed by phagocytosis. Treated individuals must be protected from stress during the four to six weeks post treatment. (B504) 
  • Melarsomine dihydrochloride (deep intramuscular injection) can also be used. (B504)
  • In severe infections, surgical removal of adult worms has been carried out. (B46)
• To kill microfilariae:
  • Ivermectin, 0.05 mg/kg, or milbemicin oxime, 0.5 mg/kg. (B504)
  • Note: ivermectin is not effective against the adult worms. (B504)
• Note: It has been suggested that Ursus maritimus - Polar bears in zoos, in a heartworm endemic area where this disease has been reported as a problem in pinnipeds, if found to be ELISA-positive for heartworm antigen, should be treated appropriately as indicated by the severity of the disease determined by e.g. radiography. (D315.3.w3)

Prophylaxis (not required in bears)

• Ivermectin, 0.006 mg/kg once monthly from one month into the time of exposure to mosquitoes until one month after the mosquito season ends. (B504)
• Diethylcarbamazine, 6.6 mg/kg orally daily from the start of the mosquito season until two months after the end of the mosquito season. (B504)
  • Do not give to dogs with microfilaraemia; severe or fatal shocklike reaction may occur. (B504)
• Note: It has been suggested that Ursus maritimus - Polar bears in zoos, in a heartworm endemic area where this disease has been reported as a problem in pinnipeds, should be treated prophylactically e.g. with Ivermectin 200 micrograms per kilogram orally monthly. (D315.3.w3)

Occurrence

• Dirofilaria immitis was found in the heart (right ventricle) and pulmonary artery of 8% of bears during a survey of Ursus americanus - American black bear from six states in the southeastern USA, July 1973-November 1976. They were present in four of the 13 bears from the coastal plane of North Carolina, and were seen only in bears of at least 2.5 years of age. (J1.14.w10)
• Microfilariae were found in the blood of 17% of 24 bears examined during a survey of Ursus americanus - American black bear from six states in the southeastern USA, July 1973-November 1976. (J1.14.w10)
• Dirofilaria immitis were detected in the lungs and heart of 15.0% of 46 Ursus americanus - American black bears examined during a survey in Florida, October 1976-April 1981. No microfilariae were present in the adult female nematodes. (J381.50.w1)

In Lagomorphs

Occurrence

• Infection was detected as an incidental finding in a laboratory rabbit Oryctolagus cuniculus domesticus - Domestic European rabbit as an aberrant host. (B614.11.w11, J11.68.w1)

• Environmental Assessment
• History & Documentation
• Physical Examination of Mammals
• Necropsy of Mammals
• Imaging in Lagomorph Diagnosis and Treatment
• Clinical Pathology of Lagomorphs
• Treatment and Care
• Environmental and Population Management
• Preventative Medicine for Mammals
• Chapter 2 - B36 Field Manual of Wildlife Diseases - Specimen Collection and Preservation
• Chapter 3 - B36 Field Manual of Wildlife Diseases - Specimen shipment
• Appendix A - B36 Field Manual of Wildlife Diseases - Sample specimen history form

• Bears (Ursidae - Bears (Family))
• Ursus americanus - American black bear
• Ursus maritimus - Polar bear
• Rabbits (Leporidae - Rabbits and Hares (Family))
  • Oryctolagus cuniculus domesticus - Domestic European rabbits [aberrant host]

(List does not contain all other species groups affected by this disease)

Brigitte Reusch BVet Med (Hons) CertZooMed MRCVS (V.w127); Lord Soulsby of Swaffham Prior MA,PhD,DSc (H c: Mult) , AM(H c),DVM&S (H.c),DVMS(H.c) ,DVM(H,c) F.Med Sci.,Hon FRCVS F.Inst Biol. F.R.C.Path (H.c), DVSM, MRCVS (V.w135)