Heart and Lung worm in Dogs and Cats

Describe the following for Dirofilaria immitis.

• Final Host

• Intermediate Host

• Site

• Distribution

• A nematode (roundworm) parasite

• Final host: dog, wild dogs, cat, ferret, sea lion, (human, accidental FH, zoonotic)

• Intermediate host (IMH): mosquito - has an indirect life cycle

• Site: cardiovascular system

• right ventricle, posterior vena cava, pulmonary artery

• Distribution: USA, warm temperate zones (Europe), tropics

• Not endemic in UK - identified in pets travelled or imported from Southern and Eastern Europe

Describe the life cycle of D. immitis.

• The eggs often hatch internally within the adult (MF) → released from female into the blood circulation

Describe the pathogenesis of D. immitis in dogs.

• Depends on worm burden

• Low numbers, little apparent ill effect

• High numbers, lots of problems

• Pathology associated with adults in heart

• Adult females are 20-30cm long

• Impede blood flow

• Giving rise to chronic congested right-sided heart failure

• Pulmonary embolism

• Adults blocking vessels

• Important if treat to kill adult worms

  • Need to consider level of the infection → Is it safe to administer treatment?

• Vena cava syndrome (blocked)

• Endocarditis in valves, pulmonary endarteritis

• Local inflammatory response

• Glomerulonephritis

• Deposition of immune complexes in kidney

What are the main clinical signs of D. immitis?

• Cardiovascular dysfunction

• Listless/gradual loss of condition

• Exercise intolerance

• Chronic soft cough

Important to confirm by diagnosis

How exactly can D. immitis be diagnosed?

• Clinical signs and history

— Has the dog been in an endemic area?

• Radiography or Echocardiography

• Detection of Mf in blood

• Wet blood film- (Mf around 300 um long)

• Stain dry blood film

• Detection of circulating antigen by ELISA

• Immunochromatographic test

• Referred to as SNAP test (10 mins)

• Detects Ag released from female adult worms

What do antigen tests detect?

When can diagnosis or testing for heartworm?

• Antigen tests can detect a single female but not a male worm

• Antigen is secreted by adult female worm

• Start testing by 6-7 months of age (PPP ~ 6 months)

• More sensitive to do SNAP test than Mf detection, particularly if animal on prophylaxis

• Mf can be concentrated from blood to help detection

• More chance to detect on the blood smear

• Prophylaxis often stops the L1, L3 stages which makes it more difficult to detect Mf

• Some dogs have occult infections (adults but no Mf)

• And ~7% dogs have Mf but no antigen- so best to use both tests

Describe the distribution of D. immitis?

• Major pathogen in USA

• Spread rapidly over last two decades

• Climate change?

• Present in Southern & Eastern Europe

• Only ever found in UK in imported/travelled dogs

• Mosquitoes present in UK that can transmit infection (but rarely warm enough to develop)

What are the main factors effecting the epidemiology of D. immitis in dogs?

• Density of dogs

• Numbers of strays & wild canids

• Mf can be present for long periods

• Poor immunity

• Poor owner compliance for prophylaxis

What are some of the factors affecting epidemiology of D-immitis mosquitoes?

• Distribution and spread of susceptible vectors

• Environmental change/urban 'heat islands'

• Introduction/spread of new vector species

• Capacity for rapid population increase

• Short developmental period Mf-L3, 10-14 days

• Temperature dependent development

• Stop development if < 14°C (57°F)

• Some mosquitoes can survive for months

• Infection rates can vary 2-19%

• But can be much higher in vicinity of infected dogs

What is the best control of D. immitis?

• Prophylaxis is the basis of control, since control is more simple than treatment!

- Tropics, treat all year round

- Temperate zones with mosquito season, treat 1 month before to at least 2 months after

- Recommended to treat year round - better compliance

How can D. immitis be treated prophylactically?

• Macrocyclic lactones (e.g. ivermectin/milbemycin-orally or selamectin/moxidectin topically) monthly

• kill L3 and L4 up to ~6 weeks p.i.

• DEC (diethylcarbamazine) daily (not widely used)

• Prophylactic drugs do not kill adult worms

• MLs may have sterilising effect on female worms - but will not kill them, can reduce their ability to produce Mf

• But some evidence of treatment failures - resistance to drugs?

How should prophylaxis be given to dogs?

• Check Mf status of dog FIRST, Ideally dogs should be Mf-ve pre-treatment

• Ivermectin and particularly DEC also kill microfilariae at prophylactic doses

• If Mf+ve dog treated - can induce anaphylactic shock partic. if high Mf counts

• Topical moxidectin seems to limit adverse reactions

• Prophylaxis targets the L3/L4

• In endemic areas, puppies should be on prophylaxis by 8 weeks of age

• Combine with mosquito repellent/ectoparasiticide

Other than prophylactic treatment, how can heartworm parasite be prevented?

• Keep animals indoors at peak mosquito biting times- usually dusk

• Insecticide collars or spot-on

• Attempt to reduce mosquito population, but very difficult

Ultimately, best to use multiple control measures

What parasite does D. immitis harbor?

• What might targeting this parasite allow us to do?

• D. immitis harbours a bacterial endosymbiont - Wolbachia

• Killing Wolbachia with doxycycline sterilizes female- eventual death of adult worms

• Does not happen immediately

• Combination of IVM and doxycycline kills adult D. immitis

What are the current treatment recommendations for D. immitis?

• AHS* now recommends combination treatment with ML and doxycycline prior to adulticidal therapy (Moxy-Doxy)

• ML clears Mf and prevents new infections

• Dox reduces pathology associated with dead adults, shrinks the worm mass

• Followed by melarsomine (Immiticide®) to kill adult worms

• Not licensed in UK

• Exercise must be restricted

For injected animals - surgical removal of adult worms in specialized heartworm clinics

Is there concern for D. immitis in cats?

• Not well adapted to cats

• Very pathogenic in cats

• Adult worms live for shorter period of time compared to dog

• Mf are transient

• Prevalence generally 5-15% of that in local dog population

Briefly compare dog and cat infections with D. immitis.

What is the treatment and control of D. immitis in cats?

• Pathology in cats relates to the inflammatory response to worms/worm death and not from biomass of adults, as in dogs

• Pathology related to the cat’s immune response

• If no clinical signs, can allow spontaneous recovery, with monitoring

• Melarsomine is very toxic in cats

• Chemoprophylaxis using ML compounds

Describe the main features of Angiostrongylus varosum.

• Host

• IMH

• Site

The lungworm of dogs, sometimes called french heartworm or small heartworm of dogs

• Host: dog, fox

• IMH: molluscs - snails and slugs

• Site: right ventricle, pulmonary artery

• Small brown worms - 2.5cm - less pathogenic due to size compared to D. immitis

Describe the life cycle of A. vasorum.

• Coughed up and swallowed after entering the lungs

Describe the pathogenesis of A. varosum.

• Chronic infection

• Pathology associated with adults in large vessels and eggs in pulmonary capillaries

• Chronic congestive cardiac failure

• Fibrosis in arteries - immune response to the infection

• Lung mottled, penetrating L1

• Bleeding (coagulation defect)

Infection often causes lower fibrinogen levels

What are the clinical signs of A. varosum.

• Early on - asymptomatic

• Later - increased respiratory rate/ cough on exercise

• Heavy worm burden, symptoms in resting dog

• fainting

• s.c. haematoma

• Can be associated with brain and spinal cord haemorrhage

Describe the diagnostic tests for A. vasorum.

• Diagnosis Baermann on faecal sample

• But sporadic, so should sample on 3 days to ensure detection

• L1, characteristic tail with small spine

• PCR

• Antigen test (IDEX)

• Detects antigen in the blood stream of the infected dog

• Not compromised by intermittent shedding of larvae in faeces

• And a PCR test (but only used in research)