Category:Dog Nematodes

Nematodes of Dogs

In Britain, the most important nematode of dogs is the ascarid, Toxocara canis. Almost all puppies harbour this worm, which in large numbers can cause serious disease during the first weeks of life. T. canis larvae can also invade human tissues - impairment of vision is a possible outcome.

Other veterinary clinical problems associated with nematodes, such as hookworm and whipworm, are largely confined to large kennels or dogs in rural areas. Overseas, however, there are two nematode diseases of major significance in small animal practice. These are the hookworm, Ancylostoma, and the canine heartworm, Dirofilaria.

	Small Intestine	Caecum	Lungs	Heart
Nematode Species	Toxocara canis Toxascaris leonina Uncinaria stenocephala Ancylostoma caninum (rare in UK)	Trichuris vulpis	Filaroides (Oslerus) osleri	Angiostrongylus vasorum Dirofilaria immitis (not in UK)

Nematodes of Dogs - LUNGWORMS

Treatment of Hookworms, Whipworms and Lungworms in Dogs and Cats

Compound	Trade-Name	Hookworms	Whipworm	Lungworm
Piperazine Pyrantel Pyrantel + Febantel Fenbendazole Mebendazole Flubendazole Nitroscanate Selamectin Milbemycin Moxidectin Emodepside	various (high dose needed) Strongid (dogs only) Drontal (various combinations) Panacur Telmin Flubenol Lopatol (dog only) Stronghold Milbemax Advocate Profender (cat only)	++ +++ +++ +++ +++ +++ +++ +++ +++ +++ +++	- - +++ +++ +++ +++ - - +++ +++ -	- - - - - - - ? + ? -

Nematodes of Dogs - CANINE HEARTWORM

• Dirofilaria immitis is one of the most important causes of morbidity and mortality in dogs in many regions of the world that have a warm, humid climate, including parts of southern Europe, USA and Australia.
• The presenting signs are usually those of heart failure, but sudden collapse may occur in heavily infected dogs.
• The endemic zone for canine heartworm disease is spreading as people increasingly travel with their pets.
• Strains of D. immitis are adapting to cooler climates.
• It is not endemic in the UK, but more infected dogs are likely to be imported now that the quarantine regulations have been relaxed.
• It has a very long prepatent period, so clinical signs may not appear for many months after importation.
• Although primarily a canine parasite, cats and ferrets can become infected.
• Owners taking their pets into endemic regions require advice on how the disease can be prevented.

Dirofilaria immitis:

• a filarial worm
• females: up to 30cm long; males: up to 15cm long
• life-span 5-7years
• up to 250 worms may establish in the heart and pulmonary arteries
• produce microfilariae, not eggs.

Microfilariae:

• in peripheral circualtion
• periodicity - maximum numbers in blood evening/night
• greater than 300µm long
• life-span 2years
• present in approximately 60% of infected dogs
• microfilariae are absent from the circulating blood if:
  • only immature worms present
  • only one worm present
  • only one sex
  • microfilariae killed by immune response (in 15% of dogs)
  • females sterilised by chemotherapy (e.g. ivermectin).

Intermediate hosts:

• many, but not all, species of mosquito.

Local Epidemiology:

• determined by feeding preferences of local species, and population density.
• up to 45% of non-protected dogs infected in some parts of USA.

In mosquito:

• microfilariae → L1 → L2 → infective L3
• this takes 1week at 30°C, or 4weeks at 18°C - there is no development below 14°C.
• when mosquito next feeds:
  • L3 moves to mouthparts
  • up to 12 L3 deposited on skin
  • enter body via puncture wound.

In dog:

• larvae migrate through connective tissues and moult twice
• immature adults (L5) are 1-5cm long → caudal distal pulmonary arteries in 4months → diffuse eosinophilic reaction in lung parenchyma, then migrate back towards right ventricle
• start producing microfilariae 6-7months post-infection.

Zoonotic hazard:

• human infection can occur, but few cases are diagnosed
• this usually happens when a radio-opaque plaque is detected in the lung, and further investigation shows it to be caused by a trapped D. immitis larva.

Pathology

Worms produce:

• substances that are:
  • antigenic
  • immunomodulatory
  • pharmacologically active.

Lesions are:

• not confined to the location of the worms
• also caused by shear stress of high blood flow.

Severity:

• not associated with the number of worms
• exacerbated by exercise (i.e. by high blood flow rate)
• sedentary dogs often asymptomatic - symptoms most commonly associated with racing greyhounds.

Acute prepatent disease:

• immature adult worms in caudal distal pulmonary arteries
• leads to intense diffuse eosinophilic reaction, which in turn leads to coughing.

Chronic disease:

• mature worms in right heart and pulmonary arteries
• endothelial swelling and sloughing
• increased permeability → inflammation → periarteritis
• platelets/white blood cells activated → thrombosis
• proliferation of smooth muscle, thickening of media:

→ impairment of blood flow

→ pulmonary hypertension

→ right ventricular strain

→ right ventricular hypertrophy and right-sided heart failure

• insufficient blood pumped through pulmonary capillary bed → insufficient preload for left ventricle.

Post Caval Syndrome (Dirofilarial haemoglobinuria):

• can be acute or chronic
• heavy heartworm infestation:
  • entangled clumps of worms → impaired closure of tricuspid valve → post-caval stagnation → hepatic congestion and hepatic failure
• this is accompanied by increased red blood cell fragility, haemolytic anaemia and haemolobinuria.

Clinical signs:

• often sudden onset severe lethargy and weakness, but:
• signs variable, reflecting multiple system dysfunction - pulmonary circulation, heart, liver and kidneys:
  • lung damage (severe pulmonary hypertension; thromboembolism)
  • heart failure (right-sided congestive)
• therefore, not pathognomonic
• acute prepatent = coughing
• chronic = exercise intolerance, sometimes with ascites
• acute post caval syndrome = collapse (dyspnoea, pale mucous membranes or jaundice, haemoglobinuria)

Diagnosis:

• Physical examination:
  • signs of heart disease
  • lung involvement
• Radiography:
  • enlargement of right heart, main pulmonary arteries; arteries in lung lobes with thickening and tortuosity; inflammation in surrounding tissues
• ECG:
  • right axis deviation → deep S waves
• Echocardiography:
  • if post caval syndrome suspected - right ventricular enlargement with worms in ventricle appearing as parallel lines.

Clinical pathology:

• needed alongside physical examination and other tests to determine treatment strategy and prognosis.

Parasite detection:

• methods for demonstrating microfilariae in blood:
  • wet blood smear (okay for quick look, but insensitive) = D. immitis not progressively motile
  • Knott's test = red blood cells lysed; stained sediment examined
  • micropore filter = blood forced through; microfilariae held on filter; stained and examined
  • antibody detection ELISA = not reliable in dogs, but it is the best for cats (although some false positives)
  • antigen detection ELISA (using specific antigen from adult female worm) = reliable positives from 5-7months post-infection in dogs; although occasional false negatives occur → not useful for cats
• the immunochromatographic test (ICT) uses coloured gold colloidal particles tagged to monoclonal antibodies to visualise the presence of adult worm antigen - performance similar to antigen detection ELISA, but quicker and easier to do (but not as quantitative as some ELISAs are)
• operator error can give false positives, therefore best to confirm result with another test.

Chemotherapy:

• three treatment objectives needing different approaches:

1) Adulticidal

• risk that dead worms → thromboembolism → respiratory failure
• therefore, hospitalise and strict exercise restriction for at least 3weeks post-treatment
• organic arsenicals for adulticidal therapy:
  • Thiacetarsamide (2.2mg/kg IV bid for 2days) - hepatotoxic; skin sloughing
  • Melarsomine (2.5mg/kg IM sid for 2days) - generally safer, but greater risk of thromboembolism

NB - Ivermectin preventative doses over 16months reduces adult worm numbers

2) Microfilaricidal

• start 3-6weeks after adulticidal therapy:
  • Ivermectin (50µg/kg)
  • Milbemycin oxime (0.5mg/kg)

NB - risk of reaction to dead microfilariae in sensitised animals (lethargy, retching, tachycardia, circulatory collapse) - observe for 8hours post-treatment

3) Preventative (prophylactic)

• objective = kill migrating L4 before they reach the heart
• monthly treatments are 100% effective and safe if used properly, but often fail because of inadequate owner compliance
• test for adult infection/microfilarie before start and annually thereafter:
  • Ivermectin (6µg/kg monthly) - blocks maturation of larvae; these die only after several months
  • Selamectin (6mg/kg monthly)
  • Moxidectin (injectable formulation - 0.17mg/kg gives 6months protection)
  • Milbemycin oxime (0.5mg/kg monthly) - care → kills microfilarie, therefore risk of reaction
  • DEC (diethylcarbamazine) daily - care → kills microfilarie, therefore severe risk of reaction

Treatment of Post Caval Syndrome:

• surgical removal with forceps via jugular vein
• usually very successful, but:
• do not crush or fragment worms

→ massive release of antigen

→ cardiac failure and acute respiratory distress

→ rapid death

A typical therapy protocol:

1) Pre-treatment evaluation

2) Adulticide: 4-6weeks restricted exercise

3) Microfilaricide: 3weeks after adulticide

4) Initiation of monthly preventative treatments

5) Check for microfilariae after 2weeks

6) Check for adults (ELISA) 4-6months after adulticide, and before start of each subsequent mosquito season.