Document 6431548

Surveillance Vol.17 No.4 1990
Canine heartworm disease
Canine heartworm (Dirofilaria immitis) infestation has been detected in New Zealand
only in dogs recently introduced to this countryfrom parts of the world where the infestation is endemic. It has not been detected in
dogs resident in this county.' Two factors,
however, make establishment of Dirofilaria
infestation in New Zealand dogs a distinct
possibility.
The two factors which could make
possible the establishment of D. immitis
in New Zealand are:
1 The marked increase in prevalence in
North America and Australia and its
spread to more temperate regions
within these continents over the last
several years.
2 The presence of suitable mosquito
vectors (Aedes notoscriptus and Culex
quinquefasciatus) in parts of New Zealand.'
Consequently, technical staff of the
MAF Animal Health Laboratories routinely screen all canine blood films for
microfilariae and attempt to positively
identify any present. In reality, this involves differentiating microfilariae of
Dipetalonema reconditum from those of
Dirofilaria, as Dipetalonema infestation is
endemic in New Zealand dogs.
Recently Ruakura Animal Health
Laboratory received a blood and urine
sample for routine clinical pathology examination from a dog suspected of being
in renal failure. The results of severe
azotaemia, isosthenuria and a non-responsive anaemia confirmed this diagnosis. Occasionalmicrofilariaewere also
noted in the blood film but positive identification was not possible because of the
low numbers present. A formalinised
EDTA blood sample was requested and a
Knott's test performed. On morphological grounds this sample appeared to contain both Dirofilaria and Dipetalonema
microfilariae. This caused some concern
as the subject was a working sheep dog
from Gisborne,born in the district. It was
therefore considered necessary to investigate the case further.
However, in the meantime the dog
had been euthanatised because of its deteriorating clinical condition. A postmortem performed at the laboratory revealed a chronic interstitial nephritis, a
suppurative prostatitis and left-sided
heart failure from a chronic vegetative
lesion on the left atrioventricular valve.
No adult Dirofilaria were seen, nor were
there any pulmonary vascular myointimal proliferations characteristicof heartworm infestation? Serum tested for canine heartworm antigen was negative.
Blood samples were collected from
five other dogs on the suspect farm. Two
had microfilariaein their peripheral blood
and those were positively identified as
Dipetalonema from length and width
measurements (Table 1). All five were
seronegative to canine heartworm antigen.
In summary, the initial tentative diagnosis of Dirofilaria immitis infestation in a
sheep dog from Gisborne was ruled out
because:
1 There were no adult worms present
(no adulticide treatment had been
given).
2 There was none of the vascular pathology commonly seen in heartworm
disease.
3 Serology of the initial case and five incontacts was negative.
4 Microfilariae seen in two of five incontacts were confidently identified
as Dipetalonema reconditum.
References
1
2
3
Belford, C J, McSporran, K D, 1987: Canine
heartworndisease. Surveillance 14(4):14-15.
Robinson, W F, Maxie, M G, 1985: The
Cardiovascular System. In Pathology of Domestic Animals, Third Ed, Jubb, K, Kennedy, P
C, Palmer, N. Academic Press, Inc: 46-48.
Redinton, B C, Jackson, R F, Goodwin Seymour, W, Otto, G F 1977: The various microfilariae found in dogs in the United States. In
Proceedings of the Heartworm Symposium,
1977, Otto, G F (ed.) Bonner Springs, Veterinary Medicine Publishing CO: 14-21.
R S Ellison
K G Townsend
Ruakura Animal Health Laboratory
A M McKellow
Palmerston Road Veterinary Clinic
Gisborne
F J A Neilson
MAF Quality Management
Gisborne
I Table 1: Measurementsof microfilariaerecovered from two in-contact dogs
1 AnimalID
li
Length (10
Width (p)
Mean
Range
Mean
250
247
248 -252
243 -252
4.95
4.95
308
263
295 - 325
250 - 288
Range
Reference Measurements'
D.immitis
D. reconditum
5.0 - 1.5
4.5 - 5.5
Surveillance 17(4) 9