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(Excerpts from a 2021 Zoetis Technical Bulletin by Professor Rebecca Traub BSc BVMS (Hons) PhD of Melbourne University)
How often should dogs be dewormed?
Deworming intervals are highly dependent on six factors, namely;
1. The age of the dog
2. The prepatent period of the parasite
3. The efficacy of the anthelmintic formulation against the adult parasite
4. The efficacy of the anthelmintic formulation against the larval stages of the parasite
5. The persistence (or residual) efficacy of the anthelmintic formulation against larval stages of the parasite, and
6. The infection pressure of these parasites in your locality.
Transplacental and to a lesser extent, lactogenic infection of Toxocara canis and lactogenic infection of Ancylostoma caninum, the most pathogenic of the canine hookworms, can establish in the gut of pups as early as 10 to 14 days after birth. Lactogenic larvae can provide a continuous source of infection to pups for up to 5 weeks post-parturition, that develop to patency 2-3 weeks later.1
The aim of deworming fortnightly until at least 8 weeks is to therefore target adult worm burdens as they mature and in turn, lower with the risk of morbidity and mortality in pups. It is imperative the dam be treated using the same deworming regime as pups during this time.
Prepatent periods of the intestinal nematodes can vary according to their species and routes of infection. For Ancylostoma spp. and Uncinaria stenocephala, the prepatent period is 14-18 days irrespective of the route of transmission. For T. canis, the prepatent period varies from 3 weeks for paratenic-host sourced infections, to 5 weeks following the ingestion of embryonated eggs. The prepatent period of Trichuris vulpis on the other hand is 10-12 weeks and can only be acquired through the ingestion of embryonated eggs in the environment, which explains why it is an uncommon parasite of dogs under the age of 6 months. In theory, deworming intervals should closely follow the prepatent period of the target parasites, as re-infection can occur soon after treatment is administered.
Therefore, except for T. vulpis, 3-monthly deworming intervals leave a large susceptibility gap of 7-10 weeks in which dogs may be actively shedding eggs of hookworms and T. canis into the environment.
It is therefore recommended that at a minimum, we deworm pups fortnightly, starting at 2 weeks of age and then monthly from 8 weeks of age.
In addition, daily removal of faeces from the environment will eliminate the risks of further exposure to the dog, other animals (including cats) in the household, and humans.
Why did my patient not respond to deworming? It remained positive for eggs a mere week after treatment.
If a dog is dewormed with an anthelmintic that has a high efficacy against adult worms (adulticide) but not against developing larvae (L3s, L4s and juvenile worms) then a one-off treatment of the animal may result in the dog developing patent, egg-producing adult worms in its gut significantly earlier than its stated prepatent period.
For example, for Ancylostoma spp. regardless of the route of infection, 4th stage larvae are recovered from the intestinal tract on Days 3-7 following infection and moult to immature adults between Days 7-11.2 If an adulticidal-only product with 100% efficacy is administered, this will not prevent the maturing L4s or immature adults from developing to patent egg-producing adults within a week of treatment. Similarly, for T. canis, an adulticidal-only product with 100% efficacy may result in L4s or immature adults developing to patency within two weeks of treatment.
The information on whether an anthelmintic product is efficacious against adult, immature adults and larval (L3 or L4) stages of a parasite can be sourced directly from its label.
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References:
1. Burke, T.M. and Roberson, 1985. Int J Parasitol. 15, 485-490.
2. Yoshida, Y. et al, 1974. J Parasitol 60, 636-641.
