Internal and External Parasites in Dogs and Cats

Internal Parasites

Internal parasites (also known as worms) are frequently found in dogs and cats, yet often may not be noticed in your pet’s stools. The most common internal parasites include roundworms, tapeworms, hookworms and whipworms. These worms can cause problems such as vomiting, diarrhea and weight loss. In severe cases, small animals may suffer from stunted growth, malnutrition, and even the possibility of pneumonia. Worms can spread between your pets and have the potential to infect humans too.

We recommend routine deworming of all dogs and cats, regardless of whether they remain indoors or whether they go outdoors frequently. Puppies and kittens are particularly susceptible to infection and deworming should be started around the time of their first vaccination.

Oral tablets or liquid medications are available for effective deworming. Please speak to the primary care veterinarians at CityU VMC to discuss deworming protocols for your pets.


Heartworm is a parasite that can infect dogs (uncommonly cats and rarely humans). The parasite is transmitted through mosquito bites and the larval worms then travelling through the blood vessels to the heart. Here, the worms grow and reproduce over months to years and cause damage to the heart and lungs. Eventually, these can cause problems such as coughing, exercise intolerance, breathing difficulties, weakness, weight loss, abdominal distension and in even cause death.

Regular heartworm prevention is recommended, and available preventative products include monthly tablets, monthly spot-ons or yearly injections. Note that if your dog is a collie-related breed, please be aware that some preventatives are contraindicated or should be used with caution due to potential genetic mutation in these breeds. There is genetic testing available, which can be discussed with the primary care veterinarians at CityU VMC, as well as which prevention strategy is most appropriate for your pets.

Even with stringent prevention, a yearly blood test for heartworm is recommended. Should your pet be overdue their heartworm prevention or if your puppy has not started heartworm prevention by 6 months of age, please arrange an appointment with the primary care veterinarians at CityU VMC for a blood test prior to giving any heartworm prevention products.


Ectoparasites can be more commonly known as external or skin parasites.
Fleas and ticks are common ectoparasites of dogs and cats. Others include sarcoptes mites, demodex mites, ear mites and less commonly cheyletiella mites or lice.
Fleas can cause itchiness, hair loss and may transmit intestinal worms. They can spread between dogs, cats and often humans too. Mites and lice can also result in skin problems in your pet but tend not to infect humans.

Ticks usually do not cause skin problems but they can spread a range of diseases to mammals. “Tick fever” is a common disease seen in dogs in Hong Kong, and results from parasites being spread through tick bites. Tick fever may be associated with signs such as lethargy, weakness, fever, reduced appetite, pale gums, bleeding (internal and external), dark-coloured urine, etc. If you are suspicious of tick fever in your dog, please see us as soon as possible.

A number of preventative products are available for ectoparasites, including spot-on treatment, oral tablets, collars and sprays. Heavy infestations may require additional medicated shampoos, ear drops and environmental decontamination. Please speak to the primary care veterinarians at CityU VMC to discuss which prevention strategy or treatment is most appropriate and safe for your pets.