External Parasites in Dogs and Cats

by Bruce W. Little, DVM


Although they can be a problem the entire year, spring is usually the time our thoughts turn to external parasites on our dog and cat family members. It is true the beginning of warm weather in most parts of the United States will produce an environment conducive to the proliferation of parasites on our pets, in the house and any other place that is frequented by our furry friends. Most parasites will attack pets at any time of the year if the conditions are right to do so; therefore, it is necessary to be diligent for external parasites all year. Movement of people and pets from all parts of the country throughout the year will perpetuate parasite infestations, albeit sometimes with less vigor than when the warm weather comes in northern locations. It is for this reason the Companion Animal Parasite Council (CAPC) recommends keeping pets on parasiticides continuously throughout the year.

Any discussion about external parasites would be incomplete if it did not discuss ticks on dogs and cats. However, this blog published an extensive article titled, How to Protect Your Pets from Tick-borne Diseases, to which you can refer for complete information about ticks. Suffice it to say; however, that ticks are one of the most damaging parasites in which pets can become involved, and they can carry major diseases both for the pet itself and their human partners. You should check your dog or cat every day and if you find ticks present take the pet to the veterinarian for testing and treatment immediately.

Fleas are tiny, wingless insects that travel by jumping or hopping from one place to another. Fleas can jump as high as two feet, certainly high enough to land on most dogs and cats, and they can jump repeatedly as many as 10,000 times without stopping. Nature has provided them with a special springing mechanism that allows them to gain access to many places that a non-flying insect cannot. The most common flea is called the cat flea, but contrary to its name, it is the flea most frequently found on dogs and humans, as well as cats. Pets usually pick up these fleas outdoors; however, they can become infested in kennels, when dogs come to visit that are infested with fleas, or if the fleas were carried into the house on clothing, bedding or other materials that contain fleas, flea larvae or flea eggs.

Why is it important to control fleas on our pets? First, a flea infestation with just a few fleas can cause scratching by the pet to the extent that the pet causes self-inflicted skin rash or irritation. Sometimes, if left untreated, these irritations become infected causing major sores, usually on the rump area of the dog or cat. Sometimes these irritations will cause biting and scratching leading to draining wounds that attract flies who lay eggs that hatch into larvae called maggots that feed off the skin of the dog causing deep and severe wounds. Severe flea infestations can frequently lead to anemia due to the loss of large amounts of blood. However, the major reason there is need to control fleas is to prevent flea bite dermatitis. When the flea bites, it secrets a protein through its mouth parts that prevents the blood from coagulating so they can extract a blood meal. Many dogs, and sometimes cats, develop a reaction to this protein which can cause a severe allergic dermatitis. This condition may cause symptoms including fever, loss of appetite, incessant scratching that leads to further skin damage leading to more difficult problems for the pet. Also, fleas can attack humans who live in the home causing skin rash and scratching. The life cycle of fleas includes the egg, larvae and the adult fleas. A female flea can lay as many as 20 to 50 eggs per day. These eggs fall from the pet and land in bedding or carpeting and hatch into larvae in about 2 to 5 days. Flea larvae are small, hairy worm-like creatures that feed off of the dried blood that is created when the adult fleas bite the pet, and on excrement of the adult fleas. After 8 to 15 days the larvae spin a cocoon in which they pupate and develop into adult fleas. At normal room temperature, the entire life cycle can occur in about 18 days; however, void of optimal conditions, fully formed fleas can remain in their cocoon for up to 12 months. This is another reason why current science dictates that flea season lasts all year, and most external parasites on pets should be treated and managed all year around.

New and safer, more effective products aimed at controlling adult fleas on pets have made flea management possible without pesticide sprays, shampoos and dusts in most situations. Management of fleas on pets must occur in conjunction with regular, thorough cleaning of pet resting areas indoors and out. Once fleas infest a home, control will require a vigilant program that includes vacuuming, eliminating fleas on pets, and cleaning up and possibly treating areas outdoors where pets frequent. Newer products to control fleas on dogs and cats are applied either orally or topically to the body of the pet and are much more effective than the products that were available even a few years ago. Dr. Michael Dryden, Kansas State University College of Veterinary Medicine veterinary parasitologist, states that, “veterinarians can now prescribe a flea control product with residual speed of kill”, meaning that the product kills the flea before they have an opportunity to bite extensively. Dryden goes on to say, “We can now kill fleas fast enough where not enough protein is injected into the saliva of the flea to elicit an allergic reaction in the dog. We can manage flea allergies by using products with this residual speed of kill. We don’t know how many pets are affected by flea allergies, but that number is pretty high. It is the most common allergy in pets.”

Another external parasite that can cause problems in your pet are mosquitos. The mosquito bite not only causes scratching and irritation to the skin, but it is the carrier of the heartworm parasite larvae that can cause severe damage to the heart and circulatory system of dogs and cats once it establishes residence there. It is best to control mosquitos in your area by eliminating all standing water and keeping the screens on doors and windows under good repair. Mosquitos tend to be in greater abundance during and immediately after times of rain and flooding, so precautions should be taken to eliminate all areas where mosquitos propagate. Insecticide sprays and dusts may help to control the mosquito population, but it is usually wise to have a veterinarian’s prescription for insect and parasite control since some of the paraciticides will control mosquitos and other parasites as well.

Demodex mange mites can be found in both dogs and cats. The demodicosis may be local or generalized and may or may not cause itching or loss of hair. These mites infest the hair follicles and sebaceous glands. In dogs, localized demodicosis is usually found on the head and limbs in puppies less than six months old, while in cats the lesions usually occur in the groin area, ventral chest or on the limbs. Generalized demodicosis is caused by an overgrowth of the mites and can cause more severe symptoms thought to be associated with immune defect or underlying systemic disease. Demodex mange mites have not been shown to cross-infest between dogs and cats, nor are they transmitted to people. Most cases of localized demodicosis resolve spontaneously without treatment; however, there is an ointment that a veterinarian can prescribe if needed. Generalized demodicosis is much more difficult to control and may require extended and aggressive therapy to resolve the disease.

Sarcoptic mange mites infest a wide range of mammalian hosts, including dogs and other canids, human beings, horses and cattle. Cats are rarely infested with this mite. Sarcoptic mites burrow into the skin and produce an intensely itchy dermatitis with thickening of the skin and loss of hair that may cause self-mutilation. These mites typically are found on the margins of the ears, lateral elbows and lateral parts of the hind legs of affected animals, although lesions are also common on the flanks and ventral areas of the body, especially in more severe cases. In small dogs, it is not unusual for the mites to transfer to their human handlers while carrying the dog on their hip or in their arms. Infestations initially produce dry, crusted lesions that become pruritic (itchy) and frequently develop a serous exudate. Diagnosis may be confirmed by a skin scrapping, however, these mites are hard to find sometimes. Symtomatic treatment may be needed to confirm the diagnosis. There are several approved topical ointments that effectively treat this condition. People can develop a self-limiting infestation with Sarcoptic mange from dogs. The lesions that are produced will be highly pruritic, but usually clear without the need for specific treatment for the mite infestation. If lesions persist or are particularly uncomfortable, a dermatologist should be consulted.

Ear mite infestations are not uncommon in dogs and cats that do not receive routine veterinary care. These mites can be transferred between hosts by close contact. Clinical signs include shaking of the head, scratching at the ears, and inflammation of the ear canals and accumulation of copious amounts of cerumen (ear wax) with frequent purulent exudates, depending on the secondary infectious agents that are present. Cats may have what appears to be significant lesions with blood in the ear canal, yet only one or two mites are present. Other cats may have very clean ear canals and as many as 50 or so mites present. The degree of severity varies widely in cats. Cats may have as many as 1,000 ear mites in each ear, while the numbers in dogs are much lower. Mites are diagnosed by your veterinarian with an otoscope or by taking a swab smear and looking at it under the microscope. Treatment consists of products manufactured specifically for ear mites that are applied directly into the ear canal. Cleaning of the ear canal is always recommended and treating for secondary infections should be implemented. On rare occasions, people have been diagnosed with ear mite infestations.

There are two types of lice that infest dogs and cats. The sucking louse feeds in a sucking manner much the same as the mosquito and the chewing louse feeds on tissue debris on the surface of the body. Lice can survive only 3 to 7 days if separated from the host. A female lays several eggs daily throughout her life which lasts about 30 days. The eggs hatch into a nymph (called a nit) then go through two molts before becoming an adult louse. Usual signs of louse infestation include irritation and damage caused by the host such as rubbing, scratching and biting of infested areas. Heavy infestations may cause severe pruritus, restlessness, intense scratching and sometimes hair loss. Infestations are more prevalent in very young, old or debilitated animals or animals that are maintained in unsanitary conditions. Chewing lice are found in dogs and cats around the world, while sucking lice are most common in the cold climates. Lice on dogs and cats are fairly restricted parasites. They are not shared between dogs and cats and are not transmitted from pets to people. So, your kids do not get head lice from the dog or cat, or vice versa. The effects of louse infestation are usually minimal to non-existent other than a scruffy hair coat. Cats can develop heavy infestations resulting in restlessness, scratching, a ruffled coat and sometimes hair loss. You will probably have to take your pet to the veterinarian to confirm a diagnosis of lice infestation. Lice are treated with many of the same products that veterinarians use to treat flea infestations. Treated animals should be placed in a clean living space, and treatment may be repeated in a week to ensure any nymphs hatching from eggs are also killed. The eggs and immature stages will die over time due to drying out. Infested pets should be placed in quarantine and treated about a week after the first treatment before coming into contact with other pets. These parasites pose virtually no public health risk because canine and feline lice do not infest people and human lice do not infest pets.

Dogs left in a kennel or tethered outside during fly season can be attacked by flies, usually on the tips of their ears. These bites cause head rubbing and scratching until the skin on the tip of the ear is irritated that attracts even more flies that bite. This can be prevented by placing insect repellent on the ears during the time they are outside to repel the flies.

For continued control and preventation of external parasites on your household pets, consult your veterinarian for diagnosis, treatment and advice on eradicating the problem and preventing reoccurrence of parasite infestations. It will be cheaper in the long run because many insect and parasite prevention products simply are not effective and many do not perform as well as others. Your veterinary professional maintains state-of-the-art information and products to fight these conditions. Your pet will thank you!

Thanks for reading, and be sure to check back for more articles from Bruce W. Little, DVM on Veterinarians.com, and follow him on Twitter @DrBruceLittle!