Mosquito-Borne Diseases in Dogs

Mosquitoes have long been the carriers of many types of disease. But for the heartworm parasite, they are an essential link in its life cycle. When a female mosquito bites an animal infested with heartworms, it can ingest the tiny larvae of the parasite with the blood it consumes.

Within the mosquito, the larvae develop into infective larvae, which are then transmitted into a dog when the same mosquito bites him. This allows the infective larvae to migrate to the heart and blood vessels of the dog and grow into adults. Without the mosquito, the heartworm larvae cannot develop into an infective stage, and could not infect your dog.

While the itch of a mosquito bite may only be mildly annoying, these parasitic bugs can transmit infections into the bloodstream of your dog which can have fatal consequences. Mosquitoes prey on the blood of animals, and can pick up viruses, parasites, and bacteria from them, only to infect the next animal they bite. Symptoms of these diseases may not appear until the infection is severe, but can include various digestive, respiratory, and neurological signs.

The signs of a mosquito-borne disease go beyond the reaction of the body to the bite, such as itching and bumps on the skin. These signs will go away, but if your dog has contracted an infection from the mosquito, those symptoms can be quick or slow to appear. Symptoms of a mosquito-borne disease can include:

• Fever
• Diarrhea
• Coughing 
• Shortness of breath
• Weight loss
• Decreased appetite
• Lethargy 
• Depression
• Loss of stamina
• Abdominal pain
• Swollen abdomen or legs
• Jaundice
• Weakness
• Abnormal aggression 
• Increased heart rate
• Painful lymph nodes 
• Disorientation
• Stupor 
• Incoordination
• Difficulty walking
• Tremors
• Abnormal head posture
• Circling
• Muscle spasms 
• Uncontrolled, recurring eye movements 
• Convulsions
• Seizures 
• Organ failure

Types

The types of diseases that mosquitoes can transfer to dogs include parasitic infestations, viral infections, and bacterial infections. The most common include:

Heartworm

Heartworms are parasites that live in the heart and blood vessels of dogs. Once they have multiplied, they can block the flow of blood to essential organs and disrupt heart function. They can also invade the lungs, liver, and kidneys. A heartworm infestation is difficult to detect, as signs are generally absent for years. Once symptoms of coughing, shortness of breath, and anemia occur, the infestation is quite progressed and is often life-threatening.

West Nile Virus

Mosquitoes contract this virus from birds, and in turn, transmit it to other animals, most commonly horses. This viral infection often causes no signs in dogs, and does not result in serious illness. However, young, old, and immune-compromised dogs are at a higher risk of developing neurological signs, which can include difficulties walking, convulsions, tremors, circling, and a decreased appetite. 

Eastern Equine Encephalitis Virus

This virus commonly affects Passerine birds, horses, and other equids, but it has been known to infect dogs. The virus, sometimes called “sleeping sickness,” is hard to diagnose and can be fatal. Signs of fever and diarrhea can progress into uncontrolled eye movements, depression, and seizures within 1 to 2 days. 

Venezuelan Equine Encephalomyelitis Virus

Commonly seen in wild rodents, this virus causes a decrease in both red and white blood cells in the blood of dogs. Although some dogs can successfully develop antibodies and show no signs of the infection, others may experience fever, weight loss, and diarrhea, which can progress to neurological symptoms including muscle spasms, incoordination, and convulsion. Death often follows the onset of these neurological signs.

Tularemia

Francisella tularensis is the organism responsible for this bacterial disease commonly found in rabbits and rodents. Once inside an animal, the bacteria is collected by the lymph nodes, where a systemic infection can follow involving the lungs, liver, spleen, and bone marrow. The bacteria then forms masses and abscesses in the liver. It can be transmitted to a dog through the ingestion of an infected animal, or through an infected mosquito bite. Painful lymph nodes, high fevers, pain, jaundice, and organ failure are signs of this sometimes fatal disease.

A mosquito survives on the blood of animals. When a mosquito bites the skin of an animal, it draws in blood. Viruses, parasites and bacteria that are present in that blood can be taken in as well, infecting the mosquito. The infected mosquito then flies off to find another animal to extract blood from. This time when it bites, it transfers the virus, parasite, or bacteria is has contracted into the animal it is feeding from, thereby infecting the animal. 

While mosquitoes contract and transmit many different kinds of blood-borne diseases, often, the diseases are species specific, meaning that only some of these diseases can infect dogs.

Due to the range of symptoms and the possible causes, your veterinarian will start with a physical exam, which may include a neurological exam. Blood is drawn for testing, and this can reveal heartworm-related proteins. If heartworm is suspected, further testing will confirm the diagnosis and determine the severity of the infestation. These can include further blood tests, X-rays, an EKG, and an echocardiogram, all which can analyze the state of the heart, blood vessels and lungs. 

A urinalysis and fecal testing may also be performed in an attempt to narrow down a cause. While there is no test that can specifically identify a tularemia infection, blood tests can reveal abnormal white and red blood cell counts, while a urinalysis can reveal blood in the urine, all of which can point to this cause. Mosquito-borne viruses are difficult to diagnose, and samples will need to be confirmed through laboratory testing. Often, a presumptive diagnosis can be made with the visible symptoms.

For dogs diagnosed with heartworms, there are treatments available, but your dog will still have associated risks. Often, by the time symptoms of a heartworm infestation are noticed, the condition is quite progressed, and may be fatal within weeks to months. Injectable medications to kill the heartworms can be given, which will cause death and migration of adult worms into the lungs. This often leads to symptoms of coughing if the infection is severe. Complete rest for your dog is required throughout treatment, and any adverse reactions need to be reported to your veterinarian immediately. In cases where the infection is too advanced, treatment may only be prescribed to address organ damage. Antibiotics, anti-inflammatories, and other supportive care can be given.

If your dog can successfully clear the heartworms from his body, he may need continued care to address the heart and other organ damage that may have occurred. This can come in the form of special diets, diuretics, and various heart medications.

Currently, there are no cures or treatments for the West Nile Virus, Eastern Equine Encephalitis Virus, or Venezuelan Equine Encephalomyelitis Virus. Treatment is mainly supportive, and can include fluid therapy, anti-inflammatories, and anticonvulsants. Tularemia can be treated with aggressive supportive care during hospitalization, which includes antibiotics and fluid therapy. The best chance for recovery is early treatment.

Your dog’s recovery will depend on what type of disease he has contracted and the severity of the infection. The prognosis for these types of diseases can vary considerably, and your veterinarian will discuss your dog’s chances of recovery based on his particular case. The earliest treatment is administered, the better the prognosis will be. Be sure to follow all your veterinarian’s instructions after any heartworm treatments to ensure your dog can safely recover. Once he is cleared of worms, actively engage in routine heartworm prevention to protect your dog from re-infection.

There are vaccines available for West Nile Virus and Venezuelan Equine Encephalomyelitis Virus. While these vaccines cannot help your dog recover, they can protect him from becoming infected with these possibly fatal diseases. You can decrease your dog’s chances of becoming exposed to mosquito-borne viruses through management techniques to lower mosquito populations on your property. These can include eliminating standing water, using hole-free window screens in home windows, and using pet safe mosquito repellants to protect your dog.