A dog can be considered weak when it seems unusually tired, lethargic, and uninterested in normal activities and pastimes. Weakness is a vague term, and can result from many underlying causes. Because many of these causes are chronic or life-threatening, you should always consult your veterinarian when your dog seems abnormally weak.

It is normal for any dog to be tired after a bout of strenuous physical exercise, or during a hot day, but when your dog’s energy remains low following a rest period, or if there is no apparent cause, your dog must immediately be professionally examined.

Pathological weakness can occur in dogs for a huge number of reasons. Most of these are systemic health problems, not injuries to a particular part of the dog’s body. 

Often confused with the less serious kennel cough, canine distemper is an extremely contagious virus that impacts the respiratory, digestive, and nervous systems. It can be spread by any kind of excretions—feces, urine, mucus—from a dog up to 20 feet away. It can be air-borne. Symptoms can appear in your dog’s respiratory, digestive, and nervous systems, but runny nose and eyes are the most common symptoms. Even previously vaccinated dogs can be susceptible to canine distemper; puppies from 3-4 months old are the most vulnerable to canine distemper, but all dogs are vulnerable to this virus. Canine distemper is very common anywhere dogs are housed in close quarters, such as animal shelters.

Caused by a parasitic worm that is transmitted by mosquito bites, heartworm disease can cause severe lung disease, heart failure, organ damage, and death. It can occur in any of the United States, but is most common in coastal states from the Gulf of Mexico to New Jersey. Besides fatigue, symptoms include a light but persistent cough, decreased appetite, and weight loss. 

Congestive heart failure can occur on either the left or right side of a dog’s heart. In left-sided congestive heart failure, the opening in the dog’s ventricle and aorta is too narrow, forcing those organs to work too hard to pump blood, eventually causing their collapse. In either right- or left-sided congestive heart failure (or both at once), your dog may experience faintness, shortness of breath, and an inability to exercise, which translates to the observer as weakness. Coughing and anorexia are also common symptoms. If left untreated, the tissues become increasingly depleted of oxygen, the heart fails, and the dog dies. Subaortic stenosis, an inherited heart defect that can lead to congestive heart failure, is more common in Golden Retrievers, Newfoundlands, and Rottweilers. Congestive heart failure can also be caused by heartworms, valve degeneration, dilated cardiomyopathy, or a tumor in the heart.

When a dog cannot connect glucose and insulin, typically because the dog’s pancreas is not properly producing insulin, it develops diabetes. Because of insulin deficiency, the dog can no longer metabolize carbohydrates. The most common symptoms are excessive drinking, urination, and eating (along with weight loss); weakness, vomiting, seizures, and UTIs can develop as the disease progresses. Older dogs, female dogs, and obese dogs are at higher risk of developing diabetes mellitus, as well as Poodles, Bichons Frises, Pugs, and Dachshunds, among other breeds. 

Many drugs legitimately prescribed to your dog can be poisonous if taken in excess, while any amount of such human medications as ibuprofen are dangerous to, and must never be ingested by, your dog. Besides acting weak and lethargic, a poisoned dog may vomit, drool, exhibit lack of coordination and an irregular heartbeat, have seizures, and, in more serious cases, go into a coma. Dogs of any age, sex, or breed can be poisoned by ingesting too much of their own prescribed medication or any amount of a medication meant for humans.

Canine distemper, heartworms, congestive heart failure, diabetes mellitus, and drug poisoning are all very serious conditions which require immediate veterinary evaluation and treatment. 

Following blood testing, checking secretions, and possibly bone marrow testing, with a confirmed diagnosis of distemper, your veterinarian will instruct you on general measures to take to keep your dog healthy. Antibiotics can be used to encourage recovery, but there is no certain cure for distemper. Dogs whose complications are limited to gastrointestinal or respiratory disease are more likely to achieve a strong recovery than are dogs who have developed more advanced neurological damage. 

Once your dog has been infected with heartworms, the only solution is to kill the heartworms. The drugs ivermectin or milbemycin can be taken orally, or the drug melarsomine dihydrochloride can be injected into the back muscles. The latter is painful, and may need to be accompanied by pain medications. While receiving treatment for a heartworm infection, your dog must be confined, as physical activity while under treatment could cause an embolism, or blockage in a lung artery, which can be deadly.

There is no cure for congestive heart failure or diabetes mellitus, so your focus will be on symptom management and lifestyle change. Dogs with a mild case of congestive heart failure may not require treatment, but dogs with moderate to serious congestive heart failure may be prescribed beta blockers, and will need to have their general activity level controlled. A dog with diabetes mellitus will need daily doses of insulin, a switch to a high fiber and high protein diet, and daily monitoring of glucose levels.

When a dog has ingested poison, the poison must be removed from the dog’s system. You may be able to induce vomiting by administering hydrogen peroxide to the dog. In the case of recent poisoning, your veterinarian may also try to induce vomiting, give the dog activated charcoal to absorb the poison, or may pump your dog’s stomach. In any case of drug poisoning, the dog will be given fluids to flush the poison from its system and normalize its blood pressure.

Canine distemper, heartworms, and drug poisoning can all be prevented, and in every case prevention is much easier than treatment. Keep your dog vaccinated against canine distemper, and be careful of coming into contact with other dogs that might be infected. Your veterinarian can prescribe heartworm prevention medicine if you plan to visit a high-risk geographical area; a veterinarian may recommend that dogs living full-time in these areas take heartworm prevention medicine year-round. Make sure to administer your dog’s medicine as prescribed, and keep any human medications safely out of reach. 

The best preventive measure for diabetes mellitus is to keep your dog at a healthy weight. If the cause is genetic, diabetes mellitus cannot be prevented, only managed when it develops; this is also the case with congestive heart failure.

The cost of treating weakness in your dog can range from $800 to $3000. Treatment of drug poisoning costs an average of $800. The average cost of treating canine distemper and heartworms is $1800. The average cost of treating congestive heart failure is $2500, while at $3000, diabetes mellitus is the most expensive of these conditions to treat.