Toxoplasmosis in Animals

This is a protozoan parasite affecting many hosts but only completing its life cycle in the feline. It may affect many organs and therefore cause many clinical signs.


Bacteria is neither a protozoa or internal worm, but is addressed in this discussion because bacteria is normal on a fecal smear as the feces are composed of eighty percent bacteria. There will be many types: rods; long and short, cocci; big and small, motile and non-motile. Because many of these bacteria are normal, only massive overgrowths will be considered abnormal. Bacterial overgrowths are seen as large numbers of the same type of bacteria in the stool smear. Campylobacter is a common, highly infectious, cause of diarrhea characterized by highly motile rods in yellowish stools. It is not uncommon to see a “bacterial overgrowth” in fecal smears from dogs with enteritis from dietary indiscretion or dietary change. Trichomonas or other protozoal infections can, also mask serious bacterial disease such as E. Coli, Salmonella, Clostridia or Shigella.


hookworms in pets

Figure 1.Hookworms can cause 0.2 ml of blood loss per day per worm

Identification of hookworm eggs should be done on fresh feces. Older feces (12 to 24 hours) may have allowed the hookworm larvae in the eggs to hatch. The developing embryos in the egg are constantly changing and all forms should be recognized.

hookworm head showing sharp teeth, which grasps the intestinal villus during feeding 

Figure 2. Hookworm head showing sharp teeth, which grasps the intestinal villus during feeding.

All hookworm eggs are oval and thin-walled (Figure 4). The contents of the egg usually do not fill the entire eggshell. The contents may vary from two overlapping spheres of a light brown color with fine grainy texture to many smaller overlapping spheres of the same color and texture. Further development results in a fully formed embryo identified as a small worm folded upon itself in the egg. Eventually, the egg will hatch and a small worm will be seen under the microscope. The hookworm egg will hatch in the feces within twenty-four hours and spread to the dirt or kennel floor. The worms are delicate and need moisture to survive. They penetrate intact skin of the dog and people in moist environments. Yard and kennel sprays (Dursban®) will kill these worms. Also, the application of sodium borate (Borax®) to graveled runs at the rate of 10 lbs. per 100 sq. ft. will kill larval stages. Smooth, inert surfaces can be sprayed with 10% bleach solution as well. Fecal material should be removed from the pet’s environment.

Hookworm Life Cycle in Dogs

hookworm life cycle in dogs

Figure 3. Hookworm life cycle showing various methods of infection.

Hookworm infection can occur by five routes: Prenatal (in-utero), milk-borne, ingestion of larvae, skin penetration and ingestion of paratenic hosts by feeding raw meat. It is thought that ingestion of larvae and active penetrations are the most common modes of transmission. Hookworms are transmitted by milk-borne infection more commonly than prenatal. After infection, the hookworm larvae may migrate through the body. In adult animals with good immune systems, these migrating forms will encyst in the tissues during migration. These tissue cysts may become active during pregnancy or severe stress. The migration through the tissues terminates in the lungs where the worms are coughed up the trachea, swallowed and travel down the esophagus into the stomach and pass into the small intestine where they attach to and feed on the intestinal villi. They will mate and lay eggs within two to three weeks. The eggs that are laid will be passed in the stools and rapidly hatch into larva. These larvae are the infective stages.

They may penetrate intact skin of the host or other animals. If they penetrate into the incorrect animal (paratenic host), the worm will encyst and wait until a cat or dog consumes this animal. Larva may reach great numbers in the kennel environment. Penetration of the skin of the feet by large number of larvae may result in a severe dermatitis. The larva may survive for four months in moist soil. Worming for hookworms in dogs and cats is started at two or three weeks of age. The worming is repeated every three weeks until the pet is three months of age. The worms are only killed while they are in the intestine. Therefore, animals wormed at three weeks of age can have new worms arrive in their gut as early as five weeks of age. Worms transmitted in their mother’s milk infect nursing puppies/kittens. The mother should also be wormed at the same intervals as the offspring. Pyrantel Pamoate is recommended for puppy and kitten worming. Additional medications that are effective include Ivermectin, Drontal®, and Interceptor®. Adult cats and dogs with hookworms are re-wormed in three weeks. Fecal examinations are recommended twice yearly to monitor for reinfection.
Hookworms cause anemia in young animals. The anemia is due to the ability of the hookworms to arrive in the intestine of an animal that is three to six weeks old in a synchronized attack. This infection occurs by transmission from the mother, either in-utero or through the mammary glands. The worms arrive in the gut all at once and start a feast of blood sucking. The kitten or puppy is small and defenseless and has a lowered ability to produce red blood cells. The neonate becomes anemic, weak, stops eating, and may die. Fecal samples will be bloody but negative for worm eggs. The hookworms are too young at this time to produce eggs. The pet’s stools are first darker than normal, and then they become black, tarry and foul-smelling. The pet may have frank hemorrhage or blood mixed with the dark stools. In the final stage, the pet is not eating, the body temperature is below normal and the pet is very weak. Large volumes of feces, blood, and small hookworms may be passed in the final days of the young animal’s life. The worms are visible to the naked eye but are thin as thread and less than a quarter inch long. The gums are pale to white and the pet may develop a heart murmur from the anemia. The right treatment is prevention by worming at two to three weeks of age. Early recognition, blood transfusions, worming, and supportive care are imperative for survival of a severely affected kitten or puppy. Vitamin supplements with iron to combat the anemia and re-worming in two to three weeks are also necessary. Hookworms can cause bloody diarrhea, intestinal pain, weight loss, and vomiting in adult cats and dogs.

Roundworms in Cats

microscopic appearance of roundworm egg in cats
Figure 4. Microscopic appearance of roundworm egg

Roundworm eggs (Figure 4) are larger than hookworm eggs and are round or oval in shape. The name of this worm comes from the shape of the egg. The egg wall is thick and usually rough. The rough nature may be on the outside, inside or both depending on the species of the worm. The contents of the egg are brown, grainy and fill the egg completely. The shell makes the roundworm egg almost impossible to kill in the environment. Pesticides and yard sprays do not kill this worm. This worm is spread by ingestion of the egg either directly or in paratenic hosts. The stomach acid dissolves the eggshell releasing the worm. Dog roundworms may infect cats and vice-versa.

Roundworm Life Cycle

roundworm life cycle

Figure 5. Roundworm life-cycle showing modes of infection and transport hosts

Roundworm infections can occur by four routes: Prenatal infection via transplacental migration, milk-borne infection via trans-mammary migration, ingestion of infective eggs or ingestion of paratenic hosts. (After infection, migration through the tissues, migration in the intestinal wall and migration through the liver and lung are possible.) (Figure 5) The prenatal infected animal is born with this larva in its lung tissue. These larva are coughed up and swallowed by the animals within three weeks of birth. The kitten or puppy may also be infected through the mother’s milk. Infection typically lasts four to six months and the animal will shed eggs laid by these worms for the next 6 months. Animals of all ages may be infected by ingestion of eggs from their environment, mainly from the soil. In young animals this type of infection can result in the liver-lung-migration pattern. In older animals, the worms will migrate through other body tissues and will encyst. These encysted stages will be activated at day forty-two of gestation and will migrate to the unborn fetus or to the mammary gland. The female roundworm is a very prolific egg layer with each adult female worm laying 200,000 eggs per day. This large number of infective eggs poses a human health risk. Visceral Larval Migrans is a disease whereby a human ingests an infective roundworm egg. The egg hatches into a larval stage that migrates through the body of the human. The worm usually migrates through the nervous tissue of the brain or eye. Blindness or death can result. Fortunately, roundworm eggs do not become infective immediately – they must embryonate first – this process takes three days outside the host. For this reason fresh feces are not as infective as older feces, and regular cleaning of yards and kennels will reduce the chance of infection. The biggest danger to humans is sandboxes that may have been used by cats with roundworm infections. Roundworms in cats are easily killed with most worming medications. Pyrantel Pamoate is recommended at three weeks of age and every two or three weeks until three months of age. Removal of feces from the environment is very important in the control of this and other parasites.

The pet owner may observe roundworm infection in various stages. The worms migrate through the liver to the small airways of the lungs. In the airways, the worms grow to a size that irritates the lungs and stimulates coughing. The migration of large numbers of worms may cause pneumonia. Coughing brings the worms up into the pharynx from which they are swallowed. Sometimes the pet may cough or vomit up the small (one inch), round, cream-colored, comma-shaped worm. This immature roundworm will pass through the stomach to reside in the small intestine. There it will mate and lay its eggs. Because roundworms are so large, severe infections give the puppy or kitten a “pot-bellied” appearance. These worms feed by grazing on the intestinal villi with their three lips.
Some roundworms may be passed in the stools. These worms are between one and eight inches long and may be moving. (Figure 3) They are cream color and usually form a circle and don’t tend to lay flat. Worming medications will kill the worms but occasionally large numbers of the dead worms will impact the intestine. This may lead to excessive straining which may result in an intestinal intussusception. An unthrifty pet is a typical clinical presentation of roundworm infection. There may also be vomiting, poor hair coat, diarrhea and susceptibility to secondary infection (parvovirus). Diagnosis is usually confirmed by fecal flotation.