Feline Heartworm Disease

Biology of the Infection in Cats

The cat is a resistant, but suceptable host. Thus they can get heartworms, but it takes a greater exposure than in dogs.

• If dogs not previously exposed to heartworms are infected with 100 L3 larvae, about 75 adult worms develop in almost 100 % of dogs .

• If cats not previously exposed to heartworms are infected with 100 L3 larvae, about 3-10 adult worms develop in about 75% of cats.

Thus the cat is a susceptable, but resistant host. Anywhere heartworm disease has been found in the dog, it has been observed in cats.(Countries)

(Basic review of D. immitis life cycle- dog & cat)

Incidence maps presented at World Veterinary Congress, Japan 1995 (Japan) (Italy) (US) (US SE)

The cat is more difficult to infect with heartworms than is the dog

• Less infective larvae survive to become adults, but most cats can be infected if exposed to L3 larvae.

• Experimentally, heartworm disease is more difficult to produce in cats than in dogs. Moreover, the percentage of infective larvae (L3) developing into adult worms is significantly less in cats (1-25%) than in dogs (40-90%).
• Cats are at greatest risk in a heavily endemic areas where repeated bites by infected mosquitos are common.
• Remember, the average mosquito can only transmit a maximum of 10 infective larvae, therefore an experimental infection with 100 L3 would represent 10 - 20 mosquitoes all biting a cat at the same time.
• However, experimentally, when adequate infective larvae are used, the percentage of infective larvae developing to adult Dirofilaria immitis is low, but the percentage of cats from which adult worms are recovered is high (66-90%).
• Infective larvae in the cat are poorly oriented therefore ectopic sites for the adult (brain, subcutaneous tissue, abdomen) are more common than in the dog.
• The worm burden is less in the cat (range usually 1-9 worms) compared to the dog, but up to 20 adults have been experimentally induced in a cat.
• Although the adult worms reach significant size in the cat, (female > 21 cm, male > 12 cm), their development seems to be slower in the cat than the dog.
• Experimentally, the male cat is easier to infect and the worm burden tends to be higher when exposed to the same number of L3 larvae as in female cats.

Once infected with adult D. immitis, the cat is a poor reservior

• Once infected by adult heartworms via either infective larvae (L3) or transplantation (from experimentally superinfected dogs) the natural resistance of the cat induces a shortened period of patency and a lower concentration or absent microfilaremia .

Microfilaria are bigger in width than a RBC. Although they live for up to 2 years and are present in about 60% of dogs with heartworms, but they are rarely observed in the blood stream of the cat. Therefore the cat is not considered to be a host that can provide mosquitoes with a site to acquire potential larvae. Mosquites that feed on cats rarely if ever infect other cats with heartworms.

• The average time from infective larvae being introduced into the cat until the development of circulating microfilaria in experimental infections is about 8 months and occasionally longer in the cat compared to the typical 5 to 6 1/2 months in the dog. Since the microfilaremia is transient and in very low numbers , the use of concentration tests are recommended and a negative test does not rule-out heartworm disease (see "Diagnosis" below). Microfilaremia is uncommon (< 20% of spontaneous clinical cases), inconsistent and transient when present. Infective larvae developed in about 1% of Anopheles spp. and Aedes spp. mosquitos that fed on cats with patent infections. Thus the cat is a potential but insignificant source as a reservoir for the parasite.

The life cycle would then dictate that for a cat to become infected, a mosquito must bite a species of animal with a patent infection (usually a dog) and then after proper climate conditions, bite a cat. The feeding pattern of the mosquito determines which cat get infected.

• Some species of mosquitoes will readily feed on both dogs and cats, others prefer only one species. The incidence of heartworms in cats is very high in some areas (18%) and may reflect the willingness of a species of mosquito in the area to feed on both dogs and cats.
• Thus as noted on incidence maps, there is no consistent ratio between the percent of mongrel dogs with heartworms and the percent of cats in the same area with heartworms. (Note (US SE) and (Italy)
• Which cat gets infected depends on a mosquito being willing to feed on a dog and then later feed on a cat. With over 50 species of mosquito being able to transmit heartworms, this is an important variable.

Adult parasites do not survive as long in cats as in dogs

• There is a high mortality of the L5 as they first reach the lungs, 3-4 months after infection. An acute reaction is often noted at this time.
• Evidence supports the premise that adult heartworms in the cat have a relatively short life span (probably less than 2 years) compared to the dog (approximately 5 years). Survival of adult worms after transplantation and after L3 infections would indicate that the cat does not harbor the adult as long and spontaneous recovery is much more likely in the cat than in the dog. A shortened longevity would contribute to an underestimation of the incidence of heartworm disease in the cat based on routine necropsy examination of the general population. A gradual decrease in the number of adult worms found in the heart has been noted when cats are chronologically studied. Thus the cat is a susceptible but resistant host for Dirofilaria immitis with a more transitory disease than in the dog.