Incidence of Infection Study

Project Description

Valley Fever can cause serious illness, even death, in dogs but there has been little scientific investigation into this disease in dogs. In order to be able to design a trial to test a Valley Fever vaccine for licensing in dogs, we had to know how many dogs get sick from the disease. Discovering how many dogs in southern Arizona are infected or sick with Valley Fever was the primary goal of the study. The secondary goals included understanding the relationship of the blood test results to illness in dogs, and assessing environmental and breed risk factors for infection. This study was supported by the Valley Fever Vaccine Project of the Americas.

How did we do the study?

We enrolled dogs from veterinary practices in Pima and Maricopa counties in Arizona. We studied two groups of dogs and combined the data for analysis.

Group 1: 4-6 month old puppies were determined by blood tests to be healthy and free of Valley Fever infection based on cocci serology. Owners filled out a questionnaire regarding how much time the dogs spent indoors or outdoors, size of yard, proximity to construction, where and how often exercised, and digging behavior. Owners agreed to return the puppy for Valley Fever testing two more times at 6 month intervals. Dogs that became sick during the one year course of their enrollment received a veterinary examination and extra blood tests to determine if Valley Fever was causing their illness.

Group 2: 4-18 month old puppies were tested once with a Valley Fever blood test. This group of dogs was selected by age only and were allowed to be enrolled whether healthy or sick. Owners filled out the questionnaire described above and the veterinarians submitted information on whether the dog was sick or healthy when submitting the blood for the Valley Fever test.

Analysis of infection and disease

Group 1: 124 puppies were enrolled and 104 completed at least 1 year of testing. Thirty-seven came back for an 18-month check and 7 returned for a 2-year check. Five dogs became ill with Valley Fever, including positive blood tests, during their enrollment in the study, but an additional 23 dogs had positive Valley Fever tests without being sick.

Group 2: 381 dogs were enrolled for the single blood test and questionnaire. Thirty-two had positive tests, but only 13 of the 32 were sick with Valley Fever.

Statistical analysis of all (group 1 and group 2) dogs showed that:
  • Dogs raised from birth in Pima or Maricopa county in Arizona have a 28% chance of being infected with the Valley Fever fungus by two years of age; the probability of infection is 11% in the first year of life and 17% in the second year of life.
  • Dogs raised from birth in Pima or Maricopa county have a 6% probability of becoming sick with Valley Fever by 2 years of age - 2% in the first year and 4% in the second year.

From these results, we estimate that about 4% of dogs will become sick with Valley Fever on an annual basis.

70% (42/60) of the dogs in this study had positive tests but were not sick from the infection. While we believe that most of these subclinically infected dogs go on to become permanently immune, we would have liked to follow the blood test positive, healthy dogs for several more years to determine if the infection would flare into illness in the future.

Analysis of Valley Fever serology results

Positive Valley Fever serology is reported as a titer in doubling dilutions (1:2, 1:4, 1:8, etc.). A total of 60 dogs in the 2 groups had positive Valley Fever serology and a titer. They were all analyzed together.

Only 18 dogs were classified as sick with clinical Valley Fever; the remaining 42 (70%) were healthy. These healthy dogs were classified as having subclinical infection.
  • Healthy, or subclinically infected, dogs had titers ranging from <1:2 up to 1:16
  • Sick, or clinically infected, dogs had titers of <1:2 to 1:32.

These titers overlapped significantly among dogs with clinical and subclinical Valley Fever infections. Because of this, we concluded that tests in addition to a Valley Fever titer are often necessary to diagnose clinical illness.

Analysis of risk factors for Valley Fever infection in dogs

All 485 owners filled out at least one questionnaire regarding the environment in which their dogs lived and exercised. The questionnaire asked about amount of time spent outdoors, cover of the yard (grass, gravel, etc.), size of the yard (greater or less than 1 acre), age of residence, proximity of residence to construction, number of times walked and where walked, and amount of digging. We also recorded information on the breed, sex, and age of the dogs.

A dog was considered infected if the Valley Fever serology test was positive. There was an insufficient number of clinically ill dogs (18) to analyze risk factors for a dog developing illness due to Valley Fever.

Consequently, clinically ill and subclinically ill dogs were lumped together in the following analyses of risk factors for infection.
  • Time outdoors: Compared to dogs that live primarily indoors, dogs that spend most of their time outside (80% or more) were 4.9 times more likely to be infected.
  • Yard size/roaming space: Infected dogs were 6.2 times more likely to have access to more than one acre of land when outside.

Using the questionnaire results from the dogs in group 2, multiple risk factors were analyzed together to design a statistical model to predict which factors affect a dog’s risk of infection with Valley Fever.

In this analysis, factors associated with an increased risk of infection were:
  • larger roaming or yard space
  • walking a dog in the desert
  • increasing age of the dog - for each month the dog gains in age, the risk of infection increases.
One factor was associated with a decreased risk of infection:
  • walking a dog on sidewalks

Regarding the relative protection afforded to dogs walked on the sidewalk, we predict these dogs live in a more urban area with less exposure to dust and native soils with rodent holes, factors historically associated with Valley Fever infections in dogs. Because geographic data was not available for Maricopa county, we were not able to test this hypothesis.

Regarding the relative risk of increasing age, there is probably an age when this plateaus and is no longer a risk factor for increased likelihood of infection, but we suspect it is in the range of 4-5 years. This study involved only young dogs over a relatively short period of time, so we were unable to evaluate the risk of increasing age over time.

For this study, the following analyzed factors did not show any relationship with increased or decreased risk of infection:
  • breed or gender of the dog
  • age of the home
  • proximity of the home to construction
  • yard cover (grass, gravel, etc.)
  • frequency of digging

We hope that these studies will lead to additional research on such topics as a vaccine for prevention of Valley Fever in dogs, and perhaps clinical studies of new treatments for this disease.