Heartworm Prevalence Up Again in Minnesota

University Survey Shows Heartworm Prevalence is Up Again in Minnesota

Minneapolis/St. Paul (April 2003) -- The 2002 Minnesota Heartworm Survey, conducted by the Laboratory of Parasitology, College of Veterinary Medicine at the University of Minnesota, in conjunction with the Minnesota Veterinary Medical Association, showed the number of heartworm cases continued to increase last year.  Results came from data submitted by 235 individual veterinary practices (a 50 percent response rate) from 67 of Minnesota's 87 counties.
The increase shows that dogs throughout Minnesota are at significant risk to infection if they are exposed to mosquitoes, and therefore should be placed on preventative medication.  The question that often arises is when should one initiate and stop the use of the preventative.  The use of preventative medication is very much like the purchase of insurance that our pets will not become infected with Dirofilaria immitis (heartworm).  Therefore, an extra month of prevention may be a reasonable expense.  

          Canine findings

• The number of reported cases of canine dirofilariasis in 2002 was 1,463 (0.74% of 197,857 dogs tested), up from 1,151 cases (0.59% of 193,976 dogs tested) in 1999 and 0.24% of 223,169 dogs tested in 1995.  
• The seven county metro area observed a slight increase in prevalence, from 0.19% (121,916 tested) in 1999 to 0.25% (115,156) in 2002.  
• Several counties had increases: Anoka went from 0.15% in 1999 to 0.17%; Dakota, from 0.15 to 0.35%; Scott, from 0.29% to 0.77%; Washington, from 0.24% to 0.26%.  
• The remaining metro counties experienced decreases:  Carver decreased from 0.79% in 1999 to 0.50% in 2002; Hennepin from 0.11% to 0.10%; and Ramsey from 0.24% to 0.20%.  
• The prevalence of heartworm infection in greater Minnesota increased from 1.27% in 1999 to 1.43% in 2002.

          Counties with the greatest number of cases of canine dirofilariasis were Freeborn (132), Rice (99), Isanti (76), McLeod (76), and Pine (73).  There was also an increase in the number of cases of heartworm in counties directly south of the Twin Cities along Interstate 35.  With the increased prevalence of heartworm, there was no evidence that the range (endemic counties) had actually changed.  We believe that for all practical purposes the entire state of Minnesota should be considered endemic for heartworm.

          Feline results
Twenty-two cases of heartworm in cats were reported for 2002 (0.99% of 2,229 tested), down from 79 cases from 1999.  Future surveys may be able to help us to better understand the prevalence of heartworm in cats.
          The data indicate that most Minnesota veterinarians still begin their heartworm testing in March, and about one third test year-round.  Almost two-thirds of the heartworm diagnoses were made in the months of April, May and June.  The Antigen test was used most frequently (>95% of the time), while a microfilariae identification test was used by the remaining practices.  The majority of practices (>99%) prescribe once a month preventatives.
          Minnesota continues to have a relatively low prevalence of heartworm infection compared with other states  (0.74% of dogs tested positive in 2002).  This is particularly true when compared to some Southern states, where as many as 40 to 50% of pound dogs will harbor adult heartworms.  The fact that Minnesota data relate to dogs seen by veterinarians, compared to animal shelter dogs in Southern states, must be taken into account.  

          For more information, contact: 

Bert Stromberg         612/625-7008          b-stro@umn.edu
Sue Prouty              612/625-8115          prout005@umn.edu
Gary Averbeck          612/625-7048          averb002@umn.edu