Student guest post: Unintended Consequences

Student guest post by Naomi Kirschenbaum

Although we can never know, there are estimates in the range of 15,000 displaced pets in the wake of 2005 Hurricane Katrina.  Many of the dogs found their way to shelters and homes in our community around the Monterey Bay in California.  As a local veterinarian the most notable observation I saw was how it “seemed” that so many were heartworm positive.  Six years later we have a published study finding a 48.8% prevalence of heartworm in these dogs.

This story is an example of a few important lessons.  First, how things seemed to me, in my clinical practice turned out to be 48.8% of the dogs, not all.  (Of course in our area we may have had a different subset of positive dogs, but I thought, in general, they were nearly all heartworm positive). Secondly, how long it takes for a study to be done and published.  In this case the study I referenced has a six year interval between the event and publication of data examining an aspect of concern.

Now, let’s step into the present.  I’m currently taking courses for a Masters in Public Health at the University of Iowa to branch out from my basic training in veterinary medicine.

Yesterday, in a course I’m taking we had a lecture on a group of zoonotic diseases, Trypanosomes.  This group of little single celled organisms, protozoans, causes problems all over the world.   In Africa it causes, Sleeping Sickness, in Latin America, Chagas’ disease.  We don’t hear a lot about it here in North America.

What came to my attention was a disease described in dogs, here, in the U.S. caused by one in this group called Leishmania.  Dogs are a known reservoir in areas where these diseases are endemic but these U.S. reports starting in the late 1990’s were in two breeds with whopping over representation, specifically Foxhounds1 and Neapolitan Mastiffs.

That’s weird, I thought.  I’ve been a small animal veterinarian for a long time and those are not two very common breeds.  What’s up?

The first two things you need to know have to do with our basic understanding of where this parasite lives and how it infects mammals.  It has been traditionally thought a mammal becomes infected from the bite of an insect vector (tsetse fly in Africa or sand fly in South America), which is carrying the protozoa.  Also, although this occurs more rarely, you can become infected by direct contact with the blood of an infected animal into your tissue, read blood-to-blood transmission.  This second bit of information will be important later.

As well from studying these outbreaks in Foxhounds, one research group received a donated pregnant bitch they new was infected which allowed them to examine the puppies and look to see if they were also infected.  They found Leishmania in the puppies.  This lends evidence of transmission of the organism from mother to puppies in utero. Their thought is the Leishmania protozoan circulates in the mother’s blood and crosses over the placenta to infect the developing fetuses.

An important point here is the novel idea that transmission of the infection can be vertical and DOES NOT REQUIRE A VECTOR.  This would mean you could sustain the parasite in a mammal population where it has never lived before and would not normally be expected to be able to live.

This disease is endemic in parts of Europe and these two breeds, although fairly rare here in the U.S., often are imported from Leishmania endemic areas to be incorporated into U.S. breeding stock lines.   These imported dogs are very valuable and key to their breeding programs.

The work done showing vertical transmission from mother to pup suggests we can establish the infectious agent in a host indefinitely.  So far we are lucky and the areas where these dogs live don’t have vector insects readily available.  I wouldn’t count on that lasting too long.  Between global travel and climate change alone, and if historical record of disease spread with so many other zoonotic infectious agents is any guide, it’s really, likely, just a matter of time.

So a final concern, more immediate, goes back to that second route of traditional transmission I described above, the direct contact, blood-to-blood infection.   Here’s the thing.  These dogs, the Foxhounds and Neapolitan Mastiff’s that are infected are breeding dogs.  Breeding dogs, by definition, are sexually intact.  Dogs that have their “parts” can more often get into scrapes (read: fights).  When dogs fight they really can tear each other up.  The fighting often occurs around the head, neck and ears.  All fight wounds bleed, a lot.  Ears especially bleed like stuck pigs.

People try to break up the fighting dogs.  People get bitten all the time doing this.  (Read: Do not try to break up fighting dogs yourself, but that’s another essay of it’s own).  The dog blood that is all over the dogs is now all over you.  You have an open bite wound.  The dog’s blood now is mingling with your tissue and blood.  You now have Leishmania.  This is the problem.

The good news is if you are immune-competent you should mount a good response to this insult and have a very good likelihood of clearing the infection.  It will require a significant effort calling upon both arms of your immune systems, the cellular and the humoral.  Unfortunately you will not be immune to reinfection should another exposure event occur.  The bad news is if you are in anyway immune compromised, not so good.  You are likely to get clinical illness.

I guess our best hope at this point in time is to help breeders see the need and importance of choosing disease free dogs.  Encourage them to buy and bring only dogs that they have tested and know are free of Leishmania into the U.S.  I know breeding for phenotype and working characteristics and abilities is the holy grail of breeders, but can’t we do it looking at the bigger picture, the greater good?

1Monti, Dean (June 2000). "Hunters hounded as leishmaniasis is diagnosed in Foxhounds". J Am Vet Med Assoc 216 (12): 1887, 1890.

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