Continued from last week
Last week, I had promised to commence a new theme today, since we had exhausted the topic of maladies associated with the heart and circulatory system. We dealt primarily with the malfunctions of the heart itself. However, there is one very specific parasite-based ailment that impacts so much on the heart’s function, that we have decided to include it in the discussion of heart problems.
I am referring to the heartworm infection. This disease has been dealt with before, but we have decided to revisit this sickness, because it is rearing its ugly head all over Guyana with greater incidence than ever before – possibly because of this disease’s mode of transmission which is associated directly with mosquitoes.
As the name suggests, this worm lives in the heart of dogs (and cats), primarily in the right heart chambers and in the pulmonary artery (and sometimes also in other big blood vessels). This is one wicked worm, especially because of the insidious nature of the development of the ailment it causes. By the time the symptoms show themselves the dog has been debilitated.
If the dog dies after a lingering and excruciating sickness (the heart function being compromised, all the other organ systems begin to close down), one can find – via a post mortem examination – a bundle of worms like spaghetti in the right heart chambers.
The prevalence of this disease is increasing in Guyana in proportions that are disturbing. There are certain areas in this country that one can designate heartworm enclaves. Courida Park and the Lower East Coast are two such areas. The extent of canine heartworm infection is in all probability quite huge (pan-Guyana?), but we are not researching this disease in any great detail. One or two of the veterinary fraternity may have acquired special kits and would therefore embark on an investigation on a larger scale. We will then have a better understanding as to how widespread this malady truly is. I understand that at least one commercial laboratory has the wherewithal to accurately check for heartworm. Of course, any veterinarian with a microscope can determine – with a fair degree of accuracy – the presence of heartworm in a blood sample.
I should mention that canine heartworm is to be found all over the world, especially in areas at sea level (and below) in the tropics and sub-tropics, wherever the mosquito can be found. The mosquito transmits canine heartworm!
The prevention and treatment of canine heartworm cannot really be attempted unless one understands the life cycle of the worm.
Heartworm life cycle
The tiny (can’t be seen without the aid of a microscope) immature stage (microfilaria) of the heartworm, which is in the mosquito’s mouth parts, is deposited by the mosquito on the skin. These robust microfilariae then actively penetrate the skin. These immature stages of the worm begin to grow (for almost two months) before heading for the heart. Once they have arrived in their new home – the right side heart chambers – they begin to mature and become sexually active. Then they begin to give birth to new microfilariae, which your veterinarian can discover by examining a blood sample. We are told that one female heartworm can give birth to as many as 5,000 microfilariae in one day. The veterinarian can pick up hundreds in one drop of blood.
The whole cycle – development in the mosquito, growth in the host’s tissues, circulation in the dog’s blood, further development in the heart chambers – can take up to six months. In the tropics, this cycle could be shorter (4-5 months) than in temperate and sub-tropical climates.
Of course, once undetected, the adult worms (as long as there are males and females) can continue to produce the microfilariae. The worms can live in the heart for years before even the well-cared for dog succumbs. During one post mortem (on a dog that was not a patient of mine – let me hasten to add), I found over 100 adult worms in the heart. Some of the worms measured about six inches in length. The literature tells us that the females are larger than the males and can reach lengths of one foot and can number as many as 250, with the mature females discharging thousands of young microfilariae in the bloodstream daily!
As you can see, the dog with a heartworm infestation is in big trouble.
Next week we’ll deal with the symptoms of canine heartworm disease.