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Dirofilaria immitis, heartworm
cellular organisms - Eukaryota - Fungi/Metazoa group - Metazoa - Eumetazoa - Bilateria - Pseudocoelomata - Nematoda - Chromadorea - Spirurida - Filarioidea - Onchocercidae - Dirofilaria - Dirofilaria immitis
OverviewDirofilaria immitis is a filarial parasitic nematode that infects primarily dogs and other canid species. It usually resides in the pulmonary, femoral, and hepatic arteries, as well as other blood vessels including veins and right atrium of live dogs. Heartworms cause serious vascular damages and can be fatal, especially in working dogs with high level of physical activity.
Vascular pathogenThe common name heartworm is originated from the fact that the worms are frequently found in the heart. However, it is considered to be a postmortem artifact. Heartworm is a vascular pathogen rather than an intracardiac one.
MosquitoesThe vectors of Dirofilaria immitis larvae are many mosquito species. The following is an incomplete list of mosquito species that were shown to be capable of supporting larval development of Dirofilaria immitis to the 3rd (infective) larval stage: Culex pipiens (often infects cats), Anopheles maculipennis, Coquillettidia richiardii, Aedes triseriatus, Ochlerotatus notoscriptus (prevalent in Australia), Aedes albopictus, Aedes aegypti, Culex quinquefasciatus, Aedes taeniorhynchus, Aedes scapularis, Aedes trivittatus.
HostsDirofilaria immitis is capable of infecting wide variety of mammals (cats, dogs, rodents, horses, humans), as well as birds (there is a documented case of the parasite infecting the Humboldt penguin). Human pulmonary dirofilariasis, while being a benign and relatively rare zoonosis, represents itself as a medical problem, because it being manifested as a "coin lesion" similar to pulmonary neoplasia (tumor) that warrants unnecessary invasive diagnostics and even surgery.
Development in non-canidsFrequency of infection in organisms other than dogs often depends on regionally prevalent competent mosquito species and their preferences to different hosts. Also, normal development of heartworms in hosts other than dogs is usually inhibited; they mature longer and live shorter.
Heartworm disease in catsClinically affected cats may present with a wide range of clinical signs, such as chronic coughing, laboured breathing and vomiting and some infected cats die suddenly without any premonitory signs (acute death syndrome) due to acute pulmonary arterial infarction. However, many cats are subclinically infected and infection tends to be self-limiting.
Endosymbiont WolbachiaDirofilaria immitis, like some other filarial parasites, carry endosymbiotic Wolbachia bacteria whose metabolites contact with the host's organism and can play a significant role in the survival, adaptation, and reproduction of the pathogen, as well as in the host's immune response and overall pathogenesis. Specifically, two main mechanisms of Wolbachia interfacing with the host were identified: (1) a release of Wolbachia upon death of the parasites regardless of their developmental stage of the parasite, and (2) continuous release of Wolbachia products (metabolites, excreted proteins, etc.) by heartworm females during their production of microfilariae (L1 larvae of the worm, see below). Discovery of Wolbachia changed traditional view on duality of filaria-host interactions. It presented an additional challenge for researchers to understand and conquer filarial parasites.
Accidental dissection of worms during extraction procedures performed on cats, results in an acute shock-like reaction and death. It is hypothesized that the damage to the heartworm cuticle causes the sudden release of large amounts of heartworm antigen resulting in acute systemic anaphylaxis.
United States statistics (2001)According to 2001 year survey the following states had greatest numbers of dignosed human dirofilariasis: Texas (23), Florida (16), Louisiana (10), and South Carolina (8).
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(microfilariae) During bloodmeal, the mosquito ingests some microfilariae that start their development into infective heartworm.
- L2 Microfilariae migrate from the midgut to the malpighian tubules within 48 hr after ingestion. In the ensuing 4 days of intracellular development, the parasites become shorter and thicker transforming into so-called sausage form non-motile larvae, which contain virtually all major organ systems of the parasite.
- L3 Inside the mosquito microfilariae develop into the infective larval stage (L3). At 26°C, the first molt occurs about 10 days after infection giving rise to elongated motile larvae, which in about 3 days molt and transform into mature L3 infectious larvae. Within 17 days of infection, the L3 larvae move to mosquito's mouthparts; they break out of the mouthparts during the bloodmeal, and have to find their way through the hole in the skin after the mosquito finished feeding; as many as 45% of the infective larvae are being lost on the surface of the skin and fail to enter the organism.
in the host Description of the migration of the parasite in the host slightly varies in reviewed sources.
- L3 The larvae settle at the place of the infection; in 1-12 days they molt and transform into L4.
- L4 The larvae migrate with blood and many of them settle in submuscular membranes all over the body as intermediate sites of development; between 85 and 120 days of infection L4 larvae move from the intermediate sites by entering the blood vessels and migrating to places where they ultimately lodge; if L4 larva enters an artery, it will likely to develop in the branch of the arterial tree, including pulmonary arteries; if L4 larva enters in a vein, it will likely arrive in the lungs.
- L5 L5 larvae are an immature adult worms, the last larval stage before mature adults.
- Mature worm Mature male heartworms are 12-20 cm long and 0.7-0.9 cm wide with a tapering, spirally coiled posterior end. Female worms are 25-31 cm long and 1.0-1.3 mm wide with an obruse posterior end. Overall, in the dog, it take a little over six months for the infective larvae to mature into adult worms that can live for five to seven years in the dog; in cats, it takes about eight months to mature into adult worms that live from two to three years.
(microfilariae) L1, first stage larvae that circulate in the bloodstream and infect mosquitoes. Mature female heartworms are ovoviviparous and release unsheathed motile vermiform embryos into the host's bloodstream. Microfilariae are approx. 300 μm long and 7 μm wide. Depending on the number of adult parasites they reach concentrations from 103 to 105 microfilariae per milliliter of blood. The microfilariae appear in the blood of experimentall infected dogs 190-197 days after infection. They can circulate for years in the bloodstream of canid species and infect mosquitoes.
- Initial infection, asymptomatic and usually undetected, takes place between L3 and adult stages of the parasite, before microfilariae are released into the bloodstream of the canine host and the parasites at different stages of development start dying because of the host's defense reaction or other reasons.
- Chronic asymptomatic infection is the most wide-spread type of the infection. The animal hosts (dogs and cats) do not exhibit clear signs of vascular and cardiac distress especially when the parasite burden as well as physical activity are low.
- Chronic symptomatic infection usually occurs when the parasite's burden is high and vascular deterioration and local inflammation events take place because of the parasite's activities: feeding, reproducing, leaking toxins and antigens, etc. This period can last as long as 5-7 years. Coughing, anemia, weakness, and lethargy are common.
- Acute infection can occur as a result of (1) ectopic (not typical) migration of the worms in an atypical host (e.g. cats) or after strenuous physical activity, (2) massive death and disintegration of the parasites, or (3) extremely high parasite burden. Heart and kidney failure, seizures, blindness, and ultimately death can occur.
Presence of microfilariae
- Chronic microfilaraemic infection is characterized by presense and gradual accumulation of microfilariae (L1 stage larvae) in the host's bloodstream. This type of infection occurs in canine species infected by both genders of the parasites (males and females).
- Transient microfilaraemic infection is characterized by brief occurence and subsequent clearance of the microfilariae in the bloodstream of an atypical host such as cats.
- Amicrofilaraemic infection is characterized by absense of the microfilariae in the bloodstream of an atypical host such as human. Amicrofilaraemic infection also occurs when the host was infected by one gender (male or female) of the parasite.
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