Structure and Function After blood is oxygenated in the lungs, blood returns via the pulmonary veins to the left atrium where the mitral valve controls the one-way blood flow from the left atrium to the left ventricle. The mitral valve is about 4-6 cm2 in area and consists of two cusps or leaflets that are arranged in a circular pattern along with a muscles and tendons as supporting structures (Ray, 2006; Chandrashekhar, 2009). These muscles (papillary) and tendons (chordae tendineae) are attached to the leaflet-like strings on a parachute which prevent prolapse into the atrium. These leaflets are typically thin with an eyelid-like shape (Marieb, 2010). The anterior cusp is the crescent moon shaped part of the valve and represents two thirds of the valve and rises higher than the posterior leaflet which has a larger surface area (Enriquz-Sarano, 2009). Papillary muscles attach the walls of the left ventricle to the inelastic chordate tendineae. The chordate tendineae from each papillary muscle are then affixed to the leaflets. During the diastole phase of the cardiac cycle, the left ventricle relaxes and the mitral valve opens allowing blood to flow into the ventricle from the atrium. However, during the systole phase, the ventricle contracts, the intraventricular force on the blood causes the valves to close and the tendons then cause the leaflets to seal together (Marieb, 2010). Furthermore, the tendons also keep the valves from opening the wrong direction thus preventing backflow of blood into the left atrium.
Mitral Stenosis One of the conditions affecting the mitral valve is mitral stenosis. Mitral stenosisis is the narrowing or obstruction of the mitral valve and occurs when valve openings are smaller than normal due to stiff or fused leaflets (Chandrashekhar, 2009). This obstruction impedes the blood flow from the left atrium into the left ventricle during diastole and causes blood to collect in the atrium. Since the blood is unable to empty from the atrium, when the atrium tries to contract the atrium ends up stretching. As the atrium is being stretched, the electrical pathways that cause the heart to have a stable rhythm can become disturbed creating palpitations (Chandrashekhar, 2009). It can also cause fatigue, dizziness, chest pain, and even coughing up blood (Ray, 2006).
Mitral stenosis is usually associated with a rheumatic heart disease that was caused by an untreated streptococcal infection (strep throat). However, the wide availability of antibiotics has drastically decreased its prevalence in industrialized nations (Carbello, 1993). Other causes include infective endocarditis, rheumatoid arthritis, systemic lupus erythematosus, carcinoid hearty disease, and severe calcification of the mitral annulus (Chandrashekhar, 2009).
There are several medical treatment options. One treatment is the use of medications such as calcium channel blockers and beta blockers to slow down the heart rate, thus allowing more time for the blood to move from the left atrium to the left ventricle (Chandrashekhar, 2009). While this does allow some patients to feel better, it does not slow the progression of the disease (Iung, 2002). The second medical option involves the use of anticoagulants like warfarin to thin the blood allowing it to flow easier. This option does help prevent strokes and thromboembolism, especially in patients with irregular heartbeats (Ray, 2006).
In addition to drug treatments, there are several surgical options. One surgical treatment that works well for patients whose valves do not leak to severely and are not calcified is percutaneous mitral balloon valvotomy (PMBV). Valvotomy involves using a catheter to inflate a balloon across the stenotic valve to split commissures and increase valve area (Chandrashekhar, 2009). Another surgical option is mitral valve repair or replacement (MVR). Mitro valve repair is more difficult and in many cases does not correct mitral regurgitation (Iung, 2002). On the other hand, repair has several benefits including better preservation of LV function, lower operative and long-term mortality, no long term dependency on anticoagulation medicine, and a reduced risk of infective endocarditis (Iung, 2002).
Mitral Regurgitation Mitral regurgitation is a condition in which the mitral valve does not properly close and therefore blood leaks backwards from the left ventricle into the left atrium (Carbello, 1993). It can be caused by a variety of factors such as myxmatous degeneration, rheumatic heart disease, coronary artery disease, ineffective endocarditis, cardiomyopathy, and by damage from mitral valve prolapsing or damage from heart attacks (Enriquz-Sarano, 2009). A common form of regurgitation occurs when the left ventricle broadens causing the valves to close improperly. This can be caused by prolonged high blood pressure, alcohol abuse or chronic mitral valve leakage (Ray, 2006).
In chronic mitral regurgitation there is a gradual increase in left atrial size, while left atrial and pulmonary venous pressures do not increase until late stages of the disease and may remain asymptomatic for years (Enriquz-Sarano, 2009). As the regurgitation becomes more pronounced, the increased volume overloads the left ventricle causing the left ventricle to dilate and become hyperdynamic creating an increase in afterload, contractile dysfunction, and heart failure (Iung, 2002). Due to the left atrial enlargement, chronic mitral regurgitation patients are more prone to atrial fibrillation and thromboembolism (Enriquz-Sarano, 2009). As the condition progresses additional blood is leaked back into the atrium, causing patients to become fatigued, breathless and can eventually become disabled. Unfortunately, medicine has limited benefit; however, diuretics that reduce the fluid in the body and drugs that can reduce blood pressure may offer limited help (Enriquz-Sarano, 2009). Surgery timing is critical. It should not be done too soon creating unnecessary patient risk, nor should it be performed after the heart muscle is too weak to withstand the surgery. Long-term patients can develop pulmonary hypertension and right-sided heart failure. However, prognosis is excellent if the surgery is performed before the heart is too weak especially if the mitral valve can be repaired (Ray, 2006).
Acute mitral regurgitation is unusual. It is typically caused by an infected heart valve or a trauma to the valve structure such as a chordal rupture or papillary muscle rupture (Iung, 2002). When acute and severe, the left atrial and pulmonary venous pressures rapidly increase causing pulmonary venous hypertension and congestion as well as pulmonary edema. It is usually symptomatic because of the sudden regurgitant volume load in the nondilated left atrium and ventricle. This leads to pulmonary venous hypertension and congestion and causes the patient to become extremely ill and typically necessitate an emergency replacement of the valve.
Mitral Valve Prolapse Mitral valve prolapse is a condition in which one or both of the mitral valve flaps bulge back into the left atrium during systole. This can cause the valve tissues to become floppy and stretchy, thus preventing the valve from forming a tight seal and causing the valve to leak. Mitral valve prolapse is the most common type of mitral valve disease in America. Patients with myxomatous valve disease are at an increased risk and often have elongated and thickened leaflets. Mitral valve prolapse can cause mitral regurgitation; and while mild, it can be progressive often necessitating valve repair or replacement. Patients with mitral valve prolapse are usually asymptomatic; however, in the past it has been linked to arrhythmia, atypical chest pain, dyspnea, palpitations, anxiety, syncope, etc. but testing has failed to confirm these reports. Most people diagnosed with mitral valve prolapse have no long-term medical problems associated with it and have a normal life expectancy.
Mitral valve prolapse can often times be diagnosed with a simple physical examination. The diagnosis is confirmed with an electrocardiogram as well as to determine the severity of mitral regurgitation. Patients that do not have heart rhythm disturbances, mitral regurgitation, and whose heart is otherwise healthy are thought of as low risk for long-term problems. For other patients, especially those with severe regurgitation, are considered at risk for valve infections and typically require preventative antibiotics before surgery and before dental work. Medication can also be used to help alleviate some of the symptoms, drugs such as beta blockers decrease the force with which the heart contracts and block the effects of increased adrenaline levels can help..
Diagnosis A preliminary mitral valve disease diagnosis can often be made by an experienced physician simply by listening to the heart through a stethoscope. Chest radiology can also be utilized to show pulmonary congestion and enlargement of the left arteries and left atrium. Furthermore, an electrocardiogram (EKG) can be used to show atrial fibrillation, left atrial enlargement or in an advanced case, right ventricular hypertrophy. Also, an echocardiograph can then be utilized to diagnose mitral valve disorders as well as to determine which treatment would be best and to check the progress and effectiveness of any treatment administered. A transesophageal echocardiogram (TEE) is a type of echocardiograph that is especially effective for observing the mitral valve and its operation. It utilizes a small tube with a probe on the end that is swallowed and allows observation from the esophagus. Cardiac catheterization (angiogram), radionuclide scans, and magnetic resonance imaging (MRI) can also be utilized.
Conclusion Mitral valve diseases are some of the most common forms of heart valve diseases. Mitral stenosis, although rare in industrialized countries continue to remain a severe medical problem especially in developing countries. While medical therapy can suppress some symptoms, minor surgeries such as those utilizing balloons may be an option. In other patients, heart valve surgery will be needed to either repair or replace the valves. Mitral regurgitation may accompany other heart diseases and should be surgically corrected when severe or when decreasing heart function necessitates, preferably utilizing repair over valve replacement. Mitral valve prolapse is typically not harmful but for a small minority can lead to mitral regurgitation or other problems. Patients with mitral prolapse should consider using antibiotic prophylaxis before some dental and surgical procedures.
Intestinal Parasites in Wild Rats
Mauritius is a small island of 1865 km2 situated 900 km east of the Malagasy Republic in the southern Indian Ocean. The island was found to be suitable as a port of call for traders travelling along the famous Spice Route from Europe round the Cape of Good Hope to the East Indies and was named East India trade route. The Dutch introduced sugarcane, orange trees, mango trees, rice and tobacco during their stay in Mauritius. They also brought deer from Java, rabbits, sheep, chickens and ducks. (http://mauritius.genosy.com)
The island has been much altered under various colonial powers. The Dutch, French and British cleared the dense forests that covered the island leaving very few areas of intact forest which are found in the highland and in the south-west and these have been invaded by exotic species such as wild plants and various introduced species (e.g. black rats and pigs). (John A. N. Parnell et al., 1989; http:// know-britain.com) Black rats were introduced in Mauritius probably by the Europeans in the 15th century when they were visiting the island. These rats were found in their vessels and they escaped and invaded the forests. (http://books.google.mu)
Plague was introduced in Mauritius by rats which were in Indian vessels trading with the island. The first case of genuine plague in Mauritius was discovered in Port-Louis and was confirmed by blood analysis. The victim died 24 hours after having been attacked. In February 1899 an outbreak of bubonic plague was reported in Port-Louis. This affected badly the island as the victims’ houses were burnt, affected people were sent to the segregated ranches outside of town. Suspects were put in quarantine in the suburbs of Port-Louis. This had a very bad effect on the economy and the local inhabitants. (John C. Campbell., 1899)
Recently in Mauritius, two employees of the airport of Plaisance died on 17 July 2013 and 14 August 2013 of leptospirosis. Another patient suspected of having contracted leptospirosis was admitted to a hospital in Mahebourg. The latter was working in a security company at Plaisance airport.These cases have aroused as the airport was under construction, forcing the wild rats to leave their natural habitats and live in proximity to humans. The three patients exhibited the same symptoms namely, muscle aches, vomiting and high fever.(http://en.indian-ocean-times.com)
Types of rats
Rats are highly opportunistic, prolific, mobile, troublesome and damaging rodents that have capitalized on human movements to conquer most continents and a variety of habitats. They are considered as pests in many countries as they destroy crops, eat and contaminate food, damage structures and properties, disturb the ecological cycle and transmit diseases and parasites to other animals and humans. (Andrea Paparini et al., 2012; R. M. Timm et al., 2011)
There are two main rat speciesworldwide and these are:
Rattus norvegicus, also known as the Norway or brown rat.
Rattus rattus, also known asthe roof or black rat.( http://pestcontrol)
Table 1.1: Classification of the Norway and Black rats (http://biologycorner.com)
Large and robust
Sleek and agile
Grey to white
198 – 510 grams
141 – 283 grams
Shorter than body
Longer than their heads and bodies combined
Long enough to reach eyes if folded over
They usually live in the basement or ground floor of buildings, in moist areas especially in gardens and fields, for example in sugarcane fields and near rivulets, and beneath rubbish or woodpiles.
Their nests are found above the ground in shrubs, trees and dense vegetation. They can be found in enclosed or elevated spaces in buildings. They prefer ocean-influenced, warmer climates.
Table 1.2: Characteristics of Norway and Black rats (R. M. Timm et al., 2011; http://ipm.ucdavis.edu)
Rats as disease carriers
Rats are rodents that spread diseases to humans and animals as they act as reservoir hosts for many zoonotic pathogens. (Siti Shafiyyah et al., 2012; Jesse Adams) They transmit a number of zoonoses as they harbor and disseminate the pathogens involved, either through their biological materials or via their ectoparasites. (Antoniou M et al., 2010) Some of the diseases transmitted by rats to humans and other animals are rabies, leptospirosis, murine typhus, spotted fever, lassa fever, polio, meningitis, trichinosis, rat-bite fever, salmonellosis, plague, and toxoplasmosis. (http://cowleys)
Helminthes are worm-like parasites characterized by elongated, flat or round bodies. They are also known as triploblastic metazoans. They fall under two main phyla: Platyhelminthes and Nemathelminthes. Platyhelminthes comprise the classes Cestoda (tapeworms) and Trematoda (flukes) whereas Nemathelminthes comprise the class Nematoda (roundworms). Helminths involved in the intestines of rats are cestodes and nematodes.
Table 1.3: Brief phylogenic classification of helminthes (FEG Cox, 1982, 1983)
The full classification of cestodes and nematodes can be found in the appendix.
Not separate (monoecious/ hermaphrodite).
Suckers are present.
Scolex with hooks and rostellum.
No suckers and no hooks.
Well developed buccal cavity armed with teeth or cutting plates.
Absorption by microvilli on the cuticle’s surface.
Present and complete.
Table 1.4: Characteristics of helminthes. (Dawit Assafa et al., 2004)
Little is known concerning the intestinal parasites living in the locally available wild rats. The prevalence of parasites among wild rats (Rattus norvegicus and Rattus rattus) throughout the world is well documented. A study conducted by Tung et al. (2009) showed that 93.7% parasites prevailed on 95 rodents from different localities in Taiwan. Nippostrongylus braziliensis and Hymenolepis diminuta were the main endoparasites detected in a study done by Easterbrook et al. (2008) in 162 rats in Baltimore, Maryland, USA. In 2008, Elshazly et al. found that the commonest cestode was H. diminuta and the commonest nematode was Capillaria hepatica in rodents in Egypt. 999 feral rats were examined by Singh and Chee-Hock in 1971 and 450 were positive for nematode parasites. Capillaria hepatica was reported in wild rodents collected from different states in peninsular Malaysia by Liat et al. in 1977. From 151 house rats, Rattus rattus diardii collected from five different localities, examined in Kuala Lumpur, Leong et al. (1979) recovered nineteen species of parasites, H. diminuta and N. brasiliensis being the predominant species. (Siti Shafiyyah et al., 2012)
In a study conducted on 41 Norway and Black rats collected from two regions of Gaza, the prevalence ofintestinalparasites was 58.5%, consisting of Strongyloides eggs, G. lamblia, Hymenolepis diminuta, E. histolytica/dispar, Syphacia obvelata, Isospora, Acanthocephala and Heligmonoides josephi. (Al Hindi AI,Abu-Haddaf E., 2013)
In a surveillance programme conducted in625 wildrodents in 51 different areas of Cyprus, 92 animals were detected with Cysticercus fasciolaris, Hymenolepis diminuta, and Physalloptera spp. 12 different Salmonella spp. and serotypes were also detected in the intestine of 56rats.(Antoniou M et al., 2010)
Two species of cestodes (H. nana, H. diminuta), two genera (Raillietina species I, Raillietina species II) and one unidentified Hymenolepididae and the nematodes Streptopharagus species and Monanema nilotica were demonstrated on 220 Nilerats caughtin different regions of Sudan during the period January 2003-January 2006. (Fagir DM,El-Rayah el-A., 2009)
Rats and mice are known to be carriers of at least 35 diseases. They transmit swine and poultry diseases which also affect humans. The following are reservoirs of rodents:
Bacterial zoonoses such as leptospirosis, salmonellosis, salmonella typhimurium, tularaemina, yersiniosis, rat-bite fever plague, meliodosis and lyme disease.
Rickettsial zoonoses such as Siberian tick typhus, rickettsial pox and flea-borne typhus.
Viral zoonoses such as Argentinian haemorrhagic fever, Venezuelan equine encephalitus, lymphocytic choriomeningitis, haemorrhagic fever with renal syndrome and lassa fever.
Parasitic zoonoses such as echinococcossis due to E. multilocularis, leishmaniasis, schistosomiasis, angio strongyliasis, toxoplasmosis and Trichinelliosis.
Mycotic zoonoses such as histoplasmosis. (Jesse Adams)
So far there is no record available on these intestinal helminths and the other organisms in wild rats in Mauritius.
Dog ownership, dog behaviour and transmission ofEchinococcusspp. in the Alay Valley, southern Kyrgyzstan
Bioinformatic prediction of epitopes in the Emy162 antigen ofEchinococcus multilocularis
Characteristics of Cestodes
Tapeworms are flattened, elongated organisms, and consist of segments called proglottids. They are usually 2 to 3 mm long and may reach up to 10 m. They may have three to several thousand proglottids.
Cestodes are hermaphroditic. Pseudophyllidean tapeworms’ eggs exit through a uterine pore found in the center of the ventral surface. The female system of cyclophyllidean tapeworms includes a uterus without a uterine pore.
Figure 2(a): Structure of tapeworms. (http://pathobio.sdu.edu.cn)
Cestodes possess a scolex bearing the organs of attachment, a neck which is the region of segment proliferation, and a strobila which is a chain of proglottids. Elongation of the strobila occurs when new proglottids add in the neck region. The immature segments are nearest to the neck and the sex organs are not fully developed. The more posterior segments are mature. The terminal segments are termed gravid as the uterus is egg-filled.
The scolex contains the brain or cephalic ganglion. Externally, the scolex consists of a bothria, rostellum or acetabula. A rostellum is a retractable, cone-like structure found on the anterior end of the scolex and it may be hooked. Bothria are long and narrow with weakly muscular grooves that are present in pseudophyllidean tapeworms. Acetabula are characteristic of cyclophyllideans.
Adult tapeworms do not have an alimentary canal. Substances enter the tapeworm across the tegument which possesses numerous microvilli.
A body cavity is absent.
Pseudophyllidean tapeworms’ eggs are operculated, whereas those of cyclophyllideans are not. All tapeworm eggs contain an oncosphere. That of pseudophyllidean tapeworms is called a coracidium as it is ciliated externally. The eggs of cyclophyllideans are released only when gravid proglottids are shed into the intestine. Some proglottids disintegrate to release eggs that are voided in the feces whereas other proglottids are passed intact.
The egg develops into a procercoid stage in its first immediate host and in its next intermediate host it develops into a plerocercoid larva. It becomes an adult worm in the definitive host. Depending on the species, the oncosphere of cyclophyllidean tapeworms develops into a cysticercus larva, cysticercoid larva, coenurus larva, or hydatid cyst in specific intermediate hosts and finally become adults in the definitive host.
Figure 2(b): Generalized life cycle of tapeworms. (http://skola.okuladocs)
Table 2.5: Characteristics of Cestodes (Gilbert A. Castro: http://ncbi.nlm.nih.gov)
Characteristics of Nematodes
Nematodes are cylindrical organisms. The body wall consists of an outer cuticle which may have longitudinal ridges called alae.
Nematodes are usually bisexual. Males are smaller than females. They have a curved posterior end, and some species possess copulatory structures, such as spicules or a bursa, or both. The males possess one or two testes that lead into a seminal vesicle and eventually into the cloaca. The female system is made up of reflexed ovaries where each ovary is continuous, with an oviduct and uterus. The uteri join to form the vagina that opens to the exterior through the vulva.
During copulation, sperm is transferred into the vulva of the female and it enters the ovum. The zygote secretes a fertilization membrane which gradually thickens to form the chitinous shell.
Figure 3(a): Structure of nematodes. (A) Female. (B) Male. (http://pathobio.sdu.edu.cn)
They possess neither suckers nor hooks. The buccal cavity may be absent or minuteto spaciousthat may be armed with teethor jaws.
The alimentary canal is complete, with both mouth and anus. The lips bear bristles which are sensory papillae. The esophagus has a different shape in different species. The intestine is composed of a layer of columnar cells with microvilli on their luminal surface.
The body cavity, pseudocoelom, is enclosed by a body wall containing longitudinally-arranged muscles It contains the flattened gut and various reproductive organs.
Below the shell is a second membrane that acts as a selective barrier so that the egg is impervious to all substances except oxygen and carbon dioxide. A third proteinaceous membrane is secreted in some species as the egg passes down the uterus and is deposited outside the shell.
Females lay eggs that are passed into the external environment. They undergo three developmental stages, L1, L2, and L3, before the nematode is again infective to another host. A first stage larva develops inside the egg which hatches. The hatching process is controlled by several factors such as temperature and moisture levels of the environment which stimulate the enclosed larvae to hatch by secreting enzymes to digest the egg membranes, and exerting pressure against the weakened membranes to rupture them and escape. The newly hatched L1 feeds on bacteria and grows. The molting process allows further growth of the larva. During each life cycle nematodes molt four times, each occurring at the end of every larval stage. Moulting separates the first and second larval stages (L1 and L2), the second and third larval stages (L2 and L3), the third and fourth larval stages (L3 and L4) and the fourth larval stages and immature adults (L4 and L5). The L5 grows to the limit of its new cuticle and develops into a sexually mature adult.
Figure 3(b): Generalized life cycle of intestinal nematodes. (http://pathobio.sdu.edu.cn)
Table 2.7: Characteristics of Nematodes (Gilbert A. Castro: http://ncbi.nlm.nih.gov; http://cal.vet.upenn.edu; http://pml-nematode.org.uk; http://userwww.sfsu.edu)