Align-ju STUDENT
Life cycle
In most cases nematodes have direct life cycles , but some species have vector hosts or intermediate hosts that are paratenic ( where no parasite development occurs ) [ 1 ] .
In nematodes with a direct life cycle , eggs are dispersed in faecal matter , from which larval stages emerge . These go through multiple moults and develop into adults that can again lay eggs ( L1 – L5 – adult ). In most cases the L1 – L3 stages live outside the host . L3 is ingested by the end host ( EH ), which continues the life cycle . However , in some species L1 – L3 may develop inside a paratenic host or an intermediate host , such as the mosquito in the case of heartworm . A paratenic host differs from an intermediate host in that it does not contribute to the life cycle but merely holds the larval stage ( L3 ) until it is ingested by the EH [ 2 ] .
Once inside the EH , larval migration usually occurs . This varies by species , from minor to more extensive hepatic – tracheal migration . In the latter , the larvae cross from the small intestine to the portal vein to enter the liver and exit via the hepatic vein , vena cava , heart and lungs [ 1 ] . Some will migrate up the trachea , be coughed up and then swallowed again to enter the gastrointestinal tract and become adults to lay eggs . In most cases , adults inhabit the small or large intestine , but adult heartworms inhabit the heart or pulmonary artery [ 3 ] .
T . canis and T . cati
Toxocara is a genus of roundworms . T . canis infects dogs and T . cati infects cats . T . canis is zoonotic and is considered to be the most common ascarid parasite . Adult worms are found in the small intestine [ 1 ] . T . canis can have a significant impact on the health of EH puppies , in which hepatic – tracheal larval migration is most common . However , in an EH older than 6 months , the larvae tend to remain dormant in body tissues , such as the liver and muscle . In the case of an adult bitch , the larvae may be activated to migrate to the placenta by a change in hormones [ 2 ] . This will allow the larvae to complete their life cycle in the newborn puppies . This larval migration can also result in larvae maturing to adults in the mother , resulting in an increased output of eggs in faeces ( Figure 1 [ 4 ] , page 52 ).
Consequently , the most common route of infection for puppies is transplacental , which is why it is important that bitches are wormed during pregnancy . If the bitch is left untreated , the larvae cross the placenta and migrate via the liver to the lungs of the puppies , where they are coughed up , ingested and remain in the small intestine to develop into adults by the time the puppy is 3 weeks old [ 2 ] . Alternatively , puppies can be infected through transmammary transmission from the bitch ' s milk [ 1 ] . When worming , it is essential to check that the anthelmintic used is effective against larval and adult stages of the nematodes , as dormant larval stages may not be sensitive to treatment . The timing of treatment can also play a role – for example , treating the bitch prepartum and treating the puppies at 2 weeks of age . Usually , the pathogenic effect on the older host is limited . However , in the young dog , heavy infections can result in pneumonia due to larval migration , or enteritis due to the presence of adult roundworms .
Treatment
A number of preparations are used to treat nematode infections . They include fenbendazole , milbemycin , moxidectin , praziquantel and selamectin [ 5 ] .
The mechanism of action of most anthelmintics is to prevent the parasite maintaining its position on the gut wall where it feeds . This happens in two ways :
1 . The active ingredient ( such as pyrantel , piperazine or ivermectin ) has a paralytic effect on the worms , so they cannot maintain their feeding position on the gut wall and are evacuated from the host .
2 . The active ingredient ( such as benzimidazole or niclosamide ) affects the energy metabolism or nutrient absorption of the worms , so they are starved and then evacuated from the host .
It is possible for nematodes to become resistant to anthelmintics . This is attributed to overuse , such as treatment when the presence of worms has not been confirmed , as well as incorrect dosing , which is why it is important to have an accurate weight for the animal . Underdosing would pre-select survival of the most resistant worms , while overdosing could result in unnecessary environmental exposure .
It is helpful to carry out microscopic faecal examination to identify the eggs , which can determine the species of helminth , as some species have characteristic egg morphology ( Figure 2 [ 6 ] , page 52 ). A faecal egg count ( FEC ) before and after treatment will enable the effectiveness of the anthelmintic to be assessed . Generally , there should be an egg count reduction of at least 90 % after treatment [ 3 ] .
Not all nematode infections can be identified by a FEC . Heartworm , for example , will require serology or blood film examination , together with clinical examination . Heartworm is not endemic to the UK , but it should be a consideration in dogs that have travelled to endemic regions , such as southern Europe [ 3 ] .
T . canis infection is significant to human health , as humans can act as a paratenic host when eggs are ingested . Children are particularly susceptible . Larvae can cause abdominal pain if they migrate to the abdomen ( visceral larva migrans ), or blindness if they migrate to the eye ( ocular larva migrans ) [ 1 ] .
Volume 39 ( 2 ) • April 2024
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