There are various different flea treatments for dogs. Some involve bathing the dog, others are mixed into the food, some liquids are brushed into the fur and there are collars that repell fleas.
Since you want to treat street dogs, the best treatment is one that you only have to apply once. In my personal opinion, the best options are:
- Flea collars. These are strips of rubbery kind of plastic that are drenched in a stinky liquid that repells fleas and ticks. There's a list of different flea collars by The Spruce Pets (not affiliated). You need to measure the neck circumference of each dog and buy the correct size. A collar that's too long can be cut to size. Most collars last 3 months before they should be replaced. The disadvantage is that fleas might still infest the rear end of the dog where the odor is less intense. There's also the risk that the collar can get caught and trap the dog. Some collars are designed to break in such a situation, but there's no guarantee.
- Anti flea topical. These are little vials of a liquid you apply to the the fur in the neck. It contains a substance that is poisonous to fleas and most products recommend wearing gloves while applying them. The poison is actually absorbed into the body of the dog and kills any fleas that drink its blood. You need to measure or guess the weight of the dog and apply the correct amount of topical solution to the fur. Most topicals last only one month before they should be reapplied. Some dogs feel ill and vomit the first few days after applying the topical.
- Anti flea powder. You dust the fur with this powder and try to work it into the deeper layers where the fleas live. It only kills the fleas that are already there but doesn't repell new fleas. It also has to be applied more often than the other solutions.
There are also some home remedies you can try if you cannot find any "professional" product in your area. But to be honest I am not a huge fan of these home remedies. Most of them have to be applied every few days, some contain substances that can cause discomfort to the dog (when it licks its own fur) and some contain substances that are poisonous to dogs (again, when it licks its own fur). Of the many options found on the internet this is the list that I personally think is the safest:
- Mix 50:50 apple cider vinegar and water, dip a wash cloth into the solution and rub it over the fur. Don't apply it to the face, only from the neck down.
- Cut one lemon into small pieces and boil it in 1 liter of water. Let it sit over night. The next day you can either fill it into a spray bottle to spray the fur of the dogs or apply it with a wach cloth, as described above. Don't apply it to the face, only from the neck down.
- Crush a bunch of fresh peppermint in your hands (to release the oil) and rub it over the coat. The natural peppermint oil repells fleas and the dogs will smell nice ;)
Warning: Spraying insect spray onto the dogs is dangerous to their health and not a good flea treatment.